JOURNAL OF PETROLOGY
VOLUME 52
NUMBER 3
PAGES 603^641
2011
doi:10.1093/petrology/egq094
Deep Mafic Roots to Arc Volcanoes: Mafic
Recharge and Differentiation of Basaltic Andesite
at North Sister Volcano, Oregon Cascades
MARIEK E. SCHMIDT* AND ANITA L. GRUNDER
DEPARTMENT OF GEOSCIENCES, OREGON STATE UNIVERSITY, CORVALLIS, OR 97333, USA
RECEIVED JUNE 10, 2009; ACCEPTED DECEMBER 10, 2010
ADVANCE ACCESS PUBLICATION FEBRUARY 2, 2011
The deep crustal magmatic history of arc volcanoes is obscured by diversity in mantle inputs, modest isotopic contrast between magma
and wall-rock, and overprinting processes in the middle and upper
crust. To identify and quantify processes in the deep arc crust, we
investigated the evolution of the mafic composite North Sister
Volcano, the oldest and most mafic of the Three Sisters Volcanic
Field of the central Oregon Cascade arc. Here, intra-arc extension
limits the degree of magma interaction with the mid- to upper crust
and the range in primitive magmas delivered from the mantle is
known. North Sister Volcano has produced low-K basaltic andesitic
magmas (0·5^0·8 wt % K2O) for 400 kyr during four
central-vent eruptive stages and along the late, 11km long Matthieu
Lakes Fissure. Although restricted in bulk composition (53^55 wt
% SiO2), North Sister basaltic andesites from different stages cluster into elemental and isotopic groups. Over time, North Sister basaltic andesites generally have decreasing compatible elements, such
as Ni (from 112 to 40 ppm), and increasing Al2O3 and TiO2.
Concurrently, incompatible elements remain the same or decrease
(e.g. from 302 to 247 ppm Ba). Isotopic variations at North Sister
are small, but systematically progress toward more mantle-like
ratios with time; 87Sr/86Sr decreases (from 0·70369 to 0·70356),
and 144Nd/143Nd increases (from 0·51285 to 0·51292). We present a
multi-stage petrological model for the evolution of North Sister
magmas to account for: (1) the generation of low-K basaltic andesite; (2) geochemical variations within the eruptive stages; (3) evolution of the magma system over time to more mantle-like
compositions. The earliest and most isotopically ‘crust-like’ (highest
87
Sr/86Sr and lowest 143Nd/144Nd) North Sister magma is consistent with two-component mixing of regionally typical mantle-derived,
low-K tholeiites with partial melts of the crust. Crustal melts must
be high in SiO2 and Al2O3, and most probably result from
low-degree melting of plagioclase^clinopyroxene amphibole-bearing
gabbro at high pressure. Variations in highly compatible elements
within compositional groups (e.g. 60 ppm Ni within a single
group) reflect fractionation of plagioclase, olivine, and clinopyroxene
and recharge by more primitive basaltic andesite that overprint
longer-term variations between groups. To understand the evolution
of the North Sister basaltic andesite magmas through time, we use
an energy-constrained model that balances assimilation of refractory
gabbroic wall-rocks and abundant recharge by mantle-derived
low-K tholeiites. These complementary processes allow Sr and Nd
isotopic ratios to become more like those of the regional basalts while
maintaining high Ni concentrations. Low-K basaltic andesites like
those of North Sister Volcano are found along the Oregon Cascade
arc and they imply that low-K tholeiitic magmas interact with a refractory mafic underplate along its length. Dominantly basaltic andesite volcanoes are common in arcs and provide insight into the
extensive, albeit compositionally cryptic mafic underplating and
intraplating that affects arc crust.
*Corresponding author. Present address: Department of Earth
Sciences, Brock University, St. Catharines, ON, Canada L2S 3A1.
Telephone: (905)-688-5550 X3527. E-mail: [email protected]
The Author 2011. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oup.com
KEY WORDS: Cascade arc; basaltic andesite; deep arc crust; Oregon;
petrologic modeling
I N T RO D U C T I O N
Andesites are the signature compositions in arcs and have
been the focus of many petrogenetic models weighing the
relative importance of the mantle versus crustal inputs
(e.g. Smith & Leeman, 1987; Grove et al., 2002) and the
JOURNAL OF PETROLOGY
VOLUME 52
NUMBER 3
MARCH 2011
Fig. 1. (a) Tectonic map of the Cascadia subduction zone, modified after Schmidt et al. (2008). North Sister of the Three Sisters Volcanic Field is
located at the intersection of the north^south-trending High Cascade Graben, indicated by normal faults, and the Brothers Fault Zone. MM,
Mt. Meager; MC, Mt. Cayley; MG, Mt. Garibaldi; MB, Mt. Baker; GP, Glacier Peak; MR, Mt. Rainier; SH, Mt. St. Helens; MA, Mt.
Adams; SM, Simcoe Mountain; MH, Mt. Hood; MJ, Mt. Jefferson; NV, Newberry Volcano; CL, Crater Lake; MMc, Mt. McLaughlin; ML,
Medicine Lake; MS, Mt. Shasta; LA, Lassen Peak. (b) Simplified geological map on a digital elevation model base map of North Sister,
Little Brother, the Matthieu Lakes Fissure (MLF), and Holocene scoria cones modified from Schmidt & Grunder (2009). Unpatterned areas
are undifferentiated Middle Sister, moraine, glacial ice (gl.), alluvial, and ash deposits. Small open circles are sample locations.
roles of crystallization versus assimilation (e.g. Gill, 1981;
Grove & Kinzler, 1986; Tepper et al., 1993). Although the
prominent andesitic to dacitic edifices of arcs attract attention, seemingly monotonous, basaltic andesitic composite
volcanoes are of comparative volume and represent a distinct class of arc volcano. Examples include Volca¤n
Villarrica in the Southern Andean Arc (250 km3 over 100
kyr; Clavero & Moreno, 2004), Klyuchevskoi Volcano in
the Kamchatka Arc (300 km3 over 7 kyr; Kozhemyaka,
1995), and Volca¤n Arenal in the Central American Arc
(7 km3 over 7 kyr; Wadge et al., 2006). The volumetric
dominance of evolved mafic magmas at such volcanoes requires a balance between persistent influx of mantle melts
and crustal processing. We here describe and model the
evolution of magmas from one such consistently mafic
composite center, North Sister Volcano, the oldest and
most mafic of the three main edifices in Three Sisters
Volcanic Field of the central Oregon Cascade arc (Fig. 1).
North Sister has erupted 40 km3 of monotonous
low-K2O basaltic andesite (52·5^55 wt % SiO2 and
0·5^0·8 wt % K2O) over 400 kyr (Schmidt & Grunder,
2009). The basaltic andesitic composition of this volcano is
maintained by recharge-dominated processes and implies
modification of the deep arc crust by emplacement of
mafic magmas and complementary cumulates.
The deep arc crust is a difficult place to access geologically, but it is a likely site of substantial interaction between
mantle-derived basalts and the crust (e.g. Bergantz, 1989;
Mu«ntener et al., 2001; Petford & Gallagher, 2001). The prevailing paradigms for andesite generation, the ‘MASH’
model of Hildreth & Moorbath (1988) and the ‘hot zones’
model of Annen et al. (2006), are based on magma
604
SCHMIDT & GRUNDER
ARC VOLCANO ROOTS
evolution in the deep crust. Mantle-derived basalts stall at
depth and acquire crustal affinities such as higher SiO2,
d18O, and 87Sr/86Sr. Tectonically exposed sections of arc
lower crust, such as the Talkeetna arc in SE Alaska
(DeBari & Coleman, 1989) and the Jijal Complex of
Kohistan, Pakistan (Jan & Howie, 1981; Jagoutz et al.,
2007) include thick assemblages of ultramafic and gabbroic
cumulates, reflecting significant mass and heat fluxes from
the mantle. Yet fundamental questions remain with respect
to the heat and mass budgets required to maintain an arc
magmatic system and how the deep crust and the
magmas produced there evolve with time.
Deep crustal processes are complicated to unravel because the decompression or fluid-fluxed melting of the
underlying, probably heterogeneous sub-arc mantle delivers diverse basaltic magmas to the deep crust.
Moreover, elemental and isotopic contrasts are modest between the basaltic melts and the deep arc crust, which
makes crustal interaction difficult to fingerprint. In addition, processes during ascent and possible stalling in the
mid- to upper crust may generate further diversity and obscure the deep crustal heritage.
The central Oregon Cascade Range (Fig. 1a) is an ideal
setting to investigate igneous processes in the deep arc
crust because extension has facilitated the eruption of
abundant mafic magmas (Hughes & Taylor, 1986; Sherrod
& Smith, 1990). The extensional environment limits the
degree of interaction with the mid- to upper crust. Also,
primitive basaltic magmas of the Oregon Cascades are
well characterized (e.g. Bacon et al., 1997; Conrey et al.,
1997; Schmidt et al., 2008; Rowe et al., 2009) and constrain
the compositions of potential mantle input.
In this study, we track magma compositions over the
400 kyr eruptive history of North Sister Volcano (Fig. 1b;
Schmidt & Grunder, 2009). Our goals are: (1) to quantitatively describe the origin of low-K2O basaltic andesitic
magmas at North Sister in the deep crust; (2) to understand the origin of compositional variation within discrete
temporal clusters of lavas and dikes; (3) to assess how the
overall temporal evolution of the North Sister system reflects the evolution of the deep arc crust. We test the relative contributions of primitive mantle inputs, fractional
crystallization, and assimilation of partial melts of ultramafic or gabbroic crust. The North Sister magmatic
system offers a window to deep arc crustal processes and
is a template for understanding the generation and evolution of mafic intermediate magmas.
THE CENTR A L OREGON
C A S C A D E A RC
Volcanism along the Cascade arc (Fig. 1a) is the result of
oblique NW subduction of the Juan de Fuca Plate beneath
North America. The youthfulness of the down-going plate
(Wilson, 1988), a paucity of seismicity in the slab (Weaver
& Baker, 1988), high heat flow (Blackwell et al., 1990), and
extension within the arc (Hughes & Taylor, 1986) have led
to characterization of the Oregon portion of the arc as a
‘hot’ subduction zone. A large accretionary wedge at the
Cascadia margin suggests that little to no sediment is
being subducted at present (Fleuh et al., 1998).
The discontinuous north^south-trending High Cascade
Graben bounds most Quaternary volcanism in Oregon
(Fig. 1a). Intra-arc extension is caused by a combination of
clockwise rotation of the Siletz Terrane fore-arc block and
westward encroachment of the Basin and Range extensional province. Rates of extension are 51mm a1 (Wells et al.,
1998), but are sufficient to allow the passage of abundant
mafic magmas along normal faults. Voluminous basaltic
andesite and basalt magmas have formed strings of small
shield volcanoes and monogenetic vents that align parallel
to the graben structure. More felsic arc stratovolcanoes,
such as South Sister, Mt. Mazama, and Mt. Jefferson, are
also found in the central Oregon Cascades, but their total
eruptive volume is small (180 km3 since 2 Ma) relative to
the mafic volcanism (2500 km3 since 2 Ma; Sherrod &
Smith, 1990; Hildreth, 2007).
The High Cascade Graben is widest and has the highest
density of volcanic vents around the Three Sisters Volcanic
Field (Guffanti & Weaver, 1986; Conrey et al., 2004). The
region includes five composite volcanoes (North, Middle,
and South Sisters, Broken Top, and the Husband) as well
as numerous smaller scoria cones, shields, and domes.
More silicic (dacitic to rhyolitic) magmatism in the Three
Sisters region is fairly recent (within the past 40 kyr;
Calvert et al., 2005) and is centered at the Middle and
South Sisters. The high density of volcanic vents around
the Three Sisters is probably related to the intersection of
the High Cascade Graben and the Brother’s Fault Zone,
an ESE^WSW-trending dextral transform zone at the
northern margin of the Basin and Range Province in eastern Oregon (Fig. 1a; Lawrence, 1976). Late Tertiary and
Quaternary bimodal volcanism of the High Lava Plains
broadly coincides with the Brother’s Fault Zone
(MacLeod et al., 1975; Jordan et al., 2004).
Crustal structure
A change in seismic velocity from 47·9 to 7 km s1 at
about 40 km depth beneath the Oregon Cascade volcanic
arc is inferred to indicate the location of the Moho and
the maximum depth of plagioclase stability (Stanley et al.,
1990). The Moho depth is apparently constant from the
fore-arc into the back-arc (Stanley et al., 1990; Trehu et al.,
1994; Brocher et al., 2003). Between 30 and 40 km depth,
the lower arc crust of the central Oregon Cascades probably consists of mafic granulites and underplated mafic to
ultramafic intrusions, on the basis of seismic refraction
and magnetotelluric studies (Stanley et al., 1990).
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JOURNAL OF PETROLOGY
VOLUME 52
Mu«ntener & Ulmer (2006), among others, have suggested that the petrological transition from mantle peridotite to mafic cumulates in the deep crust lies beneath the
seismic Moho. Tectonically exhumed sections of arc lower
crust, such as the Tonsina section of the Jurassic Talkeetna
arc, SE Alaska (DeBari & Coleman, 1989) and the Jijal
complex in Kohistan, Pakistan (Jan & Howie, 1981;
Jagoutz et al., 2007) expose significant sections of ultramafic and gabbroic cumulates, which probably crystallized
from sills of mantle-derived magma in the deep crust.
The bulk of the crust upon which the Oregon Cascade
arc is built has been accreted to North America since the
Cretaceous. The Columbia Embayment (Fig. 1a) is defined
by Bouguer gravity anomalies (Couch & Riddihough,
1989) and is thought to be oceanic in origin (Miller, 1989;
Burchfiel et al., 1992). The largest and most coherent block
of accreted crust is the Eocene age Siletz Terrane, a fossil
oceanic plateau (Trehu et al., 1994; Parsons et al., 1999).
The southern and western margins of the Siletz Terrane
are unresolved, but may run through the Three Sisters
region (Fig. 1a; Trehu et al., 1994). Arc plutons and mafic
dikes related to intra-arc extension probably make up a significant portion of the mid- to upper crust (Hughes &
Taylor, 1986; Stanley et al., 1990), although the degree to
which such mafic intrusions have replaced or ‘homogenized’ the upper crust is unknown.
Range of mafic magmas
Mafic magmas in the Cascade arc include assorted basaltic
andesitic and basaltic magmas. Relatively primitive
Cascade basalts with Mg-numbers greater than 60 have
yielded important insights into mantle melting regimes
and mass fluxes from the subducted slab and mantle
wedge (e.g. Bacon, 1990; Leeman et al., 1990, 2005; Bacon
et al., 1994, 1997; Borg et al., 1997; Conrey et al., 1997;
Reiners et al., 2000; Grove et al., 2002, 2006; Schmidt et al.,
2008; Rowe et al., 2009).
At least three, to as many as six, distinct primitive
magma compositions (including four basalts, high-Mg
basaltic andesite, and high-Mg andesite) have been identified in the Cascade arc (Bacon et al., 1997; Conrey et al.,
1997; Leeman et al., 2005; Schmidt et al., 2008). As summarized by Schmidt et al. (2008), end-member primitive basaltic magmas are: (1) arc-typical and alkali-rich calc-alkaline
basalts (CAB); (2) incompatible element-depleted low-K
tholeiites (LKT; also called high-alumina olivine tholeiite;
HAOT; e.g. Bacon et al., 1997); (3) high field strength
element-enriched (HFSE-rich) basalts also called ocean
island basalts (OIB; e.g. Bacon et al., 1997) or within-plate
basalts (WIP, e.g. Conrey et al., 1997); (4) rare strongly
alkali-enriched absarokites (ABS, Fig. 2).
The degree of diversity among the primitive basalt
end-members varies along the strike of the Cascade volcanic arc. The least variability is found in the central part
of the Cascades, which is undergoing extension and
NUMBER 3
MARCH 2011
Fig. 2. Chondrite-normalized rare earth element (REE) abundance
patterns for normal mid-ocean ridge basalt (N-MORB) and primitive
Cascade arc basalt end-members, including CAB, LKT, HFSE-rich,
and ABS (Bacon et al., 1997; Schmidt et al., 2008). CC is a CAB from
Cayuse Crater. Chondritic REE are from Sun & McDonough (1989).
Abbreviations as in the text.
includes the Three Sisters Volcanic Field. Here primitive
magmas representing partial melts of the mantle are limited to LKT, relatively incompatible element-poor CAB,
and rare, small-volume ABS in the fore-arc. Basalts from
this part of the arc span the narrowest range in 87Sr/86Sr
and define the Central Segment of the Cascade arc
(Schmidt et al., 2008).
More voluminous than basalts in the central Oregon
Cascades are basaltic andesites of two main types, low-K
and medium-K. Most mafic volcanoes, including scoria
cones, shields and larger composite volcanoes, are dominated by a single basaltic andesite type. North Sister is constructed of low-K basaltic andesite, whereas the nearby,
smaller, Little Brother shield volcano (Fig. 1b) consists of
medium-K basaltic andesite. The low-K basaltic andesite
(also called the North Sister-type by Hughes & Taylor,
1986; Conrey et al., 2001) is depleted in incompatible elements (0·6 wt % K2O), compared with the medium-K
basaltic andesite (1wt % K2O; called Mt. Washingtontype by Hughes & Taylor, 1986; Conrey et al., 2001). Both
basaltic andesite types are parental to more silicic compositions found at the major stratovolcanoes (e.g. Conrey
et al., 2001, 2004).
North Sister Volcano
North Sister Volcano (Fig. 1b) is the most mafic and longest
lived of the Three Sisters Volcanic Field. It is a composite
volcano that has produced 40 km3 of compositionally restricted low-K basaltic andesitic magma (52·5^54·9%
SiO2) from 400 to 50 ka. In addition to North Sister
Volcano, we include in our study the nearby, small shield
606
SCHMIDT & GRUNDER
ARC VOLCANO ROOTS
volcano Little Brother (0·8 km3), and a string of
vents called the Matthieu Lakes Fissure (MLF; 0·4 km3)
that transects North Sister (Fig. 1b; Schmidt & Grunder;
2009).
Most eruptions at North Sister were dike-fed and produced scoria and agglutinated lava flows. Glacial ice intermittently covered the volcano and significantly eroded the
edifice. Eruptions from beneath ice produced thick accumulations of palagonitic tuff. North Sister Volcano was
constructed over four eruptive stages, which are distinguished on the basis of changes in eruption style. In addition, clustering among the compositional data and
40
Ar/39Ar ages confirm the field-based stratigraphy. The
four stages are: Stage 1, the Lower Shield (400 ka);
Stage 2, the Glacial Stage (182^99 ka); Stage 3, the Upper
Shield Stage (80 ka); Stage 4, the Stratocone Stage
(70^55 ka; Schmidt & Grunder, 2009). Early feeder dikes
(Stages 1 and 2) are radial to the edifice, whereas later
ones (Stages 3 and 4) are cross-cutting and trend more
north^south, paralleling the High Cascade Graben. This
change in dike orientations probably reflects waning
magma supply and advancement of the regional extensional regime to the North Sister center (Schmidt & Grunder,
2009).
The waning of activity at North Sister coincides with
a shift of activity to the MLF, which was active at 75
and 20^11 ka. The MLF (Fig. 1b) is a 411km long series
of thick, dike-fed lavas and scoria cones that parallel
regional faults (N to N158E; Schmidt & Grunder, 2009).
Three splays of the MLF (East, Main, and West) are
compositionally distinct and are younger toward the
west. The 20^11 ka Main splay transects the North
Sister edifice (Schmidt & Grunder, 2009). Construction of
the Middle Sister (37^14 ka) was contemporaneous
with the MLF and produced 12 km3 of basaltic andesitic to rhyodacitic magmas (Hildreth, 2007), marking a
southward movement of the focus of volcanism. The most
recent volcanic activity in the North Sister region
formed Collier Cone and Yapoah Crater along the MLF
(Fig. 1b).
The Little Brother shield (0·8 km3 total volume) lies
2 km west of the North Sister summit. Little Brother was
constructed between 153 and 90 ka during the Glacial
Stage of North Sister. Little Brother consists of a central
cone of palagonitic tuff that has been capped by a shield
of many thin lava flows. A later N158E-trending dike
cross-cut the north flank of Little Brother at 48 ka
(Schmidt & Grunder, 2009), mimicking the transition to
fissure eruption in time at North Sister.
A N A LY T I C A L T E C H N I Q U E S
A total of 171 samples were analyzed by X-ray fluorescence
(XRF) in the GeoAnalytical Laboratory at Washington
State University^Pullman using the protocol and
instrument described by Johnson et al. (1999). Sixteen representative samples were analyzed for trace elements by inductively coupled plasma mass spectrometry (ICP-MS) at
Washington State University. Samples were first chipped
in an alumina jaw crusher and then powdered in an
Fe shatterbox swing mill. The powders were then
digested using methods modified from Crock & Lichte
(1982). Samples were run on a Sciex Elan 250 ICP-MS
system with a Babington nebulizer, H2O-cooled spray
chamber and Brooks mass flow controllers. One acid
blank and three standards (BCR-P, GMP-01 and
MON-01) were run with each set of unknowns. Representative major and trace element compositions are given in
Table 1. A complete suite of analyses has been given by
Schmidt (2005).
Phenocrysts, including olivine, plagioclase feldspar, pyroxene, and spinel from selected North Sister and MLF
samples were analyzed using the Oregon State
University’s CAMECA SX-50 electron microprobe
(Table 2 and Electronic Supplement I, available at http://
www.oxfordjournals.petrology.org).
Analyses of 14 whole-rock powders were conducted at
the University of Colorado at Boulder for Sr and Nd isotopic ratios (Table 1). Sr and Nd were separated using conventional techniques (Sr separated using SrSpec resin).
87
Sr/86Sr ratios were determined using four-collector
static mode measurements. Thirty measurements of
SRM-987 during the study period yielded mean
87
Sr/86Sr ¼ 0·71032 2 (2s). Measured 87Sr/86Sr were corrected to SRM-987 ¼ 0·71028. Measured 143Nd/144Nd were
normalized to 146Nd/144Nd ¼ 0·7219. Analyses were
dynamic mode, three-collector measurements. Thirtythree measurements of the La Jolla Nd standard during
the study period yielded a mean 143Nd/144Nd ¼
0·511838 8 (2s).
Seven plagioclase separates were analyzed for oxygen
isotopes (Table 1) (four from North Sister, two from the
MLF, one from Little Brother) and one olivine separate
from a primitive CAB from Cayuse Crater, a cinder cone
and flow on the south flank of Broken Top Volcano, which
is part of the Three Sisters Volcanic Field, located 13 km
SE of North Sister Volcano. Mineral oxygen isotope analyses, reported in the d notation relative to VSMOW, were
conducted at Washington State University using a laser
fluorination system (Sharp, 1990, 1992) with BrF5
(Borthwick & Harmon, 1982) as the oxidizing agent. Raw
data were corrected to a NBS-28 (African glass sand)
value of 9·59ø, using the garnet standard UWG-2
(Valley et al., 1995), with a value of 5·8ø, as an in-house
standard. Replicate analyses of UWG-2 analyzed with the
samples typically show a daily standard deviation of
0·05ø. O2 liberated during fluorination of the 2^3 mg
samples was analyzed directly as oxygen gas without conversion to CO2.
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NUMBER 3
MARCH 2011
Table 1: Representative whole-rock analyses for North Sister, Little Brother, and Matthieu Lakes Fissure
Sample:
Stage:
NS-02-66
Group:
wt %
SiO2
Al2O3
TiO2
FeO*
MnO
CaO
MgO
K2O
Na2O
P2O5
Unnorm. total
ppm
Ni
Cr
V
Sc
Rb
Cs
Sr
Ba
Pb
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Y
Zr
Hf
Nb
Ta
Th
U
87
Sr/86Sr
143
Nd/144Nd
d18O
Modal %
Pl
Ol
Opx
Cpx
Hbd
Opaque minerals
Vol. %
Phenocrysts
Glomerocrysts
Groundmass
52·5
18·4
1·20
8·1
0·14
9·1
5·9
0·66
3·7
0·26
98·17
98
54
179
27
9
0·30
579
274
3·20
11·6
25
3·2
14·6
4·0
1·42
4·0
0·66
4·1
0·85
2·2
0·32
1·88
0·30
22·1
111
2·82
7·1
0·44
1·00
0·39
0·70369
0·51285
5·5
NS-02-751
Lower
Shield
1
54·7
18·3
0·97
7·2
0·13
8·3
5·8
0·74
3·6
0·20
97·90
NS-02-871
Lower
Shield
1
54·4
18·7
0·99
7·2
0·12
8·6
5·4
0·74
3·7
0·20
95·81
NS-02-1111
Lower
Shield
1
54·7
18·7
1·03
7·1
0·13
8·5
5·1
0·78
3·8
0·21
97·78
NS-01-1
Glacial
Stage
2a
53·1
19·0
1·03
7·5
0·13
9·2
5·7
0·62
3·7
0·19
99·35
108
130
154
20
8
93
78
160
23
11
83
71
167
21
9
104
55
172
20
6
580
294
4
16
34
593
275
4
31
27
596
299
1
17
14
580
234
2
26
27
18
106
17
106
18
111
19
94
6
6
6
6
2
2
3
—
0·70360
0·51288
NS-03-141
Glacial
Stage
2a
53·6
19·1
1·05
7·2
0·13
9·3
5·2
0·62
3·7
0·19
99·37
82
56
198
25
7
0·21
591
241
2·91
9·3
20
2·7
12·4
3·4
1·22
3·5
0·58
3·6
0·72
1·9
0·27
1·65
0·26
18·8
87
2·34
4·9
0·30
0·82
0·31
0·70358
0·51287
5·6
NS-03-1531
Glacial
Stage
2a
54·8
18·9
1·01
7·0
0·12
8·5
5·1
0·70
3·8
0·19
99·18
72
62
185
23
9
576
258
3
—
19
19
104
6
2
0·70356
0·51289
NS-02-29
Dike
NS-02-331
Dike
NS-02-381
Dike
NS-02-471
Dike
2a
2a
2a
2a
53·4
18·8
1·08
7·6
0·13
9·1
5·4
0·66
3·7
0·19
98·79
75
56
186
27
7
0·24
582
248
3·03
9·7
21
2·7
12·6
3·6
1·29
3·7
0·61
3·7
0·74
2·0
0·28
1·72
0·27
20·0
93
2·50
5·0
0·31
0·87
0·33
0·70357
0·51290
5·6
53·4
18·7
1·03
7·4
0·13
9·1
5·7
0·65
3·6
0·19
98·40
53·2
18·6
1·02
7·6
0·13
9·2
5·9
0·6
3·5
0·18
96·87
54·1
18·3
1·08
7·5
0·13
8·9
5·3
0·74
3·7
0·21
97·39
89
93
182
24
7
112
97
172
29
9
64
86
186
21
9
567
250
4
12
13
561
243
2
19
6
539
287
2
15
16
18
94
18
94
20
107
6
5·9
2
1
6
—
0·70356
0·51289
11·4
3·3
—
—
—
—
7·6
1·7
—
—
—
—
9·9
0·7
—
—
—
—
17·9
3·7
0·6
—
—
—
16·7
3·5
—
—
—
—
6·7
1·8
—
—
—
—
16·3
5·7
—
—
—
—
16·5
1·1
—
—
—
—
14·7
—
85·3
5·1
4·2
90·7
10·6
—
89·4
18·5
3·7
76·8
11·3
8·7
80·0
6·1
2·4
89·0
13·0
8·9
78·0
11·9
9·1
73·3
(continued)
608
SCHMIDT & GRUNDER
ARC VOLCANO ROOTS
Table 1: Continued
Sample:
Stage:
Group:
wt %
SiO2
Al2O3
TiO2
FeO*
MnO
CaO
MgO
K2O
Na2O
P2O5
Unnorm. total
ppm
Ni
Cr
V
Sc
Rb
Cs
Sr
Ba
Pb
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Y
Zr
Hf
Nb
Ta
Th
U
87
Sr/86Sr
143
Nd/144Nd
d18O
Modal %
Pl
Ol
Opx
Cpx
Hbd
Opaque minerals
Vol. %
Phenocrysts
Glomerocrysts
Groundmass
NS-01-161
Glacial
Stage
2b
53·38
18·95
1·058
7·61
0·129
9·13
5·20
0·66
3·70
0·19
98·69
74
57
181
22
7
580
253
1
18
31
19
97
6
2
NS-03-164
Glacial
Stage
2b
NS-03-165
Upper
Shield
3–4
NS-03-132
Stratocone
3–4
NS-03-135
Stratocone
3–4
54·6
18·9
0·99
7·0
0·12
8·6
5·2
0·69
3·7
0·19
99·58
54·5
18·3
1·09
7·4
0·13
8·7
5·2
0·77
3·7
0·23
99·29
54·1
19·0
1·14
7·4
0·13
9·0
4·6
0·70
3·8
0·20
98·87
53·7
19·2
1·07
7·1
0·13
9·1
5·1
0·65
3·8
0·19
99·65
89
62
165
23
8
0·13
590
251
2·77
10·4
22
2·8
13·1
3·4
1·24
3·5
0·56
3·4
0·69
1·8
0·26
1·57
0·25
18·0
95
2·55
5·4
0·34
0·97
0·35
64
82
181
25
10
0·28
553
294
3·54
12·1
26
3·3
14·9
4·0
1·35
3·9
0·64
3·8
0·77
2·1
0·30
1·82
0·28
20·5
110
2·87
6·7
0·42
1·12
0·39
51
41
28
26
8
0·26
600
265
3·24
10·4
23
3·0
13·9
3·8
1·38
3·9
0·65
3·9
0·80
2·1
0·30
1·85
0·29
20·9
98
2·69
5·5
0·33
0·92
0·35
78
43
25
25
7
0·22
595
247
3·10
9·6
21
2·8
12·9
3·5
1·26
3·7
0·60
3·6
0·73
2·0
0·28
1·74
0·27
19·4
90
2·48
5·0
0·31
0·88
0·32
15·6
5·6
15·6
5·6
78·8
NS-02-46
Dike
NS-02-92
MLF East
NS-02-931
MLF East
NS-02-941
MLF East
NS-02-120
MLF East
NS-02-101
MLF Main
3–4
53·8
19·7
1·16
7·4
0·13
9·1
4·0
0·70
4·0
0·20
99·23
35
30
199
26
7
0·25
609
250
3·05
9·7
21
2·8
13·4
3·7
1·33
3·9
0·64
3·8
0·79
2·1
0·30
1·80
0·29
20·6
95
2·57
5·0
0·31
0·86
0·34
0·70356
0·51292
6·0
53·3
20·0
1·16
7·5
0·13
8·0
4·1
0·69
4·0
0·22
98·75
45
18
179
22
6·6
0·13
638
258
3·10
10·1
22·2
3·0
14·0
3·7
1·40
3·8
0·62
3·8
0·78
2·1
0·29
1·82
0·28
20·7
99
2·63
5·3
0·32
0·85
0·33
0·70354
0·51290
5·9
23·3
1·8
23·1
1·7
Tr.
Tr.
0·2
19·9
5·4
74·7
54·9
17·8
1·57
8·9
0·15
7·5
3·7
0·85
4·5
0·27
96·95
54·8
18·5
1·37
8·3
0·15
7·4
3·8
0·89
4·4
0·32
95·47
10
7
262
19
7
32
12
178
18
7
546
305
—
9
16
561
308
3
19
32
24
124
25
134
7
8
—
—
53·6
18·2
1·46
9·0
0·16
7·8
4·9
0·68
3·9
0·26
98·96
56
13
228
25
4·8
0·09
605
305
3·25
10·8
24·3
3·40
16·1
4·4
1·61
4·5
0·74
4·5
0·90
2·4
0·34
2·05
0·32
23·7
94
2·66
5·2
0·33
0·63
0·31
6
9
121
23
20·1
0·52
489
480
5·36
18·9
40·0
5·1
23·0
5·9
1·90
5·8
0·94
5·7
1·17
3·1
0·45
2·78
0·44
31·2
183
4·70
9·9
0·63
2·32
0·84
0·70365
0·51288
6·0
7·2
0·9
0·2
23·7
1·3
Tr.
5·0
1·0
Tr.
0·9
Tr.
Tr.
Tr.
17·9
6·9
75·1
10·1
4·2
85·1
25·8
7·6
2·4
90·0
2·1
97·9
74·2
8·1
1·9
59·7
17·0
1·33
7·0
0·14
5·5
2·5
1·41
5·1
0·37
97·85
1·4
0·4
Tr.
0·4
(continued)
609
JOURNAL OF PETROLOGY
VOLUME 52
NUMBER 3
MARCH 2011
Table 1: Continued
Sample:
Stage:
wt %
SiO2
Al2O3
TiO2
FeO*
MnO
CaO
MgO
K2O
Na2O
P2O5
Unnorm. total
ppm
Ni
Cr
V
Sc
Rb
Cs
Sr
Ba
Pb
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Y
Zr
Hf
Nb
Ta
Th
U
87
Sr/86Sr
143
Nd/144Nd
d18O
Modal %
Pl
Ol
Opx
Cpx
Hbd
Opaque minerals
Vol. %
Phenocrysts
Glomerocrysts
Groundmass
NS-03-115
MLF Main
55·2
17·3
1·53
8·9
0·16
7·7
4·0
0·93
4·1
0·28
98·62
11
23
287
30
13·0
0·46
552
349
4·23
14·4
30·3
3·9
17·6
4·6
1·58
4·7
0·75
4·6
0·93
2·5
0·36
2·17
0·34
24·5
129
3·42
7·9
0·51
1·57
0·58
4·7
0·8
0·4
0·4
NS-03-1221
MLF Main
56·9
17·2
1·50
8·4
0·15
6·7
3·3
1·10
4·5
0·31
99·72
NS-02-1071
MLF West
57·6
17·9
1·05
7·1
0·12
7·1
3·7
0·97
4·3
0·14
98·66
6
9
248
29
16
23
35
177
28
15
511
400
2
15
28
504
310
25
146
18
101
9
6
3
2
Tr.
3·4
1·1
0·3
Tr.
26
32
Tr.
5·0
1·2
93·8
100
2·9
2·0
95·1
NS-02-169
MLF South
58·0
16·9
1·43
8·0
0·15
6·2
3·0
1·24
4·8
0·34
98·39
3
6
185
23
17·1
0·33
503
430
5·18
16·7
35·4
4·6
20·5
5·3
1·74
5·3
0·84
5·1
1·04
2·8
0·40
2·51
0·39
27·7
157
4·12
8·4
0·54
1·99
0·73
LB-02-2
Little
Brother
LB-03-22
Little
Brother
LB-03-23
Little
Brother
CC-02-1
Cayse
Crater
FLR03-1
Foley
Ridge
XRF SD
52·8
17·7
1·41
8·5
0·15
8·7
5·6
0·98
3·7
0·47
97·74
53·0
17·8
1·44
8·3
0·16
8·7
5·4
0·98
3·7
0·48
98·76
53·2
17·8
1·43
8·2
0·15
8·7
5·3
0·96
3·8
0·48
99·30
51·94
16·53
1·053
7·85
0·150
9·15
8·62
0·65
3·13
0·221
49·08
16·94
1·510
9·57
0·175
9·83
9·03
0·40
3·07
0·332
0·09
0·07
0·004
0·01
0·001
0·01
0·10
0·07
0·05
0·002
59
91
202
27
12·9
0·38
681
429
4·34
20·9
44·1
5·7
24·7
5·9
1·92
5·8
0·90
5·3
1·05
2·9
0·40
2·47
0·39
28·1
161
3·90
13·9
0·85
1·76
0·59
58
93
192
28
13·1
0·41
669
429
4·66
21·4
45·1
5·7
25·3
6·1
1·97
5·9
0·94
5·5
1·12
3·0
0·42
2·59
0·40
29·3
167
4·09
14·5
0·88
1·76
0·60
71
97
195
25
12·8
0·30
647
410
4·23
20·1
42·3
5·4
23·7
5·8
1·85
5·5
0·86
5·1
1·04
2·8
0·39
2·39
0·37
27·1
155
3·84
13·1
0·79
1·71
0·58
0·70376
0·51287
5·7
1·3
0·3
Tr.
Tr.
Tr.
0·3
9·5
7·1
9·2
2·8
11·8
3·7
0·3
0·4
0·4
1·0
0·8
98·2
4·5
12·5
83·0
2·3
9·6
87·5
5·7
10·1
84·1
153
411
185
31·3
10·9
0·39
431
224
2·76
10·01
21·31
2·76
12·48
3·39
1·23
3·66
0·61
3·77
0·78
2·10
0·30
1·84
20·49
93
2·41
6·67
0·42
1·27
0·44
0·70354
0·51294
5·4
176
359
229
35·7
4·3
0·20
398
231
2·27
11·77
25·95
3·58
16·37
4·58
1·64
4·95
0·82
5·18
1·05
2·88
0·40
2·56
27·82
116
3·00
7·39
0·51
0·98
0·23
0·70356
0·512922
1
2
5
2
1
1
9
2
10
10
1
1
0·5
2
ICP-MS SD
1·61
0·28
0·01
3·24
1·76
0·09
0·10
0·14
0·03
0·12
0·06
0·02
0·06
0·01
0·07
0·01
0·05
0·01
0·03
0·01
0·5
0·04
0·13
0·02
0·03
0·01
*Total Fe presented as FeO.
1
Samples analysed by XRF only; other samples analysed by XRF and ICP-MS.
XRF major element analyses were normalized to 100%. Whole-rock powders were analysed at the Washington State University
GeoAnalytical Laboratory; 2s errors for isotopic measurements are 0·00002 for 87Sr/86Sr, 0·000008 for 143Nd/144Nd, and 0·1 for
d18O. CC-02-1 is a CAB from Cayuse Crater and FLR03-01 is an LKT from a canyon-filling flow that now forms Foley Ridge in the
western Cascades (Schmidt et al., 2008; Rowe et al., 2009). Modal percentages are based on point counting 500–700 nodes at 1 mm grid
spacing (Schmidt & Grunder, 2009). Pl, plagioclase; Ol, olivine; Cpx, clinopyroxene; Hbd, pseudomorphed hornblende; Tr., trace.
610
SCHMIDT & GRUNDER
ARC VOLCANO ROOTS
cores range from An67·4^91·6 in the lowest SiO2 MLF
magmas to An80·4^89·3 in the highest SiO2 MLF magmas.
Rare pseudomorphs of amphibole phenocrysts are found
in the East splay lavas and are completely converted to
orthopyroxene, clinopyroxene, plagioclase, and opaque
oxides; they range in size from 0·01 to 1·5 cm.
Titanomagnetite phenocrysts up to 1mm in size are found
in samples from the Main splay of the MLF.
Little Brother lavas (51·8^53·8 wt % SiO2) are distinguished from those from North Sister by conspicuous
2^3 mm glomerocrysts of plagioclase and olivine that
have a distinctive granular, sugary texture. Glomerocrysts
are typically twice as abundant as single phenocrysts of
olivine and plagioclase (Table 1).
Table 2: Representative mineral compositions from North
Sister and the MLF
North Sister
ol
opx
MLF
plag
chrom
SiO2
39·0
53·6
45·0
1·5
Al2O3
0·1
1·8
33·5
17·4
TiO2
0·03
0·36
2·44
ol
cpx
39·7
plag
51·3
47·6
0·1
0·02
3·0
32·6
4·1
0·01
0·96
11·37
FeO*
16·9
16·6
0·6
33·5
16·7
8·9
0·7
CaO
0·2
1·5
17·0
0·2
0·1
19·8
15·5
MgO
43·9
26·1
0·1
13·2
44·8
15·5
0·1
K2O
0·02
Na2O
1·8
Cr2O5
Total
Ti-mag
72·9
3·0
0·08
0·2
Major and trace element variations
2·6
26·9
100·1
100·0
98·0
95·3
101·3
99·5
99·2
91·47
R E S U LT S
Petrography of the samples
Basaltic andesitic lavas (52·5^55·0 wt % SiO2) erupted at
North Sister Volcano are generally crystal-poor with 10 to
rarely as much as 20 vol. % phenocrysts of plagioclase,
olivine, orthopyroxene, clinopyroxene
(Table
1;
Schmidt & Grunder, 2009). The most abundant phenocryst
phase is plagioclase feldspar, which is typically 10 times as
abundant as olivine. Plagioclase phenocrysts are generally
0·2^1·0 mm and rarely up to 2 mm. Anorthite contents
range from as high as An92 in cores to An39 at the rims
(Electronic Supplement I). Olivine phenocrysts are small
(0·05^0·8 mm, rarely up to 2 mm) and range from Fo84 to
Fo54 (Electronic Supplement I). Glomerocrysts of plagioclase with or without olivine make up 0^15% of the crystal
cargo. Orthopyroxene and clinopyroxene phenocrysts are
found only in samples with 454 wt % SiO2 and typically
make up 51 vol. % of the total rock. Samples with lower
SiO2 concentrations may contain trace amounts of Mg^
Al chromite included within olivine crystals or as a minor
phenocryst phase. The groundmass varies from glassy to
very finely holocrystalline and diktytaxitic with plagioclase, olivine, and magnetite microlites. The concentration
of H2O in an undegassed olivine-hosted melt inclusion is
3·5 wt % (Mercer & Johnston, 2008).
The MLF magmas (53·1^59·7 wt % SiO2) are mineralogically more diverse than those erupted from
North Sister. The phenocryst assemblage consists of
plagioclase, olivine, clinopyroxene, orthopyroxene,
titanomagnetite, amphibole (Table 1). Crystal contents
in MLF lavas decrease (56 to 3 vol. %) with increasing
SiO2 (Schmidt & Grunder, 2009). Plagioclase feldspar
The North Sister, MLF, and Little Brother volcanic systems are compositionally distinct (Figs 3 and 4). North
Sister basaltic andesitic magmas are notable not only for
their low K, relative to the medium-K basaltic andesites
of the Cascades, but also for high Al2O3 and low concentrations of incompatible elements [i.e. P and light rare
earth elements (LREE); Fig. 5; Schmidt et al., 2008, compilation]. Heavy REE (HREE) concentrations at North
Sister are particularly low and range to lower than those
in primitive CAB and LKT (Fig. 4a). Small positive Eu
anomalies in North Sister magmas (Fig. 4) are like those
found in primitive basalts of the central Oregon Cascades
(e.g. Bacon et al., 1997). Positive Eu anomalies are absent
in basaltic andesite from Little Brother and in more
evolved (higher SiO2) MLF rocks (Fig. 4).
The greatest degree of compositional variability within
the North Sister suite is found among the compatible
trace elements, such as Ni and Cr, which allows distinction
of clusters of data points on variations diagrams (Fig. 6)
that correspond to the stratigraphic packages of the four
central-vent eruptive stages of North Sister (Schmidt &
Grunder, 2009). Compositional ranges of the eruptive
stages (1, lower shield; 2, glacial stage; 3, upper shield; 4,
stratocone) delineate four compositional groups (1, 2a, 2b,
and 3^4). We divide the voluminous glacial stage into two
compositionally distinct groups (2a and 2b) that are
found on opposite sides of the edifice and have 40Ar/39Ar
age ranges of 182^142 ka and 120^99 ka, respectively, but
with no evidence for a change in eruptive style (Schmidt
& Grunder, 2009). The last two stages are separated by an
erosional surface and change in eruptive style [thin lavas
of the upper shield stage (3) and thicker, agglomerate
lavas and scoria of the stratocone stage (4)], but they are
compositionally similar and are combined into group 3^4.
The stratigraphically lowest sample is the most primitive
sample from North Sister, based on SiO2 (NS-02-66; SiO2
52·5 wt %, Table 1). Glaciated distal lavas of North Sister
have little or no stratigraphic context and have been
linked to compositional groups only if their compositions
611
JOURNAL OF PETROLOGY
VOLUME 52
NUMBER 3
MARCH 2011
In total, the sampled eruptive record of North Sister is
nearly continuous since 400 ka until 50 ka, excepting
a significant (100 kyr) unconformity that separates
stages 1 and 2 (compositional groups 1 and 2a; Fig. 7). This
unconformity corresponds to a major glacial event at
250 ka and does not necessarily reflect a hiatus in eruptive activity (Schmidt & Grunder, 2009).
For the North Sister suite in general, Al2O3, CaO, FeO*,
and TiO2 increase with time over a restricted range of
SiO2 (although the stratigraphically lowest sample,
NS-02-66, has the highest FeO* and TiO2; Fig. 7a^e). The
range of K2O concentrations is small with values of 0·8 wt
% occurring throughout the suite, but with low values (to
0·55 wt %) more common in younger samples (Fig. 7f).
There are coherent subparallel trends of decreasing Ni
with increasing SiO2 (Fig. 6) within each compositional
group, and in general Ni decreases with time (Fig. 7g).
This highlights a significant pattern within the North
Sister dataset. We find two types of trends: variations
within compositional groups, and differences between
groups that follow the overall temporal evolution of the
magma system.
The following summarizes other important aspects of
NS-02-66 and the four compositional groups as well as
the compositions of the MLF and Little Brother.
NS-02-66
The NS-02-66 sample was found in a low drainage on the
north side of the edifice and represents the oldest sample
of lava from the North Sister system. It has the lowest
SiO2 and highest MgO concentrations (Table 1) and, in
that sense, is the most primitive. The sample was used as
the starting composition for the high-pressure phase equilibrium experiments of Mercer & Johnston (2008).
NS-02-66 also has the highest FeO* and TiO2 contents
(Figs 6b and 7d, e), but its Ni and Cr contents (98 and
54 ppm, respectively) are not higher than those in later
groups (Figs 6a, d, and 7 g, h). The sample is similar to
the rest of North Sister in terms of most incompatible elements, except that it has slightly higher REE concentrations
(Fig. 4b).
Fig. 3. SiO2 variation diagrams illustrating compositional differences
between samples collected from North Sister, Little Brother, and
MLF. Concentrations are in wt %, unless otherwise indicated.
(a) MgO; (b) Ba; (c) FeO* (total Fe as FeO) vs SiO2.
unambiguously plotted within obvious data clusters in
variation diagrams (e.g. Fig. 6) and field relations suggest
a link (e.g. rocks were found down-slope of a
well-constrained section of lavas). Dikes were linked to
nearby lavas based on compositional similarity or using
40
Ar/39Ar ages to allow assignment to eruptive stages
(Fig. 7). Dikes commonly are located near or cross-cut
lavas of similar composition (Schmidt & Grunder, 2009).
Group 1
Group 1 lavas are mainly found low on the northern flank
of the volcano and contain 454 wt % SiO2, which is at
the high end of the North Sister spectrum, but have high
MgO and Ni concentrations (5·1^5·8 wt % and
87^107 ppm, respectively; Figs 6a and 7a). 40Ar/39Ar ages
determined for two of these lavas (500·8 116·9 ka and
311·3 106·2 ka; Fig. 7l; Schmidt & Grunder, 2009) are significantly older than the rest of North Sister. Group 1 includes the highest Cr concentrations analyzed at North
Sister (134 ppm; Figs 6d and 7 h). FeO*, TiO2, and V are
relatively low in group 1 (Figs 6b and 7d, e, i).
Concentrations of CaO range to the lowest, whereas those
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Fig. 4. Chondrite-normalized rare earth element (REE) patterns. (a) The range of REE in basaltic andesites from North Sister and Little
Brother plotted along with primitive LKT and CAB. LKT C894 (Bacon et al., 1997) and CAB CC are used in some later differentiation
models. (b) North Sister data for single samples are compared with data for the east and Main splays of the MLF. North Sister sample
NS-02-66 (thicker line) is indicated. Chondritic REE abundances are from Sun & McDonough (1989).
of Sr range to the highest values at North Sister (Figs 6c, e,
and 7c, j). The large ion lithophile element (LILE) abundances, such as K2O and Ba (Fig. 7f, k), are restricted and
at the high end of the spectrum relative to the rest of the
North Sister suite.
Group 2a
Group 2a is the most voluminous and includes a large
number of NE^SW-striking dikes of similar composition.
40
Ar/39Ar ages for group 2a range from 180 to 155 ka
(Schmidt & Grunder, 2009). A few of the dikes have the
highest Ni (up to 112 ppm) found at North Sister (Figs 6a
and 7 g) and are probably parental compositions for
group 2 because they lie at the end of coherent variations
of decreasing Ni with increasing SiO2, CaO and Sr
(Fig. 6a, c and e). With respect to Ni and SiO2, group 2a
encompasses the greatest range in composition of the
North Sister groups (50 ppm variation in Ni and
2 wt % variation in SiO2; Figs 6a and 7a, g).
Concentrations of Cr are also variable and we recognize
three trends: a high-Cr, a low-Cr, and an intermediate Cr
trend (Fig. 6d). Lavas of the three trends are stratigraphically intercalated (Fig. 7h).
Group 2b
Group 2b samples were collected from lavas making up the
SE ridge of North Sister and have 40Ar/39Ar ages ranging
from 114 to 120 ka (Schmidt & Grunder, 2009). Group 2b
has a restricted range in SiO2 and Ni (Figs 6a and 7a, g)
and, like group 1, ranges to lower concentrations of TiO2
and V (Figs 6b and 7e, i). In a plot of CaO vs Ni, most
group 2b samples lie along a lower, horizontal linear
trend that is dominated by group 1 (Fig. 6c).
Group 3^4
Group 3^4 is the youngest (70^55 ka; Schmidt &
Grunder, 2009) and most evolved group. Samples range to
the highest SiO2 (53·4^54·8 wt %) and lowest Ni and Cr
concentrations (e.g. 87^35 ppm Ni; Figs 6a^d and 7a, g,
h) at North Sister. Group 3^4 trends toward higher TiO2
and FeO* concentrations with decreasing MgO (Figs 6b
and 7d, e). Concentrations of Al2O3, CaO, and K2O span
the full range of the North Sister dataset (e.g. 0·59^0·79 wt
% K2O; Figs 6c and 7b, c, f). Even with the variability
seen in these elements, group 3^4 mimics coherent trends
defined by earlier groups in plots of Ni vs SiO2 and CaO
vs Ni (Fig. 6a and c).
Matthieu Lakes Fissure (MLF)
The MLF consists of three N158E-trending splays (East,
Main, and West). 40Ar/ 39Ar age ranges are 75^55 ka for
the East splay and 11^19·5 ka for the Main splay
(Schmidt & Grunder, 2009). The West splay is undated but
is relatively unglaciated, indicating that it is the youngest.
Together, the MLF defines compositional trends emanating from the cluster of North Sister data to more evolved
compositions. With increase in SiO2 to 59·5 wt %, K2O increases to 1·5 wt %, and Ba to nearly 500 ppm, and MgO
and Ni have attendant decreases to 2·5 wt % and 1ppm,
respectively (Figs 3, 5 and 6a). REE increase and the positive Eu anomaly disappears with increasing SiO2
(Fig. 4b). FeO*, TiO2, and V increase rapidly (e.g. up to
9·5 wt % FeO) up to about 55 wt % SiO2 and then
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Coherent compositional trends are observed within the
East and Main splay datasets (Figs 3 and 6), whereas the
West splay is more variable, ranging to lower TiO2 and
higher Sr concentrations (Fig. 6b and e). At its low-SiO2
end, the East splay is variable with respect to Ti and Sr,
exhibiting (1) a composition very similar to North Sister
group 2a, (2) a Sr-rich composition (Fig. 6e), and (3) a
TiO2-rich composition (1·48% TiO2, Fig. 6b).
Little Brother
The Little Brother shield volcano was constructed mainly
between 90 and 153 ka (Schmidt & Grunder, 2009) and
consists of medium-K basaltic andesites. Magmatism was
renewed when a later, medium-K, north^south-striking
dike (48 ka) cross-cut Little Brother. Compared with
North Sister, Little Brother magmas have comparable concentrations of SiO2, CaO, and MgO, but are richer in
FeO* and incompatible elements (e.g. K, P, Ba, Ti, REE
and Sr; Figs 3, 4a and 6). No systematic changes in composition with stratigraphic position are observed.
Sr, Nd, and O isotopes
Fig. 5. SiO2 variation diagrams (concentrations in wt %) for North
Sister, Little Brother, and MLF compared with the range of Cascade
arc magmas (compilation by Schmidt et al., 2008). North Sister
magmas have (a) high Al2O3, (b) low K2O, and (c) low P2O5 relative to the rest of the Cascade arc.
decrease with further increasing SiO2 (Figs 3c and 6b), reflecting the appearance of titanomagnetite phenocrysts in
the Main splay. Crystal contents generally decrease from
24% to 0^2% with increasing SiO2 (Table 1).
The East splay has the lowest SiO2 (53^55% SiO2) and
highest Ni (76^6 ppm) and the Main splay is the most
evolved (55^59 wt % SiO2; 1^14 ppm Ni; Fig. 6a and b).
The isotopic ratios of 87Sr/86Sr and 143Nd/144Nd at North
Sister, Little Brother, and the MLF are similar to the isotopically restricted primitive basalts erupted within the
Central segment of the Cascade arc (Fig. 8; Schmidt et al.,
2008). Variations in Sr and Nd isotopic ratios are small at
North Sister Volcano (Table 1), but are systematic with
time. Compositional groups form clusters in a plot of
87
Sr/86Sr vs 143Nd/144Nd (Fig. 8a); the 87Sr/86Sr of three
group 2a samples and two group 1 samples do not vary outside analytical uncertainty. Over the course of the history
of the central-vent eruptions at North Sister Volcano,
143
Nd/144Nd increases (0·51285^0·51292) and 87Sr/86Sr decreases (0·70369^0·70357). The earliest and most primitive
NS-02-66 composition is isotopically least like primitive
basalts from the Central Segment of the Cascade arc
(Schmidt et al., 2008), whereas the sample of group 3^4 is
isotopically more like the LKT and CAB of the Central
segment. This trend toward more mantle-like Sr and Nd
isotopic ratios with increasing differentiation is contrary
to the usual trends of assimilation of continental crust
(e.g. DePaolo, 1981). The changes in Sr and Nd isotopic
composition at North Sister are not accompanied by significant changes in the concentrations of Sr and Nd
(Fig. 8b and c). Oxygen in North Sister magmas becomes
heavier (d18O 5·5^6·0ø) with decreasing MgO (Fig. 8d).
As for the elemental trends, Little Brother stands apart
from the North Sister and MLF trends; Little Brother has
higher 87Sr/86Sr and Sr. Two MLF samples demonstrate
decreasing 143Nd/144Nd (0·51290^0·51288), increasing
87
Sr/86Sr (0·70354^0·70365), and d18O (5·9^6·0ø) with
increasing SiO2 and decreasing MgO (Table 1, Fig. 8).
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Fig. 6. Variation diagrams (concentrations in wt % unless otherwise indicated) illustrating differences between North Sister compositional
groups, the East, Main, and West splays of the MLF, and Little Brother. North Sister dikes and glaciated, distal lavas with little stratigraphic
context are represented by separate symbols. (a) Ni vs SiO2; (b) TiO2 vs MgO; (c) CaO vs Ni; (d) Cr vs Ni; (e) Sr vs Ni.
DISCUSSION
Modeling the evolution of the North
Sister system
The North Sister Volcanic system was active for 400 kyr
and is characterized by basaltic andesitic magmas
(52^55 wt % SiO2) with low abundances of incompatible
elements and some differentiation or contamination indices varying systematically in composition with time (e.g.
decreasing Ni and 87Sr/86Sr; Figs 7 and 8). Minor andesite
was produced along the later MLF, and no basalts were
erupted (Schmidt & Grunder, 2009).
In this section, we present models that describe the influence of igneous processes (e.g. fractional crystallization
and assimilation of partial melts of the crust and the relative importance of different fractionating minerals).
Ultimately, our models provide insights into (1) how a balance of igneous processes repeatedly produces seemingly
monotonous intermediate magmas over a long period of
time and (2) how mantle-derived basalts transform the
deep crust.
Magmatic processes may engender compositional diversity at volcanoes; for example, by fluid-fluxed versus
decompression mantle melting beneath arcs (e.g. for the
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Fig. 7. Compositional variations for the central-vent stages of North Sister with time. Samples with 40Ar/39Ar dates are indicated by white boxes
around the symbol. All other samples are plotted in order, according to their stratigraphic position or cross-cutting relationships. Plots illustrate
(a) SiO2; (b) Al2O3; (c) CaO; (d) FeO*; (e) TiO2; (f) K2O; (g) Ni; (h) Cr; (i) V; (j) Sr; (k) Ba variations with time. (l) 40Ar/39Ar ages with
2s errors (Schmidt & Grunder, 2009; A. T. Calvert, unpublished data). The 40Ar/39Ar ages for the oldest stage, the Lower Shield, have large
2s errors, but the two ages overlap at 400 ka (Schmidt & Grunder, 2009).
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Fig. 8. Isotopic variation diagrams for the North Sister compositional groups, Little Brother, MLF, and primitive basalts from the Central segment of the Cascade arc that includes Medicine Lake, Crater Lake and central Oregon (Schmidt et al., 2008). Primitive basalt types include
LKT, CAB, and ABS. LKT ‘C894’ (Bacon et al., 1997) and CAB ‘CC’ are used in some differentiation models. (a) Plot of 143Nd/144Nd vs
87
Sr/86Sr demonstrates a decrease in 87Sr/86Sr and increase in 143Nd/144Nd with time in the North Sister basaltic andesites. Average 2s error
bars are indicated. (b) 87Sr/86Sr vs Sr concentration. (c) 143Nd/144Nd vs Nd concentration. (d) d18O vs MgO (wt %).
Cascades: Bacon, 1990; Reiners et al., 2000; Leeman et al.,
2005; Grove et al., 2006), or through fractionation and contamination at various depths during magma ascent. They
may lead to compositional homogenization, such as by extensive magma recharge (e.g. Volca¤n Arenal; Streck et al.,
2002) or by self-contamination (basaltic andesites of the
Tatara^San Pedro Volcanic Complex; Dungan &
Davidson, 2004; dacites of the Aucanquilcha Volcanic
Cluster; Grunder et al., 2008), or by processing in the crust
as envisioned by Hildreth & Moorbath (1988). Although
igneous diversity makes the inference of potential magmatic processes more clear, the processes leading to homogeneity may have the more important mass and thermal
imprints on the crust.
North Sister is deceptively simple in that it erupted compositionally restricted basaltic andesitic magmas for several hundred thousand years. We examine whether the
limited diversity among the North Sister magmas is the
result of processes operating at multiple depths (mantle,
deep crust, and mid- to upper crust). We present a
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Fig. 9. Illustration of the multi-stage approach to model the basaltic andesitic magma system at North Sister Volcano. The central Oregon
Cascade crustal section showing the geothermal gradient is modified after Stanley et al. (1990). Trends within the data link to processes at different levels within the crust. From the bottom, (a) we first determine the mantle input composition, either a primitive LKT (black circles) or
CAB (black triangles). Nd vs MgO demonstrates the range of possible mantle inputs. (b) In the lower crust, separate processes generated the
starting composition (NS-02-66) and the trend with time (differences between North Sister groups). NS-02-66 is isotopically the most ‘crust-like’
composition (lowest 143Nd/144Nd, highest 87Sr/86Sr) and is modeled as a mixture between a primitive basalt and a partial melt of the crust.
Through time, North Sister magmas trend towards more mantle-like isotopic ratios (higher 143Nd/144Nd), but with more evolved major and
trace element compositions (higher Al2O3 and lower Ni), away from the mixing trend to make NS-02-66. (See energy-constrained models in
text.) (c) Variations within compositional groups are linked to crustal processing at various stages of the overall evolution. The within-stage processes are mainly replenishment by North Sister basaltic andesite, fractional crystallization of olivine, plagioclase, and clinopyroxene, and
minor crustal assimilation.
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Open-system processes are implied for the long-term evolution of the North Sister magmatic system by the variation in isotope compositions (e.g. 143Nd/144Nd of 0·51287^
0·51292; Fig. 8a).
In the following discussion we address the following
questions.
Table 3: Potential crustal assimilant compositions
Sample or cap. no.: 478
77
256
114
152
207
231
211
W/D:
Init.
D
W
D
W
D
W
W
P (kbar):
n.a.
1
1
3
3
6·9
6·9
6·9
Melt % (F):
0
19·0
6·1
20·7
23·9
21·0
28·5
48·5
SiO2
52·47 73·29 79·72 76·56 74·24 72·31 71·36 69·22
Al2O3
15·29 13·14 12·81 13·11 16·2
15·3
(1) What is the origin of the incompatible element-poor
North Sister basaltic andesites? How is the earliest,
most primitive North Sister basaltic andesite generated? What is the primary mantle-derived basaltic
input to North Sister? What is the nature of the crustal contaminant(s)?
(2) What processes generated variations within the North
Sister compositional groups and the MLF? Are these
variations distinct from the underlying differences between groups?
(3) How were the temporal variations between compositional groups generated at North Sister? How
has North Sister become isotopically more like the
regional primitive basalts with time, but paradoxically more compositionally evolved as reflected in
decreasing Ni and Sr and increasing SiO2? What
mass fluxes are required for North Sister to maintain
a nearly constant, yet intermediate character for
400 kyr?
(4) And finally, what constraints can we make about the
depth, temperature and H2O concentrations of the
North Sister magmas? Do the erupted products of
North Sister Volcano truly fingerprint deep crustal
processes?
18·23 19·79
TiO2
1·74
0·66
0·32
0·43
0·35
0·59
0·21
0·21
FeO*
11·79
3·07
1·01
2·39
1·89
2·93
1·37
1·12
MnO
0·22
0·1
0·08
0·08
0·11
0·06
0·07
0·03
CaO
9·21
4·05
1·98
2·72
3·52
3·76
4·87
6·10
MgO
5·29
1·49
0·5
0·49
0·43
0·87
0·18
0·04
K2O
0·16
0·88
1·43
0·92
0·54
0·96
0·39
0·30
Na2O
2·55
3·33
2·12
3·21
2·57
3·05
3·00
3·05
P2O5
0·49 —
0·04
0·07
0·12
0·18
0·08
0·20
Representative experimental glasses were produced by partial melting an LKT-like greenstone (sample 478) by Beard
& Lofgren (1991). Cap. no. is experimental capsule
number. W/D, water saturated or dehydration melting conditions. The fO2 conditions of these experiments were 1–2
log units above the nickel–nickel oxide (NNO) buffer. Glass
compositions that are the product of 20% partial melting
(77, 256, 114, 152, 207, 231) were used in calculating
AþFC in Fig. 11. Glasses 114 and 256 were used in
AþFC models of group 2a and MLF, respectively.
multi-stage petrological model (Fig. 9) that tracks the injection of a parental mantle-derived basalt into the lower
crust, its transformation into a basaltic andesite, the evolution of the magma system with time, and re-equilibration
of the magma during transport to the surface. Because no
evolved (high-SiO2) magmas were erupted during the
central-vent stages of North Sister, and the lavas are
crystal-poor, we think it likely that the magma reservoir
stayed well above the solidus at least during the development of a particular stage. Deep crustal liquid lines of descent have been constrained by high-pressure phase
equilibria experiments (5^20 kbar, 0^15% H2O) on a
North Sister basaltic andesite (sample NS-02-66) by
Mercer & Johnston (2008).
The North Sister dataset defines two main types of
trend: changes within groups and changes between groups
over time. First, major and trace element data form arrays
with little to no change in Sr and Nd isotope compositions
within the various groups. For example, 143Nd/144Nd data
for group 2a are within error of each other (three samples,
Table 3), but range over 50 ppm Ni, suggesting
(closed-system?) processes that do not affect Nd isotopic
ratios. Second, the Sr and Nd isotopic ratios become steadily more like those of the regional basaltic magmas over
400 kyr (for the seven samples analyzed; Table 1).
The origin of North Sister basaltic andesite
In general, the origin of intermediate magmas has been
attributed to: (1) high-degree hydrous mantle melting (e.g.
Tatsumi, 1982; Grove et al., 2002; Parman & Grove, 2004);
(2) partial melting of (amphibolitized) lower crust driven
by mantle-derived heat inputs (Conrey et al., 2001; Ownby
et al., 2008); (3) fractional crystallization of a basaltic
parent in the upper or lower crust (e.g. Gill, 1981; Sisson &
Grove, 1993; Mu«ntener et al., 2001; Annen & Sparks, 2002);
(4) mingling of mafic and more fractionated silicic melts
in the upper crust (Reubi & Blundy, 2009); (5) assimilation
of crustal partial melts by mantle-derived basalts in the
deep crust (e.g. Hildreth & Moorbath, 1988; Petford &
Gallagher, 2001; Annen et al., 2006).
Basaltic andesites at North Sister are neither primary, as
the MgO concentrations and Mg-number are too low
(5·9^4 wt % and 35^45, respectively) to be in equilibrium
with the sub-arc mantle, nor are they direct partial melts
of the crust because their SiO2 concentrations are too low
(52·5^55 wt %; Beard & Lofgren, 1991; Sisson et al., 2005).
North Sister magmas also cannot be simple crystal fractionates from basaltic parents because compatible trace
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element concentrations are high, including Ni (up to
120 ppm) and Cr (up to 130 ppm), and comparable with
those of regional primitive basalts. For example, 37%
olivine fractionation could generate a 4 wt % increase in
SiO2 concentration (48·5^52·5 wt %), but would yield a
magma with less than 1ppm Ni. The involvement of older
crust in the petrogenesis of at least the earliest North
Sister magmas is implied by their relatively elevated
87
Sr/86Sr. We will explore processes acting in the crust, but
first we explore the primary mantle input.
What is the primary mantle-derived basaltic input?
The parental basalts that contributed to North Sister are
essentially restricted to mid-ocean ridge basalt (MORB)like LKT and more arc-typical CAB (Fig. 4a). These are
common compositions found all along the arc, but the
LKT are particularly abundant in the central Oregon
Cascades (Bacon et al., 1997; Conrey et al., 1997; Schmidt
et al., 2008). The differences between CAB and LKT are
subtle and truly primitive end-member magmas are rare.
HFSE-rich basalts (also called OIB or WIP) found in
northern part of the Cascade arc have not been found in
the central segment that hosts North Sister and, in any
case, are too enriched in Nb and LREE to be parental to
North Sister magmas. The volumetrically minor, fore-arc
absarokite is also an unsatisfactory starting point because
it has extreme enrichments in LILE and LREE (Schmidt
et al., 2008; Rowe et al., 2009).
The North Sister magmas are calc-alkaline (as defined
by FeO*/MgO vs SiO2; Miyashiro, 1974) and have REE
patterns more similar to the CAB than to the LKT
(Fig. 4a). Most of the primitive (high-Mg) CAB are too enriched in incompatible trace elements, such as K, Ba, or
La, to be parental to North Sister (Bacon et al., 1994, 1997;
Conrey et al., 1997; Schmidt et al., 2008; Rowe et al., 2009).
A CAB that is incompatible element-poor may play a role
at North Sister, such as one from Cayuse Crater from the
southern end of the Three Sisters Volcanic Field (‘CC’ in
Figs 4a and 8; sample CC-01-1 inTable 1). This composition
was selected as a potential mafic end-member because it is
similar to North Sister in terms of REE and has a high
Mg-number (66·2); however, its high SiO2 (51·9 wt %) suggests that it may have been affected by crustal processing.
LKT is the only primitive basalt type that consistently
has lower concentrations than North Sister basaltic andesites in nearly all incompatible elements, particularly LILE
and LREE (but excepting HREE; Fig. 4), making it the
most likely parent, provided that the HREE behave compatibly during (deep) crustal processing. We chose LKT
sample C894 from Crater Lake (Figs 4 and 8; Bacon
et al., 1997) to represent a mafic end-member because it is
primitive (47·4% SiO2 and Mg-number ¼ 65·1) and has
particularly low LILE abundances (e.g. 46 ppm Ba).
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What is the nature of the crustal contaminant?
To evaluate the nature of the crustal contaminant, we focus
on the earliest, most primitive, yet most isotopically
evolved sample from North Sister (NS-02-66). Simple
two-component mixing models were calculated to fit this
composition between two end-members: a central Oregon
Cascades basalt (LKT or CAB) and a corresponding theoretical (crustal) melt (AA or AB respectively, Fig. 10). As
a starting point, to constrain the composition of the
theoretical crustal melt, we assume NS-02-66 is a (somewhat arbitrary) mixture of 70% primitive basalt and
30% melt of unknown composition. Chemical components
that were considered for these mass-balance models include SiO2, Al2O3, K2O, Nd, Sr, 143Nd/144Nd, and
87
Sr/86Sr (Fig. 10).
The initial basalts (CAB and LKT) have different SiO2
concentrations and this causes the greatest discrepancy between the resulting crustal melts (69·6 wt % SiO2 for
model melt AA that mixes with the LKT vs 54·5 wt %
SiO2 for A B that mixes with the CAB; Fig. 10a).
Neglecting SiO2, inasmuch as the Cayuse Crater sample
may not be primitive, this simple exercise indicates that
the theoretical crustal melt has high Al2O3 (22^24%)
and Sr (915^1100 ppm). High-Al2O3 silicic melts may
occur under hydrous, deep (6·9 kbar) crustal conditions,
where the stability of plagioclase is reduced, leading
to either plagioclase-free fractional crystallization or preferential melting of plagioclase (Table 3; Beard & Lofgren,
1991; Mu«ntener et al., 2001). The theoretical crustal
melt must also have higher 87Sr/86Sr and lower
143
Nd/144Nd than the basalts, suggesting that it is a partial
melt of older crust. Our model for the origin of North
Sister basaltic andesite is consistent with the deep crustal
hot zone hypothesis of Annen et al. (2006), where heat and
H2O released by crystallizing basalt in the lower crust
preferentially melts plagioclase from the surrounding
wall-rock.
Despite the simplified nature of these models, some important observations can be made about the evolution of
the North Sister system. First, the mixing models recreate
only the NS-02-66 composition and do not fit later North
Sister data, particularly with respect to the trend of
decreasing 87Sr/86Sr and increasing 143Nd/144Nd with time
(Fig. 10b), and neither do they recreate the variations
within compositional groups. Second, North Sister compositional groups progress away from the modeled mix
through time in isotope vs element diagrams. A plot of
143
Nd/144Nd vs Nd (Fig. 10c) demonstrates the
near-constant Nd content with steadily increasing
143
Nd/144Nd with time. Third, the youngest sample (group
3^4) mimics the mafic end-members in some respects,
such as in Nd and 143Nd/144Nd (Fig. 10c), but not in other
respects, such as in plots of 87Sr/86Sr vs Sr or 87Sr/86Sr vs
Al2O3 (Fig. 10d and f).
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Fig. 10. Two-component mixing models to generate the NS-02-66 composition with a primitive basalt and a theoretical crustal contaminant.
(a) Primitive end-members are an LKT from Crater Lake (C894; Bacon et al., 1997) and a CAB from Cayuse Crater (Schmidt et al., 2008).
The theoretical crustal contaminants were calculated by fitting a mixing curve so that NS-02-66 is a mixture of 70% primitive basalt and
30% contaminant. The theoretical crustal melt end-members AA and AB were derived by mixing with LKT and CAB, respectively. (b)
87
Sr/86Sr vs 143Nd/144Nd mixing model. Tick marks represent 5% mixing. North Sister compositional groups are indicated. North Sister evolves
away from the mixing trend in (c) 143Nd/144Nd vs Nd (ppm) and (d) 87Sr/86Sr vs Sr (ppm). The Central Cascades Sr mantle array is defined
by trend in Fig. 8b of primitive basalts from Medicine Lake, Crater Lake, and central Oregon basalts (Schmidt et al., 2008). Plotted against
Al2O3 are (e) 143Nd/144Nd and (f) 87Sr/86Sr, where North Sister systematically evolves away from the mixing curve over time.
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Processes generating variations within
compositional groups
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MARCH 2011
Table 4: Mineral^melt partition coefficients (D values)
The within-group variations are distinct from the underlying differences between groups; the differences between
groups track the isotopic evolution of the North Sister
system. Clustering of compositional groups in Sr and Nd
isotope space suggests fractional crystallization or
self-contamination with little to no mixing with an isotopically distinct component (Fig. 8a). The groups parallel
one another in major and trace element variation diagrams (Fig. 6). Whatever conditions or processes caused
these variations, they must have persisted over the entire
lifespan of the volcano. We focus modeling on one group
to demonstrate the range of processes that may have affected all North Sister magmas. We address group 2a because it is stratigraphically well constrained, represents
the most voluminous stage of North Sister Volcano, and
has varied magma compositions (Fig. 6). We also present
magma differentiation models for the MLF because it has
trends that are distinct from those of most North Sister
magmas.
The models developed were formulated to investigate
how major and trace element concentrations change as a
function of the fractionating or residual mineral assemblage, variably coupled with models including assimilation
and mafic recharge. Major element concentrations were
calculated by subtracting bulk mineral compositions
(assuming no solid solution; Table 2) and adding a contaminant or recharge magma at a defined ratio, rr or ra
[(mass recharge magma or assimilant melt)/(mass crystallized)] at 1% increments. Trace elements were modeled as
recharge (R) or assimilation (A) plus fractional crystallization (AþFC or RþFC) by incrementally summing
the composition of the liquid, Cliq derived by Rayleigh
fractional crystallization with a batch of an assimilant
melt or a recharge magma with composition Ca or Cr at a
given rr or ra. The formulation of the AþFC and RþFC
trace element differentiation models is described in the
Appendix. Mineral^melt trace element distribution coefficients (D values) are in Table 4. The AþFC or RþFC
equations differ from the AFC equations of DePaolo (1981)
where Cliq/Cinit is asymptotic with r, approaching infinity
near r ¼1, meaning that AFC cannot be applied for ra
near or greater than unity. Several workers have demonstrated diverse cases where assimilation ratios (ra) must
have been greater than unity (e.g. Grove et al., 1988;
Reiners et al., 1995; Fowler et al., 2004). In addition, the
AþFC formulation allows substitution of a recharge
magma (R) for A as RþFC, where the ratio of mass of recharge magma to the mass crystallized (rr) may be greater
than unity.
Our preferred models do not represent unique solutions
to the observed trends. Rather, they illustrate a range of
potential processes, their relative importance, and the
cpx
opx
spinel1
0·00
0·00
0·00
0·00
0·01
0·13
0·00
0·02
0·2
1
0·5
1·23
0·07
0·00
0·19
0·01
2·58
0·03
0·01
0·41
0·10
DNi
6·8
0·06
5
7·3
15
DCr
4·20
0·08
3·8
4·22
153
amph
plag
DBa
0·43
0·46
DSr
0·49
1·83
DV
3
DNd
DY
ol
15
0·73
0·02
19
0·25
0·00
Partition coefficients are after compilation by Claeson &
Meurer (2004) and the GERM website (http://earthref.
org/GERM/index.html; Philpotts & Schnetzler, 1970;
Ewart et al., 1973; Matsui et al., 1977; Dostal et al.,
1983; Blundy & Shimizu, 1991; Nielsen et al., 1992;
Beattie, 1993; Hart & Dunn, 1993; Bédard, 1994; Dunn
& Sen, 1994; Hauri et al., 1994; Sisson, 1994; Chazot
et al., 1996; Canil & Fedortchouk, 2001; Meurer &
Claeson, 2002).
1
Spinel compositions vary between chromite-dominated
North Sister and titanomagnetite-dominated MLF. Bulk
partition coefficients for element i (Dibulk ) in magma
differentiation
models
were
calculated
as
P
i
Dibulk ¼ Dimineral Xmineral , where Kdmineral
is the partition
coefficient of element i and Xmineral is the molar proportion
for each mineral.
minerals that may have affected the North Sister and
MLF magma compositions.
Modeling group 2a
To first constrain where processing of group 2a magmas
occurred, we focus on the effect of the changing plagioclase, pyroxene, and olivine stability with pressure on
Al2O3 and MgO concentrations (Fig. 11). Trends originating from a 2a dike sample (NS-02-38, Table 1) characterize
magma differentiation (FC and AþFC) occurring at
three levels within the crust (‘deep’, ‘mid’, and ‘shallow’;
Table 6). AþFC models incorporate (ra ¼ 0·25) partial
melts of basaltic protoliths under hydrous or dehydration
conditions at the three levels (Table 3; Beard & Lofgren,
1991). In the ‘deep’ crust, plagioclase feldspar is unstable
under hydrous conditions and clinopyroxene and orthopyroxene are the dominant minerals (e.g. 43% H2O and 12
kbar crystallization experiments of Mu«ntener et al., 2001),
leading to increasing Al2O3 with decreasing MgO concentrations (Table 6). The ‘deep’ trajectories define an upper
boundary for Al2O3 concentrations for the North Sister
data as well as a possible means to generate the high
Al2O3 found in some later North Sister and MLF samples.
The ‘shallow’ assemblage (Table 6) is constrained by
modal phenocryst abundances in the North Sister lavas of
plagioclase4olivine4magnetite (Table 1). Steeply inclined
arrays within group 2a parallel the shallow-level FC
model. The ‘mid’ crustal mineral assemblage (Table 6) is
622
SCHMIDT & GRUNDER
ARC VOLCANO ROOTS
Fig. 11. Al2O3 vs MgO (wt %) for North Sister and the MLF with
compositional group 2a indicated. Differentiation trajectories (FC
and AþFC) are plotted originating from the initial composition of
sample NS-02-38 (Table 1) using representative mineral compositions
(Table 2) at proportions characteristic of ‘deep’, ‘mid’, and ‘shallow’
crustal depths (Table 6). Each line represents 10% crystallization.
The assimilants (A) are experimental glasses (Beard & Lofgren,
1991) for three depths (6·9, 3, and 1 kbar) that were generated under
‘dry’ dehydration or ‘wet’ hydrous conditions (Table 3) and added at
a ratio ra of 0·25.
less constrained, but we assume that plagioclase, olivine,
and clinopyroxene are present at proportions intermediate
to the ‘deep’ and ‘shallow’ assemblages. The ‘mid’ crustal
10% crystallization trend line is shorter and more horizontal (decreasing MgO with modest decrease in Al2O3)
than the other trajectories (Fig. 11), providing a mechanism
to fan out the data and mask higher degrees of
differentiation.
Group 2a is the most variable in terms of the compatible
trace elements Ni and Cr (112^54 ppm Ni and 101^46 ppm
Cr; Fig. 6d), which are sensitive to different minerals (e.g.
olivine vs chromite). On the basis of Cr and Ni variations,
we define three trends within group 2a [(1) a high-Cr lineage, (2) a low-Cr lineage, and (3) an intermediate lineage
(Figs 6d and 12)] that we fit to models of FC, AþFC, and
RþFC. Parental magma compositions for each lineage, as
well as recharge magma and assimilant melt compositions,
are listed in Table 5. Major element compositions for the
assimilated crustal melt (Ca), are from dehydration melting
experiments on an LKT-like greenstone at 3 kbar
(Table 3; Beard & Lofgren, 1991) with trace element compositions calculated by 20% batch melting of basalt
(CC-02-1). If basaltic magmas recharged the magma
system, they would shift the Sr and Nd isotopic
composition. So instead, the most primitive 2a dike
(sample NS-02-38, Tables 1 and 5) with high Cr and Ni
and low SiO2 concentrations was selected to be the parental and recharge magma composition (Table 5).
The ‘deep’ and ‘shallow’ FC models bound the North
Sister data in most variation diagrams and are plotted
along with preferred solutions (FC, AþFC, and RþFC)
for the three Cr lineages (Fig. 12). Our preferred FC mineral assemblages (Table 6) were decided by selecting from
dozens of systematic trials runs. The FC assemblages for
the ‘mid’ crust are more olivine-rich than the pyroxenedominated ‘deep’ and plagioclase-dominated ‘shallow’ assemblages. Also presented in Table 6 are the preferred
ratios (ra and rr) for the AþFC and RþFC models. The
‘shallow’ FC model is the worst fit with regard to Ni,
SiO2, and CaO/Al2O3 (Fig. 12a and b) and therefore fractional crystallization of the phenocryst assemblage did not
generate the compositional diversity observed in North
Sister magmas. For CaO/Al2O3 to decrease as is observed
among group 2a lavas (0·05 decrease over a 1wt %
change in MgO), some fractional crystallization of clinopyroxene must have occurred (Fig. 12b and f). The
pyroxene-dominated ‘deep’ FC trend replicates the
marked decrease in Ni and decreasing CaO/Al2O3 over
the first 5% crystallization for the lower SiO2 magmas
among group 2a. This suggests that North Sister magma
compositions record differentiation at a range of depths,
from the deep crust (10 kbar) to mid-crust (3 kbar).
We interpret the separation between the high-Cr and
low-Cr lineages to have been caused by 1^2% fractional
crystallization of Cr-rich clinopyroxene. Reported values
of partition coefficients for Cr in clinopyroxene (DCr
cpx ) in
basaltic to andesitic magmas range from 1·5 to 30 (e.g.
Hart & Dunn, 1993; Hauri et al., 1994; Skulski et al., 1994)
and depend on temperature, pressure, composition,
oxygen fugacity, and water content of the magma. The
compatibility of Cr in clinopyroxene appears to increase
with the SiO2 content of the melt. Mercer & Johnston
(2008) reported DCr
cpx of 30 (1s standard deviation of 15)
for near-liquidus augite in North Sister basaltic andesite
over a range of conditions (5^20 kbar and 0^15% H2O).
Their data suggest that DCr
cpx is lower in the hydrous (5%
H2O) lower pressure (5 kbar) runs. To account for Cr concentration elevated by 20 ppm in the high-Cr lineage,
we use a lower DCr
cpx (3·8; Hart & Dunn, 1993; Table 4,
Fig. 12c and g) to fit both the trace (Cr) and major elements (CaO/Al2O3). A reduced affinity of Cr in clinopyroxene for the high-Cr lineage is consistent with either wetter
or more shallow conditions than for the low-Cr lineage.
Chromite fractionation could also affect the differences
in the high- and low-Cr lineages. Mg^Al chromite has
been found as inclusions in olivine crystals and rare phenocrysts (Table 2), but it cannot account for the attendant
CaO/Al2O3 decreases with decreasing MgO that are
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JOURNAL OF PETROLOGY
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Table 5: North Sister model compositions
Sample:
NS-02-38
NS-01-1
NS-02-38
NS-03-115
NS-02-94
NS-02-120
2a high-Cr
2a low-Cr
2a intermediate
MLF Main
MLF East
MLF East
Cinit-1
Cinit-23
Cinit
Cr
Cinit1
Ca2
Cr
Cinit
Ca4
wt %
SiO2
53·2
76·56
53·1
53·4
53·2
55·2
79·7
54·8
53·6
Al2O3
18·6
13·11
19·0
19·0
18·6
17·3
12·8
18·5
18·2
TiO2
1·01
0·43
1·03
1·05
1·01
1·53
0·32
1·37
1·463
FeO*
7·6
2·39
7·5
7·5
7·6
8·9
1·0
8·3
9·0
CaO
9·2
2·72
9·2
9·1
9·2
7·7
2·0
7·4
7·8
MgO
5·9
0·49
5·7
5·4
5·9
4·0
0·5
3·8
4·9
K2O
0·64
0·92
0·62
0·64
0·64
0·93
1·43
0·89
0·68
Na2O
3·5
3·21
3·7
3·7
3·5
4·1
2·1
4·4
3·9
ppm
Ni
112
46
104
91
112
Cr
97
66
55
50
97
11
10
32
56
V
172
137
172
173
Ba
243
546
234
240
172
287
196
178
228
243
339
612
308
Sr
561
299
580
581
294
561
518
223
561
571
Starting (Cinit), assimilant (Ca), and recharge (Cr) melt compositions for AþFC and RþFC modeling of three trends
(high-Cr, low-Cr, and intermediate-Cr) within group 2a and the Main and East splays of the MLF.
1
Cinit and Cr were modeled as equivalent compositions for the group 2a high-Cr lineage.
2
The Ca major element composition is from glass 114 from Beard & Lofgren (1991)’s dehydration partial melting experiments of an LKT-like greenstone at 3 kbar and 20% melting (Table 3). Trace element concentrations of Ca were calculated
as a 20% batch melt with a residual assemblage approximated by the CIPW norm of CC-01-1 (53 plag, 10 olivine, 12 cpx,
21 opx, 4 magnetite).
3
The initial composition for the intermediate lineage (Cinit-2) was derived by 3% FC of the low-Cr lineage (Table 6).
4
The major elements of Ca for the MLF Main splay trend are from glass 256 from partial melting experiments of Beard &
Lofgren (1991) at 1 kbar and 9008C of an LKT-like greenstone. Trace elements were calculated as a 10% partial melt
of a basalt with plagioclase4olivine4clinopyroxene4magnetite.
consistent with clinopyroxene. The high-Cr lineage also
cannot be the result of chromite accumulation because
decreases in SiO2 or increases in Ni are not observed
(Fig. 6a).
After separation of magmas into the high- and low-Cr
lineages, the lineages followed similar differentiation
paths (Fig. 12c and g). Both the high-Cr and low-Cr lineages may be fitted by simple FC, with or without a small
amount of assimilation (ra50·25) (Fig. 12). Assimilation
(ra50·25) may be necessary to account for slight enrichment of incompatible elements such as Ba (Fig. 12d), but is
unlikely to be volumetrically significant owing to the restricted ranges in incompatible trace elements and isotopic
ratios. The addition of recharge (RþFC) shortens fractionation trend lines and buffers compatible elements (e.g. Ni
and Cr) at more primitive concentrations (Fig. 12a and c).
The intermediate lineage is defined by a broad swath between the high-Cr and low-Cr lineages in a plot of Cr vs
Ni (Fig. 12g). This lineage includes samples with the highest
SiO2 and therefore cannot be the result of simple
two-component mixing between the high- and low-Cr lineages. The initial composition for the intermediate lineage
(Table 5) is a daughter of the low-Cr FC model, derived
by 3% crystallization. Fractional crystallization would
not lead to an 40 ppm increase in Cr with 40 ppm decrease in Ni. Our preferred differentiation RþFC model
involves fractional crystallization of plagioclase, olivine,
and clinopyroxene with recharge by a primitive high-Cr
2a magma (NS-02-38, rr 2) to maintain high compatible
trace elements while simultaneously increasing SiO2
(Fig. 12e).
Modeling the MLF
We next model the Main and East splays of the MLF by
FC, AþFC, and RþFC (Appendix). Evidence for upper
crustal differentiation of MLF magmas is largely geological; SiO2 increases southward along the length of the East
(from 53 to 55 wt % SiO2 over 4 km) and Main (from 55
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SCHMIDT & GRUNDER
ARC VOLCANO ROOTS
Fig. 12. Magma differentiation models for North Sister group 2a, which are divided into the high-Cr, low-Cr, and intermediate lineages.
Preferred FC, AþFC, and RþFC paths (Appendix) are calculated using partition coefficients in Table 4 and mineral proportions as in
Table 6. Models for the high-Cr lineage are in plots (a) Ni vs SiO2, (b) CaO/Al2O3 vs MgO, (c) Cr vs Ni, and (d) Ba vs Ni. Single-mineral
cpx
FC paths (cpx, ol, pl, chrom) are also shown. The partitioning of Cr in clinopyroxene may vary and we present FC trajectories for DCr ¼ 3·8
cpx
and DCr ¼ 30. Tick marks represent 10% crystallization. Also shown are the ‘deep’ and ‘shallow’ fractional crystallization paths (Fig. 11,
Table 6). The AþFC path adds a silicic partial melt component (Table 5) at a rate (ra) of 0·25 (Table 6). For the RþFC path, the initial and recharge magmas are the same composition (Table 5) and recharge occurs a rate (rr) of 4·0. Calculated differentiation paths for 2a low-Cr and
intermediate lineages are in (e) Ni vs SiO2, (f) CaO/Al2O3 vs MgO, (g) Cr vs Ni, and (h) Ba vs Ni. The high-Cr FC paths from (a)^(d) are
shown for comparison. Our preferred RþFC model for the intermediate-Cr lineage originates from the low-Cr trend and is recharged by the
initial high-Cr magma (Table 5) at rr ¼ 2.
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Table 6: Representative and preferred mineral assemblages,
ra, and rr values for FC, AþFC and RþFC differentiation
models
ol
cpx
opx plag chrom Ti-mag amph ra
rr
‘shallow’
16 —
—
83
—
1
—
0–0·25
‘mid’
42
8
—
50
—
—
—
0–0·25
‘deep’
— 60
40
—
2a high-Cr
30 14·5 —
55
0·5
—
—
2a low-Cr
28 20
—
50
2
—
—
—
2a intermediate 20 20
—
60
—
—
—
—
MLF Main
8 20
—
58
—
14
MLF East
25 —
—
65
—
—
0–0·25
0–0·25 0, 4
2
0–0·25
10
—
6
The ratios ra and rr refer to the AþFC and RþFC models,
respectively. For the FC models, ra and rr are zero.
to 60 wt % SiO2 over 8 km) splays. The West splay is less
systematic (Fig. 6). Crystal contents also decrease with
increasing SiO2 in MLF magmas (from 44 to 3 vol. %;
Table 1), suggesting systematic crystal^liquid separation.
Simple, euhedral phenocrysts and glassy groundmass textures along with geochemical variations support fractional
crystallization in either a zoned magma chamber or along
dike conduits in the upper crust (Schmidt & Grunder,
2009). Ni and CaO/Al2O3 variations among MLF
magmas are inconsistent with fractional crystallization of
two pyroxenes in the deep crust (FC ‘deep’ in Fig. 12a
and b). MLF andesites are unlikely to be partial melts of
mafic crust because they lie at the end of a continuous
trend with respect to SiO2, and Ni contents are too low
(as low as 1ppm at 60 wt % SiO2).
The Main splay forms nearly straight trends with a few
exceptions in SiO2 and MgO variation diagrams (Figs 6,
and 13a, b). Concentrations of TiO2 and V increase with
decreasing MgO in East splay lavas, but these elements decrease with decreasing MgO in the Main splay lavas
(Fig. 6b). This inflection in TiO2 vs MgO correlates with
the appearance of Ti-magnetite phenocrysts that are up to
1mm in size in Main splay lava. The importance of magnetite is emphasized by plotting trace element models as a
function of the V concentration (Fig. 13).
Our preferred model for the Main splay trends involves
40% FC of plagioclase, clinopyroxene, Ti-magnetite, and
olivine (at 58:20:14:8). Significant magnetite fractionation
(14%) is necessary to account for the marked decrease in
TiO2 with decreasing MgO (Fig. 13b). Because the
87
Sr/86Sr of MLF lavas varies just outside the analytical uncertainty (0·70354^0·70356, Fig. 8a), the Main splay probably assimilated a very small amount (up to ra ¼ 0·25) of
NUMBER 3
MARCH 2011
crust with Sr isotopic ratios similar to the Little Brother
lavas (Table 1). Two outliers from the Main splay trend
have low concentrations of Sr (Fig. 13c) that are probably
the result of plagioclase removal.
The East splay of MLF is more complex than the Main
splay because it has three potential mafic inputs: one that
is like North Sister group 2a, one that has high TiO2 and
FeO* (NS-02-120; Table 1) and one with high Al2O3 and
Sr (Fig. 6e). The origin of the high-Al2O3 and -Sr mafic
end-member can be resolved either by accumulation of
plagioclase feldspar or by plagioclase-absent fractional
crystallization of North Sister-like magma under deep, hydrous conditions (Fig. 10). The origin of the high-TiO2
end-member is more speculative, but it may have formed
by closed-system, olivine-dominated fractional crystallization. High-TiO2 compositions are also found in West splay
lavas, as well as among distal North Sister lavas, indicating
that the conditions for forming TiO2-rich mafic magmas
are not isolated to the East splay. The more evolved samples of the East splay form near-linear arrays in variation
diagrams and increase rapidly in TiO2 (from 1·37 to
1·57% TiO2 from 3·8 to 3·5% MgO) and decrease slightly
in K2O, suggesting that a high-TiO2, low-K2O magma recharges the system (rr ¼ 6; Table 5). Amphibole phenocrysts in some East splay lavas suggest higher H2O
(5 wt %) and lower temperatures (Moore &
Carmichael, 1998) than for the North Sister lavas. For
these models, we use hornblende instead of clinopyroxene
as a fractionating phase. Our preferred model for the East
splay includes fractional crystallization of plagioclase, olivine, and amphibole (65:25:10) along with significant recharge by the high-TiO2 mafic end-member (rr 6;
Fig. 13e^h, Table 6).
Summary of the 2a and MLF models
In light of the many unknown variables, the solutions presented for group 2a and the Main and East splays of the
MLF are not unique, but they represent realistic differentiation scenarios. These models rely on variable mineral^
melt partition coefficients derived from literature
(Table 4), which contribute to the uncertainty. Even so, all
variations within groups may be explained by relatively
simple models of fractional crystallization, with or without
recharge and/or minor crustal assimilation. Trends within
the 2a group may be explained by 10^20% fractional crystallization of plagioclase, olivine, clinopyroxene, þ chromite in the mid- to deep crust. The Main splay of the
MLF is the result of 40% fractional crystallization of
plagioclase, clinopyroxene, magnetite, and olivine. The
East splay involves significant recharge and 20% fractionation of plagioclase, olivine, and amphibole at more
shallow levels of the crust. Magma recharge does not involve adding primary mantle-derived basalt and instead
mixes primitive basaltic andesite or, as in the case of the
East splay of the MLF, a high-TiO2 basaltic andesite.
626
SCHMIDT & GRUNDER
ARC VOLCANO ROOTS
Fig. 13. Calculated differentiation paths (AþFC and RþFC) for the MLF using the mineral compositions and partition coefficients inTables 2
and 4, respectively. Magma compositions, preferred mineral assemblages, ra, and rr are given in Tables 5 and 6. Preferred models for the Main
splay are presented in plots of (a) K2O vs MgO, (b) TiO2 vs MgO, (c) Sr vs V, and (d) Ba vs V. Tick marks represent 10% crystallization.
The MLF East trend is presented in plots of (e) K2O vs MgO, (f) TiO2 vs MgO, (g) Sr vs V, and (h) Ba vs V.
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JOURNAL OF PETROLOGY
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Because SiO2, incompatible trace element, and isotopic
compositions are essentially constant within compositional
groups, wall-rock assimilation was minor within groups
for the North Sister magmas. Some assimilation of the
upper crust (ra ¼ 0·25) is necessary for the MLF Main
splay.
Importantly, the processes modeled for the variations
within groups do not involve isotopically distinct components and cannot account for the decreasing 87Sr/86Sr and
increasing 143Nd/144Nd over 400 kyr. The variations
within compositional groups arose mainly by fractional
crystallization over a range of pressures as the magmas
traversed the crust, thereby overprinting their deeper history, including basaltic andesite generation and the
long-term evolution of the North Sister system. The
magma thus carries the compositional record of a deeper
differentiation history than indicated by the phenocryst assemblage (see Annen et al., 2006).
How were temporal variations generated?
Simple models of fractional crystallization or crustal assimilation cannot account for systematic changes in composition (decreasing 87Sr/86Sr and increasing 143Nd/144Nd,
while Ni concentrations slightly increase and then decrease) over the course of 400 kyr. This is because
open-system processes of recharge and crustal assimilation
are energetically linked. Magma recharge involves adding
heat and mass, and affects rates of assimilation and crystallization. The mass and total enthalpy of the magma,
crystals, wall-rock, and recharge magma must be incorporated into open-system models tracking changes in trace
element and isotopic compositions with variable assimilation and crystallization rates. The EC-E‘RwAFC model,
standing for Energy-Constrained Eruption, Recharge,
with variably efficient (w) Assimilation and Fractional
Crystallization, does this elegantly (Bohrson & Spera,
2001, 2003; Spera & Bohrson, 2001, 2002, 2004). The
EC-E’RwAFC model calculates the mass and composition
of assimilated wall-rock melt by requiring that energy be
conserved between the magma, country rock, and crystals.
Our goal for this section is to develop an energetically
balanced model that fits general trends in trace element
and isotopic compositions for the North Sister system
through time. The EC-E’RwAFC model was not applied
in previous sections because it does not track major element concentrations. We weigh isotopic data more heavily
in the evolution between groups and so have applied the
model.
The energy-constrained model
EC-E’RwAFC models a composite magma system made up
of four sub-systems: the magma body (m), country rock
(a), a reservoir of recharge magma (r) that may be added
to the magma body, and a reservoir of erupted material
(e) that may be removed (Bohrson & Spera, 2001, 2003;
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MARCH 2011
Spera & Bohrson, 2001, 2002, 2004). The boundaries between these sub-systems may be open, closed, or diffuse,
allowing heat and/or matter to move between them. The
boundary between the magma and the wall-rock is permeable to a fraction (w) of produced melt; heat can move
freely across the boundary. During recharge or eruption
episodes, the boundary between the magma chamber and
the recharge or erupted reservoirs are completely open to
heat and mass transport. Each sub-system has its own
mass (Mm, Ma, Mr, Me) and thermal properties, and isotopic (87Sr/86Sr and 143Nd/144Nd) and trace element (Sr,
Nd, Ba, Ni, and Cr) compositions, and trace element distribution coefficients for each element (Bohrson & Spera,
2001, 2003; Spera & Bohrson, 2001, 2002, 2004).
EC-E’RwAFC is a set of 4 þ t þ i þ s coupled nonlinear
differential equations, where the number of trace elements,
and radiogenic and stable isotopic ratios modeled are t, i
and s, respectively. These equations are a function of
magma temperature and end at a user-defined equilibrium
temperature (Teq), when the temperature of the magma
equals that of a calculated mass of wall-rock. Igneous processes within the magma system cause changes in enthalpy
(h, Table 7) and may change the total enthalpy of the
system depending on energy and mass-transport mechanisms (Spera & Bohrson, 2001, 2004).
Conceptual model and input parameters
A large number of variables must be specified for the
EC-E’RwAFC model (Table 7) and are constrained by our
understanding of the North Sister magma system.
Compositional parameters were chosen to reflect a conceptual model: a magma chamber of North Sister basaltic andesite is recharged by mantle-derived LKT and
assimilates partial melts of gabbroic wall-rock. The starting point for these models is the earliest and most mafic
composition (NS-02-66). Its liquidus temperature was
determined by pMELTS (Ghiorso & Sack, 1995; Asimow
& Ghiorso, 1998), assuming a pressure of 10 kbar (35 km)
and 2 wt % H2O, to be 12908C.
For simplicity, we present models that test the influence
of recharge mass (Mr) and the thermal and compositional
characteristics of the assimilant. The recharge magma
composition is like that of the LKT in the two-component
mixing models to generate the North Sister basaltic andesite (Fig. 10; Bacon et al., 1997), but with slightly higher incompatible element concentrations (Ba and Sr). Models
for recharge by CAB are not presented because they are
too enriched in incompatible trace elements to maintain
the incompatible element-poor nature of North Sister
magmas. Non-dimensional masses relative to the initial
0
) of the four subsystems
mass of the magma (M=Mm
(magma body, wall-rock melt, recharge magma, and eruptive products) are plotted as a function of temperature in
Fig. 14a. As temperature decreases, recharge LKT magma
(Mr) is added linearly until it totals three or one times the
628
K )
1
Lower limit temperature
629
0·3
0·512846
0·4
Isotopic ratio, em0 (143Nd/144Nd)
DNd
98
4·2
54
2·42
Ni (ppm)
DNi
Cr (ppm)
DCr
3·92
100
6·0
300
0·16
300
LKT
3·8
249
6·0
300
0·16
100
0·3
0·512953
10
1
0·70346
350
assimilant (a2)
Recharge
3·8
249
4·6
174
0·11
150
0·2
0·512953
10
1
0·70346
350
magma (r)
Mass erupted material at Teq, Me
Recharge mass at Teq, Mr
Total mass of the magma body (Mm)
Integrated Ma*/Ms
Mass of erupted material, Me
Mass of recharge magma, Mr0
Mass of solids, Ms
Mass of cumulates, Mcm
Mass of assimilant partial melt, Ma*
Resulting mass characteristics at Ni ¼ 35 ppm:
1484
396000
1370
270000
1484
1·67
0·06
0·30
1·73
0·81
0·21
0·02
Ta 5 T s
0.5
3
1
1·75
0·40
0·30
1·75
0·83
0·21
0·12
Ta ¼ Ts
Thermal and thermodynamic parameters are after Fowler et al. (2004). (See text for abbreviations.) Bulk distribution coefficients (DSr, DNd, DBa, DNi, and DCr) reflect
a crystallizing assemblage of plagioclase 4 clinopyroxene amphibole and values in Table 4. The initial magma composition is sample NS-02-66 (Table 1).
0·23
DBa
274
0·5128
15·2
Nd (ppm)
Ba (ppm)
1
1
DSr
10
0·7038
0·70369
600
magma (m)
560
Enriched
assimilant (a1)
Initial
Isobaric specific heat of recharge magma, Cp,r (J kg1 K1)
Isotopic ratio, em0 (87Sr/86Sr)
Sr (ppm)
1100
1800
Equilibration temperature, Teq
Compositional parameters
940
Solidus temperature, Ts
1320
Crystallization enthalpy of recharge magma, hr (J kg1)
1320
Recharge magma liquidus temperature, Tl,r
Recharge magma initial temperature, Tr
Isobaric specific heat of assimilant, Cp,a (J kg1 K1)
850 or 940
Fusion enthalpy, ha (J kg1)
Isobaric specific heat of magma, Cp,m (J kg
Assimilant initial temperature, Ta0
0
396000
Initial mass of the magma (Mm0)
1
Crystallization enthalpy, hm (J kg1)
1350
1300
1290
Input mass characteristics
Thermodynamic parameters
Assimilant liquidus temperature, Tl,a
Magma initial temperature, Tm
0
Magma liquidus temperature, Tl,m
Thermal parameters
Table 7: EC-E 0RwAFC parameters
SCHMIDT & GRUNDER
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JOURNAL OF PETROLOGY
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0
initial mass of the magma (Mm
) at Teq (black and orange
curves respectively). The amount of erupted material is
kept constant (Me ¼ 0·5 at Teq) for all models presented.
Two gabbroic wall-rock compositions are modeled. One
is enriched in Ba and Sr and has Sr and Nd isotope ratios
like the silicic end-member (Fig. 10) used in the
two-component mixing models for the generation of parental North Sister basaltic andesite (a1; blue lines in
Fig. 14). The other is isotopically like LKT (a2; red lines)
and has lower Ba and Sr. The wall-rock liquidus temperature was estimated to be greater than the liquidus of the recharge magma (13508C and 13008C, respectively). The
initial temperature of the wall-rock was modeled as either
being equal to the solidus (Ta0 ¼ Ts) or less than the solidus
(Ta0 5Ts) (Table 7 and Fig. 14). Other thermodynamic parameters follow the EC-E’RwAFC application for the Isle
of Skye Igneous Province of Fowler et al. (2004; Table 7).
Once thermal, mass, thermodynamic, and compositional characteristics were entered, we calculated compositional paths of magma differentiation. Partition coefficients
were varied iteratively to best fit the observed trends and
linked to values presented in Table 4 by dozens of
simulations.
Results of EC-E’RwAFC modeling
The calculated mass of anatectic wall-rock melt (Ma*)
and its influence on the composition of the magma depends on the initial temperature of the wall-rock (Ta0 )
relative to the solidus temperature (Ts) of the system
(Fig. 14a). The Ma* is 140% greater for the case of
Ta0 ¼ Ts than for Ta0 5Ts. More wall-rock melt (Ma*) at a
higher initial temperature leads to higher concentrations
of the incompatible elements Ba and Nd in the magma.
Also, Sr and Nd isotopes are more strongly influenced by
the assimilant for Ta0 ¼Ts than Ta0 5Ts (Fig. 14b).
The computational endpoint in EC-E’RwAFC models is
a user-defined equilibrium temperature (Teq), when the
temperature of the magma equals the temperature of the
wall-rock. The petrological end-point, however, was determined by compositional criteria when the models become
unrealistic. We defined the lower limit temperature as
when the modeled Ni concentration reached the lowest Ni
concentration of North Sister (35 ppm) or 1184^11758C, depending on the wall-rock composition and recharge rate
(Fig. 14). In all models, the lower limit temperature coincides with a marked increase in anatectic melt (Ma*;
Fig. 14a). Also for the enriched assimilant (a1) models, the
maximum 143Nd/144Nd and minimum 87Sr/86Sr occurs at
the lower limit temperature (Fig. 14b). For the LKT-like
assimilant (a2) models, 143Nd/144Nd continues to increase
and 87Sr/86Sr steadily decreases after the lower limit temperature because the isotopic compositions of the recharge
magma and the wall-rock are the same.
The preferred bulk distribution coefficients (Table 7) reflect a gabbroic fractionating assemblage of clinopyroxene,
NUMBER 3
MARCH 2011
plagioclase and minor amphibole. These models indicate
that olivine cannot play a significant role in the evolution
of the magma because Ni concentrations are high over the
entire eruptive history. Minor amounts of amphibole (up
to 10%) are necessary to maintain low REE abundances
(Nd; Fig. 14d). Although some North Sister lavas with
454% SiO2 contain orthopyroxene phenocrysts, they are
not common. Amphibole is even more rare and has been
identified in only a handful of MLF lavas. The phenocryst
assemblage of plagioclase and olivine reflects later equilibration in the mid- to upper crust and is not important to
the generation and evolution of North Sister magmas.
Another outcome of these models is that significant recharge rates are necessary to reproduce the isotopic and
Nd, Cr, and Ni variations with time (Fig. 14). For Ni to
remain high, the recharge magma must be ‘primitive’
(300 ppm Ni; Table 7) and so cannot have significantly
fractionated an ultramafic assemblage prior to mixing.
0
High recharge rates (Mr =Mm
¼3) successfully increase
143
144
87
Nd/ Nd and decrease Sr/86Sr, while maintaining
nearly constant Nd concentrations, regardless of wall-rock
composition or initial temperature. For the lower recharge
0
mass model (Mr =Mm
¼1, where Ta0 5Ts and LKT assimilant), the Sr and Nd isotopic composition of the magma
does not reach the values of later North Sister magmas
(Fig. 14c). Ni and Cr among samples of groups 1 and 2a
are higher than the initial composition (NS-02-66), which
also supports recharge by high-Ni and Cr basaltic magma
(Fig. 14e).
To investigate the influence of the assimilant, we compare the results of melting an isotopically crustal wall-rock
(a1, blue) and a depleted LKT-like wall-rock (a2, red;
Fig. 14). Both wall-rock compositions generate realistic differentiation trends that reproduce the North Sister variation with time. The LKT-like assimilant a2, however,
causes further increase in 143Nd/144Nd (Fig. 14b) and decrease in 87Sr/86Sr with continued decreasing temperature
past the lower limit. Assimilation alone of incompatibleelement-poor wall-rock melt cannot account for the variation with time because crustal melts do not contain Ni or
Cr concentrations sufficient to maintain elevated levels in
the magma while it crystallizes mafic minerals. Also, such
a melt would not have Sr and Nd abundances great
enough to significantly affect the isotopic ratios of the
0
magma (e.g. Mr =Mm
¼1 model, orange lines). An incompatible element-poor assimilant like a2 (e.g. 150 ppm Ba)
coupled with abundant LKT recharge would sustain low
incompatible element concentrations over the evolution of
a magma system. This suggests that the crustal assimilant
could be different from the one that initially contributed
to the generation of North Sister basaltic andesite and
that high basalt flux from the mantle is remaking the
lower crust with time, which in turn becomes a crustal
contaminant.
630
SCHMIDT & GRUNDER
ARC VOLCANO ROOTS
Fig. 14. EC-E0 RwAFC models (Bohrson & Spera, 2001, 2003; Spera & Bohrson, 2001, 2002, 2004) for the evolution of the North Sister basaltic
andesitic magma system with time. Input thermal, thermodynamic, and compositional parameters and resulting mass characteristics are
given in Table 7. Compositional parameters are meant to reproduce a magma chamber of North Sister basaltic andesite (m) that is recharged
by mantle-derived LKT (r), and assimilates a partial melt of gabbroic wall-rock with either an isotopically enriched (a1; blue lines) or
LKT-like (a2; red lines) composition. The initial temperature of the country rock was modeled as either less than or equal to the solidus
(continued)
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JOURNAL OF PETROLOGY
VOLUME 52
These models simplify the differentiation of a complex
magma system over a long period to a single liquid line of
descent and there are some notable excursions from the
observed data. For instance, group 2b lavas (not analyzed
for isotopes) share some chemical characteristics with
group 1 lavas, such as lower CaO and higher Ba, but at
lower Ni concentrations (Figs 6c and 14e). Consequently,
group 2b does not lie along the calculated EC-E’RwAFC
differentiation path, perhaps implying renewed interaction
with a more enriched assimilant like (a1). We interpret the
higher Ba among the more evolved (higher SiO2) 2a
magmas (Fig. 14e) to reflect crustal overprinting as discussed in a previous section (Fig. 12). Also our energyconstrained model does not match the exact shape of the
evolution curve in 143Nd/144Nd vs 87Sr/86Sr (Fig. 14c) and
this may suggest changing isotopic composition of the basaltic or crustal input.
Independent mass-balance calculations of major elements verify the plausibility of the EC-E’RwAFC models.
We estimated SiO2 and Al2O3 concentrations of the
magma by summing the LKTand high-silica melt components and subtracting crystals at the mass proportions at
the lower limit temperature (Fig. 14a). Reasonable major
element concentrations are found with a recharge magma
of 45% SiO2 and a crystallizing assemblage containing
50% plagioclase, 40% pyroxene, and 10% amphibole,
and are consistent with the distribution coefficients.
Depending on the initial temperature of the wall-rock,
higher SiO2 and Al2O3 abundances were calculated for
Ta0 ¼ Ts models (57·2 and 19·0 wt %, respectively) than
for Ta0 5Ts (55·9 and 18·7 wt %), reflecting additional
wall-rock melt with 65 wt % SiO2. Al2O3 concentrations
are relatively constant over the history of North Sister, suggesting involvement of plagioclase, although the upper
range of Al2O3 increases over time (Fig. 7b). The most
evolved (lowest Ni) composition in the differentiation
trend (NS-02-46) contains the highest Al2O3 of the North
Sister magmas (19·7 wt %; Table 1), but also has relatively
low SiO2 (53·8 wt %) and lies along the ‘deep’
plagioclase-free trajectory in AþFC models (Figs 10
and 11). This suggests that plagioclase disappeared from
the fractionating assemblage.
NUMBER 3
MARCH 2011
In summary, while the model presented here simplifies
North Sister’s evolution to a single magma differentiation
trend over its 400 kyr history, EC-E’RwAFC is an effective way to evaluate the mass and heat budget necessary to
maintain a depleted, mafic composition over a long
period of time. The case where the initial temperature of
the assimilant is lower than the solidus temperature
(Ta05Ts) is preferred because it produces the least
wall-rock melt, allowing the recharge magma to have
greater influence and shift isotopic compositions toward
higher 143Nd/144Nd and lower 87Sr/86Sr (Fig. 14c) without
greatly affecting SiO2 and Al2O3 concentrations. This
case also sustains low and nearly constant incompatible
element concentrations, while compatible elements such as
Ni increase and then decrease through time. The basaltic
andesites of North Sister are fundamentally mixtures of
mantle and crustal melts, but once the primitive North
Sister magma forms, there can be little contribution from
high-Sr, high 87Sr/86Sr crustal melts. The mafic character
of the magma is sustained by continuous recharge with incompatible element-poor basalt, which in turn modifies
the composition of the lower crust through sill
emplacement.
Volumes
The chemical models of the generation and evolution of
the North Sister magma system have mass consequences
on magma, cumulate, and wall-rock volumes (Fig. 15).
The calculated volume of magma necessary to make the
40 km3 erupted at North Sister (Schmidt & Grunder,
2009) is based first on the two-component mixing model
for the origin of North Sister basaltic andesite (70% LKT
and 30% silicic melt; Fig. 10) and second on the mass characteristics at the lower limit temperature (11808C for
Ta0 5Ts) by EC-E’RwAFC modeling to replicate North
Sister’s trend with time (Fig. 14a; Table 7). Later crustal
overprinting was not included in these estimates because
it accounts for relatively small amounts of fractionated material (between 5 and 15%). In any event, the volume estimates are minima, inasmuch as not all the magma
produced was erupted and later, minor fractionation is
omitted.
Fig. 14. Continued
temperature of the system (Ta0 < Ts or Ta0 ¼ Ts ). The total mass of recharge magma added to the system (Mr) is three or one times the initial
0
). (a) Modeled non-dimensional mass characteristics relative to the initial mass of the magma body plotted as a
mass of the magma body (Mm
function of temperature (8C). The recharge mass (Mr) is either three or one and is represented by black or orange lines, respectively. The
end-points for the models are the lower limit temperature where Ni in the magma equals the lowest Ni found in North Sister. The lower limit
temperatures are 11828C or 11758C for Mr ¼ 3 or 1, respectively. (b) Selected compositional characteristics (Ba, Ni, and 143Nd/144Nd) are
plotted as a function of temperature with the lower limit temperature indicated. (c) EC-E0 RwAFC trends are plotted in 143Nd/144Nd vs
87
Sr/86Sr. North Sister compositional groups are shown, as is the field of primitive LKTs from the Central Segment of the Cascades arc
0
¼ 1) does not reach the later compositional groups, indicating that high rates of re(Schmidt et al., 2008). The lower recharge model (Mr =Mm
charge are necessary to maintain the system. (d) Nd concentrations are relatively constant over the history of North Sister magma system
whereas 143Nd/144Nd increases. High rates of recharge are also necessary to maintain constant Nd concentrations. The models involving a2
curve toward higher Nd concentrations than the a1 models. (e) Fields of North Sister compositional groups, dikes, and the EC-E0 RwAFC
models plotted in Ba vs Ni (ppm).
632
SCHMIDT & GRUNDER
ARC VOLCANO ROOTS
Fig. 15. Histogram of volumes required to generate and maintain the
North Sister magma system (40 km3 of erupted material; Schmidt &
Grunder, 2009). These volumes reflect (a) mixing LKT with a crustal
and (b) the EC-E’RwAFC model of high recharge to replicate North
Sister’s variation with time for the case of Ta0 < Ts at its lower limit
temperature (Fig. 14). Nondimensional masses for each km3 of North
Sister basaltic andesite are also indicated.
The amount of LKTrecharge is nearly double the initial
North Sister magma volume. We modeled a constant recharge rate (0·58 km3 kyr1), although we acknowledge
that this does not reflect declining eruption rates at North
Sister Volcano from 0·18^0·12 km3 kyr1 during the first
two stages (36 km3 over 200^300 kyr) to 0·08 km3 kyr1
during the last two stages (4 km3 over 50 kyr; Schmidt
& Grunder, 2009). The recharge rate probably also slowed
45^65% over the last two eruptive stages. According to
these models, accumulation of plagioclase and pyroxene
in the crust beneath North Sister counter-balances magma
recharge to maintain a steady-state composition. The total
amount of crystals generated (4100 km3) is nearly 50%
the amount of recharge magma to maintain the North
Sister system. Our calculations emphasize that despite
subtle compositional variability, significant amounts of
mass flux and crustal processing are necessary for the persistence of a mafic magma composition.
Deep crustal origin and processing of the
North Sister magmas
We now address under what conditions the North Sister
basaltic andesitic magmas formed and evolved as constrained by our multi-stage petrological models (Fig. 9).
Several lines of evidence support a deep crustal origin.
The relative homogeneity of North Sister basaltic andesite is sustained over 400 kyr by repeated injection of
a mantle-derived LKT. Its contribution is indicated by
high Ni and constantly low incompatible elements, including Nd and Ba among the North Sister magmas (Fig. 14d
and e). Deep crustal staging would facilitate the
occurrence of such an undifferentiated, incompatible
element-poor melt.
For a mantle-derived LKT to transform into North
Sister basaltic andesite, it must mix with an Al- and
Sr-rich melt component (20 wt % Al2O3; Fig. 10) that
probably formed under hydrous, high-pressure conditions.
Small positive Eu anomalies in North Sister magmas
(Fig. 4) are consistent with this model; however, they are
common among the more primitive Cascades basalts and
may instead have been caused by Eu2þ exclusion from residual clinopyroxene (Donnelly-Nolan et al., 1991; Bacon
et al., 1997). This Al-rich melt probably did not result from
hydrous fractional crystallization of a basaltic magma
(Mu«ntener et al., 2001; Sisson et al., 2005) because it must
have higher 87Sr/86Sr and lower 143Nd/144Nd than the regional basalts, indicating that it is probably a partial melt
of an older protolith. Under water-saturated conditions
and at pressures greater than 7 kbar, plagioclase can entirely melt out of the basaltic crust at 48508C, leaving
behind a clinopyroxene-bearing refractory residuum (e.g.
Beard & Lofgren, 1991). An H2O content of 3·5 wt % was
determined in undegassed olivine-hosted melt inclusions
in North Sister basaltic andesites (Mercer & Johnston,
2008) and is consistent with water contents calculated by
the pressure-dependent plagioclase^melt hygrometer
(Lange et al., 2009) that are in the range of 1·0^3·6 and
1·8^4·3 wt % H2O for 1 and 10 kbar, respectively. At
43·5 wt % H2O and pressures 410 kbar, plagioclase is no
longer the liquidus phase for North Sister magmas
(Fig. 16; Mercer & Johnston, 2008).
Persistent elevated Ni contents in parental North Sister
basaltic andesite (to 120 ppm) precludes significant fractionation of olivine and orthopyroxene. Although olivine
is always found as a phenocryst in North Sister lavas and
modestly contributed to variations within compositional
groups, it is not near the liquidus at 43·5 wt % H2O at
pressures 45 kbar (Fig. 16; Mercer & Johnston, 2008). One
consequence of the relatively late formation of olivine is
that the study of olivine-hosted melt inclusions in North
Sister basaltic andesite can reveal only the later crustal history of the magma.
Instead of olivine, clinopyroxene fractionation is likely
and would not greatly affect Ni concentrations (Fig. 14).
Augite is the first phase to crystallize from North Sister
basaltic andesite at 3·5 wt % H2O and between 10 to 19
kbar (35^62 km depth; Mercer & Johnston, 2008; Fig. 16).
The depletion of HREE in North Sister relative to the
LKT parents is consistent with residual clinopyroxene
(e.g. Hack et al., 1994; Mercer & Johnston, 2008), be it fractionating or residual in the partial melting of crustal contaminants. Fractionation of amphibole (up to 10% in the
energy-constrained models) would further enhance
HREE depletion in the daughter magmas. Garnet did not
play a role in differentiation of North Sister magmas
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JOURNAL OF PETROLOGY
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NUMBER 3
MARCH 2011
Fig. 16. Liquidus phase diagram for NS-02-66 from hydrous (0^15 wt % H2O), high-pressure (5^20 kbar) experiments by Mercer & Johnston
(2008). At point A, LKT magmas assimilate Al-rich partial melts of the crust to generate North Sister basaltic andesite near the seismic
Moho. High-pressure crystals are resorbed at B. Then near-liquidus crystallization (C) and degassing-induced crystallization (D) occur as a
batch of North Sister magma moves upward through the crust to the surface.
because (1) LREE and HREE are not strongly fractionated (gently sloping pattern in Fig. 4; e.g. McKenzie &
O’Nions, 1991) and (2) garnet is the liquidus phase for basaltic andesite magma at 418 kbar, which is greater than
the crustal thickness (Fig. 16; Mercer & Johnston, 2008).
Seismic studies further limit the depth of North Sister basaltic andesite generation by locating the Moho or maximum depth of plagioclase stability at 40 km (Stanley
et al., 1990; Brocher et al., 2003). We therefore suggest
that the North Sister basaltic andesite magma was
generated and in large part evolved at 35^40 km depth
and 3·5^4 wt % H2O (Fig. 16). Resorption of the
clinopyroxene-rich high-pressure assemblage during
ascent of North Sister magmas from the deep crust along
an adiabat would account for the lack of clinopyroxene
phenocrysts (Mercer & Johnston, 2008; Fig. 16).
The fractionating assemblage (clinopyroxene, plagioclase, and minor amphibole) derived for the evolution of
North Sister magmas would produce gabbroic cumulates
in the deep crust similar to those observed in tectonically
exhumed arc-related lower crustal ultramafic and gabbroic
cumulates. For example, a section of lower crust of the
Jurassic Talkeetna Arc, SE Alaska formed at 9·5^11 kbar
(28^33 km) and includes 2 km of gabbronorite (DeBari &
Coleman, 1989; DeBari & Sleep, 1991). Fractional crystallization experiments have replicated the ultramafic to gabbroic cumulates of lower arc crust exposures in Kohistan,
Pakistan (Mu«ntener & Ulmer, 2006; Jagoutz et al., 2007).
As in other arcs, fractional crystallization occurs in the
deep crust of the Oregon Cascade arc. However, this
study at North Sister highlights the importance of a
balance between recharge and fractional crystallization
to produce a near-constant or ‘steady-state’ magma
composition.
Implications for the Oregon Cascade
arc crust
Since 8 Ma, intra-arc extension of the Oregon Cascade
arc has led to increased heat flow and has focused magmatism to within the High Cascade Graben (Fig. 1a). LKT
634
SCHMIDT & GRUNDER
ARC VOLCANO ROOTS
magmas generated by decompression melting of the
sub-arc mantle are temporally linked to the initiation and
propagation of the High Cascade Graben (Conrey et al.,
2004). These magmas are in addition to the abundant
LKT supplied to the evolving North Sister system, and together imply significant contributions toward the accumulation of mafic to ultramafic lithologies in the deep crust.
East^west seismic profiles across this portion of the
Cascades indicate little to no crustal thickening associated
with the volcanic arc (Trehu et al., 1994; Brocher et al.,
2003). The modest relief of the Oregon Cascade Moho
may reflect extension and ductile flow of lower crustal
lithologies coupled with intraplating of low-K magmas to
form ultramafic to gabbroic cumulates. Accretion of LKT
magmas to the lower Cascade arc crust along with hydrous
melting of plagioclase has also altered the isotopic characteristics of the crust with time. The crustal contaminant
probably became increasingly refractory, so that it contributed lesser amounts of Al-rich, high-87Sr/86Sr melt.
An 200 km long alignment of low-K basaltic andesitic
volcanoes lies along the axis of the High Cascade Graben
and includes the North and Middle Sisters, MLF, the Mt.
Bachelor Chain, and Belknap Crater (Gardner, 1994;
Conrey et al., 2004; Hildreth, 2007). The axial occurrence
of low-K basaltic andesitic magmatism suggests that a
complementary cumulate intraplate underpins the extensional portion of the Cascade arc.
the North Sister magmas (0·70369^0·70356; Table 3).
However, the rate of change in 87Sr/86Sr [d(87Sr/86Sr)/dt]
for the last 25 years at Arenal is roughly 5000 times faster
than for the integrated eruptive history of North Sister
Volcano and represents an instantaneous snapshot of an
evolving magma system. Detailed modeling of the current
eruption of Arenal by Ryder et al. (2006) demonstrates a
steady state between basaltic recharge and eruption (3:1
ratio of recharge to eruption). On the basis on MELTS
models (Ghiorso & Sack, 1995), Ryder et al (2006) suggested
that staging of basaltic andesitic magmas occurs in the
mid-crust (4 kbar) with little later crystal^liquid fractionation despite their high crystal contents (Streck et al.,
2002).
As at Arenal, the North Sister magma system was at a
compositional steady state, but deeper in the crust and
with a ratio of 2:1 of recharge to crystallization as represented by the within-suite variations. Overall, the North
Sister magma system underwent four such episodes, with
each successive one being less crustally contaminated. The
more protracted and repeated steady state of North Sister
was facilitated by the hotter regime deeper in the crust,
which requires less recharge to thermally maintain the
system. It is the high rate of recharge that leads to compositionally monotonous mafic magma systems.
Tatara^San Pedro
Rates, periodicity, and evolution of mafic
arc volcanoes
We here compare the compositional variability and mass
flux rates at North Sister with other long-lived arc volcanoes. We present three examples: Volca¤n Arenal in the
Central American Arc, the Tatara^San Pedro Volcanic
Complex in the Southern Volcanic Zone of the Andean
Arc, and Mount Adams in the Cascade arc. These comparisons give insights into the rates and periodicity of magmatic processes.
Volca¤n Arenal
Volca¤n Arenal first erupted at 7 ka and has since built an
edifice with a total volume of 7 km3 (Wadge et al., 2006).
Since 1968, Arenal has continuously erupted a nearly
homogeneous, low-K2O (0·55^0·7 wt %) basaltic andesite
(Reagan et al., 1987; Ryder et al., 2006). Recharge by basaltic magmas over the last 25 years of its continuing eruption
have been identified by zoned Cr contents in clinopyroxene
and spinel phenocrysts (Streck et al., 2002, 2005) and by
whole-rock compositional changes, such as increasing
SiO2 and decreasing 87Sr/86Sr with time (from 0·70381 to
0·70378; Ryder et al., 2006). Whole-rock major element compositional trends with time over the continuing eruption
of Arenal are broadly similar to trends over the entire
400 kyr lifespan of North Sister Volcano (e.g. Fig. 7), although the overall change in 87Sr/86Sr is greater among
The Tatara^San Pedro Volcanic Complex in the Southern
Andes has produced four cycles of mafic magmas (48^
55 wt % SiO2) that are predominantly basaltic andesitic.
Each cycle eventually evolved to more diverse and silicic
composition (52^68 wt % and up to 76 wt % SiO2) since
770 ka (Ferguson et al., 1992; Feeley & Dungan, 1996;
Dungan et al., 2001). These cycles occur over regular
intervals of 150^200 kyr with recharge being the principal process during the basaltic andesite stage. Although
compositional variations are more subtle at North Sister
Volcano, we also observe two cycles of restricted composition followed by broader ranges of magma composition
(group 1 to 2a and group 2b to 3^4 and MLF; Fig. 7).
Cycle intervals at North Sister are 150^200 kyr, similar
to those at Tatara^San Pedro.
The cause of these restricted to diverse cycles every
150^200 kyr at both volcanic centers may be linked to periodicity of the mantle input, sufficient to develop
multi-stage processing. Alternatively, Tatara^San Pedro
and the Three Sisters are both built on crust that is
40 km thick (Stanley et al., 1990; Bohm et al., 2002). The
periodicity may instead reflect characteristic scales of heat
transfer in the arc crust, such as the time for heat to transfer from the base to the mid-crust and then to return to
steady state for a given thickness of crust (3·75^5·0
kyr km1 crust).
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JOURNAL OF PETROLOGY
VOLUME 52
Mount Adams
The cycles of alternating restricted and more diverse
magma compositions are not found at long-lived andesitic
stratovolcanoes such as Mount Adams (Fig. 1a; Hildreth
& Lanphere, 1994), where mafic magmas erupted peripherally to the main andesitic to rhyodacitic volcanic edifice.
Volcanic activity in the Mount Adams region began at
940 ka with major cone-building episodes at 500, 450,
and 30 ka. At 111^122 ka, a pulse of peripheral basaltic
(LKT) magmatism coincided with a pulse of andesite and
dacite activity at the central vent (Hildreth & Lanphere,
1994). This is consistent with a zone of thermal perturbation within the mid- or upper crust that was rejuvenated
by the basaltic pulse. This thermally perturbed zone accumulated or produced more silicic magmas, but punctuated
cone-building episodes may imply that the zone intermittently seized up and became fully solidified. Such a
long-lived zone of thermal perturbation does not appear
to exist in the mid- to upper crust at North Sister, Arenal,
or Tatara^San Pedro despite high rates of recharge. At
these centers, efficient conduits (e.g. faults of the High
Cascades graben) allowed more mafic magma compositions to ascend rapidly without significant crustal interaction. However, that is not the whole story because the
High Cascades Graben faults extend into southern
Washington in the vicinity of Mount Adams as well as
other Cascade stratovolcanoes, such as Crater Lake
(Bacon et al., 1989), where partial melt zones reside in the
mid- to upper crust. More silicic compositions (rhyodacite
and rhyolite) at Middle and South Sisters have arisen in
the past 40 kyr (Calvert et al., 2005), and this suggests that
400 kyr and tens of km3 of through-going magma were
required to sufficiently perturb the thermal regime to generate silicic magmas.
So why was the low-K basaltic andesite magmatism of
North Sister so stable for such a long period of time, whereas Mount Adams developed a thermally anomalous zone
in the mid- to upper crust? To answer this, we compare
the isotopic compositions of Mount Adams and North
Sister Volcano. Significant diversity among Sr, Nd, Pb, Hf,
Os, and O isotopes among Mount Adams lavas (e.g.
0·702889^0·703854 87Sr/86Sr; Jicha et al., 2009) indicate
that the lower crust in that part of the arc is isotopically
heterogeneous and implies assimilation of an assortment
of crustal domains with different ages (Hart et al., 2003;
Jicha et al., 2009). The range in 87Sr/86Sr of North Sister
Volcano is much narrower (0·7035^0·7038) and mirrors
the limited isotopic ranges for the Central segment of the
arc as a whole (Schmidt et al., 2008). The wall-rock surrounding the North Sister magma system must therefore
be relatively homogeneous, young and/or refractory, probably derived by crystallization of low-K magmas, thereby
buffering the magma composition and preventing evolution of higher SiO2 compositions. Thus, the compositional
NUMBER 3
MARCH 2011
stability of long-lived basaltic andesitic magma systems
implies mafic crustal growth and underplating.
CONC LUSIONS
North Sister Volcano erupted over 400 kyr and provides
a window into the magmatic processes and evolution of
the deep arc crust of the Oregon Cascades. The low-K basaltic andesites that make up North Sister Volcano are apparently monotonous, but retain a subtle and important
record of extensive processing of mantle-derived magmas
by extensive magma recharge, fractionational crystallization of a clinopyroxene-bearing assemblage, and modest
assimilation of aluminous crustal melt derived from mafic
and ultramafic wall-rocks.
North Sister basaltic andesites define four successive
compositional groups that correspond to stratigraphically
defined eruptive stages that span tens of thousand years to
100 kyr (Schmidt & Grunder, 2009). Variations between
compositional groups track the evolution of the North
Sister magma system overall and differ from trends within
compositional groups. In particular, the Nd and Sr isotopic characteristics of the North Sister basaltic andesites
become increasingly like those of regional primitive basalts
with time (Fig. 8), whereas there is essentially no isotopic
variation within groups (Fig. 12). To address these
between-group and within-group differences, we develop
a multi-stage model (Fig. 9) that tracks the inception of
North Sister basaltic andesite magmatism via magma differentiation in the deep crust and magma modification to
eruption. The results of this model are as follows.
636
(1) The earliest and most mafic North Sister basaltic andesite composition (NS-02-66) may be generated by
two-component mixing of a mantle-derived LKTand
an Al-rich melt with high 87Sr/86Sr and low
143
Nd/144Nd (Fig. 10). The Al-rich melt probably
formed by hydrous partial melting of gabbroic
wall-rock at high pressures (7 kbar).
(2) Over the 400 kyr history of North Sister, the basaltic
andesite magma evolves away from the initial basalt^
crust mixing curve, to Sr and Nd isotopic ratios more
like those of the ambient basalts. Using energyconstrained modeling (Spera & Bohrson, 2001, 2002,
2004), differentiation for North Sister overall is dominated by recharge with a low-K tholeiite. Recharge is
balanced by fractional crystallization of clinopyroxene þ plagioclase, and relatively minor assimilation
of a lower crustal melt (Fig. 14) with an increasingly
basaltic isotopic character and depleted incompatible
trace element composition, such as might be derived
from minor melting of accreted cumulates.
(3) Variations within groups are consistent with extensive
recharge by relatively primitive basaltic andesite
coupled, at a ratio of 2:1, with fractional
SCHMIDT & GRUNDER
ARC VOLCANO ROOTS
crystallization of a clinopyroxene-bearing assemblage
at a range of levels within the crust. Plagioclase and
olivine, ubiquitous phenocrysts in the North Sister
basaltic andesite, did not play a major role in magma
genesis, as their removal would deplete Al2O3 and Ni
more than observed. The olivine^plagioclase assemblage formed at pressures less than 5 kbar based on
comparison with the phase equilibrium experiments
of Mercer & Johnston (2008). The three Cr lineages
to some degree occur in all groups and attest to distinct compositional overprints that developed at different levels within the crust (deep vs shallow).
(4) Andesites of the Matthieu Lakes Fissure, which transects North Sister Volcano, evolved from a North
Sister basaltic andesite composition dominantly by
fractional crystallization, with minor assimilation at
shallow levels within the crust.
Mass consequences of within- and between-group modeling require significant reworking of the lower crust as a
consequence of the recharge necessary to maintain the
depleted basaltic andesitic magmas and to create the increasingly refractory and isotopically primitive cumulate
intraplate that is capable of producing the depleted,
Al-rich contaminating melts. Formation of a refractory
mafic underplate may armor the magma system, thereby
limiting the degree to which partial melting and wall-rock
assimilation may occur.
Other monotonous basaltic andesitic composite volcanoes include Arenal (Costa Rica), Klyuchevskoi (Russia)
and Tatara^San Pedro (Chile), which are among the most
productive on Earth. Limited ranges in composition for
such volcanoes imply that basaltic recharge dominates
their evolution and signify formation of complementary
cumulates and mafic crustal growth.
AC K N O W L E D G E M E N T S
We thank Lang Farmer, who provided Sr and Nd analyses,
Peter Larson, who aided in oxygen isotope analyses, and
Michael Rowe, who provided primitive basalt samples
from the central Oregon Cascades. Thoughtful and thorough reviews by Becky Lange (who read the manuscript
twice), Michael Dungan, Wendy Bohrson, and Brian
Cousens significantly improved the paper. We also thank
Rick Conrey and Ed Taylor for access to their earlier dataset from North Sister, and Wes Hildreth, Andy Calvert,
and Judy Fierstein for discussions about the geological
and geochronological context.
FUNDING
This work was supported by National Science Foundation
grants EAR-0230359 and EAR-0506869 to A.L. Grunder
and a 2002 Jack Kleinman Graduate Research Grant to
M.E. Schmidt.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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A P P E N D I X : F O R M U L AT I O N O F
A þF C A N D R þF C T R AC E
E L E M E N T D I F F E R E N T I AT I O N
MODELS
To model trace element variations within compositional
groups we developed a model of assimilation (or recharge)
plus fractional crystallization (AþFC or RþFC). The
AþFC trace element differentiation model was calculated
for a given element i by summing the concentration of the
FC
)
liquid derived by Rayleigh fractional crystallization (Cliq
with batch of contaminant melt (A) at a given ratio ra
(mass assimilated/mass crystallized). The Rayleigh fractional crystallization equation is
FC
i
¼ Cinit
F D1
Cliq
ðA1Þ
where Cinit is the initial concentration and F is the fraction
of melt remaining. The contribution of the contaminant A
is a function of its concentration (Ca), ra, and the mass
crystallized (1 ^ F):
A ¼ Cai ra ð1 FÞ:
ðA2Þ
By summing equations (A1) and (A2) and normalizing to
100%, we obtain the concentration of element i in the
i
) with the equation
liquid (Cliq
i
Cliq
¼
i
ðCinit
F D1 Þ þ Cai ra ð1 FÞ
:
1 þ ra ð1 FÞ
ðA3Þ
i
After 1% crystallization, the Cliq
is put back into equation
i
) for another
(A3) as the next initial concentration (Cinit
1% crystallization. At ra ¼ 0, the equations reduce to the
Rayleigh fractional crystallization, equation (A1).
One benefit of this formulation is that we can substitute
a recharge magma (R) whose concentration of element i
i
. This is repre(Cri ) may or may not be equivalent to Cinit
sented by the equation
i
¼
Cliq
i
ðCinit
F D1 Þ þ ðCri rr Þð1 FÞ
:
1 þ rr ð1 FÞ
ðA4Þ
A graphical representation of the AþFC model (Fig. A1)
demonstrates how concentrations change as a function
of distribution coefficients, r values, and Ca/Cinit. Also
Fig. A1. Representation of the AþFC trace element differentiation
model (colored lines). The composition of the melt relative to its initial composition (Cliq/Cinit) vs melt fraction (F) is plotted. (a) and
(b) illustrate varying trace element partitioning behavior from incompatible (D ¼ 0·1) to compatible (D ¼10), respectively. Changing r
(0, 0·5, and 0·98) and assimilant composition (Ca/Cinit) also affect the
AþFC model results. For comparison, the AFC model (DePaolo,
1981) is also illustrated for varying r values and Ca/Cinit ¼1.
presented is a comparison with the AFC model of
DePaolo (1981) at varying r values and Ca/Cinit ¼1. AFC
represents a limiting case of simultaneous removal of infinitesimal amounts of crystalline material and addition of
infinitesimal partial melt. According to the AFC formulai
i
=Cinit
is asymptotic to r and approaches infinity
tion, Cliq
near r ¼1 at constant F. At r ¼ 0·98, the composition of
the melt markedly changes over the first few per cent of
crystallization and is followed by constant Cliq/Cinit at
lower melt fractions. This implies that the AFC formulation therefore may not be applied to ra near or greater
than unity and may not be translated to include recharge,
when rr is likely be 41.
641