JOURNAL OF
PETROLOGY
Journal of Petrology, 2017, Vol. 58, No. 1, 85–114
doi: 10.1093/petrology/egx006
Original Article
Pleistocene to Holocene Growth of a Large
Upper Crustal Rhyolitic Magma Reservoir
beneath the Active Laguna del Maule Volcanic
Field, Central Chile
Nathan L. Andersen1*, Brad S. Singer1, Brian R. Jicha1, Brian L. Beard1,
Clark M. Johnson1 and Joseph M. Licciardi2
1
Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706, USA; 2Department of Earth
Sciences, University of New Hampshire, Durham, NH 03824, USA
*Corresponding author. E-mail: [email protected]
Received June 17, 2016; Accepted January 26, 2017
ABSTRACT
The rear-arc Laguna del Maule volcanic field (LdM) in the Andean Southern Volcanic Zone, 36 S, is
among the most active latest Pleistocene–Holocene rhyolitic centers globally and has been inflating
at a rate of > 20 cm a–1 since 2007. At least 50 eruptions during the last 26 kyr allow for a thorough
interrogation of changes in the physical and chemical state of this large, 20 km diameter, silicic
system. Trace element concentrations and Sr, Pb and Th isotope ratios indicate that the mafic precursors to the LdM rhyolites result from mixing between partial melts of garnet-bearing mantle
and crust in Th-excess and partial melts of garnet-free crust in U-excess. The 238U/230Th ratios of
the LdM lavas are decoupled from the slab fluid signature, similar to several recently studied frontal arc volcanic centers in the Southern Volcanic Zone. A narrow range of radiogenic isotope compositions and increasing isotopic homogeneity with differentiation indicate that silicic magma is
generated by magma hybridization and crystallization in the upper crust with limited involvement
of older, radiogenic material. New 40Ar/39Ar and 36Cl ages reveal a wide footprint of silicic volcanism during the early post-glacial (25–19 ka) and Holocene (c. 8–2 ka) periods, but focused within a
single eruptive center during the interim period. Subtle temporal variations in trace element compositions and two-oxide temperatures indicate that these eruptions, issued from vents distributed
within a similar area, tapped at least two physically discrete rhyolite reservoirs. This compositional
distinction favors punctuated extraction and ephemeral storage of the erupted magma batches.
Frequent mafic recharge incubates this long-lived, growing shallow silicic magma reservoir above
the granite eutectic, which favors magma interactions over rejuvenation of near- to sub-solidus
silicic cumulates. A long-term rate of mass addition—extrapolated from surface deformation accumulated over the past decade—is comparable with those that have produced moderate- to largevolume caldera-forming eruptions elsewhere.
Key words: rhyolite; Andes Southern Volcanic Zone; magma chamber; geochronology; radiogenic
isotopes
INTRODUCTION
Large silicic volcanic systems are of great interest because
they generate caldera-forming eruptions that disperse
enormous quantities of ash over a vast area.
Heterogeneities in the resulting pyroclastic fall and flow
(ignimbrite) deposits are often interpreted to reflect the
structure of the pre-eruption magma reservoir (e.g.
Hildreth, 1981). The composition and ages of major and
accessory phases can provide records of magma accumulation, crystallization, and mixing on both short (100–102
C The Author 2017. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: [email protected]
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86
year) and long (104–106 year) timescales (e.g. Vazquez &
Reid, 2004; Charlier et al., 2005, 2008; Wark et al., 2007;
Costa, 2008; Reid, 2008; Reid et al., 2011; Wotzlaw et al.,
2013, 2015; Chamberlain et al., 2014a, 2014b).
Complementing these records are studies of smaller preand post-caldera silicic eruptions that record the longer
thermochemical context that produced the calderaforming system, particularly when the earlier or subsequently erupted material is physically distinct from the
caldera-forming system or the caldera-collapse event produces a structural realignment of the shallow magma system (Metz & Mahood, 1985, 1991; Sutton et al., 2000;
Charlier et al., 2005; Smith et al., 2005, 2010; Simon et al.,
2007; Wilson & Charlier, 2009; Bachmann et al., 2012;
Barker et al., 2015).
The archetype model of voluminous silicic magma
systems involves crystallization of mafic to intermediate
forerunners in the middle to upper crust, yielding an
intermediate to silicic crystal mush—an extensive
crystal-rich (>60% solid) reservoir containing evolved
interstitial melt. Crystal-poor eruptible magma bodies
are assembled by progressive extraction and accumulation of melt from these crystal-rich domains (Bachmann
& Bergantz, 2004; Hildreth, 2004) or remelting of silicic
cumulate during magma recharge events (Mahood,
1990; Wolff et al., 2015; Evans et al., 2016). The relative
importance of these mechanisms varies between
caldera-forming systems as well as within zoned ignimbrites produced during individual events (e.g. Vazquez
& Reid, 2004; Charlier et al., 2005; Bindeman et al.,
2008; Wotzlaw et al., 2013, 2015; Chamberlain et al.,
2014a, 2014b; Evans et al., 2016).
Departures from the model of progressive rhyolite
extraction have been noted at large silicic systems such
as Taupo Volcano and Yellowstone involving a greater
proportion of remelting of silicic forerunners and the
amalgamation of distinct rhyolite melts, potentially catalyzed by extensional tectonics (Smith et al., 2004, 2010;
Charlier et al., 2005, 2008; Wilson et al., 2006; Shane
et al., 2007, 2008; Bindeman et al., 2008; Wilson &
Charlier, 2009; Allan et al., 2013; Bégué et al., 2014;
Storm et al., 2014). Brief repose periods following the
eruption of compositionally distinct pre-caldera rhyolites, durations of zircon crystallization, and crystal residence based on solid-state diffusion kinetics indicate
that the assembly of 102–103 km3 eruptible rhyolite
magma bodies in these systems occurred more rapidly
than predicted by models of progressive melt extraction
(Charlier et al., 2008; Allan et al., 2013; Bindeman &
Simakin, 2014; Wotzlaw et al., 2015). Thus, understanding the mechanisms of rhyolite genesis in a particular
system can inform predictions of the processes and
timescales of the formation of a future, potentially large
eruptible silicic magma body.
The importance of lower crustal differentiation in
producing basalts and andesites in arc settings is well
recognized (e.g. Hildreth & Moorbath, 1988; Ownby
et al., 2011); it has also been proposed that silicic
magma is generated in the lower crust by partial
Journal of Petrology, 2017, Vol. 58, No. 1
melting of the deep crust (up to 70% depending on the
magma flux and lithology of the crust), fractional crystallization of hydrous basalt, and mixing of the resulting
differentiates and crustal melts. Shallow systems are
assembled incrementally from these lower crustal ‘hot
zones’ (Annen et al., 2006), but undergo limited chemical differentiation following shallow magma emplacement. Thus, the volume of eruptible magma is primarily
a function of the magma flux to the upper crust (e.g.
Glazner et al., 2004; Annen et al., 2006; Annen, 2009;
Gelman et al., 2013).
The investigation of pre-caldera silicic eruptions can
provide clues to the physical and thermal evolution that
sets the stage for the assembly and eruption of a voluminous silicic magma reservoir. Pre-caldera eruptive records can be limited owing to infrequent eruptions,
poorly resolved geochronology, burial or destruction by
subsequent caldera-forming events (Metz & Mahood,
1991; Stix & Gorton, 1993; Wilson et al., 2009).
Nevertheless, such records have proven useful in identifying changes in the mafic flux to the upper crust, the
amalgamation of previously discrete magma reservoirs,
and placing limits on the longevity of the subsequent
caldera-forming reservoir (e.g. Metz & Mahood, 1991;
Simon et al., 2007; Bindeman et al., 2008; Wilson &
Charlier,
2009;
Chamberlain
et
al.,
2014b).
Understanding the recent magmatism at historically active rhyolitic volcanic centers (e.g. Miller, 1985; Hildreth,
2004; Smith et al., 2005; Castro & Dingwell, 2009;
Hildreth & Fierstein, 2012; Rawson et al., 2015) allows
for the interrogation of the structure of the magma reservoir, the petrogenesis of rhyolites, the physical and
thermal processes preceding the recent eruptions, and
their evolution through time. Such systems are potential sites of caldera-forming eruptions and, taken together, this information is valuable in evaluating the
possible style of future eruptions and establishing a
context in which to better interpret seismic, magnetotelluric, geodetic, and gravity observations (e.g. Singer
et al., 2014).
The rear-arc Laguna del Maule (LdM) volcanic field
(Fig. 1) produced two dacitic to rhyodacitic calderaforming eruptions during the mid-Pleistocene. A recent
concentration of silicic volcanism has yielded at least 50
rhyolitic eruptions in the last 26 kyr; thus LdM is among
the most frequently erupting active rhyolitic volcanic
centers globally (Hildreth et al., 2010; Fierstein et al.,
2012; Sruoga, 2015). This remarkable spatial and temporal concentration of rhyolite eruptions since the last
glacial maximum, locally dated at c. 24 ka based on the
age of glaciated and unglaciated lava flows at LdM
(Singer et al., 2000), has encircled the lake in the central
LdM basin and is unprecedented in the southern Andes
(Fig. 2; see also Table 1; Hildreth et al., 2010; Singer
et al., 2014). Hildreth et al. (2010) presented several lines
of evidence suggesting that these eruptions are derived
from an integrated silicic magma system, most prominently: (1) rhyolite lavas erupted 10–12 km apart have
nearly identical major and trace element compositions,
Journal of Petrology, 2017, Vol. 58, No. 1
87
100 km
Santiago
c
u-C
hile
Talca
Laguna
del Maule
TataraDomuyo San Pedro
Nevados de Chillan
Arge
nti
na
Concepción
Chil
e
7.4
r
m/y
Per
-36º
Tre
nch
-34º
Tupungato
San Jose
MaipoDiamante
Calabozos
Caldera
Cerro AzulQuizapu
Puelche
Volcanic
Field
Antuco
-38º
Lonquimay
Llaima
Villarrica
Mocho-Choschuenco
-40º
Puyehue-Cordón Caulle
Osorno
-74º
-72º
-68º
-70º
Fig. 1. Regional map of the SVZ between 33 and 41 S showing
the location of Laguna del Maule. Selected frontal arc volcanos
(triangles) and caldera systems and silicic volcanic centers
(dark gray fields) are labeled for reference. The velocity of the
Nazca plate relative to the South American plate is calculated
using MORVEL (DeMets et al., 2010).
suggesting that they are derived from a single homogeneous reservoir; (2) inclusions of mafic magma in rhyodacite lavas are common, whereas mafic eruptions
have been rare and peripheral since the beginning of
post-glacial rhyolite volcanism, indicating that a broad,
low-density magma body is blocking the ascent of
mafic magma. Consequently, the numerous postglacial silicic eruptions at LdM may represent a high
temporal resolution sampling of the evolution of a
large, shallow magma system.
Several geophysical methods document continuing
volcanic unrest within the LdM basin that remains active at the time of this writing. Geodetic data since 2007,
obtained by continuous global positioning system
(GPS) and interferometric synthetic aperture radar
(InSAR), record uplift at a rate in excess of 20 cm a–1,
among the fastest measured at a volcano not actively
erupting (Fournier et al., 2010; Feigl et al., 2014; Le
Mével et al., 2015). A model of an inflating sill at 5 km
depth produces the best fit of the measured deformation pattern, with an estimated volume increase of
003–005 km3 a–1 between 2007 and 2014 (Le Mével
et al., 2016). This probably transient rate is one to two
orders of magnitude greater than the late Pleistocene to
Holocene eruptive fluxes at the Southern Volcanic Zone
(SVZ) frontal arc centers Mocho–Choshuenco and
n Caulle (Singer et al., 2008; Rawson
Puyehue–Cordo
et al., 2015) and the average eruptive flux at LdM over
the last 15 Myr (Hildreth et al., 2010). During the same
period of time, frequent seismic swarms have occurred
at similarly shallow depths near the Nieblas (rln) and
Barrancas (rcb) rhyolite flows, which are among the
youngest in the volcanic field (Fig. 2; Singer et al.,
2014). Initial gravity and magnetotelluric studies also
suggest the presence of a shallow, possibly growing,
magma system beneath the area of deformation at LdM
(Singer et al., 2014; Miller et al., 2016). More recent geodetic and geomorphological observations indicate that
the rate of uplift and inflation slowed slightly in 2013 (Le
Mével et al., 2015) and that dozens of similar inflation
episodes have probably occurred throughout the
Holocene (Singer et al., 2015).
The post-glacial eruptive chronology at LdM is currently defined by only four 40Ar/39Ar ages obtained
nearly two decades ago (Singer et al., 2000) and the
positions of lava flows relative to a paleoshoreline
marking the highstand of the lake produced when the
outlet gorge was dammed by the early rle rhyolite flow.
Consequently, the age relations of eruptions occurring
on opposite sides of the lake have been inferred based
only on geomorphological features such as the extent
of weathering and degree of pumice cover, hindering
the interpretation of the temporal record. New 40Ar/39Ar
and 36Cl surface exposure ages for late Pleistocene and
post-glacial LdM lavas that refine the eruptive sequence
are presented in this study. New whole-rock trace element compositions, Sr, Pb, and Th isotope ratios, and
mineral thermobarometry are evaluated in the framework of this new geochronology to examine the temporal evolution of the rhyolite and rhyodacite magma
compositions. Models of magma evolution spanning
the last 150 kyr in the central LdM basin (earlier eruptions are sparse) are used to interrogate the continuity
and integration of the LdM magma system, the nature
and depth of the processes contributing to its evolution
through time, and the implications for the continuing
volcanic unrest.
GEOLOGICAL SETTING
The Quaternary LdM volcanic field is situated on the
crest of the Andes at 36 S in the Southern Volcanic
Zone (SVZ) of central Chile (Fig. 1). Between 32 and
37 S, the arc is characterized by a gradient in crustal
thickness from 35 km in the south to 60 km in the north
(Gilbert et al., 2006; Tassara et al., 2006; Tassara &
Echaurren, 2012). This near doubling in thickness correlates with a transition from dominantly basaltic andesite to amphibole-bearing intermediate products and
well-documented gradients in trace element and radiogenic isotope composition (Hildreth & Moorbath, 1988).
Distinctively, the segment of the arc between 34 and
37 S hosts several large Quaternary silicic volcanic centers in addition to LdM: the Maipo–Diamante Caldera
(Sruoga et al., 2012), the Calabozos Caldera (Hildreth
et al., 1984; Grunder & Mahood, 1988), Puelche
Volcanic Field (Hildreth et al., 1999), and Domuyo
88
Journal of Petrology, 2017, Vol. 58, No. 1
Rio M
aule
1
1700800
1900
N
C
rdne
rle
bec
igsp
2200
23
240000
2828
igcb
3122
Paso
Pehuenche
25
00
bbc27002800
igcb
2874
rca
115
2600
2500
2400
25.7 ka
rdcn
3.5 ka
la
Cajón Chico de Bobadilla
2000
rdno
rep
-36.0º
obadil
0
260
0
270
rle ig
de B
rande
ajón G 2100
igcb
2300
19.0 ka
3080
00
rdop
00
rddm
23
ram
asm rdam
anc
acn aan
mnp
rdnp 8.0 ka dlp
rdep
mcp
os
aci
27
e
od
3056
mvc
2994
2600
2500
250
2400
Ca
lle
230
00 2
5
jón
C
NA
1.9 ka
rcb
3037
0
280
0
3000
2888
00
288000
2600
2700
Arr
oyo
14.5 ka
Arr
oy
oC
ura
mi
lio
Rhyolite
Rhyodacite
Andesite
5
Contour interval 50 m
all elevations masl
0
240
0
230
2200
2000
Pu
ent
ed
e T 2100 rcb-py
ierr
a
11.4 ka
-70.5º
-70.4º
Central Laguna del Maule Volcanic Field
Post Glacial Eruptions
rng
2700
2600
00
25
rcb-d
GE
L.
Negra
2162
de la
2800
0
29900
290
2900
rcb-d
I
NT
-7 0 . 6 º
00
2400
2500
Ca
Laguna
Fea
2486
0
26
2700
HI
km
2700
27
270
2300
AR
0
30000000
0
290 0
280
2600
LE
-36.2º
2600
5.6 ka
de
Tr
on
co
so
2800
270000
26
la
rdac
y
Aro
de
ras
l
Pa
2.1 ka
rcd
20.0 ka
A.
00
30 0
0 2800
29
2700
3175
0
rln
280
rap
22.4
29000 ka
2855
0
0
dlp mct
240
230
00
240
0
25000
26
rdct
0
290 3000
25 2600
00
apo
23
rsl
3.3 ka
0
2700
2162
00
2883
Laguna
del
Maule
apj
rdcd
300
0
29
3031
2 80 00
lved
igsp
-36.1º
22
epú
2767
rsl
aam
mpl
apv
00
asp
A. S
0
260
a
24
rdsp
250
0
2889
L.
Cari
Launa
2680
rcl
rcn
Parva
115
Arroy
o
1600
Pleistocene Eruptions
Rhyolite
Dacite/Rhyodacite
Andesite
Basalt/Mafic Andesite
Pleistocene ignimbrites
igcb - 990 ka; igsp - 1.5 Ma
3.5
Volcanic Vent
Lava Flow Direction
Pumice
Highway 115
International Boundary
Eruption age [ka]
Sample Locations
Center of Deformation
Fig. 2. Simplified geological map of the central basin of the LdM volcanic field [after Hildreth et al. (2010)] showing sample locations; unit names and abbreviations are listed in Table 1. Eruption ages are determined by 40Ar/39Ar except for the 36Cl age of unit
rdcd; uncertainties associated with the 40Ar/39Ar ages are given in Table 2 and 36Cl data are given in the Supplementary Data. The
center of uplift near the southwestern lake shore is an approximate location based on the InSAR model of Feigl et al. (2014).
Journal of Petrology, 2017, Vol. 58, No. 1
89
Table 1: Laguna del Maule eruptive units mapped in Fig. 2
Abbreviation*
aam
acn
anc
apj
apv
asm
asp
bbc
bec
dlp
igcb
igsp
mcp
mct
mnp
mpl
mvc
ram
rap
ras
rca
rcb
rcb-d
rcb-py
rcd
rcl
rcn
rdac
rdam
rdcd
rdcn
rdct
rddm
rdne
rdno
rdnp
rdop
rdsp
rep
rle
rle-ig
rln
rsl
Unit name*
Eruption age†
Andesite of Arroyo Los Mellicos
Andesite of Crater Negro
Andesite north of Crater Negro
Younger andesite of West Peninsula
Older andesite of West Penisula
Andesite south of Arroyo Los Mellicos
Andesite of Laguna Sin Puerto
n Bobadilla Chica
Basalt of Volca
Basalt of El Candado
Dacite of Laguna del Piojo
Ignimbrite of Cajones de Bobadilla (rhyodacite)
Ignimbrite of Laguna Sin Puerto (dacite)
Andesite of Crater 2657
Andesite of Arroyo Cabeceras de Troncoso
Andesite north of Estero Piojo
n Puente de la Laguna
Andesite of Volca
n de la Calle
Andesite of Volca
Rhyolite of Arroyo Los Mellicos
Rhyolite of Arroyo de Palacios
lveda
Rhyolite of Arroyo de Sepu
n Atravesado
Rhyolite of Cajo
Rhyolite of Cerro Barrancas
Cerro Barrancas Dome Complex (rhyolite)
Cerro Barrancas Pyroclastic Flow (rhyolite)
Rhyolite of Colada Divisoria
Rhyolite of Cari Launa
Rhyolite of Cerro Negro
Rhyolite of Arroyo de la Calle
Rhyodacite of Arroyo Los Mellicos
Rhyodacite of Colada Dendriforme
Rhyodacite of Northwest Coulee
Rhyodacite of Arroyo Cabeceras de Troncoso
Rhyodacite of Domo del Maule
Rhyodacite NE of Loma de Los Espejos
Rhyodacite NW of Loma de Los Espejos
Rhyodacite north of Estero Piojo
Rhyodacite west of Presa Laguna del Maule
Rhyodacite of Laguna Sin Puerto
Rhyolite east of Presa Laguna del Maule
Rholite of Loma de Los Espejos
Espejos ignimbrite (rhyolite)
Rhyolite of Colada Las Nieblas
Rhyolite south of Laguna Cari Launa
254 6 15 ka
post-glacial
post-glacial
211 6 34 ka
pre-glacial
post-glacial
<35 ka
153 6 7 ka
618 6 36 ka
pre-glacial
990 6 13 ka
1484 6 15 ka
post-glacial
post-glacial
post-glacial
54 6 21 ka
1521 6 65 ka
post-glacial; >19 ka
224 6 20 ka
19–20 ka
710 6 13 ka
multiple flows; 114–19 ka
145 6 15 ka
114 6 11 ka
21 6 13 ka
<33 ka
4660 6 56 ka
200 6 12 ka
post-glacial; >19 ka
80 6 08 ka
35 6 23 ka
202 6 41 ka
114 6 14 ka
post-glacial; >19 ka
post-glacial; >19 ka
post-glacial
pre-glacial
<35 ka
257 6 12 ka
190 6 07 ka
post-glacial; >19 ka
Late Holocene
33 6 12 ka
*Abbreviations and unit names after Hildreth et al. (2010).
†Ages are from Singer et al. (2000), Hildreth et al. (2010), Birsic (2015), and this study; all
the 11864 Ma Alder Creek Sanidine (Jicha et al., 2016).
Volcanic Complex (Miranda et al., 2006; Chiodini et al.,
2014), each situated in the rear-arc relative to the basaltto andesite-dominated frontal arc volcanoes (Fig. 1).
Owing to repeated glaciation and the remote, rugged
terrain, it is not well appreciated that the productivity of
Pliocene to Holocene silicic volcanism in this northern
sector of the SVZ is comparable with that of the Andean
Central Volcanic Zone (Hildreth et al., 1984, 1999).
Hildreth et al. (2010) documented the most recent 15
Myr of volcanic activity at LdM, which comprises more
than 350 km3 of lava, tephra, and pyroclastic deposits
ranging in composition from basalt to high-silica rhyolite erupted from at least 130 vents. The Quaternary
eruptions overlie Paleogene to Neogene volcanic and
volcaniclastic rocks and Pliocene to Mesozoic plutons
and sedimentary strata (Nelson et al., 1999; Hildreth
et al., 2010). LdM volcanic products are of tholeiitic to
40
Ar/39Ar ages are calculated relative to
calc-alkaline, medium- to high-K compositions typical
of SVZ frontal arc volcanoes. Hildreth et al. (2010) found
evidence for neither systematic variation in the slab signature across the volcanic field nor any significant contribution of back-arc, alkaline compositions. Basaltic
andesite to andesite dominates much of the preserved
eruptive history of LdM, but silicic (dacite–rhyolite)
eruptions have occurred throughout the volcanic field
during the Pliocene and Pleistocene (Hildreth et al.,
2010). Two silicic ignimbrites are preserved in the LdM
lake basin (Fig. 2), the 15 Ma two-pyroxene dacite Sin
Puerto Ignimbrite (igsp) and the 990 ka biotite rhyodacite Bobadilla Ignimbrite (igcb) (Birsic, 2015). Of these,
only the Bobadilla caldera structure partially survived
the subsequent glaciation and erosion. Two middle
Pleistocene rhyolitic lavas are preserved near the northeastern shore of the lake, the 710 6 13 ka Rhyolite of
90
Cajon Atravesado (rca) and the 4660 6 56 ka Rhyolite
of Cerro Negro (rcn). The latter contains the most
evolved compositions in the volcanic field (Hildreth
et al., 2010).
Singer et al. (2000) determined the timing of the last
glacial retreat to be between 254 6 12 ka and 232 6 06
ka based on 40Ar/39Ar age determinations (recalculated
to an Alder Creek Sanidine age of 11864 Ma; Jicha
et al., 2016) for four eruptions, including one glaciated
and three unglaciated lavas at approximately equal elevation in the LdM basin. This age is consistent with the
moraine records east of the Andes between 47 and
46 S based on 3He, 10Be, and 26Al cosmogenic exposure, 40Ar/39Ar, and 14C ages indicating that the last glacial maximum occurred prior to 23 ka with deglaciation
well under way by 165 ka (Kaplan et al., 2004; Hubbard
et al., 2005; Clark et al., 2009; Hein et al., 2010). The
post-glacial volcanism is concentrated in the LdM lake
basin, producing 36 silicic domes and coulees and dozens of explosive eruptions from at least 24 vents encircling the lake (Fig. 2; Hildreth et al., 2010; Fierstein et al.,
2012; Sruoga, 2015). Ten andesite flows emplaced since
the glacial retreat, primarily along the western lakeshore, are of subordinate volume. Basaltic andesite is
rare since the most recent deglaciation and the youngest true basalt is the 618 6 36 ka basalt of El Candado
(bec) erupted north of LdM (Fig. 2; Hildreth et al., 2010,
recalculated to an Alder Creek Sanidine age of
11864 Ma; Jicha et al., 2016).
Silicic eruptions at LdM were explosive and effusive
and generally of modest volume (<13 km3; Hildreth
et al., 2010; Fierstein et al., 2012). Continuing tephrostratigraphic investigations (Fierstein et al., 2012; Sruoga,
2015) both within the LdM basin and of distal deposits
in Argentina, are not discussed in detail here. However,
of particular note, Fierstein et al. (2012) have identified a
voluminous explosive eruption that produced flow and
fall deposits up to 6 m thick in Argentina 30 km south
and east of LdM accounting for an order of magnitude
greater volume than any single event mapped in the
central basin by Hildreth et al. (2010). This explosive
event pre-dates the rle lava flow that dammed the lake
and thus is among the earliest post-glacial rhyolite
eruptions. However, its vent location and eruption age
remain uncertain.
Rhyolite flows preserved in the LdM basin are vitrophyric and carry 5% modal phenocrysts; the rhyolite
of Arroyo Palacios (rap) and all but the latest of the
Barrancas complex (rcb) flows are notably aphyric.
Phenocrysts, when present, are dominantly plagioclase,
subordinate biotite, Fe–Ti oxide, sparse quartz, accessory zircon, apatite, and very rare FeS inclusions in
magnetite; several rhyolites also contain scarce amphibole. With the exception of the rhyodacite of Arroyo de
la Calle (rdac) the rhyodacite lavas are concentrated in
the western and northwestern basin. They are vitrophyric to micro-pumiceous and nearly all carry a phenocryst load of 10–25%, greater than any of the rhyolites;
only the rhyodacites of the Northwest Coulee (rdcn) and
Journal of Petrology, 2017, Vol. 58, No. 1
Laguna Sin Puerto (rdsp) are crystal poor. The phenocryst assemblage is similar to that of the rhyolites but
all lack quartz and contain amphibole. Most rhyodacite
lavas contain fine-grained, partly glassy, basaltic andesite inclusions, frequently with quench textures, up to
40 cm in diameter in the rhyodacites of Colada
Dendriforme (rdcd) and NW of Loma de Los Espejos
(rdno), but more commonly 1–10 cm in diameter.
Similar inclusions are rare in the Rhyolite of Arroyo Los
Mellicos (ram) mini-dome but have not been found in
any other rhyolite.
NEW 40Ar/39Ar AND 36Cl AGES AND REVISED
ERUPTION SEQUENCE
An effort to document the LdM eruptive sequence based
on the tephra stratigraphy and soil 14C ages is currently
under way (Fierstein et al., 2012; Sruoga, 2015).
However, the construction of a 14C chronology at LdM is
challenging owing to a dearth of organic material.
Whereas 14C ages typically have lower uncertainties,
where suitable material is lacking, 40Ar/39Ar and 36Cl
ages offer alternative methods to date young volcanic
eruptions. Twenty-six 40Ar/39Ar incremental heating experiments, performed at the WiscAr Geochronology Lab
(see Supplementary Data for details; supplementary
data are available for downloading at http://www.pet
rology.oxfordjournals.org) yield plateau ages, all but one
containing more than 75% of the released 39Ar, and support 12 eruption ages (Fig. 3; Table 2). We attempted to
determine 40Ar/39Ar ages for nearly all post-glacial lavas.
However, owing to their youth and high atmospheric Ar
contents, LdM products commonly yield small fractions
of radiogenic 40Ar (40Ar*). Micropumiceous rhyodacites
and commonly vesiculated and glassy andesite flows
nearly all produced high 36Ar signals from which 40Ar*
could not be resolved. Dense rhyolitic obsidian more
commonly yields plateau ages; however, only approximately 50% of such samples produced resolvable ages.
Recoil of 39Ar during irradiation of volcanic glass can result in spurious ages. This effect is mitigated for the LdM
lavas by a short irradiation time; age plateaux characteristic of recoil (i.e. decreasing apparent age with increasing step heating temperature) are only sporadically
observed for sample aliquots subjected to longer duration irradiation (see Supplementary Data). Several experiments display anomalously high ages in the low or
high temperature steps. However, this behavior is consistent neither throughout the LdM sample suite, nor between aliquots prepared from single samples. The cause
of these discordant steps is not clear, but they account
for less than 5% of the gas in single experiments and do
not bias the reported ages. Inverse isochrons for all samples yield 36Ar/40Ar intercepts within uncertainty of the
atmospheric ratio of Lee et al. (2006), indicating that excess Ar is not significant. The isochron and plateau ages
for each experiment are indistinguishable at 2r uncertainty; thus the more precise plateau ages are preferred.
Journal of Petrology, 2017, Vol. 58, No. 1
40
30
91
4.0
Southern Cari Launa Rhyolite (rsl)
3.3±1.2 ka
3.5
3.0±1.6 ka
3.7±2.1 ka
20
3.0
10
2.5
0
0.4
0.6
0.8
Espejos Rhyolite (rle)
19.0±0.7 ka
20
18.4±1.1 ka
19.5±0.9 ka
10
50
40
0.2
0.4
0.6
0.8
1.0
Rhyolite East of Presa
Laguna del Maule (rep)
25.7±1.2 ka
4.0
19.1±0.8 ka
Ar/39Ar0=296.4±2.6
n=18
3.0
2.0
1.0
0.0
3.5
0
3
40
6
9
26.2±2.6 ka
Ar/39Ar0 = 296.5±9.8
n = 13
3.0
2.5
2.0
20
0
0
40
4.0
30
10
5.3±2.9 ka
Ar/39Ar0 = 293.8±8.6
n = 10
0.4 0.8 1.2 1.6 2.0 2.4
5.0
17.7±2.5 ka
30
0
0
60
2.0
0.0
1.0
36
Age [ka]
40
0.2
Ar/40Ar x 103
-10
0
50
40
25.8±1.3 ka
25.0±3.0 ka
0.2
0.4
0.6
0.8
Cumulative 39Ar Fraction
1.5
1.0
1.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
39
Ar/40Ar
Fig. 3. Example 40Ar/39Ar age spectra and inverse isochrons for units rep, rle and rsl; values for all samples are available in the
Supplementary Data. Plateau steps are colored boxes and ellipses; discordant, excluded steps are light gray. All uncertainties are6 2r and include the analytical and J uncertainties.
The eruption of the rle flow dammed the northern
outlet of the lake, causing the lake level to rise to
200 m above its modern level and cutting a prominent
shoreline into all low-lying older rocks (Hildreth et al.,
2010; Singer et al., 2015). To constrain better the duration of the lake highstand, we undertook 36Cl surface
exposure age determinations of the rdcd lava flow,
which overruns the paleoshoreline is several places but
did not produce a resolvable 40Ar/39Ar age, and the
shoreline itself where it is notched into the igcb ignimbrite along the north shore of the lake (36Cl methods
and results are in the Supplementary Data).
The new age determinations are discussed in conjunction with the observations made during fieldwork
in support of the present work and by Hildreth et al.
(2010) to improve the chronology of the post-glacial
eruptions. Whereas LdM erupted regularly following
the last glacial maximum, the rhyolitic volcanism is
clustered in two periods of high eruption frequency.
An early post-glacial (EPG) group erupted prior to the
damming of the outlet gorge at 19 ka. This was followed by a period of relative calm in much of the lake
basin during the latest Pleistocene, with rhyolitic activity limited to the Barrancas complex in the SE basin.
Finally, silicic eruptions encircled the lake during the
Holocene (Fig. 4).
Early post-glacial eruptions
The earliest of the recent silicic units erupted shortly
prior to deglaciation, forming the rhyolite east of the
Presa (dam) (rep) at 257 6 12 ka in the northwestern
LdM basin. All subsequently erupted silicic units are
unglaciated, including the early voluminous pyroclastic
event (Fierstein et al., 2012) and numerous andesite and
rhyodacite flows and domes concentrated in the western and northwestern LdM basin. A single unglaciated
andesite flow (apo) erupted in the south; this glassy,
vesiculated lava did not produce resolvable 40Ar*. The
flow is largely buried by lake deposits and a pumice fan,
but apparently was erupted prior to the damming of the
lake. Rhyolite flows erupted on three sides of the lake in
a relatively short time interval at the end of the EPG
lveda rhyolite (ras) in the SE [which dirperiod: the Sepu
ectly overlies the 200 6 12 ka rhyodacite of Arroyo del
la Calle (rdac)], the 224 6 20 ka Palacios rhyolite (rap),
and the 190 6 07 ka Espejos rhyolite (rle).
Latest Pleistocene to Holocene eruptions
Volcanic activity waned throughout much of the LdM
basin following the end-EPG eruptions. The latest
Pleistocene eruptions were restricted to the Barrancas
center (rcb) on the southeastern rim of the lake basin.
An early episode of dome building is dated at 145 6 15
92
Journal of Petrology, 2017, Vol. 58, No. 1
Table 2: Summary of 40Ar/39Ar experiments
Sample no.
K/Ca total
Total fusion
age [ka] 62r
40
Ar/36Ari 6 2r MSWD
Isochron
age [ka] 62r
n
39
Ar %
MSWD Plateau
age [ka] 62r
Rhyolite of Cerro Barrancas, eat summit flow (rcb)
AR-267
581
24 6 21
3015 6 61
AR-267
580
44 6 31
323 6 33
Combined isochron n ¼ 19:
3010 6 57
099
024
100
87 6 58
-224 6 99
13 6 80
6 of 9
842
5 of 10
768
Weighted mean n 5 2:
103
085
17
16 6 07
23 6 09
19 6 06
Rhyolite of Colada Divisoria (rcd)
77 6 32
LdM-249*
506
518
32 6 27
LdM-249*
LdM-249
500
25 6 21
Combined isochron n ¼ 19:
3052 6 81
3007 6 51
3002 6 56
3011 6 34
012
019
008
028
-18 6 39
-05 6 10
09 6 27
03 6 03
6 of 10
832
7 of 8
984
6 of 6
1000
Weighted mean n 5 3:
065
027
013
035
28 6 23
13 6 25
22 6 19
21 6 13
Rhyolite of South Cari Launa (rsl)
ALDM-13-17
670
17 6 18
ALDM-13-17
660
30 6 16
Combined isochron n ¼ 10:
2856 6 169
2982 6 114
2938 6 86
116
025
063
86 6 57
32 6 41
53 6 29
4 of 7
801
6 of 8
962
Weighted mean n 5 2:
160
020
065
37 6 21
30 6 16
33 6 12
Rhyodacite of the Northwest Coulee (rdcn)
LdM-12-27
146
-12 6 24
2947 6 84
154
57 6 45
5 of 7
897
138
35 6 23
Rhyolite of Cerro Barrancas, northern flow (rcb)
LdM-210†
531
73 6 19
3088 6 105
LdM-210
525
39 6 24
2987 6 162
520
106 6 28
2991 6 85
LdM-210*
*
526
82 6 32
3015 6 127
LdM-210
LdM-210
512
56 6 14
3310 6 379
141
295
040
090
063
-22 6 24
26 6 29
85 6 59
31 6 27
19 6 19
5 of 6
5 of 6
8 of 9
7 of 9
7 of 7
982
979
976
861
1000
243
218
034
077
114
52 6 27
27 6 31
90 6 24
49 6 30
57 6 12
Combined isochron n ¼ 27:
2987 6 32
157
56 6 13
Weighted mean n 5 4:
147
56 6 11
Cerro Barrancas Pyroclastic Flow (rcb-py)
CB-Curamilo A
618
126 6 49
CB-Curamilo A
588
100 6 51
Combined isochron n ¼ 18:
318 6 86
2949 6 58
2987 6 48
047
13
12
-88 6 86
16 6 10
114 6 71
12 of 15
841
5 of 10
673
Weighted mean n 5 2:
12
14
003
115 613
113619
114 6 11
Cerro Barrancas Dome Complex (rcb-d)
CB-2
490
137 6 16
080
145 6 15
2974 6 27
080
158 6 36
Rhyolite of Loma de Los Espejos (rle)
LdM-60
690
207 6 25
LdM-60
710
178 6 12
LdM-60
200
182 6 17
Combined isochron n ¼ 18:
2951 6 100
2045 6 1169
2992 6 50
2964 6 26
131
055
023
073
203 6 76
207 6 16
183 6 16
191 6 08
6 of 8
909
4 of 6
912
8 of 9
993
Weighted mean n 5 3:
117
093
021
084
177 6 25
195 6 09
184 6 11
190 6 07
Rhyodacite of Arroyo de la Calle (rdac)
LdM-213
205
209 6 18
LdM-213
203
199 6 18
Combined isochron n ¼ 13:
2941 6 97
2861 6 120
2920 6 80
059
046
113
226 6 33
224 6 37
220 6 26
7 of 7
1000
5 of 7
939
Weighted mean n 5 2:
062
134
128
212 6 15
188 6 18
200 6 12
Rhyolite of Arroyo Palacios (rap)
LdM-12-23
620
224 6 20
2934 6 129
047
236 6 36
049
224 6 20
Andesite of Arroyo Mellicos (aam)
LdM-194
034
323 6 140
LdM-194
031
316 6 70
Combined isochron n ¼ 14:
3033 6 86
3027 6 47
3030 6 41
087
099
076
130 6 99
105 6 96
108 6 63
9 of 9
1000
5 of 6
829
Weighted mean n 5 2:
091
152
108
288 6 125
233 6 81
245 6 61
Rhyolite East of Presa Laguna del Maule (rep)
LdM-12-32
750
247 6 34
2919 6 280
LdM-12-32
700
270 6 13
2966 6 106
Combined isochron n ¼ 13:
2965 6 98
082
042
060
263 6 58
263 6 28
262 6 26
8 of 8
1000
5 of 6
834
Weighted mean n 5 2:
073
035
056
250 6 30
258 6 13
257 6 12
9 of 9
7 of 7
1000
1000
Weighted mean plateau ages in bold are preferred; 2r uncertainties include the analytical and J uncertainties.
*Monitored with the 28.201 Fish Canyon Sanidine (Kuiper et al., 2008); all other experiments were monitored with the 1.1864 Ma
Alder Creek Sanidine (Jicha et al., 2016);
†
high MSWD; not included in weighted mean
ka and is followed by an explosive event that produced
pyroclastic flow deposits extending SE away from the
lake into Argentina (Fig. 2). A dense vitric clast from this
pyroclastic deposit gave an age of 114 6 11 ka. These
earliest products of the Barrancas complex are exposed
on its southern and eastern flanks and, therefore, are
not subject to shoreline erosion. Continued activity at
the Barrancas complex produced a series of rhyolite
flows, the northernmost of which, along with the rdcd
rhyodacite flow, erupted near the western lake shore;
these are the youngest units at sufficiently low elevation
within the lake basin to be subject to, but not affected
by, shoreline erosion. The youngest of the three northern rcb flows yields an 40Ar/39Ar age of 56 6 11 ka;
40
Ar* could not be resolved from either of the underlying flows. The rdcd flow yields a whole-rock 36Cl surface
exposure age of 80 6 08 ka. The ages of the rdcd and
northern rcb flows are consistent with a whole-rock 36Cl
Journal of Petrology, 2017, Vol. 58, No. 1
93
(a)
5 km
rdno
rdne
(c)
deglaciation
rcd
rcl
rsl
lake
high stand
rle
rep
ram
rdam
aam
asm
acn
rdnp
apo
apj
mnp anc
rdep
East
apo
rcb summit
rln
aan
ras
rcb
rdac
rap
rng
rcb-py
rcb-d
Early
Post glacial
25.7 - 19.0 ka
ras
rdac
rap
rdep
(b)
South
rdcd
asp
rdsp
rdnp
mnp
asm
rdcn
apj
acn
anc
aan
rcl
rsl
rdcd
West
rcd
rdsp
asp
rdcn
rln
mcp
rle
ram
rdam
rdne
rdop
mct
rcb-d
Latest Pleistocene
to Holocene
14.5 - ≤ 1.9 ka
rcb
rng
rcb-py
rcb-d
rcb-py
aam
rep
30
North
25
20
15
Age [ka]
10
5
0
Fig. 4. Post-glacial eruptive sequence of central LdM basin lavas. Fill colors are the same as in Fig. 2. (a) The distribution of EPG
eruptions—those erupted prior to and including the rle flow that dammed the outlet gorge producing the highstand of the lake. (b)
The distribution of latest Pleistocene to Holocene eruptions. (c) The relative eruptive sequence constrained by 40Ar/39Ar ages from
Singer et al. (2000) and this study; the timing of the drawdown of the lake highstand is constrained by a 95 6 01 ka 36Cl surface exposure age of the highstand shoreline cut into igcb tuff. Black outlined boxes are 40Ar/39Ar and 36Cl ages, with the width corresponding to the 2r uncertainty. Gray outlined boxes are inferred eruption age ranges based on field relationships; the widths are set
relative to the nominal ages of the constraining events.
surface exposure age of 95 6 01 ka for the shoreline
cut into igcb in the northern lake basin.
The middle to late Holocene saw rhyolite eruptions
from four centers in the southern and eastern lake
basin (Fig. 4). A significant explosive eruption from the
Cari Launa complex (Fierstein et al., 2012) was followed by the older of two Cari Launa rhyolite flows
(rsl) at 33 6 12 ka, the Rhyolite of Colada Divisoria
(rcd) at 21 6 13 ka, and the small rcb flow east of the
Barrancas summit at 19 6 06 ka. Neither the uppermost western rcb flow nor the rhyolite of Colada
Las Nieblas (rln) produced resolvable 40Ar*, but on the
basis of their similar lack of pumice cover and
uneroded morphology, they are probably of comparable age to the rcd and eastern summit rcb lavas and
thus are among the most recent eruptions in the volcanic field.
Outside the Holocene south–SE rhyolite focus, the
rhyodacite of the Northwest Coulee (rdcn) erupted from
a vent near the crest of the NW basin wall and extends
nearly down to the current lake level 350 m below. This
prominent flow is dated at 35 6 23 ka and is mantled
by the andesitic cinder ring of Laguna Sin Puerto (asp),
which was subsequently intruded by the rhyodacite of
Sin Puerto (rdsp). These eruptions likewise emanated
from a vent on the crest of the NW basin wall. Two
small andesitic fissure eruptions, the andesite of Crater
2657 (mcp) and the andesite of Arroyo Cabeceras de
Troncoso (mct), occurred 6 km west of the SW lakeshore. The ages of these eruptions are not well constrained; however, mcp scoria blankets the post-glacial
rhyodacite south of Estero Piojo (rdep) mini-domes to
the north, but not the mct craters, indicating that both
are younger than rdep and, although they are at a
94
Journal of Petrology, 2017, Vol. 58, No. 1
25
Central LdM basin
Greater LdM
Pleistocene ignimbrites
20
Th [ppm]
1000
(a)
(d)
T-SP
800
Sr [ppm]
30
15
10
600
400
200
5
0
0
500
1000
(b)
400
(e)
T-SP
800
K/Rb
Zr [ppm]
T-SP
300
200
100
400
200
0
0
15
25
(c)
La/Yb
9
T-SP
6
15
10
5
3
0
45
(f)
20
12
Rb/Y
600
50
55
60
65
SiO2 [wt. %]
70
75
80
45
50
55
65
60
SiO2 [wt. %]
70
75
80
Fig. 5. Variation of selected trace elements with SiO2 for lava and pumice erupted in the central LdM basin during approximately
the last 150 kyr. Data for the 15 Myr history of the entire volcanic field, including the Pleistocene igcb and igsp ignimbrites (Hildreth
et al., 2010; Birsic, 2015) and T-SP (Dungan et al., 2001) are plotted for comparison. The typical 2r uncertainties associated with the
central LdM data are smaller than the symbols. The central LdM data show less dispersed ranges and trends relative to the larger
LdM volcanic field and T-SP. The REE and Y ratios of the igcb and igsp ignimbrites notably diverge from those of the post-glacial silicic lavas. Plots of major element variation are available in the Supplementary Data.
higher elevation than the high strandline, possibly postdate the rle eruption as well (Hildreth et al., 2010).
WHOLE-ROCK GEOCHEMICAL RESULTS
Major and trace elements
Lavas erupted during the last 150 kyr in central LdM
range from basalt to high-silica rhyolite. Primitive lavas,
rare throughout the SVZ, are absent from central LdM
as indicated by the modest Mg# (53) and low K/Rb
ratios (369–242) of the basalt and mafic andesite samples. The major and trace element evolution of central
LdM generally mirrors that of the entire 15 Myr eruptive history of the larger volcanic field (Hildreth et al.,
2010) and the frontal arc Tatara–San Pedro complex (T–
SP; Dungan et al., 2001). Central LdM trace element
compositions form narrow arrays in elemental variation
plots compared with the range observed in the volcanic
field as a whole (Fig. 5). The Pleistocene LdM ignimbrites igcb and igsp are notably enriched in rare earth
elements (REE), particularly middle REE (MREE), Y, and
Zr compared with the post-glacial silicic lavas.
Whereas many major and trace elements, such as
K2O, MgO, Th, U, Rb, and Pb, evolve monotonically
with increasing SiO2, several display prominent inflections in variation diagrams (Fig. 5 and supplementary figures). Between 52 and 60% SiO2, high field
strength elements (HFSE) (except Ti), large ion lithophile elements (LILE) (except Sr), light REE (LREE),
heavy REE (HREE), and Y increase with increasing SiO2.
Between 60 and 68% SiO2, Zr and LREE level off and
TiO2, MREE, Y, and P2O5 begin to decrease. Ba concentrations increase to 65% SiO2 but vary little in the more
evolved lavas. Between 68 and 70% SiO2, Zr concentrations begin to decrease and the depletion of Sr with
increasing SiO2 becomes greater.
Sr and Pb isotope ratios
The Sr and Pb isotope compositions of the central LdM
units, measured at the University of Wisconsin–Madison
ICP–TIMS Isotope Laboratory [Sr by thermal ionization
mass spectrometry (TIMS) and Pb by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS); see the Supplementary Data for details], display
Journal of Petrology, 2017, Vol. 58, No. 1
95
15.75
Mz intrusions
ement
Pz bas
Pb/ 204Pb
15.65
207
s
sion
ntru
Ni
Pε-
sed
c
al ar
ront
ry f
erna
t
Qua
15.55
MORB
SA-N
15.45
18.2
18.3
NHRL
18.4
206
18.5
Pb/ 204Pb
18.6
18.7
18.8
Fig. 6. The 206Pb/204Pb and 207Pb/204Pb ratios of central LdM
basin lavas (red squares); data are given in Table 3. Also
shown is the Northern Hemisphere Reference Line (NHRL;
Hart, 1984), the composition of South Atlantic N-MORB (SANMORB; Douglass et al., 1999), Mesozoic (Mz) and Paleogene
to Neogene (Pe–N) intrusive rocks (Lucassen et al., 2004),
Paleozoic (Pz) intrusive and metamorphic basement (Lucassen
et al., 2004), SVZ sediments (Hildreth & Moorbath, 1988;
Lucassen et al., 2010; Jacques et al., 2013), and Quaternary
SVZ frontal arc lavas (Davidson et al., 1987; Gerlach et al.,
1988; Hildreth & Moorbath, 1988; Hickey-Vargas et al., 1989;
McMillan et al., 1989; Jacques et al., 2013; Holm et al., 2014).
The LdM lavas yield a narrow range of 207Pb/204Pb isotopic
ratios compared with the frontal arc edifices and are distinct
from those of the Paleozoic to Mesozoic basement, indicating
that any assimilation was of younger, more primitive crust.
limited variation. Ratios of 87Sr/86Sr range from 070407
to 070422, 206Pb/204Pb from 18615 to 18646, 207Pb/204Pb
from 15606 to 15622, and 208Pb/204Pb from 38521 to
38565 (Fig. 6; Table 3; Supplementary Data Figs A6 and
A7). Whereas the 207Pb/204Pb ratio does not vary coherently with major or trace element composition, higher
87
Sr/86Sr, 206Pb/204Pb, and 208Pb/204Pb ratios are correlated with increasing SiO2 (Supplementary Data Fig. A7).
The late Pleistocene to early post-glacial andesites apj
and aam have elevated 87Sr/86Sr ratios similar to those
of the silicic eruptions, but slightly less radiogenic
206
Pb/204Pb and 208Pb/204Pb ratios compared with the
more mafic units. In contrast, quenched mafic inclusions
in the northern rhyodacite domes rdno and rdne have
87
Sr/86Sr ratios similar to those of the basalts and mafic
andesite lavas, but higher 206Pb/204Pb and 208Pb/204Pb
ratios. The 87Sr/86Sr ratio of the modest-volume andesite
scoria eruption asp is similar to that of apj, aam, and the
silicic eruptions, but also has the most radiogenic
206
Pb/204Pb and 208Pb/204Pb ratios of this sample suite.
The range of the central LdM 87Sr/86Sr ratios is notably narrow compared with regional volcanic centers
(Fig. 7). The LdM volcanic field as a whole has a wider
range of 87Sr/86Sr ratios of 070388–070435 and one
high outlying ratio, 070483, from the 430 ka rhyolite of
Cerro Negro (rcn; Hildreth et al., 2010). The Miocene
Risco Bayo–Huemul plutonic complex exposed beneath
the Tatara San Pedro volcanic complex contains volumetrically minor domains with 87Sr/86Sr ratios significantly greater (>07050) than those of juvenile lavas in
the SVZ (Nelson et al., 1999). No lava with a comparably
radiogenic Sr isotope ratio has erupted in the central
LdM since the middle Pleistocene. The range of central
LdM is also similar to, but slightly narrower than, those
found at the nearby Pleistocene silicic centers including
the Puelche Volcanic Field (070386–070440; Hildreth
et al., 1999) and the Loma Seca Tuff and associated
lavas (070380–070433; Grunder, 1987).
Th isotopes
The Th isotopic compositions, measured by MC-ICP-MS
at the University of Wisconsin–Madison ICP–TIMS
Isotope Laboratory (see Supplementary Data for details), span a narrow range with modest disequilibrium
in both U- and Th-excess (Fig. 8; Table 4). The agecorrected (230Th/232Th)0 activity ratios of the LdM lavas
range from 0773 to 0808, among the lowest yet measured in the SVZ. The rhyolites and rhyodacites display a
modest U-excess, up to 5%, and a narrow range of
(230Th/232Th)0 ratios, 0793–0808. The mafic lavas show
a greater diversity of Th isotopic compositions. The
(230Th/232Th)0 ratios of mafic lavas are nearly all lower
and have a 50% larger range, 0773–0800, than those of
the silicic eruptions. Most are in 2–5% Th-excess.
Quenched mafic inclusions hosted in units rdno and
rdne and the basaltic andesite lava mpl are in 3–4%
U-excess and have low (230Th/232Th)0 ratios spanning a
similar range to the Th-excess lavas (Fig. 8).
THERMOMETRY AND BAROMETRY
Two-oxide thermometry
The compositions of Fe–Ti oxides were determined by
electron microprobe at the University of Wisconsin–
Madison (see the Supplementary Data for details).
In the LdM rhyolites and rhyodacites, the ulvöspinel
content of magnetite ranges from Ulv13 to Ulv25 and
hematite content of ilmenite from Hm25 to Hm31. The
average Fe–Ti oxide compositions of the apj andesite
flow are Ulv50 and Hm15. Oxides in both the rhyolites
and rhyodacites span the compositional range
observed in the silicic units; however, the highest
ulvöspinel contents found in rhyolites are limited to the
products of the Cari Launa (rcl, rsl) center.
Fe–Ti oxide temperatures calculated using the calibration of Ghiorso & Evans (2008) are 760–850 C for the
rhyolites, 796–854 C for the post-glacial rhyodacites,
and 760 C for the late Pleistocene rhyodacite rddm
(Fig. 9). Silicic units yield an f O2 119–132 log units
above the Ni–NiO buffer (NNO). Oxides from the
Younger Andesite of the Western Peninsula (apj) gave a
temperature of 1017 C and f O2 03 log units greater
than NNO. The range of temperatures produced for
multiple oxide pairs from each sample is 35 C for all
but three samples, commensurate with the 630 C uncertainty typically ascribed to the two-oxide thermometer (Ghiorso & Evans, 2008). The later erupted Cari
Launa rhyolites and unit rdcd produced temperature
96
Journal of Petrology, 2017, Vol. 58, No. 1
Table 3: Whole-rock Sr and Pb isotopic compositions
Sample
Unit
LDM-12-25
LDM-12-19
ALDM-13-09
LDM-12-34
LDM-12-31
LDM-12-15
LDM-12-23
LDM-13-13
ALDM-13-14
LDM-12-07
LDM-12-08
LDM-12-11
LDM-12-17
LDM-12-17i
LDM-12-27
ALDM-13-10
LDM-12-03
ALDM-13-01
LDM-12-33
LDM-12-33i
LDM-12-16
ALDM-13-08
LDM-12-32
LDM-12-04
LDM-12-30b
LDM-12-21
ALDM-13-17
Standard analyses
NIST SRM-987
NBS-981
NBS-982
BCR-2
AGV-2
aam
apj
asp
bec
mnp
mpl
rap
rcb
rcb
rcd
rcl
rdac
rdcd
rdcdi
rdcn
rddm
rdne
rdnei
rdno
rdnoi
rdnp
rdsp
rep
rle
rle p
rln
rsl
87
87
Sr/86Sr
070419
070419
070419
070412
070409
070408
070418
070419
070420
070422
070419
070420
070418
070410
070413
070420
070421
070407
070420
070412
070411
070414
070420
070419
070420
070420
070419
071028
070505
070402
2SE
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
000001
2SD
000001
000002
000002
206
Pb/204Pb
207
2SE %
Pb/204Pb
2SE %
208
Pb/204Pb
2SE %
n
2
2
2
1
1
1
2
2
2
2
1
1
3
1
2
1
1
1
2
1
1
1
1
3
1
1
2
18618
18623
18648
18623
18622
18623
18638
18636
18633
18632
18634
18638
18640
18621
18630
18633
18636
18630
18635
18637
18631
18636
18637
18637
18637
18636
18634
000005
000004
000004
000006
000005
000008
000005
000004
000005
000006
000005
000005
000004
000006
000004
000006
000006
000010
000003
000003
000006
000004
000004
000004
000004
000005
000004
2SD
15613
15612
15614
15611
15613
15621
15614
15615
15612
15612
15613
15614
15613
15614
15612
15613
15616
15606
15613
15617
15612
15614
15612
15613
15613
15615
15613
000005
000004
000004
000006
000005
000007
000006
000004
000005
000006
000005
000005
000005
000007
000005
000005
000005
000011
000003
000003
000007
000005
000005
000004
000004
000005
000004
2SD
38532
38540
38570
38533
38538
38550
38557
38557
38549
38549
38552
38553
38557
38534
38538
38547
38551
38535
38551
38561
38546
38537
38550
38550
38554
38560
38554
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
000004
2SD
16940
36754
18756
18861
0004
0026
0029
0041
15496
17161
15628
15619
0004
0006
0009
0005
36720
36749
38720
38535
0011
0015
0100
0044
30
22
11
5 (Sr), 7(Pb)
6 (Sr and Pb)
Sr/86Sr ratios are reported as measured; age correction is inconsequential for these young samples.
Central LdM
Basin
Rhyolite
Rhyodacite
Basalt - Andesite
Mafic Inclusions
spreads greater than 60 C. The younger Cari Launa lava
flow (rcl) and associated pumice cone produced a similar range of temperatures that in aggregate is 812–
884 C; the lowest temperature in this range is more
than 2r from the mean. Excluding this temperature narrows the range to 845–884 C. Unit rdcd produced a
similarly wide range of 823–889 C; all calculated temperatures are within two standard deviations of the
mean.
0.7044
Sr/ 86Sr
Amphibole and plagioclase crystals in five rhyodacite
lavas (rdac, rdne, rdno, rdcd, and rdcn) were analyzed
by electron microprobe at the University of Wisconsin–
Madison. The plagioclase compositions are utilized to
estimate the magma water content required for the
amphibole barometer calibration of Putirka (2016); a
more thorough interrogation of the plagioclase compositions will be the subject of a future contribution.
The anorthite content of plagioclase rims ranges from
An19 to An43. Using the hygrometer of Waters & Lange
(2015), the plagioclase rim and rhyodacite whole-rock
compositions yield a mean water content for each unit
ranging from 45 to 50 wt % at 850 C and 250 MPa; a
grand mean of 48 wt % is adopted for the amphibole
calculations. The Waters & Lange (2015) hygrometer requires an estimate of the crystallization pressure, but is
Greater LdM
Loma Seca Tuff
Tatara - San Pedro
Puelche Volcanic Field
0.7046
87
Amphibole thermobarometry
SVZ 36º S
0.7042
0.7040
0.7038
0.7036
0
200
400
600
Sr [ppm]
800
Fig. 7. Comparison of the central LdM basin 87Sr/86Sr as a function of Sr content with those of nearby volcanic centers including T-SP (Davidson et al., 1987), the rear-arc Puelche volcanic
field (Hildreth et al., 1999), the Calabozos Caldera complex–
Loma Seca Tuff (Grunder, 1987), and older eruptions throughout the LdM volcanic field (Hildreth et al., 2010). The regional
data are plotted age corrected; the age correction is insignificant for the central LdM lavas and these ratios are plotted as
measured values. The central LdM lavas show a notably narrow range compared with these nearby systems.
Journal of Petrology, 2017, Vol. 58, No. 1
1.0
97
Laguna del Maule
Puyehue - Cordon Caulle
Llaima
Quizapu
Osorno and small Puyehue centers
33º - 41º S historic mafic eruptions
(a)
0.850
(b)
(230Th/232Th)o
0.825
0.9
Mafic lava
Mafic inclusion
Rhyodacite
Rhyolite
Rhyolite Glass
0.800
0.8
eq
eq
u il
uil
in e
ine
0.775
0.7
0.7
0.9
1.1
(238U/232Th)
1.3
0.750
0.70
0.75
0.80
(238U/232Th)
0.85
0.90
Fig. 8. Equiline plots of age-corrected Th isotope activity ratios for central LdM lavas and pumice erupted in the last 150 kyr. (a) The
LdM data compared with those measured at other SVZ volcanic systems (Hickey-Vargas et al., 2002; Sigmarsson et al., 2002; Jicha
et al., 2007; Reubi et al., 2011; Ruprecht & Cooper, 2012). Central LdM lavas have among the lowest (230Th/232Th)0 activity ratios yet
measured in the SVZ. (b) Detail equiline plot of the LdM data including the Th-excess mafic lavas and U-excess silicic products and
rhyodacite-hosted mafic enclaves. The uncertainties in the (230Th/232Th)0 data include those of the ages used to correct the measured ratios for decay since eruption. Dashed tie-lines connect mafic inclusions to their host rhyodacite.
relatively insensitive to this parameter. Over a range of
100–900 MPa, the calculated water content varies by
only 015 wt %. Thus, the inclusion of a pressure estimate in the hygrometry calculation does not bias the
amphibole barometry.
The LdM amphiboles are pargasite to magnesiohornblende based on the classification scheme of
Hawthorne et al. (2012). Amphibole formulae based on 23
oxygen atoms, pressures, and temperatures are calculated using the method of Putirka (2016). The equilibrium
melt SiO2 is calculated to assess equilibrium with the host
magma; amphiboles that deviate by more than 4 wt %
from the host composition, the uncertainty associated
with the equilibrium SiO2 estimate, are not included in the
pressure calculations (Putirka, 2016). The resulting dataset
comprises 12–38 amphibole analyses for each unit and
yields average crystallization pressures of 190–250 MPa
with uncertainties of 30–50 MPa (Fig. 9). These pressures
are consistent with those calculated by the less precise,
but magma composition-independent, barometer calibrations of Ridolfi et al. (2010) and Ridolfi & Renzulli (2012).
Pressure- and magma composition-independent amphibole thermometry produces a range of 828–933 C, which
overlaps the two oxide temperatures from the rhyodacite
lavas, but also extends to higher temperatures.
DISCUSSION
The narrow compositional arrays of the central LdM
basin lavas suggest a common magmatic origin
(Hildreth et al., 2010). However, divergent correlations
among radiogenic isotope ratios and inflections in the
trajectory of trace element variation diagrams suggest
distinct differentiation pathways involving diverse crustal assimilants and crystallizing assemblages. In the following sections we explore the following: (1) the
processes that have contributed to the geochemical
characteristics of the LdM lavas, particularly the sources
of U- and Th-excess; (2) whether these processes deviate significantly from those inferred at frontal arc volcanoes; (3) the processes promoting the more
homogeneous isotopic compositions of the rhyolites
compared with the mafic samples; (4) the temporal coherence of the thermo-chemical evolution of the LdM
magma system; (5) the implications for the structure
and state of the modern magma reservoir.
Crustal contributions to mafic magmas
Frontal arc centers in the central and southern SVZ
commonly show relatively narrow ranges of radiogenic
isotope ratios, despite trace element evidence for significant crustal interaction, owing to limited isotopic
contrast between the primary mafic magmas and the juvenile crust (e.g. Davidson et al., 1987; Dungan et al.,
2001). Uranium-series isotopes are a sensitive tracer of
magma evolution in arc systems as they provide information about the nature of mantle and crustal components, the processes leading to their mixing, and in
some cases the timescales of these processes (e.g.
Hickey-Vargas et al., 2002; Turner et al., 2003, 2010;
Jicha et al., 2007, 2009; Reubi et al., 2011; Ankney et al.,
2013). Mafic lavas in U-excess are common in arc settings and are often attributed to the flux of slab fluids to
the mantle wedge (e.g. Turner et al., 2003). Less common Th-excess continental arc magmas are generally
thought to reflect a garnet signature inherited from the
98
Journal of Petrology, 2017, Vol. 58, No. 1
Table 4: Whole-rock and glass 230Th–238U compositions
Sample
LDM-12-25
LDM-12-19
ALDM-13-09
LDM-12-34
LDM-12-15
LDM-12-31
LDM-12-23
LDM-13-13
ALDM-13-14
LDM-12-07
LDM-12-08
LDM-12-11
LDM-12-17
LDM-12-17i
LDM-12-27
ALDM-13-10
LDM-12-03
ALDM-13-01
LDM-12-33i
LDM-12-16
ALDM-13-08
LDM-12-32
LDM-12-04
LDM-12-30b
LDM-12-21
ALDM-13-17
LDM-12-07
LDM-12-04
LDM-12-21
ALDM-13-14
LDM-12-08
Standard analyses
BCR-2
AGV-2
Unit
Age (ka)
aam
254 6 15
apj
211 6 34
asp
<35
bec
618 6 36
mnp
<24
mpl
54 6 21
rap
224 6 20
rcb
<3
rcb
145–56
rcd
22 6 12
rcl
<33
rdac
200 6 12
rdcd
800 6 084
rdcd i
800 6 084
rdcn
35 6 23
rddm
114 6 14
rdne
257–190
rdne i
257–190
rdno i
257–190
rdnp
<24
rdsp
<35
rep
257 6 12
rle
190 6 07
rle pum
190 6 07
rln
<3
rsl
33 6 12
rcd glass
21 6 12
rle glass
190 6 07
rln glass
<3
rcb glass
145–56
rcl glass
<35
Th (ppm) U (ppm) (238U/232Th)
920
804
794
309
406
676
2205
1880
2097
2059
2014
1999
1960
339
1520
1822
1636
539
616
1534
1697
2342
2350
2325
1908
2089
2176
2367
2025
2116
2078
227
203
207
077
109
165
595
505
561
549
541
542
539
088
412
494
436
142
167
618
461
632
632
620
514
556
581
636
542
566
560
587
601
168
186
0748
0765
0792
0754
0814
0742
0819
0815
0812
0810
0815
0822
0834
0784
0822
0823
0809
0798
0823
0828
0824
0819
0816
0810
0817
0808
0811
0815
0812
0812
0817
2SD
0869
0938
2SE
(230Th/232Th)
2SE
0004
0005
0005
0005
0005
0004
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0781
0778
0790
0765
0800
0770
0800
0799
0799
0798
0799
0798
0798
0798
0799
0815
0799
0778
0783
0802
0804
0802
0803
0808
0800
0797
0794
0803
0798
0794
0801
2SD
0876
0946
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0005
0002
0002
0006
0005
(230Th/232Th)0 2SE* (230Th/238U)0 n
0790
0781
0790
0773
0798
0788
0795
0798
0798
0798
0798
0793
0797
0798
0798
0802
0797
0773
0774
0799
0804
0797
0800
0808
0800
0796
0794
0801
0798
0793
0801
0008
0007
0005
0012
0008
0020
0007
0005
0006
0005
0005
0007
0005
0005
0006
0027
0007
0008
0008
0009
0005
0008
0007
0007
0005
0005
0005
0007
0005
0006
0005
1057
1021
0998
1025
0981
1062
0970
0980
0982
0986
0979
0964
0956
1017
0971
0974
0985
0968
0941
0965
0976
0973
0980
0998
0980
0985
0979
0982
0982
0977
0980
1
1
2
1
1
1
1
1
1
2
1
1
2
1
1
1
1
1
1
1
1
1
2
1
1
2
1
1
1
1
1
6
5
*(230Th/232Th)0 uncertainty includes that of the eruption age.
mantle or lower crust owing to its affinity for U over Th
(DU/DTh ¼ 23–129; e.g. Rubatto & Hermann, 2007; Qian
& Hermann, 2013). In the SVZ, correlations among
fluid-mobile trace elements, 10Be/9Be, and U-excess in
frontal arc basalts have been interpreted as a slab fluid
control of the primary Th isotope signature (HickeyVargas et al., 2002; Sigmarsson et al., 2002).
However, subsequent U-series studies of several SVZ
centers, including LdM, call into question the ubiquity of
this relationship. The enrichment of fluid-mobile elements in the SVZ is modest compared with volcanic arcs
globally (e.g. Ba/Th < 300) and is only weakly correlated
with U-excess (Fig. 10; Supplementary Data Fig. A8).
Moreover, correlations between fluid-mobile element enrichment and U-excess can result from crustal assimilation rather than variations in the slab fluid signature
(Reubi et al., 2011). Whereas the addition of slab fluids to
the mantle wedge plays an important role in promoting
U-excess at some frontal arc centers, several mechanisms could contribute to their decoupling in the SVZ: (1)
long magma residence (>350 kyr) following the addition
of the fluid component to the mantle wedge allows the
U-excess signature to decay away (Hickey-Vargas et al.,
2002); (2) the addition of a Th-enriched sediment melt to
the mantle wedge would mitigate the fluid-derived U enrichment (Jacques et al., 2013).
In the absence of significant fluid-derived U enrichment, 3–6% partial melting of garnet lherzolite mantle
(e.g. Ottonello et al., 1984), with a composition
estimated as the average of that of Palme & O’Neill
(2003), will yield Th-excess similar to that measured in
the LdM lavas (Fig. 10). However, these low extents of
melting favor silica-undersaturated melts inconsistent
with the silica-saturated to -oversaturated lavas erupted
at LdM. Thus, the Th excess at LdM most probably reflects a greater extent of mantle melting and a contribution from garnet-bearing crust (GBC). The 207Pb/204Pb
ratios of the LdM lavas are distinct from those of the
more radiogenic Paleozoic to Mesozoic basement, indicating that this crustal component must be relatively
primitive (Fig. 6; Luccassen et al., 2004).
Models of lower crust melting are calculated using experimental phase equilibria and partition coefficients
from the literature (see the Supplementary Data for
model parameters). The composition of the lower crust
is estimated using the global average of Rudnick & Gao
(2003); the narrow range of the (230Th/232Th)0 ratios of
the LdM lavas suggest that the initial U/Th ratio of the
crustal component is similar to that observed at LdM,
and thus the estimated crustal composition is adjusted
accordingly. Batch melting of GBC (e.g. Berlo et al., 2004;
Hora et al., 2009) and the formation of garnet during dehydration melting of initially garnet-free amphibolite
(Wolf & Wyllie, 1993; Ankney et al., 2013) have been proposed to explain Th-excess in continental arc settings.
The latter, although appropriate for the large Th-excess
observed in Cascade lavas (Jicha et al., 2009; Ankney
et al., 2013; Wende et al., 2015), yields large Th-excess
Journal of Petrology, 2017, Vol. 58, No. 1
0
(a)
M
Ld olite
y
Rh
Log fO2
-13
n
p
200
Bi
sh
op
Tu
ff
Holocene
EPG
300
Gl
-16
+2
ui
as
s
M
QF
tos
100
O
Mt
.
-15
a
NN
ua
Or
-14
Rhyodacite lavas
rdcn
rdcd
rdac
rdne
rdno
(b)
ec
S
ma
Lo ff
Tu
P [MPa]
-12
M
e
Ld acit
d
yo
h
R
S
-11
VT
T
-10
99
-17
650
700
750
800
T [ºC]
850
900
400
800
850
900
950
T [ºC]
Fig. 9. Results of mineral thermobarometry for central LdM eruptions. (a) T–f O2 plot for central LdM silicic eruptions. Fields show
the range of temperatures and oxygen fugacities for the Loma Seca Tuff (Grunder & Mahood, 1988), Bishop Tuff (Hildreth &
Wilson, 2007), Glass Mountain rhyolites (Metz & Mahood, 1991), post-Oruanui rhyolites (Sutton et al., 2000) and the Valley of Ten
Thousand Smokes rhyolites (VTTS; Hildreth, 1983). Reference T–f O2 curves for the nickel–nickel oxide buffer (NNO) and 2 log units
above the quartz–fayalite–magnetite buffer (QFM þ 2) are shown, illustrating the highly consistent T–f O2 buffering of the LdM eruptions. (b) Temperatures and pressures derived from amphibole compositions for LdM rhyodacite lavas. The pressure calculation
assumes a magma with 48 wt % H2O based on plagioclase hygrometry (Waters & Lange, 2015). Each point is a single spot analysis
and has uncertainties of 630 C and 6160 MPa (Putirka, 2016). The bars on the left of the plot are the average pressure and associated uncertainty for each unit. The pressures of the Holocene lavas are nominally 50–60 MPa less than, but within uncertainty of,
those of the EPG units.
and HREE depletions inappropriate for the SVZ
(Supplementary Data Fig. A3). Mixing of 10% partial
melts of garnet-bearing crust and mantle reasonably reproduces the range of Th-excess and REE compositions
found at LdM; however, the presence of U-excess mafic
lavas requires an additional explanation (Figs 10 and 11).
LdM mafic lavas in U-excess could be interpreted as
reflecting the slab fluid signature only partially overprinted in the lower crust. However, these samples are
enriched in incompatible elements relative to the basalts
and mafic andesites in Th-excess, indicating that the
U-excess mafic lavas have experienced greater interaction with a crustal component. In contrast to garnet
production by amphibolite dehydration, the formation of
clinopyroxene during the melting of plagioclase- and
amphibole-bearing crust (garnet-free crust; GFC) can
produce U-excess (Fig. 11; Beard & Lofgren, 1991; Berlo
et al., 2004). Holocene intermediate lavas at T-SP were
produced, in part, by the melting of hornblende-bearing
mafic intrusions similar to T-SP xenoliths (Costa &
Singer, 2002). A 10% dehydration melt of this material
yields 6% U-excess, commensurate with the range
observed in the LdM lavas (Fig. 10).
Mixing among the mantle, GBC, and GFC endmembers, each produced by 10% partial melting, can
explain the Th isotope and trace element diversity of
the LdM mafic lavas. Variation of the Th isotope ratios
with the Zr/Th and La/Yb ratios forms offset arrays with
the largest Th-excess, found in units mpl and aam,
associated with higher La/Yb and lower Zr/Th ratios.
This offset is consistent with variable mixing, 5–30%, of
the GBC and mantle melts. Additional mixing with a
10% partial melt of GFC yields the range of U-series disequilibrium observed in the LdM mafic lavas (Fig. 11).
Thus, despite a relatively limited range in isotopic compositions, the LdM lavas reflect extensive interactions
between mantle-derived melts and the continental
crust. Moreover, these processes vary little from those
inferred at frontal arc centers throughout the SVZ (e.g.
Davidson et al., 1987; Hildreth & Moorbath, 1988;
McMillan et al., 1989; Dungan et al., 2001; Costa &
Singer, 2002; Jicha et al., 2007). Thus, whereas the concentration of rhyolite at LdM is exceptional, the underlying mafic magmatic processes are not.
Shallow vs deep origin of rhyolite
The LdM silicic lavas are depleted in Ti, P, Sr, and Y,
have negative Eu anomalies, and have lower Dy/Yb
ratios relative to the andesites (Fig. 5; Supplementary
Data Fig. A5). These trends indicate a shift in the differentiation regime from that of mafic magmas primarily
influenced by assimilation of crustal melts. Annen et al.
(2006) suggested that the majority of compositional diversity of volcanic rocks is imparted by lower crustal
processes. This model is inconsistent with the relatively
shallow crystallization pressures determined by amphibole barometry at LdM. However, it is possible that the
amphibole is late crystallized and does not capture the
high-pressure differentiation history of the silicic magmas. Differences in phase equilibria and the composition of potentially assimilated rocks between the deep
and shallow crust would impart predictable, divergent
geochemical trends during the generation of silicic
magma that are compared with the LdM compositions
to judge the plausibility of differentiation in the lower vs
upper crust.
We utilize Rhyolite-MELTS (Gualda et al., 2012)
to simulate fractional crystallization of an andesitic
LdM parental magma at a range of pressures
(150–1050 MPa), initial water contents (1–6 wt %), and
f O2 buffers (QFM to QFM þ 2, where QFM is quartz–
100
Journal of Petrology, 2017, Vol. 58, No. 1
(a)
equiline
350
300
Quizapu
LdM
Llaima
Villarica
Villarica SEC
Puyehue
Puyehue SEC
Osorno
250
Ba/Th
200
150
100
50
0.6
0.8
1.0
1.2
1.4
1.6
1.8
(238U/ 230Th)0
(b)
equ
ilin
e
0.84
(230Th/232Th)0
0.82
Garnet 2
lherzolite
melting
0.80
0.78
LdM
silicic
lavas
3 4 6 10 15
30
5
10 1520
Garnet-bearing
crust melting
15 10
0.76
0.74
0.65
0.70
0.75
0.80
(238U/232Th)
5
Garnet-free
crust melting
0.85
0.90
Fig. 10. Sources of U-series disequilibrium in central LdM
lavas. (a) Plot of SVZ U-series disequilibrium data for mafic
lavas compared with the Ba/Th ratio, an indicator of fluid enrichment. Volcanic centers, including small eruptive centers
(SEC) associated with larger edifices, are listed in the legend in
geographical order from north (Quizapu) to south (Osorno)
along the arc (Fig. 1). Some centers display evidence of coupling between fluid enrichment and U-excess; however, this
correlation may also result from crustal overprinting of the
slab signature (Reubi et al., 2011). The range of Ba/Th in the
Th-excess lavas is similar to that in U-excess and thus a strong
coupling between fluid enrichment and U-series disequilibrium
is not evident in the SVZ. Data sources: Sun (2001), HickeyVargas et al. (2002), Jicha et al. (2007), Reubi et al. (2011) and
Ruprecht & Cooper (2012). (b) The U-series disequilibrium expected during melting of the garnet-bearing mantle, garnetbearing lower crust, and garnet-free crust (see the
Supplementary Data for model parameters). The Th-excess apparent in the mafic LdM samples (red squares) can be produced by melting with residual garnet in either the mantle or
lower crust. U-excess in several mafic andesites and the silicic
lavas reflects the overprinting of the garnet signature by partial
melting of garnet-free crust rather than U enrichment imparted
by a subduction fluid (see text).
fayalite–magnetite) to evaluate the conditions in which
the LdM rhyolite magma formed. Each model is cooled
from the calculated liquidus to c. 700 C, depending on
model convergence at low melt fractions. The variation
of SiO2 and MgO of the LdM lavas is best reproduced
by shallow, oxidizing conditions and a moderate initial
water content. High pressures, water contents and
reducing conditions promote the early stabilization of
pyroxene at the expense of plagioclase and magnetite,
producing large depletions in MgO over a narrow range
of SiO2, inconsistent with the LdM compositions (Fig.
12). Moreover, Gaulda & Ghiorso (2013) argued that the
increasing stability of quartz with depth precludes the
generation of rhyolite by high-pressure fractional
crystallization.
MELTS is not well calibrated for hydrous intermediate to silicic compositions saturated in amphibole.
However, in this case, the SiO2/MgO ratio of LdM
amphibole (26–34) is between those of orthopyroxene
(2–3) and clinopyroxene (33–44) predicted by MELTS
such that the crystallization of either two pyroxenes or
amphibole would have a similar impact on the magma
SiO2/MgO ratio. Whereas some model misfit may result
from the prediction of pyroxene rather than amphibole
crystallization, the agreement between the MELTS modeling and amphibole barometry indicates that the suppression of plagioclase and magnetite crystallization is
the more important factor. Thus, MELTS simulations of
hydrous systems must be interpreted with caution, but
can yield useful first-order phase equilibrium constraints even when amphibole is present.
The physical plausibility of a viscous rhyolite magma
ascending >30 km through the crust is questionable
(e.g. Rubin, 1995). Even if it were possible, the similarity
of the rhyolite 87Sr/86Sr ratios to those of the mafic
and rhyodacite lavas (Fig. 7) weigh against a deep crust
origin. Following differentiation in the lower crust, Srdepleted rhyolite would then traverse the crustal column that includes highly radiogenic Paleozoic to
Mesozoic rocks (Lucassen et al., 2004; Supplementary
Data Fig. A6). The inevitable assimilation of even small
amounts (<5%) of this material would produce higher
and more variable 87Sr/86Sr ratios in the rhyolites than
observed. The more radiogenic 87Sr/86Sr ratios,
>07046, of the mid-Pleistocene rcn rhyolite erupted in
the eastern LdM basin and the most-evolved domains
of the Miocene plutonic complex beneath T-SP (Nelson
et al., 1999; Hildreth et al., 2010) potentially reflect assimilation of this material; however, the modestly radiogenic, homogeneous 87Sr/86Sr ratios of the post-glacial
rhyolites do not. Taken together, the isotope ratios of silicic LdM lavas, the incongruity between the predicted
phase equilibrium and the LdM major element compositions, and shallow crystallization pressures recorded
by amphibole barometry rule out generation of the LdM
rhyolites in the lower crust.
Shallow hybridization and fractional
crystallization
The narrow range of Th isotope ratios and uniform
U-excess of the silicic lavas contrast with the more varied mafic compositions (Fig. 8). Fractionating Th from U
in the upper crust to produce the silicic compositions
from a parental melt in Th-excess is not
Journal of Petrology, 2017, Vol. 58, No. 1
1.15
(a)
(238U/230Th)0
1.10
(b)
10% GFC
melt
50
1.05 LdM
silicic
lavas
1.00
50
10
30
10%
GBC
melt
0.85
10
50 3
20
10%
0 20
10
30
Zr/Th
50
0
30
10
50
20
30
La/Yb
Mantle melt + 30% GBC melt
(c)
10
20
equiline
10%
GBC
melt
10
5
melt
40
30
10%
mantle
melt
5 mantle
LdM
silicic
lavas
50
10
equiline
10
0.90
10% GFC
melt
50
30
30
0.95
sample/chondrite
101
Mantle melt + 5% GBC melt
(d)
100
10
% GFC melt
10
30
50
1
La Ce
Nd
Sm Eu
Dy
Yb
La Ce
Nd
SmEu
Dy
Yb
Fig. 11. A mixing model to explain the variation of U-series disequilibrium and the trace element composition of the mafic LdM
lavas. The mixing endmembers are 10% melts of garnet lherzolite mantle, garnet-bearing crust (GBC), and garnet-free crust (GFC)
(see the text and Supplementary Data). (a) and (b) show the variation of U-series disequilibrium with the Zr/Th and La/Yb ratios produced by first mixing mantle and GBC melts. Subsequent mixing with a 10% melt of garnet-free crust produces the range of Thand U-excess observed in the LdM mafic samples (red squares). The offset arrays of LdM data are consistent with varying mixing
proportions of the mantle and GBC end-members. (c) and (d) show chondrite-normalized (Sun & McDonough, 1989) REE patterns
produced by 10%, 30%, and 50% mixing of the GFC endmember with a melt composed of 5% or 30% mixing of GBC with the mantle melt, compared with those of the mafic LdM lavas (gray field).
straightforward. Crystallization of major phases will not
significantly increase the U/Th ratio, but accessory
phases such as apatite, titanite, allanite, and monazite
have greater leverage (Berlo et al., 2004). Of these, only
apatite is common at LdM. Rare, possibly xenocrystic,
titanite has been recovered by heavy liquid separation
from the large, early tephra eruption, but not from any
other LdM rhyolite; neither allanite nor monazite are
present. The crystallization of sufficient apatite or titanite to produce U-excess from a Th-excess mafic magma
is not consistent with the P2O5 and MREE compositions
of the LdM lavas: fractionation of 03% titanite
(DTh ¼ 187, DU ¼ 7, DDy ¼ 935, DYb ¼ 393; Bachmann
et al., 2005) or 32% apatite (DTh ¼ 282, DU ¼ 19;
Condomines, 1997) is required to produce the observed
change in the U/Th ratio. The crystallization of these
phases in this quantity would decrease the Dy/Yb ratio
by a factor of seven and the P2O5 composition by 14 wt
%, respectively; both are approximately four times
greater than the variation observed in the central LdM
lava compositions. Thus, the crystallization of accessory
phases cannot account for the U-excess observed in the
silicic lavas.
The eruption of mafic magma in Th-excess and
evolved magma in U-excess has been observed at
several volcanoes in the Andes, Cascades, and Alaska
(Garrison et al., 2006; Jicha et al., 2007; Turner et al.,
2010; Ankney et al., 2013). This transition has variously
been ascribed to mixing with a U-excess endmember
derived from small degrees of partial melting with residual accessory phases, hydrothermal alteration of
assimilated wallrock, and variation in the contribution
of a subduction component through time. The garnetfree crustal component evident at LdM offers an alternative explanation. The requirement of garnet in the
production of Th-excess limits this process to the lowermost crust. Thus, only rapidly ascending magmas
would preserve a garnet-derived Th isotope signature.
Those that stall in the middle to upper crust and further
differentiate will have greater opportunity to interact
with GFC and acquire U-excess. Amphibole, common in
arc crust, is produced both by direct crystallization and
by reaction between clinopyroxene and ascending hydrous melt. Costa et al. (2002) advocated the latter
mechanism for the generation of amphibole beneath
T-SP and it also probably occurs at LdM. The subsequent melting of amphibole-bearing crust has been proposed as an important source of melt and volatiles in
volcanic arcs more generally (e.g. Davidson et al., 2007,
2013); thus, the production of clinopyroxene during
102
4
Model Conditions
210 MPa, 3% H2O, QFM+2
210 MPa, 3% H2O, QFM
MgO [wt. %]
3
210 MPa, 5% H2O, QFM+2
600 MPa, 3% H2O, QFM+2
2
1
(a)
0
55
60
1.0
65
70
SiO2 [wt. %]
phase fraction
75
80
mt
cpx
0.9
0.8
0.7
plag
0.6
opx
0.5
melt
0.4
bt+
qtz
0.3
0.2
0.1
0
210 MPa, 3% H2O, QFM+2
1050
950
(b)
850
1.0
opx
0.9
cpx
750
0.8
phase fraction
amphibole dehydration may be an under-appreciated
source of U-excess in intermediate to evolved continental arc magmas.
The evolution of the major and many trace element
compositions from the andesitic to silicic magmas is consistent with the fractionation of the plagioclase þ
amphibole þ biotite þ Fe–Ti oxide þ apatite 6 zircon assemblage observed in the rhyodacite and rhyolite lavas.
The saturation of zircon yields prominent inflections in the
evolution of the Zr concentration (Fig. 5); deviations from
the expected closed-system evolution would favor more
extensive open-system processes. Zr and Th are similarly
incompatible in major phases and thus, prior to zircon saturation, fractional crystallization would produce comparable enrichments in both elements. In central LdM, the
modest difference in the Zr concentrations of the rhyodacite and andesite lavas is incongruent with the two-fold
difference in the Th concentrations.
We first consider a model of zircon-free fractional
crystallization of an andesite parental magma utilizing a
range of Zr partition coefficients, the anhydrous mineral
assemblage predicted by the best-fit MELTS model (Fig.
12), and a hydrous mineral assemblage in which amphibole crystallizes in place of pyroxene (Table 5). None of
these fractional crystallization pathways are able to produce the variation in Zr composition of the intermediate
LdM lavas (Fig. 13). The zircon saturation temperature
of most of the post-glacial rhyodacites is less than but
within uncertainty of the two-oxide temperature, indicating they may have been zircon saturated—based on
the zircon saturation model of Watson & Harrison
(1983); none are zircon saturated using the model of
Boehnke et al. (2013). Thus, the Zr contents of the rhodacite lavas could be produced by fractional crystallization including a small but increasing modal per cent
zircon or could reflect open-system processes.
The two-oxide temperature of the andesite apj is
1017 C, several hundred degrees higher than the zircon
saturation temperature of this lava (Watson & Harrison,
1983; Boehnke et al., 2013). The onset of zircon saturation during cooling is evaluated by combining the
major element composition–crystallinity–temperature
relationship predicted by MELTS with a zircon-free fractional crystallization model of the Zr content. The crystallizing andesite magma saturates zircon after cooling
c. 260 C, resulting in 47% crystallization and reaching a
maximum Zr concentration of 305 ppm. This Zr content
is 15% greater than that of the central LdM rhyodacites,
indicating that they evolved under dominantly zirconundersaturated conditions (Fig. 13).
Moreover, the conclusions of this model are consistent with the amphibole thermometry. The amphibole
temperatures and equilibrium melt SiO2 compositions
define an SiO2–temperature evolution that deviates
somewhat from the relationship predicted by MELTS.
Nevertheless, the comparison of the zircon saturation
temperatures of the LdM lavas and the amphibole crystallization temperatures indicates that zircon was not
saturated in the LdM magma until it reached c. 70%
Journal of Petrology, 2017, Vol. 58, No. 1
0.7
plag
0.6
mt
0.5
0.4
gt+
qtz
melt
0.3
0.2
0.1
0
600 MPa, 3% H2O, QFM+2
1050
950
T [C]
(c)
850
750
Fig. 12. Comparison of rhyolite-MELTS (Gualda et al., 2012) fractional crystallization simulations with the SiO2–MgO variation of
central LdM lavas to evaluate the effect of crystallization at a
range of pressure, H2O, and f O2 conditions. (a) Four representative MELTS simulations. These models are not exhaustive of the
range of conditions considered but rather were selected to illustrate the effect of changes to each parameter (see text). The bestfit model (thick black line) involves a low-pressure, moderate
water content, and oxidizing conditions, consistent with the mineral thermobarometry (Fig. 9). Higher pressures, water content,
and more reducing conditions produce significant depletions in
MgO at intermediate SiO2 contents that strongly contrast with
the LdM data. (b) and (c) illustrate the contrasting crystallizing assemblage produced at 210 and 600 MPa. Higher pressures, as
well as high H2O and more reducing conditions, stabilize pyroxene early at the expense of magnetite and plagioclase and produce MgO-depleted magmas. Mineral abbreviations: mt,
magnetite; cpx, clinopyroxene; opx, orthopyroxene; plag, plagioclase; bt, biotite; gt, garnet; qtz, quartz.
Journal of Petrology, 2017, Vol. 58, No. 1
103
SiO2 (Fig. 13). Thus, whereas some LdM rhyodacites
may have saturated zircon prior to eruption, it was a
late-crystallizing phase, and the rhyodacite Zr contents
are primarily the result of open-system processes rather
than zircon fractionation. The rhyolite compositions are
consistent with an additional 20–35% crystallization of
an intermediate hybridized magma, assuming fractionation of the mineral assemblage observed in the silicic
LdM lavas (Fig. 13; Table 5).
The array of rhyodacite Th–Zr compositions does not
readily implicate an LdM lava composition as the silicic
mixing endmember. It is relatively enriched in most incompatible trace elements, similar to the LdM rhyolites,
but not depleted in Zr (Fig. 13). The isotopic composition of the LdM lavas weighs against significant remobilization of existing silicic crust, such as the plutons
beneath T-SP or the Pleistocene LdM ignimbrites.
Whereas the mafic LdM lavas span the entire range of
Sr and Pb isotopic compositions measured in the central basin, reflecting the diversity imparted by lower
crustal interactions, and the rhyodacites nearly so, the
rhyolites exhibit more homogeneous isotope ratios despite the wide spatial distribution of vents (Fig. 14).
Significant contributions from the modestly more radiogenic and isotopically diverse upper crust would yield
higher, and probably more variable, 87Sr/86Sr ratios in
the LdM silicic lavas than observed.
The silicic end-member could be the product of relatively closed-system differentiation (Fig. 13). However,
no magma with a composition consistent with this evolution has erupted in the LdM since the 990 ka Bobadilla
ignimbrite. Moreover, a silicic mixing endmember
derived from closed-system differentiation would inherit the more varied isotopic ratios of the mafic lavas
and would not promote the increasingly homogeneous
isotopic ratios in the more evolved magmas.
Instead, the high temperature of the intruding mafic
magma could promote the resorption of zircon during
magma hybridization, thereby enriching the Zr content
relative to the rhyolitic magma. Zircon has not been
identified in thin sections of the LdM rhyodacites so the
presence of rare, partially resorbed zircon cannot be
confirmed. However, the abundance of mafic inclusions
in the rhyodacite lavas records the shallow mixing and
mingling of mafic and silicic magmas. Moreover,
plagioclase phenocrysts display a range of textures
ranging from relatively homogeneous to complexly
zoned, including resorption surfaces with overgrowths
and mottled and sieved cores reflecting varied and
complex thermal histories (Fig. 15). In contrast, plagioclase in the rhyolite lavas is only weakly zoned; this is
probably the result of efficient melt extraction from the
zones of magma hybridization, followed by a limited degree of cooling and crystallization prior to eruption.
Taken together, the inferred trace element and isotopic
composition of the silicic endmember and the outcropto mineral-scale textures of the rhyodacite lavas favor
self-assimilation—hybridization of intruding mafic to
intermediate magma with the post-glacial silicic reservoir including the resorption of zircon, rather than assimilation of the upper crust (Figs 13 and 14).
Extensive rhyolitic magma was probably not available during the early growth of the LdM system. Thus,
the initial stages of magma reservoir development may
have involved remobilizing remnants of midPleistocene episodes of silicic magmatism and shallow
silicic intrusions or production of silicic magma by
closed-system processes (Fig. 13). Geochemical evidence of such magma has not yet been identified and
they may have never produced an eruption. As the
post-glacial silicic system grew progressively larger, the
assimilation of young, hybridized rhyolite overtook any
contribution of the older material or highly fractionated
magma. Through self-assimilation, the increasing size
and homogeneity of the LdM magma system is a
coupled and self-reinforcing process.
Temporal evolution of the LdM magma system
The temporal and spatial distribution of LdM eruptions
favors a laterally integrated shallow silicic magma system (Hildreth et al., 2010) and offers clues to its structure and variations in the magmatic focus through time.
Eruptions in the southern and eastern LdM basin were
dominantly rhyolitic, excepting the apo andesite,
throughout post-glacial times, whereas volcanism in
the NW is characterized by a wider range of
Table 5: Partition coefficients and phase proportions used in fractional crystallization models
Phase
plagioclase
orthopyroxene
clinopyroxene
amphibole
magnetite
biotite
apatite
zircon*
Partition coefficients
Zr
Th
0001–001
0026–014
013–041
023–093
0025–035
005
001
00006
004–022
004–029
001–025
005–042
001–05
108–282
6–18
Fractionated phase proportions (%)
Intermediate
anhydrous
59
5
20
16
09
007
Intermediate
hydrous
Silicic
59
69
25
16
10
5
15
09
008
09
007
*The Zr content of zircon is assumed to be stoichiometric.
Data sources: Luhr & Carmichael (1980); Bacon & Druitt (1988); Dunn & Sen (1994); Ewart & Griffin (1994); Sisson (1994); Brenan
et al. (1995); Bindeman et al. (1998); Villemant (1988); Sano et al. (2002); Blundy & Wood (2003); Bachmann et al. (2005).
104
Journal of Petrology, 2017, Vol. 58, No. 1
400
1200
(a)
LdM
ig.
GBC
melt
FC
zrc
resorption
20
ns
e
en
mantle
melt
0
0
p
ioc
M
FC, 0.08% zrc
700
zircon undersaturated
zircon saturated
saturation not determined
10
20
Th [ppm]
l
900
800
30
o
lut
mo
de
1000
80
10
LTS
T [°C]
fre
e
c-
20
GFC
melt
200
100
ME
7% zrc
60
amphibole
thermometry
LdM lava zrc
saturation T
1100
FC, 0.0
zr
Zr [ppm]
300
(b)
3)
(198
W&H
13)
B(20
zrc under-saturated
30
600
58
63
68
SiO2 [wt. %]
zrc saturated
73
Fig. 13. Comparison of fractional crystallization and magma mixing contributions to the upper crustal genesis of the LdM silicic
lavas and evaluation of zircon saturation during magma differentiation. (a) The variation of Th and Zr concentrations in central LdM
lavas. The trace element compositions of the mafic lavas are dominated by mixing between partial melts of the mantle, GBC, and
GFC (Fig. 11). Zircon-free fractional crystallization (zrc-free FC) of a parental andesitic magma produces an enrichment in Zr greater
than observed in the LdM rhyodacites; the dashed lines show the range of models produced using the low and high partition coefficients reported in the literature and hydrous vs anhydrous fractionating assemblages (Table 5). The temperature evolution of the
best-fit MELTS simulation in Fig. 12 predicts that the magma system will saturate in zircon at 305 ppm Zr, indicating that the flat Zr
evolution of the rhyodacites is not due to the fractionation of a small modal fraction of zircon. Instead, the rhyodacite compositions
are most consistent with mixing between intermediate and silicic LdM magmas (green line), the latter enriched in Zr by the resorption of zircon (green diamond). The rhyolite compositions are consistent with an additional 20–35% crystallization of a hybridized
intermediate magma. (b) The SiO2–temperature evolution of the MELTS model calculated from the amphibole compositions compared with the SiO2–zircon saturation temperature relationship of the LdM lavas—calculated using the calibration of Watson &
Harrison (1983)—and predicted by the MELTS fractional crystallization model using the zircon saturation calibration of both
Watson & Harrison (1983) [W&H(1983)] and Boehnke et al. (2013) [B(2013)]. Both the model and mineral data predict that the
magma saturates in zircon at c. 70% SiO2, consistent with the inflection in the whole-rock SiO2–Zr variation (Fig. 5).
compositions (Fig. 4). The common andesite eruptions
in the west and NW during EPG time suggest that the
upper crustal magma system was thinner there relative
to the south. Similarly, the numerous rhyodacite eruptions in the NW carry abundant, large mafic inclusions,
whereas they are rare and small in the lone post-glacial
rhyodacite eruption in the south, rdac. Taken together,
magmatism in the central LdM has been focused in the
southern basin since before the last glacial maximum,
resulting in a well-developed mush and a preponderance of rhyolitic eruptions.
In the NW, the most recently erupted rhyolite is the
190 ka rle flow; subsequent eruptions of any composition are scarce until the late Holocene (Fig. 4). The most
recent northern eruptions, units rdcn and rdsp,
occurred after a local hiatus of as much as 15 kyr. These
geographical differences in the eruption frequency and
physical and compositional characteristics of the eruptive products indicate that the crystal mush, well developed to the south, either thins or is discontinuous
beneath the NW portion of the lake basin. Renewed volcanism in the NW during the Holocene produced units
rdcn and rdsp, suggesting a recent expansion of the
magmatic footprint at LdM and potentially lateral
growth of the active silicic magma system. The amphibole crystallization pressures of the Holocene rdcn and
rdcd lavas are nominally 50–60 MPa less than those of
the EPG rhyodacites, although this difference is within
the uncertainty of the barometer calibration (Fig. 9).
These results suggest, but cannot prove, that the lateral
growth of the LdM system may have been accompanied by the shallowing of active magmatism.
Whereas spatial distinctions in the distribution of
mafic and silicic eruptions are readily apparent, compositional differences among the post-glacial rhyolites
are subtle, but coherent in time rather than with vent location. Holocene rhyolites are enriched in Y and MREE
compared with the EPG rhyolites (Fig. 16). Two-oxide
temperatures vary similarly. The eruption temperature
ranges of the EPG (737–801 C) and Holocene (781–
850 C) rhyolites overlap; however, the Holocene temperatures are consistently at the higher end of the total
range, suggesting an increase in magma reservoir temperature with time (Fig. 16). That the earlier erupted
rhyolite is cooler and more evolved precludes linking
the EPG and Holocene compositions by a progressive
differentiation or mixing process.
The variation of most trace elements in the rhyolites
defines a single liquid line of decent; in contrast, the
Holocene enrichments in Y and MREE define opposing
trends with SiO2 compared with the earlier erupted
rhyolites. These trace elements show flat or decreasing
trends with SiO2 in the EPG rhyolites but increasing
trends in the Holocene rhyolites (Fig. 16) and thus are
Journal of Petrology, 2017, Vol. 58, No. 1
0.7044
(a)
LdM ignimbrites
M
pluioce
to ne
ns
0.7042
80
87
Sr/ 86Sr
0.7043
60
40
20
80
0.7041
60
0.7040
0
0.7043
105
100
40
20
200
300 400
Sr [ppm]
20
40
500
600
(b)
Sr/ 86Sr
0.7042
60
87
80
0.7041
20
0.7040
18.61
18.62
206
40
60
18.63
Pb/ 204Pb
18.64
Fig. 14. The effect of magma hybridization on Sr and Pb isotope ratios; symbols are the same as in Fig. 7. Curves illustrate
mixing between high and low 87Sr/86Sr mafic magma and an
average rhyolite composition. The isotopic diversity of the
mafic magmas, inherited from lower crust interactions, is
largely preserved by the rhyodacites. The comparatively narrow range of the rhyolite isotope ratios is produced by hybridization and homogenization within an integrated magma
system. The fields in (a) show the range of 87Sr/86Sr ratios for
igcb, igsp, and the Risco Bayo–Huemul plutons, plotted as
measured (Nelson et al., 1999; Hildreth et al., 2010).
Assimilation of this material would yield higher and more varied 87Sr/86Sr ratios in the post-glacial rhyolites than observed,
favoring a model of self-assimilation within the post-glacial
magma reservoir.
also inconsistent with progressive eruption from a
zoned magma reservoir. Instead, the compositional differences reflect discrete magma bodies that, remarkably, produced eruptions over a comparably wide area,
similar to those inferred for the Mamaku and Ohakuri
ignimbrites and rhyolites following the 254 ka calderaforming Oruanui eruption in the Taupo Volcanic Zone
(Sutton et al., 2000; Vandergoes et al., 2013; Bégué
et al., 2014; Barker et al., 2014, 2015). Rhyolites of distinct composition were erupted c. 20 kyr apart, from
vents separated by only 2 km (e.g. rap and rln). In contrast, coeval rhyolites nearly identical in composition
erupted more than 10 km apart during both the EPG
(e.g. rap and rle) and Holocene (e.g. rln and rcd). Rather
than being the products of small, short-lived, isolated
magmatic systems, the temporally coherent, spatially
extensive rhyolitic eruptions imply the extraction of
chemically distinct magma from a long-lived, compositionally evolving, upper crustal source region.
Long-term variations in rhyolite composition, temperature, and mineralogy can be driven by variations in
the lower crust temperature in response to the basalt
flux from the mantle and changes in the supply of slab
fluids (Deering et al., 2008, 2010). However, at LdM, the
relatively short duration of rhyolitic volcanism and
nearly invariant f O2 buffering indicate that the subtle
differences in trace element composition and temperature are more probably related to the upper crust processes of rhyolite differentiation and extraction. Hildreth
(2004) proposed that trace element variations among
broadly homogeneous rhyolites can reflect the variable
stability of accessory phases. Similarly, Barker et al.
(2014, 2015) attributed the diversity of post-Oruani silicic magma compositions at Taupo volcano, in part, to
the resorption of amphibole, clinopyroxene, and zircon.
At LdM, extraction of a volatile-rich rhyolite would
leave behind a relatively water-poor cumulate mush
(Wolff et al., 2015). The repeated intrusion of mafic
magma would promote the resorption of amphibole or
late crystallized, cryptic titanite, resulting in MREE- and
Y-enriched magma (e.g. Deering et al., 2011). Thus, the
eruption of compositionally distinct rhyolites over time
may reflect long-term changes in the phase equilibrium
and temperature of the plutonic mush induced by the
aggregate effect of at least 26 kyr of mafic intrusions
into the upper crust. Alternatively, the composition of
each rhyolite could reflect the ephemeral effect of each
most recent magma recharge episode. In this case,
compositional differences between one set of coeval
rhyolites and the next could be a record of the response
to and size of the mafic incursions, but not necessarily
of the long-term dynamics and thermo-chemical state
of the magma reservoir. Protracted extraction or residence in the crust would tend to average out subtle
compositional differences; thus, the preservation of
compositional distinctions among the LdM rhyolites
favors rapid melt segregation and only brief storage.
Whereas there is scarce evidence for physical interaction between the erupted rhyolite and intruding mafic
to intermediate magma, the extraction of crystal-poor
rhyolite could nevertheless be catalyzed by magma recharge in the lower reaches of the magma reservoir.
Increasing temperatures would raise the porosity of the
crystal mush and, along with the exsolution of volatiles
from the mafic magma, increase the buoyancy of the
rhyolitic liquid (e.g. Barker et al., 2016).
Structure and dynamics of the magma reservoir
The combination of the basin-wide progression generally
from andesite to rhyolite, the importance of magma hybridization in rhyolite petrogenesis, and the temporal coherence of variable rhyolite compositions suggests the
physical configuration of the LdM magma system
106
Journal of Petrology, 2017, Vol. 58, No. 1
(a)
(b)
rdne
(d)
(g)
(c)
rdcd
(e)
(h)
rdcn
(f)
(i)
(j)
Fig. 15. Textural evidence of open-system processes in LdM rhyodacites. (a–c) Outcrop photographs of rdne, rdcd, and rdcn showing representative examples of chilled mafic inclusions, highlighted by the arrows. (d–j) BSE images of representative rhyodacite
plagioclase textures including sieved and mottled cores, resorption surfaces, and oscillatory zoning—all indicative of varied and
complex thermal histories. In contrast, rhyolite plagioclase crystals, not shown, are dominantly homogeneous. The scale bar in
each image represents 100 lm.
illustrated in Fig. 17: it comprises an integrated magma
source zone, sustained during at least the last 26 kyr. This
region is spatially extensive and intercepts the ascent of
diverse mafic magmas that promote magma hybridization, resorption of accessory phases, and the segregation
of crystal-poor melt batches. In the south, this magma
mingling and mixing is limited to the base of the crystal
mush, resulting in little physical interaction between the
recharge magma and the erupted rhyolite batches.
Thinning of the system to the north allows for penetration
of mafic magma to shallower levels, thereby promoting
the eruption of mingled and hybridized magma. Crystalpoor rhyolite is periodically extracted and stored only
briefly prior to eruption. The composition of these erupted
magma batches reflects the longer-term homogenization
in the upper crust by magma hybridization, temporal variation in the thermochemical state of the magma reservoir,
and possibly compositional characteristics imparted during melt extraction.
The repeated generation of compositionally and isotopically distinct rhyolite magma batches is an increasingly recognized feature of long-lived silicic magma
systems. These systems have produced a range of
eruptive behavior including the sequential eruption of
diverse rhyolites, the coeval eruption of spatially and
compositionally distinct magmas, and the pre-eruption
amalgamation of several magma bodies, yielding voluminous ignimbrites characterized by isotopically and
compositionally diverse phenocrysts (Bindeman et al.,
2008; Deering et al., 2008; Charlier & Wilson, 2010;
Klemetti et al., 2011; Storm et al., 2011, 2014; Barker
et al., 2014, 2015; Bégué et al., 2014; Wotzlaw et al.,
2015; Evans et al., 2016; Myers et al., 2016; Rubin et al.,
2016). The compositional continuity of the distributed
rhyolite eruptions through time observed at LdM and
Taupo, post-Oruanui, (Sutton et al., 2000; Barker et al.,
2014) demonstrates the remarkably lateral continuity
possible in silicic systems and the short timescales over
which compositional distinctions can be produced.
Neither LdM nor Taupo have erupted high-SiO2 rhyolite with extreme depletions in Sr and Ba, large negative
Eu anomalies, and low temperatures that characterize,
for example, the Glass Mountain rhyolites erupted at
Long Valley (Metz & Mahood, 1991; Hildreth & Wilson,
Journal of Petrology, 2017, Vol. 58, No. 1
900
(a)
Silicic Lavas
EPG
L. Pleistocene
& Holocene
LdM ignimbrites
6
Sm [ppm]
ne
ce d
sto
lei tren
P
t ene
tes
La oloc
&H
4
(c)
rhyolite
rhyodacite
875
Two-oxide T [ºC]
8
107
850
825
800
775
750
EPG trend
725
EPG
700
30
2
24
(b)
Y [ppm]
20
10
Eruption age [ka]
0
(d)
LdM ignimbrites
23 - 46 ppm Y
22
Latest Pleistocene
& Holocene
20
18
16
14
65
70
SiO2 [wt. %]
75
5 km
Fig. 16. (a, b) Comparison of Sm and Y concentrations for EPG and Holocene silicic eruptions and central LdM ignimbrites igcb and
igsp (Hildreth et al., 2010; Birsic, 2015) indicating that two compositionally distinct post-glacial rhyolite bodies were erupted in central
LdM. The enrichment of the Holocene rhyolites in MREE and Y is consistent with the resorption of cryptic titanite and/or amphibole.
The destabilization of these phases could be in response to either repeated mafic intrusion or the ephemeral effect of each most recent
recharge event. Error bars corresponding to the 2r analytical uncertainty are smaller than the symbol size. (c) Temporal variation in
two-oxide temperatures. The Holocene rhyolites are subtly hotter than those erupted in the EPG, whereas rhyodacite temperatures
vary little during post-glacial times. Eruption ages were determined by 40Ar/39Ar or 36Cl, or were estimated from stratigraphic relationships (Table 1; Fig. 4). Pink and orange symbols are rhyolites and rhyodacites, respectively. Vertical error bars are the range of temperatures produced by touching pairs or the minimum and maximum determined by combinations of isolated titanomagnetite and
ilmenite crystals, with the tick indicating the average; the uncertainty in the thermometer calibration is 630 C (Ghiorso & Evans,
2008). (d) Map showing the distribution of silicic lavas erupted during the EPG and latest Pleistocene to Holocene.
2007). Such compositions are indicative of a eutectic
mineral assemblage saturated in two feldspars and
quartz. The crystallinity of eutectic systems is more sensitive to temperature than those saturated in plagioclase
and quartz, resulting in more variable trace element
compositions in response to large changes in crystallinity during both cooling and remelting of crystal mushes
(e.g. Mahood, 1990; Sutton et al., 2000; Bindeman &
Simakin, 2014). Recharge by hotter magma is implicated
in the generation of eruptible rhyolite reservoirs in both
sanidine-bearing and sanidine-free systems but possibly
by different physical mechanisms. The melting of fertile,
sanidine-bearing mush or hydrothermally altered silicic
precursors contributed to the caldera-forming magma
reservoirs in Long Valley (Chamberlain et al., 2014a;
Evans et al., 2016), San Juan (Bachmann et al., 2005;
Wotzlaw et al., 2013), and Yellowstone (Bindeman et al.,
2008; Bindeman & Simakin, 2014; Wotzlaw et al., 2015)
systems. In sanidine-free systems, the thermal input of
magma recharge catalyzes the extraction of crystal-poor
rhyolite and resorption of some minerals, but not remelting on the scale observed in eutectic systems (Barker
et al., 2014, 2015, 2016; Singer et al., 2016). A number of
factors such as the local and regional tectonics, the crustal lithology, the depth of the magma system, and its
volatile contents contribute to the dynamics of rhyolite
generation. However, the minerology-dependent response of the shallow reservoir to magma recharge may
also have significant implications for the varied mechanisms and timescales of the generation of eruptible rhyolitic magma bodies and the growth of their source
regions; this is worthy of further investigation.
The similarity between the rhyolite volcanism at
LdM and following the Oruanui eruption in Taupo is
striking and suggests similar underlying dynamics.
Owing to active rifting and a high flux of mantlederived melt, the rhyolite productivity of the TVZ is remarkable globally (Wilson et al., 2009). Tectonic extension is often suggested as a catalyst for rhyolite
volcanism (e.g. Hughes & Mahood, 2011) and the concentration of silicic volcanism behind the frontal arc in
the SVZ, and at LdM in particular, may be related to
back-arc extension (Folguera et al., 2012). However,
widespread extensional structures are not observed at
LdM and thus the effect of local to regional extension
cannot be confirmed.
108
Journal of Petrology, 2017, Vol. 58, No. 1
xtl-poor
IMPLICATIONS FOR THE CONTINUING UNREST
2 km
(a) Early Post Glacial
NW
xtl-rich
SE
2 km
Laguna del Maule
melting
mingling
mixing
intermediate forerunners
to rhyolite flare-up
shallow, eruptible
mingled melt
crystal-poor rhyolite
holding zone
crystal-poor melt
extraction
Mafic magma from lower crust
mantle melt + crust melt
(b) Holocene
Laguna del Maule
growth of magma mush
accommodated by
surface deformation
rejuvenation of
northern source zone
crystal-poor melt
extraction
Mafic magma from lower crust
mantle melt + crust melt
(c) Modern configuration
ongoing uplift >20 cm/yr
Laguna del Maule
continued magma intrusion
promotes surface deformation
shallow
seismicity
?
?
crystal-poor melt
extraction?
Mafic magma from lower crust
mantle melt + crust melt
Fig. 17. Conceptual cross-sections of the structure and temporal evolution of the LdM magma system. The three panels
do not represent specific moments in time, but rather summarize important facets of the magma system during each eruptive
episode. The shallow LdM magma system comprises an extensive crystal-rich magma source zone that extends beneath
most, if not all, of the lake basin. Throughout post-glacial time,
mafic magmas ascending from deeper in the crust are intercepted, providing a source of mass, heat, and volatiles preventing the system from cooling to the granite eutectic.
Hybridization and crystallization yield isotopically homogeneous rhyolite (Fig. 14) that is segregated into eruptible, crystal-poor bodies that fed the post-glacial rhyolite eruptions. (a)
During the EPG, the abundant eruption of mafic and mafic inclusion-bearing rhyodacite lavas in the NW suggests that the
mushy rhyolite source zone thins compared with the southeastern basin where similar products are not observed. The
highly consistent trace element compositions of rhyolites
erupted in the north and south suggest, but do not require, that
the erupted reservoir was integrated throughout the LdM
basin. (b) During the latest Pleistocene to Holocene, less common mafic eruptions suggest that growth of the northern
magma system increased its capability to intercept ascending
The continuing inflation at LdM is interpreted as a response to magma emplacement in the shallow crust
(Feigl et al., 2014; Le Mével et al., 2016; Miller et al.,
2016). The uplift of the southern lake highstand paleoshoreline of >60 m implies repeated similar deformation episodes throughout the Holocene, consistent
with the emplacement of a significant volume of
magma into the shallow crust (Singer et al., 2015).
Zircon crystallization ages suggest that the 600 km3,
rhyolitic Bishop Tuff magma body accumulated at a
rate of 75 km3 ka–1 for 80 kyr prior to its eruption
(Chamberlain et al., 2014b). At LdM, if the rate of volume addition modeled to explain the modern uplift is
taken as the growth rate of the silicic magma system
and the average length of a deformation episode is
taken to be 10 years, the integrated volume increase
would be 05 km3 per inflation episode (Le Mével et al.,
2016). The physical significance of magma emplacement rates derived from zircon crystallization intervals
is a matter of debate. At a minimum, they probably represent an average of many punctuated, high-flux periods rather than protracted steady-state mass addition.
To achieve a long-term average flux at LdM of similar
magnitude would require 15 magmatic episodes, similar to the one occurring today, every thousand years.
Frequent shallow intrusion of magma at LdM, with an
average recurrence interval of decades to centuries, is
consistent with the repeated eruption of rhyolite since
the last glacial maximum and the dramatic deformation
of the highstand paleoshoreline during the last 95 kyr.
Wilson & Charlier (2009) suggested that long zircon
crystallization histories record inheritance of antecrysts
during the growth of magmatic mush and not the accumulation of eruptible magma. Rates of melt extraction
leading up to rhyolite eruptions can reach several km3
a–1 as inferred for the Oruanui and a post-caldera eruption at Taupo (Allan et al., 2013; Barker et al., 2016). The
rate of volume addition inferred from geodesy at LdM is
not of this remarkable magnitude, but is similar to the
more modest rates of rhyolite extraction of other
Holocene Taupo rhyolites (Barker et al., 2016). Thus,
whereas the rate of uplift today at LdM is globally remarkable (Le Mével et al., 2015), the potential rates of
magma. The lack of Holocene rhyolite eruptions in northern
LdM suggests that the segregation of melt was limited to the
southern basin. (c) The focus of magma intrusion may have
migrated during the Holocene as the modern inflation center is
NW of the most productive Holocene rhyolite center,
Barrancas, and the areas of maximum shoreline deformation
(Singer et al., 2015). The continuing crustal deformation and
shallow seismicity concentrated near the rcb and rln rhyolites
(Feigl et al., 2014; Le Mével et al., 2015; Singer et al., 2015) reflects magma intrusion and the movement of melt and fluid,
consistent with the magmatic processes inferred throughout
post-glacial times. Consequently, the future segregation of
eruptible, crystal-poor rhyolite appears likely. However, that
such a magma body currently exists, and if so, its extent and
volume, is the subject of a continuing geophysics investigation
(Singer et al., 2014).
Journal of Petrology, 2017, Vol. 58, No. 1
long-term reservoir growth and continuing melt extraction are comparable with those inferred beneath productive rhyolitic systems that produced calderas
elsewhere.
Mixing between existing reservoirs and intruding
magma has been found to precede silicic eruptions by
as little as weeks to years (e.g. Druitt et al., 2012; Till
et al., 2015; Singer et al., 2016). The duration of extraordinary inflation at LdM has already exceeded these
shortest temporal estimates. Volcanic inflation episodes
usually conclude without eruption, and the most recent
geodetic observations suggest that the rate of uplift at
LdM is beginning to decrease (Le Mével et al., 2015).
There is no evidence to suggest that the current unrest
is anything but a continuation of the longer-term processes operating at LdM that produced significant deformation of the highstand paleoshoreline during the
Holocene and frequent eruptions since 26 ka. Future
rhyolite eruptions are likely; however, that an eruption
is imminent is not at all clear. Whether these future
events will continue to be of modest volume or if the
system is building towards a larger eruption remains an
open question and is the subject of continuing geophysical surveys and numerical modeling investigations
(Singer et al., 2014).
SUMMARY AND CONCLUSIONS
The post-glacial concentration of rhyolite volcanism at
LdM is fundamentally the product of magmatism
throughout the thickness of the crust little different
from that inferred at SVZ frontal arc volcanoes.
Mantle-derived basalt mixes with two lower crustal
components prior to shallow emplacement. Whereas
the eruptive expression of this magmatism along the
frontal arc is dominantly mafic to intermediate in the
SVZ, rear-arc systems such as LdM yield more silicic
compositions—possibly catalyzed by regional back-arc
extension. Upon ascending to the upper crust this
mafic magma mingles and mixes with pre-existing
silicic material followed by fractional crystallization
yielding the rhyolitic compositions. The combination
of self-assimilation and the plagioclase þ quartz-saturated silicic magma buffers the modestly evolved,
broadly homogeneous compositions.
The rhyolite eruptions are clustered in two pulses,
both of which produced activity throughout the LdM
basin. Distinct trace element compositions and twooxide temperatures indicate that at least two crystalpoor magma bodies were extracted during post-glacial
times. The temporal correlation of increasing temperature and enrichment in trace elements compatible in
titanite and amphibole suggests that melt extraction is
catalyzed by mafic recharge that promotes resorption
of accessory and hydrous phases within a crystal-rich
reservoir.
The petrological model supported by the geochemical data is consistent with shallow magmatism that
probably produced episodes similar to the continuing
109
unrest throughout post-glacial time. Extrapolating the
volume change estimated for the modern inflation
yields a rate of mass addition consistent with that which
produced the rhyolitic Long Valley caldera-forming
eruption. However, that this unrest is foretelling either a
future caldera-forming event at LdM or an imminent
eruption of any particular style is not clear.
SUPPLEMENTARY DATA
Supplementary data for this paper are available at
Journal of Petrology online.
ACKNOWLEDGEMENTS
Wes Hildreth generously contributed samples and has
been a source of insight since the outset of this project.
Luis Torres Jara is thanked for invaluable guidance and
logistical support for navigating the Laguna. Meagan
Ankney, Allison Wende, and John Fournelle are
thanked for analytical assistance in obtaining the radiogenic isotope and electron microprobe data. Amanda
Houts is thanked for laboratory assistance with chlorine
extraction. Robert Finkel and Susan Zimmerman are
thanked for careful 36Cl accelerator mass spectrometry
measurements and data reduction at CMAS-LLNL. This
work greatly benefited from many fruitful discussions
with Hélène Le Mével, Judy Fierstein, Paty Sruoga, Wes
Hildreth, and the LdM research group. Simon Barker,
Jörn-Frederik Wotzlaw, and Chad Deering are thanked
for insightful reviews, and Gerhard Wöerner for editorial handling. This research is supported by the US NSF
(EAR-1322595, EAR-1411779 to B.S.S.), the Geological
Society of America (9791-12, 10016-13 to N.L.A.), the
Wisconsin Alumni Research Foundation (WARF), and
University of Wisconsin Department of Geoscience gift
funds.
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