JOURNAL OF PETROLOGY
VOLUME 52
NUMBER 12
PAGES 2335^2363
2011
doi:10.1093/petrology/egr047
Tracking Open-system Differentiation during
Growth of Santa Mar|¤ a Volcano, Guatemala
BRAD S. SINGER1*, KATY E. SMITH1, BRIAN R. JICHA1,
BRIAN L. BEARD1, CLARK M. JOHNSON1 AND NICK W. ROGERS2
1
DEPARTMENT OF GEOSCIENCE, UNIVERSITY OF WISCONSIN, MADISON, WI 53706, USA
2
DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCE, OPEN UNIVERSITY, MILTON KEYNES MK7 6A, UK
RECEIVED JANUARY 25, 2011; ACCEPTED SEPTEMBER 14, 2011
Prior to the AD 1902 Plinian eruption of 8 km3 of dacite and subsequent growth of the 41km3 Santiaguito dacite dome complex,
Santa Mar|¤ a volcano grew into an 8 km3 composite cone over 75
kyr in four phases (at 103^72, 72, 60^46, and 35^25 ka). The
1902 eruption occurred after an 25 kyr period of repose in growth
of the composite cone.To provide context for processes that ultimately
led to the 1902 eruption, we present geochemical and isotopic (Sr,
Nd, Pb, U-series) data from lavas of the composite cone for which
ages are constrained by 40Ar/39Ar dating. The four cone-building
phases comprise basaltic to basaltic-andesite lava (51·4^56·1%
SiO2) whose major- and trace-element compositions suggest that
crystallization was important in differentiation. Relative to other
Central American arc volcanoes, these lavas also have large 238U
excesses and high 207Pb/204Pb ratios that imply melting of a mantle
wedge modified to an unusual extent by fluid from subducted crust
and sediment of the Cocos plate. Major- and trace-element and isotopic variations over time imply that mafic recharge and magma
mixing were prevalent during early phases of cone-building, whereas
assimilation processes were more dominant during the latest stage
of cone growth. Indeed, some early erupted basalts have lower
143
Nd/144Nd and higher 87Sr/86Sr ratios than more SiO2-rich basaltic andesites that erupted during the final phase of cone-building.
These features point to an assimilant that is not typical continental
crust and instead may be more like mid-ocean ridge basalt with respect to major- and trace-element composition and Sr, Nd, Pb, and
U^Th isotope ratios. Energy-constrained modeling of a parental
basalt that undergoes crystal fractionation, assimilation and periodic
recharge with basalt in the lower crust can reproduce lava compositions erupted during phases I^III and the early part of phase IV.
Modeling further indicates that assimilation within the lower crust
of partially melted garnet-amphibolite metabasalt, without basaltic
recharge, may produce the youngest cone-forming lavas in phase IV.
*Corresponding author. E-mail: [email protected]
These models link the 8 km3 of cone growth over 75 kyr to the mass
flux of magma into the crust. Our findings suggest an along-arc
magma flux into the lower crust beneath Santa Mar|¤ a of
420 km3 km1 Myr1, which is higher than anticipated in recent
numerical^thermal approaches to basalt^crust interaction.
Consequently, the thermal incubation period needed to produce
hybrid basaltic-andesite magma may be only a few tens of thousand
years.
KEY WORDS: Guatemala; Santa Mar|¤ a volcano; isotopes; geochronology; basaltic andesite
I N T RO D U C T I O N
Identifying source components involved in magma genesis
beneath active arc volcanoes and the processes that
modify magma composition en route to the surface
remain challenging, yet they are critical to understanding
geochemical cycling, crustal evolution, and volcanic hazards. Geochemical and isotopic observations of along- and
across-arc volcano transects (e.g. Hildreth & Moorbath,
1988; Carr et al., 1990, 2003, 2007b; Ryan et al., 1995;
Kelemen et al., 2003; Singer et al., 2007), intensive studies
of well-dated single volcanic centers (e.g. Hildreth &
Lanphere, 1994; Singer et al., 1997; Jicha & Singer, 2006;
Escobar-Wolf et al., 2010), development of geochemical and
numerical models of deep crustal processes (e.g. Hildreth
& Moorbath, 1988; Dufek & Bergantz, 2005; Annen et al.,
2006), and petrological models of magma ascent-driven
crystallization (Blundy & Cashman, 2005; Blundy et al.,
2006) have each sharpened the focus of current research
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JOURNAL OF PETROLOGY
VOLUME 52
on subduction zone magmatism. A major question is how
much of the geochemical and isotopic composition of
arc lavas reflects protracted storage, assimilation, and
mixing in so-called deep crustal ‘hot zones’ (Hildreth &
Moorbath, 1988; Annen et al., 2006), and what fraction reflects modification at shallower depths as magma ascends
and crystallizes relatively rapidly on its way to the surface
(Blundy et al., 2006).
The Santa Mar|¤ a^Santiaguito volcanic complex,
Guatemala, is a relatively simple volcanic center whose
history includes a long period of basaltic to basalticandesitic cone growth, followed by repose for several thousand years, and explosive eruption of dacite in 1902 (Rose,
1987a). It is an ideal system in which to link the record of
historical eruptions to a 40Ar/39Ar-based chemostratigraphy of earlier cone-building (Escobar-Wolf et al., 2010).
We combine the temporal constraints from 40Ar/39Ar
dating with new whole-rock major- and trace-element geochemical data, as well as Sr, Pb, Nd, and U^Th isotopes,
to investigate the long-term evolution of magma over the
lifetime of the volcanic system. The present study focuses
on the magmatic evolution recorded by the cone-forming
lavas of Santa Mar|¤ a volcano. A subsequent paper (Singer
et al., 2011, and in preparation) focuses on the origin of the
1902 dacite.
Tectonic, geological, and petrological
setting
Subduction of the Cocos plate at 7^8 cm year1 beneath
the Caribbean plate has generated the Central American
Volcanic Arc (CAVA) between Guatemala and Costa
Rica. Santa Mar|¤ a, near the western end of the CAVA, is
one of 39 active frontal-arc volcanic centers (Fig. 1).
Crustal thickness beneath the arc varies from 440 km at
its northwestern (Guatemala) and southeastern (Costa
Rica) ends, to 30 km beneath its center in Nicaragua
(Carr, 1984; Carr et al., 2003, 2007a). Santa Mar|¤ a volcano
sits atop the Chortis block, which comprises several terranes that range from Paleozoic continental crust of the
Central Chortis Terrane to Mesozoic oceanic crust of
the Southern Chortis Terrane, and Cretaceous ophiolites,
that crop out along the Motagua fault (Fig. 1; Beccaluva
et al., 1995; Hoernle et al., 2002; Rogers 2007).
Overlying these rocks are undivided volcanic rocks of late
Cenozoic age, including deeply eroded remnants of a
Plio-Pleistocene volcanic edifice.
CAVA lavas define systematic arc-wide geochemical
patterns; for example, Ba/La ratios are at a maximum in
Nicaragua and are lowest in Guatemala and Costa Rica,
whereas La/Yb ratios define the opposite trend (Carr
et al., 1990, 2007a; Patino et al., 2000). B/La ratios and
(238U/230Th) activity ratios are positively correlated with
Ba/La and are inferred to reflect an increase in slab or
sediment flux to the mantle wedge in the center of the arc
(Leeman et al., 1994; Ru«pke et al., 2002; Walker et al., 2007;
NUMBER 12
DECEMBER 2011
Jicha et al., 2010). The majority of geochemical data within
the CAVA are from lavas erupted in the central and
southern parts of the arc, and relatively few are from
Guatemala. The thicker, more lithologically diverse crust
in Guatemala allows for a potentially more complex petrological evolution of magma (e.g. Halsor & Rose, 1991),
and probably plays a critical role in the large explosive
eruptions of dacite and rhyolite at volcanoes including
Santa Mar|¤ a, Cerro Quemado, and the Atitla¤n Caldera
(Fig. 1). Despite the thick crust beneath Santa Mar|¤ a volcano, Jicha et al. (2010) used Sr and U-series isotope data
to show that the flux of slab fluid into the mantle below
Guatemala is as great as or greater than in other parts of
the CAVA where crust is relatively thin (e.g. Nicaragua),
a finding that contrasts with earlier models (Patino et al.,
2000; Carr et al., 2003). Moreover, Jicha et al. (2010)
inferred that the crust beneath Santa Mar|¤ a may include
mid-ocean ridge basalt (MORB) and is not limited to
metamorphic and granitic rocks, suggesting that the
Chortis block may also contain deeply buried fragments
of oceanic crust.
Evolution of Santa Mar|¤ a^Santiaguito
volcano
40
Ar/39Ar dating of 15 lava flows, together with paleomagnetic data and stratigraphic observations of subtle unconformities, acquired by Escobar-Wolf et al. (2010), indicates
that Santa Mar|¤ a volcano, an 8 km3 mafic composite cone
(Fig. 2), grew episodically over a c. 75 kyr interval via four
eruptive phases at 103^72 ka (phase I), 72 ka (phase II),
60^46 ka (phase III), and 35 ka (phase IV), at an average
rate of 0·12 km3 kyr1 (Rose, 1987 a). There is no geological
or historical record of eruptive activity at Santa Mar|¤ a following the last significant episode of cone building at
about 35 ka, indicating that a 35 kyr period of repose preceded the great eruption of 1902 (Escobar-Wolf et al., 2010).
On 25 October 1902, 8 km3 (Dense Rock Equivalent) of
pumice and ash was erupted in a Plinian column from
Santa Mar|¤ a during the second largest eruption of the
twentieth century, killing an estimated 8750 people
(Williams & Self, 1982; Witham, 2005). The eruptive products consist of 95% dacitic pumice lapilli, blocks, and ash,
followed by 5% alternating mafic scoria, lapilli and ash,
as well as lithic fragments (mainly diorites) scattered
throughout the deposit. This eruption did not occur from
the central summit vent, but rather from the SW flank of
the cone, where it created an explosion crater that
removed 0·5 km3 of material exposing in its walls most of
the cone-building lava sequence. In 1922, a vent SW of the
1902 crater began to erupt dacitic lava and ash, forming
the Santiaguito dome complex, which is now 41km3 and
continues to be active today (Rose, 1972, 1987b; Harris
et al., 2003; Escobar-Wolf et al., 2010).
The Santa Mar|¤ a^Santiaguito system is strongly bimodal wherein the early erupted, cone-forming lavas and
2336
BASALTIC ANDESITE AT SANTA MARI¤A
SINGER et al.
90° 45’
91° 30’
14° 45’
Cerro
Siete
Quemado
Chicabal OrejasQuetz.
City
Tzanjuyub
San
Pedro
Atitlán
Caldera
Santa María
E
LIZ
BE
Maya
Block
GUATEMALA
North American
Plate
lt
au
aF
u
Caribbean
tag
Mo
Plate
Chortis
Block
Santa
María
Amatitlán
Caldera
Atitlán &
Tolimán
HONDURAS
15°
Guatemala
City
N
85°
90°
MEXICO
EL S
ALV
ADO
R
Agua
Fuego &
Acatenango
NICARAGUA
Pacaya
DSDP 495
0
?
Chorotega
Block
Cocos Plate
10°
0
50 km
200 km
COSTA
RICA
Fig. 1. Digital elevation model and map of western Guatemala adapted from Escobar-Wolf et al. (2010). Quetz., Quetzaltenango. Inset: tectonic
setting of Santa Mar|¤ a volcano modified from Carr et al. (2007a) and Geldmacher et al. (2008).
pyroclastic flows are basaltic andesite, and the historically
erupted tephra and lava are primarily dacitic (Rose et al.,
1977; Escobar-Wolf et al., 2010). Petrological studies by
Rose (1972) and Rose et al. (1977) proposed that the late
cone lavas and the subsequent 1902 and post-1902 dacites
could be formed by fractional crystallization of a mafic
parent magma. Rose (1987a) later suggested that the
Santa Mar|¤ a lavas began as mantle-derived basalt that
evolved toward basaltic-andesite composition in a crustal
magma body, where inputs of crustal melts increased as
the cone grew larger and the lithostatic load on the crust
rose, thereby inhibiting eruptions and trapping magma in
the deep crust. More recently, Escobar-Wolf et al. (2010)
used 40Ar/39Ar geochronology, paleomagnetic directions,
and unconformities in the stratigraphy to document the
episodic nature of cone growth at Santa Mar|¤ a. These
data, combined with major- and trace-element compositions, allowed Escobar-Wolf et al. (2010) to correlate eruptive episodes to magma compositional changes.
Building upon the 40Ar/39Ar-based temporal framework
established by Escobar-Wolf et al. (2010), this study focuses
on evolution of basaltic to basaltic-andesite lavas that
make up the four phases of the main cone growth.
We present seven new 40Ar/39Ar age determinations, new
major- and trace-element compositions of 27 whole-rock
samples from the cone lavas, as well as Pb, and Nd isotope
compositions of samples from all four phases of cone building. The Sr and U^Th isotope data for these samples were
presented by Jicha et al. (2010) and are used here alongside
new data to explore the temporal evolution of this system.
We present also analyses, including Sr, Nd, Pb, and U^Th
isotopes, of three diorite fragments collected from the
1902 airfall deposit to aid in characterizing the basement
under the volcano. Using this combination of 40Ar/39Ar
geochronology, geochemical, and isotopic data, together
with modeling via the EC-RAxFC (energy constrainedrecharge, assimilation, fractional crystallization) algorithm (Bohrson & Spera, 2007), we aim to quantify
open-system processes and test whether the model of a
deep-crustal hot zone, fueled by repeated injection of
basalt (e.g. Hildreth & Moorbath, 1988; Dufek &
Bergantz, 2005; Annen et al., 2006) is appropriate for the
formation of volatile-rich dacite that erupted explosively
after several thousands of years of repose. Specifically,
given constraints on the volumes and rates at which lava
flows were added to the Santa Mar|¤ a cone, the
2337
Fig. 2. Geological map of Santa Mar|¤ a^Santiaguito volcanic system, modified from Escobar-Wolf et al. (2010). 40Ar/39Ar-dated samples are labeled with sample numbers and ages. Cone lava
(labeled within 1902 eruption crater) ages are weighted averages of several lavas forming each eruptive phase. Not all sample sites are shown (blue diamonds).
JOURNAL OF PETROLOGY
VOLUME 52
2338
NUMBER 12
DECEMBER 2011
SINGER et al.
BASALTIC ANDESITE AT SANTA MARI¤A
EC-RAxFC models can provide estimates of the amount
and rate of magma emplacement into the crust and heat
outputs that helped drive magmatic differentiation.
Accordingly, major- and trace-element, and isotopic analyses for 24 samples from 1902 eruptive products and
10 samples from the Santiaguito dome complex were
undertaken as part of a parallel study of the historical
eruptions; these data are used graphically in the present
study, and are the focus of a subsequent paper (Singer
et al., 2011 and in preparation).
S A M P L E S A N D A N A LY T I C A L
M ET HODS
Whole-rock geochemistry
Samples on which this study is based include the 103 ka
phase I lava on the north flank of the cone, and 24 samples
from a stratigraphic sequence of lava flows exposed in the
NW wall of the 1902 eruption crater; these 24 samples
were collected together with those of Escobar-Wolf et al.
(2010), but the whole-rock powders used for geochemical
analysis are not identical. In addition, five samples are
from lava flows on the SE flank of the cone, three are
from Volca¤n del Valle (Fig. 2) and three are diorite blocks
from the 1902 airfall deposit. Whole-rocks (500 g) were
crushed (tungsten carbide piston crusher), powdered (aluminum oxide ceramic shatterbox and puck), and analyzed
for major (X-ray fluorescence) and trace elements (inductively coupled plasma mass spectrometry, ICP-MS) at
the Open University, UK following the methods of
Rogers et al. (2006); the data are reported in Table 1 and
Supplementary Data Electronic Appendix 1 (available for
dowloading at http://www.petrology.oxfordjournals.org).
Further details on analytical procedures, including data
from standard rocks, precision, and accuracy, are reported
in Supplementary Data Electronic Appendices 2 and 3.
40
Ar/39Ar age determinations
Escobar-Wolf et al. (2010) used the 40Ar/39Ar incremental
heating method to date 15 lava flows, including a single
lava of phase I, and three packages of lava flows forming
phases II, III, and IV that crop out in the 1902 crater
wall. Using procedures identical to Escobar-Wolf et al.
(2010), 40Ar/39Ar ages were determined for five lavas on
the southern flank of SM, as well as two from Volca¤n del
Valle (map units Qvv, Qsmf, and Qta, Fig. 2) at the
University of Wisconsin (UW)-Madison Rare Gas
Geochronology Laboratory. Groundmass separated from
each sample was packed into copper foil, loaded into aluminum disks, and irradiated adjacent to 28·02 Ma Fish
Canyon sanidine (Renne et al., 1998). Furnace incremental
heating experiments, including replicates for three of the
seven samples, employed an automated gas extraction
system linked to a MAP 215-50 mass spectrometer that
provided data used to calculate a weighted mean plateau
age and associated 2s analytical uncertainty for each
sample using procedures and plateau age criteria of
Singer et al. (2008). Isochron ages agree with plateau ages
and do not indicate evidence that excess argon is present
in any of the lavas; therefore, we consider the plateau ages
to give the most precise estimate of the time elapsed since
eruption (e.g. Singer et al., 2008).
Pb, Sr, and Nd isotopes
A representative subset of samples from the four phases of
cone growth were analyzed for 87Sr/86Sr (16 samples),
208
Pb/204Pb, 207Pb/204Pb, 206Pb/204Pb (14 samples), and
143
Nd/144Nd (16 samples) at the UW-Madison Radiogenic
Isotope Laboratory via thermal ionization mass spectrometry (TIMS, VG Sector 54 instrument). Unspiked samples
(100 mg) were digested in HF, HNO3, and HCl acids
in Teflon beakers. Lead fractions were separated via
anion-exchange columns in HBr and HCl. Sr and Nd fractions were separated via cation-exchange columns using
HCl for Sr, and HCl and a-HIBA for Nd. The same aliquot
of sample was used for all three separations for Pb, Sr,
and Nd. Procedural blanks averaged 104 pg for common
Pb, 509 pg for Sr, and 137 pg for Nd, all of which are
negligible.
Lead isotope ratios were measured by static multicollector TIMS using single Re filaments and silica gel
and the reported Pb isotope ratios and errors are based
on 50, 5 s integrations with a 208Pb ion signal of 2 1011
A. Instrumental mass bias was corrected empirically
based on Pb isotope analysis of NIST SRM-981 (n ¼10)
and SRM-982 (n ¼11) run under similar conditions to
those for samples. The pooled average mass fraction
correction based on the measured 207Pb/206Pb analysis
of SRM-981 and the measured 208Pb/206Pb ratio of
SRM-982 was 0·148 0·045% per a.m.u. Nine samples
were analyzed twice and the reproducibility of these samples was within the analytical uncertainty of the empirically determined mass fractionation factor. Strontium isotope
ratios were measured using a multi-collector dynamic
TIMS analysis of samples that had been loaded on Ta filaments with H3PO4. Strontium isotope ratios and errors
are based on 120, 5 s integrations with an 88Sr ion signal of
3 1011 A, using internal normalization for exponential
mass fractionation and an 86Sr/88Sr ratio of 0·1194. Repeat
analysis of NIST SRM-981 yielded an average 87Sr/86Sr
ratio of 0·710267 0·000018 (2SD; n ¼ 30). Duplicate samples of the USGS rock standard BCR-1 were processed
through the entire analytical procedure and the measured
87
Sr/86Sr ratios were 0·705021 0·000011 and 0·705028 0·000010 (2SD). Neodymium was analyzed as NdOþ
using a multi-collector dynamic TIMS analysis, where samples were loaded on Re filaments with silica-gel and
H3PO4, and an O2 gas bleed to produce a source pressure
of 5 107 mbar. Mass analysis was done with a mass 160
2339
JOURNAL OF PETROLOGY
VOLUME 52
NUMBER 12
DECEMBER 2011
Table 1: Major- and trace-element and Sr, Nd, Pb isotope data for selected Santa Mar|¤ a lavas
Santa Mara cone lavas
Sample:
SM-07-07
SM-05-01
SM-05-02
SM-05-04
SM-05-05
SM-05-06
SM-05-07
SM-05-09
SM-05-13
Unit name:
SM flank
SM cone
SM cone
SM cone
SM cone
SM cone
SM cone
SM cone
SM cone
SM cone
Erupt. seq.:
1
2
3
5
6
7
8
10
14
16
Age (ka):*
103
75
71
75
71·3
60·2
53·9
46
37·8
35·4
SiO2 (wt %)
SM-05-15
53·97
52·10
51·55
51·43
52·88
52·89
52·70
53·39
52·57
TiO2
0·89
1·03
0·99
0·98
0·88
0·87
0·88
0·89
0·90
54·05
0·82
Al2O3
18·32
18·95
19·01
18·92 s
18·83
18·71
18·67
18·57
18·70
18·58
Fe2O3
8·92
9·33
9·38
9·34
9·27
9·29
9·27
9·19
9·32
8·71
MnO
0·15
0·15
0·15
0·15
0·14
0·15
0·15
0·15
0·14
0·14
MgO
4·72
4·85
5·31
5·32
5·32
5·43
5·39
5·42
5·58
4·67
CaO
7·97
9·10
9·52
9·42
8·29
8·38
8·26
8·10
8·49
7·67
Na2O
3·75
3·69
3·62
3·51
3·72
3·73
3·70
3·79
3·69
3·97
K2O
1·19
0·93
0·84
0·82
0·90
0·89
0·90
0·96
0·88
1·07
P2O5
0·23
0·24
0·23
0·22
0·23
0·22
0·22
0·23
0·21
0·25
LOI
0·07
0·03
0·25
0·21
0·34
0·25
0·07
0·32
0·33
0·19
100·18
100·38
100·36
99·89
100·12
100·31
100·21
100·37
100·16
99·75
33·6
47·8
52·1
51·6
37·5
40·6
34·3
42·8
51·7
49·0
Total
Cr (ppm)
Mn
1257
1200
1222
1185
1154
1183
1184
1188
1150
1144
Ni
23·1
19·4
23·2
22·6
31·7
33·2
34·3
36·1
37·9
29·7
Rb
22·5
13·5
12·5
12·3
13·0
13·3
13·7
13·5
12·9
13·6
Sr
570
Y
22·6
Zr
124
Nb
Ba
3·46
571
653
19·7
101
4·25
510
662
638
19·0
18·1
93
90
3·99
481
4·77
459
563
17·9
103
2·81
466
570
18·7
104
2·81
457
569
18·5
105
2·83
465
564
18·2
111
3·00
503
566
572
18·4
17·9
99
123
2·79
3·14
456
541
La
11·31
10·24
9·63
9·32
8·33
8·40
8·46
8·58
8·58
9·02
Ce
24·90
22·89
22·15
21·39
19·36
19·35
19·59
20·00
19·76
21·37
13·99
Nd
15·60
14·45
13·66
13·27
13·01
13·00
13·08
13·29
13·27
Eu
1·13
1·09
1·05
1·02
1·01
1·01
1·02
1·03
1·03
1·04
Yb
1·99
1·75
1·68
1·61
1·55
1·58
1·60
1·61
1·56
1·60
Lu
0·30
0·26
0·25
0·24
0·23
0·24
0·24
0·24
0·24
0·24
Hf
3·13
2·51
2·33
2·32
2·65
2·64
2·65
2·76
2·56
3·07
Pb
7·23
5·63
5·12
4·88
5·00
5·75
5·33
5·93
4·30
6·64
Th
2·16
1·16
1·09
1·05
0·91
0·93
0·94
0·98
0·92
1·13
0·40
U
0·86
0·49
0·46
0·45
0·41
0·41
0·44
0·40
0·50
206
18·677
18·652
18·668
18·672
18·701
18·709
18·696
18·703
18·709
207
15·589
15·587
15·606
15·602
15·604
15·613
15·598
15·620
15·608
208
38·420
38·381
38·461
38·449
38·483
38·516
38·463
38·540
38·505
Pb/204Pb
Pb/204Pb
Pb/204Pb
87
Sr/86Sr
0·70395
0·70402
0·70400
0·70403
0·70394
0·70401
0·70394
0·70394
0·70393
0·70392
2s
0·000010
0·000011
0·000020
0·000008
0·000008
0·000013
0·000010
0·000010
0·000011
0·000020
143
0·512918
0·512860
0·512863
0·512863
0·512916
0·512890
0·512907
0·512903
0·512896
0·512900
2s
0·000008
0·000008
0·000009
0·000009
0·000008
0·000010
0·000007
0·000009
0·000009
0·000009
Nd/144Nd
(continued)
2340
BASALTIC ANDESITE AT SANTA MARI¤A
SINGER et al.
Table 1: Continued
Santa Mara cone lavas
Other volcanoes
Sample:
SM-05-20
SM-05-22
SM-07-01
SM-09-11
SM-09-10
SM-09-17
SM-07-08
SM-09-05
SM-09-06
SM-09-12
Unit name:
SM cone
SM cone
SM cone
SM flank
SM flank
SM flank
V. Valle
V. Valle
V. Valle
Qta
Erupt. seq.:
20
22
24
25
26
27
Age (ka)*
33·1
36·5
38
31·1y
29·3y
24·6y
163
388·9y
328·6y
441·1y
SiO2 (wt %)
54·12
54·13
54·74
54·78
55·86
56·14
53·02
62·80
64·36
TiO2
0·86
0·86
0·86
0·75
0·74
0·80
0·91
0·56
0·45
0·97
Al2O3
18·76
18·72
18·63
18·52
19·01
18·72
19·02
16·65
15·66
19·53
Fe2O3
8·73
8·79
8·63
8·37
8·09
7·97
8·97
5·24
4·18
10·03
MnO
0·15
0·15
0·15
0·15
0·15
0·14
0·15
0·10
0·09
0·16
MgO
4·39
4·29
3·85
4·32
3·64
3·89
4·54
2·02
1·64
4·62
CaO
7·88
7·79
7·51
7·46
7·27
7·15
8·68
4·73
3·82
8·88
Na2O
3·79
3·87
3·73
4·32
4·01
3·96
3·33
3·88
3·60
3·43
K2O
1·09
1·12
1·17
1·13
1·10
1·19
1·03
2·39
2·76
0·80
P2O5
0·22
0·23
0·24
0·23
0·24
0·23
0·20
0·14
0·13
0·20
LOI
0·11
0·12
0·79
0·23
0·12
0·07
0·41
0·50
2·54
0·03
Total
99·89
99·83
100·29
99·80
99·99
100·27
100·25
99·01
99·21
99·78
Cr (ppm)
18·7
18·3
16·5
13·7
10·1
7·5
32·2
4·0
4·1
13·8
Mn
1244
1208
1233
1168
1242
1121
1163
866
769
Ni
20·4
18·7
10·2
15·0
8·6
9·1
15·1
3·5
3·4
Rb
16·0
19·6
22·3
19·2
19·4
21·6
13·2
53·5
78·9
Sr
552
Y
19·4
Zr
122
Nb
Ba
3·29
611
560
20·8
123
3·37
575
555
22·4
125
3·41
567
561
19·4
131
3·34
543
524
21·3
129
3·23
531
524
18·9
146
3·67
552
541
17·2
102
2·96
498
377
16·7
147
4·71
842
335
16·4
133
4·62
854
51·19
1271
10·1
12·6
539
20·4
83
2·41
384
La
9·57
10·59
11·67
10·38
9·83
10·91
7·64
14·13
14·47
7·43
Ce
21·54
23·52
25·41
22·84
21·65
23·88
16·86
29·33
27·83
16·34
Nd
13·80
14·67
15·49
14·30
13·82
14·33
11·91
13·91
13·55
11·85
Eu
1·03
1·08
1·11
1·05
1·06
1·05
0·93
0·82
0·73
1·03
Yb
1·85
1·94
2·11
1·83
2·07
1·77
1·60
1·66
1·63
1·90
Lu
0·28
0·29
0·32
0·29
0·32
0·27
0·24
0·27
0·27
0·29
Hf
3·05
3·08
3·15
3·06
3·08
3·38
2·63
3·69
3·39
2·17
Pb
6·59
6·64
7·13
5·23
4·71
5·34
6·13
8·83
9·92
2·94
Th
1·49
1·66
2·49
1·17
1·10
1·19
1·21
4·52
4·94
0·86
U
0·65
0·68
0·97
0·47
0·50
0·49
0·55
1·52
1·89
0·37
206
18·677
18·722
18·664
18·700
18·709
18·691
207
15·578
15·619
15·591
15·599
15·592
15·600
208
38·396
38·543
38·421
38·460
38·448
38·471
Pb/204Pb
Pb/204Pb
Pb/204Pb
87
Sr/86Sr
0·70398
0·70399
0·70382
0·70386
0·70385
0·70397
0·70399
0·70396
0·70392
0·70395
2s
0·000008
0·000010
0·000010
0·000020
0·000020
0·000020
0·000010
0·000020
0·000023
0·000020
143
0·512912
0·512894
0·512902
0·512921
0·512926
0·512881
0·512864
0·512881
0·512865
0·512872
2s
0·000008
0·000009
0·000010
0·000009
0·000009
0·000007
0·000008
0·000008
0·000008
0·000009
Nd/144Nd
For complete major- and trace-element data for all lavas analyzed in this study, see Electronic Appendix 1.
*Ages measured via 40Ar/39Ar geochronology, see Escobar-Wolf et al. (2010) for complete age summary.
y
Ages measured via 40Ar/39Ar geochronology, this study, Table 2.
Abbreviations: Erupt Seq ¼ eruptive sequence relative to SM-07-07 as first-erupted cone lava, SM ¼ Santa Mara,
V. Valle ¼ Volcán del Valle, LOI ¼ loss on ignition.
All analyses are on whole-rock powders.
Errors for Sr and Nd isotope measurements are 2-SE based on in-run statistics. Errors for Pb isotope measurements are
largely controlled by uncertainty in the empirically determined fractionation factor which is 0.09%/amu.
2341
JOURNAL OF PETROLOGY
VOLUME 52
(144Nd16O) ion signal of 1 1011 A and is the average of
100, 5 s integrations. Correction for oxygen isotopes was
done using 18O/16O of 0·002110 and 17O/16O of 0·000387
and exponential normalization to a 146Nd/144Nd of 0·7129.
The measured 143Nd/144Nd for two in-house Nd standards
(Ames I and Ames II) are 0·512149 0·000014 (n ¼15;
2SD) and 0·511974 0·000016 (n ¼15; 2SD), respectively.
The measured 143Nd/144Nd for La Jolla Nd was
0·511844 0·000015 (n ¼ 6; 2SD). Duplicate samples of the
USGS rock standard BCR-1 were processed through the
entire analytical procedure and the measured 143Nd/144Nd
ratios were 0·512633 0·000019 and 0·512632 0·000007
(errors are 2SE in-run statistics).
U and Th isotopes
Fourteen of the 40Ar/39Ar-dated cone lavas that span the
four eruptive phases, as well as the range of major- and
trace-element and Sr^Nd^Pb isotope compositions, were
selected for U^Th analysis. A mixed tracer containing
229
Th and 235U was added to 100^350 mg of whole-rock
prior to digestion via HF, HNO3, and HCl acids in a combination of Teflon beakers and Parr bombs. Chemical separation of U and Th was achieved by passing the samples
through anion-exchange columns (1·2 ml, Teflon) using
BioRad AG1X8 200^400 mesh resin. The Th fraction was
then passed through a second set of columns (700 ml,
Teflon) to further enhance purification.
Solution-based U and Th isotope measurements were
obtained by multi-collector ICP-MS using a Micromass
IsoProbe system at UW-Madison’s Radiogenic Isotope
Laboratory following the procedures of Jicha et al. (2009).
Uranium cuts were dissolved in dilute HNO3 to a concentration of 30 ppb and aspirated using an Aridus
desolvation nebulizer system fitted with a self-aspirating,
concentric-flow nebulizer tip with an uptake rate of
45^60 ml min1. Uranium isotopes (238U, 235U) were measured on Faraday detectors with a total ion intensity of 6^
9 V (235U þ 238U); 30, 10 s integrations were measured for
each U cut. Thorium cuts and Th standard solutions were
aspirated similarly (diluted to 100 ppb), but were mixed
with a U-500 solution (238U/235U ¼1·0003) to monitor
instrumental mass fractionation while measuring Th isotopes. Faraday detectors were used for 229Th and 232Th,
and a Daly detector fitted with a WARP filter was used to
measure 230Th. Total ion intensity for all species measured
in the Th cut with U-500 was 20^50 V (230Th þ 232Th þ
235
U þ 238U); 40, 10 s integrations were measured for each
Th cut. Before and after analysis of each sample, the
IRMM-035 standard mixed with U-500 was used as a
thorium reference standard to monitor internal precision
throughout the analytical session and to determine, via interpolation, the Daly^Faraday gain for each sample measurement. The Daly^Faraday gain is the 232Th/230Th ratio
corrected for machine mass bias divided by the true
232
Th/230Th ratio. Solution and rock standards including
NUMBER 12
DECEMBER 2011
AThO, BCR-1, and AGV-1 were analyzed, along with variable concentration IRMM-035 and IRMM-036 solutions,
to monitor accuracy, reproducibility, and external precision (Supplementary Data: Electronic Appendix 4).
Additionally, we analyzed an in-house rock sample,
FB-819, which has a high U/Th concentration ratio (0·46)
and is 37·9 Ma; this sample plots on the equiline, confirming accuracy of the procedure. The mean 232Th/230Th
values for IRMM-035 (87799 1006; 2SD; n ¼15) and
IRMM-036 (326 300 2126; 2SD; n ¼ 8) are indistinguishable from consensus values (Sims et al., 2008).
R E S U LT S
40
Ar/39Ar geochronology
Of the five samples dated from the southern flank of Santa
Mar|¤ a (Fig. 2), an andesite and a basalt from the Qta map
unit of Fig. 2 gave ages of 441·1 14·7 and 424·6 6·3 ka
that are too old to be associated with the main cone
(Fig. 3, Table 2). Moreover, a high-SiO2 andesite from
Volca¤n del Valle (Rose, 1987b; unit Qvv, Fig. 2) yielded discordant spectra that suggest an age of 389 ka (Fig. 3).
These results, together with an age of 163 49 ka determined by Escobar-Wolf et al. (2010), expand the duration
of volcanic activity preserved beneath the Santa Mar|¤ a
cone. Thick forest cover and limited outcrop make it difficult to link the Qta lavas we have dated at older than
400 ka on the south flank with those of Volca¤n del Valle.
In contrast, three basaltic-andesite lava flows mapped as
unit Qsmf (Fig. 2) that were interpreted by Rose (1987b)
to have erupted from flank vents high on the southern
side of the main cone, yield 40Ar/39Ar plateau ages of
31·1 6·8, 29·3 8·0, and 24·6 6·1 ka, which are, on
average, slightly younger than the 35 2 ka mean age
determined by Escobar-Wolf et al. (2010) from six cone
lavas representing phase IV (Figs 2^4). The new
40
Ar/39Ar ages therefore suggest that the Qsmf flows represent the youngest portion of phase IV eruptive activity.
The major- and trace-element compositions of these
Qsmf flows are similar to those of other phase IV lavas
(Table 1, Supplementary Data Electronic Appendix 1),
and thus some phase IV eruptions took place as recently
as 25 ka. Therefore, the repose period between the culmination of cone-building and the AD 1902 dacite eruption
may have lasted about 25 kyr; that is, 10 kyr shorter than
proposed by Escobar-Wolf et al. (2010) on the basis of
dating only lavas exposed in the 1902 crater wall.
Major- and trace-element compositions
Santa Mar|¤ a cone lavas are calc-alkaline basalts and basaltic andesites (51·4^56·1wt % SiO2) that are ortho- and
clinopyroxene, olivine, plagioclase, and titanomagnetite
phyric. The 1902 eruption produced dacitic ash, lapilli,
and blocks of pumice (64·7^67·2 wt % SiO2), followed by
scoria, lapilli and ash (53·5^55·9% SiO2). Lithic fragments
2342
SINGER et al.
160
Age (ka)
Age (ka)
120
BASALTIC ANDESITE AT SANTA MARI¤A
SM-09-17
24.6 ± 6.1 ka
n = 14 of 14
240
180
80
120
40
60
0
0
180
120
1200
SM-09-10
29.3 ± 8.0 ka
n = 13 of 15
1000
800
60
600
0
400
SM-09-11
31.1 ± 6.8 ka
n = 13 of 15
SM-09-12
438.3 ± 14.6 ka
n = 8 of 8
200
0
Age (ka)
800
SM-09-13
422.0 ± 6.3 ka
n = 5 of 8
800
600
600
400
400
200
200
SM-09-05
388.9 ± 22.7 ka
n = 5 of 8
0 0 10 20 30 40 50 60 70 80 90 100 0 0 10 20 30 40 50 60 70 80 90 100
39
39
Cumulative Ar Released (%)
Cumulative Ar Released (%)
Fig. 3. 40Ar/39Ar age spectrum diagrams for six lava flows. Weighted mean plateau ages include 2s analytical uncertainties. n refers to the
number of heating steps included for final calculation of the plateau age, illustrated by the arrows.
including gabbro, diorite, granodiorite, granite, quartz
monzonite, and metamorphic rocks were also incorporated
into the 1902 deposit (Rose, 1987a). Lavas that we have
sampled from the Santiaguito dome complex are dacitic
(63·0^65·7 wt % SiO2). The major-element trends of
decreasing TiO2, Al2O3 Fe2O3, MgO, and CaO, and
increasing Na2O and K2O, with increasing SiO2 (Fig. 5)
are typical of calc-alkaline volcanoes, although Rose
(1987a) noted that Santa Mar|¤ a lavas have higher Na2O
than other CAVA lavas at a given SiO2 content. These
plots also illustrate the bimodality of the Santa Mar|¤ a^
Santiaguito system, where the 1902 eruption products and
Santiaguito dome lavas mainly have higher SiO2 contents
compared with the earlier cone-forming lavas. Our new
major-element data are consistent with those obtained by
Rose et al. (1977) and Escobar-Wolf et al. (2010), apart from
discrepancies in MgO content, which are minor enough
to be attributed to sample heterogeneity (our data are
from sample powders prepared independently from those
of previous studies).
Trace-element trends for Santa Mar|¤ a lavas are less uniform than major element trends; with increasing SiO2,
Rb, Zr, Hf, Nb, Th and U increase, whereas Sr, Ni, Sc, V,
and Cr decrease (Fig. 5). Most trends are non-monotonic,
which suggests the influence of other petrogenetic processes besides simple crystal fractionation. Nevertheless, as
with the major-element variation diagrams, the bimodality
of the system is well displayed in the trace-element variation. It should be noted that the composition of plutonic
fragments scatters both in and around the lava and tephra
compositions.
As cone-building took place, SiO2 broadly increased
slightly, with three periods marked by slight decreases
during phases II and III, and the early part of phase IV
(Fig. 6). MgO is relatively constant during phases I^III
but jumps to higher values during the early part of phase
IV and then declines during the waning part of phase IV
(Fig. 6). These patterns are consistent with the chemostratigraphic data of Rose et al. (1977). Ni exhibits a temporal
pattern similar to that of MgO with concentrations
2343
JOURNAL OF PETROLOGY
Table 2:
Sample
40
NUMBER 12
DECEMBER 2011
Ar/39Ar ages of seven lava flows
K/Ca
Total fusion
Temperature
total
age (ka) 2s
Range (8C)
Age spectrum
39
Ar %
SM-09-10
VOLUME 52
n
Age 2s (ka)
Isochron analysis
40
MSWD
Ar/36Ari 2s
Age (ka) 2s
MSWD
0·24
26·0 13·4
780–1225
97·49
25·6 11·1
0·45
6 of 7
296·6 3·8
19·80 23·0
0·48
0·24
34·2 13·8
720–1250
99·61
33·2 11·5
0·60
7 of 8
294·8 3·5
36·00 19·9
0·70
29·3 8·0
SM-09-11
0·25
31·2 12·7
780–1225
0·25
36·2 15·5
720–1250
100·0
97·51
30·9 8·6
0·44
7 of 7
296·8 3·7
25·50 17·4
0·42
31·4 11·1
0·32
6 of 8
295·0 4·3
33·20 20·4
0·38
31·1 6·8
SM-09-17
0·28
22·4 9·2
780–1225
100·0
21·8 7·6
0·33
7 of 7
295·2 7·4
22·00 15·5
0·40
0·27
31·6 11·8
800–1250
100·0
29·6 10·1
0·14
7 of 7
297·1 7·5
26·90 15·8
0·13
SM-09-12
0·19
445·3 18·1
720–1250
100·0
438·3 14·6
0·78
8 of 8
301·2 9·8
429·60 19·3
0·60
SM-09-13
1·30
430·8 6·7
720–1250
84·83
422·0 6·3
0·53
5 of 8
300·8 17·2
417·70 14·7
0·56
SM-09-05
0·003
308·7 29·4
720–1250
81·14
388·9 22·7
1·75
5 of 8
292·5 3·8
415·70 38·7
1·23
SM-09-06
0·06
429·4 33·2
720–1250
73·31
328·6 53·3
3·28
3 of 8
272·4 72·5
862·0 1676·1
4·73
24·6 6·1
Relative Eruptive Sequence
Ages calculated relative to Fish Canyon Sanidine (28·02 Ma, Renne et al., 1998) and using the decay constants of
Steiger & Jäger (1977). Ages in bold are preferred number of heating steps included in final age calculation; MSWD,
mean square of weighted deviates.
Phase IV 35-25 ka
Phase III 60-46 ka
Phase II 72 ka
Phase I 103-72 ka
South
flank
lavas
Cone
lavas
from
crater
wall
section
in stratigraphic
order
120
100
40
80
60
40
39
Ar/ Ar Age (ka)
20
0
Fig. 4. Plot of 40Ar/39Ar age vs eruptive sequence constrained mainly by the stratigraphy of the 1902 crater wall for Santa Mar|¤ a cone and flank
lavas. The four-phase history reflects stratigraphic breaks and paleomagnetic directions associated with the dated flows (see Escobar-Wolf
et al., 2010).
2344
BASALTIC ANDESITE AT SANTA MARI¤A
SINGER et al.
1922 - Present
Santiaguito Dome
Mafic inclusions
1902 Fall Deposit
Dacite pumice
Bas. And. scoria
Plutonic fragments
Cone-forming lavas
35-25 ka
60-46 ka
72 ka
103-72 ka
1.4
35
1.0
25
0.6
0.2
15
5
TiO2
Ni
8
160
6
140
120
4
100
2
80
MgO
20
18
16
14
12
10
8
10
8
6
4
2
Zr
Fe2O3
La
60
50
4.5
40
30
3.5
20
2.5
10
Na2O
2.2
3
1.8
2
Rb
1.4
1
1.0
Th
K2 O
50
55
60
65
50
55
60
65
SiO2 (wt. %)
SiO2 (wt. %)
Fig. 5. Major- and trace-element variation diagrams for lava and tephra from Santa Mar|¤ a (major-element oxides are in wt % and
trace-element concentrations are in ppm).
increasing modestly during the first three phases of cone
growth, then decreasing four-fold during the latter part of
phase IV (Fig. 6). Ratios of highly incompatible trace elements including K/Rb, U/Th, Th/Nb, and Zr/Hf also exhibit
small shifts during phases I^III and much larger variability during the latter portion of phase IV (Fig. 6).
The rare earth element (REE) patterns in Santa Mar|¤ a
cone lavas are remarkably parallel to one another
2345
JOURNAL OF PETROLOGY
VOLUME 52
NUMBER 12
DECEMBER 2011
present
Santiaguito
1922
dome
1902
SM
main
cone
3525 ka
6046 ka
72 ka
103 ka
SiO2 (wt %)
50
60
55
MgO (wt %)
1.2
65
2.0 2.8
3.6 4.4 5.2
6.0
present
Santiaguito
1922
dome
1902
SM
main
cone
3525 ka
6046 ka
72 ka
103 ka
Th (ppm)
Ni (ppm)
0
10
20
0.5
30
1.0
1.5
2.0
2.5
present
Santiaguito
1922
dome
1902
SM
main
cone
3525 ka
6046 ka
72 ka
103 ka
280
present
Santiaguito
1922
dome
1902
SM
main
cone
K/Rb
U/Th
380
480
580
680
0.25
0.30
0.35
0.40
0.45
3525 ka
6046 ka
72 ka
103 ka
0.2
Zr/Hf
Th/Nb
0.4
0.6
0.8 38
40
42
Fig. 6. Temporal evolution of selected major and trace elements. Data arranged in relative stratigraphic sequence, with
straints noted. Symbols as in Fig. 5.
(Supplementary Data Appendix Fig. 1a). La/Yb ratios of
cone lavas are 4·1^6·2, with most of this range defined by
the phase IV lavas, whereas with one exception the historical dacites have La/Yb ratios of 7·2^8·5 (Fig. 7). The REE
44
40
39
Ar/ Ar age con-
concentrations and La/Yb ratios of the cone-forming
lavas are within the range of other mafic lavas from
Guatemala (Feigenson & Carr, 1986; Carr et al., 1990,
2003, 2007b; Walker et al., 2007; Bolge et al., 2009).
2346
BASALTIC ANDESITE AT SANTA MARI¤A
700
0.8
600
0.6
500
0.4
Th/Nb
K/Rb
SINGER et al.
400
0.2
300
10
20
30
40
0.5
50
Rb (ppm)
1.0
1.5
2.0
2.5
3.0
Th (ppm)
44
43
42
6
41
Zr/Hf
La/Yb
8
40
4
39
5
10
15
La (ppm)
85
20
105
125
145
Zr (ppm)
165
38
Fig. 7. Plots of Rb (ppm) vs K/Rb, La (ppm) vs La/Yb, Th vs Th/Nb and Zr vs Zr/Hf. Symbols as in Fig. 5.
The cone and flank lavas have slight negative Eu anomalies (Eu/Eu* ¼ 0·89^0·98, Supplementary Data Appendix
Fig. 1a) that do not vary in a regular sense over time, but
show the greatest variability during phases II and IV, signaling slight fluctuations in the importance of plagioclase
fractionation.
Samples from Volca¤n del Valle have similar major- and
trace-element compositions and include a high-silica andesite, a dacite, and one sample (SM-07-08) that falls
within the geochemical range of the Santa Mar|¤ a cone
lavas. However, because Volca¤n del Valle is far older than
Santa Mar|¤ a volcano, data from these samples are not discussed further.
Sr, Pb, and Nd isotopes
Cone lavas have 87Sr/86Sr ratios between 0·70382 and
0·70403, and 143Nd/144Nd between 0·512863 and 0·512926
(Figs 8 and 9) that are typical of those measured at other
Guatemalan frontal arc volcanic centers (Supplementary
Data Appendix Fig. 2). Although the correlations are not
strong, early erupted lavas with the lowest SiO2 and highest MgO contents have the highest 87Sr/86Sr ratios and
lowest 143Nd/144Nd ratios, whereas a few of the last-erupted
lavas have the highest SiO2 and lowest MgO contents, yet
these have the lowest 87Sr/86Sr and highest 143Nd/144Nd
ratios (Figs 8 and 9). During the first three phases of cone
growth, 87Sr/86Sr varies little, but phase IV lavas show a
two-fold greater spread in 87Sr/86Sr values that includes
the least radiogenic compositions (Fig. 9). Although there
is significant spread in 143Nd/144Nd during phases III, III,
and IV, the mean value of each phase increased as the
cone grew (Fig. 9).
The ratios of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb
in the cone lavas are in the range of 18·65^18·72, 15·59^
15·62, and 38·38^38·54, respectively (Table 1), and these
overlap the values for the only two frontal arc lavas from
Guatemala for which Pb isotope data are available
(Feigenson et al., 2004). The 207Pb/204Pb and 208Pb/204Pb
ratios of the Santa Mar|¤ a cone lavas plot well above the
Northern Hemisphere Reference Line (NHRL; Hart,
1984) and are more radiogenic than other frontal arc
lavas from the western portion of the CAVA (Guatemala,
El Salvador, Nicaragua; Walker et al., 1995; Feigenson
et al., 2004; Fig. 10). Moreover, at a given 206Pb/204Pb
ratio, the 207Pb/204Pb and 208Pb/204Pb ratios of the cone
lavas are more radiogenic than those for plutonic rocks
from Guatemala, including dioritic fragments in the 1902
Santa Mar|¤ a airfall deposit, but are within the range for
Cocos plate sediments from Deep Sea Drilling Project
(DSDP) Site 495 (Feigenson et al., 2004; Figs 1 and 10).
Temporally, the 206Pb/204Pb (and 207Pb/204Pb and
208
Pb/204Pb) ratios become more radiogenic during
2347
VOLUME 52
NUMBER 12
DECEMBER 2011
0.70405
0.70405
0.70395
0.70395
0.70385
0.70385
0.70375
0.70375
0.51296
0.51296
0.51292
0.51292
0.51288
0.51288
143
Nd/144Nd
87
86
Sr/ Sr
JOURNAL OF PETROLOGY
0.51284
50
55
60
1
65
2
3
4
5
0.51284
6
MgO (wt %)
SiO2 (wt %)
Fig. 8. Variation of 87Sr/86Sr and 143Nd/144Nd vs SiO2 and MgO. Symbols as in Fig. 5.
phases II and III and the early part of IV, whereas the
latter part of phase IV is characterized by relatively large
variability in Pb isotope compositions (Fig. 9). Pb isotope
ratios are not strongly correlated with SiO2 or MgO.
U^Th isotopes
Measured (230Th/232Th) and (238U/232Th) activity ratios
in Table 3 were corrected for time since eruption using the
40
Ar/39Ar ages obtained by Escobar-Wolf et al. (2010) and
in this study (Table 2). The cone lavas have 238U excesses
between 3 and 26%, and the latter values are among the
highest found in the CAVA (Jicha et al., 2010). Two samples
from the Santa Mar|¤ a cone collected by Rose (1977, 1987a)
were analyzed for U^Th isotopes by Walker et al. (2007),
and these fall within the range observed in this study
(Fig. 11). The U^Th isotope compositions of Santa Mar|¤ a
cone lavas are distinct from those of volcanic rocks erupted
elsewhere along the CAVA. For example, lavas in
Nicaragua plot near the equiline with (230Th/232Th) activity ratios of 2^3 (outside the scale of Fig. 11), whereas lavas
from Costa Rica plot mainly with 230Th excess within the
MORB range (Thomas et al., 2002). Data from other centers along the volcanic front in Guatemala (e.g. Pacaya,
Fuego) plot near the equiline, and contain only slight
230
Th or 238U excesses (Walker et al., 2007; Fig. 11). The dioritic fragments found in the 1902 airfall deposit from
Santa Mar|¤ a plot together on the equiline at very low
(238U/232Th) and (230Th/232Th) ratios of about 0·9, which
are distinctly lower than those for lava flows and tephra at
Santa Mar|¤ a or elsewhere in the CAVA (Fig. 11).
Temporally, the (230Th/232Th) ratios increase slightly
from phase I to the early part of phase IV, but decline
during phase IV with the exception of the summit-forming
lava at the top of the sequence, which is similar to the historical dacites (Fig. 9). The (238U/230Th) activity ratioça
measure of 238U excessçalso shows a slight increase from
phase I to the early part of phase IV, with a decline
during the latter part of phase IV toward values similar
to those of the historical dacites (Fig. 9).
DISCUSSION
Importance of open-system differentiation
in the petrologic evolution of Santa
Mar|¤ a cone lavas
The 40Ar/39Ar geochronology provides a precise temporal
framework within which the geochemical and isotopic
datasets can be integrated and interpreted. Here we summarize the key observations and inferences that arise from
(1) major-element compositions, (2) trace-element compositions, (3) Sr^Nd^Pb isotope ratios, and (4) U^Th isotope
disequilibrium in the cone-forming lava flows. In turn,
these inferences underpin quantitative modeling of processes that may be responsible for the compositional and
isotopic evolution of the main cone.
2348
SINGER et al.
BASALTIC ANDESITE AT SANTA MARI¤A
present
Santiaguito
1922
dome
1902
SM
main
cone
3525 ka
6046 ka
72 ka 87Sr/86Sr
103 ka
0.7037
0.7038
present
Santiaguito
1922
dome
1902
SM
main
cone
0.7040 0.51285
0.7039
0.51290
144
Nd/ Nd
0.51295
3525 ka
6046 ka
72 ka
103 ka
18.65
present
Santiaguito
1922
dome
1902
3525 ka
206
18.69
204
Pb/ Pb
18.73
6046 ka
238
230
72 ka ( U/ Th)
103 ka
1.0
1.1
0.9
1.2
230
1.3
1.4
232
( Th/ Th)
0.85
equiline
SM
main
cone
143
0.95
1.05
1.15
1922 - Present
Santiaguito Dome
Mafic inclusions
1902 Fall Deposit
Dacite pumice
Bas. And. scoria
Plutonic fragments
Cone-forming lavas
35-25 ka
60-46 ka
72 ka
103-72 ka
Fig. 9. Temporal evolution of selected isotope ratios. Symbols as in Fig. 5.
The major-element composition of cone-forming lavas
that erupted between 103 and 25 ka ranges from basaltic
to basaltic andesite (51·4^56·1% SiO2), whereas a compositional gap separates these lavas from the historical dacitic
tephra and lava (63^66% SiO2). Previous work by
Rose et al. (1977) and Rose (1987a) favored an origin for
the silicic magmas dominated by fractional crystallization,
perhaps modulated to a small degree by crustal melting
and assimilation. Our data show three successive intervals
of decreasing SiO2, each beginning with a parent of slightly higher SiO2 (Fig. 6). This might suggest that rather
than fractional crystallization alone, the plumbing system
was recharged, perhaps more than once, with relatively
mafic magma that mixed with extant magma throughout
this period. If correct, the recharge events may be reflected
by the somewhat irregular increase in Ni throughout this
period, and buffering of MgO at a nearly constant level
(Fig. 6). Whereas the decreasing SiO2 of the first five lavas
of phase IV appears to follow a recharge pattern similar
to that of phases II and III, this is followed in the middle
of phase IV by lavas that show a more systematic trend
toward increasing SiO2, and decreasing MgO and Ni
(Fig. 6). These features imply that during the latter half of
phase IV either fractional crystallization became more
effective, replenishment with SiO2-poor, MgO-rich
magma waned, or the mantle input to the system shifted,
or that a combination of these processes may have propelled differentiation.
Some features of the trace-element variations are consistent with a prominent role for fractional crystallization
2349
JOURNAL OF PETROLOGY
VOLUME 52
NUMBER 12
DECEMBER 2011
15.8
15.6
s.
207
Pb/204Pb
15.7
Santa Maria cone (this study)
DSDP 495 sediments
Granite Plutons Guatemala
Diorite, 1902 Santa Maria
Metamorphic Rocks Guatemala
Western CAVA Front lavas
Costa Rica CAVA Front lavas
cos
te
Pla
sed
Co
NHRL
15.5
15.4
18.4
18.6
18.8
206
19.0
19.2
19.4
/204
Pb Pb
Fig. 10. 207Pb/204Pb vs 206Pb/204Pb for lavas from the Central American volcanic arc, including data from Santa Mar|¤ a cone lavas of this study.
The Pb isotope field of sediments from DSDP Site 495 (Fig. 1), and analyses of granites and metamorphic rocks from Guatemala are from
Feigenson et al. (2004) and Walker et al. (1995). Diorite fragments from the 1902 Santa Mar|¤ a eruption are from this study. NHRL, Northern
Hemisphere Reference Line for MORB, from Hart (1984).
Table 3: U^Th data for whole-rock samples of Santa Mar|¤ a cone-forming lavas
Sample
U (ppm)
Th (ppm)
(230Th/232Th)
(230Th/232Th)
2s
(238U/232Th)
2s
age-corrected
SM-07-01
0·940
2·347
1·188
1·176
0·007
1·216
0·007
SM-05-22
0·644
1·533
1·126
1·067
0·006
1·274
0·008
SM-05-20
0·631
1·513
1·141
1·098
0·007
1·264
0·008
SM-05-15
0·492
1·104
1·195
1·135
0·007
1·353
0·008
SM-05-13
0·383
0·859
1·208
1·148
0·007
1·354
0·008
SM-05-10
0·413
0·934
1·213
1·165
0·007
1·341
0·008
SM-05-09
0·414
0·941
1·198
1·126
0·007
1·335
0·008
SM-05-07
0·391
0·879
1·207
1·117
0·007
1·347
0·008
SM-05-06
0·395
0·874
1·208
1·088
0·007
1·371
0·008
SM-05-05
0·390
0·886
1·203
1·079
0·006
1·337
0·008
SM-05-04
0·431
1·009
1·200
1·105
0·007
1·297
0·008
SM-05-02
0·434
1·013
1·186
1·084
0·007
1·298
0·008
SM-05-01
0·440
1·026
1·197
1·095
0·007
1·300
0·008
SM-07-07
0·760
1·881
1·167
1·075
0·006
1·226
0·007
relatively deep within the crust. For example, the quasilinear trends in Fig. 5 and parallel REE and multi-element
patterns in Supplementary Data Appendix Fig. 1 show
little evidence for a negative Eu anomaly that might reflect
shallow crystallization of plagioclase. The relatively large
range of K/Rb, Th/Nb, and Zr/Hf ratios (Figs 6 and 7)
argues that open-system processes involving partial melting and assimilation of crust have affected the compositions of the erupted magmas (e.g. Davidson et al., 1987;
Hildreth & Moorbath, 1988; Walker et al., 2007).
2350
BASALTIC ANDESITE AT SANTA MARI¤A
SINGER et al.
1.5
1.4
Cone-forming lavas
35-25 ka
60-46 ka
72 ka
103-72 ka
1922 - Present
Santiaguito Dome
Mafic inclusions
1902 Fall Deposit
Dacite pumice
Other CAVA Volcanoes
Bas. And. scoria
Guatemala
Diorite fragments
Santa Maria (Walker)
Costa Rica
e
in
uil
eq
1.2
MORB
1.1
230
( Th/232Th)
1.3
1.0
OIB
0.9
0.8
0.8
0.9
1.0
1.1
1.2
238
1.3
1.4
1.5
232
( U/ Th)
Fig. 11. (238U/232Th) vs (230Th/232Th) equiline diagram. 230Th values of Santa Mar|¤ a samples are age-corrected. Data from volcanoes in Costa
Rica and Guatemala are fromThomas et al. (2002) and Walker et al. (2007). It should be noted that data from volcanoes in Nicaragua plot outside
this diagram to the upper right near the equiline. 2s uncertainties are equivalent to symbol size.
Moreover, we note that the variability of these and other
trace-element ratios is by far greatest among the late
phase IV lavas that formed the culmination of cone
growth (Figs 6 and 7), suggesting that magma^crust interaction was likely to have been pervasive at this time.
Walker et al. (2007) observed a weak positive correlation of
Th/Nb with (230Th/238U) ratios in six whole-rock lavas
from the Guatemalan volcanic front and attributed
this to assimilation of Th-rich partial melts of Mesozoic
metamorphic or granitic rocks found in southeastern
Guatemala. Indeed, at Santa Mar|¤ a, four of the late phase
IV lavas have Th/Nb ratios between 0·4 and 0·7 that are
far higher than other, mainly earlier, cone-forming lavas,
and much higher than those analysed by Walker et al.
(2007). However, as discussed below, the high 87Sr/86Sr
and low 143Nd/144Nd ratios of the metamorphic and granitic rocks analyzed by Walker et al. (1995, 2007), and the low
207
Pb/204Pb ratios of the granites in Guatemala (Walker
et al., 1995; Feigenson et al., 2004) preclude their involvement as important assimilants beneath Santa Mar|¤ a.
Although the 87Sr/86Sr and 143Nd/144Nd ratios in the
cone-forming lavas fall within the published ranges for
this part of the CAVA (e.g. Walker et al., 1995, 2007; Carr
et al., 2007b; Jicha et al., 2010), the observation that lavas
with the highest SiO2 and lowest MgOçcomprising
some of the last-erupted cone lavasçare among those
with the lowest 87Sr/86Sr and highest 143Nd/144Nd ratios
(Figs 8 and 9) is unexpected. Moreover, Jicha et al. (2010)
observed that the 87Sr/86Sr ratios are inversely correlated
to the degree of excess 238U in the cone-forming lavas,
leading to the hypothesis that as Santa Mar|¤ a grew, assimilation of a MORB component within the deep crustç
characterized by low 87Sr/86Sr and old enough to be in
secular equilibrium in the 238U^230Th systemçcontributed in a quasi-progressive manner to the compositional
evolution of these magmas.
The Pb isotope ratios of the Santa Mar|¤ a cone lavas
are also consistent with a crustal contribution to a
mantle-derived arc basalt. However, the high 207Pb/204Pb
ratios similar to those of DSDP Site 495 sediments
(Fig. 10) and the low Ce/Pb ratios (3·1^4·8) of the
cone-forming lavas may reflect addition of Pb to the
mantle wedge by fluid released from the subducting
Cocos plate slab and overlying sediment (Hofmann et al.,
1986). A large flux of fluid-mobile Pb from the subducted
slab and sediment into the mantle wedge is consistent
with the unusually large 238U excess in the Santa Mar|¤ a
cone lavas (Jicha et al., 2010). Moreover, the high concentration of Pb in the cone-forming lavas (45 ppm) may explain why there are no clear trends in Pb isotope ratios
over time; if the wall-rocks are MORB-like and thus contain 50·5 ppm Pb, the isotopic composition of these
2351
JOURNAL OF PETROLOGY
VOLUME 52
magmas would be insensitive to small amounts of assimilation. The hypothesis of a MORB-like assimilant stands in
stark contrast to previous models of magma^crust interaction in Guatemala in which silicic, large ion lithophile
element-enriched magmas reflect either assimilation of a
more typical continental crustal lithology, such as the
Mesozoic metamorphic or grantic rocks (e.g. Walker et al.,
1995; Cameron 1998; Walker et al., 2007), or partial melting
of arc basalt recently intruded into the lower crust (e.g.
Vogel et al., 2006).
The large 238U excesses in the Santa Mar|¤ a lavas
prompted Jicha et al. (2010) to conclude that the parental
magmas entering the crustal plumbing system beneath
Santa Mar|¤ a volcano reflect melting of a mantle wedge
that was strongly enriched by slab-derived fluid. Five of
the early cone-forming lavas of phases II and III have
among the highest 238U excesses (Fig. 9), which provides
unusual leverage in quantifying not only the U^Th isotope
composition of crustal components involved in the assimilation process, but also the duration of this process. The
youngest phase IV lavas decrease in (238U/230Th), suggesting that the assimilant is older than 350 ka and thus probably lies along the equiline (Fig. 9). These phase IV lavas
also have low 87Sr/86Sr ratios and high SiO2 contents
(Figs 6 and 9), forming the basis for arguments by Jicha
et al. (2010) that the youngest cone lavas reflect assimilation
of a partial melt that was derived from relatively mafic
oceanic crust. Jicha et al. (2010) proposed that MORB,
perhaps similar to that found in Cretaceous ophiolites
that crop out south of the Motagua fault and along the
Motagua suture (Beccaluva et al., 1995; Alvarado et al.,
2007; Fig. 1) may be present within the Paleozoic^
Mesozoic metamorphic (Martens et al., 2007) and plutonic
(Patino, 2007) rocks that form the Chortis Block beneath
Santa Mar|¤ a.
To develop a more quantitative understanding of the
crystallization, partial melting, and assimilation processes
that occurred during the growth of Santa Mar|¤ a, and
rigorously test the model outlined by Jicha et al. (2010), we
have undertaken phase equilibria and thermodynamically
constrained mass-balance modeling of the major- and
trace-element, and isotopic variations recorded by the
cone-forming lavas. In the following sections, we highlight
how these models are employed, and use them to constrain
the physical conditions of magma evolution, crustal heating, the composition and mineralogy of the end-members
that participated in open-system processes, as well as the
temporal changes in these processes.
Major-element modeling
The temporal trends in major- and trace-element, and isotopic compositions indicate multiple episodes of recharge
and magma^crust interaction; however, we begin by attempting to constrain the role of fractional crystallization
using the MELTS algorithm (Ghiorso & Sack, 1995;
NUMBER 12
DECEMBER 2011
Asimow & Ghiorso, 1998). We chose sample SM-05-04 as
a parent magma composition for the MELTS models
because it contains the lowest SiO2 (51·4 wt %) and K2O
(0·82 wt %), has high MgO (5·3 wt %) (Table 1), and is
one of the earliest erupted of the cone-forming lavas based
on an 40Ar/39Ar age of 75 13 ka (Escobar-Wolf et al.,
2010). Two-oxide and two-pyroxene thermometry based on
phenocrysts in the dacitic pumice and basaltic-andesite
scoria of the 1902 eruption suggest that the scoriaçwhich
is remarkably similar in bulk composition to the
cone-forming lavas (Fig. 6)çequilibrated at about 10208C
at an fO2 equivalent to Ni NiO þ 2 log units (Singer et al.,
2009). We take this temperature estimate as the lower limit
for daughter melts and use the fO2 estimate (equivalent to
about QFM þ 2·3 log units, where QFM is the quartzç
fayalite^magnetite buffer) as a guide to appropriate starting conditions for the parent magma used in the MELTS
models, each of which were created assuming perfect
fractional crystallization of minerals that formed during
cooling.
A range of pressure, temperature, H2O content, and
f O2 values were used as starting conditions for a suite of
more than a dozen MELTS models, seven of which best
approach the compositions of the cone-forming lavas:
these are illustrated in Supplementary Data Appendix
Fig. 3. These seven models span pressures from 7 to 1 kbar;
both isobaric and dynamic models that involved decompression were evaluated. H2O in the parent magma was
varied between 0 and 3 wt %, with five models using a
value of 2 wt %. An fO2 of QFM þ 2 log units was used
in five of these seven models; other models assumed
an f O2 one log unit higher or lower than QFM þ 2
(Supplementary Data Appendix Fig. 3). Depending on the
starting pressure and H2O content, MELTS calculated a
liquidus temperature for the parent magma between 1255
and 11308C. Phenocryst phases predicted by MELTS include combinations of clinopyroxene, orthopyroxene,
plagioclase, olivine, magnetite, and ulvo«spinel that are
similar to those observed in the cone-forming lavas.
Despite a wide range of starting parameters, MELTS
models using the early erupted low-SiO2 and K2O
magma as a potential parent fail to reproduce the covariation of the major oxides in the later erupted lavas of
phases III and IV. In particular, these models capture neither the high MgO contents of phase III and early phase
IV lavas nor the relatively minor decrease in FeO* as
SiO2 increases from 51·5 to 56% (Supplementary Data
Appendix Fig. 3). The inability of these fractional crystallization models to reproduce the observed major-element
trends does not disprove crystal fractionation as an important process. Instead, we suggest that open-system processes, including recharge of the magmatic plumbing
system with MgO- and FeO*-rich mantle-derived basalt
during cone-building phases II and III and the early part
2352
SINGER et al.
BASALTIC ANDESITE AT SANTA MARI¤A
of phase IV, and assimilation of SiO2- and K2O-rich partial melts of crustal wall-rocks during the latter part of
phase IV, operated in tandem with fractional crystallization and strongly influenced the major- and trace-element
and isotopic composition of the cone-forming lavas.
Trace-element and isotopic evolution
Proposed assimilants and melting environments
Walker et al. (2007) argued that the 230Th excesses observed
in Guatemalan lavas from volcanoes other than Santa
Mar|¤ a are a result of melting and assimilation of plutonic
or metamorphic rocks at depths above the garnet stability
field, with residual zircon and perhaps magnetite, because
there is no clear evidence for garnet in the trace-element
compositions. Alternatively, Vogel et al. (2006) suggested
that silicic magmas in Central America are generated
from partial melting of penecontemporaneous medium- to
high-K2O, calc-alkaline basalts that ponded or crystallized
in the middle crust. In light of these studies, we consider
partial melts of the crustal compositions of Walker et al.
(1995) and high-K basalts in the middle crust (Vogel et al.,
2006) in our trace-element and isotopic modeling.
Evidence for intermediate-granitic composition crust beneath Santa Mar|¤ a comes from diorite fragments contained in the 1902 airfall deposit (Figs 5, 7, and 8) and we
consider these materials, together with other plutonic
rocks in Guatemala, as potential assimilants. Our modeling also considers partial melts of MORB-like crust
[normal (N)-MORB and enriched (E)-MORB] (Jicha
et al., 2010), as well as ocean island basalt (OIB), based on
observed OIB signatures in lavas elsewhere along the
CAVA (Hauff et al., 2000; Hoernle et al., 2002; Gazel et al.,
2009). Moreover, models were designed to test whether
the chemical and isotopic composition of cone-forming
basaltic-andesite magmaçtypical of the CAVA and other
arcsçis acquired in the deep crust, as suggested locally
by Halsor & Rose (1988), and globally by Hildreth &
Moorbath (1988) and Annen et al. (2006).
With the aim of modeling to capture the full range of
trace-element behavior displayed by the cone-forming
lavas, the composition of the parent magma was chosen to
include the lowest concentration of each incompatible
element measured in any of these lavas; Sr was set at
650 ppm, similar to several phase II lavas. The assumed
concentrations for the parent magma are presented in
Table 4 and Supplementary Data Appendix 5. The
models focus on generating a daughter magma similar to
the summit-forming cone lava SM-07-01, which has the
highest concentrations of incompatible elements (Fig. 6)
and was erupted late during phase IV. The REE compositions of potential assimilants described above are plotted
along with the assumed parent and daughter (target)
magma compositions in Fig. 12. Because partial melting of
crustal rocks will enrich incompatible elements in the
melt, wall-rocks that have incompatible element contents
that greatly exceed those in the target magma are unlikely assimilants; these include OIB, E-MORB, the diorite
fragments, granites and metamorphic rocks from SE
Guatemala. Although the REE pattern of the target
magma could be generated via assimilation of a very
small amount of melt from these lithologies into the
parent magma, the progression from early erupted basalt
with relatively high 87Sr/86Sr (0·70403) and low SiO2
(51^52 wt %) to late-erupted basaltic andesite with lower
87
Sr/86Sr (0·70382) and higher SiO2 (54^56 wt %) is not
consistent with these assimilants (Jicha et al., 2010).
Moreover, the Santa Mar|¤ a cone lavas have 207Pb/204Pb
and 206Pb/204Pb ratios that plot well above the plutonic
rocks from Guatemala, including the granites of Walker
et al. (1995) and Feigenson et al. (2004) and the diorite fragments in the 1902 airfall deposit that we have analysed
(Fig. 10). The diorite fragments (and presumably the granites) have high Pb contents of 5·6^8·4 ppm similar to, or
higher than, the cone-forming lavas. Thus, partial melts of
local granitic or dioritic crust would contribute a significant amount of Pb to the magmas, but it would be too
low in 207Pb/204Pb for these rocks to be considered as reasonable assimilants. Alternatively, the low 87Sr/86Sr ratios
in the relatively SiO2-rich lavas erupted late during phase
IV (Fig. 9) lead us to favor assimilation of partially
melted N-MORB, which has relatively low incompatible
element contents, a 87Sr/86Sr ratio lower than the parent
magma, and a U^Th isotope composition that plots on
the equiline in the MORB field.
EC-RAxFC model parameters
The EC-RAxFC algorithm (http://magma.geol.ucsb
.edu/papers/ECAFC.html; Spera & Bohrson, 2004;
Bohrson & Spera, 2007) was used to model crystallization
of a basaltic parent magma that heats, partially melts,
and assimilates lower or middle crust. Model parameters,
including initial and liquidus temperatures (estimated
from MELTS models in Appendix Fig. 3), specific heats,
enthalpies of fusion and melting, isotopic and traceelement compositions of the parent magma, assimilant
and recharge magma, bulk distribution coefficients,
and equilibration temperature are given in Table 4 and
Supplementary Data Appendix 5. All models assume
10308C as the equilibration temperature, which falls
within the range determined by Singer et al. (2009) for
Santa Mar|¤ a basaltic-andesite scoria using two-oxide
thermometry. Distribution coefficients are from the
GERM database (http://earthref.org/GERM/) and represent an average of the range of published Kd values for
each element for a basaltic system undergoing crystal fractionation within wall-rock comprising garnet amphibolite
(10% garnet, 20% amphibole, 70% clinopyroxene) or
amphibolite (20% amphibole, 80% clinopyroxene).
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JOURNAL OF PETROLOGY
VOLUME 52
NUMBER 12
DECEMBER 2011
Table 4: Selected EC-RAxFC parameters
Thermodynamic parameters
Value
Geochemical parameters*
tlm, liquidus T of magma
12008C
Element:
Sr
Nd
U
Th
13
In magma
tmo, initial T of magma
12508C
Concentration (ppm)
tla, liquidus T of assimilant
12008C
Bulk Dmo
tao, initial T of assimilant
7758C
In assimilant
1·1
tlr, liquidus T of recharge magma
12008C
Concentration (ppm)
tro, initial T of recharge magma
12008C
Bulk Dao
ts, solidus T
9008C
In recharge magma
140
0·2
teq, equilibration T
10308C
Concentration (ppm)
cpm, specific heat of magma
1450 J kg1 K1
Bulk Dro
cpa, specific heat of assimilant
1370 J kg1 K1
cpr, specific heat of recharge magma
1450 J kg1 K1
650
0·36
0·83
0·55
0·05
0·18
9·62
0·2
0·7
1·5
0·01
0·015
0·36
0·83
0·05
0·18
230
13
1·2
0·55
Sr/86Sr
Isotope ratio:
1
650
87
143
Nd/144Nd
238
U/232Thy
Th/232Thy
hm, enthalpy of crystallization of magma
396000 J kg
In magma
0·70403
0·51286
0·4265
171500
ha, enthalpy of melting of assimilant
270000 J kg1
In assimilant
0·70320
0·51318
0·3669
161000
hr, enthalpy of crystallization of
396000 J kg1
In recharge
0·70403
0·51286
0·4265
171500
recharge magma
Initial size of magma body
1·0
X, normalized proportion of wallrock mixed into
evolving magma body
1·0
Mro, mass of recharge magma added during
recharge event
1·0
*Geochemical parameters are selected examples from an N-MORB assimilation model. N-MORB values from Klein (2003).
Complete geochemical parameters for this and other models are listed in Supplementary Data Electronic Appendix 6.
yIt should be noted that U–Th isotope ratios are abundance ratios.
See Spera & Bohrson (2004) and Bohrson & Spera (2007) for complete listing of parameters and definitions.
Dimensionless values are all normalized to the magma body ‘size’ of 1·0. D, bulk D value for magma (Dmo), assimilant,
(Dao), recharge magma (Dro) (Rollinson, 1993).
Rock/Chondrite
1000
N-MORB
E-MORB
OIB
Lithic fragments
Crust (SE Guatemala)
parent magma (72 ka)
daughter magma (35-25 ka)
100
10
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Fig. 12. Chondrite-normalized REE patterns of the parent magma (black circles) and the target daughter magma (open circles) used for modeling. Also plotted are various possible assimilants: N-MORB (Klein, 2003); E-MORB (Klein, 2003); OIB (Sun, 1980); crustal compositions
from SE Guatemala (Walker et al., 1995); two dioritic lithic fragments (this study). Chondrite values are from Sun & McDonough (1989).
2354
SINGER et al.
BASALTIC ANDESITE AT SANTA MARI¤A
0.70405
1000
0.70400
0.70395
800
0.70390
600
87
Ta* (°C)
1.8
1.6
1.4
1.2
1.0
0.8
0.6
Sr/86Sr
0.51298
0.51294
143
Mm
144
Nd/ Nd
2.5
2.0
0.6
1.5
0.4
1.0
Th (ppm)
Ma*
1.0
0.8
0.6
0.4
0.2
0.51290
0.51286
0.8
0.2
0.70385
238
Ms
232
U/ Th
0.5
0.425
0.420
0.415
0.410
0.405
0.400
1200 1150 1100 1050 1000
1250 1200 1150 1100 1050
T of magma body (°C)
T of magma body (°C)
Lower crust, no recharge
Lower crust, with recharge
Mid crust, no recharge
Fig. 13. Three EC-RAxFC models: lower crust without recharge, lower crust with recharge and mid-crust without recharge. The models track
thermodynamic and geochemical parameters throughout cooling of the magma body. Ta*, temperature (8C) of the assimilant body (wall-rock).
Dimensionless parameters are relative to an initial mass of the parent magma of 1·0 and include: Mm, mass of magma body; Ma*, mass of
assimilant melt incorporated into the magma body; Ms, mass of cumulate solids formed. Arrows indicate down-temperature direction of
model progress.
EC-RAxFC model calculations: lower (40^45 km) vs
middle (30^35 km)crust
The thermodynamic pathways followed in three
EC-RAxFC models, each assuming that partially melted
N-MORB is the assimilant, are illustrated in Fig. 13. In
the first model a batch of magma intrudes lower crustal
rocks, initially at a temperature of 7758C, without
recharge. This model implies that during cone growth,
40% by mass of the parent magma crystallizes as it cools
from 1250 to 10308C. Simultaneously, the temperature of
the lower crustal solid wall-rock increases from 775 to
10308C, such that partial melting begins once it is heated
beyond the solidus at 9008C. By the time the daughter
melt has cooled to 10308C and reached a trace-element
2355
JOURNAL OF PETROLOGY
VOLUME 52
and isotopic composition similar to the late phase IV cone
lavas, 20% by mass of the N-MORB assimilant has contributed to the final magma composition (Fig. 13).
A second model involves the same parent magma losing
heat to lower crustal wall-rock of garnet amphibolite, initially at 7758C, but also incorporates episodic recharge
throughout the earlier stages of cone building. In this
model, three pulses of recharge magma that resemble the
parent magma were introduced to the system during
phases I^III of cone growth when the magma body had
cooled to 12008C, 11758C and 11508C. The recharge has a
notable effect on the thermodynamic pathwayçwall-rock
melting above the 9008C solidus begins at a higher
magma temperature than in the non-recharge modelç
but a moderate effect on the trajectories of trace-element
concentrations and isotope ratios (Fig. 13). The recharge
model doubles the amount of magma in the system, which
ultimately results in assimilation of a greater mass of
N-MORB partial melt (71% more by mass), and forms
more cumulate solids (50% more by mass), compared
with the same model without recharge.
A third model was designed such that assimilation
occurs in the mid-crust outside the garnet-stability range
(35^30 km). Because the wall-rock is initially only at
6008C in this model, most of the thermal energy of the
parent magma is consumed by heating the wall-rock,
with partial melting to a small extent occurring only as
the parent magma cools from 1050 to 10308C (Fig. 13).
Assimilation of the limited quantity of partial melt produced in this model fails to shift trace-element concentrations and isotopic ratios of the parent magma enough to
match the range of cone-forming lavas.
Ascent of mantle-derived basalt into the lower crust, followed by prolonged storage, including fractional crystallization, wall-rock melting, assimilation, and mixing is
a theme common to many models of subduction zone
magmatism (e.g. Hildreth & Moorbath, 1988; Dufek &
Bergantz, 2005; Annen et al., 2006; Ownby et al., 2010).
If the range of trace-element and isotopic compositions
in the cone-forming lavas of Santa Mar|¤ a was acquired
via magma^crust interactions, a deep crustal origin is supported by our models.
EC-RAxFC model calculations: garnet-bearing
lower crust?
The EC-RAxFC algorithm may be used to estimate the
lower-crustal mineral assemblage, using the bounds on
assimilant (N-MORB) and depth (lower crust, 40 km).
Importantly, the daughter magma is reproduced when
D41 for the mid- to heavy REE, consistent with partial
melting and assimilation in the presence of 10% garnet.
Our finding that assimilation may have involved residual
garnet in the lower crust contradicts the hypothesis of
Walker et al. (2007) that crustal melting did not occur at
depths where garnet is stable. Walker et al. (2007) based
NUMBER 12
DECEMBER 2011
their conclusion on the lack of correlation between 230Th
and trace-element ratios such as Tb/Yb, La/Yb, and Sr/Y:
high values for these ratios and an excess of 230Th would
be indicative of residual garnet. Assimilation of an
N-MORB partial melt using Kd values characteristic of
an amphibolite mineral assemblage where partition
coefficients increase from low- to high-REE but remain incompatible (i.e. DREE51) fails to yield a daughter magma
that is comparable with the most evolved basaltic-andesite
lava on Santa Mar|¤ a (Fig. 14, Supplementary Data
Appendix 6). These models are consistent with a
garnet-amphibolite or -granulite metabasalt in the lower
crust beneath Santa Mar|¤ a volcano.
Isotopic and trace-element trajectories for the
N-MORB-assimilant models, with and without recharge,
are shown in Figs 15 and 16. For the elemental concentrations and isotopic ratios modeled, the recharge trajectories
initially trend in the same direction as the non-recharge
models, but shift to a retrograde pathway as the recharge
magma batches pull the evolved magma back toward the
parent magma composition (Fig. 15). Figure 15 illustrates
that the compositions of many, but not all, lavas associated
with phases I^III and the earliest part of phase IV may
be created by a combination of crystal fractionation and
assimilation, accompanied by recharge. For some elements,
including U and Th, the EC-RAxFC models with, or without, recharge can reproduce some of the late-erupted
phase IV lavas (Fig. 15a), yet for other elements, such as
Rb, the modeling suggests that concentrations in magmas
erupted late during phase IV are primarily the result of
AFC processes and less affected by recharge (Fig. 15b).
In a (238U/232Th) vs (230Th/232Th) plot, the path for the
model involving recharge is nearly identical to that for the
model with no recharge; both shift the parent magma
with a large 238U excess toward the N-MORB assimilant
located on the equiline at the time partial melting begins
(Fig. 16). Because the EC-RA xFC algorithm takes a
thermodynamically constrained, mass-balance approach,
it does not consider radioactive decay. Thus, it must be
kept in mind that in addition to crystal fractionation, assimilation, and recharge processes that can shift the
(238U/232Th) ratios horizontally, the vertical spread of
compositions in the equiline diagram may also reflect ingrowth of 230Th. Three phase III lavas and three early
stage IV lavas have (230Th/232Th) activity ratios that may
reflect a minimal amount of assimilation and recharge, followed by the ingrowth of 230Th for several thousand
years. If this interpretation is correct, a large fraction of
the magma erupted during the early part of cone
growth may reflect incomplete processing, partial solifidication, and tens of thousand years of storage in a lower
crustal MASH (melting, assimilation, storage and homogenization) zone (Hildreth & Moorbath, 1988) prior to
eruption.
2356
BASALTIC ANDESITE AT SANTA MARI¤A
SINGER et al.
100
Rock/Chondrite
N-MORB (assimilant)
EC-RAFC result: grt-bearing
EC-RAFC result: amphibolite
parent magma (72 ka)
daughter magma (35-25 ka)
10
7
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Fig. 14. Chondrite-normalized REE plot with the parent magma and target daughter compositions and the N-MORB assimilant composition
(Klein, 2003). Two EC-RAxFC models are plotted: one used Kd values for a lower crust of amphibolite and the other used Kd values appropriate
for a garnet-amphibolite. The model with residual garnet best matches the target daughter composition. Chondrite normalization values are
from Sun & McDonough (1989). grt, garnet.
Evolution of the mantle wedge?
The EC-RA xFC models we have presented assume that
much of the trace-element and isotopic variability of the
Santa Mar|¤ a cone lavas reflects open-system interactions
between magma and crust. This view, however, does not
consider secular changes in the composition of the mantle
wedge from which the parental basalts originate that
could affect the amount of crustal assimilation required to
explain the cone-forming lavas. The exceptionally large
238
U excesses and high 207Pb/204Pb ratios of the Santa
Mar|¤ a cone lavas signal an unusually large flux of fluid
from the subducted slab and sediments into the mantle
wedge below Santa Mar|¤ a (Jicha et al., 2010). Such a large
contribution from the Cocos plate to the mantle wedge
may mediate the extent of partial melting, but as fluid
mobile elements including U, Pb, and Sr are removed by
ascending melts, the excess 238U and elevated ratios of
207
Pb/204Pb and 87Sr/86Sr initially in the wedge may diminish. Although we favor the view that intracrustal processes are mainly responsible for the variations in the
Santa Mar|¤ a lavas, to the extent that partial melting transfers U, Pb and Sr from the mantle wedge into magmas
and leaves behind a mantle with progressively lower 238U,
207
Pb/204Pb, and 87Sr/86Sr ratios over time, the amount of
crustal assimilation predicted in our models may be
reduced.
Time scales of cone-building, magma flux,
and differentiation
The thermal models of Dufek & Bergantz (2005) and
Annen et al. (2006) rely on assumptions regarding the
mass of basaltic magma and rate at which it is emplaced
into the lower crust. Although estimating magma flux
through subduction zones is hampered because many variables are unknown, here we can couple the well-dated
record of cone growth at Santa Mar|¤ a with the
EC-RAxFC models to make first-order estimates of
magma flux into the lower crust. As an example, the
EC-RAxFC model that includes recharge suggests that as
the system evolved the mass of magma (Mm) increased
180% relative to the initial body of parent magma, whereas the mass of cumulate crystals formed (Ms) exceeds
that of the initial magma intrusion by 10% (Fig. 13).
During 75 kyr of cone building, 8 km3 of lava erupted.
Assuming that 100% of the residual melt created by the
EC-RAxFC process was erupted to form the cone, the
total magma input into the lower crust was about 14 km3,
whereas a more realistic scenario may be that only 20%
of the residual melt was erupted (White et al., 2006), in
which case 40 km3 of magma entered the lower crust
during the 75 kyr. We consider this to be a maximum
estimate of the magma input to the lower crust for three
reasons: (1) we may have overestimated the amount of recharge; (2) secular changes in the isotopic composition of
the mantle wedge could imply that we have overestimated
the amount of assimilation; (3) the initial wall-rock temperature in our lower crustal models is only 7758C; a
higher temperature reflecting recent intrusions, or heating
of the crust by magmas prior to cone-building would
lower the amount of magma required in our models. The
areal footprint of the 8 km diameter Santa Mar|¤ a cone
(Fig. 2) is 5 107 m2 and given the magma volume estimate of 40 km3 and duration of cone-building of 75 kyr
we can roughly estimate the flux of magma into the lower
crust. A maximum estimate might integrate over the
actual footprint of the volcano for 75 kyr, and this yields a
2357
JOURNAL OF PETROLOGY
(a)
VOLUME 52
NUMBER 12
DECEMBER 2011
3.0
Th (ppm)
2.5
2.0
1.5
1.0
0.25
0.45
0.65
0.85
U (ppm)
(b)
700
EC-RAxFC Models
Assimilant = N-MORB
no recharge
with recharge
650
K/Rb
600
550
500
450
10
15
20
25
Rb (ppm)
Fig. 15. (a) Plot of U (ppm) vs Th (ppm) with two N-MORB assimilant model trajectories, one with recharge and one without. (b) Plot of Rb
(ppm) vs K/Rb with the same two models. Symbol spacing along the model paths represents 98C temperature steps of the cooling magma body.
magma flux of 1·07 102 m3 m2 year1. A minimum estimate might integrate over a larger area and longer
time period. For example, if we double the width of
the area and increase the duration to 100 kyr, to reflect
uncertainty as to whether the 40Ar/39Ar data encompass
the entirety of cone-building, the magma flux is about
0·40 102 m3 m2 year1.
Another, more customary way to estimate magmatic
flux for comparison with along-arc eruptive flux estimates
is to integrate over a distance along the arc front. There
are at least eight, and possibly nine, frontal arc volcanoes
spanning 140 km from Chicabal to Pacaya with an average
spacing of about 20 km (Fig. 1). If we integrate over 20 km
for 75 kyr, the result is an along-strike magma flux
2358
BASALTIC ANDESITE AT SANTA MARI¤A
SINGER et al.
Assimilant
composition
before
melting
1.2
MORB
230
( Th/232Th)
eq
ui
lin
e
1.3
1.1
1922 - Present
Santiaguito Dome
Mafic inclusions
1902 Fall Deposit
Dacite pumice
Bas. And. scoria
Cone-forming lavas
35-25 ka
60-46 ka
72 ka
103-72 ka
EC-RAxFC Models
no recharge
with recharge
1.0
1.0
1.1
~50 ka
AF
C±
R
C
1.2
232
( U/ Th)
1.3
1.4
238
Fig. 16. (238U/232Th) vs (230Th/232Th) plot with the same two models as in Fig. 15. The recharge and non-recharge models are nearly identical.
Also plotted is the solid (before partial melting) assimilant composition of N-MORB used in the models. The large arrow indicates the trajectory of AFC (assimilation^fractional crystallization) and RC (recharge). If stored in the crust for several thousand years, compositions generated by these models may be modified by 230Th ingrowth (vertical arrow).
estimate beneath Santa Mar|¤ a of 27 km3 km1 Myr1.
Integrating over 100 kyr yields a magma flux of 20
km3 km1 Myr1. An alternative approach is to use the
actual footprint of the volcano as a guide to the lateral dimensions of the crustal plumbing system. At Santa Mar|¤ a,
this results in an along-arc magma flux into the base of
the crust over the last 75 kyr of 66 km3 km1 Myr1; integrating over 100 kyr reduces this estimate to
50 km3 km1 Myr1. Although each of these estimates is
several times larger than those based on geophysical imaging of arc crust coupled with age constraints on the
long-term duration of arc formation (e.g. Crisp, 1984;
White et al., 2006), it should be kept in mind that we are
considering a short, 5105 year, time scale for a single
well-characterized frontal arc volcanic center. The extent
to which our magma flux estimates for Santa Mar|¤ a may
typify several other larger composite basaltic andesitedominated volcanoes in Guatemala, for example, San
Pedro, Toliman, Atitlan, Fuego, and Agua (Fig. 1) is unknown. We note, however, that even allowing for a 50%
uncertainty in the value of 40 km3 for the mass of magma
inferred from the EC-RAxFC modeling, our lowest estimates of magma flux (0·40 0·20 102 m3 m2 year1,
or 20 10 km3 km1 Myr1 of arc length) are several times
greater than the values assumed in the thermal modeling
of Dufek & Bergantz (2005) and Annen et al. (2006).
Using the thermal model of Hawkesworth et al. (2000), intrusion, cooling, and crystallization of 40 km3 of mafic
magma within the lower crust during the initial 65^75 kyr
of cone-building could generate 180 MW of thermal
energy that would be available to melt wall-rock and promote assimilation during the final phase of cone-building.
Using a 20 km volcano spacing, we estimate the
along-arc extrusive flux during the 75 kyr of cone-building
at Santa Mar|¤ a to be 5 km3 km1 Myr1. It should be
noted that this value increases to about 9 km3 km1 Myr1
if the 1902 dacite eruption and Santiaguito dome lavas are
considered. The latter places Santa Mar|¤ a at the high end
of the range of 5^9 km3 km1 Myr1 estimated for volcanoes in Nicaragua and Costa Rica (Carr et al., 2007b). We
find this particularly striking when one considers that the
much thicker crust in Guatemala might be expected to act
as a low-density filter that retards magma ascent. A relatively large flux of mantle-derived basalt into the lower
crust beneath Santa Mar|¤ a may help to explain several features, including the high eruptive flux, the heating
required to drive assimilation, and the clear, undiluted,
slab-fluid signature carried in the form of large 238U
excesses and elevated 207Pb/204Pb ratios in all the Santa
Mar|¤ a cone lavas. The implications of this finding for
better understanding elemental fluxes through the CAVA
warrant further attention.
2359
JOURNAL OF PETROLOGY
VOLUME 52
CONC LUSIONS
Major- and trace-element compositions, 40Ar/39Ar ages,
and Sr, Pb, Nd, and U^Th isotope ratios are used to constrain the timing, extent, and location of crystallization,
crustal assimilation, and magmatic recharge processes
beneath Santa Mar|¤ a volcano during the entire conebuilding period. Below we summarize the conclusions of
this study in terms of the length of time required to build
the Santa Mar|¤ a cone, the evidence for magma recharge
and assimilation of the lower crust, the time scales of magmatic processes, and the insights gained by studies focused
on single volcanoes.
(1) The duration of cone-building at Santa Mar|¤ a
is longer than previously thought based on three new
40
Ar/39Ar ages that, together with the ages of
Escobar-Wolf et al. (2010), suggest the final eruptions took
place between 35 and 25 ka, thereby reducing the length
of repose preceding the 1902 dacite eruption from 35 to
25 kyr. Cone-building lavas erupted between about 103
and 25 ka and provide a well-dated, flow-by-flow record of
at least 75 kyr of magmatic processes.
(2) Major- and trace-element and isotopic variations
imply that mafic recharge and magma mixing were prevalent during early phases of cone-building, whereas AFC
processes were more dominant during the latest stage of
cone growth. Specifically, successive intervals of decreasing
SiO2, each beginning with a parent of slightly higher
SiO2, suggest that the plumbing system was recharged,
perhaps more than once, with relatively mafic magma.
The recharge events may also be reflected by an irregular
increase in Ni throughout this period, and buffering of
MgO at a nearly constant level. This is followed in the
middle of the last phase of cone-building by lavas that
show a more systematic trend toward increasing SiO2,
and decreasing MgO and Ni. We infer that during the
latter half of phase IV either fractional crystallization
became more effective, replenishment with MgO-rich
magma waned, or the mantle input to the system shifted,
or that a combination of these processes may have propelled differentiation.
Isotopic variations confirm that open-system processes
are prevalent and that they have had a significant effect
on magmas erupted during phase IV. Many of the early
erupted basalts with 51^52 wt % SiO2 have higher
143
Nd/144Nd and lower 87Sr/86Sr ratios than more SiO2rich basaltic andesites (54^56 wt % SiO2) that erupted
during the final period of cone-building. These features indicate that the assimilant is not typical continental crust,
but more like MORB in composition. EC-RAxFC modeling (Bohrson & Spera, 2007) of a parental basalt that
undergoes crystal fractionation, assimilation and periodic
recharge with basalt in the lower crust can reproduce lava
compositions erupted during phases I^III and the early
part of phase IV. Similar modeling shows that assimilation
NUMBER 12
DECEMBER 2011
within the lower crust of partially melted garnet amphibolite metabasalt, without mafic recharge, may produce the
youngest cone-forming lavas in phase IV.
(3) If correct, our model indicates that repeated magma
recharge into a lower crustal domain over a period of 75
kyr may sufficiently heat the crust to induce partial melting. The physical insights revealed through EC-RAxFC
modeling link the 8 km3 of growth of the 40Ar/39Ar-dated
composite cone to the mass flux of magma into the crust.
Our findings suggest that the frequency and amount of
magma injected into the lower crust beneath Santa Mar|¤ a
were higher than anticipated in the numerical^thermal
models of Dufek & Bergantz (2005) and Annen et al.
(2006). Consequently, the incubation time needed to produce hybrid magmas that comprise melts derived from
both fractional crystallization of a parent magma and
wall-rock anatexis in this deep crustal MASH zone may
be as little as 50^75 kyr, which is at the low end of predictions made using these numerical^thermal models.
Moreover, U^Th isotope compositions allow for the possibility that some of the early formed magmas may have
been stored or preserved in isolation from later-formed
melts in the lower crustal MASH zone for several tens of
thousand years prior to eruption.
(4) The key physical aspects of how typical
basaltic-andesite magmas are generated and modified beneath the CAVA highlighted in this study are not likely to
be revealed by regional along-arc or across-arc studies of
basaltic to andesite lava compositions (e.g. Carr et al.,
2007b; Walker et al., 2007). Regional studies clearly are important for broad context, but our findings demonstrate
that detailed study of volcanoes that expose a large
fraction of the eruptive history, combined with careful
40
Ar/39Ar geochronology, and precise isotopic dataç
including short-lived U^Th disequilibriumçcan provide
new insight into magma fluxes and the pace of petrological
processes and crustal modification.
AC K N O W L E D G E M E N T S
We thank Bill Rose, Ru«diger Escobar-Wolf and the geologists and staff of INSIVUMEH, Guatemala for guidance
in the field, and John Hora for laboratory assistance.
Thorough and insightful reviews by Jim Walker, Phillipp
Ruprecht, Georg Zellmer, and Jennifer Garrison helped
us to clarify many issues and improve the paper in important ways. We heartily thank each of them for their
comments.
FU NDI NG
This work, which includes the MS thesis research of
K.E.S., was supported by National Science Foundation
grant EAR-0738007 to B.S.S. and the Weeks fund
2360
SINGER et al.
BASALTIC ANDESITE AT SANTA MARI¤A
administered by the University of Wisconsin-Madison
Department of Geoscience.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
R EF ER ENC ES
Alvarado, G. E., Dengo, C., Martens, U., Bundschuh, J., Aguilar, T.
& Bonis, S. (2007). Stratigraphy and geologic history.
In: Bundschuh, J. & Alvarado, G. E. (eds) Central America: Geology,
Resources and Hazards. London: Taylor & Francis, pp. 345^394.
Annen, C., Blundy, J. D. & Sparks, S. J. (2006). The genesis of intermediate and silicic magmas in deep crustal hot zones. Journal of
Petrology 47, 505^539.
Asimow, P. D. & Ghiorso, M. S. (1998). Algorithmic modifications extending MELTS to calculate subsolidus phase relations. American
Mineralogist 83, 1127^1131.
Beccaluva, L., Bellia, S., Coltori, M., Dengo, G., Giunta, G.,
Mendez, J., Romero, J. & Siena, F. (1995). The northwestern
border of the Caribbean plate in Guatemala: New geological and
petrological data on the Motagua ophiolitic belt. Ophioliti 20, 1^15.
Blundy, J. D. & Cashman, K. (2005). Rapid decompression-driven
crystallization recorded by melt inclusions from Mount St. Helens
volcano. Geology 33, 793^796.
Blundy, J. D., Cashman, K. & Humphreys, M. (2006). Magma
heating by decompression-driven crystallization beneath andesite
volcanoes. Nature 443, 76^80.
Bohrson, W. A. & Spera, F. J. (2007). Energy-Constrained Recharge,
Assimilation, and Fractional Crystallization (EC-RAxFC): A
Visual Basic computer code for calculating trace element and isotope variations of open-system magmatic systems. Geochemistry,
Geophysics, Geosystems 8, 2007GC001781.
Bolge, L. L., Carr, M. J., Milidakis, K. I., Lindsay, F. N. &
Feigenson, M. D. (2009). Correlating geochemistry, tectonics,
and volcanic volume along the Central American volcanic front.
Geochemistry, Geophysics, Geosystems 10, 2009GC002704.
Cameron, B. I. (1998). Melt generation and magma evolution in southeastern Guatemala. PhD thesis, Northern Illinois University,
DeKalb, IL.
Carr, M. J. (1984). Symmetrical and segmented variation of physical
and geochemical characteristics of the Central American volcanic
front. Journal of Volcanology and Geothermal Research 20, 231^252.
Carr, M. J., Feigensen, M. D. & Bennett, E. A. (1990). Incompatible
element and isotopic evidence for tectonic control of source
mixing and melt extraction along the Central American arc.
Contributions to Mineralogy and Petrology 105, 369^380.
Carr, M. J., Feigenson, M. D., Patino, L. C. & Walker, J. A (2003).
Volcanism and geochemistry in Central America: Progress and
problems. In: Eiler, J (ed.) Inside the Subduction Factory. American
Geophysical Union, Geophysical Monographs 138, 153^174.
Carr, M. J., Patino, L. C. & Feigenson, M. D. (2007a). Petrology and
geochemistry of Lavas. In: Bundschuh, J. & Alvarado, G. E. (eds)
Central America: Geology, Resources, and Hazards. London: Taylor &
Francis, pp. 565^590.
Carr, M. J., Saginor, I., Alvarado, G., Bolge, L. L., Lindsay, F. N.,
Milidakis, K., Turrin, B. D., Feigenson, M. D. & Swisher, C. C.,
III (2007b). Element fluxes from the volcanic front of Nicaragua
and Costa Rica. Geochemistry, Geophysics, Geosystems 8,
2006GC001396.
Crisp, J. A. (1984). Rates of magma emplacement and volcanic output.
Journal of Volcanology and Geothermal Research 20, 177^211.
Davidson, J. P., Dungan, M. A., Ferguson, K. M. & Colucci, M. T.
(1987). Crust^magma interactions and the evolution of arc
magmas: The San Pedro^Pellado volcanic complex, southern
Chilean Andes. Geology 15, 443^446.
Dufek, J. & Bergantz, W. (2005). Lower crustal magma genesis and
preservation: A stochastic framework for the evaluation of basalt^
crust interaction. Journal of Petrology 46, 2167^2195.
Eggins, S. M., Woodhead, J. D., Kinsley, L. P. J., Mortimer, G. E.,
Sylvester, P., McCulloch, M. T., Hergt, J. M. & Handler, M. R.
(1997). A simple method for the precise determination of 40
trace elements in geological samples by ICPMS using enriched
isotope internal standardization. Chemical Geology 134, 311^326.
Escobar-Wolf, R. P., Diehl, J. F., Singer, B. S. & Rose, W. I. (2010).
40
Ar/39Ar and paleomagnetic constraints on the evolution of
Volcan Santa Mar|¤ a, Guatemala. Geological Society of America
Bulletin 122, 757^771.
Feigenson, M. D. & Carr, M. J. (1986). Positively correlated Nd and
Sr isotope ratios of lavas from the Central American volcanic
front. Geology 14, 79^82.
Feigenson, M. D., Carr, M. J., Maharaj, S. V., Juliano, S. & Bolge, L.
L. (2004). Lead isotope composition of Central American volcanoes: Influence of the Galapagos plume. Geochemistry, Geophysics,
Geosystems 5, 2003GC000621.
Gazel, E., Carr, M. J., Hoernle, K., Feigenson, M. D., Szymanski, D.,
Hauff, F. & van den Bogaard, P. (2009). Galapagos-OIB signature
in southern Central America: Mantle refertilization by arc^hot
spot interaction. Geochemistry, Geophysics, Geosystems 10,
2008GC002246.
Geldmacher, J., Hoernle, K., van den Bogaard, P., Hauff, F. &
Klu«gel, A. (2008). Age and geochemistry of the Central American
forearc basement (DSDP Leg 67 and 84): Insights into Mesozoic
arc volcanism and seamount accretion on the fringe of the
Caribbean LIP. Journal of Petrology 49, 1781^1815.
Ghiorso, M. S. & Sack, R. O. (1995). Chemical mass transfer in magmatic processes. IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid^
solid equilibria in magmatic systems at elevated temperatures and
pressures. Contributions to Mineralogy and Petrology 119, 197^212.
Halsor, S. P. & Rose, W. I. (1988). Common characteristics of paired
volcanoes in northern Central America. Journal of Geophysical
Research 93, 4467^4476.
Halsor, S. P. & Rose, W. I. (1991). Mineralogical relations and magma
mixing in calc-alkaline andesites from Lake Atitla¤n, Guatemala.
Mineralogy and Petrology 45, 47^67.
Harris, A. J. L., Rose, W. I. & Flynn, L. P. (2003). Temporal trends
in lava dome extrusion at Santiaguito 1922^2000. Bulletin of
Volcanology 65, 77^89.
Hart, S. R. (1984). A large-scale isotope anomaly in the Southern
Hemisphere mantle. Nature 309, 753^757.
Hauff, F., Hoernle, K., Tilton, G., Graham, D. W. & Kerr, A. C.
(2000). Large volume recycling of oceanic lithosphere over short
time scales: geochemical constraints from the Caribbean Large
Igneous Province. Earth and Planetary Science Letters 174, 247^263.
Hawkesworth, C. J., Blake, S., Evans, P., Hughes, R., MacDonald, R.,
Thomas, L. E., Turner, S. P. & Zellmer, G. (2000). Time scales of
crystal fractionation in magma chambersçintegrating physical,
isotopic and geochemical perspectives. Journal of Petrology 41,
991^1006.
Hildreth, W. & Moorbath, S. (1988). Crustal contributions to arc magmatism in the Andes of central Chile. Contributions to Mineralogy and
Petrology 98, 455^489.
2361
JOURNAL OF PETROLOGY
VOLUME 52
Hildreth, W. H. & Lanphere, M. A. (1994). Potassium^argon geochronology of a basalt^andesite^dacite arc systemçThe Mount
Adams volcanic field, Cascade Range of southern Washington.
Geological Society of America Bulletin 106, 1413^1429.
Hoernle, K., van den Bogaard, P., Werner, R., Lissinna, B., Hauff, F.,
Alvarado, G. & Garbe-Scho«nberg, D. (2002). Missing history
(16^71 Ma) of the Gala¤pagos hotspot: Implications of the tectonic
and biological evolution of the Americas. Geology 30, 795^798.
Hofmann, A. W., Jochum, K. P., Seufert, M. & White, W. M. (1986).
Nb and Pb in oceanic basalts: new constraints on mantle evolution.
Earth and Planetary Science Letters 79, 33^45.
Jicha, B. R. & Singer, B. S. (2006).Volcanic history and magmatic evolution of Seguam Island, Aleutian Island arc, Alaska. Geological
Society of America Bulletin 118, 805^822.
Jicha, B. R., Hart, G. L., Johnson, C. M., Hildreth, W. & Beard, B. L.
(2009). Isotopic and trace element constraints on the petrogenesis
of lavas from the Mount Adams volcanic field, Washington.
Contributions to Mineralogy and Petrology 157, 189^207.
Jicha, B. R., Smith, K. E., Singer, B. S., Beard, B. L. & Johnson, C. M.
(2010). Crustal assimilation no match for slab-fluids beneath
Volca¤n de Santa Mar|¤ a, Guatemala. Geology 38, 859^862.
Kelemen, P. B., Yogodzinski, G. M. & Scholl, D. W (2003). Alongstrike variation in the Aleutian Island arc: Genesis of high Mg#
andesite and implications for continental crust. In: Eiler, J (ed.)
Inside the Subduction Factory. American Geophysical Union, Geophysical
Monographs 138, 223^276.
Klein, E. M. (2003). Geochemistry of the igneous oceanic crust.
In: Rudnick, R. L., Holland, H. D. & Turekian, K. K. (eds)
Treatise of Geochemistry Volume 3: The Crust. San Diego, CA: Elsevier,
pp. 433^463.
Leeman, W. P., Carr, M. J. & Morris, J. D. (1994). Boron geochemistry
of the Central American arc: Constraints on the genesis of
subduction-related magmas. Geochimica et Cosmochimica Acta 58,
149^168.
Martens, U., Ortega-Obrego¤n, C., Estrada, J. & Valle, M. (2007).
Metamorphism and metamorphic rocks. In: Bundschuh, J. &
Alvarado, G. E. (eds) Central America: Geology, Resources, and
Hazards. London: Taylor & Francis, pp. 485^522.
Ownby, S. E., Lange, R. A., Hall, C. M. & Delgado-Granados, H
(2010). Origin of andesite in the deep crust and eruption rates in
the Tanc|¤ taro^Nueva Italia region of the central Mexican arc.
Geological Society of America Bulletin, doi: 10.1130/B30124.1.
Patino, L. C. (2007). Intrusive rocks. In: Bundschuh, J. & Alvarado, G.
E. (eds) Central America: Geology, Resources, and Hazards. London:
Taylor & Francis, pp. 549^564.
Patino, L. C., Carr, M. J. & Feigenson, M. D. (2000). Local and regional variations in Central American arc lavas controlled by variations in subducted sediment input. Contributions to Mineralogy and
Petrology 138, 265^283.
Renne, P. R., Swisher, C. C., Deino, A. L., Karner, D. B., Owens, T.
L. & DePaolo, D. J. (1998). Intercalibration of standards, absolute
ages and uncertainties in 40Ar/39Ar dating. Chemical Geology 145,
117^152.
Rogers, N., Thomas, L., MacDonald, R., Hawkesworth, C. &
Mokadem, F. (2006). 238U^230Th disequilibrium in recent basalts
and dynamic melting beneath the Kenya rift. Chemical Geology 234,
148^168.
Rogers, R. D. (2007). Piercing lines between southwest Mexico and
the Chortis Block of northern Central America; constraints on
Cretaceous positions of the Chortis Block. Eos, Transactions, Americn
Geophysical Union 88, Supplement, U53A-07.
Rollinson, H. (1993). Using Geochemical Data: Evaluation, Presentation,
Interpretation. New York: John Wiley.
NUMBER 12
DECEMBER 2011
Rose, W. I. (1972). Santiaguito Volcanic Dome, Guatemala. Geological
Society of America Bulletin 83, 1413^1434.
Rose, W. I. (1987a). Santa Mar|¤ a, Guatemala: A bimodal soda-rich
calc-alkalic stratovolcano. Journal of Volcanology and Geothermal
Research 33, 109^129.
Rose, W. I. (1987b).Volcanic activity at Santiaguito volcano,1976^1984.
In: Fink, J. (ed.) The emplacement of silicic domes and lava flows,
Geological Society of America, Special Papers 212, 17^27.
Rose, W. I., Grant, N. K., Hahn, G. A., Lange, I. M., Powell, J. L.,
Easter, J. & Degraff, J. M. (1977). The evolution of Santa Mar|¤ a volcano, Guatemala. Geology 85, 63^87.
Ru«pke, L. H., Morgan, J. P., Hort, M. & Connolly, J. A. D. (2002).
Are the regional variations in Central American arc lavas due to
differing basaltic versus peridotitic slab sources of fluids? Geology
30, 1035^1038.
Ryan, J. G., Morris, F., Tera, F., Leeman, W. P. & Tsvetkov, A. (1995).
Cross-arc geochemical variations in the Kurile arc as function of
slab depth. Science 270, 625^627.
Sims, K. W. W., Gill, J. B., Dosseto, A., Hoffman, D. L., Lundstrom, C.
C., Williams, R. W., Ball, L., Tollstrup, D., Turner, S., Prytulak, J.,
Glessner, J. J. G., Standish, J. J. & Elliot, T. (2008). An interlaboratory assessment of the thorium isotopic composition of synthetic and rock reference materials. Geostandards and Geoanalytical
Research 32, 65^91.
Singer, B. S., Jicha, B. R., Harper, M. A., Naranjo, J. A., Lara, L. E.
& Moreno-Roa, H. (2008). Eruptive history, geochronology, and
magmatic evolution of the Puyehue^Cordon Caulle volcanic complex, Chile. Geological Society of America Bulletin 120, 599^618.
Singer, B. S., Jicha, B. R., Leeman, W. P., Rogers, N. W., Thirlwall, M.
F., Ryan, J. & Nicolaysen, K. E. (2007). Along-strike trace element
and isotopic variation in Aleutian Island arc basalt: Subduction
melts sediments and dehydrates serpentine. Journal of Geophysical
Research 112, 2006JB004897.
Singer, B. S., Jicha, B. R., Beard, B. L., Johnson, C. M., Smith, K. E.,
Fournelle, J. H. & Rogers, N. W. (2011). Lying in wait: 40Ar/39Ar
and 238U-230Th constraints on time scales of dacite formation
beneath Santa Maria Volcano, Guatemala. Abstracts of the
International Union of Geodesy and Geophysics 21st General
Assembly. Melbourne, Australia.
Singer, B. S., Greene, S. E., Smith, K. E., Jicha, B. R., Fournelle, J.,
Johnson, C. M., Beard, B. L. & Rogers, N. W. (2009). Petrologic,
Sr and Th isotope insight to evolution of the 1902 dacite, Volca¤n
Santa Mar|¤ a, Guatemala. Eos, Transactions, American Geophysical
Union 90(52), Fall Meeting Supplement, Abstract V43H-03.
Singer, B. S., Thompson, R. A., Dungan, M. A., Feeley, T. C.,
Nelson, S. T., Pickens, J. C., Brown, L. L., Wulff, A. W.,
Davidson, J. P. & Metzger, J. (1997). Volcanism and erosion during
the past 930 k.y. at the Tatara^San Pedro complex, Chilean
Andes. Geological Society of America Bulletin 109, 127^142.
Spera, F. J. & Bohrson, W. A. (2004). Open-system magma chamber
evolution: an energy-constrained geochemical model incorporating
the effects of concurrent eruption, recharge, variable assimilation
and fractional crystallization (EC-RAxFC). Journal of Petrology 45,
2459^2480.
Steiger, R. H. & Ja«ger, E. (1977). Subcommission on geochronology:
Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36, 359^362.
Sun, S. S. (1980). Lead isotopic study of young volcanic rocks from
mid-ocean ridges, ocean islands and island arcs. Philosophical
Transactions of the Royal Society of London, Series A 297, 409^445.
Sun, S. S. & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and
processes. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in
2362
SINGER et al.
BASALTIC ANDESITE AT SANTA MARI¤A
the Ocean Basins. Geological Society, London, Special Publications 42,
313^345.
Thomas, R. B., Hirschmann, M. M., Cheng, H., Reagan, M. K. &
Edwards, R. L. (2002). (231Pa/235U)^(230Th/238U) of young mafic
volcanic rocks from Nicaragua and Costa Rica and the influence
of flux melting on U-series systematics of arc lavas. Geochimica et
Cosmochimica Acta 66, 4287^4309.
Vogel, T. A., Patino, L. C., Eaton, J. K., Valley, J. V., Rose, W. I.,
Alvarado, G. E. & Viray, E. L. (2006). Origin of silicic magmas
along the Central American volcanic front: Genetic relationship
to mafic melts. Journal of Volcanology and Geothermal Research 156,
217^228.
Walker, J. A., Carr, M. J., Patino, L. C., Johnson, C. M.,
Feigenson, M. D. & Ward, R. L. (1995). Abrupt change in magma
generation processes across the Central American arc in southeastern Guatemala: flux dominated melting near the base of the
wedge to decompression melting near the top of the wedge.
Contributions to Mineralogy and Petrology 120, 378^390.
Walker, J. A., Mickelson, J. E., Thomas, R. B., Patino, L. C.,
Cameron, B., Carr, M. J., Feigenson, M. D. & Edwards, R. L.
(2007). U-series disequilibria in Guatemalan lavas, crustal contamination and implications for magma genesis along the Central
American subduction zone. Journal of Geophysical Research 112,
2006JB004589.
White, S. M., Crisp, J. A. & Spera, F. J. (2006). Long-term volumetric
eruption rates and magma budgets. Geochemistry, Geophysics,
Geosystems 7, 2005GC001002.
Williams, S. & Self, S. (1982). The October 1902 plinian eruption of
Santa Mar|¤ a Volcano, Guatemala. Journal of Volcanology and
Geothermal Research 16, 33^56.
Witham, C. S. (2005). Volcanic disasters and incidents: A new database. Journal of Volcanology and Geothermal Research 148, 191^233.
2363