JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 1
PAGES 203±219
2004
DOI: 10.1093/petrology/egg086
Chemical versus Temporal Controls on the
Evolution of Tholeiitic and Calc-alkaline
Magmas at Two Volcanoes in the
Alaska---Aleutian Arc
RHIANNON GEORGE1*y, SIMON TURNER1y, CHRIS
HAWKESWORTH1, CHARLES R. BACON2, CHRIS NYE3,
PETE STELLING2 AND SCOTT DREHER4
1
DEPARTMENT OF EARTH SCIENCES, WILLS MEMORIAL BUILDING, UNIVERSITY OF BRISTOL,
BRISTOL BS8 1RJ, UK
2
US GEOLOGICAL SURVEY, MS 910, 345 MIDDLEFIELD ROAD, MENLO PARK, CA 94025, USA
3
ALASKA VOLCANO OBSERVATORY, ALASKA DIVISION OF GEOLOGY AND GEOPHYSICAL SURVEYS,
794 UNIVERSITY AVENUE SUITE 200, FAIRBANKS, AK 99709, USA
4
DEPARTMENT OF GEOGRAPHY, GEOLOGY AND ANTHROPOLOGY, INDIANA STATE UNIVERSITY,
TERRE HAUTE, IN 47809, USA
RECEIVED SEPTEMBER 18, 2002; ACCEPTED JULY 22, 2003
The Alaska---Aleutian island arc is well known for erupting
both tholeiitic and calc-alkaline magmas. To investigate the
relative roles of chemical and temporal controls in generating
these contrasting liquid lines of descent we have undertaken a
detailed study of tholeiitic lavas from Akutan volcano in the
oceanic Aleutian arc and calc-alkaline products from Aniakchak
volcano on the continental Alaskan Peninsula. The differences
do not appear to be linked to parental magma composition. The
Akutan lavas can be explained by closed-system magmatic
evolution, whereas curvilinear trace element trends and a large
range in 87 Sr/86 Sr isotope ratios in the Aniakchak data appear
to require the combined effects of fractional crystallization,
assimilation and magma mixing. Both magmatic suites preserve
a similar range in 226 Ra---230 Th disequilibria, which suggests
that the time scale of crustal residence of magmas beneath both
these volcanoes was similar, and of the order of several thousand
years. This is consistent with numerical estimates of the time
scales for crystallization caused by cooling in convecting crustal
magma chambers. During that time interval the tholeiitic
Akutan magmas underwent restricted, closed-system, compositional evolution. In contrast, the calc-alkaline magmas beneath
Aniakchak volcano underwent significant open-system
*Corresponding author. E-mail: [email protected]
y
Present address: GEMOC, Department of Earth and Planetary
Sciences, Macquarie University, Sydney, N.S.W. 2109, Australia.
compositional evolution. Combining these results with data
from other studies we suggest that differentiation is faster in
calc-alkaline and potassic magma series than in tholeiitic series,
owing to a combination of greater extents of assimilation,
magma mixing and cooling.
KEY WORDS:
time scales
uranium-series; Aleutian arc; magma differentiation;
INTRODUCTION
The existence of the tholeiitic and calc-alkaline lines
of liquid descent in arc magma suites has long been
recognized (e.g. Miyashiro, 1974); many studies have
investigated the role of parental magma composition
and/or the effects of crustal assimilation in determining
which evolutionary path is followed (e.g. Grove &
Baker, 1984, and references therein). Fewer studies
have sought to constrain the time scales involved in
magmatic differentiation and how those might vary
with magma composition and differentiation path,
Journal of Petrology 45(1) # Oxford University Press 2004; all rights
reserved
JOURNAL OF PETROLOGY
VOLUME 45
largely because this has only become possible recently
through the application of U-series isotopes. Such
investigations remain in their infancy and most to
date have concentrated on within-plate lavas (e.g.
Reagan et al., 1992; Bourdon et al., 1994; Condomines
et al., 1995; Thomas, 1999; Vigier et al., 1999; Cooper
et al., 2001).
Several studies of volcanic arc rocks have sought to
use mineral---whole-rock isochrons to obtain crystallization ages. However, complexity arises from the
potential for the ages obtained to reflect the entrainment of old cumulate crystals, and mixing between
crystal aliquots of different age (Volpe & Hammond,
1991; Volpe, 1992; Schaefer et al., 1993; Heath et al.,
1998). Thus, it is often more straightforward to
look for correlations between U-series disequilibria
and indices of differentiation in whole rocks [see
Hawkesworth et al. (2000) for a recent discussion].
Trends of decreasing (226 Ra/230 Th) with increasing
SiO2 in several along-arc suites of rocks have suggested
time scales ranging from hundreds to several thousand
years for differentiation from basaltic to andesitic and
dacitic magmas, respectively (Turner et al., 2000,
2001a). However, such interpretations implicitly
assume that the primary disequilibria were similar for
all volcanoes. This is clearly an oversimplification, and
so the next step is to investigate how (226 Ra/230 Th)
varies with indices of differentiation within individual
volcanoes whose erupted products span a range of
compositions. Interpretation in terms of differentiation
time scales also requires the assumption that parental
magmas for a particular volcano have similar initial
disequilibria.
Here we present the results of a combined geochemical, radiogenic isotope and U---Th---Ra isotope
disequilibria study of tholeiitic rocks from Akutan
volcano in the oceanic Aleutian arc and calc-alkaline
eruptive products of Aniakchak volcano on the continental Alaskan Peninsula. The aim was to investigate the relative roles of primary magma composition,
crustal interaction and magma residence times in
producing these two different liquid lines of descent.
These data are then compared with the results from a
study of the evolution of potassic magmas from a
rear-arc volcano (Sangeang Api) in the Sunda arc
(Turner et al., 2003b) allowing comparison between the
three major liquid lines of descent (tholeiitic, calcalkaline and potassic) commonly observed in arc
magmas. Our evaluation of the results leads us to
hypothesize that magma residence times beneath arc
volcanoes are broadly similar, and that it is the rate
of cooling and extent of assimilation and magma
mixing that exert the primary controls on the efficiency of differentiation and which liquid line of
descent is followed.
NUMBER 1
JANUARY 2004
THE ALASKA---ALEUTIAN
ISLAND ARC
The Alaska---Aleutian island arc forms the northernmost segment of the circum-Pacific subduction system
and transgresses both continental (Alaska) and oceanic
(Aleutian) crust (Fig. 1a). Formed by the NE-directed
subduction of the Pacific plate, the arc comprises some
40 historically active volcanoes, which erupt a diverse
range of magmas (Myers et al., 1985; Fournelle et al.,
1994; Kay & Kay, 1994). A recent along-arc U-series
study by George et al. (2003) investigated the relative
contributions of components from the subducting plate
and the time scales of their transfer and provides the
background to the present work.
A key feature of the Alaska---Aleutian arc is that it is
characterized by islands that frequently contain adjacent tholeiitic and calc-alkaline volcanoes and many
individual volcanoes have erupted both tholeiitic and
calc-alkaline products at different times in their history
(e.g. Myers et al., 1985; Nye, 2000). Not surprisingly,
there has been much interest in and debate about the
origin of this juxtaposition and magmatic diversity. In
one group of models, Myers et al. (1985) proposed that
calc-alkaline centres form in an early, immature stage
of conduit development where significant lithospheric
debris is incorporated into the magma during transit.
They argued that these centres subsequently evolve
into tholeiitic ones as the conduit becomes increasingly
thermally and chemically preconditioned, reducing
the extent and effects of assimilation. On the other
hand, Kay & Kay (1994) have long maintained that
there is a tectonic control, whereby the magmas that
feed the high-volume tholeiitic centres pass quickly
through crust which is under extension at the ends of
segments. These magmas enter shallow-level magma
chambers where crystallization occurs at low pressures
and minimal crustal interaction takes place. Conversely, calc-alkaline centres develop where there is little
extension and small volumes of magma pass slowly
through the crust and undergo both crystallization
and more extensive assimilation at greater depths.
AKUTAN VOLCANO
Akutan volcano is a composite stratovolcano (Romick
et al., 1990) formed on oceanic crust at the eastern end
of the Aleutian chain (Fig. 1a). The volcano is highly
active, and has an active intra-caldera cinder cone
(Fig. 1b) and a history extending back to the Pleistocene, including a 510 km3 caldera-forming eruption
that occurred 1611 years BP (Richter et al., 1998;
weighted mean of six calibrated radiocarbon age determinations). Young andesitic lava flows, some of which
were erupted in 1978 and 1929 [see Richter et al.
204
GEORGE et al.
TWO VOLCANOES IN ALASKA±ALEUTIAN ARC
Fig. 1. (a) Map of the Alaska---Aleutian arc showing locations of Akutan and Aniakchak. (b) Map of Akutan island showing the main
Holocene eruptions. (c) Map of Aniakchak caldera showing the extent of the 1931 flows. [Note change of scale between (b) and (c).]
205
JOURNAL OF PETROLOGY
VOLUME 45
(1998) for age constraints], cover the 2 km diameter
caldera floor south and north of the cinder cone and
extend several hundred metres downslope through
a gap in the crater rim. Flows extruded in 1947 blanket
the central portion of the NW end of the island at
Lava Point, where about 4 km2 of jagged aa basalt
occurs adjacent to several cinder cones. The majority
of the erupted products range from basalt to andesite in
composition. The pre-eruptive H2O contents of some
of the older magmas are estimated to be around 5%
on the basis of the presence of pargasitic amphibole
(Romick et al., 1990) but this is likely to be a maximum. A minimum H2O content would be 2% and
this is the estimate we will use in viscosity calculations
below. A trachytic dyke (68% SiO2), related to a late
cinder cone, demonstrates that more evolved magmas
are also occasionally produced in this magmatic system
and a high-silica pyroclastic flow, of unknown age and
apparent offshore origin, containing rhyolite obsidian
clasts (72% SiO2), is exposed along the southern shore
of the island (Richter et al., 1998). The compositional
affiliation of the magmas is inferred to have changed
through time from tholeiitic to transitional tholeiitic,
which Romick et al. (1990) suggested may reflect fractionation at increasingly lower pressures, possibly
beginning at a depth of 25 km and becoming shallower over time. There is little field or petrographic
evidence for mixing and within-suite variations can be
explained by closed-system fractionation (Romick et al.,
1990). Pyroxene geothermometry studies indicate
temperatures around 1120 and 950 C for basaltic
andesites and dacites, respectively (Romick et al.,
1990), and this range is consistent with our own estimates based on calculations performed using the
MELTS algorithm (Ghiorso & Sack, 1995). The samples analysed in this study are basaltic andesites from
the post 1400 years BP period and include samples from
the historical 1910, 1929 and 1978 eruptions, which
were of the order of 01 km3 (Simkin & Siebert, 1994).
They typically contain 1---3 mm sized phenocrysts in the
proportions 10---20% plagioclase, 2---5% clinopyroxene
and 1---4% orthopyroxene, with trace amounts of
olivine and opaques set in a cryptocrystalline groundmass (Richter et al., 1998). A xenolith (found in lavas
scattered on ridges) and a rhyolitic obsidian clast
(denoted by the `glass' in Table 1) from the pyroclastic
flow were also analysed to expand the compositional
range of the lavas investigated.
ANIAKCHAK VOLCANO
Aniakchak volcano lies to the east of Akutan on the
Alaskan Peninsula (Fig. 1a), where numerous domes,
flows and cones occupy the interior of a 10 km diameter
caldera (Fig. 1c). This volcano is predominantly
NUMBER 1
JANUARY 2004
calc-alkaline but erupted tholeiitic lavas between 450
and 240 ka (Nye et al., 1993). The flows and tuffs of the
pre-caldera volcano consist mainly of basaltic andesite,
two-pyroxene andesite and dacite. Ash flows from the
caldera-forming eruption at 3430 10 years BP
(Miller & Smith, 1987) may total 450 km3 (Miller &
Smith, 1977, 1987), and range in composition
from andesite to rhyodacite with whole-rock SiO2
contents as high as 704% (Dreher, 2002). The postcaldera volcanic rocks include andesite and basaltic
andesite but are predominantly crystal-poor (20%),
plagioclase---pyroxene dacites erupted from intracaldera cones. The largest cone is Vent Mountain,
which is 25 km in diameter and has erupted mainly
dacitic magmas with an SiO2 content ranging from 61
to 67% (Neal et al., 1992). For these rocks, Fe---Ti oxide
thermometry indicates pre-eruptive temperatures of
around 855---860 C (Bacon et al., 1997). Melt inclusion
studies of basaltic andesite from Blocky Cone, the
dacite `pink pumice' from Half Cone, and the 1931
dacite indicate that these magmas had pre-eruptive
H2O contents in the range 3---4% at depths of 3---5 km
(Bacon, 2002). Macroscopic evidence for mingling
between andesite and dacite magmas is common in
the caldera-forming ignimbrite, and magma mixing is
inferred from petrographic and chemical data to have
been a major process throughout the post-caldera history of the volcano (Neal et al., 1992; Bacon et al., 1997;
Bacon, 2002). For the purposes of this study, we analysed samples ranging from basaltic andesite to rhyodacite in composition. Two are from the zoned
3430 years BP caldera-forming eruption, four are post
3430 lavas, one a Vent Mountain lava and eight samples belong to the 1931 eruptions. The 1931 event
produced 03---05 km3 of tephra zoned from dacite
to andesite, and minor andesite and dacite lava flows
(Neal et al., 2001).
ANALYTICAL TECHNIQUES
Major element compositions were determined by
X-ray fluorescence spectroscopy at the GeoAnalytical
Laboratory of Washington State University ( Johnson
et al., 1999) and are reported here normalized to 100%
on a volatile-free basis. Trace elements were measured
by inductively coupled plasma mass spectrometry
(ICP-MS) at Durham University, where powders
were digested using standard HF---HNO3 treatments,
ensuring that no fluoride residues formed. Internal
drift was monitored by spiking with Rh, In and Bi
before dilution to 35% HNO3. Solutions were analysed on a Perkin---Elmer---SCIEX Elan 600 system
using a cross-flow nebulizer. Oxide interferences for
most analyses were much less than 25% of the total
signal. Corrections were made using oxide/metal ratios
206
GEORGE et al.
TWO VOLCANOES IN ALASKA±ALEUTIAN ARC
Table 1: Geochemical and isotopic data for Akutan lavas
Sample:
96PSXEN 96PS09
Age (years/AD): -------
96PS12
96PS01
AK81-35 96PS28
Historical Historical Historical 51870
41400
96PS32
96PS30
96PS13
96PS18
96PS23
96PS17
GLASS
1910
41910
1929
1978?
1978
1978?
?
wt %
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
P2O5
42.19
0.69
52.75
1.19
55.82
0.92
57.19
1.04
57.03
1.06
59.65
0.99
57.03
1.03
57.31
1.03
55.61
1.13
54.33
1.08
55.37
1.15
55.70
1.13
72.49
0.23
26.09
9.96
17.80
11.35
17.70
9.03
18.24
7.81
18.21
7.96
16.49
8.10
18.09
7.90
18.17
7.80
17.09
9.36
18.03
9.69
17.06
9.50
17.11
9.28
13.76
2.60
0.11
5.64
0.22
4.00
0.21
3.80
0.20
2.51
0.21
2.59
0.21
2.63
0.20
2.85
0.20
2.67
0.21
3.77
0.19
3.59
0.22
3.69
0.22
3.66
0.08
0.14
14.11
1.12
8.48
3.45
0.62
7.61
7.27
4.60
0.94
7.44
4.34
0.94
5.59
4.83
1.26
7.24
4.52
0.93
7.14
4.54
0.93
7.64
4.11
0.88
7.74
4.27
0.76
7.65
3.91
0.83
4.29
0.86
7.67
4.17
0.86
4.83
4.92
0.14
0.17
0.20
0.21
0.24
0.20
0.20
0.21
0.32
0.21
0.20
0.03
0.07
0.03
0.90
ppm
Sc
7
28
21
21
20
20
21
21
27
29
27
27
5
343
290
199
139
160
96
148
136
237
262
260
235
1
Cr
49
11
26
25
2
5
3
9
31
29
7
3
0
Ni
12
2
4
0
3
0
0
0
2
3
0
1
0
116
V
Cu
12
Zn
52
Ga
19.3
0.97
Rb
Sr
567
2.1
4.6
Y
Zr
Nb
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Ta
Pb
Th
U
Sr/86 Sr
143
38
0.69
1.57
0.23
1.05
0.32
0.30
0.35
0.06
0.37
0.08
0.21
0.03
0.20
0.03
0.15
0.04
Hf
87
0.09
0.11
Nd/144 Nd
0.97
0.080
0.041
0.70351
-------
80
17.4
13.24
410
27.4
65.8
76
55
53
49
56
53
101
104
108
86
28
77
75
87
77
88
86
87
86
85
61
18.4
16.71
18.4
20.40
76
18.9
19.1
30.48
18.6
20.68
18.4
20.50
18.5
19.69
19.0
16.38
19.2
19.22
19.0
19.35
388
28.0
89.2
419
34.8
98.0
1.80
2.08
20.74
407
34.3
99.3
345
418
41.9
140.4
1.19
1.32
1.68
0.86
236
5.29
326
6.92
13.24
2.11
17.39
2.65
19.25
2.97
7.86
19.5
3.01
10.47
25.54
10.38
3.34
12.36
3.73
14.25
4.40
14.28
4.45
1.17
4.03
1.21
4.16
1.42
5.05
0.69
4.42
0.71
4.51
0.95
2.68
344
4.57
2.00
347
35.2
98.4
1.80
2.07
365
34.4
95.1
1.78
2.00
334
426
28.5
86.3
1.64
1.63
32.9
93.4
1.80
1.84
323
379
34.1
94.9
1.83
1.97
7.86
7.80
3.85
19.56
3.03
8.30
20.56
3.23
7.61
19.29
3.00
18.76
2.91
19.19
2.96
18.05
5.46
14.47
4.44
14.17
4.43
14.48
4.46
15.69
4.83
13.81
4.38
14.22
4.35
1.42
5.17
1.62
6.25
1.42
5.24
1.45
5.10
1.40
5.12
1.37
5.05
1.34
5.01
1.37
5.09
0.87
5.54
0.88
5.57
1.04
6.69
0.88
5.60
0.88
5.57
0.87
5.65
0.81
4.87
0.86
5.37
0.86
5.49
0.96
2.78
1.20
3.43
1.18
3.43
1.44
4.14
1.19
3.47
1.19
3.46
1.20
3.42
1.01
2.84
1.15
3.31
1.18
3.36
0.42
2.69
0.44
2.85
0.54
3.44
0.53
3.47
0.66
4.25
0.54
3.45
0.53
3.46
0.54
3.44
0.44
2.85
0.52
3.32
0.52
3.36
0.43
1.99
0.46
2.64
0.55
2.87
0.54
2.967
0.66
4.13
0.54
2.88
0.55
2.92
0.55
2.87
0.44
2.55
0.53
2.75
0.55
2.85
0.10
7.03
0.14
7.43
0.14
9.45
0.19
10.11
0.14
9.51
0.13
9.06
0.14
9.17
0.13
3.20
0.14
8.87
0.14
7.57
1.195
0.629
1.706
0.872
1.865
1.027
0.70349
0.51305
0.70346 0.70348
0.51305 -------
7.98
314
370
19.36
3.02
------9.34
464
1.81
2.05
416
346
7.79
7.86
344
2.59
3.08
35.3
98.7
331
1.872
0.979
2.830
1.481
1.847
0.980
1.857
0.980
1.828
0.948
1.547
0.799
1.673
0.900
1.757
0.932
0.70358
0.51305
0.70352
0.51311
0.70349
0.51301
0.70350
0.51320
0.70349
0.51304
0.70349
0.51316
0.70348
0.51300
0.70348
0.51306
207
17.6
119.83
49
63.1
500.2
15.13
6.05
1129
34.16
76.92
10.52
41.92
9.98
0.99
9.11
1.57
9.65
2.08
6.22
1.02
6.82
1.08
13.54
1.08
19.14
13.899
6.181
0.70353
0.51317
JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 1
JANUARY 2004
Table 1: continued
Sample:
96PSXEN 96PS09
Age (years/AD): ------206
Pb/204 Pb
207
Pb/
204
208
Pb/204 Pb
230
(
Th/
Pb
232
Th)
(238 U/232 Th)
(238 U/230 Th)
(226 Ra/230 Th)i
96PS12
96PS01
AK81-35 96PS28
Historical Historical Historical 51870
41400
18.943
15.636
18.975
15.599
18.954
15.592
18.963
15.596
18.981
15.602
-------
38.674
1.357
38.568
1.318
38.540
1.430
38.549
1.349
38.559
1.348
-------
1.632
1.145
1.597
1.211
1.514
1.085
1.721
1.276
1.701
1.261
-------
-------
-------
-------
-------
------1.394
1.605
1.139
-------
96PS32
96PS30
96PS13
96PS18
96PS23
96PS17
GLASS
1910
41910
1929
1978?
1978
1978?
?
18.954
15.581
18.949
15.579
18.959
15.596
18.957
15.606
18.963
15.591
18.930
15.556
18.799
15.557
38.510
1.347
38.500
1.350
38.559
1.359
38.580
1.345
38.528
1.324
38.437
1.358
38.337
1.366
1.710
1.195
1.732
1.186
1.699
1.158
1.619
1.165
1.626
1.228
1.874
1.185
1.377
0.988
1.038
1.081
1.062
1.175
1.034
1.156
-------
Total Fe as FeO.
measured on matrix-matched standard solutions, and
calibration was achieved using matrix-matched international (USGS W2) and in-house reference materials.
Total procedural blanks for all elements were negligible
for all analyses, based on digestion of 01 g of powder.
Sr, Nd and Pb were separated using cation and anion
exchange separation techniques following standard
HF---HNO3 dissolutions at the Open University (OU),
the NERC Isotope Geosciences Laboratory (NIGL) or
at Adelaide University (AU). Sr and Nd isotopes were
analysed by thermal ionization mass spectrometry
(TIMS) at OU, NIGL and AU and corrected for
within-run mass bias to 86 Sr/88 Sr ˆ 01194 and
144
Nd/146 Nd ˆ 07219. All Sr and Nd data are reported
relative to values of 071025 for NBS 987 and 051184
for La Jolla. Uncertainties, as determined from the 2s
reproducibility of the NBS 987 (OU and NIGL), La
Jolla (AU) and J&M (OU) standards during the
course of analysis, were 28 ppm for Sr and 21---25 ppm
for Nd. Pb isotopes were analysed either at the OU,
on a Nu-Instruments multi-collector ICP-MS system
using Tl for internal mass bias correction (Belshaw et al.,
1998), or at AU by TIMS where ratios were corrected
for 1% per atomic mass unit mass fractionation using
the recommended values of NBS 981 (Todt et al.,
1996). Reproducibility is estimated at 02% 2s. Total
procedural blanks were less than 1 ng, 500 pg and
500 pg for Sr, Nd and Pb, respectively.
U---Th separation was carried out on standard
HF---HCl---HNO3 dissolutions to which a mixed
229
Th---236 U tracer had been added. Samples were
treated with HCl and H3BO4 to ensure sample-spike
equilibration and to eliminate fluorides. U and Th
were isolated using anionic exchange resin, with
HNO3, HCl and HBr as elutants, and then loaded
onto degassed Re filaments along with colloidal graphite and an HNO3---H3PO4 solution, respectively. Th
and U concentrations and (234 U/238 U) ratios were
determined to 05% at the OU by TIMS system
fitted with an RPQ II energy filter for high abundance
sensitivity (van Calsteren & Schwieters, 1995). The
(230 Th/232 Th) measurements were made on the NuInstruments multi-collector ICP-MS system at the
OU using techniques and reproducibility reported by
Turner et al. (2001b). Decay constants used in the
calculation of activity ratios (denoted by parentheses)
were l230 Th ˆ 91952 10 ÿ6 , l232 Th ˆ 4948 10 ÿ11
and l238 U ˆ 1551 10 ÿ10 . (234 U/238 U) ratios in all
samples are within error of unity, suggesting that
sub-solidus (seawater) alteration has not modified the
primary compositions of the samples.
Ra was separated from samples with known historical eruption ages using techniques identical to those
described by Turner et al. (2000). Powders were
weighed to yield 50 fg of Ra, and spiked with 228 Ra
to achieve a 228 Ra/226 Ra ratio of 1. Ra was preconcentrated using a double pass through cation
exchange resin, using HCl, H2O and HNO3 as elutants. Ra and Ba were then separated by chromatographic separation using ElChrom Sr-spec resinTM and
HNO3 as the elutant (Chabaux et al., 1994). Samples
were loaded onto degassed Re filaments with a
Ta---HF---H3PO4 activator solution (Birck, 1986). Samples were analysed dynamically by TIMS at the OU.
Analytical precision was better than 1% (2s). Repeat
analyses of a sample from Mt. Lassen and an in-house
standard (ThITS) were used to assess the accuracy and
reproducibility of the analyses, which is estimated to be
13% for (226 Ra/230 Th) ratios. Total procedural
blanks were below detection limits (501 fg/g). The
decay constant used to calculate 226 Ra activities was
l226 Ra ˆ 4332 10 ÿ4 .
RESULTS
The new analytical data obtained as part of this study
are presented in Tables 1 and 2. These and existing
major element analyses are plotted in Figs 2 and 3 with
208
Table 2: Geochemical and isotopic data for Aniakchak volcanic rocks
Sample:
98AC2D
AC38B
94CNA11
92CNA22
NA94-5
NA94-7
94AMC 2
97ANB-44
97ANB-27
97ANB-33
97ANB-26
97ANB-32
NA93-94a
92CNA06
92CNA05
Age (yr/AD):
3500
3500
pre-3500
post-3500
post-3500
post-3500
post-3500
1931
1931
1931
1931
1931
1931
1931
1931
70.40
0.54
53.24
1.19
52.52
1.17
53.59
1.20
54.54
1.34
65.05
0.94
57.78
1.26
58.18
1.25
61.67
1.07
62.55
1.03
63.83
0.94
66.40
0.78
67.06
0.78
68.00
0.70
16.46
7.39
14.92
2.63
17.60
9.55
18.10
9.39
18.07
8.69
16.97
9.80
15.98
4.73
16.68
7.79
16.77
7.49
16.54
5.87
16.44
5.57
16.26
5.07
15.89
4.05
15.66
3.97
15.52
3.65
MnO
0.21
0.19
4.29
8.53
0.20
3.75
8.44
2.10
5.00
0.18
1.84
4.48
0.16
3.11
6.80
0.20
2.28
5.32
0.19
1.23
3.79
0.20
3.19
7.02
0.21
4.37
10.10
0.18
3.99
9.52
0.17
2.90
6.29
0.14
0.59
1.98
0.18
MgO
1.16
3.44
0.15
1.05
3.18
0.15
0.86
2.92
K2O
4.69
1.54
5.74
2.96
3.41
1.71
3.03
0.94
3.44
1.10
3.53
1.19
5.32
2.48
4.32
1.37
4.36
1.40
4.88
1.75
4.89
1.85
5.05
2.01
5.55
2.32
5.36
2.55
5.39
2.63
P2O5
0.68
0.10
0.30
0.20
0.22
0.25
0.31
0.38
0.43
0.42
0.38
0.33
0.24
0.23
0.19
TiO2
Al2O3
FeO
CaO
Na2O
209
ppm
Sc
23
11
25
32
31
32
14
25
26
21
20
16
16
12
12
V
113
4
239
366
359
337
63
196
184
105
92
66
41
38
32
Cr
11
1
21
39
73
27
-------
9
41
9
20
9
11
80
0
Ni
35
9
20
18
13
10
-------
1
2
0
3
0
0
1
0
Cu
19
6
102
91
76
79
12
20
17
10
9
8
9
8
8
Zn
103
81
18.8
91
81
99
86
98
108
108
104
91
117
79
78
18.8
43.81
19.2
21.22
88
19.8
20.5
27.67
19.4
61.41
19.6
30.11
17.9
62.36
17.8
64.36
Ga
Rb
19.2
33.82
Sr
483
Y
37.7
144.8
Zr
Nb
Cs
Ba
9.77
1.52
531
Ce
18.79
41.80
Pr
5.92
La
73.44
226
53.1
296.4
17.68
3.46
973
29.22
62.00
8.39
384
36.2
195.4
12.54
1.25
414
372
24.3
95.5
5.72
0.96
304
24.59
21.0
32.61
21.9
46.06
21.3
48.10
19.0
46.68
21.3
57.20
422
396
300
429
457
448
423
352
381
256
241
26.8
108.1
30.5
120.1
45.7
245.1
32.3
124.5
35.0
134.1
42.0
180.5
42.2
187.1
38.5
181.2
45.6
221.8
44.6
243.1
44.5
248.8
6.37
1.15
363
7.40
1.40
384
14.77
2.78
772
7.71
1.39
471
8.36
1.48
512
11.27
2.09
667
11.74
2.25
695
11.14
2.16
661
13.68
2.66
799
14.19
2.94
798
14.62
2.99
809
17.60
39.39
9.80
22.22
11.87
26.26
12.48
27.53
23.49
51.27
14.60
32.18
16.13
35.66
20.27
44.84
20.91
45.22
19.24
41.78
23.48
50.58
23.40
50.27
23.15
49.62
5.60
3.17
3.75
3.94
7.08
4.56
5.04
6.25
6.30
5.77
6.94
6.84
6.68
TWO VOLCANOES IN ALASKA±ALEUTIAN ARC
58.40
1.44
SiO2
GEORGE et al.
wt %
Table 2: continued
98AC2D
AC38B
94CNA11
92CNA22
NA94-5
NA94-7
94AMC 2
97ANB-44
97ANB-27
97ANB-33
97ANB-26
97ANB-32
NA93-94a
92CNA06
92CNA05
3500
3500
pre-3500
post-3500
post-3500
post-3500
post-3500
1931
1931
1931
1931
1931
1931
1931
1931
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
210
Lu
Hf
Ta
Pb
U
87
Sr/86 Sr
Nd/144 Nd
206
Pb/204 Pb
207
Pb/204 Pb
208
Pb/204 Pb
230
(
Th/232 Th)
(238 U/232 Th)
(238 U/230 Th)
(226 Ra/230 Th)i
14.14
3.93
16.44
4.42
17.59
4.72
29.81
7.66
20.16
5.36
22.36
5.92
27.13
7.05
27.14
7.00
24.85
6.31
29.59
7.47
28.66
7.19
28.08
7.01
2.02
6.74
1.04
2.09
1.50
6.24
0.98
1.20
4.12
0.66
1.33
1.99
7.56
1.20
6.22
1.01
2.11
7.49
1.21
1.79
7.03
1.15
2.03
7.12
1.13
1.79
5.53
0.88
1.87
6.03
0.97
2.06
4.45
0.73
1.48
4.99
0.80
1.67
8.11
1.33
7.08
1.15
1.71
6.89
1.12
6.16
1.26
3.50
1.74
5.05
5.88
1.22
3.42
4.11
0.85
2.39
0.92
2.62
4.94
1.04
2.90
7.47
1.54
4.45
1.10
3.09
5.76
1.19
3.34
1.42
3.98
6.89
1.43
4.02
1.30
3.66
7.32
1.54
4.34
1.51
4.30
0.53
3.35
0.53
5.25
0.85
0.52
3.37
0.54
0.36
2.36
0.37
2.59
0.42
0.44
2.86
0.45
0.69
4.52
0.72
3.01
0.47
0.51
3.26
0.52
3.98
0.63
0.63
4.03
0.64
3.72
0.58
0.67
4.42
0.71
4.45
0.71
3.69
0.62
5.54
1.09
11.00
4.96
0.76
5.25
2.58
0.62
3.56
0.41
4.22
3.18
0.48
4.42
6.38
0.95
8.98
0.49
5.17
3.60
0.55
5.58
0.72
7.41
4.87
0.75
7.72
0.71
7.24
5.78
0.87
9.11
0.91
9.13
4.410
2.094
0.70324
1.914
0.898
0.70327
1.066
0.70325
2.548
1.144
0.70331
4.649
2.571
0.70336
1.289
0.70346
3.038
1.381
0.70341
1.916
0.70346
4.438
2.009
0.70346
1.947
0.70345
5.339
2.389
0.70351
2.623
0.70337
3.356
1.475
0.70333
0.51313
8.33
0.80
7.59
7.070
3.117
0.70340
2.94
2.202
2.813
0.51304
0.51299
0.51311
5.975
2.694
0.70342
18.847
15.620
18.876
15.583
18.911
15.614
18.883
15.585
18.887
15.580
18.899
15.599
18.882
15.590
0.51307
18.876
15.572
38.412
1.449
38.465
1.338
38.444
1.461
38.383
1.457
38.359
1.350
38.331
1.427
38.393
1.411
38.594
1.428
38.468
1.404
38.569
1.410
38.486
1.416
38.451
1.396
38.528
1.407
38.487
1.399
38.429
1.420
1.489
1.028
1.409
0.854
1.441
0.986
1.401
0.961
1.406
1.041
1.509
1.057
1.595
1.130
1.415
0.991
1.460
1.040
1.425
1.010
1.373
0.970
1.478
1.059
1.358
0.965
1.468
1.049
1.381
0.972
1.113
1.167
1.073
1.055
1.050
1.046
1.039
1.005
Total Fe as FeO.
0.51312
5.832
6.46
1.23
9.39
18.866
15.565
-------
0.51313
4.285
6.33
0.67
4.43
0.71
0.51311
18.838
15.547
-------
0.51305
4.231
4.78
0.68
6.97
1.47
4.25
18.839
15.559
-------
0.51307
4.77
0.57
7.10
18.854
15.565
-------
0.51296
3.31
0.62
6.18
0.51306
18.865
15.582
-------
0.51298
0.47
6.85
18.890
15.588
-------
0.51298
0.40
5.33
18.874
15.573
-------
0.51308
4.51
JANUARY 2004
143
24.23
6.31
NUMBER 1
Th
34.74
8.54
VOLUME 45
Yb
26.14
6.71
JOURNAL OF PETROLOGY
Sample:
Age (yr/AD):
GEORGE et al.
TWO VOLCANOES IN ALASKA±ALEUTIAN ARC
Fig. 2. (a) K2O vs SiO2 variation diagram for the Akutan and
Aniakchak samples. Xenoliths and prehistoric Akutan lavas (&)
are compiled from this study and Romick et al. (1990). High- and
low-K boundaries from LeMaitre et al. (1989). (b) FeO /MgO vs
SiO2 diagram with the tholeiitic---calc-alkaline dividing line from
Miyashiro (1974). Akutan samples with SiO2 565% plot on the
tholeiitic side of the line and define a tholeiitic trend directed towards
the trachytic dyke [SiO2 678%, FeO /MgO ˆ 553 from Richter
et al. (1998)], whereas those from Aniakchak form a calc-alkaline
trend, which is shallower than the divide. Fields for the Aleutian
(pale grey) and Alaskan (dark grey) volcanics are shown (Nye &
Turner, 1990; Johnson et al., 1996; George et al., 2003; J. Myers &
T. McElfrsh, unpublished data, 2003).
fields of regional data from the Aleutian islands and
Alaskan Peninsula included for reference [Nye &
Turner, 1990; Johnson et al., 1996; George et al., 2003;
J. Myers & T. McElfrsh, unpublished data, 2003
(available at http://www.gg.uwyo.edu/aleutians/
index.htm)]. The Akutan samples have been
Fig. 3. (a) 143 Nd/144 Nd vs 87 Sr/86 Sr, (b) 207 Pb/204 Pb vs 206 Pb/204 Pb,
and (c) (230 Th/232 Th) vs (238 U/232 Th) diagrams showing the compositional range of Akutan and Aniakchak compared with Aleutian and
Alaskan data (references as in Fig. 2b). Deep Sea Drilling Project
(DSDP) Site 183 sediment average from Plank & Langmuir (1998),
NE Pacific MORB from Langmuir et al. (1992), Northern Hemisphere Reference Line (NHRL) from Hart (1984).
subdivided into historical and prehistoric subsets
and the Aniakchak samples into pre-1931 and 1931
eruptive products. It should be noted that, to maintain
an expanded scale, the xenolith (422% SiO2) and
211
JOURNAL OF PETROLOGY
VOLUME 45
rhyolitic obsidian (725% SiO2) from Akutan have not
been plotted in all of the diagrams. Excluding these
two samples, the Akutan lavas that were young enough
for a short-lived isotope study are all basaltic andesites
and andesites with a compositional range between 528
and 597% SiO2 and between 40 and 25% MgO.
Trace element abundances are similarly restricted in
the basaltic andesites, with Th varying between 12
and 28 ppm and Sr having a range from 426 to
345 ppm. In contrast, the Aniakchak samples exhibit
a much greater compositional range, with SiO2 ranging from 525 to 704% and MgO from 44 to 06%.
Th contents in these rocks extend from 19 to 71 ppm
and Sr from 483 to 226 ppm.
On a K2O vs SiO2 diagram (Fig. 2a), the historical
Akutan lavas form an array at low K2O relative to the
Aniakchak samples, whereas the rest of the two datasets overlap. According to the definition of Miyashiro
(1974), tholeiitic and calc-alkaline suites converge at
low silica, but tholeiitic suites show FeO/MgO vs SiO2
trends that are as steep as or steeper than the line in
Fig. 2b. At intermediate silica contents, the distinction
refers to which side of the line a suite of lavas plot. At
high silica, MgO decreases rapidly with differentiation
and so FeO /MgO increases rapidly, leading to curved
trends, which can, again, cross the line (Miyashiro,
1974). When the regional Aleutian---Alaska dataset is
plotted on an FeO/MgO vs SiO2 diagram (Fig. 2b),
both data fields cross the tholeiitic---calc-alkaline dividing line at low silica but the oceanic (Aleutian) field is
clearly steeper than the continental (Alaskan) one. The
same general distinction is observed between Akutan
and Aniakchak volcanoes when data in addition to
those of this study are considered. If only lavas with
565% SiO2 are considered, then even though some of
the Aniakchak samples cross to the tholeiitic side of the
Miyashiro line at low silica, they straddle it and overall
form a tight array, which is clearly much shallower
than the tholeiitic---calc-alkaline dividing line. The
Akutan lavas analysed in our study scatter but suggest
a trend that is distinctly steeper than the Aniakchak
field and that is roughly parallel to, and lies on the
tholeiitic side of the line. Although the Aniakchak data
re-cross the line at higher silica, the high FeO/MgO
trachyte dyke (and rhyolitic obsidian) suggests that
Akutan products have higher FeO , Al2O3 and FeO/
MgO at a given silica content (see arrows in Fig. 2b).
Thus, according to the definition of Miyashiro (1974),
and irrespective of the origins of the within-suite compositional variations that will be discussed further
below, the young Aniakchak samples are classified
as calc-alkaline, whereas the young Akutan lavas are
tholeiitic (see also Romick et al., 1990).
Compatible trace element concentrations are similarly low in both analysed suites, but incompatible
NUMBER 1
JANUARY 2004
trace element concentrations serve to distinguish
between the products of the two volcanoes. Briefly,
Aniakchak samples are significantly more enriched in
incompatible trace elements such as Rb and Zr,
whereas Sr concentrations are similar. The Aniakchak
rocks are also more enriched in the light rare earth
elements, with La/YbN ranging from 14 to 21 in the
Akutan lavas compared with 30 to 40 at Aniakchak.
The Sr and Nd isotope data are shown in Fig. 3a.
Whereas the data broadly encompass a similar range
in 143 Nd/144 Nd (051320---051296), in detail the
Aniakchak samples have lower average 143 Nd/144 Nd
and exhibit a much larger range in 87 Sr/86 Sr
(070324---070351) compared with the range in 87 Sr/
86
Sr in the Akutan rocks (070346---070358). In Pb
isotope space, the Akutan samples typically have
higher 206 Pb/204 Pb (1893---1898, excluding the obsidian) than the Aniakchak rocks (1884---1891) but
span a similar range in 207 Pb/204 Pb (1556---1564 and
208
1555---1562,
respectively)
and
Pb/204 Pb
(3844---3867 and 3833---3859, respectively, excluding
the obsidian). In Fig. 3b, the Akutan and Aniakchak
data form two sub-parallel arrays that are steeper than,
and displaced above the Northern Hemisphere Reference Line. The data from these two volcanoes generally
fall within the fields of the regional data from George
et al. (2003) but with the Aniakchak analyses showing
more isotopic diversity than those from Akutan.
The U-series data are also given in Tables 1 and 2,
and the U---Th isotope data are displayed on the
equiline diagram in Fig. 3c. The Akutan lavas have
lower (230 Th/232 Th) and higher (238 U/232 Th) activity
ratios ( 135 and 15---19, respectively) than the
Aniakchak products and, overall, form a flattish
array to the right of the equiline, similar to the other
oceanic sector (Aleutian) data, with the 238 U-excesses
reaching 30%. In comparison, the Aniakchak samples
have (230 Th/232 Th) ˆ 135---157 and (238 U/232 Th) ˆ
134---160 and have (238 U/230 Th) ratios ranging from
085 to 113, thus straddling the equiline (Fig. 3c)
along with the other continental sector (Alaskan)
rocks. There is no correlation between (238 U/230 Th)
and SiO2 (not shown) in the samples from either volcano. The rocks from both volcanoes preserve a very
similar range of moderate Ra-excesses (1---18%) and
are, therefore, largely distinguished by their (238 U/
230
Th) ratios. The preservation of 226 Ra-excesses suggests that the U---Th isotope variations are likely to be
primary signatures, unaltered by post-eruptive ageing.
INTERPRETATION
Key aspects to be drawn from the preceding section are
that the lavas erupted from Akutan volcano belong to
the tholeiitic magmatic series and have a restricted
212
GEORGE et al.
TWO VOLCANOES IN ALASKA±ALEUTIAN ARC
compositional range, excepting rare highly differentiated compositions such as the trachyte dyke. In contrast, eruptive products at Aniakchak commonly are
calc-alkaline and exhibit a much more complete
compositional range. At the low-SiO2 end of the compositional range both lava suites converge in extent of
226
Ra-excess. There is a clear distinction between the
two volcanoes in U---Th isotope systematics. A striking
observation from Fig. 3c is that this is mirrored regionally in that the oceanic Aleutian magmas have low
(230 Th/232 Th) and sizeable 238 U-excesses whereas the
continental Alaska Peninsula magmas can have either
238
U-excesses or 230 Th-excesses and tend generally to
have higher (230 Th/232 Th) (George et al., 2003). The
Akutan lavas exhibit the strongest slab fluid signal
identified with 238 U-excesses and would accordingly
be expected to be the most oxidized; nevertheless, it is
the Ankiakchak rocks that show greater iron depletion
relative to silica increase (Fig. 2b), a calc-alkaline
characteristic that typically reflects early magnetite
fractionation under oxidizing conditions. The explanation of this apparent paradox probably lies in the fact
that none of the analysed samples is compositionally
primitive (i.e. all are differentiated magmas) and that
the Aniakchak magmas, at least, appear to have interacted with continental crust.
Relative roles of crystal fractionation and
mixing
The large range in radiogenic isotope ratios observed
in the Aniakchak products is suggestive of crustal
assimilation. This is explored further on a plot of
87
Sr/86 Sr vs SiO2 in Fig. 4. In this diagram the Akutan
samples form no clear co-variation between 87 Sr/86 Sr
and SiO2; we note in passing that 87 Sr/86 Sr is no higher
in the rhyolitic obsidian than in the basaltic andesites.
In contrast, the pre-1931 Aniakchak products show a
broad positive correlation between 87 Sr/86 Sr and SiO2
(Fig. 4) indicative of mixing of at least two isotopic
components, one from relatively non-radiogenic mantle and the other from comparatively radiogenic crust.
This is important because Grove & Baker (1984) have
argued that crustal assimilation may play a crucial role
in leading a magma batch to follow a calc-alkaline
evolutionary path. This trend is most clearly apparent
when the full Aniakchak data set is considered; 87 Sr/
86
Sr ratios of the 1931 samples span a larger range than
those of the Akutan rocks, yet show no correlation
between 87 Sr/86 Sr and SiO2 (although there is a weak
positive correlation if the two low-87 Sr/86 Sr, smallvolume Slag Heap and Doublet Crater dacite lavas
are excluded) (Fig. 4). Although it is not possible to
determine unambiguously the extent to which assimilation was involved in the evolution of these magmas
Fig. 4. Variation of 87 Sr/86 Sr with SiO2 for the Akutan and
Aniakchak samples, contrasting the range in 87 Sr/86 Sr and their
correlation with SiO2 in the Aniakchak rocks with the limited
isotopic and compositional range in the Akutan lavas.
on the basis of the 87 Sr/86 Sr data alone (but see below),
assimilation and mixing are inferred from petrographic
and field data to have been important (Neal et al.,
1992; Bacon et al., 1997; Bacon, 2002). In an associated
regional study, George et al. (2003) concluded that
much of the variation in radiogenic isotopes in primitive lavas along the Aleutian---Alaska arc is determined
by the relative contributions from the subducting slab
to the mantle wedge source region. However, we suggest that this source component balance is unlikely to
change significantly beneath an individual volcano on
the time scale over which the analysed Aniakchak
products were erupted (3400 years). Therefore, it is
suggested that the 1931 magmas probably evolved
from parental liquids similar to those of earlier postcaldera magmas (Dreher, 2002).
Given the evidence that mixing was an important
process in the compositional evolution of the Aniakchak magmas, it is important to distinguish whether
this could be entirely due to two end-member mixing
or whether crystal fractionation was also important.
On many binary diagrams the 1931 Aniakchak data
form linear arrays that could be consistent with two
end-member mixing between andesite and dacite magmas (Neal et al., 1992; Bacon et al., 1997; Bacon, 2002).
However, Dreher (2002) has shown that diagrams of
V, Eu, TiO2, FeO and P2O5 vs SiO2 in pre-, syn- and
post-caldera Aniakchak rocks all show inflected trends.
These provide evidence that two end-member assimilation or magma mixing alone cannot explain all of
the data and that crystal fractionation must also have
played a role in producing the compositional range, at
least below 62% SiO2. Figure 5 shows that there are
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JOURNAL OF PETROLOGY
VOLUME 45
Fig. 5. P2O5 and Sr vs SiO2 variation diagrams, showing the inflections that argue against purely two end-member assimilation or
magma mixing in the Aniakchak suite.
also inflections on plots of P2O5 and Sr vs SiO2 (and
also TiO2 vs SiO2, which is not plotted) for the young
Aniakchak products analysed in this study. Because
87
Sr/86 Sr ratios also increase across this compositional
range, it appears that assimilation of material having
elevated 87 Sr/86 Sr, possibly siliceous partial melts of
crustal wall rocks, by the magma in the storage reservoirs or conduits, must also have been involved.
To place more precise constraints on the compositional evolution of the magmas we have modelled the
change in Ba/Th ratio with Eu/Eu in Fig. 6a. In
gabbroic assemblages, such as those that characterize
the phenocryst assemblages in the Ankiakchak and
Akutan samples, plagioclase is the principal phase
that incorporates Ba and Eu2 ‡ (and Ra, see below)
and thus controls the evolution of Ba/Th and Eu/Eu
ratios. The partition coefficient of these trace elements
can be accurately determined for a given temperature
and bulk composition (Blundy & Wood, 1994).
Figure 6a shows that crystal fractionation vectors using
appropriate partition coefficients cannot reproduce the
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Fig. 6. (a) Ba/Th vs Eu/Eu compared with a calculated fractionation vector (dotted line), which shows that the decrease in Ba/Th in
the Aniakchak samples cannot be accounted for by crystal fractionation alone. The partition coefficient for Ba into plagioclase at the
temperature of the Aniakchak 1931 dacite magma (855 C; Bacon
et al., 1997) was calculated using the model of Blundy & Wood
(1994). The best-fit line (dashed) requires DBa ˆ 3, which is impossible for a gabbroic fractionating assemblage with currently accepted
KD values. (b) (226 Ra/230 Th) vs Eu/Eu with an equivalent model
fractionation vector, which, again, cannot simulate the Aniakchak
data, implying that both mixing and the time taken for fractional
crystallization are important factors in controlling the decreases in
(226 Ra/230 Th) observed in the data. The Aniakchak data lie close to a
mixing hyperbola calculated assuming two-component mixing
between andesitic and dacitic end-members (continuous line). The
Akutan lavas span a similar range in (226 Ra/230 Th) to those from
Aniakchak, suggesting that a comparable storage time was spent in
high-level magma chambers if initial (226 Ra/230 Th) ratios were similar. Modelling assumes an assemblage composed of 30% clinopyroxene (with DRa ˆ 17 10 ÿ6 , DTh ˆ 0013, DBa ˆ 4 10 ÿ5 ,
DEu ˆ 07, DSm ˆ 075 and DGd ˆ 058) and 70% plagioclase (with
DRa ˆ 009, DTh ˆ 1 10 ÿ9 , DBa ˆ 07, DEu ˆ 38, DSm ˆ 011 and
DGd ˆ 005). The parental composition was that of sample 97ANB 27
in both diagrams (open circle with black dot).
rate of decrease of Ba/Th with decreasing Eu/Eu
observed in the Aniakchak data; a fractionation
model forced through the data requires DBa ˆ 3,
which is several times higher than any Ba partition
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GEORGE et al.
TWO VOLCANOES IN ALASKA±ALEUTIAN ARC
coefficients yet measured or predicted for plagioclase.
The implication is that an additional process must
be controlling the compositional evolution of the
Aniakchak magmas and, given the discussion above,
we infer that this is the effect of magma mixing, and
possibly assimilation combined with crystal fractionation. It should be noted that this result from Fig. 6a is
just as applicable to the 1931 suite as to the older
Aniakchak rocks, even though the 87 Sr/86 Sr evidence
for assimilation was less clear in the 1931 lavas. In
contrast, we have found no requirement in our data
for assimilation or magma mixing in the young Akutan
lavas and the closed-system fractionation vector in
Fig. 6a could easily simulate the majority of the Akutan
array, so long as an appropriately different parental
magma starting composition were chosen. A similar
conclusion was reached by Romick et al. (1990) for
within-suite variations in the Akutan lavas, although
their data indicate a need for temporal changes in
parental magma compositions between the older
(1---2 Ma) and younger Akutan lava sequences.
226
Ra---230 Th constraints on magmatic time
scales
Current models suggest that 226 Ra-excesses in island
arc volcanic rocks reflect additions from the subducting
slab (e.g. Gill & Williams, 1990; Turner et al., 2001a,
2003a; Sigmarsson et al., 2002), and the lack of withinsuite variations of source component indicators, such as
Zr/Nb ratio, suggests that there is no reason to assume
that the parental magmas for the young lavas from
each volcano varied significantly in the last few thousand years. Thus, much of the variation in the preserved (226 Ra/230 Th) ratios is likely to record the effects
of ageing of the magmas prior to eruption such that
they can be used to constrain the time scales of differentiation (e.g. Vigier et al., 1999; Cooper et al., 2001;
Turner et al., 2001a). Ba provides a close, though not
exact (see below), chemical analogue for Ra and so the
plot of (226 Ra/230 Th) vs Eu/Eu in Fig. 6b is essentially
equivalent to the Ba/Th vs Eu/Eu plot of Fig. 6a with
the crucial difference that (226 Ra/230 Th) will decrease
toward secular equilibrium, (226 Ra/230 Th) ˆ 1, as a
function of time and at a rate proportional to the halflife of 226 Ra (1600 years). In practice, because Ra has
an ionic radius 5% larger than that of Ba it will be
more incompatible than Ba (Wood et al., 1999). This is
illustrated in Fig. 6b, where the fractionation vector
from Fig. 6a has been transcribed, taking into account
the greater incompatibility of Ra relative to Ba. As
can be seen, (226 Ra/230 Th) ratios change little with
fractionation because Th is also very incompatible in
gabbroic assemblages (Fig. 6b). Consequently, the
large range in (226 Ra/230 Th) ratios observed in both
the Akutan and Aniakchak datasets cannot be ascribed
to crystal fractionation and must, additionally, reflect
radioactive decay.
Several important results emerge from Fig. 6b. First,
the historical products of Akutan and Aniakchak
encompass almost identical ranges of (226 Ra/230 Th),
even though the extent of primary 226 Ra---230 Th disequilibria generated in the mantle wedge is not well
constrained. Second, the most mafic lavas analysed
for 226 Ra from each suite have similar (226 Ra/230 Th)
ratios and major and compatible trace element compositions. In the simplest interpretation this suggests
that the pre-eruption residence times of magmas
beneath these two volcanoes were similar and the
range in (226 Ra/230 Th) ratios can be used to calibrate
the actual time scales involved. It should be noted that
if the Ra---Th disequilibria in the primary magmas
were significantly greater than that observed in the
most primitive lavas ( 12) then the overall transfer
time of the U-series signal from the subducting plate
could be longer. As shown in Fig. 6b, the implied range
of magma residence times is of the order of several
thousand years. Such estimates are similar to numerical estimates of the time scales for crystallization as a
result of cooling in crustal magma chambers (e.g. Marsh,
1989), rather than crystallization caused by decompression and degassing, which can result in much more
rapid crystallization time scales approaching those of
eruptive periodicity (Blundy & Cashman, 2001).
The Ra---Th isotope data permit that the time scale
for fractionation below 62% SiO2 might have been
similar at both Akutan and Aniakchak. Above 62%
SiO2, the Aniakchak data appear to be linear on many
plots and it has proved hard to distinguish unambiguously whether fractionation or mixing played the
dominant role in producing this part of the compositional array. For example, there is a good correlation
between indices of differentiation, such as Eu/Eu , and
(226 Ra/230 Th) within the Aniakchak samples in Fig. 6b
whereas the Akutan samples show a much more
restricted compositional variation over the same
range in (226 Ra/230 Th). However, both fractionation
accompanied by ageing and mixing curves can reproduce these trends (see Fig. 6b). If mixing involves large
volumes of a siliceous end-member that was in
226
Ra---230 Th equilibrium, then the decreases in
226
( Ra/230 Th) with increasing differentiation will
reflect a combination of assimilation or magma mixing
and the time taken for differentiation. Thus, we consider the time scale implications of two end-member
models for the Aniakchak data:
(1) If fractionation was the dominant process, then it
is striking that similar durations for magma evolution
are inferred for magma suites representing very
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JOURNAL OF PETROLOGY
VOLUME 45
different amounts of differentiation beneath these two
volcanoes.
(2) However, if mixing was responsible for the
Aniakchak data beyond 62% SiO2, and the evolved
end-member was in, or close to, 226 Ra---230 Th equilibrium, then the formation of this part of the compositional range could have been much faster than the few
thousand years indicated in Fig. 6b. In this latter
model, the andesite is formed largely by fractional
crystallization of basalt and its 226 Ra-excess reflects
that of the parental basalt and the time taken for this
fractionation. In principle, the dacitic end-member
that mixes with the andesite may be either a differentiation product of a similar andesite or a partial or
wholesale melt of upper-crustal rocks or a previous
dacite. However, it would take 8000 years for a
magma to reach 226 Ra---230 Th equilibrium. It seems
unlikely that any magma could have remained liquid
in the inferred shallow-level reservoir for this period of
time or to have survived the 3430 years BP calderaforming eruption. Therefore, partial melting of reservoir wall rocks seems the most likely model for this
dacitic end-member.
Whichever model ultimately proves to be correct, it
would appear that the greater compositional range
observed at Aniakchak volcano was produced in a
similar or shorter time than the more restricted compositional range observed at Akutan. Magma mixing
and assimilation are inferred to have played a key role
in the differences between the two volcanoes, the primary result being that differentiation was faster in the
magma system beneath Aniakchak volcano.
DISCUSSION AND COMPARISON
WITH OTHER SYSTEMS
As the inferred time scales for both volcanic systems
appear to be appropriate for crystallization driven by
cooling, the principal controls on the extent of differentiation will be the rate of cooling and the efficiency of
crystal---liquid separation mixing. The rate of cooling
will be a function of magma volume and temperature
relative to the country rock temperature and is likely to
be increasingly slow at greater depths within the crust.
The magma volumes of both the recent eruptive products and caldera sizes are greater at Aniakchak than
at Akutan (Simkin & Siebert, 1994; Neal et al., 2001),
even though overall Akutan is a larger volcano (considering the submarine part of the edifice). Country
rock temperatures are poorly constrained and the
larger chamber might be expected to cool more slowly.
Additionally, greater heat loss may be implicit at
Aniakchak if melting of wall rocks was necessary to
produce the increases in 87 Sr/86 Sr ratios during
NUMBER 1
JANUARY 2004
differentiation and given that hydrothermal fluid
circulation would have been promoted by fractures
related to caldera collapse.
A simple crystal settling model provides a maximum
likely time for production of evolved magmas. If differentiation occurs by crystal settling in a convecting
magma chamber, the rate of crystal settling (tsettle)
will be controlled by the radius of the crystals (r),
their density contrast with the magma (Dr) and the
magma viscosity (m) (Martin & Nokes, 1988):
tsettle ˆ 9Hm=2gDrr2
where H is the height of the magma chamber and g is
the acceleration due to gravity. Taking two andesites
with similar compositions from Akutan (96PS30) and
Aniakchak (97ANB-44) for illustrative purposes, and
magmatic temperatures and water contents of 970 C
and 2 wt % and 855 C and 4 wt %, respectively
(Romick et al., 1990, and discussion above; Bacon et al.,
1997; Bacon, 2002), we calculate densities (Bottinga
& Weill, 1970) and viscosities (Shaw, 1972) of
2490 kg/m3 and 123 104 Pa s for the Akutan
magma and 2516 kg/m3 and 103 105 Pa s for the
Aniakchak magma. Assuming a gabbroic fractionating assemblage with a density of 3000 kg/m3 and
crystals with a radius of 10 ÿ3 m, this leads to predicted
crystal settling times of the order of 850 and 6900 years
for Akutan and Aniakchak, respectively, in magma
chambers of 10 km3 size, which is the order of
magnitude of the volumes of the recent eruptives and
caldera-forming eruptions of these two volcanoes.
Although these calculations are clearly subject to
large uncertainties, because of the sensitivity of the
density and viscosity estimates to the magmatic
temperatures and water contents (which are poorly
constrained, especially at Akutan), it nevertheless
seems likely that crystal---liquid separation by settling
would require time scales that are significant relative
to the residence times inferred from the 226 Ra---230 Th
disequilibria data. Moreover, it would appear that
crystal---liquid separation is predicted to be at least as
fast at Aktuan yet these lavas show the more restricted
compositional range. Either the greater compositional
variation observed at Aniakchak reflects some other,
more efficient, mechanism of crystal---liquid separation, such as gas-driven filter-pressing (Sisson & Bacon,
1999) or the extent of assimilation and magma mixing
evident in the Aniakchak samples exerted a primary
control on the difference in degrees of differentiation
experienced beneath the two volcanoes. Although the
former hypothesis cannot be tested using our data, we
favour the latter given the good evidence for mixing at
Aniakchak.
As outlined in the Introduction, many different models have been proposed to account for the distinction
216
GEORGE et al.
TWO VOLCANOES IN ALASKA±ALEUTIAN ARC
between the tholeiitic and calc-alkaline magmatic
series, although we believe this is the first to investigate
possible temporal differences. It seems unlikely that the
liquid line of descent reflects parental magma compositions, as these appear to be similar beneath Akutan and
Aniakchak and the Akutan system is, if anything, the
most influenced by addition of oxidizing fluids from the
subducting slab. Comparison of recent erupted
volumes and caldera size suggests that the Akutan
system currently has a smaller volume than that at
Aniakchak, which is the opposite of the Kay & Kay
(1994) model. Equally, there is little evidence that
Akutan is a longer-lived volcano than Aniakchak, and
the temporal changes from calc-alkaline to tholeiitic
and back to calc-alkaline at Aniakchak (Nye et al.,
1993) and from tholeiitic to transitional tholeiitic at
Akutan are not predicted by the Myers et al. (1985)
model. Our results suggest, in fact, that the time scales
of magmatic evolution were probably similar in the
two systems but that differentiation was more efficient
in the calc-alkaline system, arguably because of a combination of greater extents of assimilation and cooling.
The role of assimilation---magma mixing in the calcalkaline system is a common feature of all of the Grove
& Baker (1984), Myers et al. (1985) and Kay & Kay
(1994) models.
Finally, in Fig. 7 we compare the Akutan and
Aniakchak (226 Ra/230 Th) arrays with the results of
two other recent studies. The rate of differentiation is
proportional to the slope of the arrays on the (226 Ra/
230
Th) vs Th diagram; the shallower the slope the
more differentiation achieved per unit of 226 Ra
decay, or time. Binary mixing could produce similar
arrays in this diagram or curves if the mixing endmembers had notably different Th contents. A
suite of tholeiitic lavas from the 1978 eruption of
Ardoukoba in the Asal rift (Vigier et al., 1999) define
an array with a slope that is indistinguishable from that
formed by the Akutan transitional tholeiites. Thus,
despite the difference in tectonic setting, it would
appear that differentiation was restricted in both
tholeiitic systems despite the magmas apparently residing in crustal magma chambers for several thousand
years. Also depicted in Fig. 7 is the slope for a suite of
potassic lavas from Sangeang Api volcano in the rear
of the east Sunda arc (Turner et al., 2003b). The slope
of this array is intermediate between those of the
tholeiitic suites and the calc-alkaline lavas from
Aniakchak, suggesting that differentiation, as measured by Th concentration, was more rapid in the
potassic suite than in the tholeiitic ones, although less
rapid than in the calc-alkaline suite. Turner et al.
(2003b) have argued that assimilation (but not binary
mixing) was also involved in the evolution of the
Sangeang Api lavas.
Fig. 7. Diagram of (226 Ra/230 Th) vs Th (used as an index of differentiation) comparing the slopes of the Akutan lavas (&) with tholeiitic lavas from the Asal rift from Vigier et al. (1999), and the
curvature of the 1931 Aniakchak samples (*) with that of high-K
calc-alkaline lavas from Sangeang Api in the rear east Sunda arc
(Turner et al., 2003b). The calc-alkaline and potassic suites both have
shallower slopes, indicating more rapid differentiation than the two
tholeiitic suites (see text for discussion).
CONCLUSIONS
The recent magmas erupted from Akutan and
Aniakchak volcanoes belong to the tholeiitic and calcalkaline magma series, respectively. The Akutan lavas
exhibit little within-suite variation in SiO2 or 87 Sr/86 Sr
and can be explained by closed-system magmatic evolution. They are characterized by 238 U-excesses
whereas those samples from Aniakchak straddle the
U---Th isotope equiline. Because the size of 238 U-excess
will reflect both the oxygen fugacity and the Th content of the mantle wedge, this may provide evidence for
more oxidizing conditions beneath Akutan (Turner
et al., 2003a). However, the rate of iron depletion is
less pronounced here than that at Aniakchak, suggesting that this may not reflect a difference in primary
magma oxidation state. Curvilinear trace element
trends and a large range in 87 Sr/86 Sr isotope ratios in
the Aniakchak data appear to require the combined
effects of fractional crystallization, assimilation and
magma mixing. The products of both volcanoes preserve a range in 226 Ra---230 Th disequilibria, which suggests that the time scale of crustal residence of magmas
beneath these volcanoes was similar, and of the order of
several thousand years. During that time interval the
tholeiitic Akutan magmas underwent restricted,
closed-system, compositional evolution; a similar
observation has been made for within-plate tholeiitic
lavas from the Asal rift (Vigier et al., 1999). In contrast,
calc-alkaline magmas beneath Aniakchak volcano
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JOURNAL OF PETROLOGY
VOLUME 45
underwent significant compositional evolution over a
similar range in 226 Ra---230 Th disequilibria. Potassic
lavas from Sangeang Api appear to be intermediate
between Aniakchak and Akutan in their differentiation
rate. Thus, it appears that differentiation may be more
rapid in the calc-alkaline and potassic systems as a
result of a combination of greater extents of assimilation and cooling.
ACKNOWLEDGEMENTS
David Bruce, Julian Pearce, Louise Thomas, Mabs
Gilmour and Geoff Nowell are all thanked for their
assistance with the analytical work. We would like to
thank Jake Lowenstern and an anonymous reviewer
for internal US Geological Survey reviews, and we
gratefully acknowledge the formal reviews by John
Gamble, James Myers and Nathalie Vigier, as well as
editorial assistance from Marjorie Wilson. During the
course of this work R.G. was supported by NERC
grant GR3/11701 to S.T. and C.J.H., and S.T. was
funded by a Royal Society University Research
Fellowship.
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