JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 8
PAGES 1375±1400
2003
Magma Storage Region Processes Inferred
from Geochemistry of Fe±Ti Oxides in
Andesitic Magma, Soufriere Hills Volcano,
Montserrat, W.I.
J. D. DEVINE1*, M. J. RUTHERFORD1, G. E. NORTON2y AND
S. R. YOUNG2z
1
DEPARTMENT OF GEOLOGICAL SCIENCES, BROWN UNIVERSITY, PROVIDENCE, RI 02912, USA
2
MONTSERRAT VOLCANO OBSERVATORY, MONGO HILL, MONTSERRAT, W.I.
RECEIVED JUNE 15, 2002; ACCEPTED FEBRUARY 26, 2003
heating to the overlying conduit, which carries the magma
through the upper arc crust. In this model, the magma chamber
is being emptied from the bottom, at the contact between
pre-existing andesite and newly intruded basalt.
Analyses of Fe±Ti oxides help constrain models of magma
storage region processes for the Soufriere Hills Volcano,
Montserrat (W.I.), and provide clear evidence of the nature
of transient heating events in the magma storage region. To
constrain timescales of magma heating and remobilization, the
TiO2 zoning patterns in a time series of natural titanomagnetites were compared with those produced in controlled phase
equilibrium experiments on the andesite. Most samples of andesite erupted from 1995 to 2002 contain titanomagnetite crystals
with uniform core compositions (TiO2 78 wt %). Many
crystals are characterized by rimward increases in TiO2, interpreted to be Ti diffusion gradients caused by heating of the
andesite by invading basaltic magma. Some andesites erupted
during periods of the highest observed mass eruption rate, however, contain titanomagnetite with uniformly low TiO2 contents
from core to rim. The observation that no Ti diffusion gradients, and no elevated core TiO2 contents, occur in the vast
majority of titanomagnetite grains in magma batches that were
erupted more than 2 years after the onset of the present eruption
strongly suggests, first, that heating of the batches of andesite
occurred just before eruption, and, second, that injection of
basaltic magma has continued throughout the eruption. Heat,
but little mass, may be transferred from the invading basalt to
the andesite in the magma storage region by injection of dikes or
formation of sills. Ponding of basaltic magma at the base of a
pre-existing andesitic magma storage region is the simplest
explanation consistent with observations. The Fe±Ti oxide
data strongly suggest that an internal conduit within the andesitic magma storage region carries magma from the zone of
The 1995-to-present (December 2002) eruption of the
Soufriere Hills Volcano, Montserrat (W.I.), is the
latest in a series of andesitic, dome-forming eruptions
in a history spanning 4175 kyr (Roobol & Smith, 1998;
Harford et al., 2002). The current eruption is thought
to have been triggered by injection of magma of basaltic or basaltic andesite composition into a pre-existing
andesitic magma storage region (SiO2 57±61 wt %;
Devine et al., 1998a), judging from the presence of
ubiquitous `mafic inclusions' (51 vol. %) with diktytaxitic textures found in the andesite (Murphy et al.,
1998, 2000). The most recent eruptions before the new
activity, which generated magma of generally similar
composition, occurred at 400 and 3950 years BP
(Young et al., 1996; Roobol & Smith, 1998), and
there were seismic crises in the 1890s, 1930s, and
1960s that are thought to have been caused by
magma movement (Shepherd et al., 1971). It seems
*Corresponding author. Telephone: 401-863-2560. Fax: 401-8632058. E-mail: [email protected]
yPresent address: British Geological Survey, Keyworth
NG125 GG, UK.
zPresent address: 1506 Cordova Street, Coral Gables, FL 33134, USA.
Journal of Petrology 44(8) # Oxford University Press 2003; all rights
reserved
KEY
WORDS:
titanomagnetite
andesite; diffusion; geothermometry; magma;
INTRODUCTION
JOURNAL OF PETROLOGY
VOLUME 44
likely that magma mixing processes have been important throughout the recent history of the volcano.
The mineral assemblage of the newly erupted andesite includes 45±55 wt % phenocrysts of plagioclase
(An48±93), amphibole, orthopyroxene, titanomagnetite, and minor quartz and ilmenite. Clinopyroxene
occurs as small grains in the groundmass and as
fine-grained reaction rims on quartz and some
orthopyroxene phenocrysts. Apatite and sulfide are
accessory phases.
Following an initial phreatic stage, the eruptive
style has been largely extrusive, except for ash venting
episodes (Norton et al., 2002) and several episodes of
vulcanian explosive eruptions (Druitt et al., 2002). The
first explosive eruption occurred on 17 September
1996, following a partial dome collapse, and there
were two cyclic series of explosions in early August
and then in September and October 1997 (Miller
et al., 1998; Voight et al., 1998). Occasional dome
collapse episodes have resulted in pyroclastic f lows
that have, in many cases, reached the coast. These
flows, and tephra fall deposits, were sampled when
conditions permitted.
One component of the volcanic hazard monitoring
program of the Montserrat Volcano Observatory has
involved petrographic examination and chemical analysis of dome lava and tephra fall samples, carried out
at Brown University (USA) and the University of
Bristol (UK). The aim has been to complement monitoring that has included earthquake seismology,
ground deformation studies (global positioning system,
electronic distance measuring, gravity), dome volume
measurement, and gas emissions monitoring [correlation spectrometry (COSPEC) and Fourier transform
infrared spectroscopy (FTIR); see references in special
issues of Geophysical Research Letters, vol. 25, nos 18 and
19 (S. R. Young et al., eds), and Geological Society of
London Memoir 21 (T. H. Druitt & B. P. Kokelaar, eds),
for descriptions of methods and results].
This paper uses Fe±Ti oxides to constrain models of
magma storage region processes. The rationale was to
monitor Fe±Ti oxide compositions to try to detect
whether or not `global' heating of the andesite was
occurring, i.e. gradual heating of the entire magma
body. Some important questions relating to global
heating are: What are the relative volumes of (1)
invading basalt, (2) andesite resident in the magma
storage region, and (3) erupted andesite? How is the
energy required to remobilize the andesite transferred
from basalt to andesite? How soon does remobilized
andesite erupt after being heated? If the cumulative
volume of injected basalt becomes large (e.g. comparable with the andesite resident in the magma storage
region), how much heat can be transferred from basalt
to overlying andesite in the absence of extensive
NUMBER 8
AUGUST 2003
magma hybridization, and what would be the timescale of such heat transfer? What is the likelihood of
eventual extensive hybridization? What would be the
potential effect of open-system behavior, with respect
to volatile species in basalt such as H2O, CO2, and
SO2, on the degree of vapor saturation of remobilized
andesite? In our view, the occurrence of global heating
would require a fundamental reassessment of volcanic
hazard zonation maps (e.g. Wadge & Isaacs, 1989),
because it might indicate that a relatively large volume
of andesite has been remobilized, which could potentially be ejected in an explosive eruption larger than
any observed to date. Therefore, we monitored Fe±Ti
oxide chemistry of eruptive products to assess the
nature of magma heating and remobilization.
Fe±Ti oxides generally change composition much
faster than do silicates in response to magma system
temperature rises that may be caused by magma mixing (e.g. Venezky & Rutherford, 1999). They are
therefore sensitive probes of magma storage region
heating episodes that take place on timescales of a few
days to months before eruption of affected magma
batches. Silicate phenocrysts, with the possible exception of amphibole, generally may not change composition (or become resorbed) fast enough to record
changes in magma chamber temperatures that take
place on such short timescales. So the Fe±Ti oxides
may provide the only clear evidence of the nature of
transient heating events in the inaccessible magma
storage region. For example, Nakamura (1995) used
Fe±Ti oxide analyses to show that the 1991 eruption of
Unzen Volcano ( Japan) was caused by a series of
injections of andesitic magma into an overlying dacitic
magma chamber, emphasizing that the injections continued after the onset of the eruption.
In the present study, the TiO2 concentration zoning
patterns in natural mineral grains are compared with
those produced in titanomagnetite grains in controlled
experiments designed to determine the phase relations
of the natural andesite (Rutherford & Devine, 2003).
The comparisons are then used to constrain estimates of timescales of dynamic magma heating and
remobilization.
PREVIOUS RESULTS
The bulk composition of erupted andesite has
remained within a relatively narrow range (SiO2
57±61 wt %) over the course of the eruption, with no
apparent systematic change in composition with time
(Devine et al., 1998a). Analyses of the cores of coexisting titanomagnetite and ilmenite crystals in recent
Soufriere Hills Volcano products were used to infer
pre-eruptive temperature and oxygen fugacity of the
andesitic magma in the storage region [T 840 C;
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DEVINE et al.
MAGMA STORAGE REGION PROCESSES
log fO2 NNO ‡ 1 (where NNO is nickel±nickel
oxide); Devine et al., 1998a] using the algorithm of
Andersen & Lindsley (1988) as amended by Andersen
et al. (1991). Pre-eruptive melt water contents of 47
wt % (Barclay et al., 1998; Devine et al., 1998a) suggested that the top of the magma chamber was located
at depths in excess of 5±6 km at a pressure of 130 MPa
[crustal density distribution model assumed to be
similar to that determined for Mount St. Helens by
Williams et al. (1987)], a depth estimate that is in
accord with estimates derived from the lower bound
of the vast majority of earthquake hypocenter locations
(5±6 km; Aspinall et al., 1998).
One observation that is consistent with recent
reheating of the andesite is that quartz phenocrysts
are either partially resorbed or are mantled by finegrained reaction rims of clinopyroxene. Quartz crystals, although rare (1±2 grains per thin section), occur
in nearly all samples over the 47 years of the eruption.
In addition, the complex compositional zoning of
plagioclase phenocrysts supports the inference that
there have been numerous episodes of heating and
cooling of the andesite (Devine et al., 1998a). Reversely
zoned plagioclase microphenocrysts in andesites
(Murphy et al., 1998, 2000) also provide evidence for
recent reheating. The presence of andesite-derived
xenocrysts in mafic inclusions and the presence of
mafic magma-derived pargasitic amphibole crystals
in the andesite (Rutherford & Devine, 2003) indicate
that some magma mingling and hybridization have
occurred, even though they have not yet been manifested as a monotonic mixing trend in whole-rock
major element analyses. Variations in magma bulk
chemistry have been observed, however, that are larger
than analytical error, as has been confirmed by independent analyses of the same samples by the Brown
and Bristol research groups.
RESULTS
Petrographic observations and analytical
results
Two-oxide disequilibrium
Electron microprobe analyses of Fe±Ti oxides in a time
series (1995±2002) of andesites were used in two-oxide
geothermometry (Andersen & Lindsley, 1988). Rimto-rim transects of titanomagnetite grains were also
obtained to detect the potential development of nearrim Ti diffusion gradients. Our sample selection
favored tephra fall deposits. Magma ascent rate influences the nature of eruption products as well as eruptive style. For oxide minerals, slow magma ascent
provides opportunities for near-surface oxidation and
slow cooling, with formation of exsolution lamellae of
ilmenite or titanohematite within titanomagnetite
crystals. For example, the first-out magma of the current eruption had a reddish-buff color as a result of
such oxidation. Tephra fall deposits are less affected by
conduit processes such as plagioclase microlite crystallization and oxidation, so they provide the clearest
insight into the pre-eruptive magmatic conditions in
the storage region.
The cores of large ilmenite and titanomagnetite
grains in oxide pairs in Soufriere Hills Volcano andesites are relatively homogeneous, and the relatively low
TiO2 contents of the titanomagnetites have suggested
temperatures within the range 835±850 C (log fO2
ÿ118, or NNO ‡ 11; Devine et al., 1998a). Analyses
of rare ilmenite±titanomagnetite in-contact pairs, however, reveal Ti zoning in the titanomagnetites (Fig. 1).
In contrast, the titanomagnetites of in-contact oxide
pairs from many other well-studied eruption products
(e.g. the Minoan eruption of Santorini; the 18 May
1980, eruption of Mount St. Helens; the 1902 eruption
of Mt. Pelee) are homogeneous (Venezky &
Rutherford, 1999; Pichavant et al., 2002).
Typical Ti concentration gradients in titanomagnetite phenocrysts in contact with ilmenite phenocrysts
are illustrated in Figs 1a and b, and 2a and b. Twooxide geothermometry calculations (examples of which
are given in Table 1) initially suggested that the higher
concentrations of Ti in titanomagnetite lying closest to
the interface with the ilmenite were recording transient
temperatures of up to 900 C. Experimental investigation of amphibole stability in the Soufriere Hills
Volcano andesite has shown, however, that amphibole
crystals thermally decompose to a mixture of melt and
anhydrous crystals (Cpx, Plag, Fe±Ti oxide) in experiments held at 880 C for only 48 h (P ˆ 130 MPa, water
saturated; Rutherford & Devine, 2003). Such thermal
decomposition of amphibole is observed only rarely in
the natural samples, indicating that heating of the
andesite by the invading basalt has not exceeded
the thermal stability limit of amphibole, which is
855 C at 130 MPa, under water-saturated conditions
(Rutherford & Devine, 2003). We conclude that what
initially appeared to be diffusion gradients in titanomagnetite crystals abutting ilmenite grains are actually
reaction fronts: as a result of the andesite being heated
by invading basalt, the ilmenite is in fact being replaced
by titanomagnetite along an advancing reaction front.
The replacement reaction of ilmenite by titanomagnetite is best illustrated in Fig. 3, which is a colorized
TiKa X-ray dot map of an ilmenite±titanomagnetite
contact zone. The ilmenite crystal (red) has a reaction
rim (green) of 10 mm thickness of high-Ti titanomagnetite. The abutting titanomagnetite phenocryst
has a low-Ti core (dark blue) and higher-Ti rim compositions (light blue), the latter characterized by true
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JOURNAL OF PETROLOGY
VOLUME 44
Fig. 1. In-contact titanomagnetite±ilmenite pairs, sample MVO291
(29 September 1997, explosive eruption). (a) TiO2 vs distance from
titanomagnetite rim, transect in direction of ilmenite crystal core.
[Note the gradient in TiO2 concentration in titanomagnetite near
contact with the ilmenite, but essential lack of TiO2 gradient at the
titanomagnetite crystal rim, which is in contact with melt (now glassbearing groundmass). Note also the slightly lower TiO2 concentration in ilmenite near the contact with titanomagnetite; in most cases,
however, the composition of the ilmenite is effectively uniform across
the grain.] *, ILM±MT pair 3 (see below). (b) Apparent temperature (Celsius) vs distance from titanomagnetite rim for ILM±MT
pair 3 above, estimated by five methods. Geothermometer algorithm
of Andersen & Lindsley (1988), as amended by Andersen et al.
(1991): *, solution model of Stormer (1983), with error bars calculated by the program (see text); &, solution model of Anderson
(1968); ^, solution model of Carmichael (1967); ~, solution
model of Lindsley & Spencer (1982). !, geothermometer of Ghiorso
& Sack (1991). Average two-oxide temperature estimated on the
basis of all measured crystal core compositions and averages of the
four estimation methods of Andersen & Lindsley (1988) is equal to
830 10 C. For reasons discussed in the text, temperature estimates plotted in other figures throughout the rest of this paper are
those based on the Stormer solution model alone.
diffusion gradients. The high-Ti reaction rim surrounding the ilmenite phenocryst is interpreted to be
a mixture of sub-micron scale domains of titanomagnetite and relict ilmenite (which could not be resolved
using the electron microprobe).
Transects of the titanomagnetite illustrated in Fig. 3
reveal relatively high TiO2 concentrations at the
NUMBER 8
AUGUST 2003
crystal rim, where in contact with groundmass
(Fig. 4). Two-oxide geothermometry calculations,
using the titanomagnetite rim composition for the
spinel phase, and the average ilmenite composition
for the rhombohedral phase, yield apparent temperature estimates that are again in excess of the thermal
stability limit of amphibole in the andesite, and so are
considered unrealistic. Therefore, although Ti diffusion gradients at the rims of titanomagnetite grains
are produced by, and are therefore evidence of,
magma reheating, it is not possible to obtain meaningful temperature estimates from them. On the other
hand, two-oxide temperature and oxygen fugacity
estimates based on phenocryst core compositions are
believed to be valid.
The accuracy and precision of two-oxide temperature estimates have been investigated by several
research groups (e.g. Frost & Lindsley, 1991; Ghiorso
& Sack, 1991). We have previously determined,
however, that the closest match between two-oxide
temperature estimates for experimental charges and
thermocouple-measured temperatures in our hydrothermal experiments (5 C accuracy) is obtained
when the solution model of Stormer (1983) is used in
conjunction with the temperature±fO2 algorithm of
Andersen & Lindsley (1988) (see Geschwind &
Rutherford, 1992; Gardner et al., 1995; Cottrell et al.,
1999). Use of other solution models can result in either
higher or lower temperature estimates relative to the
Stormer±Andersen formulation (Fig. 1a). In some
cases, the estimated temperatures using different solution models are more than 10 C lower than the estimates obtained by our preferred method [e.g. the
solution model of Anderson (1968)]. The potentially
systematic error in our two-oxide temperature estimates for natural sample phenocryst core pairs could
possibly be greater than 10 C. The precision of the
estimates is generally 20 C, as calculated by the
Andersen & Lindsley (1988) algorithm (Fig. 1b).
Time series of Fe±Ti oxide analytical transects
Petrologic monitoring of the newly erupted andesite by
our group included estimation of magma ascent rates
from amphibole breakdown and estimation of magma
heating from Fe±Ti oxide geochemistry on a near realtime basis. The results of representative analytical
transects of the oxides are presented in Fig. 2 and in
Figs 5±11, which are arranged in order of eruption
date. The scales of the vertical axes are the same in
each figure (unless otherwise noted), which allows one
to easily assess subtle changes in zoning patterns that
have occurred as the eruption has progressed. In our
view, there is no other `index' that would provide an
adequate description of the significant variability of
1378
DEVINE et al.
MAGMA STORAGE REGION PROCESSES
Fig. 2. Analytical transects of titanomagnetite crystals in Soufriere Hills Volcano andesite sample MONT 153 (vesiculated pumice, 17
September 1996, explosive eruption). (a) Ilmenite±titanomagnetite in-contact oxide pair; TiO2 content (connected ) and `apparent'
temperatures (connected *) vs distance (from the grain interface) across the titanomagnetite grain; `apparent' temperatures calculated
using the method of Andersen & Lindsley (1988) (see text for caveat about temperature calculations; also for interpretation of TiO2
concentration gradients). (b) Ilmenite±titanomagnetite in-contact oxide pair [see caption for (a)]. (c)±(f) rim-to-rim analytical transects of
titanomagnetite phenocrysts that are in contact with melt (now glass-bearing groundmass).
zoning patterns within a single sample (e.g. Fig. 8).
Finally, a caveat applies to the figure axes labeled
`Apparent T (C)'; as indicated above, near-rim titanomagnetite compositions may give spuriously high temperature estimates, because there is no ilmenite in
equilibrium with this magnetite.
September 1996
Tephra from the first explosive eruption on 17 September 1996, which was triggered by a major dome collapse, contain a diversity of titanomagnetite grains,
some of which lack Ti diffusion gradients at the crystal
rim (MONT153; Fig. 2a and f ), some of which had
hints of rim Ti diffusion gradients (Fig. 2c and d), and
some of which had slight rim Ti diffusion gradients
(Fig. 2b and e).
September±October 1997
Pumice fall sample MVO573, a single clast of which was
used as an experimental starting material (Rutherford
& Devine, 2003), is typical of tephra produced by the
cyclic series of eruptions in September and October
1997. Of 12 randomly analyzed titanomagnetite phenocrysts (Fig. 5), 10 had no Ti diffusion gradients and two
had only slight gradients at the crystal rims (Fig. 5b
and f). Sample MVO291, a pumice fall deposit produced by an explosive eruption on 29 September 1997,
(i.e. during this same period), also has titanomagnetites that lack Ti rim diffusion gradients or are characterized by weak gradients.
November 1999
The volcano was in an eruptive hiatus from about
March 1998 until April 1999, when the onset of ash
venting episodes and minor explosions signalled that
the eruption was not over (Norton et al., 2002).
Samples of ash produced by the explosions were
examined for a juvenile component. Although glassy
clasts and vitreous amphibole fragments were present,
it was not immediately clear whether they were newly
erupted material, or just young ash from earlier
eruptions that had been carried aloft by phreatic
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NUMBER 8
AUGUST 2003
Table 1: Example geothermometry calculations for Fig. 1
(MVO291, ILM±MT pair 3)
Point 8
Oxide
Wt %
Element
Cation frac.
Formula/3 sites
Spinel phase
7.65
1.97
Ti
Cr2O3
0.00
Cr
Fe2O3
52.70
36.20
TiO2
Al2O3
FeO
MnO
MgO
Total
0.56
1.17
100.25
Al
Fe3 ‡
Fe2 ‡
Mn
Mg
X(Usp) spinel
0.223
0.203
0.215
Carmichael model
X(Usp) spinel
0.218
Lindsley model
X(Usp) spinel
X(Usp) spinel
0.0717
0.0289
0.22
0.09
0.0000
0.4943
0.3774
0.00
1.48
1.13
0.0059
0.0217
0.02
0.07
0.4036
0.0032
0.0002
0.81
0.01
0.00
0.1893
0.3546
0.38
0.71
0.0101
0.0390
0.02
0.08
Stormer model
Anderson model
Rhombohedral phasey
42.86
0.22
Al
Cr2O3
0.02
Cr
Fe2O3
20.09
33.86
TiO2
Al2O3
FeO
MnO
MgO
Total
0.95
2.09
100.08
Ti
Fe3 ‡
Fe2 ‡
Mn
Mg
X(Ilm) rhomb
0.800
0.789
0.807
Carmichael model
X(Ilm) rhomb
0.807
Lindsley model
X(Ilm) rhomb
X(Ilm) rhomb
Stormer model
Anderson model
Pair passes Bacon & Hirschmann (1988) equilibrium test
X(Usp) sp
X(Ilm) rh
T ( C)
log fO2
Solution model
ÿ11.88 0.14
ÿ11.92 0.14
ÿ12.05 0.15
Stormer
Lindsley
919
ÿ12.02 0.15
ÿ10.51
Element
Cation frac.
Formula/3 sites
0.223
0.203
0.800
0.789
832 19
0.215
0.218
0.807
0.807
823 19
823 18
825 19
Anderson
Carmichael
Ghiorso & Sack
Point 16
Oxide
Wt %
Spinel phase
9.85
1.84
Ti
Cr
Fe2O3
0.08
48.75
FeO
37.98
Fe2 ‡
TiO2
Al2O3
Cr2O3
Al
Fe
3‡
1380
0.0919
0.0269
0.28
0.08
0.0008
0.4552
0.00
1.37
0.3941
1.18
DEVINE et al.
MAGMA STORAGE REGION PROCESSES
Point 8
Oxide
Wt %
Element
MnO
0.85
Mn
MgO
1.20
100.56
Mg
Total
Cation frac.
Formula/3 sites
0.0089
0.0222
0.07
0.4004
0.0031
0.80
0.01
0.0001
0.1960
0.00
0.39
Mn
0.3508
0.0106
0.70
0.02
Mg
0.0390
0.08
X(Usp) spinel
0.284
Stormer model
X(Usp) spinel
Anderson model
X(Usp) spinel
0.263
0.276
Carmichael model
X(Usp) spinel
0.278
Lindsley model
0.03
Rhombohedral phasez
TiO2
Al2O3
Cr2O3
Fe2O3
FeO
MnO
MgO
Total
X(Ilm) rhomb
X(Ilm) rhomb
X(Ilm) rhomb
X(Ilm) rhomb
42.35
0.21
Ti
Al
0.01
20.72
Cr
33.36
1.00
Fe2 ‡
2.08
99.74
Fe
3‡
0.793
0.782
Anderson model
0.801
0.800
Lindsley model
Stormer model
Carmichael model
Pair passes Bacon & Hirschmann (1988) equilibrium test
X(Usp) sp
X(Ilm) rh
T ( C)
log fO2
Solution model
ÿ11.24 0.14
ÿ11.28 0.13
ÿ11.42 0.14
Stormer
0.284
0.263
0.793
0.782
879 19
0.276
0.278
0.801
0.800
869 19
870 19
ÿ11.39 0.14
ÿ10.45
871 19
923
Anderson
Carmichael
Lindsley
Ghiorso & Sack
Calculated using the algorithms of Andersen & Lindsley (1988; as amended by Andersen
et al., 1991), and Ghiorso & Sack (1991); solution models from Carmichael (1967), Anderson
(1968), Lindsley & Spencer (1982), and Stormer (1983).
yComposition of ilmenite in crystal core.
zComposition of ilmenite close to interface with titanomagnetite.
explosions. On 9 November 1999, an explosion
occurred that was clearly too large to have been caused
merely by phreatic processes. Fe±Ti oxides from the
ash produced by that explosion (samples MVO1125
and MVO1126) were analyzed, and transects of titanomagnetites (Fig. 6) suggested diverse origins for the
respective grains. Some grains contained elevated
TiO2 contents (Fig. 6b and e), relative to the low
TiO2 contents (58 wt %) of the cores of phenocrysts
in explosively erupted tephra from the 1995±1998
phase of the eruption (e.g. Figs 2 and 5), whereas
others had lower values similar to those of earliererupted grains (Fig. 6a and d). Some grains had
begun to unmix, resulting in compositional banding
(Fig. 6f), and still others had become variably altered
to hematite (not shown). The Fe±Ti oxide data suggested that the ash was a mixture of old and new tephra
and/or lithic fragments.
March 2000
Sample MVO1175 (Fig. 7) is a fragment of dome lava
collected in March 2000, after dome extrusion resumed
at the beginning of the second eruptive phase. All
ilmenite±titanomagnetite pairs are characterized by
reaction fronts at their contact that yield spuriously
high temperature estimates (Fig. 7a±d). Many titanomagnetite phenocrysts, however, have low-TiO2
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Fig. 3. Colorized Ti Ka X-ray dot map of ilmenite±magnetite incontact oxide pair from dome lava sample MVO1208A (September
2000); ilmenite is being replaced by titanomagnetite along an advancing reaction front; the ilmenite crystal (red tones) has a reaction rim
of 10 mm thickness (green tones) composed mainly of high-Ti titanomagnetite; the abutting titanomagnetite phenocryst has a low-Ti
core (dark blue) with higher-Ti rim compositions (light blue), the
latter characterized by true diffusion gradients; the high-Ti reaction
rim surrounding the ilmenite phenocryst is interpreted to be composed of a mixture of sub-micron-scale domains of titanomagnetite
and relict ilmenite; results of analytical transects A±A0 and B±B0 are
given in Fig. 4.
(58 wt %) cores, and some have modest Ti diffusion
gradients at the rim (Fig. 7e and f ).
September 2000
Samples 1208A, 1208B, and 1208C are fragments of
dome lava erupted in September 2000. Sample 1208C
contained a large (15 cm 8 cm 5 cm), flattened,
ellipsoidal, diktytaxitic mafic inclusion that was
enclosed by andesite. Oriented thin sections were
made such that the distance from titanomagnetite
grains in the andesite to the andesite±mafic inclusion
interface could be measured. The objective was to see if
there was a systematic variation in the Ti zoning patterns in the titanomagnetite grains as a function of
distance from the mafic inclusion. The reasoning was
that quenching of the mafic blob caused by injection
into the andesite might cause transient heating of the
andesite near the blob, recorded by compositional
changes in the titanomagnetite crystals. The results
of the analytical transects are presented together in
Fig. 8.
There is considerable diversity of Ti diffusion profiles. Some grains lack diffusion gradients and also have
low-TiO2 cores, whereas others contain long, short, or
Fig. 4. Analytical transects of titanomagnetite crystal shown in
Fig. 3; dome lava sample MVO1208A (September 2000). (a) Transect A±A0 in Fig. 3, which is roughly perpendicular to the ilmenite±
titanomagnetite interface (see text for explanation of spuriously high
`apparent' temperature estimates derived from two-oxide geothermometry calculations). (b) Transect B±B0 in Fig. 3, which is roughly
parallel to the ilmenite±titanomagnetite interface, 130 mm away (note
asymmetry of rim-to-core diffusion gradients on opposite sides of the
crystal; also caveat about spuriously high `apparent' temperature
estimates derived from two-oxide geothermometry calculations).
even abrupt Ti diffusion gradients at the crystal rims,
but have `normal', low-TiO2 cores. At least one relatively small grain has an elevated core TiO2 content
and no diffusion gradient, although this may only be a
sectioning artifact. High TiO2 contents are also commonly observed in titanomagnetite microlites in the
groundmass of all andesites described above. These
high TiO2 contents cannot be used for temperature
estimates, for reasons outlined above.
The observed diversity of compositional zoning in
titanomagnetite phenocrysts in sample MVO1208C
all occurs within 7 mm of the interface with the mafic
blob, and there is no systematic variation of zoning
pattern or core Ti content as a function of distance
from the andesite±mafic inclusion interface. Titano-
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Fig. 5. Analytical transects of titanomagnetite crystals in sample MVO573 (vesiculated pumice from one of the September±October 1997
explosive eruptions). (a)±(f) Six of 12 rim-to-rim analytical transects of titanomagnetite phenocrysts that are in contact with melt (now glassbearing groundmass); only two grains, illustrated in (b) and (f), contained slight Ti diffusion gradients at the phenocryst rims, the other 10
crystals being essentially free of measurable Ti diffusion gradients (see text for discussion).
magnetite grains that have undergone different thermal histories have been brought together before, or
during, eruption. This inference also applies to the
silicate phases in the andesite.
September 2001
Dome lava samples MVO1228B (Fig. 9) and
MVO1229B (Fig. 10), which erupted in September
2001, contain titanomagnetite phenocrysts with the
most pronounced rim-to-core Ti diffusion gradients.
The rim-to-rim profiles of the titanomagnetite phenocrysts are generally symmetric, yet the TiO2 concentrations in the cores of the grains generally remain
like those that are characteristic of the andesite before
the most recent heating event. Most amphibole phenocrysts in these samples have thin or no decompressioninduced breakdown rims.
September 2002
Several textural types of dome lava were sampled
from dome collapse deposits that were extruded in
September 2002. Samples MVO1234A, MVO1234B,
and MVO1234C include moderately friable, slightly
oxidized, brownish blocks, and denser, light gray
varieties. All contain a mixture of different types of
amphibole phenocrysts that are dominated by euhedral crystals with thin or no decompression breakdown
rims, with lesser amounts of other amphibole grains
that have decompression or thermal breakdown features of variable extent and thickness. Small fragments
and/or microphenocrysts of amphibole also occur in
the groundmass (see Rutherford & Devine, 2003).
Mafic inclusions appear to be slightly more abundant
in these samples than in earlier-erupted samples. Fe±Ti
oxide phenocrysts are free of exsolution lamellae
(Fig. 11). Rare ilmenite±titanomagnetite in-contact
pairs are observed (Fig. 11a). Although TiO2 gradients
are observed in titanomagnetite crystals near the contact with ilmenite, suggestive of replacement of the
former by the latter, ilmenite crystals are still present
in eruption products more than 7 years after its onset.
Partially resorbed quartz phenocrysts are also still present. Although Ti diffusion gradients of variable extent
occur at the rims of titanomagnetite phenocrysts,
the TiO2 contents of the cores of the grains remain at
the low values obtained since the beginning of the
eruption.
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Fig. 6. Analytical transects of titanomagnetite crystals in combined samples MVO1125 and MVO1126 (lithic-rich ashes from the
9 November 1999 explosive eruption). (a)±(e) Rim-to-rim and rim-to-core transects with variable Ti diffusion gradients and absolute
concentrations (see text for discussion). (f) Titanomagnetite grain with incipient unmixing, probably caused by slow cooling within the
conduit or dome (note change of scales with respect to other figures).
DISCUSSION
Geothermometry
Our results indicate that the average TiO2 content of
titanomagnetite phenocryst cores (30 mm from the
rim) in tephra fall deposits is 778 028 wt % (see
Figs 2 and 5). Two-oxide geothermometry calculations, based on average core compositions of titanomagnetite and ilmenite phenocrysts in tephra fall
deposits, suggest that, before the recent heating event,
the andesite was at a temperature of 830 10 C and a
log fO2 of ÿ118, about 11 log units above the NNO
synthetic buffer; this is essentially the same result as
that reported by Devine et al. (1998a). Experiments
show (1) that, at 130 MPa, quartz does not crystallize
until the temperature falls to 825 C, and (2) that the
general stability of amphibole in the natural magma
indicates that the temperature in the magma is unlikely
to have been raised above 850±860 C for more than
a few hours (Rutherford & Devine, 2003). The rise in
temperature of the magma remobilized just before
eruption is therefore likely to be from 825 10 C
to 855, i.e. 30 C.
Causes of reheating of the andesitic
magma
The essential lack of Ti diffusion gradients in most
titanomagnetite grains in magma batches such as samples MVO573 and MVO291, erupted during the
August±October 1997 period of highest volume extrusion rate (10 m3 /s; Druitt et al., 2002), has implications
for the nature of magma heating episodes. Magma
erupted during that period travelled from the magma
storage region at 5±6 km depth to the surface in 52
days (Devine et al., 1998b; Rutherford & Devine, 2003);
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Fig. 7. Analytical transects of titanomagnetite crystals in sample MVO1175 (dome lava erupted in March 2000). (a)±(d) Ilmenite±
titanomagnetite in-contact oxide pairs (see caption for Fig. 2a); (e) and (f) rim-to-rim analytical transects of titanomagnetite phenocrysts
that are in contact with melt (now glass-bearing groundmass).
therefore, it was not affected by conduit processes, such
as plagioclase microlite crystallization, that might overprint phase relations in the storage region just before
eruption. Furthermore, the inferred heating that
caused remobilization of these magma batches did not
last long enough to produce TiO2 diffusion gradients in
the titanomagnetite phenocrysts.
The pronounced Ti diffusion gradients in titanomagnetite crystals in samples such as MVO1228 and
MVO1229 (erupted in September 2001) are equally
significant. In contrast to the explosively erupted samples from the September±October 1997 period, the
processes that resulted in eruption of these lava batches
did permit formation of Ti diffusion gradients. But the
question arises whether the zoning observed in such
titanomagnetite crystals is (1) produced in the
magma storage region, as a result of heating of the
host andesite by invading basalt or perhaps changes
in oxygen fugacity, or (2) produced during magma
ascent, as a result of release of latent heat caused by
microlite crystallization, which is brought on by degassing of H2O, or perhaps a combination of both processes (R. S. J. Sparks & M. Pichavant, personal
communication, 2002).
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Fig. 8. Analytical transects of titanomagnetite crystals in sample
MVO1208C (dome lava erupted in September 2000). All grains
occur in andesite that is within 7 mm of the contact with a large
mafic inclusion; wide variation in Ti diffusion gradient patterns and
absolute concentrations should be noted (see text for discussion).
Potential magma heating as a result of latent
heat effect
The efficacy of the latent heat effect depends on the
nature of heat flow in the conduit walls, and whether
the magma in the conduit is an open system with
respect to volatile species derived from magma at
depth, which may also carry heat out of the system
(Devine & Rutherford, in prep.). Modal analysis,
microprobe analyses of minerals and glasses, and mass
balance calculations suggest that the maximum temperature rise as a result of release of latent heat would
be 30±40 C ( J. D. Devine, unpublished data, 2002),
assuming the unlikely boundary condition of zero heat
loss from the magma in the conduit to the wall rocks or
the atmosphere.
Regarding the question of the magnitude of heat flow
from the magma in the conduit into the wall rocks, the
Soufriere Hills Volcano has previously been studied for
its extensive and long-lived hydrothermal system,
which included a hot water pond with magmatic
H2O-influenced isotopic characteristics located
455 km from the summit domes (Chiodini et al.,
1996; Hammouya et al., 1998). This indicates that
hydrothermal fluids are probably capable of carrying
heat and volatiles away from the conduit. In addition,
the D/H ratios of amphibole phenocrysts in some earlyerupted dome samples indicate that magma has at
times interacted with meteoric water (Harford &
Sparks, 2001), again suggesting the potential for heat
loss from the conduit as a result of hydrothermal activity. Furthermore, hundreds of thousands of earthquakes have been recorded in a laterally extensive,
45 km2 area since the onset of the present eruption,
most of them located in the 0±3 km depth range, many
characterized by the sharply impulsive onsets resulting
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from rock fracture (Aspinall et al., 1998). It is inferred
that the country rock in the immediate vicinity of the
conduit is highly fractured and therefore accessible to
hydrothermal fluids.
On the other hand, the eruptive style of the volcano
appears to depend at times on catastrophic release of
pressurization of the lava dome or upper part of the
conduit (Woods et al., 2002), suggesting that the wall
rocks of the conduit, or the sides of the growing lava
dome, may at times become impermeable, perhaps as a
result of precipitation of silica from volcanic exhalates.
At a time of frequently occurring vulcanian eruptions, in August±October 1997, however, the eruptive
dynamics suggested that leakage of volcanic gas from
the conduit through the surrounding rocks was occurring (Clarke et al., 2002a, 2002b).
Thus the rise in upper conduit magma temperature
potentially caused by release of latent heat during
microlite crystallization may well be considerably less
than the maximum 30±40 C calculated assuming
negligible loss of heat from the conduit. In addition,
experimental evidence described below suggests that
there is insufficient time for the chemical zoning
observed in natural titanomagnetites to be produced
by the latent heat effect.
Potential effect of changes in oxygen fugacity
Experimental petrologists have shown that compositions of titanomagnetites in hydrous andesitic and
dacitic magmas vary strongly with changes in oxygen
fugacity (e.g. Rutherford & Devine, 1988; Frost &
Lindsley, 1991; Martel et al., 1999). The question
therefore arises whether the compositional zoning
observed in the titanomagnetite phenocrysts in the
newly erupted andesite is due to changes in fO2 as
well as temperature (M. Pichavant, personal communication, 2002).
For changing fO2 to have a significant effect on titanomagnetite composition, the change must be in a direction essentially normal to the common fO2 buffers in
fO2±T space (e.g. Frost & Lindsley, 1991, p. 443). The
observed titanomagnetite zoning would require perhaps an order of magnitude change in fO2, which we
consider to be unlikely, at least at relatively constant T.
In fact, the variation of fO2 in island arc magmas in
general, and in the Lesser Antilles in particular, is more
or less parallel to the common fO2 buffers (e.g. Arculus
& Wills, 1980; Gill, 1981; Devine, 1987).
Investigations of the effects of temperature and fO2
changes on the compositions of coexisting Fe±Ti oxides
in simple systems indicate (1) that rising temperature
with fO2 varying parallel to the common fO2 buffers
results in an increase in the TiO2 content (i.e. mole
fraction of ulvospinel) of coexisting titanomagnetite,
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Fig. 9. Analytical transects of titanomagnetite crystals in sample MVO1228B (dome lava erupted in September 2001). (a) Ilmenite±
titanomagnetite in-contact oxide pair; TiO2 content (connected ) and `apparent' temperatures (connected *) vs distance (rim-to-rim)
across the titanomagnetite grain; `apparent' temperatures calculated using the method of Andersen & Lindsley (1988). (b)±(f) Rim-to-rim
analytical transects of titanomagnetite phenocrysts that are in contact with melt (now glass-bearing groundmass) [note more pronounced Ti
diffusion gradients, and only slightly higher core TiO2 concentrations of some grains, relative to earlier, explosively erupted samples (Figs 2
and 5)].
and a smaller decrease in the TiO2 content of coexisting ilmenite; (2) that decreasing fO2 at constant T
results in increases in the TiO2 contents of both phases
(e.g. Frost & Lindsley, 1991). The cores of ilmenite
grains are relatively invariant in composition and ilmenite in contact with titanomagnetite appears to be
being replaced by the latter. The TiO2 content of
ilmenite near such contacts is essentially the same as
that in the crystal cores (e.g. Fig. 11a), and in some
cases slightly lower, rather than higher, than that in the
crystal core (e.g. Fig. 1). This is the opposite of the
effect one would expect if the increase in titanomagnetite rim TiO2 contents were due to a significant
lowering of fO2. We infer that limited compositional
variations in ilmenite do not support the idea that the
fO2 of the andesite has been substantially decreased,
relative to the trends of the common buffers, as a result
of the recent magma mixing episode, an effect that
could potentially produce higher TiO2 contents in
coexisting titanomagnetite phenocryst rims. Small
changes in fO2 must occur, but we conclude that they
are mainly due to the magma being heated, rather
than to reduction by the invading basalt. We therefore
interpret the elevated TiO2 contents of titanomagnetite crystal rims to be due largely to a rise in the
temperature of the host magma (see also below).
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Fig. 10. Analytical transects of titanomagnetite crystals in sample MVO1229B (dome lava erupted in September 2001). (a)±(f) Rim-to-rim
analytical transects of titanomagnetite phenocrysts that are in contact with melt (now glass-bearing groundmass) (note more pronounced Ti
diffusion gradients, and only slightly higher core TiO2 concentrations of some grains, relative to earlier-erupted samples).
Constraints on conduit processes from petrographic
observation and experiments
We have conducted melting, crystallization, and isothermal decompression experiments to estimate the
effects of magma ascent processes on the compositional
zoning of titanomagnetite crystals (Rutherford &
Devine, 2003). The starting material for the experiments was coarsely crushed sample MVO573, which
is a partially glassy tephra. As indicated above, the
natural titanomagnetite phenocrysts are essentially
unzoned with respect to TiO2 (Fig. 5), so any zoning
in run products was probably produced during the
experiments.
First, isobaric heating experiments show that Ti
enrichment of titanomagnetite phenocryst rims can
be produced in a variety of ways. For example, samples
that are heated to a high temperature (say, 880 C;
P ˆ 130 MPa) for a short period of time (e.g. 2 days;
experiment M32) produce TiO2 zoning patterns in
titanomagnetite phenocrysts (Fig. 12a) similar to
those in crystals from samples heated at lower temperatures (e.g. 850 C) for longer periods of time (e.g.
2 weeks; run M56; Fig. 12b±d; compare Figs 9 and 10).
The effects of the experimental temperature rises,
such as those cited above, on the other phases can be
drastically different in the respective experiments. Petrographic observation of the natural samples can be used
to rule out some heating scenarios. Specifically, in the
first case mentioned above (heating for 2 days to 880 C
at P ˆ 130 MPa), the amphibole phenocrysts in the
experimental charge undergo extensive thermal
decomposition. Such decomposition is observed in
only a few grains per thin section in natural samples,
most other grains showing only decompression breakdown rims of variable thickness or opacitization caused
by near-surface oxidation. This strongly suggests that
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Fig. 11. Analytical transects of titanomagnetite and ilmenite crystals in samples MVO1234A, MVO1234B, and MVO1234C (dome lavas
erupted in September 2002). (a) Ilmenite-titanomagnetite in-contact oxide pair; TiO2 content (connected ) and `apparent' temperatures
(connected *) vs distance (rim-to-rim) across the titanomagnetite grain; `apparent' temperatures calculated using the method of Andersen &
Lindsley (1988). (b)±(j) Rim-to-rim analytical transects of titanomagnetite phenocrysts that are in contact with melt (now glass-bearing
groundmass) (note less pronounced Ti diffusion gradients, and slightly lower core TiO2 concentrations of some grains, relative to samples
MVO1228B and MVO1229B).
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Fig. 12. (a±f )
heating for short periods at temperatures above the
amphibole stability limit is unlikely to be the main
cause of the Ti diffusion gradients observed in most
natural titanomagnetite grains. Transient heating of
the andesite undoubtedly occurs during episodes of
basalt injection but the experiments suggest that the
high temperatures are likely to be dissipated in a matter of hours.
Second, the thicknesses of decompression breakdown
rims on amphibole phenocrysts may be used to estimate magma ascent rates and ascent times from the
magma storage region at 5±6 km depth, because an
experimentally calibrated speedometer has been determined by Rutherford & Devine (2003, fig. 9). For
example, in the most recently examined samples
(MVO1228, MVO1229, MVO1234), many amphibole phenocrysts lack decompression breakdown rims,
indicating magma ascent rates of 0017 m/s and
ascent times of 4 days from the storage region to
near-surface depths (Rutherford & Devine, 2003).
Assuming that most degassing of ascending magma
occurs in the upper 1±2 km of the 5±6 km conduit
(e.g. Melnik & Sparks, 1999, 2002), that would
indicate that any effects on the TiO2 zoning in titanomagnetite phenocrysts, which would potentially be
caused by release of latent heat as a result of microlite
crystallization, would probably have to occur in less
than 1±2 days. That is, within 1 or 2 days of the onset of
potential microlite crystallization in the upper conduit,
the magma will be expelled from the conduit and
therefore cooled considerably, effectively retarding
any currently occurring reactions (other than oxidation) as a result of rapidly decreasing, temperaturedependent Ti diffusion rates. Experiments discussed
in a later section also indicate that the pronounced Ti
gradients observed in titanomagnetites in some of the
most recently erupted samples cannot be produced in a
period of only 1±2 days.
Constraints on magma conduit processes from
decompression experiments
Decompression experiments were conducted to investigate the effects of gradually decreasing pressure, such
as may occur during slow magma ascent, on amphibole
stability and Ti zoning in titanomagnetite crystals
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Fig. 12. Analytical transects of titanomagnetite crystals in experimental samples. In (b)±(d), (h), and (j) the similarity of zoning patterns in
long-duration experiments compared with those in natural samples MVO1228B and MVO1229B should be noted. (See text for discussion.)
(Rutherford & Devine, 2003). The oxygen fugacity
was held near NNO ‡ 1. Experiment M29 involved
gradual isothermal (860 C) decompression of
MVO573 starting material from 130 to 4 MPa over
10 days. Experiment M36 involved the same isothermal decompression conditions, but the duration was 23
days. Transects of titanomagnetite phenocryst fragments in the experimental charges show only modest
near-rim enrichments in TiO2 (Fig. 12e and f ), even
though the experiments were artificially maintained at
the relatively high temperature of 860 C, requiring
external input of heat to the furnace apparatus. The
longer-duration experiment has better-developed
Ti diffusion gradients than those in the shorter
experiments, but neither profile is as pronounced as
those in some natural samples. It is concluded, because
of the weak Ti diffusion gradients observed in experiments M29 and M36, that the pronounced gradients
observed in titanomagnetite crystals in natural samples
such as MVO1228 and MVO1229 cannot be produced
by slow decompression P, T trajectories, even at the
relatively high temperature of 860 C. Although lowering of melt water content owing to decompression
drastically lowers Ti diffusion rates in the melt adjacent
to the phenocryst, the most likely explanation for lack
of pronounced gradients in the experimental titanomagnetites is that the diffusion rate of Ti through the
crystal is the most important rate-limiting factor.
Using available data for diffusion of Ti in magnetite,
Venezky & Rutherford (1999) calculated that it would
take 30 days to develop a Ti-rich rim of 20 mm thickness
on titanomagnetite at 850 C.
Some experiments were designed to simulate
rapid ascent of magma from the storage region
(P ˆ 130 MPa), followed by staging at a shallower
level (50 or 90 MPa; Rutherford & Devine, 2003).
None of the shorter-duration experiments of this sort
produced the pronounced Ti zoning observed in titanomagnetite phenocrysts in natural samples MVO1228
and MVO1229. Examples of the zoning profiles
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produced in the 50 MPa experiments are illustrated in
Fig. 12g (experiment M25; 10 day run) and Fig. 12h
(experiment M25 ‡ 1; 18 day run), and in the
90 MPa experiments in Fig. 12i (experiment M23; 5
day run) and Fig. 12j (experiment M40; 27 day run)
We conclude that magma ascent processes cannot
account for the Ti diffusion gradients observed in natural samples MVO1228 and MVO1229, because these
magma samples took 54 days to ascend from the
storage region to the surface, judging by the lack of
decompression-induced breakdown rims on amphibole
phenocrysts. The diffusion gradients in the natural
samples were therefore caused by magma storage
region processes.
Experimental constraints on timescales of heating of
andesite in the storage region
Fe±Ti oxide minerals can re-equilibrate (i.e. become
homogeneous with respect to Ti distribution) within a
few days to months of a rise in system temperature,
depending primarily on temperature and grain size
(Gardner et al., 1995; Nakamura, 1995; Venezky &
Rutherford, 1999). Therefore, the existence of Ti
diffusion gradients in Montserrat samples (e.g.
MVO1228B, MVO1229B) indicates a recent heating
event shortly before extrusion. Equally importantly,
the lack of Ti diffusion gradients in the crystal rims of
most titanomagnetite phenocrysts in explosively
erupted sample MVO573, and their preserved, low,
`original' grain TiO2 contents, indicate that heating
of that batch of magma probably happened within a
few days or weeks of eruption. The time of eruption
(September±October 1997), however, may have been
a period of the most rapid reheating and remobilization of the andesite, because a few hundred thousand
cubic meters of lava were being erupted explosively
every nine hours or so (Sparks et al., 1998; Druitt et al.,
2002). The observation that no Ti diffusion gradients,
and no elevated core TiO2 contents, occur in the vast
majority of titanomagnetite grains in that magma
batch (which was erupted more than 2 years after the
onset of the present crisis), strongly suggests that (1)
heating of the MVO573 batch of andesite occurred just
before its eruption, and (2) injection of basaltic magma
has continued throughout the eruption.
Several experiments help constrain likely timescales
of magma heating in the storage region and subsequent
eruption. Experiment M56 (MVO573 starting material; Rutherford & Devine, 2003) was run for 2 days at
870 C, then for 14 more days at 850 C at 130 MPa,
thus simulating heating by basalt injection into the
andesitic magma storage region. This experiment produced rim-to-core Ti diffusion gradients in titanomagnetite phenocryst fragments in the charge (Fig. 12b±d)
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that are essentially similar to those in natural oxide
crystals in samples MVO1228 and MVO1229 (Figs 9
and 10). Amphibole persisted as a stable phase. The
experiment was rapidly quenched, thereby freezing
in the compositional gradients that existed in the
simulated magma chamber. The natural samples
MVO1228 and MVO1229 may have been similarly
heated before rapid transport to the surface. Experimental results suggest, however, that these natural
samples could not have been briefly heated to temperatures as high as 880 C for 2 days, because this would
have resulted in extensive thermal decomposition
of amphibole grains, which is not observed in these
samples.
In contrast, experiment M21 (MVO573 starting
material), run at 860 C for only 4 days at 130 MPa,
produced no obvious Ti diffusion gradients in the rims
of titanomagnetite phenocryst fragments (Fig. 12k),
but continued heating under those conditions for a
total of 10 days (experiment M34) resulted in `rounding' and embayment of amphibole phenocryst fragments, production of very slight Ti diffusion gradients
in the rims of the largest titanomagnetite phenocryst
fragments (Fig. 12 l), and higher TiO2 concentrations,
relative to the MVO573 starting material, in the smallest titanomagnetite phenocryst fragments. The Ti
gradients in the largest phenocryst fragments in M34
(Fig. 12 l), however, do not extend to the high rim
TiO2 contents in some titanomagnetite phenocrysts
in natural samples MVO1228 and MVO1229 (Figs 9
and 10). It is also possible that the smallest titanomagnetite grains formed by recrystallization, rather
than by diffusive processes, as there are other reactions
occurring in the experiments.
It is concluded that heating periods of two or more
weeks may be recorded by some natural samples with
pronounced Ti diffusion gradients and high rim TiO2
contents in titanomagnetite phenocryst rims. It seems
highly unlikely that extensive Ti diffusion gradients
observed in samples MVO1228 and MVO1229 can
be produced at realistic temperatures in 54 days,
even at the 130 MPa pressure inferred for the magma
storage region, where diffusion rates for Ti in the melt
will probably be high relative to those prevailing in the
upper conduit, but diffusion rates in titanomagnetite
will remain relatively slow. The residence time of the
magma within the conduit is also generally too short
for such Ti diffusion gradients to have been produced
there, whether by release of latent heat or any other
process. It is therefore also concluded that reheating
and changes in Fe±Ti oxide compositions of the andesite are caused by injection of basaltic magma into an
andesitic magma storage region.
Titanomagnetite phenocrysts in the most recently
erupted lavas (MVO1234; September 2002; Fig. 11)
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lack the pronounced Ti diffusion gradients observed in
crystals from earlier-erupted samples (MVO1228,
MVO1229; September 2001; Figs 9 and 10). This
suggests that there has been no monotonic increase in
the TiO2 contents of crystals in the andesite during the
year that elapsed between eruptions of these samples.
Thus, there continues to be a lack of evidence for global
heating of the andesite. The TiO2 contents of most
crystal cores essentially remain as low as those in the
earliest eruption products.
Constraints on the magma system
Presence of magma storage regions within the arc crust
The major conclusions of our previous studies of
the andesite, and the work reported here and by
Rutherford & Devine (2003), are that, before heating
and remobilization, the erupted andesite was at a
pressure of 130 MPa, and that injection of mafic
magma caused its temperature to be raised 30 C,
from 825 C to 855 C before eruption (Devine
et al., 1998a; compare Couch et al., 2001). The energy
required to remobilize the andesite is probably supplied by transfer of heat across some sort of interface,
because only minor magma hybridization has as yet
occurred. There are a number of ways in which such
heat transfer may occur, depending on the geometry of
the magma plumbing system, and the relative fluidity
of the hydrous, mafic magma with respect to the
colder, more viscous, crystal-rich andesite. Geophysical
observations that would help constrain models of the
volcanic plumbing system are, however, few.
Monitoring of the volcano has indicated that most of
the earthquakes and ground deformation are caused
by near-surface phenomena occurring within and
around the conduit that connects the growing volcanic
dome with the storage region. Therefore, this monitoring cannot be used to infer the size or shape of the
magma storage region or the geometry of the conduits.
In contrast to volcanic centers where tectonic and
volcanic earthquakes have allowed delineation of the
lateral extent of magma chambers, such as Mount
St. Helens (Scandone & Malone, 1985) and Mount
Pinatubo (Mori et al., 1996; Pallister et al., 1996), the
relative scarcity of Montserrat volcanic earthquakes
with hypocentral depths greater than 5±6 km leaves
open questions about the size and shape of the
Soufriere Hills Volcano andesitic magma storage
region. Similarly, ground deformation studies since
the eruption began have indicated that any ground
motions caused by expansion or deflation of the
magma storage region would be masked by the much
larger near-surface motions caused by deformation of
the volcanic edifice and upper conduit (Jackson et al.,
1998; Shepherd et al., 1998; Voight et al., 1998;
compare Mattioli et al., 1998).
The dimensions of the uppermost conduit can be
inferred from a time series of measurements of extruded
magma volume, from the distribution of ballistics from
explosive eruptions, and from calculations of magma
extrusion rates and ascent rates. The upper part of the
conduit between the growing dome and the top of the
underlying magma storage region was estimated by
Robertson et al. (1998) to have a cross-sectional area
of 700 m2 , based on ballistics analysis and calculated
discharge rates of the 17 September 1996 explosive
eruption. This equates to a circle with a diameter of
30 m. An independent estimate of the conduit diameter can be obtained using magma ascent rate data
(Devine et al., 1998b) and the eruption rate calculated
from dome volume changes (Sparks et al., 1998). The
eruption rate appears to have been relatively constant
at about 19 m3 /s in the period 110±200 days into the
eruption (Sparks et al., 1998). Samples of the 12 May
1996 eruption contain hornblendes with a well-defined
population of thin decompression breakdown rims
(2 mm thick), which, using the Montserrat-specific
experimental calibration of the amphibole breakdown
magma ascent rate speedometer of Rutherford &
Devine (2003), yield estimated average ascent rates
between 0017 and 0022 m/s. Assuming conduit transport in a region where magma volume is constant (i.e.
before significant bubble formation), one can calculate
the approximate cross-sectional area of the conduit by
dividing the volume eruption rate by the ascent rate.
This calculation yields a conduit diameter of 11±12 m
for a cylindrical geometry (tacitly assuming a uniform
flow velocity; R. S. J. Sparks, personal communication,
2002), with an error of 7 m, assuming errors of
05 m3 /s in eruption rate and 0001 m/s in the
magma ascent rate. It is possible that conduit diameter
varies with depth, being 30 m diameter near the surface and 11±12 m at greater depths. The point is that
the conduit is narrow.
Using the larger estimated conduit diameter of 30 m,
and assuming that the conduit has a uniform diameter
with depth down to the top of the andesitic magma
storage region (5±6 km), 51 vol. % of the total
erupted volume of 4440 106 m3 of andesite
(Montserrat Volcano Observatory open file reports;
R. Herd, personal communication, 2002) could have
been contained at any one time in the discharge conduit before, or during, the eruption. Similarly, if one
presumes that the mafic magma that has triggered the
present eruption of andesite (from its `upper-crustal'
reservoir at 5±6 km depth) is itself derived from an
underlying, `mid-crustal' basaltic magma reservoir
(say, 10 km deep; depth calculation from Devine &
Rutherford, in prep.), by flow up a lower conduit of
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JOURNAL OF PETROLOGY
VOLUME 44
similar cross-section (i.e. 30 m diameter), one may also
conclude that such a basalt-filled conduit could not
contain a sufficient volume of magma to have pushed
out the 4440 106 m3 of andesite from its uppercrustal reservoir on a volume-for-volume basis.
These observations support the inference that there is
a pre-existing, extensive reservoir of andesite in the
upper crust. Additional support for the idea is that
the eruption products of the Soufriere Hills Volcano
have been in a relatively narrow compositional range
for the last 31 kyr (Roobol & Smith, 1998), implying
that magmatic processes have produced some sort of
quasi-steady state within the andesite range. The relatively low Al contents of amphiboles in the andesites
indicate a relatively narrow range of water pressures
averaging 130 MPa (Rutherford & Devine, 2003),
an estimate consistent with H2O contents of melt inclusions in plagioclase phenocrysts (Barclay et al., 1998;
Devine et al., 1998a). This suggests that the andesitic
magma storage region, although of unknown dimensions, does not extend over a significant pressure range,
and possibly has the general shape of an oblate
ellipsoid. It is probably intermittently connected by a
narrow conduit to an underlying magma storage
region filled with hydrous, high-Al basalt, as illustrated
schematically in Fig. 13.
From time to time, the andesitic reservoir is invaded
by injections from the basaltic magma storage region,
either because of density instability in the latter, or
because the basaltic reservoir has been injected with a
new aliquot of mantle-derived, primitive basaltic melt.
Petrologic observations (i.e. relatively narrow range of
Al contents in natural amphibole crystals; narrow range
of melt inclusion estimated water contents; pronounced
oscillatory zoning of plagioclase crystals; Devine et al.,
1998a) and experiments (e.g. variation of Al contents
in experimental amphiboles with pressure; Rutherford
& Devine, 2003) rule out models of the Soufriere Hills
Volcano plumbing system that involve polybaric,
decompression crystallizaton of ascending andesitic mush
columns of the type proposed by Blundy & Cashman
(2001). Extensive petrographic evidence for magma
mixing in all eruptive products of the Soufriere Hills
Volcano, i.e. the ubiquitous 1 vol. % mafic inclusions
and mineral disequilibrium features, strongly suggests
that the andesite is derived by fractional crystallization
of a hydrous basaltic parent magma (see below); that is,
the andesite is not a primary hydrous partial melt of
sub-arc mantle (see Carmichael, 2002). Magma mixing
and crustal assimilation processes do occur, but the
most important differentiation process is fractional crystallization (e.g. Devine, 1995).
One possible explanation for the predominance of
andesite in past eruptive products is that the inferred
andesitic reservoir is large enough to assimilate, by
NUMBER 8
AUGUST 2003
Fig. 13. Schematic cross-section of the Lesser Antilles arc crust
beneath Montserrat showing inferred volcanic plumbing. The proposed layering and density structure of the arc crust are an extrapolation of the seismic refraction study of Boynton et al. (1979). It is
proposed that ascending mantle-derived hydrous basaltic melts stagnate at the interface between the intermediate- and lower-crustal
layers as a result of the effect of the density structure on melt buoyancy (Devine, 1995). An andesitic magma storage region has been
developed at the interface between the upper- and intermediatecrustal layers (see text for discussion).
intimate hybridization, inputs of potentially hundreds
of millions of cubic meters of basaltic magma, without
producing measurably large changes in the average
composition of andesitic magma in the reservoir.
There is as yet no geophysical evidence for or against
the conjecture that such a large magma chamber
actually exists (e.g. attenuation of shear waves;
J. B. Shepherd, personal communication, 2002).
Another possibility is that only a small portion of the
total basaltic magma that is input into the uppercrustal reservoir actually mixes with the andesite (i.e.
it is only the 1 vol. % comprising the mafic inclusions
observed in the erupted andesite); the rest of the basalt
mainly transfers the heat required to mobilize the
andesite.
Heat transfer models for the two-magma system
There are at least two viable models of processes in
which heat, but not mass, may be transferred from the
invading basalt to the andesite in the inferred magma
storage region. One model postulates that dike-like
injections of basaltic magma into the andesite reservoir
provide ample surface area for heat transfer; but viscosity contrast prevents extensive mechanical mixing of
1394
DEVINE et al.
MAGMA STORAGE REGION PROCESSES
increasingly larger volume of basaltic magma, which
has ponded at the base of the reservoir (Fig. 14b). As
the volume of basalt increases, the cumulative amount
of heat transferred to the andesite should increase. This
heating might be recorded by a general increase in the
TiO2 contents of the titanomagnetite phenocrysts in
the andesite.
Nature of mafic magma injections
Fig. 14. Schematic illustration of possible evolution of the andesitic
magma reservoir. (a) Onset of the present eruption, with small
batches of injected basalt `ponding out' (i.e. stagnating as a result
of rheological or density contrast) at the base of the andesitic magma
storage region, causing remobilization of a thin boundary layer of
andesite; `ponded basalt' refers to the most recent injection increment; `remobilized andesite' refers to the andesite layer heated by the
most recent basalt injection; shape and dimensions (60 km3 ) of the
storage region are hypothetical (note that eruptions of several volcanoes in the Lesser Antilles have exceeded 30 km3 dense rock equivalent). (b) Potential result of hypothetical continuation of the present
eruption, if injection of basaltic magma continues and begins to
collect at the base of the andesitic reservoir, rather than become
hybridized with the andesite; dimensions and layer thicknesses are
schematic (note that the inferred layering in the magma storage
region mirrors the larger-scale inferred layering of the arc crust).
the two magmas (see, e.g. Sparks & Marshall, 1986;
Blundy & Sparks, 1992). Alternatively, basalt ponds at
the base of the andesitic magma storage region, with
mostly heat being transferred across a horizontal interface (e.g. Snyder, 2000). Couch et al. (2001) have
proposed a model of the latter type for the Soufriere
Hills andesite magma chamber, in which a heated
boundary layer at the base of the andesitic magma
storage region buoyantly rises, causing `self-mixing' of
the andesite and then eruption (Fig. 14a).
We consider first the general case of heating and
remobilization of the andesitic magma by dike-like or
sill-like injections of basalt. If the remobilized magma
were to be erupted essentially immediately, the Fe±Ti
oxide minerals in the erupted magma might not have
time to complete compositional changes caused by that
recent heating (i.e. increases in titanomagnetite Ti
contents by diffusion of Ti from the melt, and eventual
rehomogenization of grains with diffusion gradients). If
a time interval of, say, 2±4 weeks occurred between the
time of heating and the time of eruption, experimental
evidence suggests that measurable Ti diffusion gradients in titanomagnetite phenocryst rims should be
developed (Venezky & Rutherford, 1999; Rutherford
& Devine, 2003).
We consider next the case of the andesite in the
magma reservoir being heated from below by an
We now examine potential geometries of mafic magma
injection into the andesite, in light of petrologic and
geophysical constraints. Basaltic magma may pond at
the base of silicic magma chambers in sill-like layers as
a result of density contrast (e.g. Snyder, 2000; Couch
et al., 2001), but this may not necessarily be the case if
the basalt has a sufficiently high H2O content (Ochs &
Lange, 1999; Lange, 2002). On the other hand, there is
no evidence to indicate whether or not dike-like injections of basalt have played an important role in the
current eruption. To date, only minor amounts of
basalt appear to have mingled with the andesite, and
there have been no episodes of basalt eruption, so if
dike-like plumbing is important, the basalt has not yet
stoped through the andesite.
One incidence of nearly simultaneous eruption of
intermediate and basaltic magma occurred in nearby
St Kitts, where basaltic cinders (SiO2 49±50 wt %)
are interbedded with andesitic tephra (SiO2 59±62
wt %) in the Mansion Series of Mt. Misery (now called
`Liamuiga'; Baker, 1968, 1969, 1980; Baker & Holland,
1972). This example, and the presence of the South
Soufriere Hills Volcano basaltic center so close to the
Soufriere Hills Volcano, points to the possible eruption
of basaltic magma from the latter. Roobol & Smith
(1998) reported banded blocks in marker bed `M',
subunit III (17 ka BP) of the Soufriere Hills Volcano,
which are composed of intimately mingled andesite
and dark basaltic andesite, but they did not report
the presence of any discrete basaltic layers. There is
as yet no evidence to support a model of dike-like
injection of basaltic magma into, or through, the andesitic magma storage region beneath the Soufriere Hills
Volcano.
One possibility for control of the geometry of basalt±
andesite interaction at the Soufriere Hills Volcano is
the relatively high viscosity of the pre-eruptive andesite
(4107 Pa s at T 840 C; Devine & Rutherford, in
prep.). The relatively stiff andesite, before invasion by
the more fluid basalt (viscosity 102 Pa s; Devine &
Rutherford, in prep.), may prevent the basalt from
ascending any higher than the transition at 5±6 km
depth from brittle failure to ductile deformation in
the country rocks that is suggested by the vertical distribution of earthquake focal depths (Aspinall et al.,
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VOLUME 44
NUMBER 8
AUGUST 2003
1998; J. B. Shepherd, personal communication,
2001). In oceanic crust, such a brittle±ductile transition appears to coincide with the 750 C isotherm
(Phipps Morgan & Chen, 1993a, 1993b; Perfit &
Chadwick, 1998; see also Fournier, 1999). In the case
of the Soufriere Hills Volcano, the high contrast
between the viscosities of the highly fluid, hydrous
basalt and the highly crystallized (50 wt %) andesite
might prevent commingling of the two magmas (see
also Sparks & Marshall, 1986). Ponding of basaltic
magma at the base of a pre-existing andesitic magma
storage region is the simplest explanation consistent
with observations.
Nature of heating of the remobilized andesite
The strict time constraints placed on andesitic magma
remobilization and eruption by the Fe±Ti oxide data
strongly suggest that the remobilized batches of andesite are relatively small (Fig. 15). This in turn suggests
that the basaltic injections are small, perhaps sill-like
bodies with thicknesses of the order of 1±2 m (Fig. 16;
e.g. Snyder & Tait, 1995). The transfer of heat from a
sill-like intrusion of basaltic melt into the base of a
silicic magma chamber has been analyzed by Snyder
(2000), so it is possible to calculate the likely thicknesses of remobilized layers of Soufriere Hills Volcano
andesite using thermal constraints developed above. In
short, a basaltic sill of 1 m thickness (say, at 1000 C)
will remobilize a layer of 830 C andesite 16 m thick,
the heat transfer probably taking just a couple of days
(Devine & Rutherford, in prep.).
Couch et al. (2001) made a similar analysis of the
heat transfer from basalt to andesite, using different
boundary conditions, which were based on the assumption that the andesite could be heated to much higher
temperatures than those inferred here. They concluded
that the heat transfer was sufficient to cause `selfmixing' of the andesite, although they stated that a
boundary layer in the andesite only 2±4 m thick
would be mobilized. In our view, this rather limited
heating may be insufficient to cause large-scale mixing
of the andesite; first, because the storage region must be
sufficiently large to have contained all the magma
erupted to date; second, because the andesite outside
the 1±2 m, or 2±4 m, boundary layer will not `see' the
heating event (Snyder, 2000); third, because the remobilized andesite in the boundary layer is apparently
erupted within a few weeks of the onset of reheating.
We agree with the Bristol group that continued injection of basaltic magma into the base of the plumbing
system over time periods as long as decades is likely to
result in global heating of all the andesite in the magma
reservoir.
Fig. 15. Schematic illustration of basaltic magma injection into the
base of the andesitic magma storage region. The 760 m scale bar is
intended to illustrate the volume of magma erupted as of September
2002, of 440 106 m3 ; 760 m is the cube root of the volume
estimate. Scale of diagram is consistent with Fig. 14. Area enclosed
by box is enlarged in Fig. 16.
Fig. 16. Schematic enlargement of the zone of basalt injection,
showing the possible geometry of sill-like intrusions of basalt (see
also Snyder & Tait, 1995). Basaltic sill-like injections (dark pattern)
are shown at the base of the reservoir of andesite (lightly stippled
pattern). A thin boundary layer of remobilized andesite (heavily
stippled pattern) is extracted by a vertical conduit that drains the
layer and feeds the overlying conduit that extends through the upper
crust. Break in scale between schematic bottom and top of andesitic
reservoir should be noted. The lower part of the diagram is equivalent to the area of the box in Fig. 15. Basalt injections may be
intermingled with partially hybridized andesite at the bottom of
the remobilized andesite layer. The thickness of the remobilized
layer and the extent of any potential magma hybridization may
vary with the size of the basaltic magma injections.
The volume heating problem is illustrated with
reference to Fig. 15, which is a schematic representation of the basaltic magma injection zone at the base of
the andesitic reservoir. The 760 m scale bar in the
figure is used to indicate the volume of magma that
has been erupted from the storage region since 1995
(440 106 m3 ). It is unlikely that andesite away from
the zone of basalt injection could be appreciably
1396
DEVINE et al.
MAGMA STORAGE REGION PROCESSES
heated by an individual injection of basalt only 1±2 m
thick. A more appropriate question is how to move
small aliquots of andesite from the heated injection
zone to the surface.
Magma ascent pathways are used over and over
again in the Lesser Antilles, with most volcanoes producing multiple eruptions from the same conduit (e.g.
Boynton et al., 1979). Certainly the previous two eruptions of the Soufriere Hills Volcano (400 and 3950
years ago) used the current conduit. Whether the conduit is a tectonic feature (e.g. controlled by conjugate
faults), or a pre-warmed pathway, is unknown. However, the Fe±Ti oxide data suggest that an internal
conduit within the andesitic magma storage region
exists, which carries magma from the zone of heating
to the overlying conduit that in turn carries the magma
through the upper arc crust. In this model, the magma
chamber is being emptied from the bottom, not the
top. The possibility that the zone of basaltic magma
injection is at the top of the andesitic magma storage
region, rather than the bottom (e.g. because the
hydrous basaltic melt is possibly less dense than the
colder, partially crystallized andesite), seems unlikely,
because there is no evidence of extensive magma
hybridization.
The model of melt extraction from the bottom of the
andesitic magma storage region satisfies the thermal
constraints imposed by the Fe±Ti oxide data, especially
the observation that many of the titanomagnetite crystals erupted more than 2 years into the eruption retain
their `original' low-TiO2 (i.e. low-temperature) core
compositions. It is possible that at times of high volume
eruption rate (August±October 1997), there was a
higher than average basaltic magma injection rate,
with potentially thicker sills, and relatively rapid accumulation and extraction of layers of heated andesite. It
is also possible that basaltic magma is displacing andesite on a volume-for-volume basis (Fig. 14b), and that
crystallization of the accumulated basalt will, over
time, result in global heating of the remaining andesite.
Continuous monitoring of Fe±Ti oxide compositions
may reveal if that is the case.
Finally, we conclude that petrographic evidence for
limited magma hybridization can be reconciled with a
lack of evidence for a monotonic change in magma
composition, which one might expect from a process
of continuing, thorough hybridization, by postulating
that hybridization takes place only within the heated
boundary layer at the base of the andesitic reservoir.
The Fe±Ti oxides indicate that any such mixtures are
quickly harvested. Slightly more or less hybridization
with time of perhaps up to 10 or 20 wt % basaltic
magma with andesite in the boundary layer could
account for the variability and range of SiO2 contents
observed during the course of the eruption.
CONCLUSIONS
Fe±Ti oxide compositions in 1995±2002 Soufriere Hills
Volcano eruption products have been used in conjunction with experimentally determined phase relations of
the andesitic magma (Rutherford & Devine, 2003) to
infer the nature of heating and remobilization of the
andesite by invading mafic magma. Comparison of
zoning observed in natural titanomagnetite crystals
with mineral zoning produced in controlled experiments constrains the timescales of magma heating
and remobilization. Most batches of magma are heated
within 4 weeks of eruption; in some cases, perhaps
just days before eruption. The cores of most zoned
titanomagnetite crystals have retained their pre-eruption TiO2 contents, suggesting that: (1) the magma
remained at the pre-eruptive temperature of 830 C
until heated and remobilized; (2) insufficient time had
elapsed between heating and eruption to allow complete re-equilibration; (3) global heating of all the
andesite in the mid-crustal storage region at about
5±6 km depth (P 130 MPa) has not yet occurred.
The andesite is probably heated and remobilized by
relatively small, sill-like injections of basaltic magma
into the base of the andesitic magma storage region.
The heated boundary layer in the andesite adjacent to
the basaltic injections rises from the bottom to the top
of the andesitic magma storage region, and then up
through the mid-crustal conduit that connects the
storage region with the surface. The same ascent pathway appears to have been used by the magmas erupted
at 400 years and 3950 years BP.
The conclusion that global heating of the andesite in
the storage region has not occurred indicates that there
is no evidence suggesting that the probability of large
explosive eruptions is increasing (or decreasing).
Therefore, there is no compelling evidence that a
change in the present volcanic hazard zonation map
for Montserrat is required.
The present eruption is inferred to have been triggered by injections of hydrous, high-Al basalt, probably derived from its own storage region underlying
the andesitic reservoir (410 km?). The driving force of
the eruption is a function of processes occurring in the
basaltic reservoir. Basaltic melt may potentially be
ejected from its storage region as a result of several
factors. It is probably hydrous, so fractional crystallization will drive up the H2O content of residual
liquid, making it increasingly less dense, more buoyant
and gravitationally unstable. Crustal fracturing might
then allow the basalt to rise and either intersect an
overlying more evolved reservoir (such as Soufriere
Hills), or erupt directly at the surface (e.g. South
Soufriere Hills). This idea is consistent with the observation that volcanic eruptions in the Caribbean region
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JOURNAL OF PETROLOGY
VOLUME 44
may follow large earthquakes (Carr, 1977). Alternatively, the ejection of evolved, hydrous, high-Al basalt
from its storage region could be caused by injections of
parental basaltic melts derived directly from the
mantle wedge. In either case, the real cause of the
1995±present eruption of the Soufriere Hills Volcano
may be once or twice removed from processes occurring
within the andesitic magma storage region.
ACKNOWLEDGEMENTS
This work was funded by the UK Department for
International Development (formerly Overseas Development Administration) and administered by the
British Geological Survey and the Montserrat Volcano
Observatory. This work benefited from helpful discussions with Steve Sparks, John Shepherd, Don Snyder,
and Yan Liang. Thorough reviews by Steve Sparks,
Michel Pichavant, and Peter Kokelaar helped improve
the manuscript. The assistance of MVO staff in obtaining the samples used in this and previous studies is
gratefully acknowledged. Charlie Mandeville, of the
American Museum of Natural History, is thanked for
assistance in obtaining the X-ray dot map. Published
by permission of Director, British Geological Survey
(NERC).
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