JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 8
PAGES 1455±1475
2003
Experimental Constraints on the Conditions of
Formation of Highly Calcic Plagioclase
Microlites at the SoufrieÁre Hills
Volcano, Montserrat
S. COUCH1,2*, C. L. HARFORD1, R. S. J. SPARKS1 AND
M. R. CARROLL3
1
DEPARTMENT OF EARTH SCIENCES, BRISTOL UNIVERSITY, BRISTOL BS8 1RJ, UK
2
SCHOOL OF ENVIRONMENTAL SCIENCE, UNIVERSITY OF EAST ANGLIA, NORWICH NR4 7TJ, UK
3
DIPARTMENTO DE SCIENZE DELLA TERRA, UNIVERSITA DI CAMERINO, 62032 CAMERINO, ITALY
RECEIVED NOVEMBER 2, 2002; ACCEPTED JANUARY 24, 2003
High-pressure and -temperature experiments on a bulk-rock
composition representative of the groundmass of the Soufriere
Hills Volcano andesite have allowed the phase equilibria of the
system to be determined; these are then compared with the
natural samples. Experimental conditions varied from 825 to
1100 C and from 5 to 225 MPa; the main phases observed
were clinopyroxene, crystalline silica, amphibole and plagioclase. A relationship between plagioclase microlite size and
anorthite content is identified in samples of the natural andesite. Large crystals (460 mm2 in area) have cores of An60±75,
whereas small crystals (560 mm2 in area) have cores of
An40±60. Experimental results show that if the magma is
heated to 4950 C the high-anorthite microlite crystals can
form at magma chamber pressures without any need for a
change in bulk composition. It is proposed that convective
self-mixing occurs within the magma chamber. Geothermometry of coexisting plagioclase±amphibole pairs confirms the
complex crystallization history of the natural samples. Analysis of natural glass samples has identified compositional
variations that can be related to the crystallinity of the sample
and also the groundmass plagioclase composition. Rapidly
erupted pumice samples have high glass contents, lower SiO2
glass compositions and plagioclase microlites that are large
in size (460 mm2 ) and have a high anorthite content
(4An60). Slowly erupted dome samples are highly crystalline
and contain numerous plagioclase microlites of variable size
and composition.
The magma erupted at Soufriere Hills Volcano,
Montserrat, is a typical orogenic arc andesite, which is
crystal rich and has a diverse range of mineral textures
indicative of a complex history of magmatic evolution.
The petrology and geochemistry of the andesite have
been described by Devine et al. (1998a) and Murphy et al.
(2000). Those workers proposed that a variety of disequilibrium features can be accounted for by recent
reheating of the andesitic magma by basaltic intrusions.
Several observations are consistent with recent
reheating. Many plagioclase phenocrysts have resorption surfaces, and calcic overgrowth rims (Fig. 1a
and b). Plagioclase microphenocrysts generally have
more calcic compositions than the phenocryst cores.
Orthopyroxene phenocrysts have thin overgrowth
rims with compositions that indicate high crystallization temperatures (up to 970 C) in comparison with
estimates for core compositions (840±870 C) (Barclay
et al., 1998; Murphy et al., 2000). Fe±Ti oxide pairs
show evidence for heating (Devine et al., 1998a) and
there are amphibole phenocrysts replaced by intergrowths of pyroxene, plagioclase and titanomagnetite
*Corresponding author. Present address: Department of Earth
Sciences, Bristol University, Bristol BS8 1RJ, UK. E-mail:
[email protected]
Journal of Petrology 44(8) # Oxford University Press 2003; all rights
reserved
KEY WORDS:
self-mixing
glass evolution; experiment; Montserrat; plagioclase;
INTRODUCTION
JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 8
AUGUST 2003
Fig. 1. Backscattered SEM images of selected Montserrat samples. Pl, plagioclase; Px, pyroxene; Amph, amphibole. (a) and (b) variations in
plagioclase phenocryst zonation; (c) groundmass texture in dome sample; (d) variations in amphibole texture from unreacted (right-hand
crystal) to complete reaction (top-left crystal).The numerous plagioclase inclusions within the amphibole crystal should be noted.
(Devine et al., 1998b; Murphy et al., 2000). The andesite contains a small proportion ( 1 wt %) of mafic
inclusions with abundant acicular amphibole and plagioclase with a diktytaxitic texture characteristic of
rapid crystallization. These observations led Murphy
et al. (2000) to propose that the andesite has been
heated and remobilized by intrusion of hydrous basalt
magma into the chamber. Other observations are consistent with an open-system replenishment by basalt;
notably, the increase in discharge rate with time in the
1995±1998 phase of dome growth (Sparks et al., 1998)
and SO2 fluxes higher than can easily be reconciled by
degassing of the andesitic magma (Young et al., 1998;
Edmonds et al., 2001). Effects of decompression during
magma ascent have been recognized, including microlite growth (Sparks et al., 2000; Fig. 1c) and the formation of amphibole reaction rims (Devine et al., 1998b;
Fig. 1d). Experimental constraints on magmatic conditions (Barclay et al., 1998), in particular the stability
of amphibole and apparent coexistence of quartz and
amphibole, suggest that the overall magma chamber
conditions did not exceed 830 C, with water pressures
5130 MPa. Water contents of 4±5 wt % in melt
inclusions indicate water pressures of 110±140 MPa
(Barclay et al., 1998). Murphy et al. (2000) accounted
for the overall homogeneity of the bulk magma
throughout the eruption and the complex microscale
textural and mineralogical variation as a consequence
of convective stirring in a chamber heated from below
and cooled by assimilation of wallrocks of similar
igneous material. This concept received further support from Harford & Sparks (2001), who observed that
some amphibole phenocrysts have heavy dD isotopic
compositions (±6 to ‡30%), which indicate exchange
with hydrothermal meteoric water at temperatures
below the solidus. These isotopic heterogeneities
could be preserved only if these altered materials had
been recently assimilated. Harford & Sparks (2001)
estimated residence times of the altered amphiboles as
only several years. Zellmer et al. (2003) estimated residence times for plagioclase phenocrysts at magmatic
temperatures of a few decades to a few centuries based
on the disequilibrium distribution of Sr in complexly
zoned crystals.
1456
COUCH et al.
MICROLITE FORMATION, SOUFRIERE
HILLS VOLCANO
In this paper new experimental results are presented,
together with further geochemical data and new
textural observations, to develop a more complete
understanding of andesitic magma petrogenesis at
Montserrat. Groundmass microlites are compared
with plagioclase generated in the experiments, and
coexisting amphibole±plagioclase pairs have been analysed to estimate temperatures and pressures of equilibration. Although the concept of open-system basalt
replenishment is retained, the new experimental data
provide additional constraints that help to explain
previously ambiguous features, in particular the calcic
compositions of plagioclase overgrowth rims and
microphenocrysts. The textural and mineralogical
characteristics of the andesite are attributed to a process of self-quenching (Couch et al., 2001), in which
andesite at the base of the chamber is heated to high
temperatures and is then cooled as it is mixed into the
chamber interior by convection. Subsequent magmatic
evolution can be explained by a polybaric crystallization with late-stage crystallization as a result of
degassing during magma ascent (Blundy & Cashman,
2001). The possibility that the magma may be heated
during ascent as a result of latent heat release from
decompression-induced crystallization is considered.
Together these results and interpretations indicate a
complex open-system and polybaric crystallization
history of the Soufriere Hills magma.
The Soufri
ere Hills eruption and its
magmatic products
The Soufriere Hills eruption began in July 1995 after
3 years of precursory seismicity. The eruption has thus
far been characterized by four main phases (Robertson
et al., 2000): a first phase of earthquakes and phreatic
explosions ( July±November 1995); a sustained phase
of dome growth (November 1995±March 1998) with
eruption of 03 km3 (dense rock equivalent) of andesite
magma; a third phase of residual volcanic activity
(March 1998±November 1999) with ash-venting,
numerous minor explosions and occasional pyroclastic
flows caused by dome collapse but no dome growth; and
the current fourth phase of dome growth (November
1999 to the time of writing, December 2002) with
production of a further 022 km3 of magma (information provided by the Montserrat Volcano Observatory), accompanied by dome collapse pyroclastic
flows and some explosive eruptions. Two main kinds
of eruptive product have been identified: (1) pieces of
dome rock sampled by pyroclastic flows formed by
dome collapse; (2) pumice clasts erupted in Vulcanian
explosions. In general, pumices have higher glass
contents than dome rocks, although the latter have
highly variable glass contents. These variations have
been attributed to fluctuations in magma discharge
rate and residence time in the dome (Sparks et al.,
2000).
Petrological background
The petrology and geochemistry of the andesite have
been documented continuously throughout the eruption (Devine et al., 1998a; Murphy et al., 2000). There
are no systematic temporal variations in bulk magma
composition or phase assemblages. There are minor
variations, however, of bulk composition, with SiO2
varying between 584 and 624 wt % (Murphy et al.,
2000). Layering on a centimetre scale is common in
lava blocks, but the different coloured layers do not
show any compositional contrasts outside analytical
error. Rocks vary from dome lavas with low vesicle
contents (5±15% typically) to highly vesicular
pumices. There is no correlation of vesicularity with
bulk-rock chemistry. The andesite is crystalline, with
45±55 wt % phenocrysts (4300 mm in length) and 10±
15 wt % microphenocrysts (300±100 mm in length) by
modal analysis and variable proportions of microlites
(5100 mm in length). The distinction between microphenocrysts and microlites is arbitrary, and is defined
by the minimum crystal size that can be point-counted
accurately by optical microscope (Murphy et al., 2000).
The andesite contains phenocrysts and microphenocrysts of plagioclase (35±40 wt %), and also amphibole
(6±10 wt %), orthopyroxene, titanomagnetite and
minor quartz. Clinopyroxene (505 wt %) is present
only as microphenocrysts. Quartz displays either
resorbed boundaries or clinopyroxene reaction rims.
The plagioclase phenocrysts are compositionally and
texturally complex. There are sodic phenocrysts
(An48±58) with oscillatory zoning, reversely zoned
sodic phenocrysts and microphenocrysts (cores of
An48±58, rims of An65±80), reversely zoned dusty sievetextured crystals (rims of An70±88) and rare highly calcic
(An89±93) phenocrysts (Murphy et al., 2000). Zellmer
et al. (2003) have also reported the presence of calcic
cores (up to An80) in a few sodic phenocrysts.
Nomarski interference images and detailed compositional profiles indicate complex growth histories for
many individual crystals with oscillatory zoning and
multiple thin zones of more calcic plagioclase (Zellmer
et al., 2003). The microphenocrysts have cores of
An48±80 and some crystals have narrow more albitic
rims (An55±60).
The groundmass is composed of the same phases as
the phenocrysts and microphenocrysts, except that
amphibole is absent. The glass varies in proportion
(5±35 wt %), and its composition is high-Si rhyolite
(76±79 wt % SiO2). Mafic inclusions ( 1 wt %) are
widespread throughout the andesite.
1457
JOURNAL OF PETROLOGY
VOLUME 44
AUGUST 2003
Table 1: Whole-rock composition
(from Murphy et al., 2000) and
representative groundmass composition
of sample MVO34 (also known as
Mon6a) based on 200-rastered microprobe analyses performed by M. Murphy
METHODS
Experimental procedure
The variability in the groundmass crystallinity of the
Soufriere Hills andesite is of particular interest as it can
provide insights into late-stage crystallization processes. To determine the phase equilibria of the
groundmass, a series of experiments were carried out
as a function of pressure and temperature using a
representative groundmass composition (aMon6a).
This was estimated by averaging 200-rastered electron
microprobe analyses of the dome rock sample MVO34
(Barclay et al., 1998; Table 1), collected in February
1996. K2O can be used to indicate the degree of crystallization represented by the groundmass, as K is
nearly incompatible in the crystallizing mineral assemblage in the Soufriere Hills magma. K2O constitutes
only around 01±04 wt % of plagioclase and
018 wt % of amphibole crystals (Murphy et al.,
2000). The groundmass composition (aMon6a) represents 35 wt % of the bulk composition from mass
balance calculations.
A glass with the aMon6a composition was prepared
by A.-M. Lejeune. Reagent grade oxides or carbonates
were dried before weighing, ground in an agate mortar, slowly decarbonated and heated to 1820 K in a
platinum crucible in an electric muffle furnace. After
four successive quenches, grindings and fusions the
melt was finally quenched. Finely ground starting
material (005 g) was placed inside 25 mm diameter
Ag75Pd25 capsules, with sufficient H2O to ensure water
saturation at the P±T conditions. After welding they
were heated briefly to check they were sealed. Lowertemperature experiments (800±925 C) were carried
out in rapid-quench cold-seal pressure vessels (Carroll
& Blank, 1997), with an H2O pressurizing fluid at
pressures of 5±225 MPa. Rouse (2000) determined the
oxygen fugacity of the cold-seal set-up to be NNO ‡
13 (where NNO is nickel±nickel oxide) based on coexisting NiO±NiPd alloys (Taylor et al., 1992). This
compares with estimated fO2 of NNO ‡ 1 for the
Soufriere Hills andesite (Devine et al., 1998a). Experiments at higher pressure and temperature were carried
out in a rapid quench TZM pressure vessel, which uses
argon as its pressurizing medium. Comparison of calculated Fe3 ‡ in pyroxenes grown in the TZM and
cold-seal experiments suggests a slightly higher fO2,
not exceeding 1±2 fO2 units, in the TZM experiments.
Temperature was measured in both set-ups by K-type
(chromel±alumel) thermocouples, precise to 5 C with
accuracy checked by B-type (Pt94Rh6±Pt70Rh30) thermocouples, calibrated at the melting point of gold
(Gardner et al., 2000). Pressure was measured by either
Nova Swiss static pressure gauges, precise to 25 MPa,
or a pressure transducer, precise to 51 MPa. The
NUMBER 8
Wt % oxides
Whole rock MVO34
Groundmass aMon6a
SiO2
59.15
0.63
71.41
0.28
18.29
6.75
13.58
2.78
0.19
2.91
0.00
1.64
K2O
7.57
3.59
0.77
4.86
3.73
1.60
Total
99.85
99.88
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
experiments were left for 1±14 days to equilibrate,
and were then rapidly quenched. To test the approach
to equilibrium reversal experiments were performed,
where a sample was taken to higher pressure (50 MPa
above desired pressure) for 5 h, before being adjusted
to the desired P±T conditions and left for an appropriate time-period. Several experiments close to the
plagioclase liquidus were seeded with 2 wt % of
finely ground labradorite (An50). Excess H2O was present in all capsules at the end of the experiment.
Analytical techniques
Polished thin sections of experimental and natural
samples were analysed on a Cameca Camebax Micro
electron microprobe with SamX software and PAP
correction procedure for crystal phases at 15 kV accelerating voltage, and a JEOL JXA8600 electron
microprobe with LEMAS link software and XAF correction procedure for glass at 15 kV accelerating voltage. The JEOL system was used for glass analyses as it
has four spectrometers; this allowed analysis to be completed quickly, before significant alkali loss occurred.
Crystals were analysed at a 10 nA beam current with a
focused beam, and glass with a 2 nA rastered beam,
again to minimize alkali mobility.
Electron probe analysis of hydrous glasses can underestimate Na and overestimate Si as a result of alkali
mobility. Studies by Harford (2000) and Steen
(unpublished data, 2000) on the optimum analysing
conditions for hydrous glasses have found that a 2 nA
1458
COUCH et al.
MICROLITE FORMATION, SOUFRIERE
HILLS VOLCANO
Table 2: Summary of experimental results, including reversal experiments
Sample
Temperature
PH2O
Crystalline
Crystal
Glass
Plagioclase
Plagioclase
No. of
number
( C)
(MPa)
silica
fraction
(wt %)*
fraction
(wt %)*
fraction
(wt %)*
anorthite
plag.
contenty
analyses
n.d.
present?
sc27
1000
5
Y
n.d.
850
25
Y
sc16
875
25
Y
71.5
72.0
n.d.
28.5
28.0
45.5
49.0
n.d.
28.7 (2.1)
30.3 (1.9)
2
sc14
sc11
900
25
Y
925
25
Y
45.3
57.6
32.2
27.3
36.8 (3.6)
39.8 (1.7)
4
sc12
54.7
42.4
sc71z
1070
25
N
n.d.
n.d.
ÐÐ
ÐÐ
ÐÐ
sc81x
n.d.
89.0
n.d.
11.0
ÐÐ
55.5
ÐÐ
82.8
42.5
17.2
57.5
54.2
28.1
ÐÐ
33.5 (1.2)
35.2 (2.0)
5
33.5
29.3
66.5
70.7
22.7
19.6
43.5 (1.8)
46.0 (3.1)
47.7 (0.7)
29.9
9.3
70.1
90.7
20.7
3 .0
5
8.6
37.9
91.4
62.1
1 .3
23.8
49.8 (1.9)
62.6 (2.5)
63.2 (2.3)
26.0
27.0
74.0
73.1
16.7
17.0
7.9
11.7
92.1
88.4
1 .2
7 .1
94.6
78.5
1100
25
N
sc9
825
50
Y
sc1
850
50
Y
sc2
875
50
Y
sc74{
875
50
Y
sc3
900
50
N
sc7
925
50
N
sc80
995
50
N
sc63
950
85
N
sc61
830
100
Y
sc51
875
100
N
sc75{
875
100
N
sc73x
950
110
N
sc53
900
115
N
sc76x
970
115
N
5.4
sc10
875
125
N
sc26
875
160
N
21.5
16.0
sc69
825
175
N
23.2
84.0
76.8
sc24z
875
190
ÐÐ
n.d.
n.d.
sc57
1000
150
ÐÐ
n.d.
sc77x
875
200
ÐÐ
n.d.
sc59
875
225
ÐÐ
n.d.
44.9 (1.5)
53.0 (2.5)
51.2 (1.2)
7
3
4
5
6
4
6
7
13
5
5
5
69.8 (1.7)
60.3 (1.7)
71.6 (2.1)
14
10
11.5
58.0 (1.0)
59.9 (1.7)
52.6 (2.2)
ÐÐ
ÐÐ
ÐÐ
n.d.
ÐÐ
ÐÐ
ÐÐ
n.d.
ÐÐ
ÐÐ
ÐÐ
n.d.
ÐÐ
ÐÐ
ÐÐ
0 .0
13.1
6 .7
7
7
5
8
*Determined by mass balance calculations (PETMIX, Wright & Doherty 1970) using average compositional data for phases
present.
yAverage (and 1s) of several analyses.
zEven though below the liquidus, sluggish kinetics resulted in no plagioclase crystallization.
xSeeded experiment.
{Reversal experiment.
rastered beam at least 5 mm in width, with early analysis of Na, Si and Al, has an estimated sodium loss
of 55%, consistent with the findings of Morgan &
London (1996). Wherever possible the raster width
was 45 mm. Morgan & London (1996) found that if
the beam current is at least 10 nA, water-rich glasses
(45 wt % H2O) suffer significant alkali loss. Harford
(2000) studied glasses with 51±17 wt % H2O and
Steen (unpublished data, 2000) studied glasses with
3±9 wt % H2O. The influence of water content on Na
loss was negligible at the conditions of a 2 nA current
with a 5 mm rastered beam.
Calibration used known standards (albite for Na and
Si; Al2O3 for Al; sanidine for K; andradite for Ca;
St. John's olivine for Mg and Si; SrTiO3 for Ti; Fe
for Fe) and then was checked by analysis of secondary
standards (KK1 and Kn18). Typically, 5±10 plagioclase analyses were carried out for each experiment,
and about five analyses for all other phases.
EXPERIMENTAL RESULTS
The main experimental results are summarized in
Table 2. Representative scanning electron microscopy
1459
JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 8
AUGUST 2003
Fig. 2. Backscattered SEM images of selected experiments, as labelled with temperature and PH2O estimates. Pl, plagioclase; Gl, glass; Qz,
crystalline silica; Sp, titanomagnetite; Px, pyroxene. There is a systematic variation in plagioclase morphology from tabular at high PH2O and
temperature, to skeletal and dendritic at increasingly low PH2O and temperature. In (g) and (h) seed crystals are seen. Plagioclase
crystallization occurred in (g), as determined by the presence of an overgrowth rim on the seed. In (h), the plagioclase liquidus has been
exceeded and resorption is observed.
(SEM) images of experimental charges are shown
in Fig. 2. There is a systematic variation in the morphology of the plagioclase crystals. At high P±T
there are relatively few large crystals with tabular
morphology. At lower P±T conditions the number of
crystals increases, and the morphology tends towards
skeletal and dendritic at very low pressures and
temperatures.
1460
COUCH et al.
MICROLITE FORMATION, SOUFRIERE
HILLS VOLCANO
Fig. 3. Phase diagram of equilibrium experiments using aMon6a. All experiments were water saturated. Each quadrant represents a different
phase. Cpx, clinopyroxene; Plag, plagioclase; Amph, amphibole; Qz, crystalline silica. If black, the phase is present at the experimental
conditions. If white, the phase was not observed. All experiments contain titanomagnetite. Some experiments were seeded with plagioclase
(An50) as labelled.
Crystal phases
Figure 3 shows the phase diagram for the equilibrium
experiments. The stability of clinopyroxene, crystalline
silica, amphibole and plagioclase is dependent on the
PH2O and temperature. Amphibole is restricted to
temperatures below 875 C and water pressures greater
than 100 MPa. Crystalline silica is found at low water
pressures and variable temperatures. Spinel (titanomagnetite) is stable throughout the experimental conditions. Plagioclase is stable until high pressures and
temperatures are attained. Clinopyroxene is stable in
all experiments except at very high temperatures. The
experimental phases are the same as the natural
mineral assemblage, except for the absence of orthopyroxene, which is observed only in the natural
groundmass.
Plagioclase can be affected by sluggish growth
kinetics, especially just below the plagioclase-in
reaction. At these conditions plagioclase failed to
nucleate unless seeded, when narrow rims of high-An
plagioclase were identified on the labradorite (An50)
seed (Fig. 2g). In experiments where no rims were
seen, or where the seed was resorbed, the plagioclasein reaction is interpreted to have been exceeded
(Fig. 2h). Figure 4 shows how the anorthite content
of the plagioclase crystals changes with pressure and
Fig. 4. Diagram illustrating the changing anorthite content of
plagioclase in experiments using aMon6a as starting material, as
a function of PH2O and temperature. The anorthite value represents
the average (and 1s) of several microprobe analyses. Black
squares denote unseeded experiments; filled stars denote seeded
experiments.
temperature. For the experiments that were seeded,
the rims were analysed to determine the stable plagioclase composition. The anorthite content of plagioclase
increases with increasing water pressure and rising
temperature.
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JOURNAL OF PETROLOGY
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AUGUST 2003
Table 3: Experimental glass compositions; average (and 1s) of several analyses, renormalized to 100%
anhydrous
Sample
T
PH2O
( C)
(MPa) of analyses
Number
aMon6a Starting composition
sc14
850
25
3
sc16
875
25
5
sc11
900
25
1
sc12
925
25
7
sc81
1100
25
4
sc9
825
50
3
sc1
850
50
5
sc2
875
50
8
sc3
900
50
11
sc7
925
50
8
sc80
995
50
5
sc63
950
85
12
sc61
830 100
4
sc51
875 100
8
sc73
950 110
9
sc53
900 115
8
sc76
970 115
4
sc10
875 125
6
sc26
875 160
9
sc69
825 175
12
sc57
1000 150
10
sc59
875 225
13
Composition (wt %)
Unnormalized
total (wt %)
SiO2
TiO2
Al2O3
FeO
MgO
CaO
Na2O
K2O
71.3
79.1 (0.5)
78.8 (0.7)
0.3
0.6 (0.2)
0.4 (0.3)
13.7
11.4 (0.2)
11.5 (0.4)
3.1
0.9 (0.2)
0.67 (0.1)
1.6
0.1 (0.1)
0.1 (0.0)
78.6 (Ð
Ð)
79.0 (0.7)
71.1 (0.3)
0.2 (ÐÐ)
0.5 (0.2)
0.3 (0.1)
11.7 (ÐÐ)
11.3 (0.4)
14.2 (0.2)
1.0 (ÐÐ)
1.6 (0.4)
2.6 (0.2)
0.1 (Ð
Ð)
0.2 (0.1)
1.6 (0.1)
4.9
0.8 (0.2)
0.6 (0.1)
1.0 (ÐÐ)
3.6
2.8 (0.1)
2.9 (0.3)
2.8 (ÐÐ)
1.6
4.8 (0.2)
4.9 (0.3)
4.5 (ÐÐ)
78.6 (0.4)
78.8 (0.6)
79.3 (0.4)
0.3 (0.2)
0.5 (0.2)
0.5 (0.4)
11.7 (0.3)
11.6 (0.2)
11.2 (0.2)
0.9 (0.2)
1.0 (0.2)
1.3 (0.2)
0.6 (0.2)
0.1 (0.1)
0.2 (0.1)
1.1 (0.2)
5.1 (0.2)
0.9 (0.2)
0.9 (0.1)
3.5 (0.1)
3.7 (0.4)
3.1 (0.1)
3.3 (0.2)
3.2 (0.1)
1.5 (0.1)
4.3 (0.1)
4.0 (0.1)
78.6 (0.7)
78.8 (0.4)
73.8 (0.5)
0.4 (0.3)
0.2 (0.3)
0.3 (0.1)
11.6 (0.2)
11.6 (0.2)
13. 9 (0.3)
1.7 (0.1)
1.8 (0.1)
2.1 (0.1)
0.3 (0.0)
0.3 (0.0)
0.9 (0.1)
1.2 (0.1)
1.7 (0.1)
1.8 (0.2)
3.6 (0.1)
3.3 (0.2)
3.6 (0.1)
3.4 (0.1)
3.7 (0.1)
3.2 (0.1)
2.2 (0.1)
2.4 (0.1)
1.8 (0.1)
73.7 (0.6)
79.0 (0.8)
78.2 (0.5)
73.8 (0.9)
0.5 (0.3)
0.2 (0.0)
0.2 (0.2)
0.4 (0.3)
14.2 (0.3)
12.1 (0.5)
12.2 (0.1)
14.2 (0.3)
1.9 (0.2)
1.0 (0.1)
1.3 (0.1)
1.9 (0.2)
0.9 (0.2)
0.2 (0.0)
0.5 (0.1)
0.8 (0.1)
3.6 (0.3)
1.4 (0.3)
2.0 (0.2)
3.6 (0.1)
3.6 (0.3)
3.1 (0.0)
3.5 (0.3)
3.6 (0.1)
1.7 (0.1)
3.0 (0.1)
2.2 (0.1)
1.7 (0.1)
74.5 (0.4)
73.0 (0.6)
77.8 (0.8)
75.9 (0.9)
0.3 (0.1)
0.2 (0.2)
0.3 (0.0)
0.2 (0.3)
12.9 (0.3)
14.3 (0.4)
12.7 (0.2)
13.7 (0.3)
2.2 (0.4)
2.3 (0.1)
1.3 (0.2)
1.4 (0.3)
1.0 (0.4)
0.8 (0.1)
0.5 (0.1)
0.6 (0.1)
3.6 (0.5)
4.0 (0.1)
2.3 (0.1)
2.8 (0.2)
3.7 (0.2)
3.7 (0.2)
3.3 (0.1)
3.7 (0.4)
1.8 (0.1)
1.8 (0.1)
1.9 (0.0)
1.8 (0.1)
77.8 (0.4)
72.0 (0.6)
74.6 (0.5)
0.2 (0.2)
0.4 (0.3)
0.2 (0.0)
12.9 (0.2)
13.9 (0.2)
14.4 (0.2)
1.1 (0.2)
2.4 (0.2)
1.6 (0.1)
0.4 (0.1)
1.4 (0.1)
0.6 (0.1)
2.3 (0.1)
4.7 (0.1)
3.4 (0.1)
3.3 (0.2)
3.7 (0.1)
3.5 (0.1)
2.1 (0.1)
1.7 (0.1)
1.7 (0.1)
Glass
Glass compositions are rhyolitic and range from 71 to
79 wt % SiO2 (normalized to 100% anhydrous;
Table 3). Using the mass balance calculation program
PETMIX (Wright & Doherty, 1970), and assuming
that K2O is incompatible, glass proportions are estimated to vary from 100 to 10 wt % dependent on
experimental conditions (Table 2 and Fig. 5). There
is a negative relationship between An content of
plagioclase and K2O content of the glass (Fig. 6).
High An contents can be reproduced at hightemperature and -pressure conditions, with high proportions of glass and thus low K2O contents. The
seeded experiments that are within the stable plagioclase field differ slightly from the general trend, as they
have elevated An contents relative to K2O content.
Blundy & Cashman (2001) have applied an
approach first developed by Tuttle & Bowen (1958)
for using the glass compositions to constrain the maximum water pressure of equilibrium of a melt, based on
96.6 (0.3)
97.2 (1.3)
97.6 (ÐÐ)
97.0 (0.5)
96.8 (0.8)
94.4 (0.9)
94.3 (0.6)
94.6 (0.6)
96.0 (0.7)
95.0 (0.5)
95.7 (0.2)
94.7 (0.8)
92.3 (0.2)
94.4 (0.6)
95.8 (0.6)
94.7 (0.6)
92.8 (1.0)
94.1 (0.8)
93.6 (0.9)
92.2 (0.5)
93.0 (0.7)
90.9 (0.5)
the ternary system of quartz±albite±orthoclase. The
glass data (Fig. 7) show a linear trend from the starting
composition away from albite, tracking towards quartz
with decreasing experimental pressure and/or falling
temperature. When silica saturation is attained
the experimental glass compositions follow the silica±
feldspar cotectic towards orthoclase enrichment.
Several of the experimental glasses that have a crystalline silica phase do not plot at the expected cotectic
pressure. This may reflect the problems with analysing
glass in strongly crystalline samples, or may indicate
that the projection scheme does not permit accurate
estimation of pressure.
ANALYSIS OF NATURAL SAMPLES
Several Montserrat samples have been analysed to
compare compositional trends in the plagioclase microlites and matrix glasses with experimental results to
make inferences about crystallization conditions.
1462
COUCH et al.
MICROLITE FORMATION, SOUFRIERE
HILLS VOLCANO
Fig. 5. Contoured diagram showing the variation in crystallinity
(wt %) of the equilibrium experiments with PH2O and temperature.
Percentage crystallinity was estimated using the mass balance program PETMIX (Wright & Doherty, 1970) and average phase
compositions from microprobe analyses. The low-PH2O±lowtemperature experiments are labelled with the estimated crystallinity
to show the rapid increase in crystal content at these conditions.
Fig. 6. Graph of experimental glass K2O content against plagioclase
An content. Also plotted are natural glass and plagioclase compositions. For the natural samples the lowest observed An content is
plotted.
Fig. 7. Qz±Ab±Or ternary [after Blundy & Cashman (2001), adapted from Tuttle & Bowen (1958)], illustrating the compositions of
experimental glass norms. Experiments are labelled according to the presence or absence of crystalline silica. If a silica phase is present, the
experiments are distinguished according to PH2O conditions, as indicated. Silica±feldspar cotectics for varying PH2O from Blundy &
Cashman (2001).
1463
JOURNAL OF PETROLOGY
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NUMBER 8
AUGUST 2003
Plagioclase microlites
Between 20 and 35 microlites were analysed from each
of 14 samples, and their dimensions (length and width)
recorded. Figure 8 shows the data plotted as area of
crystal against An content. The samples have been
divided into three groups. The first and second groups
are both dome samples collected from pyroclastic flow
deposits related to dome collapse. The third group consists of pumice clasts erupted by Vulcanian explosions
when magma discharge rates were high (Sparks et al.,
1998; Druitt et al., 2002). No direct dome samples have
been collected in the eruption and so it is not possible to
know unambiguously whether a particular sample was
erupted during periods of high or low magma discharge
rate, and how long it was resident in the dome before
collapse occurred. Samples, however, have been divided
into those with a highly crystalline groundmass and
those with a glassy groundmass. In general, dome samples from early in the eruption are much more variable
in groundmass crystallinity than later in the eruption
when discharge rates were generally higher (Sparks
et al., 1998), but still very variable. The glassy dome
samples are inferred to represent lava erupted at high
magma discharge rate and with short residence time in
the dome before collapse. Many plagioclase crystals
have a narrow (55 mm) rim that is compositionally
distinct (Fig. 8). Generally, the microlites have a tabular
morphology with two dimensions being approximately
equal and the third dimension significantly longer.
Crystals with a similar length to width dimension
were not included in the data as they are likely
to represent a plagioclase cut end-on rather than
lengthwise.
All of the natural samples have a wide range of
plagioclase compositions (An40±75). However, there
are clear differences between the types of sample:
(1) pumices have a narrower range of An values,
with all crystals being more calcic than An50. The
highly crystalline dome samples have a much wider
range of plagioclase compositions, which vary from
An475 to An540.
(2) The pumices lack the small (560 mm2 ) crystals
that are numerous in most dome samples.
(3) Rims on the pumice plagioclase crystals are
only slightly less anorthitic than the cores, and never
less than An50. The larger crystals in the crystalline
dome samples have rims, which generally have much
lower An contents than their cores. These rims have
similar An contents to the smallest crystals found in
the same sample.
(4) In pumice and in dome samples, the anorthite
content increases with crystal size.
Fig. 8. Graphs of plagioclase crystal area as a function of anorthite
content. A distinction is made between core and rim analyses. (a)
Crystalline dome samples; (b) glassy dome samples; (c) pumice
samples. Prehistoric samples MVO37 (36 ka) and MVO25 (350 a)
are plotted separately to show the similarity between recent and
prehistoric eruptive products.
(5) The glassy dome samples have intermediate
characteristics between the pumice and crystalline
dome samples. There are small crystals, but their An
content does not extend to the low values observed in
the crystalline dome samples. There are more sodic
overgrowth rims on larger microlites, with a limited
compositional range, analogous to the small crystals.
(6) Two samples are from prehistoric eruptions.
MVO37 ( 36 ka pumice) and MVO25 ( 350 a
dome lava) show similar relationships to the recent
eruptive products.
A similar relationship between crystal size and composition for plagioclase microlites has been identified in
1464
MICROLITE FORMATION, SOUFRIERE
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COUCH et al.
Table 4: Summary of electron microprobe compositions of matrix glasses (sorted by increasing K2O)
compared with microprobe compositions of melt inclusions (MI), average groundmass and average whole rock
by XRF
Sample
Sample
Eruption
Est. magma
number
description
date
extrusion rate
(m3 DRE/s)
Composition (wt %)
Unnorm. WR K2O Estimated
total
(wt %)1
SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O
1s error
glass content2
(wt %)
Matrix samples
MVO58Na3 dome lava
17/09/96
1.5
MVO1205b pumice fall
17/09/96
9
MVO1205c pumice fall
17/09/96
9
MVO1205a pumice fall
17/09/96
9
MVO53
17/09/96
9
ballistic
MVO214
dome lava
13/05/97
4
MVO305a
pumice fall
Aug. 97
MVO174
pumice
19/12/96
9
2.5
MVO1109d pumice
05/06/99
0
MVO694
banded block Oct. 97
9
MVO57
ballistic
9
17/09/96
MVO47
dome lava
11/08/96
5
MVO182
dome lava
20/01/97
7
MVO37
pumice
3700 a
MVO484
dome lava
11/08/96
MVO444
dome lava
12/05/96
MVO58K4
dome lava
17/09/96
MVO344
dome lava
01/02/96
MVO254
dome lava
350 a
ÐÐ
5
1.75
1.5
1.3
ÐÐ
MVO11514 dome lava
02/02/00
MVO1814
dome lava
20/01/97
7
MVO12174 dome lava
1/02/01
3
MVO524
dome lava
01/07/96
ÐÐ
2.5
MVO2454
dome lava
04/08/97
8
81.0 0.3
76.5 0.4
77.2 0.3
77.3 0.3
76.6 0.4
77.6 0.4
77.3 0.4
77. 8 0.4
79.3 0.4
78.9 0.4
79.1 0.5
76.8 0.5
78.9 0.4
77.7 0.3
78.6 0.4
79.1 0.4
78.7 0.4
78.8 0.4
76.6 0.5
78.7 0.4
78.1 0.3
78.7 0.6
77.2 0.5
78.0 0.4
11.3
12.1
0.6
2.5
0.1
0.1
0.1
0.4
2.4
2.1
3 .9
3 .8
0.5
2.0
99.6
97.7
ÐÐ
ÐÐ
34
5
12.2
12.4
2.0
1.8
0.2
0.1
0.3
0.3
2.1
2.1
3 .8
3 .6
2.1
2.1
97.2
97.1
ÐÐ
33
4
4
2.3
1.9
0.2
0.2
0.4
0.3
2.2
2.0
3 .6
3 .8
2.1
2.2
97.7
97.3
ÐÐ
0.67
32
12.3
11.8
26
2
ÐÐ
31
4
11.9
11.7
2.2
1.9
0.1
0.2
0.3
0.2
2.0
1.9
3 .5
3 .8
2.2
2.2
97.6
96.6
ÐÐ
0.73
31
4
28
3
11.2
11.3
2.0
1.9
0.1
0.0
0.2
0.2
1.4
1.4
3 .3
3 .6
2.2
2.4
98.9
98.1
ÐÐ
0.76
30
4
27
3
11.0
12.1
2.1
2.3
0.1
0.0
0.2
0.1
1.2
1.5
3 .7
4 .1
2.4
2.6
97.0
98.7
0.68
0.77
23
2
25
1
11.5
12.0
1.8
1.7
0.0
0.1
0.2
1.2
1.5
3 .5
4 .0
2.7
2.7
97. 9
97.2
ÐÐ
25
3
ÐÐ
25
2
10.9
10.6
1.9
1.5
0.1
0.1
0.8
0.7
3 .3
3 .0
4.1
4.5
97.2
97.3
ÐÐ
16
2
ÐÐ
14
2
11.2
10.4
1.2
1.9
0.1
0.1
0.7
0.5
2 .7
2 .7
5.0
5.2
99.0
99.6
ÐÐ
0.76
13
2
0.1
12
1
11.4
10.9
2.5
1.4
ÐÐ
0.1
0.1
0.1
0.6
0.3
3 .0
2 .8
5.2
5.2
98.0
100.0
ÐÐ
0.8
15
1
13
1
11.3
10.6
1.4
1.1
0.0
0.1
0.2
0.4
1.0
3 .3
2 .4
5.3
5.4
100.6
ÐÐ
12
2
994
ÐÐ
12
1
11.3
11.2
1.9
1.5
0.1
0.1
0.5
0.4
2 .8
2 .4
5.8
6.0
98.8
98.5
ÐÐ
0.73
11
2
10
1
mean
98.2
95.7
94.7
ÐÐ
0.1
0.1
0.1
ÐÐ
0.0
0.0
Non-matrix samples
MI in quartz5
MI in plagioclase5
MVO242 plagioclase
Groundmass av.6
Whole-rock av.7
81.2 0.2
75.1 0.2
74.3 0.4
71.4 0.3
59.6 0.6
10.4
13.4
1.2
1.8
0.1
0.1
0.2
0.4
1.6
2.5
4 .4
4 .5
1.8
2.0
13.7
13.6
1.9
2.8
0.1
0.1
0.4
1.6
2.4
4.9
4 .4
3 .7
2.2
1.6
31
17.9
6.7
0.2
2.9
7.6
3 .6
0.8
1008
37
33
43
All compositions are normalized to 100% anhydrous for comparison. DRE, dense rock equivalent.
1
Whole-rock (WR) K2O contents from XRF values where available, otherwise WR K2O average of 53 analyses of 0.777
(0.08) wt %.
2
Assuming K2O slightly compatible. Assumptions used (with estimated 1s error): plagioclase 0.2 (0.03) wt % K2O, 70 (3)
wt % of whole rock; amphibole 0.18 (0.01) wt % K2O, 7 (0.5) wt % of whole rock. Errors in glass contents calculated by
error propagation.
3
Not true glassÐ
Ðfinely crystalline intergrowth of feldspar and quartz.
4
Contains groundmass silica phase.
5
Devine et al. (1998)Ð
ÐPlag melt inclusions (MI) n ˆ 26; Quartz MI.
6
Average groundmass composition from 200-rastered electron microprobe analyses of MVO34 (Barclay et al., 1998).
7
Average of WR XRF analyses for current eruption.
8
By definition.
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JOURNAL OF PETROLOGY
VOLUME 44
the Merapi dome lavas (Hammer et al., 2000) and the
Mount St. Helens dacite (Cashman, 1992).
Glass
Electron microprobe analyses of matrix glasses from
the natural samples are rhyolitic (765±810 wt %
SiO2, normalized to 100% anhydrous; Table 4). The
glass proportions are estimated to vary from 34 to 10
wt % (Table 4) by mass balance of K2O. These calculations assume an assemblage of 70 (3) wt % plagioclase containing 02 (003) wt % K2O, 7 (05) wt %
amphibole containing 018 (001) wt % K2O and
whole-rock K2O contents from X-ray fluorescence
(XRF) values (3% relative; 0023 wt %). These
calculations are consistent with estimates determined
by image analysis (Murphy et al., 2000; Couch et al.,
2003). Reliable analysis of some samples proved impossible because of lack of sufficiently large matrix glass
areas. Glass estimates for some of these samples by
image analysis (Murphy et al., 2000) indicate glass
proportions as low as 5 vol. %.
There is considerable variation in the glass composition and groundmass crystallinity of the dome lava
samples, whereas the pumices show less variation
(Table 4). The variation between two blocks from the
same pyroclastic flow deposit is as great as the variation
between lava blocks from different pyroclastic flow
deposits. For example, glass contents for lava blocks
from the January 1997 deposits vary from 5 to
25 wt %. The variability is attributed to two factors:
first, dome collapses sample regions in the dome with
different degassing and cooling histories and hence
different crystallinities; second, pyroclastic flows can
erode blocks deposited from previous flows (Cole et al.,
2002).
Some samples show heterogeneity in major element
glass composition, where there appear to be two
`glasses', such as analyses MVO58K and MVO58Na
(Table 4). Similar observations have been made for
Mount St. Helens (Cashman, 1992). These heterogeneities have been interpreted as an intimate mixture
of a K-rich true glass, and very finely crystalline intergrowths of feldspar and quartz (Blundy & Cashman,
2001). The results on Montserrat samples support this
interpretation. The K-rich zones contain chlorine,
whereas the Na-rich zones have no chlorine, consistent
with the former zones comprising glass, and the latter
zones comprising volatile-free microcrystalline regions
(Harford, 2000).
The `melt' compositions of the natural samples
follow the same trend as the experiments, when plotted
on a Qz±Or±Ab ternary (Fig. 9), with increasing Qz
away from Ab until silica saturation and then following the silica±feldspar cotectic. Some glasses from the
NUMBER 8
AUGUST 2003
crystalline dome samples plot further along the silica±
feldspar cotectic than the experimental glasses. Also
plotted are the compositions of melt inclusions from
quartz and plagioclase (Devine et al., 1998a). The plagioclase melt inclusion composition fits well within the
overall melt trend. However, the quartz inclusion composition is relatively enriched in silica compared with
both experimental and natural melt compositions. The
plagioclase melt inclusion is the least evolved (751
wt % SiO2) composition of all the natural samples
analysed, and the least evolved melt composition is
that of an explosion pumice (765% SiO2). Selected
glass compositions from natural samples have been
plotted against the most sodic plagioclase microlite
composition in the sample (Fig. 6). As the melt composition becomes more evolved (K2O increases), the
plagioclase becomes more sodic.
Plagioclase±amphibole thermometry
Amphibole phenocrysts in the andesite contain numerous plagioclase inclusions (Fig. 1d). Using the amphibole±plagioclase geothermometer of Holland &
Blundy (1994), the temperature of coexisting plagioclase and adjacent amphibole can be estimated. The
thermometer is estimated to be accurate to 2s 75 C
in the range 400±1000 C, although the precision
between inclusions is likely to be considerably better;
2s 30 C ( J. Blundy, personal communication,
2002). Many inclusions were analysed in selected
amphiboles from several samples to investigate differences in temperature within and between samples. For
each pair the analyses of the two minerals were made
as close to the contact as possible. The thermometer
utilizes two equations:
edenite ‡ 4 quartz
ˆ tremolite ‡ albite
edenite ‡ albite
ˆ richterite ‡ anorthite
thermometer A
thermometer B:
For silica-saturated rocks both thermometers can be
used; however, for silica-undersaturated rocks only
thermometer B can be used. As the upper stability of
quartz for the bulk rock has been determined as
5830 C at 130 MPa (Barclay et al., 1998), it is possible
to determine which thermometer to use depending on
the temperature result returned by the thermometers.
A water pressure of 130 MPa was assumed based upon
likely magma chamber conditions (Barclay et al., 1998;
Murphy et al., 2000).
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COUCH et al.
MICROLITE FORMATION, SOUFRIERE
HILLS VOLCANO
Fig. 9. Ternary Qz±Ab±Or diagram [after Blundy & Cashman (2001), adapted from Tuttle & Bowen (1958)] illustrating the range of glass
norms from experiments, natural samples and melt inclusions. Silica±feldspar cotectics for varying PH2O from Blundy & Cashman (2001).
Table 5 summarizes the results and Fig. 10a and b
shows SEM images of selected amphibole crystals with
estimated temperatures for individual plagioclase inclusions analysed. Estimated temperature varies both
within and between crystals. Most crystals show unsystematic variation in temperatures from 790 to 860 C.
There is no clear relationship between amphibole
crystal size and the mean temperature.
Figure 11 shows histograms of all temperature estimates and the mean temperature for each amphibole
crystal analysed. The temperatures have a wide range
from 764 to 935 C. The mean temperature, based on
all analyses, is 846 C (2s of 54 C). This is consistent
with estimates of magma chamber temperatures by
independent methods, such as Fe±Ti oxides (Devine
et al., 1998a) and orthopyroxenes (Murphy et al.,
2000). Crystals analysed from MVO37, a 37 ka
pumice, and a crystal from MVO1217 have lower
mean temperatures of 820±830 C. Plagioclase inclusions commonly range from An50 to An64 (Fig. 12).
The mean plagioclase composition, based on all analyses, is An57 (2s of An8). At water pressures of
130 MPa, the plagioclase compositions can be used to
infer temperatures of between 825 and 980 C, based on
our experimental results (Fig. 4). A few temperature
estimates in Table 5 exceed 900 C.
ESTIMATES OF LATENT HEAT
During magma ascent, volatile loss raises the liquidus
temperature of the melt, inducing crystallization,
which in turn releases latent heat. If crystallization
occurs sufficiently fast, this heat cannot be lost, and
therefore the temperature of the system will rise.
Microlites thought to be related to decompression
form up to 32% of the Montserrat andesite samples,
so this heating could be significant. The temperature
rise can be estimated utilizing existing thermodynamic
data (Robie et al., 1978). Latent heat L is calculated by
Lˆ
DHm
Cpo
where DHm is enthalpy of melting and Cpo is the heat
capacity at the temperature of interest. The DHm for
pure anorthite is 81 000 J/mol, and 59 280 J/mol for
pure albite. We assume an initial T 860 C, the Cpo
for pure An is 32767 J/mol per K, and for pure Ab
31813 J/mol per K. Calculations of the change in
magma temperature during ascent from an initial
temperature of 860 C, assuming that plagioclase
(An50) is crystallized, indicate that temperature rises
of up to 45 C are possible.
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JOURNAL OF PETROLOGY
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NUMBER 8
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Table 5: Summary of plagioclase±amphibole thermometry estimates
MVO sample
Eruption
Crystal
Crystal
Number of
number
date
number
length (mm)
pairs analysed
MVO37
3.7 ka
Max.
Min.
Mean (1s)
Max.
55.5 (3.6)
57.6 (5.7)
57.6 (3.7)
844
782
815 (18)
845
796
827 (16)
1
9
20
62
50
2
6
7
69
50
MVO25
350 a
1
5
16
65
51
MVO34
Feb. 1996
1
6
40
63
48
MVO181
MVO1217
Jan. 1997
Feb. 2001
2
5
26
64
52
3
4
14
66
51
1
5
11
59
49
2
7
8
71
50
1
8
2.5
2
MVO1218e
Jul. 2001
18
77
50
8
59
52
1
6
39
66
51
2
4
16
60
50
DISCUSSION
Experimental starting composition
There is a question as to whether aMon6a is a representative starting material. aMon6a cannot be a liquid
at the inferred magma chamber conditions (840±
870 C, 130 MPa, Barclay et al., 1998); at the inferred
water pressures, the liquidus of aMon6a is 1050 C
and the plagioclase liquidus is 950 C (Fig. 3). Therefore what is of importance is the melt composition of
experiments run close to magma chamber conditions
(860 C, 130 MPa). Experiment sc10 was run at 875 C,
125 MPa and its glass composition plots on the Qz±
Ab±Or ternary in the cluster of points that intersect the
100 MPa cotectic line in Fig. 9. This composition is
very similar to the least evolved melt compositions
observed in the pumiceous samples, and is slightly
more evolved than the melt inclusion composition
from a plagioclase phenocryst. Experiments at lower
water pressures and temperatures than sc10 all fall
along the same trend as the natural samples. Hence,
the relevant comparison of natural and experimental
data is for experiments run at inferred conditions for
the magma chamber and during magma ascent.
Magma chamber processes
Plagioclase±amphibole thermometry
The range of estimated temperatures (764±935 C)
exceeds the precision range (30 C, J. Blundy, personal communication, 2002), indicating that there
are real variations recorded by the amphibole phenocrysts. The average range of most temperature estimates (840±870 C) agrees well with estimates based
Estimated T range ( C)
An. content range
56.1 (3.1)
58.5 (3.8)
57.0 (4.1)
54.2 (3.3)
55.8 (4.6)
59.1 (7.7)
54.5 (2.3)
56.4 (3.3)
53.8 (3.1)
Min.
Mean (1s)
935
764
865 (42)
912
769
848 (26)
877
804
842 (22)
882
780
841 (28)
888
825
862 (24)
883
805
851 (37)
888
781
844 (27)
877
808
829 (23)
916
771
846 (30)
883
785
847 (24)
on other methods, suggesting that the absolute values
of the temperature estimates are reasonable. The broad
variation of temperatures around the estimated
magma chamber conditions of 840±870 C and
130 MPa is consistent with the amphiboles predominantly forming at these conditions. The localized
variation in temperatures together with variation in
plagioclase inclusion compositions suggest that amphiboles and coexisting plagioclases grew in a fluctuating
field of temperature and possibly PH2O. Taking the 2s
value about the mean of 54 C for all data as reflecting
these fluctuations suggests variations of the order of
100 C.
Several temperature estimates exceed 865 C, which
is the experimentally determined upper stability limit
of amphibole at 130 MPa water pressure (Barclay et al.,
1998). The highest temperature estimated is 935 C,
in a 350 a dome sample (MVO25). This temperature
is in excess of the sum of 865 C and the precision
(30 C) and therefore either amphibole persists metastably above its stability limit or the amphibole formed
at higher water pressures. Experimental work by
V. Buckley (unpublished data, 2002) suggests that
amphibole will break down within days if heated
above its stability field at magma chamber pressures,
limiting metastable amphibole preservation at these
conditions to very short time-scales. Rutherford et al.
(1998) experimentally determined that amphibole
was stable in the Montserrat andesite to temperatures
of 900 C at PH2O ˆ 250 MPa. The Holland &
Blundy geothermometers are dependent on pressure.
However, the difference in calculated temperature of
plagioclase±amphibole coexistence at 130 MPa and at
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MICROLITE FORMATION, SOUFRIERE
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Fig. 10. SEM images of selected amphibole crystals from Montserrat samples (see Table 5 for sample details). (a) and (b) plagioclase
inclusions analysed are marked with a black dot and the estimated plagioclase±amphibole equilibration temperature using the geothermometer of Holland & Blundy (1994) assuming a PH2O of 130 MPa. (c) and (d) plagioclase inclusions analysed are marked with a dot and the
estimated pressure of coexistence using the estimated equilibrium temperatures shown in (a) and (b) and analysed plagioclase anorthite
content. By using anorthite analyses and estimated temperature, the PH2O was estimated using experimentally derived contours of anorthite
composition (Fig. 4).
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Fig. 11. Histogram of plagioclase±amphibole equilibration temperature estimates, using the geothermometer of Holland & Blundy
(1994).
250 MPa is generally 55 C. For most of the inclusions,
which have temperature estimates of 4865 C, increasing water pressure would allow amphibole to be
stable, based on the experimental phase boundaries
determined by Rutherford et al. (1998).
The likely PH2O of each pair was estimated (Fig. 10c
and d) using the temperature estimates from the
geothermometers in conjunction with the experimentally determined contours of plagioclase anorthite
content with pressure and temperature (Fig. 4). The
experimental anorthite contours were derived from a
more evolved composition than that which would have
crystallized the amphibole phenocrysts. However,
comparison of these experimental results with similar
experiments performed using a bulk andesite composition (Rutherford et al., 1998) suggests similar anorthite
contents at the same P±T conditions. The two amphibole crystals shown in Fig. 10c and d display an apparent range of estimated water pressures from 130 to
240 MPa. As with the temperature estimates, there is
no systematic variation in estimated water pressure.
The amphibole crystals could be interpreted to have
formed over a large range of water pressures, corresponding to maximum depths of 10 km. The estimate
of magma chamber conditions (130 MPa) may therefore constrain only the top of the chamber (Barclay
et al., 1998). The observations of random inferred
water pressures, with in some cases high estimated
pressures at the crystal rim, could require a model in
which the chamber is so large that it convects over a
large depth range (5±10 km). Alternatively, the errors
associated with these calculations may be substantial.
Table 6 shows estimated pressure ranges for the crystals
shown in Fig. 10. If the precision range of 30 C is
used, the range of estimated pressures for a given pair is
limited, such that for crystals shown in Fig. 10c and d
NUMBER 8
AUGUST 2003
Fig. 12. Histogram of the compositions of plagioclase included
within amphibole phenocrysts in the Soufriere Hills andesite.
the difference between the highest pressure and lowest
pressure estimate is significant (Table 6). These results
suggest that some of the amphibole±plagioclase pairs
formed at water pressures 4130 MPa.
The overall consistency of the temperature ranges for
each crystal implies that the crystallization of amphibole and plagioclase has remained in a similar thermal
regime for considerable periods of time. The results
also indicate that quartz and most of the amphibole
crystals cannot be in equilibrium, as the amphiboles
crystallized predominantly at temperatures in excess of
the quartz liquidus (830 C).
Plagioclase microlites
Pyroxene geothermometry (Murphy et al., 2000) and
Fe±Ti oxide geothermometry (Devine et al., 1998a)
have been used to estimate the approximate temperature of the Montserrat andesite as 840±870 C. Barclay
et al. (1998) determined that the water pressure of the
magma chamber must be greater than 120 MPa, on
the basis of H2O contents of melt inclusions. Hence if it
is assumed that all the plagioclase microlites crystallized either in the magma chamber, or while ascending
to the surface, the range of An values should be An 55
(860 C, 130 MPa) to An530 (860 C, 525 MPa) based
on our experimental results. This expectation does not
correspond to the plagioclase microlite compositions
seen in natural samples, which range from An75 to
An32. The low anorthite contents can be related to
decompression and ascent, and will be discussed later.
However, the high An values of the natural samples are
difficult to reconcile with the proposed pressures and
temperatures in the magma chamber.
The process of self-mixing (Couch et al., 2001) is
invoked to explain the high anorthite contents
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Table 6: Estimation of possible pressure range for the coexistence of
plagioclase±amphibole pairs from selected amphibole phenocrysts in the
Montserrat andesite
MVO1217#1
MVO34#3
High P
Low P
High T
Low P
Analysed anorthite content
An72
An53
An66
An51
Estimated temperature ( C)
831
848
844
838
PH2O (MPa)
240
130
220
130
2s anorthite content
(based on experimental errors)
2s precision range ( C)
An68±76
An49±57
An62±70
An47±55
801±861
818±878
814±874
808±868
200±280
90±190
210±260
75±185
Estimated P range
(MPa) using precision range on T
The highest and lowest pressure estimates have been examined for each crystal. Using 2s variation on the mean of the
experimentally derived plagioclase anorthite content, and the precision estimate of the Holland & Blundy (1994) geothermometer, the range of possible pressures is calculated for each coexisting pair.
observed in the plagioclase microlites. Mafic material is
intruded into the magma chamber, ponding at the
base. With time the mafic material heats the overlying
adjacent andesite, forming a hot buoyant boundary
layer. This layer is estimated to become unstable
within tens of days. Hence plumes of hot andesite
form; these rise through the cooler material, quench
the hot magma and mix with adjacent andesite. By
mixing packets of magma of the same composition
but with different temperatures, the variation in
thermal crystallization history on a local scale can be
explained. Our experimental data indicate that parts
of this boundary layer must have exceeded 970 C to
precipitate 4An70 microlites and overgrowth rims on
phenocrysts by this process.
Magma mixing is a possible explanation for the heating and high-temperature crystallization observed in
the magma. The textures of the mafic inclusions in the
andesite show characteristic features of magmatic
quenching of hydrous basalt against cooler andesite
(Murphy et al., 2000). These inclusions could be a
source of calcic microlites. However, the mafic inclusions contain abundant fine-grained acicular amphibole with an entirely different composition compared
with the large amphibole phenocrysts in the andesite
(Murphy et al., 2000). Mixing of this magma into the
andesite should produce abundant acicular groundmass amphibole xenocrysts, but these are not observed
in the andesite.
Further evidence for localized but pervasive heating
comes from detailed study of Fe±Ti oxides by Devine
et al. (1998a), which suggests that material was heated
to 880 C. There are reverse zoned rims on orthopyroxene crystals, which have calculated temperatures
of crystallization of 880±1050 C (Murphy et al., 2000).
There is textural evidence of quartz crystals that have
resorbed edges, which could be due to reheating above
stable quartz conditions.
Another possible cause of the high anorthite plagioclase microlites is that they formed at water pressures
4130 MPa. The current estimate of magma chamber
water pressure constrains only the top of the chamber
to 130 MPa. As discussed earlier when considering
the geothermometry results of plagioclase±amphibole
pairs, at least some of the crystallization may have
taken place at greater water pressures. At greater pressure, more anorthite-rich plagioclase can crystallize at
a given temperature (Fig. 4).
The relationship between plagioclase microlite size
and anorthite content in the Montserrat samples can
be explained in part by convective self-mixing. The
cores of larger plagioclase microlites crystallized at
elevated temperatures as reflected by their high
anorthite content, along with the Ca-rich overgrowth
rims on the large plagioclase phenocrysts. The amount
of time available for crystallization at these conditions
was not long [based on the experimental results of
Couch et al. (2003) probably several days], allowing
microlites and overgrowth rims to grow, but not phenocrysts. The medium-sized microlites, which have
anorthite contents of An50, could have crystallized
at the temperature±pressure conditions estimated
for the bulk andesite (860 C, 130 MPa), after the
heating event.
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Petrological studies of Mount Pelee (Martel et al.,
2000) have identified high anorthite content plagioclase microlites, which are also likely to have formed
in the magma chamber during heating before eruption. It is therefore important to consider microlite
crystallization as a process that will occur both in the
magma chamber and during ascent.
Petrological and theoretical studies by Stewart &
Fowler (2001) have suggested that plagioclase phenocryst overgrowth rims grew shortly before eruption,
which agrees with this study. However, those workers
proposed that the overgrowth rims formed during
ascent and started to grow at a maximum depth of
900 m. The experimental results of this study show
that this interpretation cannot be correct. Overgrowths with compositions of An530 would be
expected rather than the observed high An contents
of overgrowth rims. Furthermore, if phenocryst overgrowth rim formation was an ascent-related process,
we would expect to see a correlation between ascent
rate and overgrowth rim thickness. Plagioclase phenocrysts from pumice samples would lack rims, as the
rapid ascent would prevent growth (Couch et al.,
2003), whereas dome samples, which ascended slowly,
would have overgrowth rims. No such relationship is
observed.
The similarity in plagioclase microlite sizes and compositions between recent samples and prehistoric samples (MVO37 and MVO25) suggests that processes
such as self-mixing have been a long-term feature of
the magma chamber conditions. This has implications
for the likely eruptive behaviour in the future, implying
that if the magma retains a similar composition,
material will continue to ascend and erupt in the
ways observed during the current eruption.
Ascent processes
Plagioclase
During ascent, decompression-induced crystallization
can occur in andesite magmas, with plagioclase as the
predominant crystallizing phase. The kinetics of
plagioclase nucleation and growth are sluggish, such
that there can be a delay between depressurization and
water exsolution, and the onset of crystallization (see
Couch et al., 2003). This delay can be used to understand differences in groundmass textures and compositions, and to infer variations in their likely ascent
history.
Magma that ascends sufficiently rapidly experiences
no crystallization during depressurization owing to the
nucleation delay (Couch et al., 2003). Pumices have a
groundmass containing large microlites (4100 mm2
crystal area) with high anorthite contents (4An50).
NUMBER 8
AUGUST 2003
Comparison with experimental results indicates that
microlite groundmass crystallization took place at elevated PH2O, in the magma chamber, with enough
time for large microlites to grow. Ascent must have
been sufficiently rapid to prevent decompressionrelated crystallization. Dome samples contain a wide
range of plagioclase microlite sizes and compositions.
The large, high-anorthite crystals could have formed at
elevated pressures in the magma chamber. The numerous smaller, sub-100 mm2 area crystals must have
formed at lower pressures in the conduit during ascent,
reflected in their lower anorthite contents (5An50).
The most sodic plagioclase microlites analysed have a
composition of An32, and this can be interpreted as the
crystallization products of magma ascending to very
shallow levels.
Glass
The similarity in melt composition between the pumice
samples and the experimental glasses for the inferred
magma chamber conditions provides evidence that the
rapidly decompressed samples did not undergo significant crystallization during ascent. In some cases the
melt compositions from the pumices are less evolved
than the experimental magma chamber melt (Fig. 9).
This is consistent with heating in the magma chamber,
resulting in melting of crystals and generation of a less
evolved melt.
The dome samples are variable in composition,
from those similar to the experimental magma composition, to others that have crystallized a silica phase.
This variation is due to the broad range of groundmass crystallinities reflecting decompression-induced
crystallization. The most evolved compositions sit
further along the silica±feldspar cotectic than any
experimental compositions, indicating more extensive
crystallization at either low temperatures or low pressures than the experimental conditions (5825 C or
525 MPa). The results indicate that the crystallization of quartz is confined to the shallowest parts of the
conduit and dome itself, where the water pressure is
very low. Cooling is not thought to be significant in the
crystallization of the dome (Sparks et al., 2000),
because conductive cooling affects only the outermost
parts of the dome in a crust estimated to be only metres
in thickness. Although it is possible that some samples
come from the cooled crust of the dome, the volumetric
proportion of crust in a major dome collapse is likely to
be negligible. Crystallization of quartz is also inferred
to be confined to samples formed during periods of
low discharge or that have long residence time in the
dome before a collapse. These concepts agree with
observations by Baxter et al. (1999) that cristobalite is
found in substantial amounts only in crystalline dome
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COUCH et al.
MICROLITE FORMATION, SOUFRIERE
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materials, and is absent in explosively erupted pumice
and ash.
Relationship to eruptive conditions
The difference in composition and crystallinity
observed between samples is interpreted as a consequence of differences in eruptive conditions and
magma residence time at shallow levels. Pumice clasts
are the most K2O-poor, least crystalline samples,
erupted during explosive eruptions and some dome
eruptions. During Vulcanian explosions newly arrived
magma from the uppermost several hundred metres
was probably evacuated in 1±2 min, giving insufficient
time for crystallization. During such eruptions, magma
extrusion rate is estimated at 9 m3 /s (Druitt et al.,
2002). For pumice, magma ascent from an estimated
depth of 45 km (Barclay et al., 1998) to the surface,
assuming a conduit area of 700 m2 (Druitt et al., 2002),
would have taken 4 days. During the sub-Plinian
eruption of 17 September 1996, when it is inferred
that the majority of the conduit was evacuated
(Robertson et al., 1998), magma ejected as pumice
may have spent as little as an hour in transit to the
surface.
Magmas erupted as part of the lava dome would
have experienced a relatively slow ascent. These samples are more crystalline, with estimated glass contents
of 10±31 wt %. Magma ascent from the chamber
would have taken between 5 and 80 days for typical
extrusion rates of 8±1 m3 /s, assuming a conduit area of
700 m2 (Druitt et al., 2002). Furthermore, these samples would have spent an unknown time period in the
lava dome (days to months). The amount of crystallization therefore reflects both ascent time and residence time in the dome before collapse.
CONCLUSIONS
By combining experimental work with detailed study
of natural samples, constraints can be placed on the
conditions of crystallization of the groundmass. There
are several conclusions:
(1) the composition of natural plagioclase microlites is dependent upon their size. Large microlites
have higher anorthite contents than the smaller
crystals. Dome samples display a wide range of
microlite sizes, with pumice samples lacking small
crystals (560 mm2 ). In the dome samples, the large
microlites have overgrowth rims of significantly less
anorthitic plagioclase.
(2) Microlites in natural samples are similar to
those determined by equilibrium experiments at the
inferred conditions in the magma chamber.
(3) Geothermometry of plagioclase inclusions within amphibole phenocrysts agrees with previous estimates of magma chamber temperatures and supports
the idea that there has been a complex and variable
crystallization history.
(4) Latent heat released as a result of microlite
crystallization can raise magma temperature significantly (by up to 45 C).
(5) Comparison of experimental and natural glasses
indicates that much groundmass crystallization can
occur as a result of decompression during magma
ascent. Comparison of natural samples and experiments shows that crystalline silica bearing, highly
crystalline dome lava was at equilibrium at pressures
of 25±50 MPa, representing depths significantly less
than 2 km.
With these findings several interpretations can be
made:
(1) estimates of the pressure conditions for the
coexistence of plagioclase inclusions within amphibole
suggest a complex, polybaric crystallization history.
(2) The mechanism of convective self-mixing might
explain the high anorthite content of the plagioclase
microlites (Couch et al., 2001). Hot andesite, reheated
by the intrusion of mafic material at the base of the
magma chamber, is quenched to form microlites and
overgrowth rims against the cooler interior. The extra
heat allows plagioclase with a much higher anorthite
content to crystallize, explaining the large, An-rich
microlites seen throughout the samples. Convective
self-mixing does not require direct involvement of
another more mafic magma to generate the complex
textures commonly observed in porphyritic orogenic
intermediate and silicic volcanic rocks, although mafic
magma emplaced at the base of the magma chamber
provides the likely heat source. The concept reconciles
otherwise puzzling petrological features, and also
supports the notion that magma chambers can
convect vigorously.
(3) The more sodic overgrowth rims and small
microlites (560 mm2 ) are interpreted to be the result
of decompression-induced crystallization. Magma
ascent rate determines the microlite population
both texturally and compositionally. Rapidly decompressed samples, such as pumices, had no time to
crystallize during ascent, resulting in microlites with
limited, An-rich compositions, and an absence of small
crystals. More slowly decompressed samples had
sufficient time for nucleation and growth of small
An-poor crystals and the growth of thin rims on
existing crystals.
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JOURNAL OF PETROLOGY
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(4) The similarity between prehistoric samples and
recently erupted samples suggests that similar magma
chamber and conduit processes have occurred at
Soufriere Hills for at least 36 ka.
ACKNOWLEDGEMENTS
Thanks are due to Jon Blundy and Jenni Barclay for
useful discussions, and Richard Brooker for assistance
with TZM experiments. Members of the Montserrat
Volcano Observatory are acknowledged for providing
the samples used in this study. Thorough reviews by
Alison Pawley and Caroline Martel are appreciated
and helped to improve the manuscript. S.C. and
C.L.H. acknowledge NERC studentships, and
R.S.J.S. an NERC professorship and MRC support
from Gruppo Nazionale de Vulcanologia.
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