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Journal of Petrology Volume 41 Number 1 Pages 21-42 2000
© Oxford University Press 2000
Remobilization of Andesite Magma by Intrusion of Mafic Magma at the Soufriere Hills Volcano, Montserrat, West Indies
1DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF BRISTOL, WILLS MEMORIAL BUILDING, QUEENS ROAD, BRISTOL BS8 1RJ, UK
2DÉPARTEMENT DE MINÉRALOGIE, UNIVERSITÉ DE GENÈVE, 13 RUE DES MARAICHERS, 1211 GENEVA 4, SWITZERLAND
3DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÁ DI CAMERINO, 62032 CAMERINO, ITALY
4GEOLOGY DEPARTMENT, UNIVERSITY OF LEICESTER, UNIVERSITY ROAD, LEICESTER LE1 7RH, UK
Received September 3, 1998; Revised typescript accepted June 14, 1999
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ABSTRACT |
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The 1995–1999 eruption of the Soufriere Hills volcano, Montserrat, has produced a crystal-rich andesite containing quench-textured mafic inclusions, which show evidence of having been molten when incorporated into the host magma. Individual crystals in the andesite record diverse histories. Amphibole phenocrysts vary from pristine and unaltered to strongly oxidized and pseudomorphed by anhydrous reaction products. Plagioclase phenocrysts are commonly reverse zoned, often with dusty sieve textures. Reverse zoned rims are also common on orthopyroxene phenocrysts. Pyroxene geothermometry gives an average temperature of 858 ± 20°C for orthopyroxene phenocryst cores, whereas reverse zoned rims record temperatures from about 880 to 1050°C. The heterogeneity in mineral rim compositions, zoning patterns and textures is interpreted as reflecting non-uniform reheating and remobilization of the resident magma body by intrusion of hotter mafic magma. Convective remobilization results in mixing together of phenocrysts that have experienced different thermal histories, depending on proximity to the intruding mafic magma. The low temperature and high crystallinity are interpreted as reflecting the presence of a cool, highly crystalline magma body beneath the Soufriere Hills volcano. The petrological observations, in combination with data on seismicity, extrusion rate and SO2 fluxes, indicate that the current eruption was triggered by recent influx of hot mafic magma.
KEY WORDS: Montserrat; eruption; magma mixing; mafic inclusion; sieve texture
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INTRODUCTION |
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The Soufriere Hills volcano, Montserrat, located in the Lesser Antilles island arc in the West Indies, began to erupt on 18 July 1995, after about 350 years of dormancy (Young et al., 1998b
). The eruption was preceded by a period of seismic swarms, which began in January 1992. Previous earthquake crises have occurred on Montserrat in 1896–1897, 1932–1935 and 1966–1967 (Powell, 1938; Shepherd et al., 1971). Initial phreatic and phreatomagmatic activity was followed by extrusion of an andesitic lava dome in November 1995. Pyroclastic flows, generated by gravitational collapse of the unstable dome, have been the dominant form of activity throughout the eruption, but periods of explosive activity, producing pumice flows and falls, have also occurred (Robertson et al., 1998; Young et al., 1998b). Dome growth ceased suddenly in March 1998 and further activity up to May 1999 has mainly involved gravitational collapses of the dome and periods of ash venting, with some small-scale vulcanian explosive events.
The Soufriere Hills volcano is the only active centre on Montserrat. The volcano is a composite of at least five andesitic lava domes, flanked by pyroclastic deposits. Rea (1974) identified four other volcanic centres, all inactive, the oldest dating back to about 4·5 Ma. Andesite is the predominant eruptive product of all of the inactive centres, except for the South Soufriere Hills centre, which produced mainly basalt and basaltic andesite (Rea, 1974; Baker, 1984). The South Soufriere Hills centre was active at about 125 ka, coeval with early activity at the nearby Soufriere Hills volcano (Harford et al., in preparation).
The petrology and geochemistry have been monitored throughout the current eruption (Devine et al., 1998a, 1998b; Murphy et al., 1998). The rock is a coarse-grained, porphyritic, crystal-rich, hornblende–hypersthene andesite, which has shown no consistent variation in bulk chemistry since the beginning of the eruption. Lithologic types include grey to reddened poorly vesicular lava, and moderately to highly vesicular pumice. There is no correlation between rock type and bulk chemical variation. Rare banded lava and pumice generally show no significant difference in composition between layers. Basaltic to basaltic andesite inclusions, which range in size from <1 mm to about 40 cm, form a minor (<1 wt %) but ubiquitous component of the andesite (Murphy et al., 1998). These mafic inclusions show evidence for having been molten when incorporated into the andesite, and phenocrysts in the andesite show evidence for a recent reheating event.
In this paper, we describe the petrology and geochemistry of the andesite and mafic inclusions. We suggest that mineral heterogeneity within the andesite and the presence of mafic inclusions reflect remobilization of a relatively old, cool (
850°C), highly crystalline magma body by intrusion of hotter mafic magma. This process results in mechanical mixing of crystals that have experienced very different petrogenetic histories within the same magma body. We also suggest that the current eruption was triggered by recent intrusion of mafic magma into the system.
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ANALYTICAL TECHNIQUES |
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X-ray fluorescence (XRF) analyses were performed at Leicester University using standard techniques described by Harvey et al. (1996)
. Analyses of international reference materials indicate that accuracy and precision are better than 0·5% for major elements and better than 3% for trace elements.
Electron microprobe analyses were performed at Bristol University on a JEOL JXA-8600 four-spectrometer instrument with LINK analytical X-ray analysis system and LEMAS automation. Online data reduction used the ZAF model. Run conditions were 15 kV accelerating voltage, beam current of 15 nA and minimum probe diameter (1 µm). Counting times were 15 s on the peak and 8 s on the background. Accuracy and precision for most elements are better than 2–3% relative. In the case of Ca in orthopyroxene, important for pyroxene thermometry, repeat analyses are within 0·05 wt % CaO (absolute).
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GEOCHEMISTRY |
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Representative whole-rock major and trace element analyses of andesite and separated mafic inclusions, erupted between December 1995 and January 1998, are given in Table 1 and plotted in Figs 1 and 2. Analyses of older Soufriere Hills samples, ranging in age from about 350 years to 110 ka, and of a basaltic lava sample from the extinct South Soufriere Hills centre are also presented. All discussion here refers to analyses that have been normalized to 100% volatile free with total iron (FeOT) calculated as FeO.
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The current andesite averages 59·7 wt % SiO2, with an overall between 58·4 and 62·4 wt % SiO2 from 47 samples. Only three samples have SiO2 contents higher than 61 wt %. There have been no systematic changes in bulk composition since the beginning of the eruption. Older Soufriere Hills rocks extend to slightly higher SiO2 (63·8 wt %) than the current andesite. Mafic inclusions from the current andesite typically have SiO2 contents between 51 and 54 wt %, overlapping in composition with older South Soufriere Hills lava samples [Table 1; see Table 2 of Baker (1984)]. Some mafic inclusions extend to higher SiO2 values (55–56% SiO2) as a result of bulk incorporation of host andesite.
The Montserrat rocks are low K according to the classification of Gill (1981) (Fig. 1a). The mafic rocks plot in the tholeiite field of Miyashiro (1974), whereas the andesites plot in the calc-alkaline field (Fig. 1b). The mafic inclusions have MgO between 3·8 and 4·6 wt % and have very low Ni and Cr contents (Table 1). Compatible (MgO, Al2O3, CaO, FeOT, TiO2, Sr) and incompatible (K2O, Rb, Ba, Zr) element abundances are higher and lower respectively in the andesites than in the mafic rocks, consistent with derivation of the andesite by fractionation from mafic magma of similar composition to the more mafic inclusions.
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ANDESITE PETROLOGY |
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The andesite is highly crystalline, consisting of about 45–55 wt % phenocrysts (>300 µm) and microphenocrysts (
100–300 µm), and 20–32 wt % microlites (<100 µm). The 100 µm limit on the size of microlites is arbitrary and defines the minimum size of crystals that can be point-counted accurately by optical microscopy. The upper limit on the size of microphenocrysts corresponds approximately to the maximum size of clinopyroxene crystals and also corresponds to a compositional change in plagioclase core compositions, as plagioclase crystals smaller than 250–300 µm are often significantly more calcic than the cores of larger phenocrysts.
The andesite contains about 30–35 wt % of plagioclase phenocrysts and microphenocrysts (Table 1). The other phenocrysts are amphibole (6–10 wt %), orthopyroxene (2–5 wt %), titanomagnetite (2–4 wt %) and quartz (<0·5 wt %). Clinopyroxene (<0·5 wt %) occurs only as microphenocrysts. Apatite and ilmenite occur in trace amounts. The groundmass (<100 µm) mineralogy is similar except that amphibole is absent. Groundmass glass is high-Si rhyolite (76–79 wt % SiO2). Glass proportions vary between about 5 and 25 wt %, depending on the late-stage history of individual samples. During periods of high extrusion rate, samples tend to be more glassy (up to 25 wt %) than those erupted when extrusion rates are relatively low. In the latter case, silica minerals (predominantly cristobalite) form a significant proportion of the rock (up to about 15 wt %), occurring as vesicle infills and as minute patches (<3 µm) in the groundmass, and glass proportions are reduced accordingly, as a result of late-stage microlite growth and devitrification of glass, presumably within the lava dome (Baxter et al., 1999).
Plagioclase
Plagioclase ranges in size from small (<10 µm) microlites to large phenocrysts (up to about 5 mm). Four main plagioclase populations are distinguished on the basis of composition, zoning patterns and textures (Figs 3 and 4, Table 2):
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- Type 1: sodic phenocrysts typically vary between An48 and An58, often with small-amplitude oscillatory zoning up to about An64 (Fig. 4a). Some phenocrysts have larger amplitude patchy zoning, ranging up to about An80.
- Type 2: reverse zoned large sodic phenocrysts (Fig. 4a and b) and microphenocrysts have oscillatory zoned cores, similar (An48–58) to the Type 1 crystals, with rims (30–150 µm) typically between An65 and An80. Many reverse zoned crystals remain calcic out to the edge of the grain but some have narrow (5–20 µm) more sodic rims (An55–65). Some reverse zoned crystals have outer rims with continuous normal zoning.
- Type 3: reverse zoned dusty sieve-textured crystals (Fig. 4c–e) have clear oscillatory zoned sodic cores (An48–58), similar to Type 1 and 2 crystals, with sieve-textured calcic mantles (An70–88). Many crystals have a clear, highly calcic rim (30–200 µm), similar in composition to the sieve-textured areas. Rim zonation patterns are similar to those of the Type 2 crystals. Many crystals remain highly calcic to the outer rims but some have narrow (5–20 µm) more sodic outer rims (An55–65) on the calcic rims. Other crystals have outer rims with continuous normal zoning. The sieve-textures consist of an intimate network of calcic plagioclase (1–20 µm) and minute (<1 µm) glass patches. Boundaries between sodic cores and sieve-textured regions are irregular and truncate small-amplitude oscillatory zones in the sodic cores. The sieve-textured regions are commonly located near the outer part of the crystal but some crystals are sieve-textured throughout, with small remnant sodic patches.
- Type 4: very rare highly calcic, normally zoned, clear crystals have cores between An89 and An93, and rims between An53 and An81.
The distinction between the two classes of reverse zoned crystals (Type 2 and 3) is somewhat gradational, as some crystals classified as Type 2 have very narrow regions with poorly developed or partial sieve-textures (Fig. 4b). However, the reverse zoned regions of Type 3 crystals commonly extend to more calcic compositions than in the Type 2 crystals. Together, Type 2 and 3 crystals typically form between 30 and 50% of the total plagioclase phenocryst population. Most reverse zoned crystals do not have extensive sodic overgrowths at the rims.
Plagioclase microphenocrysts have a similar range in composition to the larger phenocrysts. Calcic microphenocrysts have cores in the range An65–85. Some calcic microphenocrysts have narrow (5–10 µm) sodic rims, An55–60. Sodic microphenocrysts have similar compositions (An48–58) to the sodic phenocrysts. Groundmass plagioclase microlites range widely between about An48 and An75 but individual microlites generally have a narrow (±An5) compositional range. Some are unzoned and others are reverse zoned. Small euhedral plagioclase crystals (30–300 µm) also occur as inclusions in amphibole. Many are similar in composition to the sodic phenocrysts and the overall range is about An48–77. Plagioclase also occurs as intergrowths with pyroxene and titanomagnetite, formed by breakdown of amphibole (see below).
Pyroxene
Orthopyroxene ranges in size from small microlites (<10 µm) up to about 5 mm, occurring as individual phenocrysts, in crystal clots and as inclusions in amphibole. Three main phenocryst populations can be defined on the basis of rim composition (Fig. 5, Table 3):
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- Unzoned crystals have narrow ranges in composition, with En and Wo contents typically in the range 57–61 and 1·8–2·3, respectively.
- Reverse zoned crystals have similar cores to the unzoned crystals but the rims have higher En (62–73) and Wo (2·2–3·7) (Figs 4f and 5), with a much smaller intra-grain than inter-grain variation (Table 3). Orthopyroxene in samples erupted during the early stages of the current eruption has very narrow (<5 µm) reverse zoned rims but all samples erupted since May 1996 contain orthopyroxene with well-developed reverse zoned rims (10–25 µm). No consistent variation has been observed in the width or composition of the reverse rims since May 1996.
- Crystals with clinopyroxene rims (10–30 µm) have similar core compositions to the Type 1 and 2 crystals.
The three populations coexist in most samples. In addition, rare crystals with similar En but much lower Wo, ranging between 1·2 and 1·5, occur in a few samples (Table 3). These low-Wo crystals are more abundant in the older Soufriere Hills andesites.
Orthopyroxene occurs as groundmass microlites and as reaction products formed by breakdown of amphibole (see below). En typically ranges between 63 and 70 but Wo contents vary more, ranging from about Wo3 to Wo5, and grading into pigeonite (Fig. 5). The variations in Wo content occur as patches or discrete sectors within individual grains and some orthopyroxene microlites have pigeonite rims. Groundmass microlites and crystals in coarse amphibole breakdown textures are similar in composition but orthopyroxene in very fine-grained amphibole breakdown textures has slightly lower En, ranging between 63 and 67.
Clinopyroxene occurs as microphenocrysts (<300 µm) and microlites, as rims on orthopyroxene and as breakdown products of amphibole. Patchy and sector zoning are common with complex variations in Al, Ti, Mg, Fe, Ca and Si. En is in the range 66–73 and Wo generally ranges from 32 to 43, with an overall range of about 20–46, grading into pigeonite at the lower end of the range. Clinopyroxene in the fine-scale amphibole breakdown textures has slightly lower average En, between about 65 and 70.
Amphibole
Euhedral to anhedral amphibole phenocrysts range between about 0·5 mm and 1 cm in size, occurring as individual crystals and in clots with plagioclase, orthopyroxene and titanomagnetite. The same minerals are abundant as inclusions in amphibole. There is considerable variation in the sizes, proportions and distributions of inclusions from one amphibole crystal to another. All are calcic amphiboles (Table 4), magnesio-hornblendes in the terminology of Leake et al. (1997). Al2O3 contents are low (Table 4, Fig. 6), typically between 6 and 8%, extending up to 9% in a few crystals. Some crystals have rims with slightly lower FeT/Mg than the cores (Table 4). Small inclusions of amphibole occur in the calcic rims of some sieve-textured plagioclase. These are aluminous (up to 14 wt % Al2O3), similar in composition to amphibole in the mafic inclusions, discussed below.
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Some samples, particularly explosive pumice ejecta, contain predominantly pristine euhedral or slightly rounded amphibole, with light to dark green pleochroism. In slowly erupted dome samples, many crystals have reaction textures (Devine et al., 1998a). Three textures can be defined (Fig. 7):
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- Type 1: fine-grained (5–30 µm) intergrowths of clinopyroxene, orthopyroxene, pigeonite, plagioclase and titanomagnetite occur as rims and along the cleavages of many crystals (Fig. 7a and b).
- Type 2: crystals with a coarser-grained (30–200 µm) intergrowth of the same phases as in Type 1 occur in most samples (Fig. 7c and d). In many cases, the clinopyroxene forms an optically continuous patchwork texture replacing the original amphibole (Fig. 7c). The extent of the texture is variable but many crystals are often almost completely pseudomorphed with small patches of remnant amphibole in the cores.
- Type 3: opaque replacement [opacite, see Garcia & Jacobson (1979)] occurs at the rims and along cleavages of many crystals (Fig. 7e). In some slowly erupted samples, the amphiboles are completely pseudomorphed by opacite. The opacite texture consists of a very fine-grained intergrowth (1–10 µm) of the same phases as in Types 1 and 2, but titanomagnetite predominates over the other mafic minerals.
Many samples contain crystals with all three reaction textures. The Type 3 texture appears to form at a late stage by oxidation within the lava dome. The Type 1 texture is believed to form during ascent (Devine et al., 1998a), as amphibole becomes unstable at pressures less than 1·5 kbar and breaks down to form an anhydrous assemblage. The extent of the reaction rims can be used to estimate magma ascent rates (Rutherford & Hill, 1993; Devine et al., 1998a). Some crystals have both Type 1 and Type 3 textures, as a result of late-stage oxidation of unreacted amphibole. The coarse Type 2 texture may form on crystals that spend long periods in the conduit, outside the amphibole stability field, or may be related to heating of the andesite by intruding mafic magma (see below).
Fe–Ti oxides
Titanomagnetite occurs as individual microphenocryst and microlites, as inclusions in phenocrysts and in intergrowths formed by amphibole breakdown. In slowly erupted lava samples, titanomagnetite has very fine-scale lamellar exsolution textures and is heterogeneous in composition. Crystals formed by breakdown of amphibole are also very heterogeneous. In pumice and rapidly erupted dome samples, individual crystals are more homogeneous in composition, although there is still significant variation between grains in individual samples (Table 5). Ilmenite is present in very minor amounts, probably much less than 1% of the total Fe–Ti oxide population.
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Quartz
Quartz occurs as rounded individual crystals (0·5–2 mm), typically one or two per thin section. Some crystals have reaction rims of clinopyroxene. Quartz does not occur intergrown with other phenocrysts. Rhyolitic melt inclusions are common in quartz.
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MAFIC INCLUSION PETROLOGY |
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Mafic inclusions are a ubiquitous but minor component of the andesite, forming <1 wt % of the lava. The inclusions range in size from about 40 cm to about 1 mm in diameter (Fig. 8a and b). The inclusions most commonly have ellipsoidal shapes but some are angular (Fig. 8a). They have sharply defined smooth or crenulate contacts with the andesite host (Fig. 8b) and some have chilled margins, defined by a decrease in groundmass grain size at the inclusion–host contact.
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Most inclusions are phenocryst poor (
1–5%) and contain phenocrysts of plagioclase only. The inclusions have diktytaxitic quench-textured groundmass (see Bacon, 1986) consisting of randomly oriented, interlocking, elongate or acicular crystals (Fig. 8c–e). The groundmass framework consists of plagioclase ± clinopyroxene ± orthopyroxene ± amphibole, titanomagnetite, patches of interstitial rhyolitic glass and abundant interstitial voids, in some cases infilled by vapour-phase silica. Larger inclusions tend to have coarser-grained groundmass than small inclusions, and framework plagioclase crystals have more equant shapes (Fig. 8c and d). Amphibole is the most abundant mafic phase in larger inclusions but in small inclusions, less than a few centimetres in size, pyroxene predominates.
Large crystals of plagioclase, amphibole, orthopyroxene and quartz in the inclusions are identified on the basis of composition and/or morphology as xenocrysts derived from the andesite. In some cases, large plagioclase phenocrysts from the andesite are partially incorporated into mafic inclusions (Fig. 8e and f). The xenocrysts are very commonly resorbed, with reaction textures that are similar to but more extensive than reaction textures on phenocrysts in the andesite. Plagioclase xenocrysts have strongly developed sieve textures, which may permeate the entire crystal, in contrast to similar Type 3 crystals in the andesite, which have experienced only partial resorption. Large amphibole xenocrysts typically have extensive fine-grained reaction textures, which consist of minor olivine (Fo60) in addition to pyroxene, plagioclase and titanomagnetite (Fig. 7f). Quartz xenocrysts have wide rims of clinopyroxene.
The framework plagioclase is calcic, typically ranging from An70 to An89 in the cores (Fig. 3, Table 2). Rims are often relatively sodic, An47–58. Larger plagioclase crystals range to more calcic compositions than the framework plagioclase, up to An93. Dusty sieve-textured crystals are compositionally similar to those in the andesite but commonly have wide (up to 100 µm) sodic (An47–58) overgrowths on the calcic rims, in contrast to sieve-textured crystals in the andesite, which typically have very narrow (5–20 µm) or no sodic rims.
The groundmass framework amphibole in the mafic inclusions is highly aluminous, mostly in the range 12–14·5% Al2O3 (Table 4, Fig. 6), much higher than amphibole in the andesite (typically 6–8% Al2O3). Crystals are often heterogeneous with patchy zoning, the different segments showing complementary variations in Al and Fe. Compositions are magnesio-hastingsites in the Leake et al. (1997) classification. Na, K and Ti contents are also much higher than in amphibole in the andesite (Table 4). In some samples, crystals have been partially replaced by a lower-Al amphibole (Al2O3 between 6·5 and 9%), overlapping in composition with the large andesitic phenocrysts. The framework amphibole in the mafic inclusions generally has very narrow (<20 µm) or no reaction rims. Mafic inclusions in strongly oxidized andesite samples typically contain oxidized framework amphibole.
Pyroxene compositions are similar to those in the groundmass of the andesite (Table 3, Fig. 5). Crystals are heterogeneous with patchy, sector or lamellar zoning, with segments of orthopyroxene and pigeonite or augite and pigeonite.
Titanomagnetite compositions overlap with those in the andesite but extend to higher TiO2 (Table 5). Crystals in oxidized samples are heterogeneous with lamellar exsolution textures. The more pristine mafic inclusions have more homogeneous crystals but there is significant inter-grain variation in individual samples.
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PETROLOGY OF OLDER MONTSERRAT ROCKS |
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 TOPABSTRACTINTRODUCTIONANALYTICAL TECHNIQUESGEOCHEMISTRYANDESITE PETROLOGYMAFIC INCLUSION PETROLOGY PETROLOGY OF OLDER MONTSERRAT... GEOTHERMOMETRY AND OXYGEN...DISCUSSIONCONCLUSIONSREFERENCES |
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Several older samples from the Soufriere Hills centre have been studied for comparison with the new magma. All of the eruptive products of the Soufriere Hills volcano range between about 58 and 64 wt % SiO2. Mafic inclusions, similar to those in the current andesite, occur in all of the older lavas and pyroclastic deposits. Analyses of lava and a mafic inclusion from the Castle Peak dome (
350 years old) and of samples from pyroclastic deposits (18 ka and 3·7 ka) are given in Table 1. The Castle Peak andesite is similar in bulk composition and mineralogy to the current andesite. The 3·75 ka and 18 ka samples analysed here are slightly more silicic (62–63·8% SiO2) than the current andesite, but the mineral assemblages are very similar. Plagioclase has a wide range of compositions but large phenocrysts typically have cores between An49 and An60, as in the current andesite. Type 2 and 3 plagioclase crystals are common but are less abundant than in the current andesite. Amphibole is similar in composition to that in the current andesite. Many orthopyroxenes are similar to those in the current andesite but crystals with much lower Wo contents (averaging 1·3 and ranging between 1·2 and 1·6) are more abundant than in the current andesite.
Mafic samples from the South Soufriere Hills centre (125 ka) have an assemblage of plagioclase, olivine, clinopyroxene and titanomagnetite. Plagioclase cores range between An82 and An94. Olivine invariably has pigeonite rims. Large olivine phenocryst cores range between Fo81 and Fo84 and microphenocryst cores range between Fo62 and Fo71. Clinopyroxene has mg-number between 72 and 76. Titanomagnetite is variable in composition but some crystals have relatively high TiO2 contents (14%).
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GEOTHERMOMETRY AND OXYGEN BAROMETRY |
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Andesite magma temperatures have been estimated using the QUILF program of Andersen et al. (1993)
. Because clinopyroxene occurs only as microphenocrysts and microlites and is not in equilibrium with the orthopyroxene phenocrysts, the QUILF program has been used in single pyroxene mode, requiring input of the orthopyroxene composition only. Only phenocryst temperatures are quoted here, as groundmass pyroxenes in the andesite are heterogeneous and give inconsistent results. In single pyroxene mode, the QUILF program calculates a temperature and an equilibrium clinopyroxene composition. The accuracy of this method has been tested using experimentally produced orthopyroxenes. The temperatures of the clinopyroxene-absent experiments of Barclay et al. (1998) on Soufriere Hills andesite and of Gardner et al. (1995) on Mount St Helens dacite, which has a similar assemblage to the Montserrat andesite, are reproduced to within ±20°C or better in most cases. The thermometer is most sensitive to variations in CaO content of orthopyroxene. Analytical precision for CaO at low abundance levels is about ±0·05%, which translates to an uncertainty in temperature of ±20°C. All values quoted are calculated for a pressure of 1·5 kbar but the variation of the thermometer with pressure is insignificant here (+3–4°C/kbar). The pressure estimate is derived from the requirement that the minimum depth of the magma chamber is about 5–6 km, as amphibole is unstable at shallower depths because of the pressure dependence of water solubility in the magma (e.g. Merzbacher & Eggler, 1984). The Wo contents of orthopyroxene cores in the current andesite are mostly between 0·019 and 0·023 (0·9–1·1% CaO), corresponding to a range in calculated temperatures of about 40°C. The mean core temperature is 858 ± 18°C, calculated from 181 analyses of 33 crystals. The overall range is 812–891°C, with most values between 835 and 875°C (Fig. 9a). The rare low-Ca orthopyroxene phenocrysts give values around 750°C. The reverse zoned orthopyroxene rims invariably yield higher values than the cores, ranging between 870 and 1040°C (Fig. 9b). Although the intra-grain compositional variation of the reverse zoned rims is large, individual crystals have more homogeneous rims. The range within individual rims is usually less than 40°C. Orthopyroxene phenocrysts in the older Soufriere Hills andesites (18 500 and 3750 BP) give an average temperature of 861 ± 16°C. The low-Ca orthopyroxene phenocrysts, which are more abundant in the older samples, give an average temperature of 747 ± 7°C.
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Oxygen fugacities have also been estimated using the QUILF program. Only the titanomagnetite composition is required for the calculation, as the temperature is constrained by the orthopyroxene composition. The method is advantageous for calculation of oxygen fugacity in magmas such as the Soufriere Hills andesite where ilmenite is present in very minor abundance and it also counteracts problems caused by rapid re-equilibration of Fe–Ti oxides, particularly ilmenite (see Frost & Lindsley, 1992
). Compositions of the most homogeneous titanomagnetite crystals were used in the calculations. Estimated relative oxygen fugacities are between 1·5 and 2 log units above QFM (quartz–fayalite–magnetite) with only minor dependence on absolute temperature (Table 5).
The mafic inclusions are believed to represent phenocryst-poor, hydrous basaltic to basaltic andesite magma(s) that intruded the andesite magma reservoir (see below). The temperatures of the inclusion-forming magma(s) are not well constrained. First, the pyroxenes, which occur only in the groundmass, are believed to have crystallized by quenching (see below) and may record quench or re-equilibration rather than original magmatic temperatures. Second, most pyroxenes are heterogeneous, like those in the andesite groundmass, and give unreliable temperature estimates. The most consistent values are in the range 1020–1060°C, obtained from homogeneous orthopyroxene crystals. However, constraints on the minimum temperatures of the mafic magmas are available from experiments, as the H2O-saturated liquidus temperature of the andesite is about 1050°C at 1·5 kbar (Barclay et al., 1998). The H2O-saturated liquidus temperature of the mafic magmas would be higher than that of the andesite. If the crystal-poor mafic magmas were not H2O saturated, then they would have had even higher temperatures. Minimum temperatures of 1050°C are consistent with low-pressure liquidus temperatures of hydrous mafic magmas from the literature (e.g. Sisson & Grove, 1993). Relative oxygen fugacities of the mafic inclusion magmas are similar to the andesite values (Table 5) but probably less accurate, because of greater variability in titanomagnetite compositions.
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DISCUSSION |
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Magma mingling
The term ‘magma mingling’ as used here refers to the formation of discrete inclusions by quenching of hotter magma in a lower-temperature magma (see Bacon, 1986
). The process of complete mixing of two magmas to form a new magma, commonly containing crystals from the two end-members, is termed ‘hybridization’.
The evidence for magma mingling in the current Soufriere Hills andesite is unequivocal. The mafic inclusions are interpreted as quenched blobs of hydrous mafic magma at temperatures of around 1050°C or above, which have been intruded into cooler andesite magma at about 850°C. In itself, the occurrence of xenocrysts derived from the andesite, in some cases partially incorporated into the inclusions, demonstrates that the mafic inclusions must have been molten when formed. The quench-textured groundmasses, ellipsoidal shapes, crenulate contacts and chilled margins are all characteristic of magmatic mafic inclusions (Vernon, 1983; Bacon, 1986; Sparks & Marshall, 1986; Blundy & Sparks, 1992). The acicular character of crystals is indicative of rapid cooling rates (Lofgren, 1980). The increase in crystal size and change in crystal shape from acicular to more prismatic and equant shapes as the inclusion size increases can also be attributed to slower cooling in larger inclusions. The tendency for pyroxene to be more abundant than amphibole in small inclusions can be explained by a variety of mechanisms including preferential loss of water from small inclusions promoting pyroxene stability or possible kinetic effects related to more rapid cooling of small inclusions.
The whole-rock chemistry of the inclusions suggests that they represent quenched basaltic to basaltic andesite magma(s), with compositions similar to the South Soufriere Hills lavas. The low MgO contents and very low Ni and Cr contents suggest that the mafic magmas have experienced extensive fractionation following their presumed generation in the mantle.
Magma mingling rather than hybridization is dominant, as the inclusions generally remain coherent and have sharp contacts with the host andesite. Rarely, there is evidence for minor disaggregation and subsequent hybridization, where the mafic inclusion groundmass minerals form local streaks into the andesite near the margins of inclusions. However, there is no evidence for significant disaggregation of inclusions in a partially molten state, in contrast to, for example, the situation described by Clynne (1999) for material erupted in 1915 at Lassen Peak, California. If the inclusions had disaggregated and hybridized with the andesite, groundmass minerals that had formed within the inclusions should be present in the andesite groundmass. This is not observed. First, groundmass amphibole is completely absent from the andesite whereas it is abundant in most inclusions. Second, the compositions of groundmass plagioclase in the andesite are distinct from those in the inclusions (Fig. 3). Third, the morphologies of microphenocryst and microlite pyroxenes in the andesite are distinct from those in the mafic inclusions, the latter commonly having elongate to acicular forms and the former being more equant. Although significant disaggregation of partially molten inclusions can therefore be ruled out, it is likely that inclusions with one or more angular faces formed by brecciation of solidified larger inclusions. However, this process did not result in hybridization.
The predominance of mingling over hybridization at Montserrat can be related to the large contrast in temperature and viscosity between the cool, crystal-rich andesite and the hot, crystal-poor mafic magma, and to small proportions of mafic magma in the mixture, which results in rapid quenching of the mafic magma at temperatures close to that of the andesite host (Sparks & Marshall, 1986).
The andesite also shows abundant evidence for reheating and recrystallization at higher temperatures. Reverse zoned dusty sieve-textured plagioclase, similar to that in the Soufriere Hills andesite and mafic inclusions, has been reported from many mingled and hybrid magmas (Eichelberger, 1978; Heiken & Eichelberger, 1980; Gerlach & Grove, 1982; Kawamoto, 1992). The texture can form by reheating plagioclase above its equilibrium solidus temperature but below its liquidus. Rapid recrystallization occurs, producing a very fine-grained intergrowth of more calcic plagioclase and trapped glass. Laboratory experiments have produced similar textures (Tsuchiyama, 1985; Nakamura & Shimakita, 1996).
The reverse zoned rims on many orthopyroxene phenocrysts, which give significantly higher calculated temperatures than the cores, also indicate reheating of the magma. The calcic rims of simply reverse zoned, unresorbed, plagioclase phenocrysts and the calcic microphenocrysts can also be interpreted in terms of rising magma temperature. The rounding of quartz crystals and the presence of fine-grained clinopyroxene rims may also be related to reheating of the magma.
Some of the amphibole reaction textures may also have been produced by reheating of the magma. Rounding of some amphibole crystals may be caused by resorption as a result of reheating. Although the fine-grained rims (Type 1) on many amphibole phenocrysts in the andesite are likely to be formed by volatile loss from the magma during ascent (Devine et al., 1998a), some of the fine-grained replacement textures may also have formed as a consequence of reheating. Xenocrysts in mafic inclusions have very fine-grained reaction products that have extensively replaced the original crystal and these textures are difficult to distinguish from decompression-induced reaction textures. The coarse-grained reaction textures (Type 2) may also have formed by reheating.
History of the Soufriere Hills magma chamber
The detailed eruptive history of the Soufriere Hills centre is currently not well constrained and the following discussion is based on a small set of samples of known eruption ages from 18 ka to the present.
Several features suggest that the current andesite may be derived from a relatively old, highly crystalline magma body. First, all magmas erupted over the last 18 kyr at least are crystal rich, have very similar mineral chemistry and a restricted range in bulk compositions (58–64 wt % SiO2). Second, simple least-squares mass balance calculations, using typical crystal compositions (Tables 2–5) and glass compositions from Devine et al. (1998b) indicate that the magma was highly crystalline (at least 60–65 wt %) in the magma chamber before eruption, further crystallization of microlites occurring during ascent. Third, the calculated magma temperature of 860°C is about 200°C below the experimentally determined water-saturated liquidus temperature of 1050°C at 1·5 kbar (Barclay et al., 1998). The high crystallinity and low temperature indicate that the magma has experienced a protracted cooling history, which, together with the similarity in mineral chemistry and narrow range in bulk chemistry of all magmas erupted over the last 18 kyr, indicates that all eruptions may be tapping the same magma body. This interpretation is supported by preliminary U–Th results on mineral separates from the current and 3·75 ka rocks, which give dates of 25 ± 10 ka (Murphy et al., 1998).
In this interpretation, the magma has existed as a highly crystalline body with pluton-like mechanical properties, remaining at a temperature of about 860 ± 20°C for the last 18 kyr at least, with periodic perturbations in temperature as a result of influx of hotter mafic magma, inferred from the presence of mafic inclusions, similar to those in the current andesite, in all of the older products of the Soufriere Hills volcano. Periodic replenishment by fresh mafic magma prevents the resident magma from fully crystallizing and may result in eruption, as discussed below. Each intrusive episode involves non-uniform reheating of the magma body, which may subsequently return to steady low-temperature (860°C) state and continue to crystallize a low-temperature mineral assemblage.
Causes of magma heterogeneity
A prominent feature of the Soufriere Hills andesite is microscale heterogeneity, which is manifested by juxtaposition of crystals that record different petrogenetic histories. Diversity in compositions, zoning patterns and textures of the phenocryst phases indicates that these crystals have been mixed together from widely different magmatic environments. The key question is what these variations in crystal history imply about spatial and temporal variations in magma composition, volatile content and temperature. We interpret the observations in the context that the mafic magma, as represented by the mafic inclusions, can provide heat, mass and volatiles to the andesite magma.
The variations in rim compositions, zoning patterns and extent of resorption of plagioclase phenocrysts and the range in plagioclase microphenocryst compositions indicate that individual crystals have experienced different thermal histories. Similarly, the narrow variations in rim compositions of individual reverse zoned orthopyroxene crystals, together with the wide inter-grain range in the compositions of the reverse zoned rims, indicate that different crystals have experienced different degrees of reheating. The amount of reheating experienced by an individual crystal will depend on its proximity to the intruding mafic magma. Temperatures over 1000°C may have been reached locally, as the mafic magma is estimated to have been at a temperature of at least 1050°C. However, the overall temperature of the andesitic magma body cannot have reached more than about 880°C, as amphibole remains as a stable phase. The experiments of Barclay et al. (1998) show that amphibole becomes unstable at about 880°C. The evidence thus points to parts of the andesite having been reheated by intrusion of mafic magma to temperatures well above the original temperature at which the stable phenocryst assemblage of sodic plagioclase (An48–58), orthopyroxene, amphibole and titanomagnetite formed.
Localized, non-uniform reheating of the andesite magma by intrusion of hotter mafic magma is likely to be a consequence of the highly crystalline nature of the resident magma. The minimum crystallinity of the magma in the reservoir can be estimated from mass balance calculations to be about 60–65 wt %, which is close to the volume percent critical crystallinity at which magmas behave as solids rather than liquids (Marsh, 1981; Lejeune & Richet, 1995). Therefore, before reheating, the andesitic magma body would have been so highly crystalline that its physical properties are more aptly considered in terms of a rigid, immobile, partially molten pluton than of a mobile magma (see Marsh, 1981; Furman & Spera, 1985). Mafic magma can intrude into the still rigid highly crystalline andesite magma body, forming dykes or sills that are subsequently broken up to form quenched mafic inclusions as the host is heated and softens (Blake, 1966; Blundy & Sparks; 1992; Hallot et al., 1996).
Several processes may have contributed to inclusion formation. Angular inclusions are likely to represent fragments of larger inclusions, dykes or sills that have disintegrated in a solid or near-solid state. Rounded inclusions are likely to represent quenched blebs of magma that have been incorporated into the host in a liquid or near-liquid state, as evidenced by the relationship between inclusion size and groundmass grain size and by the presence of chilled margins on some inclusions. Mechanisms that have been invoked to explain the latter process elsewhere include forcible injection of mafic magma (Turner & Campbell, 1986) and buoyant uprise of vesiculated underplated mafic magma into an overlying more silicic host (Eichelberger, 1980).
A major paradox of remobilization of crustal magma bodies is that crystallization takes place simultaneously with melting, when the system is considered on a large scale. Huppert & Sparks (1988) presented basic fluid dynamical principles that govern the interactions when crustal rocks or, as envisaged here, a highly crystalline magma body are remobilized. Although Huppert & Sparks (1988) considered the case of a sill, the main principles are generic and do not depend on the detailed geometrical relationships. Initially, when the mafic magma intrudes, heating is localized so that a large amount of heat is transferred into a relatively small body of host rock or magma. Once the temperature exceeds a threshold value where the melt content is sufficiently high that the mechanical strength is lost, convection begins. Thus a mobile magma body forms, surrounded by partially molten, immobile magma. Melting reactions take place at the rheologically determined boundary between mobile magma and partially molten material, which still behaves mechanically as a solid. The threshold condition is usually considered to be around 60–70 vol. % crystals (Marsh, 1981; Huppert & Sparks, 1988; Lejeune & Richet, 1995). As the volume of mobile magma grows, its average temperature falls with time and it begins to crystallize. Thus, counter-intuitively, simultaneous melting and crystallization are a consequence of the fluid dynamical principles.
There are no definite constraints on the precise mechanism(s) of inclusion formation at Montserrat but the following model, illustrated schematically in Fig. 10, can explain the observations. Initial intrusion of mafic magma results in underplating of a hot mafic magma beneath the less dense resident magma and formation of mafic dykes in the highly crystalline andesite. Softening of the host andesite by reheating and partial melting permits formation of elliptical inclusions by buoyant uprise or forcible injection of hot liquid as more mafic magma intrudes. Forced convection within the andesite results in fragmentation of earlier-formed dykes to form angular inclusions.
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The heterogeneous character of the Soufriere Hills andesite mineral assemblage can be simply explained by such a model. Initially, material that is close to the locus of mafic intrusions can be heated to high temperature, approaching that of the mafic magma itself. High-temperature orthopyroxene rims and resorbed sieve-textured plagioclase may represent regions of the remobilized magma that reached very high temperatures. As the remobilized magma body grows and cools, similar crystals in the original host are less dramatically affected. Falling temperature also results in formation of overgrowth rims on plagioclase and orthopyroxene and growth of the microphenocryst assemblage. As the magma body approaches temperatures and crystal contents that are comparable with the rheological critical conditions where rigidity sets in, the last crystals to be swept into the magma may grow no fresh rims at all.
The above model will be complicated by two factors. First, the region being remobilized may itself have compositional and thermal gradients. The lower-temperature (750°C) orthopyroxene crystals, which occur in samples from all eruptions but are rare in the current andesite, may represent material from a cooler nearly solidified part of the chamber, towards the roof or walls. The quartz crystals may also be derived from a relatively cool and highly crystalline marginal part of the system. Quartz is stable in the Soufriere Hills andesite only at or below about 830°C (Barclay et al., 1998) and contains high-Si rhyolite melt inclusions (76–79% SiO2) that are generally more evolved than the groundmass glass, suggesting that parts of the magma chamber had reached a very advanced state of crystallization. Given that most orthopyroxene temperature estimates are significantly higher than 830°C, the maximum temperature at which quartz is stable in the andesite, it is likely that the reservoir was thermally zoned before influx of the mafic magma.
A second cause of variation is the extent to which melt, crystals and volatiles are transferred from the mafic magma. In general, the extent of hybridization appears to be very minor and localized to areas adjacent to mafic inclusions. Highly calcic plagioclase crystals, which are likely to represent xenocrysts derived from the mafic magma, are rare in the andesite. However, volatile transfer from the mafic magma to the host andesite is probable and may be significant. The mafic inclusions represent volatile-rich magma, evidenced by the presence of amphibole and their high vesicularity. The volatile budget in the andesite is controlled by two counteracting effects. Rhyolitic melt inclusions in quartz and plagioclase contain 4–5 wt % H2O (Barclay et al., 1998; Devine et al., 1998b) implying H2O-saturated conditions in the residual melt at 1·25–1·5 kbar. Melting of predominantly anhydrous crystals in the andesite will tend to produce a water-undersaturated melt. On the other hand, transfer of volatiles from the hydrous mafic magma can elevate H2O contents and even create an excess vapour phase. Thus, P(H2O) can fluctuate with time and position, affecting local liquidus relations and compositions of overgrowth rims.
We conclude, therefore, that mineral heterogeneity in the andesite is caused by convective mixing together of crystals that have experienced different histories both before and during replenishment of the magma reservoir by hot mafic magma.
Intrusion of mafic magma as an eruption trigger
The effect of intrusion of hot magma into a cooler magma reservoir is widely recognized as a potential eruption trigger (e.g. Sparks et al., 1977; Pallister et al., 1992; Eichelberger, 1995). There are several possible effects of addition of hot mafic magma to a reservoir containing cooler silicic magma. The simple addition of a new volume of magma to the magma chamber causes pressurization, which may lead to fracturing of the roof and ascent of magma, possibly leading to eruption. Heating of the resident magma causes exsolution of volatiles, which can also cause pressurization of the chamber. Heating can also force convective uprise of the resident magma. Transfer of volatiles from the mafic magma to the silicic magma can also contribute to pressurization of the magma chamber. All of these effects may combine to initiate a new eruption, following an intrusive episode.
There is no direct constraint on the timing of recent mafic magma intrusion events beneath the Soufriere Hills volcano, as the mafic inclusions in the current andesite may not necessarily represent magma that was recently intruded. However, the freshness of many of the inclusions and the lack of evidence from mineral compositions for re-equilibration in most of the inclusions analysed suggest that the inclusions have formed recently. The presence of compositional banding in some samples also suggests very recent convective stirring. Further evidence for recent intrusion of mafic magma is provided by the mineral chemistry of the andesite. In particular, the increase in width of reverse zoned rims on orthopyroxene phenocrysts over the course of the eruption suggests that these rims have grown very recently, as a result of reheating of the magma. The presence of calcic rims on many plagioclase phenocrysts and the abundance of calcic microphenocrysts also suggest that recent mafic magma intrusion has occurred.
The precursor seismicity, which began in 1992, may have been initiated by movement of magma from levels deep (at least 14 km) within the crust, according to a preliminary interpretation of the geophysical data (Young et al., 1998b). This may therefore reflect an intrusive pulse of mafic magma into a shallow magma chamber at about 6–7 km depth. Evidence for the existence of a shallow magma chamber comes from geophysical data recorded throughout the eruption, which locate a large number of earthquakes at about 5–7 km depth (Aspinall et al., 1998). Other seismic crises at about 30 year intervals over the last 100 years may also reflect movement of mafic magma from deep levels in the crust, intruding the shallow magma chamber. Although these events did not lead directly to eruptions, some of the mafic inclusions in the current andesite may have been formed at these times. Addition of mass and heat to the magma chamber during these earlier events may have primed the resident magma for eruption, and the recent intrusion may have acted as the final trigger.
Evidence for the eruption being driven by influx of mafic magma also comes from extrusion rate and SO2 gas flux data. The magma extrusion rate increased with time, from 0·5 m3/s in the first few weeks of activity, averaging 2·3 m