MVO/VSG - Open Scientific Meeting
27 November 1996

Petrology and geochemistry of the
Soufriere Hills Volcano, Montserrat

M.D. Murphy1, J. Barclay1, R. Macdonald2, R.S.J. Sparks1 and M.R. Carroll1

1Department of Geology, University of Bristol, Bristol, UK
2Environmental Science Division, University of Lancaster, Lancaster, UK

The current eruption at Montserrat has produced a crystal-rich (~40% by volume) andesite (56-60% SiO2) with a phenocryst assemblage of plagioclase (28-30%), amphibole (3-7%), orthopyroxene (3-5%), titanomagnetite (1.5-2%), quartz (<1%), augite microphenocrysts (<1%) and accessory apatite and ilmenite. The groundmass consists of microlites of plagioclase, orthopyroxene, augite, pigeonite, titanomagnetite and quartz in a residual high-Si rhyolite glass (77-80% SiO2). The total groundmass, including microlites, represents a low-Si rhyolite melt (71-72 SiO2) which is likely to have crystallised the microlite assemblage shortly before eruption. During the earlier stages of the eruption up to early April 1996, the proportion of residual melt was less than about 10-15%, whereas blocks from pyroclastic flows erupted since July have 20-25% melt.

Plagioclase has a range of compositions and shows complex zoning. The most abundant phenocryst population is coarse-grained (1-5 mm) and has relatively sodic cores, zoning with rims ranging An65-80. Many crystals have very fine-grained sieve-textured mantles or rims. The sieve-texture consists of a very fine-scale network of highly calcic plagioclase, An75-88, and glass patches about 1 m in width. These crystals often have clear rims of similarly calcic composition to the sieve-textured regions. In some cases there are further more sodic narrow overgrowths but many crystals which show reverse zoning remain calcic at the rims. Plagioclase microphenocrysts (70-300 m) have calcic cores, generally between An65-80 and are unzoned or show normal zoning to about An55. Plagioclase microlites have a range in composition between about An50-70 with a narrower intra-grain range. The average An content of the predominant plagioclase microlite population has increased to about An65-70 in material erupted since late July 1996 compared to the earlier material in which the predominant composition was between about An55-60. There is overlap however between the older and younger microlite compositions.

Amphibole occurs as large crystals with a narrow range in composition, with Al2O3 between 6-8%. Crystals exhibit varying degrees of reaction from narrow rims to complete breakdown forming amphibole pseudomorphs. Reaction products consist of ortho- and clinopyroxene, titanomagnetite and plagioclase which form intergrowths on a range of scales. Amphibole occurring in mafic magmatic inclusions (see below) also contains olivine in fine-grained (typically 10-50 m) but coarser-grained (up to a few hundred m) textures are common on strongly reacted crystals.

Orthopyroxene phenocrysts have a narrow range in composition with Mg# ranging mostly between 58-62. Orthopyroxene in the fine-grained amphibole reaction textures has higher Mg#, generally ranging between 64-67. Orthopyroxene in the groundmass and in the coarser-scaled amphibole reaction textures has even higher Mg#, typically between 65-70. Clinopyroxene microphenocrysts, microlites and crystals in coarser amphibole reaction textures has Mg# between 67-74 whereas clinopyroxene in the fine-scale breakdown textures has Mg# mostly ranging between 64-67.

Mafic magmatic inclusions (51-55% SiO2), ranging from <1 mm to about 40 cm, are an important component (<1-2%) of the dome lava and the pyroclastic deposits. The inclusions are predominantly elliptical although some angular inclusions also occur. The inclusions contain phenocrysts of plagioclase in a quench-textured diktytaxitic matrix consisting of a randomly oriented framework of elongate to acicular minerals with numerous interstitial voids and patches of high-Si rhyolite glass. The matrix consists of plagioclase, amphibole, orthopyroxene, augite, pigeonite and titanomagnetite in a residual high-Si rhyolite (76-80% SiO2) vesicular glass. The inclusions also contain xenocrysts of plagioclase, amphibole and orthopyroxene which are readily identified by their chemistry as being derived from the andesite. Sieve-textured plagioclase, similar in composition to that in the andesite, is abundant in the inclusions. In some cases, xenocrysts are partially incorporated into the inclusions. Some inclusions have chilled margins defined by a decrease in grainsize of the diktytaxitic groundmass at the contact with the host andesite.

The quench textures and diktytaxitic voids are characteristic of magmatic mafic inclusions which have been quenched in cooler silicic magma. Other evidence for a magmatic origin includes rounded or ellipsoidal shapes, the presence of chilled margins and the occurrence of xenocrysts, commonly resorbed and reacted, derived from the andesite. The xenocrysts demonstrate unequivocally that the mafic material was molten when incorporated into the andesite.

The mineral chemistry of the inclusions is distinct from that of the andesite. Plagioclase phenocrysts are highly calcic ranging up to An92 with more sodic rims, An47-57. Quench-textured plagioclase is slightly less calcic than the phenocrysts, ranging An70-86 with similar sodic rims. Amphibole has much higher Al2O3 (12-15%) than amphibole of andesitic origin. Pyroxenes are magnesian, similar to those in the groundmass of the andesite and in the coarser amphibole reaction products.

The older (16,000-24,000 years) volcaniclastics of the Soufriere Hills Volcano are very similar in whole-rock and mineral chemistry to the recent magma and also contain mafic inclusions. Amphibole and orthopyroxene in the andesite show little compositional variation since this time. Plagioclase core compositions in the older products are also very similar to those in the present eruption but the extent of reverse zoning and resorption appears to be much less than in the more recent eruptions.

One interpretation of the constancy of whole-rock and mineral compositions over this time is that the andesite represents an extensive near-solidus crystal mush which has resided in the present magma chamber for at least 24,000 years. Geothermometry and experimental results (Barclay et al., this volume) suggest that the andesite magma initially crystallised at temperature between about 800-875C°. The magma may have experienced a protracted crystallisation and cooling history with quartz crystallising at lower sub-liquidus temperatures than the mafic minerals. Minimum temperatures of the mafic magmas are about 1050C° prior to incorporation into the andesite.

The thermal and chemical effects on the resident andesite due to influx of hot mafic magma are presently unconstrained. Although influx of volatiles from the intruding mafic magma may have affected the mineral-melt equilibria in the resident magma, reheating is very likely to have occurred as evidenced by the strong resorption of some plagioclase. Increasing water content will produce a more calcic equilibrium plagioclase but is unlikely to cause dissolution of pre-existing grains. The fine-grained reaction rims on the amphibole are interpreted as being decompressional in origin but the coarser-grained amphibole breakdown textures are likely to be related to reheating due to intrusion of mafic magma. It is not however presently known whether temperature perturbations are localised near the mixing interface or whether there has been a gradual reheating of the resident andesite magma. There is no evidence from the whole-rock geochemistry for hybridisation (in contrast to mingling) of mafic and resident magmas. Furthermore, it is not possible to presently constrain when and how many episodes of intrusion of mafic magma have occurred.

Given the evidence for magma mixing throughout the recent history of the Soufriere Hills Volcano and the apparent increase in the effects of mixing with time, it is suggested that the andesite magma is relatively old (>24,000 y) and had cooled to near-solidus temperatures (800-825C°) prior to renewed and periodic influx of hot mafic magma. Episodes of intrusion by mafic magma may remobilise the andesite magma and result in eruption. Such episodes of renewed intrusion may be seismically detectable.

Montserrat Volcano Observatory