We report major-element, trace-element, and radiogenic-isotope analyses of 49 lavas and xenoliths as well as 35 40Ar/39Ar ages from Grenada, Kick 'em Jenny submarine volcano, and several Grenadine Islands. Grenada magmas are compositionally unusual in several respects compared to other subduction-related magmas. Particularly controversial is the extent to which compositions reflect the presence of subducted sediment in their sources or assimilation of sediment in the arc crust and the relationship between two distinct Grenada magma series, the Sr-rich and ankaramitic C series and olivine-microphyric M series, erupted on the island. New 40Ar/39Ar ages show that eruption of these magmas has been interwoven both in time and space over the past 6 million years, during which the present volcanic edifice of Grenada has been built, indicating they share a common plumbing system. Consistent with earlier studies, our data show that the two series are isotopically distinct in their most mafic examples. At least 0.6% of a subducted sediment component must be added to depleted mantle to explain M-series isotopic compositions; considerably less, similar to 0.2%, subducted sediment is present in the C-series source. Trace elements and isotope ratios of both series are best modeled by the addition of slab-derived components predominantly through silicate melts, but the addition of hydrous fluids is also required, with the C series requiring a greater fluid component than the M series. This indicates the top of the slab is at or above its solidus beneath Grenada. C-series parental magmas are generated by significantly smaller peridotite melt fractions than M-series parents and may contain a fraction of pyroxenite melt as well. Radiogenic-isotope ratios in C-series basalts correlate significantly with MgO, but oxygen-isotope ratios do not. This and other aspects of their evolution, including decreasing K2O/Na2O and Sr concentrations with decreasing MgO, cannot be explained by sediment assimilation previously proposed. Instead, these features are readily explained by mixing with evolved M-series magmas stored in the arc crust and, perhaps, assimilation of their crystallization products. Such mixing may also affect M-series magmas, but because M-series magmas constitute three-fourths of Grenada igneous products and because parent M-series magmas are notably more heterogeneous, the effects of this mixing are less obvious. Any assimilation of sediment within the arc crust has at best second-order effects of magma compositions. The geochemistry of a pillow basalt from Mayreau Island is consistent with an earlier interpretation that the Grenadine Platform consists of uplifted oceanic crust formed through Eocene backarc spreading. Union Island consists of supra-subduction andesites and basalts (erupted between 6.5 and 3 million years ago) that are similar to those of Grenada. Kick ' em Jenny volcano is a distinct magma system from Grenada, but the neighboring volcanic islet of Isle de Caille is magmatically part of Grenada. A dacite dike cutting sedimentary rocks of the Tufton Hall Formation on Grenada yielded a 40Ar/39Ar age of 37.9 +/- 0.2 Ma, suggesting the existence of supra-subduction volcanism in the region since the Eocene.

Backscattered electron (BSE) imaging and chemical analysis of minerals and glasses in andesitic tephra produced by the 11 February 2010 explosive eruption of the Soufriere Hills Volcano (SHV), Montserrat indicate that magma in the pre-existing magma storage region had been heated from approximately 825 to 835 degrees C prior to the eruption. The evidence for the heating comes from Fe-Ti oxide geothermometry, compositional zoning profiles in Ti-magnetite and in amphibole crystals, and the absence of quartz in this magma compared to eruptions in 1995-2005. Potentially, global heating of magma in the storage region may influence magma rheology and eruptive styles, and volumes of any future eruptions.

The 2004-6 eruption of Mount St. Helens produced dacite that contains 40-50 volume percent phenocrysts of plagioclase, amphibole, low-Ca pyroxene, magnetite, and ilmenite in a groundmass that is nearly totally crystallized. Phenocrysts of amphibole and pyroxene range from 3 to 5 mm long and are cyclically zoned, with one to three alternations of Fe- and Al-rich to Mg- and Si-rich layers showing little indication of phenocryst dissolution between zones. Similar-size plagioclase phenocrysts also contain several cyclic zones ranging between ~An68 and An45-35. Textural evidence indicates that amphibole, pyroxene, and ilmenite began to crystallize before the most An-rich plagioclase. Magnetite and ilmenite phenocrysts are small (less than 100 μm), vary somewhat in composition from grain to grain, and are sporadically zoned. Magnetite-ilmenite pairs yield temperatures of equilibration ranging from 820°C to 890°C and f O2 values of NNO +1 log unit. Magnetite compositions suggest that the 2004-6 magma was formed by mingling of magmas less than 5-8 weeks before eruption and that the magma last equilibrated within this temperature range. The amphibole phenocryst zoning involves approximately equal amounts of a pressure-sensitive Al-Tschermak molecular substitution and a temperature-sensitive edenite substitution in one cycle of growth. Hydrothermal experiments done on the natural dacite show that crystallization of the Fe- and Al-rich amphibole end member requires pressures of 200-300 MPa at temperatures of 900°C, conditions approaching the upper temperature limit of amphibole stability. The dacitic magma crystallizes the An68 plagioclase when the pressure drops to 200 MPa at 900°C. The magma must cool at this depth to produce a complete An68-An40 plagioclase zone and a Mg-rich layer on the amphiboles before the magma is cycled back to a high pressure, when a new layer of Fe-rich amphibole is acquired. The amphibole crystallizing in the dacite experiments at less than 200 MPa is lower in aluminum than any compositions in the natural cyclically zoned phenocrysts. The outer rim on some 2004-6 amphibole phenocrysts appears to have formed in the 100-200 MPa range, as do some phenocrysts in the May 1980 dacite pumice. Plagioclase rims of An35 in the 2004-6 magmas indicate that phenocryst growth continued until the pressure decreased to 130 MPa and that ascent was slow until this depth. Magma then entered the conduit for a relatively rapid ascent to the surface as indicated by the very thin (less than 5 μm) decompression-induced rims on the amphibole phenocrysts.

Analyses of Fe-Ti oxides help constrain models of magma storage region processes for the Soufriere Hills Volcano, Montserrat (W.I.), and provide clear evidence of the nature of transient heating events in the magma storage region. To constrain timescales of magma heating and remobilization, the TiO2 zoning patterns in a time series of natural titanomagnetites were compared with those produced in controlled phase equilibrium experiments on the andesite. Most samples of andesite erupted from 1995 to 2002 contain titanomagnetite crystals with uniform core compositions (TiO(2)similar to7.8 wt %). Many crystals are characterized by rimward increases in TiO2, interpreted to be Ti diffusion gradients caused by heating of the andesite by invading basaltic magma. Some andesites erupted during periods of the highest observed mass eruption rate, however, contain titanomagnetite with uniformly low TiO2 contents from core to rim. The observation that no Ti diffusion gradients, and no elevated core TiO2 contents, occur in the vast majority of titanomagnetite grains in magma batches that were erupted more than 2 years after the onset of the present eruption strongly suggests, first, that heating of the batches of andesite occurred just before eruption, and, second, that injection of basaltic magma has continued throughout the eruption. Heat, but little mass, may be transferred from the invading basalt to the andesite in the magma storage region by injection of dikes or formation of sills. Ponding of basaltic magma at the base of a pre-existing andesitic magma storage region is the simplest explanation consistent with observations. The Fe-Ti oxide data strongly suggest that an internal conduit within the andesitic magma storage region carries magma from the zone of heating to the overlying conduit, which carries the magma through the upper arc crust. In this model, the magma chamber is being emptied from the bottom, at the contact between pre-existing andesite and newly intruded basalt.

A study of amphiboles and associated minerals in samples of Soufriere Hills andesite erupted from 1995 to 2002 shows significant compositional variations within hornblende phenocrysts, a separate set of small pargasitic crystals in the groundmass, and two types of reaction rims on the phenocrysts. The composition of the amphiboles and coexisting phases defines the thermal history of the erupting magma. As many as seven zones (<200 mum wide) in the hornblende phenocrysts begin with a sharp increase in Mg and Si, and then change gradually to a more Fe- and Al-rich hornblende, a transition that is consistent with a temperature rise. Analyses of the hornblende phenocrysts and associated Fe-Ti oxides verify previous conclusions that the pre-eruption magma was at 130 MPa and 830 +/- 10degreesC, but was variably heated before eruption. The heating occurred within similar to30 days of eruption for all magmas erupted, based on the width of Ti-rich rims on titanomagnetite phenocrysts. Experimental phase equilibria for the andesite confirm that the natural hornblende phenocrysts would be stable between 825 and 855degreesC at a P-H2O of 130 MPa, and would be even more Al rich if crystallized at higher pressure. Pargasite is not stable in the andesite, and its presence, along with high-An plagioclase microphenocrysts, requires mafic magma mingling and hybridization with pre-existing andesite. Experimental melts of the andesite at 130 MPa and 830 and 860degreesC compare well with melt inclusions in quartz and plagioclase, respectively. Reaction rims on a few hornblende crystals in each andesite sample are rich in high-Ca pyroxene and are produced experimentally by heating the andesite above the stability limit for hornblende. Decompression-induced breakdown rims occur in some samples, and the rate of this reaction has been experimentally calibrated for isothermal andesite magma ascent at 830-860degreesC. The average ascent rate of magma during much of the 1995-2002 eruption has been >0.02 m/s, the rate that allows hornblende to erupt free of decompression-induced reaction rims.

Abstract Dome growth at Soufrière Hills Volcano halted in early March 1998. After dome growth ceased, seismicity reduced significantly, but activity related to dome disintegration and degassing of magma at depth continued. A sustained episode of pyroclastic flows on 3 July 1998 marked the single largest collapse from March 1998 to November 1999. This led to a remarkable episode of dome collapses, low-energy explosions and ash-venting that resulted in the regular production of ash plumes, commonly reaching 1.5-6 km above sea level (a.s.l), but sometimes up to 11 km a.s.l., and the development of a small block-and-ash cone around the explosion crater. During the period of this residual activity, higher levels of activity occurred approximately every five to six weeks. This periodicity was similar to the cycles observed during active dome growth during 1995 to 1998, and probably had a similar cause. The relatively high level of observed activity caused continued concern regarding volcanic hazards and their potential to impact upon the resident population. Vigorous magma extrusion resumed in November 1999. The activity of the intervening period is attributed to the continued cooling and degassing of the dome, conduit and deep magma body, the impact of rising volcanic gases in the volcanic edifice, and limited magma flow in the conduit.

The recent eruption of the Soufriere Hills Volcano in Montserrat (July, 1995, to present; October, 1997) has produced a hornblende‐bearing, andesitic lava dome. It is possible to petrologically estimate changes in ascent rates of amphibole‐bearing magmas. For certain rates of decompression, a breakdown rim of fine‐grained, anhydrous reaction products forms where amphibole is in contact with melt. The thickness of the rim varies with ascent rate. Most of the amphibole phenocrysts in the magma storage region lack breakdown rims. About 10% have 200–400 µm‐thick, coarse‐grained breakdown rims that are interpreted to be relicts of a past heating event. Study of a time series of new dome andesites showed that ascent rate increased from December, 1995 (∼0.001 m/s), to July, 1996 (∼0.008 m/s), while eruptive style remained extrusive. Ascent rate increased to >0.012 m/s in August, 1996, and the first major explosive eruption occurred on 17–18 September, 1996.

The recent eruption of the Soufriere Hills Volcano in Montserrat (July, 1995, to present; September, 1997) has produced an andesitic dome (SiO2 ∼ 59–61 wt.%). The eruption has been caused by invasion of mafic magma into a preexisting andesitic magma storage region (P ∼ 130 MPa; ≥5 km depth). The composition of the andesite has remained essentially constant throughout the eruption, but heating by the mafic magma increased the andesite temperature from ≤830°C to ≤880°C. Prior to being heated, the stable mineral assemblage in the andesite was plagioclase + amphibole + orthopyroxene + titanomagnetite + ilmenite + quartz. The rise in temperature from ≤830°C to ≤880°C (fO2 ∼ 1 log unit above NNO) has caused quartz to become unstable, and has also caused changes in silicate and Fe‐Ti oxide mineral compositions. The andesitic magma is likely saturated with an H2O‐rich vapor phase in the upper part of the magma storage region. Melt H2O content is ∼4.7 wt.%.

New experimental results are used to constrain the P, T, X(H2O) conditions of the Soufriere Hills magma prior to ascent and eruption. The experiments were performed on a powdered andesite erupted in January, 1996, at an fO2 corresponding to ∼NNO+1 with PH2O and temperatures in the range 50 to 200 MPa and 800 to 940°C. Amphibole is stable at PH2O >115 MPa and temperatures <875°C. Quartz only becomes stable at low temperatures and after high degrees of crystallization (T <840°C, >72 wt% SiO2 in residual melt) at PH2O >115 MPa. Analyses of rhyolitic glass inclusions in quartz and plagioclase from recently erupted samples indicate melt water contents of 4.27±0.54 wt% H2O and CO2 contents <60 ppm. The evolved Soufriere Hills magma would therefore be H2O‐saturated at pressures <130 MPa.These results suggest that the Soufriere Hills magma containing the stable assemblage amphibole, quartz, plagioclase, orthopyroxene, magnetite and ilmenite was stored at PH2O of 115‐130 MPa, equivalent to a minimum depth for a water‐saturated magma chamber of 5–6 km depth. Magma temperatures were initially low (820–840°C). Quartz is believed to have been destabilised by a heating event involving injection of new basaltic magma. The stability field of hornblende provides a useful upper limit (∼880°C) for the extent of this reheating.