petrologic determination of ascent rates for the 1995 1997 soufriere hills volcano andesitic magma 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 m 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

dissolution kinetics of plagioclase in the melt of the system diopside albite anorthite and origin of dusty plagioclase in andesites the textures and kinetics of reaction between plagioclase and melts have been investigated experimentally and origin of dusty plagioclase in andesites has been discussed in the experiments plagioclase of different compositions an96 an61 an54 an23 and an22 surrounded by glasses of six different compositions in the system diopside albite anorthite was heated at temperatures ranging from 1 200 to 1 410 c for 30 min to 88 h textures were closely related to temperature and chemical compositions a crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt glass and remained its original euhedral shape below the liquidus whatever the temperature the crystal melt interface became rough and often more complicated sieve like texture composed of plagioclase melt mixture in the scale of a few m was developed from the surface of the crystal inward formation of mantled plagioclase if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase from these results the following two types of dissolution have been recognized 1 a crystal simply dissolves in the melt which is undersaturated with respect to the phase simple dissolution and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt partial dissolution the amount of a crystal dissolved and reacted increased proportional to the square root of time this suggests that these processes are controlled by diffusion probably in the crystal mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so called resorbed plagioclase in igneous rocks chemical compositions and textures of plagioclase phenocrysts in island arc andesites of magma mixing origin have been examined cores of clear and dusty plagioclase were clacic about an90 and sodic about an50 respectively this result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing

magma production and growth of the lava dome of the soufriere hills volcano montserrat west indies november 1995 to december 1997 from november 1995 to december 1997 a total volume of 246 106 dre m of andesite magma erupted partitioned into 93 106 m of the dome 125 106 m of pyroclastic flow deposits and 28 106 m of explosive ejecta in the first 11 weeks magma discharge rate was low 0 5 m s from february 1996 to may 1997 discharge rates have averaged 2 1 m s but have fluctuated significantly and have increased with time three pulses lasting a few months can be recognised with discharge rates reaching 3 to 8 m s short term pulsations in growth lasting a few days reach discharge rates of over 10 m s and there are periods of days to a few weeks when dome growth is 0 5 m s discharge rate increased from may 1997 with an average rate of 7 5 m s to december 1997 the observations indicate an open magmatic system

magma ascent rates from amphibole breakdown an experimental study applied to the 1980 1986 mount st helens eruptions recent 1980 1986 mount st helens dacites contain the phenocryst assemblage plagioclase amphibole low ca pyroxene magnetite ilmenite and rare high ca pyroxene which indicates that they all originated from an 8 km deep reservoir at 900 20 c with xh2o 0 67 in fluid according to experimental data iron titanium oxide phenocryst compositions indicate that all post may 18 dacitic magmas erupted at 900 20 c except for the final lava extrusion in october 1986 the magma reservoir may have cooled to 866 c by october 1986 amphiboles in the post may 18 1980 magma contain one or more amphibole populations characterized by reaction rims of different thicknesses the development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir constant p and t and isothermal decompression experiments show that during a 900 c constant rate decompression from 8 km to the surface no reaction rim develops on amphibole in 4 days a 10 m rim develops in 10 days and a 35 m rim develops in 20 days these experimental data and histograms of rim widths in 1980 1986 mount st helens dacites show that post may 18 eruptions are composed in large part of magma represented by a population of thin rimmed amphiboles magma which ascended from the deep 8 km reservoir in 6 to 10 days the remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 m magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly millimeter scale into the erupting magma the mixing in a viscous slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population the experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post may 18 dacites of about 15 30 m h for magma represented by the thick rimmed amphiboles and from 35 to 50 m hr for magma represented by the thin rimmed crystals an ascent rate of 66 m h is indicated for the may 18 1980 eruption which contains amphiboles with no reaction rims the volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath mount st helens in 1981 1982 was equivalent to a cylinder 8 km long and 8 9 m in radius

the role of magma mixing in triggering the current eruption at the soufriere hills volcano montserrat west indies the andesite lava currently erupting at the soufriere hills volcano montserrat contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite the andesite phenocrysts have a range of textures and zonation patterns which suggest that non uniform reheating of the magma occurred shortly before the current eruption reheating resulted in remobilisation of the resident magma and may have induced eruption

remobilization of andesite magma by intrusion of mafic magma at the soufriere hills volcano montserrat west indies the 1995 1999 eruption of the soufriere hills volcano montintroduction serrat has produced a crystal rich andesite containing quenchthe soufriere hills volcano montserrat located in the textured mafic inclusions which show evidence of having been molten lesser antilles island arc in the west indies began to when incorporated into the host magma individual crystals in the erupt on 18 july 1995 after about 350 years of dormancy andesite record diverse histories amphibole phenocrysts vary from young et al 1998b the eruption was preceded by a pristine and unaltered to strongly oxidized and pseudomorphed by period of seismic swarms which began in january 1992 anhydrous reaction products plagioclase phenocrysts are commonly previous earthquake crises have occurred on montserrat reverse zoned often with dusty sieve textures reverse zoned rims are in 1896 1897 1932 1935 and 1966 1967 powell 1938 also common on orthopyroxene phenocrysts pyroxene geothermometry shepherd et al 1971 initial phreatic and phregives an average temperature of 858 20 c for orthopyroxene atomagmatic activity was followed by extrusion of an phenocryst cores whereas reverse zoned rims record temperatures andesitic lava dome in november 1995 pyroclastic flows from about 880 to 1050 c the heterogeneity in mineral rim generated by gravitational collapse of the unstable dome compositions zoning patterns and textures is interpreted as reflecting have been the dominant form of activity throughout the non uniform reheating and remobilization of the resident magma eruption but periods of explosive activity producing body by intrusion of hotter mafic magma convective remobilization pumice flows and falls have also occurred robertson et results in mixing together of phenocrysts that have experienced al 1998 young et al 1998b dome growth ceased different thermal histories depending on proximity to the intruding suddenly in march 1998 and further activity up to may mafic magma the low temperature and high crystallinity are 1999 has mainly involved gravitational collapses of the interpreted as reflecting the presence of a cool highly crystalline dome and periods of ash venting with some small scale magma body beneath the soufriere hills volcano the petrological vulcanian explosive events observations in combination with data on seismicity extrusion rate the soufriere hills volcano is the only active centre and so2 fluxes indicate that the current eruption was triggered by on montserrat the volcano is a composite of at least recent influx of hot mafic magma five andesitic lava domes flanked by pyroclastic deposits rea 1974 identified four other volcanic centres all

the dewulu reduced au cu skarn deposit in the xiahe hezuo district west qinling orogen china implications for an intrusion related gold system abstract the triassic west qinling orogen which formed by the continental collision between the south china block and north china craton following the subduction and closure of the paleo tethyan ocean hosts numerous gold deposits that have commonly considered as products of metamorphic dehydration during the oceanic subduction and subsequent continental collision however whether or not there are gold deposits that are genetically related to regional magmatism remains poorly understood here we present mineralogical compositional and geochronological data of the dewulu au cu skarn deposit in the xiahe hezuo district to understand the ore genesis and its genetic link to many coeval sediment hosted disseminated and magmatic hosted vein gold deposits in the same district which collectively reveal a possible intrusion related gold system the dewulu au cu skarn deposit in the eastern zone of the xiahe hezuo district is associated with the early triassic i type ilmenite series dewulu quartz diorite pluton that intrudes permian marine clastic and carbonate rocks ore related hydrothermal biotite separates yield a well defined 40ar 39ar plateau age of 239 9 1 4 ma 2 which agrees with a previous zircon u pb age 238 6 1 5 ma at 2 of the dewulu quartz diorite and thus demonstrates a temporal and likely genetic link between the two both prograde and retrograde skarn assemblages are well developed in the dewulu au cu deposit prograde garnet and pyroxene are compositionally dominated by grossularite and hedenbergite respectively indicating a reduced skarn system au cu mineralization is largely represented by the arsenopyrite loellingite chalcopyrite pyrrhotite bornite assemblage and is associated with retrograde skarns gold is mostly hosted in arsenopyrite loellingite and chalcopyrite and has close textural relations with native bismuth or bismuthides mineral stability relationships and fluid inclusion microthermometric data confirm that the prograde skarns formed at a low oxygen fugacity high temperature 630 c and a depth range of 2 9 to 6 5 km arsenopyrite and chlorite geothermometers indicate that the retrograde skarns and late quartz sulfide calcite assemblages formed at 550 to 345 c and

a magmatic geohygrometer application to mount st helens and other dacitic magmas compositions of andesitic to dacitic melts in equilibrium with plagioclase orthopyroxene and clinopyroxene have been experimentally calibrated as a geohygrometer application to melson s data on melt inclusions and matrix glasses from the 1980 1982 cycle of eruptions of mount st helens reveals systematic dewatering trends ca 4 1980 to 1 1982 h 20 in subliquidus intratelluric melt in pyroclastics and 2 3 1980 to 1 1982 in subliquidus melt in dome building dacites before each pyroclastic event intratelluric crystallization achieved melt h 20 contents of ca 6 at a depth of at least 8 km a necessary condition for crystallization of amphibole dewatering trends indicate that individual magma blobs or ribbons then ascended to successively higher levels over the 3 yr cycle from a minimum depth of 4 km in 1980 to 0 35 km in 1982 and pyroclastically erupted comparison of mount st helens compositions to worldwide andesites and dacites suggests that near liquidus arc magmas that crystallize plagioclase and pyroxenes contain l 2 h 2 0 on average whereas magmas in continental margins contain appreciably more h2o

discrete ice lens theory for frost heave in soils the existing segregation potential sp method for frost heave prediction in soils is semiempirical in nature and does not explicitly predict the relationship between heave rate temperature gradient and other more fundamental soil properties the sp method assumes that the heave rate is directly related to the temperature gradient at the frost front but acknowledges that the sp parameter is dependent on pressure suction at the frost front cooling rate soil type and so forth this paper extends and modifies an approximate analytical technique of gilpin and accounts for the effects of distributed phase change within the freezing fringe in both the head and mass transfer components of the formulation the approach requires as input a relationship between frozen hydraulic conductivity and temperature and predicts the discrete location of each ice lens within the freezing soil the solution can be carried out quickly on a microcomputer to obtain the heave suction at the frost front ice lens temperatur

rheology of crystal bearing silicate melts an experimental study at high viscosities the viscosity of partially crystallized mg3al2si3o12 melts has been measured under uniaxial compression in the interval 1010 1013 poise as a function of the volume fraction of crystals these inclusions are well rounded spherulites of aluminous enstatite having the same composition as the melt and whose growth rate is negligible at the temperature of the measurements the viscosity increases by less than 1 order of magnitude for crystal fractions of 40 vol and remains newtonian up to the maximum stress exerted namely 1 kbar the einstein roscoe equation 0 1 m n provides very good fits to the measurements only if either the m or n parameter is allowed to depend on temperature for modeling of magmatic processes however the widely recommended constant values m 0 6 and n 2 5 should be adequate the rheology changes abruptly when the clustered spherulites begin to oppose shear deformation at a crystal fraction of about 40 vol the viscosity becomes non newtonian with yield strengths of a few tens of bars at temperatures at which the viscosity of the melt is higher than 1010 poise as long as the crystal fraction remains lower than 70 vol the deformation proceeds in an irregular manner with a nonuniform distribution of crystals and melt the deformation becomes again regular at low stresses with lower melt fractions but samples undergo extensive fracturation along the direction of uniaxial stress similar rheology changes have been observed during the isothermal crystallization of li2si2o5 melts which produces small ellipsoidal inclusions these results suggest that the influence of solid suspensions on the rheology of magmas is primarily determined by the crystal fraction even though additional measurements would be useful to determine the possible influence of other factors such as the size distribution or the shape of the inclusions

dusty and honeycomb plagioclase indicators of processes in the uchino stratified magma chamber izu peninsula japan abstract two types of melt inclusion bearing plagioclase coexist in the uchino basalt izu peninsula japan 1 dusty plagioclase with an ab rich core mantled by a dusty zone consisting of more an rich plagioclase and fine melt inclusions and 2 honeycomb plagioclase with a central core containing larger melt inclusions the dusty plagioclase probably forms through partial dissolution of ab rich plagioclase as dusty zones truncate the concentric zonal structure of the ab rich inner core the honeycomb plagioclase is formed by skeletal growth under supercooling conditions this is indicated by the euhedral geometry of the boundary between the inner inclusion free and the outer inclusion bearing zones of the honeycomb plagioclase which has a concentric inner core bimodal distribution of the chemical compositions of phenocryst cores and assemblages in crystal clots suggest that hybridization of basaltic and andesitic magmas took place before crystallization of phenocryst rims the core rim boundaries of phenocrysts derived from the andesitic magma and the basaltic magma have a rounded and a euhedral geometry respectively this suggests the andesitic magma was heated and the basaltic magma was cooled prior to hybridization these features are consistent with the development of a thermal diffusive interface between overlying andesitic magma and underlying basaltic magma in a stratified magma chamber

geochemical evolution of st kitts and montserrat lesser antilles the volcanic rocks of st kitts have uniform 87 sr 86 sr ratios c 0 7037 and high 143 nd 144 nd ratios c 0 5130 in a suite which ranges from basalt to rhyolite trace element abundances and the flat chondrite normalized ree patterns resemble those of island arc tholeiitic suites in comparison the montserrat lavas have higher concentrations of alkaline elements and are lree enriched geochemical differences may be related to partial displacement of the axis of the volcanic arc about 9 ma this would have led to the development of the st kitts suite in an essentially new and immature island arc environment being closer to the junction with the former arc axis the composition of the montserrat lavas may reflect previous subduction related mantle enrichment and or the influence of a sedimentary component isotopic ratios and most trace elements in the redonda basalt indicate island arc tholeiite or in some respects morb affinities but it has a high sr content and is lree enriched despite its distinctive composition redonda is situated on the main axis of the volcanic arc and its lavas were evidently contemporaneous with the younger eruptions of st kitts and montserrat

the explosive eruption of soufriere hills volcano montserrat west indies 17 september 1996 on 17 september 1996 the soufriere hills volcano started a 9 hour period of dome collapse involving 11 7 106 m dre of lava after 2 5 hours of quiescence a sustained explosive eruption began estimated eruption parameters are plume height at least 11 3 km and a maximum of 15 km 180 m s for launch velocities of ballistic clasts peak explosion pressure of 27 5 mpa magma water contents of 2 5 5 magma discharge rates from 2300 to 4300 m s ejecta volume of 3 2 0 9 106 m dre ejecta consists of pumice 1160 kg m higher density vesiculated ejecta 1300 to 2000 kg m dense glassy clasts 2600 kg m breccias cut by tuffisite veins and hydrothermally altered lithics the ejecta are interpreted as a conduit assemblage with evacuation of the conduit down to depths of about 3 to 5 km the eruption was triggered by unloading of a gas pressurised conduit due to dome collapse

microgranitoid enclaves in granites globules of hybrid magma quenched in a plutonic environment microgranitoid enclaves autoliths cognate xenoliths mafic inclusions are common in high level granitoid plutons they have relatively fine grained igneous microgranitoid microstructures1 and many show evidence of having flowed in a magmatic condition2 these features counter interpretations that the enclaves represent transformed solid fragments of diverse wall rocks3 5 or restite6 11 an alternative interpretation is that the enclaves represent globules of mafic magma that have mingled commingled and quenched in the granitoid host magma12 16 a detailed review of the literature occurrence morphology and composition of micro granitoid enclaves strongly supports this view hut indicates that the enclave magmas range in composition from mafic to felsic and that magma mixing may be involved in their formation17 i summarize here the main features of microgranitoid enclaves and briefly outline the possible processes leading to their incorporation as magma globules in the host granitoid

monitoring so2 emission at the soufriere hills volcano implications for changes in eruptive conditions correlation spectrometer measurements of sulfur dioxide so2 emission rates during the current eruption of the soufriere hills volcano montserrat have contributed towards identifying different phases of volcanic activity so2 emission rate has increased from 550 td 1 6 4 kgs 1 after july 1996 with the uncertainty associated with any individual measurement ca 30 significantly enhanced so2 emission rates have been identified in association with early phreatic eruptions 800 td 1 9 3 kgs 1 and episodes of vigorous dome collapse and pyroclastic flow generation 900 to 1500 td 1 10 4 to 17 4 kgs 1 so2 emission rate has proved a useful proxy measurement for magma production rate observed so2 emission rates are significantly higher than those inferred from analyses of glass inclusions in phenocrysts implying the existence of a s rich magmatic vapour phase

degassing at the soufriere hills volcano montserrat recorded in matrix glass compositions compositions of matrix glasses from the current eruption of soufriere hills volcano indicate that decompression driven crystallization results in 20 70 wt groundmass crystallization during eruption and variable degassing variations in crystallinity and volatile contents water and chlorine of matrix glasses are attributed to variations in extrusion rates and residence times in the lava dome residual water contents in pumice clasts 0 2 0 6 wt indicate minimum pressures of 1 1 3 7 mpa in 1997 vulcanian explosions residual water contents of 1 6 wt in a ballistic block ejected in sub plinian explosive activity on 17 september 1996 imply larger pressure drops similar to20 mpa variable residual water contents in dome samples are consistent with pressure variations of up to 9 mpa in the lava dome interior large variation in chlorine contents between lava blocks compared with explosion pumice clasts indicates that shallow level processes dominate degassing low melt chlorine contents of dome samples cannot be explained by open or closed system degassing especially when crystallization is taken into account instead heterogeneous chlorine leaching by circulation of groundwater vapour in the dome is proposed variable and elevated matrix glass deltad values can also be attributed to interaction with isotopically heavy surface waters hcl emission during the current eruption can be accounted for by cl loss from the melt consistent with the melt being undersaturated in chlorine

cristobalite in volcanic ash of the soufriere hills volcano montserrat british west indies crystalline silica mostly cristobalite was produced by vapor phase crystallization and devitrification in the andesite lava dome of the soufriere hills volcano montserrat the sub 10 micrometer fraction of ash generated by pyroclastic flows formed by lava dome collapse contains 10 to 24 weight percent crystalline silica an enrichment of 2 to 5 relative to the magma caused by selective crushing of the groundmass the sub 10 micrometer fraction of ash generated by explosive eruptions has much lower contents 3 to 6 percent of crystalline silica high levels of cristobalite in respirable ash raise concerns about adverse health effects of long term human exposure to ash from lava dome eruptions

overview of the eruption of soufriere hills volcano montserrat 18 july 1995 to december 1997 the onset of phreatic volcanic activity at the soufriere hills volcano montserrat on 18 july 1995 followed a three year period of heightened volcano seismic activity beneath the island phreatic explosions gave way to continuous eruption of juvenile andesitic magma in the form of a lava dome on or around 15 november 1995 magma production rate has varied leading to changes in eruptive style an explosive eruption on 17 september 1996 followed a period of enhanced dome growth and large scale gravitational collapses from its eastern flank increasing dome volume led to stressing and overtopping of the confining crater walls to the southwest north and west during early 1997 sustained high magma production rate since june 1997 has led to three periods of major gravitational dome collapses followed by vulcanian explosive eruptions dome growth re started immediately after the cessation of the latest of these explosive phases in october 1997 and continues as of december 1997

a complex magma mixing origin for rocks erupted in 1915 lassen peak california the eruption of lassen peak in may 1915 produced four volcanic introduction rock types within 3 days and in the following order 1 hybrid magma mixing is an important process among interblack dacite lava containing 2 undercooled andesitic inclusions mediate magmas in volcanic arcs e g turner 3 compositionally banded pumice with dark andesite and light campbell 1986 philpotts 1990 and references therein dacite bands and 4 unbanded light dacite all types represent laboratory studies of synthetic analogs of silicate magma stages of a complex mixing process between basaltic andesite systems and mathematical modeling of mixing viscous and dacite that was interrupted by the eruption they contain liquids have suggested a range of possible mixing mechdisequilibrium phenocryst assemblages characterized by the coanisms e g cashman bergantz 1991 however use existence of magnesian olivine and quartz and by reacted and of these results to interpret magmatic systems is limited unreacted phenocrysts derived from the dacite the petrography and by the applicability of synthetic analogs to magmatic crystal chemistry of the phenocrysts and the variation in rock conditions and by the imprecise knowledge of the effective compositions indicate that basaltic andesite intruded dacite magma viscosity of silicate liquid crystal mixtures at magmatic and partially hybridized with it phenocrysts from the dacite magma temperatures and pressures if magmas mix incompletely were reacted cooling crystallization and vesiculation of the hybrid i e mingle features such as compositional banding or andesite magma converted it to a layer of mafic foam the decreased undercooled inclusions are usually apparent but if they density of the andesite magma destabilized and disrupted the foam mix completely mineralogical disequilibrium may be the blobs of foam rose into and were further cooled by the overlying dacite magma forming the andesitic inclusions disaggregation of only direct evidence for a mixing origin andesitic inclusions in the host dacite produced the black dacite and if the thermal and compositional contrasts between light dacite magmas formation of foam was a dynamic process two magmas are great and the ratio of silicic to mafic removal of foam propagated the foam layer downward into the magma is large there is little interaction between them hybrid andesite magma eventually the thermal and compositional and the mafic magma is undercooled e g bacon 1986 contrasts between the hybrid andesite and black dacite magmas were many undercooled inclusions contain reacted phereduced then they mixed directly forming the dark andesite magma nocrysts inherited from their host silicic magma heiken about 40 50 andesitic inclusions were disaggregated into the eichelberger 1980 and bacon 1986 pointed out host dacite to produce the hybrid black dacite thus disaggregation that undercooled inclusions are typically formed from of inclusions into small fragments and individual crystals can be hybrid magmas in general the formation of undercooled an efficient magma mixing process disaggregation of undercooled inclusions retards further mixing sakuyama 1984 inclusions carrying reacted host magma phenocrysts produces cothompson dungan 1985 sparks marshall 1986 existing reacted and unreacted phenocryst populations koyaguchi blake 1991 however fragmentation and or disaggregation of undercooled inclusions plays an important role in hybridization in some magma systems thompson dungan 1985 coulon et al 1986 clynne christiansen 1987 clynne 1989 linneman myers

insights into magmatic evolution and recharge history in capraia volcano italy from chemical and isotopic zoning in plagioclase phenocrysts plagioclase phenocrysts in dacites from the high k calc alkaline capraia volcano were investigated for major trace element and sr isotope variations in order to gain better insight into the proposed open system behaviour of the volcano repeated dissolution zones in plagioclases from the early erupted monte castello mc and later monte rucitello mr dacitic units are associated with a sharp increase in an content and variation in isotopic compositions 87sr 86sr 0 70872 0 71004 which argue for repeated magma mixing in particular petrographic and mineralogical evidence suggest that recharge with hot basaltic magma occurred repeatedly during the lifetime of the capraia volcanic system we attempted to correlate shared crystallization and resorption histories using chemical mol an fe and sr isotopic zoning in plagioclase in this correlation framework the observed mismatch between chemical and isotopic data in some phenocrysts is interpreted to reflect either growth of phenocrysts in a separate magma chamber or compositional zoning within a single magma chamber a minor role of crustal contamination is inferred instead the intra crystal variations in isotopic composition in plagioclase reflect the existence of distinct components that may have thoroughly mixed at the end of the mc plagioclase growth history the dominant radiogenic component is similar to lamproites occurring in tuscany recharge with this component occurred throughout the genesis of the mr dacites but was restricted to the early growth history of the mc dacites the mr plagioclases are inferred to have crystallized from separate more differentiated batches of magma that were likely to have evolved at the top of a zoned magma reservoir decrease in the an content after most dissolution zones in all phenocrysts and low temperature estimates from zircon saturation thermometry 800 c also emphasize the importance of crystal fractionation during the evolution of the volcano in agreement with the occurrence of a significant amount of rhyolitic material thus it is inferred that a rather large magma chamber may have existed during the first cycle of eruptive activity

late stage mafic injection and thermal rejuvenation of the vinalhaven granite coastal maine the vinalhaven intrusive complex consists mainly of coarse grained granite inward dipping gabbro diorite sheets and a fine grained granite core small bodies of porphyry occur throughout the coarsegrained granite the largest porphyry body roughly 0 5 km by 2 5 km occurs with coeval gabbro hybrid rocks and minor finegrained granite in the vinal cove complex which formed during the waning stages of solidification of the coarse grained vinalhaven granite porphyry contacts with surrounding coarse grained granite are irregular and gradational compositions of whole rocks and minerals in the porphyry and the coarse grained granite are nearly identical neighboring phenocrysts in the porphyry vary greatly in degree of corrosion and reaction indicating that the porphyry was well stirred thermal rejuvenation of a silicic crystal mush by a basaltic influx can explain the composition and texture of the porphyry comparable rejuvenation events have been recognized in recent studies of erupted rocks weakly corroded biotite phenocrysts in the porphyry require that hydrous interstitial melt existed in the granite during remelting field relations along with thermal calculations suggest that cooling and crystallization of coeval mafic magma could have generated the porphyry by thermal rejuvenation of granite crystal mush containing about 20 melt field relations also suggest that some of the porphyrymatrix may represent new felsic magma that was emplaced during remelting

petrologic evidence for pre eruptive pressure temperature conditions and recent reheating of andesitic magma erupting at the soufriere hills volcano montserrat w i the recent eruption of the soufriere hills volcano in montserrat july 1995 to present september 1997 has produced an andesitic dome sio 2 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 fo 2 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 h 2 o rich vapor phase in the upper part of the magma storage region melt h 2 o content is 4 7 wt