contrasts in morphology and deformation offshore montserrat new insights from the sea calipso marine cruise data 1 during the december 2007 sea calipso experiment we collected seismic reflection profiles offshore of montserrat off the east coast we imaged deep fans of volcaniclastic debris from three volcanoes progressively active from 2 ma to present near shelf sedimentation rates of 8 9 cm ka are approximated following cessation of local volcanic activity the fans were deposited on sediments with apparent dips towards the ese trending montserrat havers fault system mhfs in southern montserrat the mhfs encloses the soufriere hills volcano has elevated crustal blocks at roche s bluff st georges hill and garibaldi hill and extends off the west coast off the west coast the n dip of two faults supports a n dip interpretation for a major component of mhfs the belham valley fault we propose that local deformation is affected by stress redistributions consistent with a right stepping sinistral en echelon fault system but the interplay of transtension and magmatism has resulted in complex and evolving stress regimes

a rapid mechanism to remobilize and homogenize highly crystalline magma bodies the largest products of magmatic activity on earth the great bodies of granite and their corresponding large eruptions have a dual nature homogeneity at the large scale and spatial and temporal heterogeneity at the small scale this duality calls for a mechanism that selectively removes the large scale heterogeneities associated with the incremental assembly of these magmatic systems and yet occurs rapidly despite crystal rich viscous conditions seemingly resistant to mixing here we show that a simple dynamic template can unify a wide range of apparently contradictory observations from both large plutonic bodies and volcanic systems by a mechanism of rapid remobilization unzipping of highly viscous crystal rich mushes we demonstrate that this remobilization can lead to rapid overturn and produce the observed juxtaposition of magmatic materials with very disparate ages and complex chemical zoning what distinguishes our model is the recognition that the process has two stages initially a stiff mushy magma is reheated from below producing a reduction in crystallinity that leads to the growth of a subjacent buoyant mobile layer when the thickening mobile layer becomes sufficiently buoyant it penetrates the overlying viscous mushy magma this second stage rapidly exports homogenized material from the lower mobile layer to the top of the system and leads to partial overturn within the viscous mush itself as an additional mechanism of mixing model outputs illustrate that unzipping can rapidly produce large amounts of mobile magma available for eruption the agreement between calculated and observed unzipping rates for historical eruptions at pinatubo and at montserrat demonstrates the general applicability of the model this mechanism furthers our understanding of both the formation of periodically homogenized plutons crust building and of ignimbrites by large eruptions

origin of andesite in the deep crust and eruption rates in the tancitaro nueva italia region of the central mexican arc the tancitaro nueva italia region 4400 km 2 is located within the west central mexican volcanic belt and spans 80 km of arc length along the volcanic front on the basis of 65 40 ar 39 ar ages it is shown that a total volume of 326 57 km 3 of magma has erupted from 200 vents since 1 2 ma nearly a third of this volume 103 5 km 3 was erupted as crystal rich andesite from a large stratovolcano volcan tancitaro whereas the remaining two thirds 223 52 km 3 span the complete range from 51 to 63 wt sio 2 are relatively crystal poor and were erupted from peripheral vents the total proportions of erupted lava types are 6 7 basalt 18 basaltic andesite and 74 76 andesite there is no rhyolite and only a trace of dacite there is no evidence that any significant magma differentiation occurred in the upper crust along this arc segment based on phenocryst abundances and modes major and trace element data as well as phase equilibrium experiments from the literature the following mechanisms for formation of andesite in this region can be ruled out 1 crystal fractionation of basalt basaltic andesite in the upper crust 2 magma mixing of basalt basaltic andesite with dacite rhyolite and 3 assimilation of granitoid by basalt basaltic andesite instead the same body of evidence indicates that the andesites were formed primarily in the deep crust through partial melting 20 of hornblende rich 40 gabbronorite the basaltic andesites are required owing to their elevated ni concentrations to be hybrids of the andesitic partial melts with basaltic residual liquids that have undergone variable two pyroxene fractionation in the deep crust

continuous mixing of crystal mush and replenished magma in the ongoing unzen eruption zoning profiles of magnetite phenocrysts in the successively effused dacite of the ongoing unzen eruption from 1991 wereanalyzed to estimate the time scale of magma mixing the dacite was formed by mixing of relatively high and low temperature t end member magmas the magnetite phenocrysts derived from the low t magma are reversely zoned by the mixing with high t magma a diffusion calculation for reequilibration of the reverse zonings gives the time interval from magma mixing to quenching for the mixed dacite erupted from may 1991 to may 1993 the typical diffusion time was estimated to be a few months regardless of the effused sequence for 2 yr this indicates that the mixing was continuous during the effusion the invariability of the other mixing signatures such as the thickness of reaction rims around biotite phenocrysts also supports the continuous mixing model low t end member magma is estimated by mass balance calculation to be a crystal rich mush of dacitic composition these observations lead to a model wherein the highly crystallized remnant magma of the preceding activity has been mixed with the newly injected hot magma of similar bulk composition just prior to the effusion the proposed mechanism implies that this type of magma mixing is an inevitable process in periodically erupting polygenetic volcanoes

preeruptive magma viscosity an important measure of magma eruptibility 1 using a compilation of melt compositions meltwater contents temperatures and phenocryst contents the preeruptive viscosities under magma reservoir conditions are calculated for 83 erupted magmas the basaltic to rhyolitic magmas have preeruptive viscosities over the range 101 to 108 pa s although bulk sio2 content has often been used as a qualitative measure of preeruptive magma viscosity the bulk sio2 content shows a weak correlation with magma viscosity correlation coefficient r 0 5 because of a wide range of phenocryst contents from 0 to 50 vol andesitic magmas have viscosities ranging from 102 to 107 pa s which are lower or higher than those of phenocryst poor rhyolitic magmas with 105 to 106 pa s focusing on andesitic to rhyolitic magmas the r between bulk sio2 contents and magma viscosities changes to 0 1 in contrast the melt only sio2 content from a basaltic to rhyolitic melt shows a good linear correlation with melt only viscosity r 0 9 although most of the calculated viscosities of erupted magmas fall below 106 pa s as consistent with the previous compilation study this paper describes 20 examples of highly viscous magmas with 106 pa s in most cases composed of mixtures of high silica rhyolitic melt 75 79 wt sio2 and abundant phenocrysts 30 55 vol in these highly viscous magmas 9 examples have erupted following the precursory eruption of less viscous magma suggesting that precursory dike propagation and conduit formation by the less viscous magma with 106 pa s induced the following eruption of less eruptible highly viscous magmas

common origin of plagioclase in the last three eruptions of unzen volcano japan abstract megacrysts large crystals of 2 15 mm in length of plagioclases extracted from the lavas of the last three eruptions of southwestern japan s unzen volcano ad 1663 ad 1792 and ad 1991 1995 have limited 87 sr 86 sr ratios of 0 70439 0 70454 results of micro drilling analyses indicated that three of eight megacrysts showed a gradual decrease in 87 sr 86 sr ratios from the core to the rim whereas other megacrysts showed a homogeneous 87 sr 86 sr ratio this relative homogeneity contrasts sharply to the wide variation of 87 sr 86 sr ratios of matrixes of the lavas of the last three eruptions 0 71417 0 70467 0 70447 0 70450 respectively most megacrysts show isotopic disequilibrium between their outer rims and their matrixes suggesting that they did not grow in the host magmas in addition seven of the eight megacrysts of plagioclase also have similar trace element abundance ratios la nd and sr ba their similar chemical and isotopic compositions suggest that they crystallized from the same parent magma which suggests to us that they were formed prior to or during the eruption in 1663 the isotopic ratios of the 1663 eruption lavas of 0 71417 however rules out the possibility that it is the parent magma for the plagioclase megacrysts the lavas erupted 4000 and 5000 years ago have 87 sr 86 sr ratios of 0 70454 and 0 70442 respectively chen c h depaolo d j nakada s shieh y n 1993 relationship between eruption volume and neodymium isotopic composition at unzen volcano nature 362 831 834 they are inferred to be of the parent magma because of their sr isotope ratios consequently the micro analytical results seem to suggest that plagioclases with a single origin can be supplied to volcanic products of several different eruptions surviving several eruption events

mount dutton volcano alaska aleutian arc analog to unzen volcano japan abstract holocene eruptions from mount dutton a small late quaternary volcano near the tip of the alaska peninsula bear strong physical and petrologic similarities to the 1990 1995 unzen fugendake eruption in japan the volcano had a protracted phase of effusive calcalkaline andesitic 54 59 wt sio 2 cone building in the late pleistocene followed by an abrupt switch to more silicic 65 wt sio 2 lavas emplaced as a central summit cluster of steep sided domes beginning in the early holocene the flanks of the volcano are mantled by pyroclastic flows debris flows and talus formed as a result of gravitational dome collapse disequilibrium mineral assemblages including coexisting quartz and olivine in eruptive episodes ranging from the initial cone building basaltic andesite lavas to the latest holocene dacite domes suggest extensive magma mixing in addition up to meter sized pillow like cognate mafic enclaves of hornblende plagioclase glass are common in the latest of the summit dacite domes mineralogical evidence and bulk chemical data indicate the enclaves represent a high alumina basalt parent with variable and subordinate reservoir contaminant and the host lava is reservoir magma with variable and subordinate basaltic contaminant mount dutton s history and petrology can be interpreted as reflecting the monotonous repetitive intrusion of mantle derived mafic magma into a silicic crystal rich crustal reservoir during the holocene these injections resulted in the extrusion of partially crystallized viscous sticky central domes which typically failed by collapse resulting in small volume merapi type flowage deposits we speculate that slow introduction of mafic magma into the silicic chamber leads both to enclave formation and to the effusive eruption style mount dutton volcano experienced severe shallow earthquake swarms in 1984 1988 and to a lesser extent in 1991 although none of these swarms resulted in an eruption their epicenter distribution and volcanic tectonic character indicate they recorded the movement of magma at shallow depth beneath the volcano the lessons of unzen and montserrat suggest that despite limited tephra production these small volcanic centers with central dome complexes present a serious hazard to life and property

variable regimes in magma systems documented in plagioclase zoning patterns el misti stratovolcano and andahua monogenetic cones abstract magmas may experience closed or open system evolution during their ascent and evolution in the upper crust chemically closed system processes can involve heat transfer from underplating mafic magmas or decompression due to magma ascent on the other hand newly injected magmas can mix with the residing magma and hybridize plagioclase zonation patterns and electron microprobe major and trace element analysis constrain these contrasting scenarios for andesites from the 80 km3 el misti stratovolcano 5822 m and a field of relatively small monogenetic andahua scoria cones both in southern peru in particular fe concentration in plagioclase reveals valuable information within a magmatic system to explore the dominance of closed vs open system processes large and correlated anorthite fe contrasts at resorption zones identify compositional mixing related to recharge subhorizontal an fe variations record thermal or water induced effects el misti volcano a large but still relatively youthful stratovolcano is dominated during most of its approx 120 ka history by closed system effects during its evolution recharge and mixing mostly occurred cryptically as it is not recorded in drastic compositional changes in magmas or their phenocrysts a small volume erupted at el misti however is affected by strong open system behaviour as reflected in strongly correlated variations in an and fe contents of plagioclase compositions its crystal population is texturally and chemically very similar to lavas of the monogenetic cinder cones of the andahua volcanic field for the el misti magma reservoir resorption events caused by thermal effects or magma ascent are more frequent in comparison to the actual eruption frequencies many cycles of closed system evolution heat transfer and decompression therefore occurred without resulting in an eruption plagioclase zonations from lavas of the monogenetic andahua field indicate that any mafic batch that intruded the much smaller reservoirs immediately triggered an eruption by contrast a large magma system that feeds el misti volcano can easily accommodate a new batch without causing eruption

getting granite dikes out of the source region whether a dike can propagate far from a magma reservoir depends upon the competition between the rate at which propagation widens the dike and the rate at which freezing constricts the aperture available for magma flow various formulations are developed for a viscous fluid at temperature t sub m intruding a growing crack in an elastic solid the initial solid temperature equals t sub m at the source and decreases linearly with distance from the source if t sub m is the unique freezing temperature of the fluid dike growth is initially self similar and an essentially exact solution is obtained if t sub m is above the solidus temperature the solution is approximate but is designed to overestimate the distance the dike may propagate the ability of a dike to survive thermally depends primarily upon a single parameter that is a measure of the ratio of the dike frozen margin thickness to elastic thickness perhaps more intuitively one may define a minimum distance from the essentially solid reservoir wall to the point at which the host rock temperature drops below the solidus necessary for dikes to propagate far into subsolidus rock it is concluded that for reasonable material properties and source conditions most basalt dikes will have little difficulty leaving the source region but most rhyolite dikes will be halted by freezing soon after the magma encounters rock at temperatures below the magma solidus while these results can explain why granitic dikes are common near granitic plutons but rare elsewhere the potentially large variation in magmatic systems makes it premature to rule out the possibility that most granites are transported through the crust in dikes nonetheless these results highlight difficulties with such proposals and suggest that it may also be premature to rule out the possibility that most granite plutons ascend as more equidimensional bodies

an apparent polar wander path from the tarim block china abstract the apparent polar wander apw path from the tarim block consists of devonian 16 n 165 e a95 4 late carboniferous 41 n 160 e a95 4 permian 61 n 177 e a95 9 early triassic 69 n 183 e a95 11 and jurassic cretaceous 65 n 214 e a95 6 poles on the basis of this apw path we conclude that tarim rode on a plate subducting under the kazakhstan plate between devonian and permian times by the end of the permian subduction had almost finished early triassic poles from tarim siberia north china and south china are discordant indicating post early triassic relative motions among them cretaceous paleomagnetic poles from tarim north china and south china however are consistent suggesting that china has been a coherent tectonic unit since cretaceous time the shape of the apw path from tarim is similar to that of siberia except that tarim s apw path is systematically displaced to the east a 26 clockwise rotation of tarim s apw path about the euler pole 60 n and 14 e reconciles these two apw paths tarim has moved northeastward some 2000 km relative to a fixed siberia since the cretaceous this northeastward motion of tarim with respect to siberia agrees well with the overall cenozoic tectonic style of asia most of southern china has been pushed eastward to give way to the penetrating indian continent east west and northeast trending left lateral strike slip faults north of the tarim block have been documented however the exact amounts of displacement along these faults are not well known using paleomagnetic data from tarim we will also discuss the tectonic evolution of terranes in central asia and the amount of continental shortening in the tibetan plateau

pre eruptive reheating during magma mixing at quizapu volcano and the implications for the explosiveness of silicic arc volcanoes effusive explosive transitions are observed in volcanoes erupting water rich magmas that lack mineralogical evidence of degassing suggesting that gas loss occurred rapidly and ascent was fast despite the high viscosity of rhyodacitic to rhyolitic melts here we show that pre eruptive heating of water rich silicic magma by mixing with hot recharge in subvolcanic chambers controls the effusive explosive transition volcan quizapu chile emitted a 5 km3 mingled dacite andesite lava flow in a d 1846 1847 that was 130 c hotter than the 5 km3 of the same dacite that lead to a plinian eruption in a d 1932 we explain the suppression of explosive fragmentation in the 1846 1847 lavas by enhanced syneruptive magma degassing as a consequence of late reheating following mixing with andesite recharge higher magma temperatures led to the transitioning into effusive eruptive behavior by significantly accelerating volatile diffusion and lowering melt viscosity which facilitated bubble nucleation growth and coalescence while also inhibiting brittle fragmentation thus recharge by mafic magmas in subvolcanic magma chambers may reduce the risk of explosive eruptions in cases where heat can be efficiently transferred to cold volatile rich silicic magmas

dual reservoir structure at soufriere hills volcano inferred from continuous gps observations and heterogeneous elastic modeling 1 most inversions of geodetic data for volcanic systems assume homogeneous media here we examine the effect of inhomogeneous media using axisymmetric finite element models of soufriere hills volcano shv to show that homogeneous elastic crustal models underestimate both the depth and excess pressure of the deformation source continuous gps data from 1999 to 2009 are then used to infer that the shv system has multiple crustal pressure sources for a dual reservoir volcanic model a deep reservoir located within the mid crust undergoes volume changes approximately an order of magnitude larger than those of the shallow reservoir

volcanic evolution of volcan aucanquilcha a long lived dacite volcano in the central andes of northern chile volcan aucanquilcha northern chile has produced 37 km3 of dacite 63 66 wt silica mainly as lavas with ubiquitous magmatic inclusions 59 62 wt silica over the last 1 million years a pyroclastic flow deposit related to dome collapse occurs on the western side of the edifice and a debris avalanche deposit occurs on the eastern side the 6 000 m high edifice defines a 9 km e w ridge and lies at the center of a cluster of more than 15 volcanoes the aucanquilcha volcanic cluster that has been active for at least the past 11 million years the e w alignment of vents is nearly orthogonal to the arc axis a majority of volcan aucanquilcha was constructed during the first 200 000 years of eruption whereas the last 800 000 years have added little additional volume the peak eruptive rate during the edifice building phases was 0 16 km3 ka and the later eruptive rate was 0 02 km3 ka comparable dacite volcanoes elsewhere show a similar pattern of high volcanic productivity during the early stages and punctuated rather than continuous activity volcan aucanquilcha lavas are dominated by phenocrysts of plagioclase accompanied by two populations of amphibole biotite clinopyroxene fe ti oxides and or orthopyroxene accessory phases include zircon apatite and rare quartz and sanidine one amphibole population is pargasite and the other is hornblende the homogeneity of dacite lava from volcan aucanquilcha contrasts with the heterogeneity 52 66 wt silica at nearby volcan ollague which has been active over roughly the same period of time we attribute this homogeneity at aucanquilcha to the thermal development of the crust underneath the volcano resulting from protracted magmatism there whereas volcan ollague lacks this magmatic legacy

preferential eruption of andesitic magmas through recharge filtering at subduction zones andesite lavas erupt in greater abundance than the individual magmas that mix to produce them textural and geochemical analyses of andesites from mount hood oregon reveal that injection and mixing of iron and magnesium rich magma in the magma chamber can initiate the volcanic eruption andesitic volcanic rocks are common in subduction zones and are argued to play an important role in the formation and evolution of the continental crust at convergent margins1 2 3 4 andesite formation is dominated by mixing between iron and magnesium rich mafic magmas and silica rich felsic magmas1 2 4 the abundance of andesites in many subduction zones suggests they erupt in preference to the magmas that mix to produce them4 however the reasons for this remain unclear here we use textural and geochemical analyses of andesites from mount hood oregon to show that eruptions are closely linked with episodes of mafic recharge the intrusion of mafic magma into a shallow felsic magma reservoir the felsic and mafic magmas involved rarely erupt by themselves probably because the former are too viscous and the latter too dense mafic recharge overcomes these barriers to eruption and as it also promotes efficient mixing results in preferential eruption of mixed andesitic magmas the abundance of andesites therefore relates to local crustal conditions and the ability of magmas to erupt we suggest that volcanoes such as mount hood that erupt homogeneous andesitic compositions through time are those that are the most reliant on mafic recharge to initiate eruptions

petrology and fe ti oxide reequilibration of the 1991 mount unzen mixed magma a dacitic magma 64 5 wt sio2 a mixture of phenocryst rich rhyodacite and an aphyric mafic magma was erupted during the recent 1991 1995 mount unzen eruptive cycle the experimental and analytical results of this study reveal additional details about conditions in the premixing and postmixing magmas and the nature of the mixing process the preeruption rhyodacitic magma was at a temperature of 790 20 c according to fe ti oxide phenocryst cores and at a depth of 6 to 7 km 160 mpa according to al in hornblende geobarometry the mafic magma that mixed with the rhyodacite is found as andesitic 54 to 62 wt sio2 enclaves in the erupted magma and was essentially aphyric when intruded phase equilibria indicate that an aphyric andesite at 160 mpa is 1030 c h2o saturated and possibly as high as 1130 c 2 wt h2o the composition of the rhyodacite which was mixed with the andesite is estimated to lie between 67 and 69 wt sio2 using these compositions and temperatures the temperature of the unzen magma after mixing is estimated to be at least 850 to 870 c the groundmass fe ti oxide microphenocrysts and those in pargasite bearing reaction zones around biotite phenocrysts both give 890 20 c temperatures the oxide oxide contacts give temperatures of 910 20 c the 900 30 c postmixing temperatures are consistent with phase equilibria experiments which show that the magma was not above 930 c at 160 mpa our fe ti oxide reequilibration experiments suggest that the mixing of the two magmas began within a few weeks of the eruption which is a shorter time than is calculated using available diffusion data there is also evidence that some mixing took place much closer to the time of extrusion based on the presence of unrimmed biotite phenocrysts in the magma

conditions for the growth of a long lived shallow crustal magma chamber below mount pelee volcano martinique lesser antilles arc 1 the compositional homogeneity of silicic andesites emitted during the 13 500 year long last eruptive cycle of mount pelee suggests that the physical state of the magma chamber was largely unmodified during this period experimental phase equilibria on mount pelee recent products indicate that pre eruptive magma temperatures and pressures were in the range of 875 900 c and 2 0 5 kbar respectively the estimated average eruption rate was 7 5 10 4 km3 a with an average volume of about 0 3 km3 per eruption an analytical model for a spherical magma chamber indicates that a magma flux of 4 5 10 4 km3 a can maintain a magma chamber of 0 3 km3 above 875 c below mount pelee however observations of plutons suggest that magma chambers may grow by addition of sheet like intrusions with numerical simulation we show that a minimum sheet accretion rate of a few centimeters per year is required to grow a persistently active magma chamber independently of the intruded volumetric flux this minimum injection rate is higher if the heat transfer is enhanced by convection processes the limited ability of an arc crust to extend and accommodate dikes suggests that magma injections are sills or if they are dikes that most of the volume injected in the magma chamber is eventually erupted in a conductively cooling igneous body formed by sills injected at a rate of a few centimeters per year most of the injected magma completely solidifies and only a small part of the body about 10 20 is above 875 c and able to feed eruptions

the eruptive history of the mascota volcanic field western mexico age and volume constraints on the origin of andesite among a diverse suite of lamprophyric and calc alkaline lavas the mascota volcanic field is located in the jalisco block of western mexico where the rivera plate subducts beneath the north american plate it spans an area of 2000 km2 and contains 87 small cones and lava flows of minette absarokite basic hornblende lamprophyre basaltic andesite and andesite there are no contemporary dacite or rhyolite lavas new 40ar 39ar ages are presented for 35 samples which are combined with nine dates from the literature to document the eruptive history of this volcanic field the oldest lavas 2 4 to 0 5 ma are found in the southern part of the field area whereas the youngest lavas predominantly 0 5 ma are found in the northern portion on the basis of these ages field mapping and the use of ortho aerial photographs and digital elevation models it is estimated that a combined volume of 6 8 3 1 km3 erupted in the last 2 4 myr which leads to an average eruption rate of 0 003 km3 kyr and an average volume per eruptive unit of 0 1 km3 the dominant lava type is andesite 2 1 0 9 km3 followed by absarokite 1 6 0 8 km3 basaltic andesite 1 2 0 5 km3 basic hornblende lamprophyre 1 0 0 4 km3 and minette 0 9 0 5 km3 thus the medium k andesite and basaltic andesite comprise approximately half 49 of the erupted magma with twice as much andesite as basaltic andesite and they occur in close spatial and temporal association with the highly potassic lamprophyric lavas there is no time progression to the type of magma erupted a wide variety of evidence indicate that the high mgo 8 9 wt basaltic andesites 52 53 wt sio2 were formed by h2o flux melting of the asthenopheric arc mantle wedge whereas the mafic minettes and absarokites were formed by partial melting induced by thermal erosion of depleted lithospheric mantle containing phlogopite bearing veins there is only limited differentiation of the potassic magmas with none more evolved than 55 4 wt sio2 and 4 4 wt mgo this may be attributable to rapid crystallization of the mantle derived melts in the deep crust owing to their low volumes thus the andesites 58 63 wt sio2 are notable for being both the most voluminous and the most evolved of all lava types in the mascota volcanic field which is not consistent with their extraction from extensively crystallized 60 70 low volume intrusions instead the evidence supports the origin of the andesites by partial melting of amphibolitized mafic lower crust driven by the emplacement of the minettes absarokites and the high mg basaltic andesites

effects of volatiles on mixing in calc alkaline magma systems the intimate mixing between different magmas of disparate densities characteristic of some calc alkaline magma systems is explained by a new mechanism involving the emplacement of a layer of wet undersaturated mafic magma at the base of a magma chamber containing more differentiated magma turbulent transfer of heat between the mafic magma and the overlying magma leads to crystallization and exsolution of volatiles in the lower layer for an initial water content of a few per cent the bulk density of the mafic magma can become equal to that of the overlying magma causing overturning and intimate mixing of the magmas the mechanism is most probable in the low pressure environment of a high level magma chamber

volcanic gas emissions from soufriere hills volcano montserrat 1995 2009 with implications for mafic magma supply and degassing volcanic gas emissions illustrate a complex volatile budget for soufriere hills volcano montserrat fluxes of sulphur dioxide carbon dioxide hydrogen sulphide and hydrogen chloride and probably water are substantial from this arc volcano additional sources of volatiles in addition to the erupting andesite are required to satisfy the mass balance defined by gas emissions and petrological constraints mafic magma is intruded at depths of 10 km supplying volatiles by quenching crystallising and vesiculating at the andesite mafic magma interface sulphur carbon dioxide and other volatiles supplied to the system migrate to the surface and their fluxes remain high while mafic recharge at depth is sustained decompression degassing of porphyritic andesite supplies chlorine and water to the gas plume chlorine partitions into vapour from the melt during periods of active extrusion making the hcl so2 ratio of volcanic gases a reliable indicator of the onset of eruptive activity studies of this volatile budget highlight the role of deep degassing and vapour production at arc volcanoes which are important mechanisms for driving the eruption due to its effects on magma buoyancy and the advection of heat through the system

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