experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

a new method to quantify the real supply of mafic components to a hybrid andesite the eruption of soufriere hills volcano montserrat has been ongoing since 1995 the volcano is erupting a crystal rich hornblende plagioclase andesite with ubiquitous mafic inclusions indicating mixing with mafic magma this mafic magma is thought to be the driving force of the eruption supplying heat and volatiles to the andesite resident in the magma chamber as well as producing macroscopic mafic inclusions the magma mixing process involves incorporation of phenocrysts from the andesite into the mafic magma these inherited phenocrysts show clear disequilibrium textures e g sieved plagioclase rims and thermal breakdown rims on hornblende approximately 25 of all phenocrysts in the andesite show these textures indicating very extensive mass transfer between the two magma types fragments of mafic inclusions down to sub mm scale are found in the andesite together with mafic crystal clusters which are commonly found adhered to the rims of phenocrysts with disequilibrium features mineral chemistry also points to the transfer of microlites or microphenocrysts initially formed in the mafic inclusions into the andesite this combined evidence suggests that some of the mafic inclusions disaggregate during mingling and or ascent possibly due to shearing and raises the question what proportion of the andesite groundmass actually originated in the mafic inclusions and thus what is the true amount of mafic magma in the magmatic system we present a new method for quantifying the relative proportions of groundmass plagioclase derived from mafic and andesitic magma based on analysis of back scattered electron images of the groundmass preliminary results indicate that approximately 16 of all groundmass plagioclase belongs genetically to the mafic inclusions together with the crystal clusters disequilibrium phenocryst textures and mm scale inclusions there is a cryptic mafic component in the andesite of approximately 6 by volume this is significant compared with the proportion of macroscopic mafic inclusions typically 1 5 the new method has the potential to allow tracking of the mafic fraction through time and thus to yield further insights into magma hybridisation processes

control on the emplacement of the andesite lava dome of the soufriere hills volcano montserrat by degassing induced crystallization lava solidification is controlled by two mechanisms external cooling and gas exsolution the latter inducing crystallization due to increasing liquidus temperature the andesite lava dome of the soufriere hills volcano montserrat is an extrusion dominated by crystallization caused by gas exsolution where cooling is unimportant in controlling emplacement in the magma chamber the magma has an estimated viscosity of 7 106 pa s during ascent gas exsolution caused the magma to extrude in a highly crystalline state with only 5 15 residual melt viscosities in the range 1013 1014 pa s and mechanical strength 1 mpa deformation can be heterogeneous with extrusion along shear zones rheological stiffening in the upper conduit also causes large overpressures shallow seismicity and cyclic patterns of dome extrusion gas rich porphyritic andesites tend to be the least mobile kind of lava because transition from magma into hot crystalline material was reached during ascent

explosive subglacial rhyolitic eruptions in iceland are fuelled by high magmatic h2o and closed system degassing rhyolitic eruptions beneath icelandic glaciers can be highly explosive as demonstrated by quaternary tephra layers dispersed throughout northern europe however they can also be small and effusive a subglacial rhyolitic eruption has never been observed so behavioral controls remain poorly understood and the influence of pre eruptive volatile contents is unknown we have therefore used secondary ion mass spectrometry to characterize pre eruptive volatile contents and degassing paths for five subglacial rhyolitic edifices within the torfajokull central volcano formed in contrasting styles of eruption under ice 400 m thick this includes the products of the largest known eruption of icelandic subglacial rhyolite of 16 km 3 at ca 70 ka we find pre eruptive water contents in melt inclusions h 2 o mi of up to 4 8 wt which indicates that icelandic rhyolite can be significantly more volatile rich than previously thought our results indicate that explosive subglacial rhyolite eruptions correspond with high h 2 o mi closed system degassing and rapid magma ascent whereas their effusive equivalents have lower h 2 o mi and show open system degassing and more sluggish ascent rates volatile controls on eruption style thus appear similar to those for subaerial eruptions suggesting that ice plays a subsidiary role in influencing the behavior of subglacial rhyolitic eruptions

magmatic conditions and magma ascent as indicated by hornblende phase equilibria and reactions in the 1995 2002 soufriere hills magma a study of amphiboles and associated minerals in samples of soufri ere 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 5200 mm wide in the hornblende phenocrysts begin with a sharp increase in mg and si and then change gradually to a more feand 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 130mpa and 830 10 c but was variably heated before eruption the heating occurred within 30 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 855 c at a ph2o of 130mpa 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 130mpa and 830 and 860 c 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 830 860 c the average ascent rate of magma during much of the 1995 2002 eruption has been40 02 m s the rate that allows hornblende to erupt free of decompression induced reaction rims

igneous or metamorphic hornblende phenocrysts as greenschist facies reaction cells in the half dome granodiorite california the half dome granodiorite yosemite national park california is recognized in the field by euhedral fresh looking black hornblende phenocrysts up to 2 cm in length this variety of granodiorite typifies intermediate age hornblende phyric units of cretaceous nested plutonic suites in the sierra nevada batholith although only inclusions of feldspar are evident in hand samples the phenocrysts are riddled with up to 50 inclusions of every major mineral found in the host granodiorite plus metamorphic minerals formed during cooling amphibole compositions within single phenocrysts vary from actinolite with less than 1 wt al 2 o 3 to magnesiohornblende with over 8 wt elemental zoning within the amphibole is highly irregular on the micrometer scale showing patches and polygonal zones with dramatically different compositions separated by sharp to gradual transitions the chemical compositions of entire phenocrysts are equivalent to hornblende plus a small proportion of biotite suggesting that the non biotite inclusions are the result of metamorphism of the phenocrysts backscattered electron imaging shows evidence of brecciation that may have been the result of volume changes as hornblende was converted to actinolite pressure calculations using the al in hornblende barometer show unreasonably wide variations on the micrometer scale that cannot have been produced by temperature or pressure variations during crystallization these hornblende phenocrysts would thus be unsuitable for geobarometry and caution must be used to avoid similarly zoned phenocrysts in the application of the al in hornblende geobarometer

timing and climate forcing of volcanic eruptions for the past 2 500 years volcanic eruptions contribute to climate variability but quantifying these contributions has been limited by inconsistencies in the timing of atmospheric volcanic aerosol loading determined from ice cores and subsequent cooling from climate proxies such as tree rings here we resolve these inconsistencies and show that large eruptions in the tropics and high latitudes were primary drivers of interannual to decadal temperature variability in the northern hemisphere during the past 2 500 years our results are based on new records of atmospheric aerosol loading developed from high resolution multi parameter measurements from an array of greenland and antarctic ice cores as well as distinctive age markers to constrain chronologies overall cooling was proportional to the magnitude of volcanic forcing and persisted for up to ten years after some of the largest eruptive episodes our revised timescale more firmly implicates volcanic eruptions as catalysts in the major sixth century pandemics famines and socioeconomic disruptions in eurasia and mesoamerica while allowing multi millennium quantification of climate response to volcanic forcing

growth of the lava dome and extrusion rates at soufriere hills volcano montserrat west indies 2005 2008 the third episode of lava dome growth at soufriere hills volcano began 1 august 2005 and ended 20 april 2007 volumes of the dome and talus produced were measured using a photo based method with a calibrated camera for increased accuracy the total dense rock equivalent dre volume of extruded andesite magma 306 51 mm3 was similar within error to that produced in the earlier episodes but the average extrusion rate was 5 6 0 9 m3s 1 dre higher than the previous episodes extrusion rates varied in a pulsatory manner from 0 5 m3s 1 to 20 m3s 1 on 18 may 2006 the lava dome had reached a volume of 85 mm3 dre and it was removed in its entirety during a massive dome collapse on 20 may 2006 extrusion began again almost immediately and built a dome of 170 mm3 dre with a summit height 1047 m above sea level by 4 april 2007 there were few moderate sized dome collapses 1 10 mm3 during this extrusive episode in contrast to the first episode of dome growth in 1995 8 when they were numerous the first and third episodes of dome growth showed a similar pattern of low 0 5 m3s 1 but increasing magma flux during the early stages with steady high flux after extrusion of 25 mm3

behavior of halogens during the degassing of felsic magmas 1 residual concentrations of halogens f cl br i and h2o in glass matrix glass and melt inclusions have been determined in a series of volcanic clasts pumice and lava dome fragments of plinian vulcanian and lava dome forming eruptions felsic magmas from calc alkaline trachytic and phonolitic systems have been investigated montagne pelee and soufriere hills of montserrat lesser antilles santa maria santiaguito guatemala fogo azores and vesuvius italy the behavior of halogens during shallow h2o degassing primarily depends on their incompatible character and their partitioning between melt and exsolved h2o vapor however variations in pre eruptive conditions degassing kinetics and syn eruptive melt crystallization induce large variations in the efficiency of halogen extraction in all systems studied cl br and i are not fractionated from each other by differentiation or by degassing processes cl br i ratios in melt remain almost constant from the magma reservoir to the surface the ratios measured in erupted clasts are thus characteristic of pre eruptive magma compositions and may be used to trace deep magmatic processes f behaves as an incompatible element and unlike the other halogens is never significantly extracted by degassing cl br and i are efficiently extracted from melts at high pressure by h2o rich fluids exsolved from magmas or during slow effusive magma degassing but not during rapid explosive degassing because h2o and halogen mobility depends on their speciation which strongly varies with pressure in both silicate melts and exsolved fluids we suggest that the rapid pressure decrease during highly explosive eruptions prevents complete equilibrium between the diverse species of the volatiles and consequently limits their degassing conversely degassing in effusive eruptions is an equilibrium process and leads to significant halogen output in volcanic plumes

storage and interaction of compositionally heterogeneous magmas from the 1986 eruption of augustine volcano alaska compositional heterogeneity 56 64 wt sio2 whole rock in samples of tephra and lava from the 1986 eruption of augustine volcano alaska raises questions about the physical nature of magma storage and interaction beneath this young and frequently active volcano to determine conditions of magma storage and evolutionary histories of compositionally distinct magmas we investigate physical and chemical characteristics of andesitic and dacitic magmas feeding the 1986 eruption we calculate equilibrium temperatures and oxygen fugacities from fe ti oxide compositions and find a continuous range in temperature from 877 to 947 c and high oxygen fugacities nno 1 2 for all magmas melt inclusions in pyroxene phenocrysts analyzed by fourier transform infrared spectroscopy and electron probe microanalysis are dacitic to rhyolitic and have water contents ranging from 1 to 7 wt matrix glass compositions are rhyolitic and remarkably similar 75 9 76 6 wt sio2 in all samples all samples have 25 phenocrysts but lower silica samples have much higher microlite contents than higher silica samples continuous ranges in temperature and whole rock composition as well as linear trends in harker diagrams and disequilibrium mineral textures indicate that the 1986 magmas are the product of mixing between dacitic magma and a hotter more mafic magma the dacitic endmember is probably residual magma from the previous 1976 eruption of augustine and we interpret the mafic endmember to have been intruded from depth mixing appears to have continued as magmas ascended towards the vent we suggest that the physical structure of the magma storage system beneath augustine contributed to the sustained compositional heterogeneity of this eruption which is best explained by magma storage and interaction in a vertically extensive system of interconnected dikes rather than a single coherent magma chamber and or conduit the typically short repose period 10 years between augustine s recent eruptive pulses may also inhibit homogenization as short repose periods and chemically heterogeneous magmas are observed at several volcanoes in the cook inlet region of alaska

revising midlatitude summer temperatures back to a d 600 based on a wood density network annually resolved and millennium long reconstructions of large scale temperature variability are primarily composed of tree ring width trw chronologies changes in ring width however have recently been shown to bias the ratio between low and high frequency signals to overcome limitations in capturing the full spectrum of past temperature variability we present a network of 15 maximum latewood density mxd chronologies distributed across the northern hemisphere extratropics independent subsets of continental scale records consistently reveal high mxd before 1580 and after 1910 with below average values between these periods reconstructed extratropical summer temperatures reflect not only these long term trends but also distinct cooling pulses after large volcanic eruptions in contrast to trw dominated reconstructions this mxd based record indicates a delayed onset of the little ice age by almost two centuries the reduced memory inherent in mxd is likely responsible for the rapid recovery from volcanic induced cooling in the fourteenth century and the continuation of warmer temperatures until 1600

an overview of lava dome evolution dome collapse and cyclicity at soufriere hills volcano montserrat 2005 2007 1 the third episode of lava dome growth at soufriere hills volcano montserrat was characterised by higher average magma discharge rates than either previous dome growth episode at this volcano and yet fewer collapses during sustained dome growth at moderate high average rates 6 m3 s we identified 2 6 week discharge pulses that each supplied c 20 mm3 magma from depth our observations are consistent with some existing models but we explain discrepancies by a combination of higher volatile contents and higher ascent rates cycles of c 11 16 days were evident in rockfall lp rockfall and shallow lp earthquake counts related to dome growth and degassing we speculate that degassing at the conduit margins together with stick slip conduit flow may drive these cycles only one major collapse 10 mm3 occurred during the third episode on may 20 2006 as a new magma pulse entered the dome and coincided with heavy rainfall

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

magma emplacement and remobilization timescales beneath montserrat insights from sr and ba zonation in plagioclase phenocrysts received november 22 2001 accepted december 12 2002since 1995 the soufri ere hills volcano on montserrat lesserantilles island arc has erupted crystal rich andesite magmathat is geochemically very similar to the volcanic products of atleast the last 174 3kyr 1s nomarski images of plagio clase phenocrysts from six andesites erupted between 151 4ka 1s and

chlorine variations in the magma of soufriere hills volcano montserrat insights from cl in hornblende and melt inclusions the soufriere hills volcano in montserrat erupts a cl rich porphyritic andesite hcl degassing accompanies eruption and is dependent on the growth rate of the lava dome the magma contains hornblende phenocrysts that show repetitive zoning in most elements including cl on the basis of the zoning data cl oh ratios in the melt calculated from partitioning data increase rimward through each zone indicating that the phenocrysts formed under conditions of varying cl oh m an empirical relationship between a site occupancy in the hornblende and temperature implies that crystallisation of each zone is also accompanied by increasing temperature each zone ends at a resorption horizon and crystallisation recommences at lower temperature and cl oh m melt inclusion h2o and cl contents for the 8th january 2007 explosive eruption can be explained by closed system degassing with dclfl m between 5 and 30 or by open system degassing accompanied by a small amount of crystallisation however neither simple closed system degassing nor convective circulation of magma can explain the positive correlation of cl oh m with temperature we suggest that the zoning can be caused by accumulation of co2 rich vapour in the andesite probably as a result of mafic magma injection into the chamber decreasing h2o fugacity and or increasing clm result in increasing cl oh m while heat transferred with the volatiles causes the rise in temperature intermittently the accumulated fluid is lost to the surface possibly as a result of renewed eruptive activity this model requires the co2 rich fluid to be decoupled from the magma consistent with previous observations of continuous co2 emissions at the surface

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

potentially active volcanoes of peru observations using landsat thematic mapper and space shuttle imagery landsat thematic mapper tm images and space shuttle color photograph have been used to make a synoptic study of the volcanoes of southern peru 14 17 s the northernmost portin of the central volcanic zone cvz 14 28 s of the andes apart from providing consistent coverage the chief merit of the tm for this study has been the spatial resolution provided by the 30 m pixel size the optimal 20 m resolution variable lighting and viewing geometry and stereo capability of the shuttle photography provided an invaluable ancilliary data set at the resolution available subtle glacial morphological features such as valley and terminal moraines can be confidently identified and these features have been used to determine the relative ages of volcanoes volcanoes have been classified as potentially active if they have i a well preserved summit crater ii pristine lava flow texture and morphology iii flank lava flows with low albedo and iv evidence of postglacial 10 000 years activity eight major volcanoes are postulated to be potentially active most are large dominantly andesitic composite cones with edifice heights of up to 2500 m some of which threaten nearby settlements one of them sabancaya was active as recently as july 1988 other little known postglacial volcanic features include huaynaputina site of a major explosive rhyolitic eruption in 1600 a d and several fields of monogenetic scoria cones and lava flows the active volcanic front is some 200 km east of the peru chile trench and the volcanoes lie on a trenchparallel trend oblique to the ew subduction this narrow volcanic zone is thought to reflect the steep dip of the nazca plate through the zone of magma generation the break in the trend of the volcanic front in the northern extremity of the volcanic zone is thought to reflect the complexity of the crustal stress field above a major segment boundary in the subducting plate the fields of mafic monogenetic centers also occur in this region in comparison with the southern part of the cvz the general paucity of older volcanic edifices north of 17 s suggests a more recent onset of volcanism north of this latitude probably resulting from the oblique subduction of the nazca ridge and the consequent northward migration of its intersection with the pere chile trench this coupled with the lack of any large silicic caldera systems and youthful dacite domes like those found further south suggest that there are real differences between the volcanic evolution of different parts of the cvz

petrological and experimental constraints on the pre eruption conditions of holocene dacite from volcan san pedro 36 s chilean andes and the importance of sulphur in silicic subduction related magmas we present an experimental and petrological study aimed at estimating the pre eruptive conditions of a holocene dacitic lava from volcan san pedro 36 s chilean andes phase equilibrium experiments were performed at temperatures t from 800 to 950 c and mainly at 200 mpa but also at 55 150 and 406 mpa oxygen fugacity fo2 ranged from the ni nio buffer nno to 3 5 log units above nno 3 5 and water contents from 3 to 6 wt we also report several experiments where we added sulphur 0 1 1 wt s to the dacite the main mineral assemblage of the dacite hornblende orthopyroxene plagioclase is stable at 200 50 mpa 850 10 c with 4 5 5 5 wt h2o in the melt and at fo2 of nno 1 2 0 2 in accord with the crystallinity mineral proportions and t fo2 determination from fe ti oxides of the lava however biotite which is also present in the dacite is stable at these same t fo2 conditions only in experiments with 0 1 wt s added this result is in accord with the occurrence of pyrrhotite in the lava and with the presence of s in glass inclusions and biotite 300 ppm and up 170 ppm respectively moreover the zoning patterns and compositions of plagioclase phenocrysts together with the presence of high temperature minerals e g clinopyroxene in the lava suggest that the petrological history of the dacite is more complex than a single near equilibrium crystallization stage and could be explained by short lived 100 years temperature fluctuations 50 c in the magma reservoir

an experimental study of phase equilibria in the system h 2 o co 2 nacl at 800 c and 9 kbar phase equilibria in the ternary system h2o co2 nacl were studied at 800 c and 9 kbar in internally heated gas pressure vessels using a modified synthetic fluid inclusion technique the low rate of quartz overgrowth along the b and a axes of quartz crystals was used to avoid fluid inclusion formation during heating prior to attainment of equilibrium run conditions the density of co2 in the synthetic fluid inclusions was calibrated using inclusions in the binary h2o co2 system synthesised by the same method and measured on the same heating freezing stage in the two phase field two types of fluid inclusions with different densities of co2 were observed using mass balance calculations these inclusions are used to constrain the miscibility gap and the orientation of two phase tie lines in the h2o co2 nacl system at 800 c and 9 kbar the equation of state of duan et al 1995 approximately describes the p t section of the ternary system up to about 40 wt of nacl at higher nacl concentrations the measured solubility of co2 in the brine is much smaller than predicted by the eos a salting out effect must be added to the equation of state to include coulomb interaction in the model of anderko and pitzer 1993 and pitzer and jiang 1996 the new experimental data together with published data up to 5 kbar shmulovich et al 1995 encompass practically all subsolidus crustal p t conditions a feature of the new experimental results is the large compositional range in the h2o co2 nacl system occupied by the stability fields of halite co2 rich fluid h2o nacl brine the prediction of halite stability in equilibrium with co2 rich fluid in deep crustal rocks is supported by recent petrological and fluid inclusion studies of granulites

using amphibole phenocrysts to track vapor transfer during magma crystallization and transport an example from mount st helens washington abstract in order to evaluate and further constrain models for volatile movement and vapor enrichment of magma stored at shallow levels amphibole phenocrysts from 2004 2005 mount st helens dacite were analyzed for major and selected trace elements li cu zn mn and ree and li isotopes several recent studies have examined fluid mobile trace element abundances in phencryst phases and melt inclusions as a means of tracking volatile movement within subvolcanic magmatic systems and high li contents in plagioclase phenocrysts from 1980 and 2004 mount st helens dacites have been interpreted as evidence that shallow magma was fluxed by a li bearing vapor phase prior to eruption in amphibole phenocrysts zn and mn behave compatibly correlating to feo and al 2 o 3 and show no systematic change with time in contrast li and cu abundances in amphibole vary by up to 3 orders of magnitude 7 6 1140 g g and 1 7 to 94 g g respectively and do not generally correlate with either major or trace elements however they do correlate moderately well r 2 0 54 95 confidence with each other and show systematic temporal variations that are opposite to those observed for plagioclase precluding a simple 1 step diffusion model for li enrichment we propose a diffusion crystallization multi stage dcms model to explain the temporal variations and co variations of li and cu in early erupted dacite october december 2004 profiles of li isotopes in conjunction with measured 7 li intensities and core to rim increases in li concentration are characteristic of li diffusion into the amphiboles consistent with prior models of plagioclase enrichment in amphiboles from 2005 dacite average li and cu concentrations are high 260 660 g g and 29 45 g g respectively and in contrast to amphiboles from earlier erupted dacite correlate weakly with al 2 o 3 wt amphibole al 2 o 3 concentrations are an indicator of pressure with high al amphiboles crystallizing at higher pressures and we suggest that li and cu are partitioned into a fluid phase during ascent and crystallization of the magma so that amphiboles crystallizing at lower pressure have correspondingly lower li and cu concentrations however low li and cu in amphiboles from the dacite at the start of the eruption also require crystallization from a low li cu bearing melt or residence times long enough for amphiboles to re equilibrate with a li cu depleted melt estimated residence times suggest that amphiboles in early dacite could have been present since the end of the 1980 1986 eruptive episode at mount st helens

last millennium climate and its variability in ccsm4 abstractan overview of a simulation referred to as the last millennium lm simulation of the community climate system model version 4 ccsm4 is presented the ccsm4 lm simulation reproduces many large scale climate patterns suggested by historical and proxy data records with northern hemisphere nh and southern hemisphere sh surface temperatures cooling to the early 1800s common era by 0 5 c nh and 0 3 c sh followed by warming to the present high latitudes of both hemispheres show polar amplification of the cooling from the medieval climate anomaly mca to the little ice age lia associated with sea ice increases the lm simulation does not reproduce la nina like cooling in the eastern pacific ocean during the mca relative to the lia as has been suggested by proxy reconstructions still dry medieval conditions over the southwestern and central united states are simulated in agreement with proxy indicators for these regions strong global cooling is associated with large volcanic erup

magma reservoir conditions beneath tsurumi volcano sw japan evidence from amphibole thermobarometry and seismicity abstract calcic amphibole phenocrysts in the tsurumidake summit ts lava which was produced during the most recent eruption at tsurumi volcano sw japan at around 7 5 10 5 ka have been analyzed to determine the pre eruptive conditions such as temperature t pressure p oxygen fugacity fo2 sio2 content sio2melt and feo mgo ratio feo mgomelt of coexisting silicate melts in the magma reservoir beneath the volcano although most of the amphibole phenocrysts have been completely decomposed to a fine grained opaque symplectite 6 of the grains remain intact the degree of amphibole breakdown dab defined as the ratio of the area of symplectite to the area of symplectite plus relict amphibole in each phenocryst varies from 20 to 100 compositional zoning was not observed in the amphibole grains however two distinct groups of amphibole phenocrysts have been identified based on their chemical compositions group i amphiboles which are relatively si poor and 6 al rich and have a relatively high mg 100mg mg fe2 and group ii amphiboles which are si rich and 6 al poor and have a relatively low mg empirical equations for geothermobarometry oxygen barometry and chemometry that exclusively rely on the amphibole composition were applied to estimate the t p fo2 sio2melt feo mgomelt conditions of the silicate melts with which the amphibole crystals equilibrated the results show that group i and group ii amphiboles equilibrated with andesitic melts group i melts and dacitic rhyolitic melts group ii melts respectively the t p fo2 conditions of group i melts were estimated as 374 483 mpa 13 9 17 9 km depth 950 c and nno 1 3 respectively and those of group ii melts were 93 242 mpa 3 4 9 0 km depth 824 913 c and nno 0 6 1 7 respectively the estimated t p fo2 sio2melt feo mgomelt conditions were almost constant for group i melts whereas the t nno and feo mgomelt values of group ii melts decreased with increasing sio2melt the dab decreased as the temperature increased low dab grains are absent in group ii amphiboles our results suggest that 1 a deeper reservoir at a depth of 16 km stored an andesitic melt coexisting with group i amphiboles at 950 c 2 a chemically zoned reservoir at shallower depth 6 4 km was filled with dacitic rhyolitic melts that coexisted with group ii amphiboles at 824 913 c 3 the ts lava was the product of magma mixing of at least three components including the andesitic melt derived from the deeper reservoir the dacitic and rhyolitic melts from the shallower reservoir and a mafic magma derived from the lower crust and or upper mantle and 4 the ts lava eruption occurred within a month or less of the magma mixing

andesitic magma degassing investigated through h2o vapour melt partitioning of halogens at soufriere hills volcano montserrat lesser antilles magma degassing at soufriere hills volcano shv is characterised by an almost permanent so2 flux and a hcl production rate which mainly depends on dome growth rate degassing processes have been studied through textural h2o and halogen analyses of clasts collected between 1995 and 2006 on the dome and in pyroclastic flows and vulcanian eruption deposits cl br and i are strongly depleted in melts during h2o degassing with no significant cl br i fractionation whereas f is almost unaffected all magmas erupted at shv have followed a multi step degassing path from the magma chamber up to a shallow depth 1 km p 20 mpa from that depth however effusive and explosive paths are distinct vulcanian eruptions are the result of closed system degassing csd while effusive dome growth is the result of csd up to a very shallow depth 200 m p 5 2 mpa followed by open system degassing osd csd is modelled using the h2o solubility law the perfect gas law and partition coefficients of halogens between a rhyolitic melt and h2o vapour dv li gas loss characteristic of osd is modelled using a rayleigh law degassing induced crystallisation is introduced through the ratio of crystallisation and degassing rates which ranges from 150 500 dv lcl for osd ranges between 50 300 increasing with melt cl content for csd the lower effective dv lcl 20 is attributed to kinetic effects dome forming activity has a greater impact on atmospheric chemistry than vulcanian eruptions because osd is much more efficient at extracting halogens the model shows that hcl flux is a good proxy for the dome forming eruption rate comparison between model and measured gas compositions suggests a high hbr bro conversion rate bro total br 1 3 in the shv gas plume the degassing behaviour of cl br and i implies similar cl br 160 and br i 90 in initial melts volcanic clasts and high temperature gases the low cl br at shv compared to other island arcs 250 300 is attributed to a shallow pre eruptive br enrichment the almost permanent dome extrusion at shv since 1995 has likely had a significant regional atmospheric impact because of the very efficient effusive degassing and the high conversion rate of halogens into reactive species within the gas plume

h2o and halogen f cl br behaviour during shallow magma degassing processes abstract degassing paths of silicic h2o rich magmas during explosive plinian and dome forming eruptions may be represented in simple evolution diagrams relating the residual volatile content of the melt to the volume ratio of gas over melt vg vl these parameters are measurable in erupted magma fragments they are used for interpreting the h2o and halogen contents determined in a series of variously degassed volcanic clasts from the 650 y b p eruption at mt pelee martinique fwi which is characterized by the succession of a dome extrusion and a plinian activity h2o degassing of plinian pumice clasts are modeled by an equilibrium closed system evolution which allows to calculate the partition coefficients of halogen between the aqueous fluid and the rhyolitic melt d v l i 1 for f 10 1 5 for cl and 3 7 1 for br at around 900 c the strong increase of the dv li values of cl and br during dome extrusion is explained by an open system degassing model with a simultaneous crystallization of the melt some significant br enrichments relative to cl in dome fragments are attributed to interactions with an hydrothermal fluid close to seawater in composition the models allow calculation of cl and br contents of aqueous fluids generated by the different degassing paths cl contents are 0 5 mol l for closed system evolution and vary between 0 2 and 1 mol l during open system evolutions these fluids are able to extract significant amounts of chloride complexed metals and silica from the magma during dome forming eruptions they also may generate mineralization of the wall rocks or the rising dome itself e g silica precipitation which may seal the volcanic system and be responsible for the dome explosivity and the transition from dome forming to plinian eruptive style the cl br ratio of initial melts of p1 eruption 250 is much lower than the basaltic and exospheric ratios 400 suggesting that strong cl br fractionation may occur during the production or the differentiation of subduction related magmas

pre and syn eruptive geochemistry of volcanic gases from soufriere hills of montserrat west indies soufriere hills fumaroles contained magma derived volatiles before and during the eruption initiated in 1995 but also preserved a typical and quite steady hydrothermal composition chemical changes due to increased boiling and a greater input of oxidizing magmatic gas occurred only at galway s soufriere the most active fumarolic field hydrothermal buffering of the fumaroles has been favoured by their remote location 1 2 km from the eruptive vents and by a preferential degassing of the uprising magma through intrusive conduits under the crater high temperature 720 c gas collected from the extruding lava dome in feb 1996 was chemically and isotopically representative of the magmatic gas stream its composition allows assessment of average eruptive fluxes of h2o co2 and hcl which require the degassing of only 2 5 3 times more magma than erupted

reactive halogens bro and oclo detected in the plume of soufriere hills volcano during an eruption hiatus volcanic plumes are sites of dynamic chemistry involving halogen gases here we present new data on the relative abundances of so2 bro and oclo gases emitted from soufriere hills volcano shv they were collected during an eruptive hiatus but during sustained degassing at this halogen rich volcano by comparison with data from a previous study during an eruptive phase and application of the data and modeling of villemant et al 2008 we suggest that after consideration of errors either the rate of hbr conversion to bro is variable ranging from 30 to 15 and or the relative partitioning of cl and br into the gas phase from the melt changes according to eruptive activity we examine the potential implications of this for fluid melt partitioning and compare our results with data from the experimental literature our work contributes toward understanding the controls on the bro so2 ratio for volcano monitoring purposes the changes in plume chemistry with regard to bromine at the onset of lava extrusion may be large and rapid oclo was detected in the plume at shv for the first time this species has only previously been detected in emissions from mount etna using ground based methods and from puyehue cordon caulle using satellite based methods no hcho or noy species were detected in the spectra

experimental petrology of the 1991 1995 unzen dacite japan part ii cl oh partitioning between hornblende and melt and its implications for the origin of oscillatory zoning of hornblende phenocrysts high temperature pressure experiments were carried out to determine the chlorine hydroxyl exchange partition coefficient between hornblende and melt in the 1992 unzen dacite cl in hornblende and melt was analyzed by electron microprobe whereas oh in hornblende and melt was calculated assuming anion stoichiometry of hornblende and utilizing the dissociation reaction constant for h2o o 1 4 2 oh in water saturated melt respectively the partition coefficient strongly depends on the mg mg fe ratio of hornblende and is expressed as ln k1 1 4 cl oh hb cl oh melt 1 4 2 37 4 6 mg mg fe hb at 2 3 kbar and 800 850 c the twofold variation in cl content in the oscillatory zoned cores of hornblende phenocrysts in the 1991 1995 dacite cannot be explained by the dependence of the cl oh partition coefficient on the mg mg fe hb ratio and requires c 80 variation of the cl oh ratio of the coexisting melt available experimental data at 200 mpa on cl oh fractionation between fluid and melt suggest that c 1 2 1 8 wt degassing of water from the magma can explain the required 80 variation in the cl oh ratio of the melt the negative correlation between al content and mg mg fe ratio in the oscillatory zoned cores of the hornblende phenocrysts is consistent with repeated influx and convective degassing of the fluid phase in the magma chamber

amphibole perspective to unravel pre eruptive processes and conditions in volcanic plumbing systems beneath intermediate arc volcanoes a case study from ciomadul volcano se carpathians ciomadul is the youngest volcano in the carpathian pannonian region produced crystal rich high k dacites that contain abundant amphibole phenocrysts the amphiboles in the studied dacites are characterized by large variety of zoning patterns textures and a wide range of compositions e g 6 4 15 wt al2o3 79 821 ppm sr often in thin section scale and even in single crystals two amphibole populations were observed in the dacite low al hornblendes represent a cold 900 c mafic magma amphibole thermobarometry suggests that the silicic crystal mush was stored in an upper crustal storage 8 12 km this was also the place where the erupted dacitic magma was formed during the remobilization of upper crustal silicic crystal mush body by hot mafic magma indicated by simple zoned and composite amphiboles this includes reheating by 200 c and partial remelting of different parts of the crystal mush followed by intensive crystallization of the second mineral population including pargasites breakdown textures of amphiboles imply that they were formed by reheating in case of hornblendes suggesting that pre eruptive heating and mixing could take place within days or weeks before the eruption the decompression rim of pargasites suggests around 12 days of magma ascent in the conduit several arc volcanoes produce mixed intermediate magmas with similar bimodal amphibole cargo as the ciomadul but in our dacite the two amphibole population can be found even in a single crystal composite amphiboles our study indicates that high al pargasites form as a second generation in these magmas after the mafic replenishment into a silicic capture zone thus they cannot unambiguously indicate a deeper mafic storage zone beneath these volcanoes the simple zoned and composite amphiboles provide direct evidence that significant compositional variations of amphiboles do not necessarily mean variation in the pressure of crystallization even if the al tschermak substitution can be recognized suggesting that amphibole barometers that consider only amphibole composition may often yield unrealistic pressure variation

experimental constraints on the origin of chlorine emissions at the soufriere hills volcano montserrat experimental study of cl solubility in a rhyolitic melt composition corresponding to the phenocryst free matrix of the recently erupted soufriere hills andesite provides insights into the origin of cl emitted to the atmosphere during the eruption the experiments were done at 25 250 mpa 860 890 c and they involved equilibration of the estimated matrix composition 71 wt sio2 molar na2o k2o al2o3 0 6 with a cl rich aqueous fluid or aqueous fluid hydrosaline brine the cl concentrations in glasses from the experimental run products 0 48 0 68 wt cl are significantly higher than those observed in natural matrix glasses 0 20 wt cl and melt inclusions 0 30 0 07 wt cl these results indicate that if the pre eruptive montserrat magma was vapour saturated the vapour phase would have been cl poor and there was not a co existing hydrosaline brine this interpretation based on cl solubility data is supported by the observed coincidence between remote sensing measurements of cl emissions as hcl and petrological estimates of cl emissions based on the volume of magma extruded and cl contents of melt inclusions and matrix glasses with correction for groundmass crystallization during ascent the cl emissions during the soufriere hills eruption are thus proposed to simply reflect syn eruptive degassing of magma during ascent

amphibole clinopyroxene and plagioclase melt partitioning of trace and economic metals in halogen bearing rhyodacitic melts

frictional melt homogenisation during fault slip geochemical textural and rheological fingerprints abstract volcanic environments often represent structurally active settings where strain localisation can promote faulting frictional deformation and subsequent melting along fault planes such frictional melting is thermodynamically a disequilibrium process initiated by selective melting of individual mineral phases and softening of volcanic glass at its glass transition as a response to rapid frictional heating the formation of a thin melt layer on a fault plane surface can drastically accelerate or terminate slip during fault motion a comprehensive understanding of the physical and chemical properties of the frictional melt is required for a full assessment of slip mechanisms as frictional rheology depends on the contributions from selectively melted mineral and glass phases as well as the physical effects of restite fragments suspended in the frictional melt here we experimentally investigate the impact of host rock mineralogy on the compositional and textural evolution of a frictional melt during slip high velocity rotary shear hvr experiments were performed under controlled volcanically relevant coseismic conditions 1 m s 1 slip rate and 1 mpa normal stress using three intermediate dome lavas with contrasting mineral assemblages sampled from volcanic systems where fault friction is evident 1 an amphibole bearing andesite soufriere hills volcano montserrat 2 an amphibole poor dacite santiaguito dome complex guatemala and 3 an amphibole free andesite volcan de colima mexico for each sample five hvr experiments were terminated at different stages of frictional melt evolution namely 1 at the onset of melting 2 upon formation of a steady state melt layer and 3 after 5 m 4 10 m and 5 15 m of slip at steady state conditions progressive mixing and homogenisation of selective single phase melts within the frictional melt layer through double diffusion convection demonstrates the control of melt composition on slip behaviour amphiboles melted preferentially leading to consistently lower shear stress 1 mpa less than amphibole poor samples and pronounced shear weakening during the frictional melting of amphibole bearing lavas the results highlight the implications of mineral assemblages on fault slip including conduit flow processes which may influence the style of eruptions and run out distances of rapid granular flows

amphibole record of the 1964 plinian and following dome forming eruptions of shiveluch volcano kamchatka abstract shiveluch is one of the most active explosive volcanoes worldwide during the last linian eruption in 1964 and the following 1980 current time dome forming eruptions shiveluch has produced andesites and dacites sio2 60 64 wt containing variably zoned compositionally and texturally diverse amphibole phenocrysts in this work we attempt to decode the complex zoning of the amphibole crystals in the 55 year series of pumice dome rocks and mafic enclaves in order to reconstruct the most recent evolution of the volcano plumbing system the amphibole zoning in shiveluch andesites reveals correlation with the style and date of eruption high al cores mantled by low al rims in amphiboles from the 1964 plinian eruption record a drastic decrease of pressure and rapid magma ascent from the lower crust to the shallow magma chamber typically unzoned and often opacitized low al crystals from the early dome building episodes in 1980 1981 and 1993 1995 reflect magma crystallization in the shallow magma chamber complexly zoned amphiboles from andesites erupted in 2000s indicate replenishment of the shallow magma chamber with mafic magma and syn eruptive mixing processes amphibole based barometric calculations obtained by different approaches indicate that the shiveluch plumbing system is complex and comprises two mafic and silicic magma storage zones at 15 20 km and 5 6 km depths we suggest that both episodes of the plinian eruption in 1964 and the extensive dome growth in 2001 2016 were driven by influx of mafic magma in the shallow storage zone beneath shiveluch the mafic replenishment likely preceded the 1964 plinian eruption and repeatedly occurred during the period of extensive dome growth in 2001 2016 the variable styles of the recent shiveluch eruptions may be controlled by the relative volume of the mafic recharges and their thermal and viscosity effects on the efficiency of magma mixing

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

quantitative models linking igneous amphibole composition with magma cl and oh content minerals of the amphibole group are found in igneous rocks on earth and other rocky bodies since the o 3 site of amphibole can contain oh o 2 f and cl amphibole composition provides important information about water and halogen contents oxidation state and other features of its formation and alteration environments however the complexity of amphibole crystal chemistry means this information is difficult to extract furthermore it has been regular practice in the era of the electron microprobe to neglect h and fe 3 analyses for amphibole critically reducing the amount of information available in amphibole analyses in the literature we have assembled models and insights from previous work to create a methodology that allows the estimation of magmatic h 2 o and cl contents from existing amphibole analyses since the methodology requires use of a cation norm we begin with a deeper investigation of the consequences of different cation normalization schemes for amphibole analyses and provide grounds for deciding which scheme best fits a given amphibole analysis we then show how the existing model of popp and coworkers can be reversed to estimate the oh and o 3 o 2 contents of amphiboles in synthesis experiments using a synthetic data set collected from the literature 39 amphibole analyses we calibrate a partitioning model for the oh cl competition on the o 3 site of igneous amphiboles k cl x cl x oh amphibole cl oh melt ln k cl 6 59 k na a 0 679 mg 0 487 f 6 e where x signifies a mole fraction of an anion on the o 3 site cl and oh signify melt mole fractions of the anion in question on a one oxygen stolper zhang basis k na a mg and 6 fe signify the amount of each component in the amphibole in atoms per amphibole formula unit with a denoting vacancies on the amphibole a site we then combine the popp et al model with our new model to link the occupancy of the amphibole o 3 site and other crystal chemical parameters to the cl and h 2 o content of melts crystallizing amphibole the competition between oh and cl for this site can be used to calculate the melt and amphibole oh h 2 o contents as well as the speciation of fe in amphibole provided that the cl content of both the amphibole and its coexisting melt is known without analyzing either phase for h or fe 3 fe 2 while the models in this paper should be recalibrated using future experimental data this work shows that considerably more information about the volatile contents of magmas can be gleaned from amphibole than previously shown and provides additional information about the crystal chemistry of amphibole and how it affects cl partitioning into minerals of this group

excess volatiles supplied by mingling of mafic magma at an andesite arc volcano we present the results of a study of volcanic gases at soufriere hills volcano montserrat which includes the first spectroscopic measurements of the major gas species co2 and h2s at this volcano using a multisensor gas analyzer system multigas sensor the fluxes of co2 and h2s were 640 2750 t d and 84 266 t d respectively during july 2008 during a prolonged eruptive pause the flux of co2 is similar to estimates for the entire arc from previous geochemical studies while the measured h2s flux significantly alters our interpretation of the sulphur budget for this volcano the fluxes of both sulphur and carbon show considerable excesses over that which can be supplied by degassing of erupted magma we demonstrate using thermodynamic models and published constraints on preeruptive volatile concentrations that the gas composition and fluxes are best modeled by mixing between 1 gases derived from isobaric quenching of mafic magma against cooler andesite magma at depth and 2 gases derived from shallower rhyolitic interstitial melt within the porpyritic andesite the escape of deep derived gases requires pervasive permeability or vapor advection extending to several kilometers depth in the conduit and magma storage system these results provide more compelling evidence for both the contribution of unerupted mafic magma to the volatile budget of this andesitic arc volcano and the importance of the intruding mafic magma in sustaining the eruption from a broader perspective this study illustrates the importance and role of underplating mafic magmas in arc settings these magmas play an important role in triggering and sustaining eruptions and contribute in a highly significant way to the volatile budget of arc volcanoes copyright 2010 by the american geophysical union

the influence of magma ascent path on the texture mineralogy and formation of hornblende reaction rims abstract although hornblende reaction rims are widely used as a tool for evaluating magma ascent during volcanic eruptions very few studies constrain the manner in which they form this study investigates the influence of magma ascent path on the formation of hornblende reaction rims to do this we conducted isothermal 840 c decompression experiments using dacite pumice samples erupted in december 1989 from redoubt volcano alaska experiments were first held within the hornblende stability field determined through phase equilibria experiments for 3 to 5 days before being decompressed to different pressures ranging from 100 to 2 mpa for 1 to 30 days before being quenched decompression was performed in either multiple equal steps constant rate or in one single step results from multi step experiments show that reaction rims form preferentially at pressures from 10 to 40 mpa and that this favorable pressure range narrows and decreases with increased decompression duration hornblendes in multi step experiments are tightly enclosed by fine grained reaction rims composed of plagioclase and orthopyroxene crystals with high aspect ratios ranging from 1 to 12 for single step experiments reaction rims also form preferentially within a narrow pressure range 60 70 mpa where they consist of medium grained plagioclase titanomagnetite and orthopyroxene crystals with aspect ratios ranging from 1 to 6 that broadly surround subrounded hornblendes hornblendes from single step experiments dropped to lower pressures

phenocryst hosted melt inclusions record stalling of magma during ascent in the conduit and upper magma reservoir prior to vulcanian explosions soufriere hills volcano montserrat west indies the mechanics of explosive eruptions influence magma ascent pathways vulcanian explosions involve a stop start mechanism that recurs on various timescales evacuating the uppermost portions of the conduit during the repose time between explosions magma rises from depth and refills the conduit and stalls until the overpressure is sufficient to generate another explosion we have analyzed major elements cl s h2o and co2 in plagioclase hosted melt inclusions sampled from pumice erupted during four vulcanian events at soufriere hills volcano montserrat to determine melt compositions prior to eruption using fourier transform infrared spectroscopy we measured values up to 6 7 wt h2o and 80 ppm co2 of 42 melt inclusions 81 cluster between 2 8 and 5 4 wt h2o 57 to 173 mpa or 2 7 km suggesting lower conduit to upper magma reservoir conditions we propose two models to explain the magmatic conditions prior to eruption in model 1 melt inclusions were trapped during crystal growth in magma that was stalled in the lower conduit to upper magma reservoir and during trapping the magma was undergoing closed system degassing with up to 1 wt free vapor this model can explain the melt inclusions with higher h2o contents since these have sampled the upper parts of the magma reservoir however the model cannot explain the melt inclusions with lower h2o because the timescale for plagioclase crystallization and melt inclusion entrapment is longer than the magma residence time in the conduit in model 2 melt inclusions were originally trapped at deeper levels of the magma chamber but then lost hydrogen by diffusion through the plagioclase host during periodic stalling of the magma in the lower conduit system in this second scenario which we favor the melt inclusions record re equilibration depths within the lower conduit to upper magma reservoir

quantification of the intrusive magma fluxes during magma chamber growth at soufriere hills volcano montserrat lesser antilles magma fluxes in the crust control the thermal viability and mechanical stability of magma chambers we estimated the magma fluxes required to generate the negative seismic velocity anomaly observed below soufriere hills volcano montserrat growth of a magma body by accretion of andesitic sills was simulated numerically and the resulting temperatures and melt fractions were used to calculate a synthetic anomaly of seismic wave velocity which was filtered to be comparable with the velocity anomaly obtained from a tomographic experiment petrology indicates that before it was reheated remobilized and erupted the temperature of the magma residing in the chamber was about 850 c we ran simulations where convection is assumed to be low and heat transfer is mostly by conduction and simulations where convection is assumed to be vigorous enough to rapidly cool the magma chamber to 850 c in both cases magma chamber growth over the last 350 years results in tomography anomalies that are too strong unless the magma was emplaced at an unlikely low melt fraction 0 5 good fits between the modelled and the observed velocity anomaly were obtained with sills 2 5 km in radius emplaced over 6000 150 000 years depending on the temperature and melt fraction of the emplaced magma because of a trade off between intrusion dimensions and emplacement durations the volumetric magma fluxes are restricted to 7 10 4 and 5 10 3 km3 a 1 the velocity anomaly can be reproduced with a chamber containing high melt fraction magma or with a mush of crystals and melt the range of magma ages in the modelled magma chamber is much wider than the crystal residence time of the erupted andesite this suggests that the eruption taps small pockets of recently assembled magma and that the velocity anomaly is mostly due to a non eruptible mush

underestimation of volcanic cooling in tree ring based reconstructions of hemispheric temperatures a large volcanic eruption in ad 1258 1259 is expected to have caused substantial climate cooling but evidence for this effect is absent from tree ring based temperature reconstructions numerical modelling of tree growth shows that the lack of cooling is probably an artefact caused by low sensitivity to cooling in trees growing near the treeline

estimates of volcanic induced cooling in the northern hemisphere over the past 1 500 years explosive volcanism can alter global climate and hence trigger economic political and demographic change1 2 the climatic impact of the largest volcanic events has been assessed in numerous modelling studies and tree ring based hemispheric temperature reconstructions3 4 5 6 however volcanic surface cooling derived from climate model simulations is systematically much stronger than the cooling seen in tree ring based proxies suggesting that the proxies underestimate cooling7 8 and or the modelled forcing is unrealistically high9 here we present summer temperature reconstructions for the northern hemisphere from tree ring width and maximum latewood density over the past 1 500 years we also simulate the climate effects of two large eruptions in ad 1257 and 1815 using a climate model that accounts explicitly for self limiting aerosol microphysical processes3 10 our tree ring reconstructions show greater cooling than reconstructions with lower spatial coverage and based on tree ring width alone whereas our simulations show less cooling than previous simulations relying on poorly constrained eruption seasons and excluding nonlinear aerosol microphysics our tree ring reconstructions and climate simulations are in agreement with a mean northern hemisphere extra tropical summer cooling over land of 0 8 to 1 3 c for these eruptions this reconciliation of proxy and model evidence paves the way to improved assessment of the role of both past and future volcanism in climate forcing

intermittent generation of mafic enclaves in the 1991 1995 dacite of unzen volcano recorded in mineral chemistry mafic enclaves in the 1991 1995 dacite of unzen volcano show chemical and textural variability such as bulk sio2 contents ranging from 52 to 62 wt and fine to coarse grained microlite textures in this paper we investigated the mineral chemistry of plagioclase and hornblende microlites and distinguished three enclave types type i mafic enclaves contain high mg plagioclase and low cl hornblende as microlites whereas type iii enclaves include low mg plagioclase and high cl hornblende type ii enclaves have an intermediate mineral chemistry type i mafic enclaves tend to show a finer grained matrix have slightly higher bulk rock sio2 contents 56 60 wt when compared with the type iii mafic enclaves sio2 53 59 wt but the overall bulk enclave compositions are within the trend of the basalt dacite eruptive products of quaternary monogenetic volcanoes around unzen volcano the origin of the variation of mineral chemistry in mafic enclaves is interpreted to reflect different degree of diffusion controlled re equilibration of minerals in a low temperature mushy dacitic magma reservoir mafic enclaves with a long residence time in the dacitic magma reservoir whose constituent minerals were annealed at low temperature to be in equililbrium with the rhyolitic melt represent type iii enclaves in contrast type i mafic enclaves result from recent mafic injections with a mineral assemblage that still retains the high temperature mineral chemistry taking temperature ca ca na ratio of plagioclase and water activity of the hydrous unzen magma into account the mg contents of plagioclase indicate that plagioclase microlites in type iii enclaves initially crystallized at high temperature and were subsequently re equilibrated at low temperature conditions compositional profiles of mg in plagioclase suggest that older mafic enclaves type iii had a residence time of 100 years at 800 c in a stagnant magma reservoir before their incorporation into the mixed dacite of the 1991 1995 unzen eruption presence of different types of mafic enclaves suggests that the 1991 1995 dacite of unzen volcano tapped mushy magma reservoir intermittently replenished by high temperature mafic magmas

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

experimental study of chlorine behavior in hydrous silicic melts chlorine solubility in silicate melts has been investigated at 830 850 plus minus 5c and at pressures ranging from 50 to 200 mpa using both natural pantellerite rhyolite phonolite and synthetic sio sub 2 al sub 2 o sub 3 k sub 2 o na sub 2 o compositions and a stated h sub 2 o nacl kcl fluid phase near 4 molal at 100 mpa cl contents in pantelleritic melts reach a solubility plateau at initial aqueous phase molality near 2 this plateau coincides with a large immiscibility gap between aqueous and chloride rich fluids with the coexisting cl saturated aqueous phase cl ranges from 2 720 plus minus 120 ppm in rhyolite to 8 960 plus minus 85 ppm in pantellerite and reaches 6 270 plus minus 170 ppm in phonolite at 100 mpa between 50 and 200 mpa the cl content in pantelleritic melt decreases from 9 640 plus minus 200 ppm to 5 040 plus minus 150 ppm although cl solubility increases with increasing feo sup in high sio sub 2 melts it is mainly controlled by the al si and na k al molar ratios of the melt with a minimum at na k al 1 in a series of synthetic rhyolitic to pantelleritic melts the experimental results suggest that chlorine occurs as alkali chloride complexes in high sio sub 2 more melts they also indicate that cl is concentrated in the aqueous fluids in equilibrium with sio sub 2 rich melts the exact value of d depending on melt composition and melt chlorine concentration volcanic degassing will create chlorine rich hydrothermal fluids and decrease chlorine melt content less

pre eruptive volatile content degassing paths and depressurisation explaining the transition in style at the subglacial rhyolitic eruption of dalakvisl south iceland many volcanoes express a variety of contrasting eruptive styles with behavioural transitions between explosive and effusive activities understanding eruptive mechanisms is of key importance but we currently have little insight into the behavioural controls for rhyolitic eruptions beneath ice this paper addresses a subglacial rhyolitic eruption at dalakvisl in south iceland that involved both explosive and effusive activities we present measurements of pre and post eruptive magmatic volatile contents that we use to track magma degassing from the chamber to the surface and reconstruct pressure conditions during the eruption explosive and effusive phases involve significant differences in 1 pre eruptive volatile contents 2 the openness of magma degassing and 3 inferred confining pressure this final factor is potentially associated with a jokulhlaup induced decompression due to the combination of these factors dalakvisl experienced a change from effusive to explosive behaviour although we cannot currently resolve cause from effect our work points towards complex interactions between the dynamics of magma ascent and the response of overlying ice and meltwater during rhyolitic eruptions beneath ice

evaluation of gases condensates and so2 emissions from augustine volcano alaska the degassing of a cl rich volcanic system after the march april 1986 explosive eruption a comprehensive gas study at augustine was undertaken in the summers of 1986 and 1987 airborne cospec measurements indicate that passive so2 emission rates declined exponentially during this period from 380 45 metric tons day t d on 7 24 86 to 27 6 t d on 8 24 87 these data are consistent with the hypothesis that the augustine magma reservoir has become more degassed as volcanic activity decreased after the spring 1986 eruption gas samples collected in 1987 from an 870 c fumarole on the andesitic lava dome show various degrees of disequilibrium due to oxidation of reduced gas species and condensation and loss of h2o in the intake tube of the sampling apparatus thermochemical restoration of the data permits removal of these effects to infer an equilibrium composition of the gases although not conclusive this restoration is consistent with the idea that the gases were in equilibrium at 870 c with an oxygen fugacity near the ni nio buffer these restored gas compositions show that relative to other convergent plate volcanoes the augustine gases are very hcl rich 5 3 6 0 mol hcl s rich 7 1 mol total s and h2o poor 83 9 84 8 mol h2o values of d and 18o suggest that the h2o in the dome gases is a mixture of primary magmatic water pmw and local seawater part of the cl in the augustine volcanic gases probably comes from this shallow seawater source additional cl may come from subducted oceanic crust because data by johnston 1978 show that cl rich glass inclusions in olivine crystals contain hornblende which is evidence for a deep source 25km for part of the cl gas samples collected in 1986 from 390 642 c fumaroles on a ramp surrounding the inner summit crater have been oxidized so severely that restoration to an equilibrium composition is not possible h and o isotope data suggest that these gases are variable mixtures of seawater fmw and meteoric steam these samples are much more h2o rich 92 97 h2o than the dome gases possibly due to a larger meteoric steam component the 1986 samples also have higher cl s s c and f cl ratios which imply that the magmatic component in these gases is from the more degassed 1976 magma thus the 1987 samples from the lava dome are better indicators than the 1986 samples of degassing within the augustine magma reservoir even though they were collected a year later and contain a significant seawater component future gas studies at augustine should emphasize fumaroles on active lava domes condensates collected from the same lava dome fumarole have enrichments ot 107 102 in cl br f b cd as s bi pb sb mo zn cu k li na si and ni lower temperature 200 650 c fumaroles around the volcano are generally less enriched in highly volatile elements however these lower termperature fumaroles have higher concentration of rock forming elements probably derived from the wall rock

temperature covariance in tree ring reconstructions and model simulations over the past millennium spatial covariance in the simulated temperature evolution over the past millennium has been reported to exceed that of multiproxy based reconstructions here we use tree ring based temperature reco

tracing the evolution and distribution of f and cl in plutonic systems from volatile bearing minerals a case study from the liujiawa pluton dabie orogen china fluorine and chlorine play an important role in magmatic differentiation hydrothermal alteration and related mineralization processes but tracing their evolution in magmatic and especially plutonic systems is not an easy task the f and cl in melts can be estimated from f and cl concentrations in minerals provided that partitioning between minerals and melts are constrained based on available partitioning models between mineral melt mineral fluid and melt fluid a set of equations has been derived to determine f and cl concentrations in melts from the compositions of amphibole biotite and apatite the new calculation procedure has been applied to a plutonic system the liujiawa pluton eastern dabie orogen china cl and f concentrations in amphiboles biotites and apatites from different rock types gabbronorite two pyroxene diorite clinopyroxene diorite and hornblende gabbro have been determined by electron microprobe most amphiboles show a negative correlation between log cl oh and mg number and a positive correlation between log f oh and a site occupation biotites from the gabbronorite and two pyroxene diorite show a slight positive correlation between log cl oh and mg which is however not the case for the clinopyroxene diorite apatites from all the samples are rich in f and show negative correlations between cl and f concentrations in our case study we demonstrate that the cl concentration in melt remains approximately constant at 1 000 2 000 ppm over the major crystallization interval but decreases strongly at near solidus temperatures as a result of fluid exsolution the f concentration in melt remains nearly constant at ca 2 000 3 000 ppm at high temperatures as well as near solidus conditions indicating that it is not largely affected by fluid exsolution because of its strongly preferred incorporation into melt interestingly the evolution of cl and f concentrations in melt with magmatic differentiation is similar to that determined in volcanic systems suggesting that the evolution of cl and f in melts during crystallization and late magmatic stages at depth plutonic systems is similar to that observed in volcanic systems during decompression and degassing

hornblende dehydration reactions during magma ascent at soufriere hills volcano montserrat hornblende phenocrysts in recent andesites of the soufriere hills volcano display reaction rims of microcrystalline plagioclase pyroxene fe oxides and interstitial glass formed by decompression during magma ascent mass balance calculations give the following reactions with mineral proportions in agreement with modal abundances begin aligned 100 0 text hbl to 49 8 text cpx 27 6 text opx 18 2 text plag 4 5 text ti mag 100 0 text hbl to 53 9 text cpx 24 4 text opx 18 1 text plag 1 9 text ilm 1 7 text mag end aligned these reactions require an open chemical system with exchange of selected components with surrounding melt volatiles tio2 and alkalis are expelled and sio2 and feot are consumed matrix glasses fall into two compositional groups glasses in pumice are relatively rich in cao and poor in k2o and na2o compared to glasses in dome samples the former glasses formed by moderate amounts of groundmass crystallisation of plagioclase associated with rapid magma ascent in explosive eruptions the later glasses evolved in response to hornblende breakdown groundmass crystallisation and mixing of melts from different levels during slow magma ascent and extended residence time in the dome interstitial glass compositions in reaction rims reflect the compositions of the surrounding matrix glasses but show variable compositional differences mostly consistent with the proposed open system reactions

a simple magma mixing model for sulphur behaviour in calc alkaline volcanic rocks mineralogical evidence from mount pinatubo 1991 eruption the increasing recognition of anhydrite as a primary magmatic mineral in volcanic products including the important 1991 pinatubo eruption places new emphasis on the cycling of sulphur from deep sources including the upper mantle we present a simple model for the supersaturation of sulphur during mingling and mixing in calc alkaline magmas due to the quenching and oxidation of the mafic end member which may have important consequences for predicting the annual flux of igneous sulphur this model predicts the coexistence of anhydrite and sulphur dioxide gas and explains the discrepancy between erupted magma volumes and sulphur gas emissions although sulphur may exist as a number of species in volcanic emissions one of its most important phases in volcanic rocks is primary anhydrite cas0 4 a number of important historic eruptions of calc alkaline volcanoes have now been found to involve anhydrite bearing magmas these include mount lamington in papua new guinea arculus et al 1983 nevado del ruiz in colombia fournelle 1990 and el chichon in mexico rose et al 1984 the ruiz eruption involved documented magma mixing and both the nevado del ruiz and el chichon eruptions were associated with very high sulphur emissions both approximately 3 3 x 10 6 tonnes krueger 1983 williams et al 1990 we have observed magma mixing textures in detailed studies of two neighbouring calc alkaline volcanoes in northern chile at lascar matthews 1992 and tumisa gardeweg 1988 anhydrite has been found associated with these textures in pumice from lascar volcano both anhydrite and similar disequilibrium textures indicative of magma

h2o in rhyolitic glasses and melts measurement speciation solubility and diffusion dissolved h2o in silicate melts and glasses plays a crucial role in volcanic eruptions on terrestrial planets and affects glass properties and magma evolution in this paper major progress on several aspects of the h2o melt or glass system is reviewed consistency among a variety of data is investigated discrepancies are evaluated and confusion is clarified on the infrared measurement of total h2o and species concentrations calibration for a variety of glasses has been carried out at room temperature the measurements for h2o in rhyolitic glasses have undergone the most scrutiny resulting in the realization that absorptivities for the near infrared bands depend on total h2o content although the variation of the absorptivities does not seem to significantly affect the determination of total h2o it does affect the determination of molecular h2o and oh species concentrations calibration of the infrared technique for h2o in rhyolitic glasses still needs much improvement especially at high total h2o furthermore it is now almost certain that the molar absorptivities also depend on the measurement temperature in in situ studies hence it will be necessary to carry out calibrations in situ at high temperatures on h2o speciation results from two experimental approaches the quench technique and the in situ technique are very different leading to controversy in our understanding of true speciation a solution is presented to reconcile this controversy it is almost certain that the quench technique does not suffer from a quench problem but interpretation of in situ results suffered from ignoring the dependence of the molar absorptivities on measurement temperature accurate calibration at high temperatures is necessary for the quantitative application of the in situ technique to h2o speciation in silicate melts and glasses on h2o solubility in silicate melts recent experimental work has significantly expanded the t p range of solubility measurements and recent solubility models fill a gap for predicting solubility for a wide range of melt compositions i present a solubility model for rhyolitic and quasi rhyolitic melts over a wide range of t and p 500 1350 c 0 8 kbar by incorporating the role of speciation the solubility model is able to recover the experimental solubility data and has extrapolative value although the partial molar volume of h2o derived from the solubility model differs from that derived from density measurements on h2o diffusion recent studies on h2o diffusion in a quasi rhyolitic melt at 800 1200 c 0 5 5 kbar and up to 7 total h2o not only provide important new diffusion data but are also challenging earlier understanding of h2o diffusion based on data in rhyolitic glasses at 400 550 c 1 bar and 0 2 1 8 total h2o a comparison between the earlier model and recent data is made the recent high temperature diffusivities at total h2o 2 can be predicted by the earlier model however at higher total h2o the earlier model fails new work is under way to understand the diffusion mechanisms at high h2o contents

the isotope mass effect on chlorine diffusion in dacite melt with implications for fractionation during bubble growth abstract previous experimental studies have revealed that the difference in diffusivity of two isotopes can be significant in some media and can lead to an observable fractionation effect in silicate melts based on isotope mass here we report the first characterization of the difference in diffusivities of stable isotopes of cl 35 cl and 37 cl using a piston cylinder apparatus we generated quenched melts of dacitic composition enriched in cl from these we fabricated diffusion couples in which cl atoms were induced to diffuse in a chemical gradient at 1200 to 1350 c and 1 gpa we analyzed the run products by secondary ion mass spectrometry sims for their isotopic compositions along the diffusion profiles and we report a diffusivity ratio for 37 cl 35 cl of 0 995 0 001 0 09 0 02 no significant effect of temperature on the diffusivity ratio was discernable over the 150 c range covered by our experiments the observed 0 5 difference in diffusivity of the two isotopes could affect our interpretation of isotopic measurements of cl isotopes in bubble bearing or degassed magmas because bubble growth is regulated in part by the diffusive supply of volatiles to the bubble from the surrounding melt through numerical simulations we constrain the extent of cl isotopic fractionation between bubble and host melt during this process bubble growth rates vary widely in nature which implies a substantial range in the expected magnitude of isotopic fractionation but plausible growth scenarios lead to cl isotopic fractionations up to about 5 enrichment of 35 cl relative to 37 cl in the bubble this effect should be considered when interpreting cl isotopic measurements of systems that have experienced vapor exsolution

volatile variations in magmas related to porphyry cu au deposits insights from amphibole geochemistry duolong district central tibet ore forming fluid exsolution in a shallow magma chamber is a critical step in the formation of porphyry cu mo au deposits but one for which direct evidence is rarely found here ci abundance major trace element content and h isotope analysis of amphibole in diorite and barren granodiorite porphyry samples from the duolong porphyry epithermal cu au district are presented in order to reveal processes associated with fluid exsolution and magma evolution low al mg hornblende formed in ore bearing diorite at 750 860 degrees c and similar to 80 200 mpa on primitive mantle normalized diagrams these low al amphibole grains have slightly convex upward ree patterns with distinctly negative eu anomalies and negative anomalies in pb sr eu zr hf and ti anomalies suggesting crystallization from the same arc magma after plagioclase and magnetite crystallization a large variation in estimated melt h2o similar to 6 to 3 wt and ci content 0 09 0 38 wt as well as low 8d values 103 to 113 o indicate that the magma underwent large scale fluid exsolution contributing to the formation of duolong cu au mineralization additionally amphibole from a barren granodiorite porphyry shows two distinct populations distinguished by their a1 content this intrusion has low al amphibole mghornblende which formed at similar conditions to low al amphiboles in the diorite 790 870 degrees c and similar to 100 230 mpa whereas high al amphibole tschermakite formed at similar to 880 970 degrees c and degrees 210 400 mpa some high al amphibole phenocrysts have slightly convex ree patterns with no negative eu anomalies a depletion in nb zr and hf and positive sr ba and pb anomalies likely consistent with amphibole crystallization from more mafic basaltic andesitic melts low al amphibole in the granodiorite porphyry shows different compositional trends compared to those in the diorite suggesting that they crystallized from different magmas considering the evidence for mafic magma replenishment suggested by the positive correlation between al iv and mg values in high al amphibole the low al amphibole in the barren granodiorite porphyry likely crystallized from a new hybrid magma with mafic to intermediate felsic magma compositions meanwhile smaller variations in ci content 0 08 0 24 wt in low al amphibole was dominantly controlled by magma evolution shown by variable mg rather than fluid exsolution thus the low 8d values 102 to 122 o were likely inherited from evolved dioritic magmas that underwent fluid exsolution importantly the ci compositional variation of amphibole has a potential application as a powerful tool to identify ore bearing and barren intrusions within porphyry cu mo au deposits

water co2 cl and f in melt inclusions in phenocrysts from three holocene explosive eruptions crater lake oregon rare melt inclusions 100 m in diameter trapped near the boundaries of corroded patchy zones in plagioclase phenocrysts from plinian pumice of three holocene eruptions were analyzed by ir spectroscopy for molecular h 2o oh groups and co 2 and by electron microprobe for cl and f the three rhyodacitic eruptions each of which began with aplinian phase occurred over 200 yr the llao rock and cleetwood eruptions ended with degassed lava flows and the subsequent climactic eruption with voluminous ignimbrite groundmass glass and melt inclusion compositions anhydrous are similar inclusions with total h 2o concentrations of 1 2 3 4 wt are adjacent to fractures or are hourglass inclusions suggesting partial degassing caused by depressurization melt inclusions in phenocrysts in climactic ignimbrite may have vesiculated for the same reason intact inclusions in plinian pumices have total h 2o concentrations believed to represent magmatic h 2o contents wt llao rock 5 3 5 3 cleetwood 3 8 4 7 climactic 3 9 0 2 l n 6 ratios of or molecular h 2o indicate closure temperatures of 200 500oc that reflect syn or posteruptive cooling co 2 concentrations are 25 ppm h 2o and co 2 concentrations indicate saturation pressures of 1 0 1 8 kbar or depths 5 km six inclusions from climactic pumice average 400 40 l ppm f 1880 70 ppm cl f and cl concentrations of the other samples are similar and not well correlated with each other or with total h 2o location of melt inclusions near boundaries of patchy zones which are mantled by oscillatory zoned overgrowths suggests that their h 2o concentrations represent magmatic values significantly before eruption although mean h 2o concentrations of analyzed melt inclusions decrease for the three successive eruptions it is not certain that this indicates a corresponding change in magmatic h 2o content during the interval between eruptions

magnetite bubble aggregates at mixing interfaces in andesite magma bodies magnetite is a particularly favourable site for heterogeneous bubble nucleation in magma and yet only very rarely is evidence for this preserved due to the myriad of processes that act to overprint such an association the possibility of bubble magnetite aggregates in magmas carries with it interesting implications for the fluid mechanics of magma bodies and for the magma mixing process responsible for the formation of andesites we use image analysis and statistical methods to illustrate a spatial association between magnetite and bubbles in mafic enclaves there is a large range in magnetite contents in the enclaves up to 7 5 which is related to the porosity of the enclaves indicating a mechanism of enrichment of the mafic magma in magnetite in the andesite there is no spatial association between bubbles and magnetite and the magnetite content of the andesite is small we suggest a mechanism for enclave formation whereby in vapour saturated magma bubbles nucleate on magnetite upon intrusion into the base of an andesite magma body these bubble magnetite aggregates rise and sweep up other magnetites resulting in the accumulation of aggregates at the magma interface instabilities lead to the flotation of enclaves characterized by enrichment in magnetite and bubbles

the formation environment of potassic chloro hastingsite in the nakhlites mil 03346 and pairs and nwa 5790 insights from terrestrial chloro amphibole potassic chloro hastingsite has been found in melt inclusions in mil 03346 its paired stones and nwa 5790 it is some of the most chlorine rich amphibole ever analyzed in this article we evaluate what crystal chemistry terrestrial analogs and experiments have shown about how chlorine dominant amphibole chloro amphibole forms and apply these insights to the nakhlites chloro amphibole is rare with about a dozen identified localities on earth it is always rich in potassium and iron and poor in titanium in terrestrial settings its presence has been interpreted to result from medium to high grade alteration 400 c of a protolith by an alkali and or iron chloride rich aqueous fluid ferrous chloride fluids exsolved from mafic magmas can cause such alteration as can crustal fluids that have reacted with rock and lost h2o in preference to chloride resulting in concentrated alkali chloride fluids in the case of the nakhlites an aqueous alkali ferrous chloride fluid was exsolved from the parental melt as it crystallized this aqueous chloride fluid itself likely unmixed into chloride dominant and water dominant fluids chloride dominant fluid was trapped in some melt inclusions and reacted with the silicate contents of the inclusion to form potassic chloro hastingsite

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

caught in the act implications for the increasing abundance of mafic enclaves during the recent eruptive episodes of the soufriere hills volcano montserrat 1 an exceptional opportunity to sample several large blocks sourced from the same region of the growing soufriere hills lava dome has documented a significant increase in the presence of mafic enclaves in the host andesite during the course of a long lived eruptive episode with several phases in 1997 phase i mafic inclusions comprised 1 volume percent of erupted material in 2007 phase iii deposits their volumetric abundance increased to 5 7 percent a broader range of geochemically distinctive types occurs amongst the 2007 enclaves crystal poor enclaves generally have the least evolved basaltic compositions porphyritic enclaves represent compositions intermediate between basaltic and andesitic compositions the absence of porphyritic enclaves prior to phase iii magmatism at soufriere hills volcano suggests that a mixing event occurred during the course of the current eruptive episode providing direct evidence consistent with geophysical observations that the system is continuously re invigorated from depth

discussion on magma storage region processes of the soufriere hills volcano montserrat geological society london memoirs 39 361 381 the recent study by devine rutherford 2014 magma storage region processes of the soufriere hills volcano montserrat published as chapter 19 of the geological society of london memoir volume 39 focused primarily on updating the record of magmatic temperatures recorded by fe ti oxides in the andesite erupted from soufriere hills volcano however a key result of the paper was that the compositions of some plagioclase hosted melt inclusions indicate mixing of mafic magma components into the host andesite and trapping of the mixed melts within phenocrysts this interpretation is consistent with our earlier work on the chemistry of melt inclusions humphreys et al 2010 and of microlite crystal populations humphreys et al 2009 a 2013 humphreys et al 2010 proposed this interpretation by showing that a subset of soufriere hills melt inclusions and matrix glasses has anomalous concentrations of k2o and or tio2 as have mafic inclusion matrix glasses see humphreys et al 2010 fig 2 however in their appendix devine rutherford 2014 suggest that enrichment in k2o can result only from decompression crystallization and not from magma mingling they also assert that our melt inclusion dataset is fundamentally flawed and subject to faulty corrections for post entrapment correction and that our melt inclusions were actually matrix glasses finally devine rutherford 2014 doubt whether or not one can look at melt inclusion analyses and distinguish the effects of decompression crystallization from the effects of mingling with components derived from the injected mafic magma here we address the criticisms of the devine rutherford 2014 appendix drawing on additional published data to support our arguments and explanations we were unable to reproduce some of the key figures of devine rutherford 2014

magma storage region processes inferred from geochemistry of fe ti oxides in andesitic magma soufriere hills volcano montserrat w i 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 tio278 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

hcl emissions at soufriere hills volcano montserrat west indies during a second phase of dome building november 1999 to october 2000 hcl so2 mass ratios measured by open path fourier transform spectroscopy op ftir in the volcanic plume at soufriere hills volcano montserrat are presented for the second phase of dome building between november 1999 and november 2000 hcl so2 mass ratios of greater than 1 and hcl emission rates of greater than 400 t day 1 characterise periods of dome building for this volcano the data suggest that chlorine partitions into a fluid phase as the magma decompresses and exsolves water during ascent this is substantiated by a correlation between chlorine and water content in the melt derived from the geochemical analysis of plagioclase melt inclusion and matrix glasses from phase i and ii of dome growth the matrix glass from the november 1999 and march 2000 domes indicate an open system degassing regime with a fluid melt partition coefficient for chlorine of the order of 250 300 september 1997 glasses have higher chlorine contents and may indicate a switch to closed system degassing prior to explosive activity in september and october 1997 the op ftir hcl time series suggests that hcl emission rate is strongly related to changes in eruption rate and we infer an emission rate of over 13 5 kt day 1 hcl during a period of high extrusion rate in september 2000 a calculation of the hcl emission rate expected for varying extrusion rates from the open system degassing model suggests a hcl emission rate of the order of 1 4 kt day 1 is indicative of an extrusion rate of between 2 and 8 m3 s 1 monitoring of hcl at soufriere hills volcano provide a proxy for extrusion rate with changes in ratio between hcl and so2 occurring rapidly in the plume order of magnitude changes occur in hcl emission rates over the time scale of hours to days making these changes easy to detect during the day to day monitoring of the volcano mean water emission rates are calculated to range from 9 24 kt day 1 during dome building activity calculated from the predicted mass ratio of h2o hcl in the fluid at the surface and ftir derived hcl emission rates

microlite transfer by disaggregation of mafic inclusions following magma mixing at soufriere hills volcano montserrat the soufriere hills volcano on montserrat has for the past 12 years been erupting andesite with basaltic to basaltic andesite inclusions the andesite contains a wide variety of phenocryst textures and strongly zoned microlites analysis of minor elements in both phenocrysts and microlites allows us to put detailed constraints on their origins compositions of clinopyroxene from overgrowth rims on quartz and orthopyroxene and coarse grained breakdown rims on hornblende are identical to those from the mafic inclusions indicating that these rims form during interaction with mafic magma in contrast resorbed quartz and reversely zoned orthopyroxenes form during heating microlites of plagioclase and orthopyroxene are chemically distinct from the phenocrysts being enriched in fe and mg and al and ca respectively however microlites of plagioclase orthopyroxene and clinopyroxene are indistinguishable from the compositions of these phases in the mafic inclusions we infer that the inclusions disaggregated under conditions of high shear stress during ascent in the conduit transferring mafic material into the andesite groundmass the mafic component of the system is therefore greater than previously thought the presence of mafic derived microlites in the andesite groundmass also means that care must be taken when using this as a starting material for phase equilibrium experiments

magmatic controls on eruption dynamics of the 1950yrb p eruption of san antonio volcano tacana volcanic complex mexico guatemala abstract san antonio volcano in the tacana volcanic complex erupted 1950 yr b p with a pelean type eruption that produced andesitic pyroclastic surges and block and ash flows destroying part of the volcano summit and producing a horse shoe shaped crater open to the sw between 1950 and 800 yr b p the eruption continued with effusive andesites followed by a dacite lava flow and a summit dome all from a single magma batch all products consist of phenocrysts and microphenocrysts of zoned plagioclase amphibole pyroxene magnetite ilmenite set in partially crystallized groundmass of glass and microlites of the same mineral phases except for the lack of amphibole included in the andesitic blocks of the block and ash flow deposit are basaltic andesite enclaves with elongated and ellipsoidal forms and chilled margins the enclaves have intersertal textures with brown glass between microphenocrysts of plagioclase hornblende pyroxene and olivine and minor proportions of phenocrysts of plagioclase hornblende and pyroxene a compositional range obtained of blocks and enclaves resulted from mixing between andesite 866 c 22 and basaltic andesite enclaves 932 c 22 which may have triggered the explosive pelean eruption vestiges of that mixing are preserved as complex compositional zones in plagioclase and clinopyroxene rich reaction rims in amphibole in the andesite whole rock chemistry geothermometry experimental petrology and modeling results suggest that after the mixing event the eruption tapped hybrid andesitic magma 900 c and ended with effusive dacitic magma 825 c all of which were stored at 200 mpa water pressure a complex open system evolution that involved crustal end members best explains the generation of effusive dacite from the hybrid andesite amphibole in the dacite is rimmed by reaction products of plagioclase orthopyroxene and fe ti oxides produced by decompression during ascent amphibole in the andesite however lacks such rims because the andesite was at 866 22 c and the dacite was at 825 c the reaction rims indicate that the andesitic magma ascended at 0 023 m s 1 during the explosive phase of the eruption whereas the dacitic magma rose more slowly at 0 002 0 004 m s 1

igneous and shock processes affecting chassignite amphibole evaluated using chlorine water partitioning and hydrogen isotopes amphibole in chassignite melt inclusions provides valuable information about the volatile content of the original interstitial magma but also shock and postshock processes we have analyzed amphibole and other phases from nwa 2737 melt inclusions and we evaluate these data along with published values to constrain the crystallization cl and h2o content of phases in chassignite melt inclusions and the effects of shock on these amphibole grains using a model for the cl oh exchange between amphibole and melt we estimate primary crystallization oh contents of chassignite amphiboles sims analysis shows that amphibole from nwa 2737 currently has 0 15 wt h2o it has lost 0 6 wt h2o from an initial 0 7 0 8 wt h2o due to intense shock chassigny amphibole had on average 0 3 0 4 wt h2o and suffered little net loss of h2o due to shock nwa 2737 amphibole has d 3700 it absorbed martian atmosphere derived heavy h in the aftermath of shock chassigny amphibole with d 1900 incorporated less heavy h low h2o cl ratios are inferred for the primitive chassignite magma which had significant effects on melting and crystallization volatiles released by the degassing of martian magma were more cl rich than on earth resulting in the high cl content of martian surface materials

rejuvenation of the fish canyon magma body a window into the evolution of large volume silicic magma systems voluminous unzoned phenocryst rich pyroclastic deposits considered as erupted batholiths provide a unique opportunity to investigate magmatic processes in silicic magmas the fish canyon tuff a well documented example of these monotonous ignimbrites displays evidence for simultaneous dissolution of feldspars 1 quartz and crystallization of hydrous phases during gradual near isobaric reheating from 720 to 760 8c these observations along with a high crystallinity 45 and near solidus mineral assemblage suggest that the fish canyon magma cooled to a rigid crystal mush before being partly remelted prior to eruption rejuvenation was triggered by intrusion of water rich mafic magmas at the base of the fish canyon mush but the mechanisms of heat transfer remain poorly understood the growth of amphibole during reheating requires addition of mafic components but the absence of any measurable gradients and the paucity of mafic enclaves in the fish canyon magma rule out a reheating event dominated by convective mixing with a mafic magma closed system processes such as heat conduction and convective self mixing could not account for the transport of externally derived mafic components we performed numerical simulations of upward percolation of a hot low density h2o co2 fluid phase gas sparging through a crystalline framework saturated with rhyolitic melt to assess the efficiency of such a process in rejuvenating silicic mushes in open systems sparging by 20 40 km 3 of gas extracted from 3000 km 3 of mafic magma is capable of reheating 7500 km 3 of silicic crystal mush by 40 8c in 150 200 k y moreover the vertical thermal gradient after 150 k y in most of the mush is small 25 8c in the upper 65 gas sparging also produces an increase in the internal pressure of silicic crystal mushes and may lead to the formation of crystal poor rhyolites by expelling interstitial melt however our simulations predict that filter pressing driven by sparging of externally derived gas could not solely account for the generation of the most voluminous rhyolites

discrepancies between the modeled and proxy reconstructed response to volcanic forcing over the past millennium implications and possible mechanisms 1 we show that a systematic discrepancy between model simulations and proxy reconstructions of hemispheric temperature changes over the past millennium appears to arise from a small number of radiatively large volcanic eruptions past work has shown that accounting for this mismatch alone appears to reconcile inconsistencies between the overall amplitude of simulated and proxy reconstructed temperature changes we provide empirical support for the previously posited hypothesis that this discrepancy may arise from threshold growth effects in tree line proximal trees that limit their response to large volcanic cooling events such threshold responses could lead to an absence of growth rings as many as six accumulated years over the past eight centuries for some fraction of tree line proximal trees leading to a potential misalignment of volcanic cooling events in trees from climatically distinct regions and a further attenuation and smearing of the volcanic cooling signal since the high frequency component of nearly all proxy reconstructions of past hemispheric temperature change is derived from tree ring data this bias would likely impact nearly all such reconstructions we show here that the discrepancy may have led to an underestimation bias in past studies attempting to infer equilibrium climate sensitivity from proxy temperature reconstructions of the past millennium

1738 years of mongolian temperature variability inferred from a tree ring width chronology of siberian pine 1738 years ad 262 1999 of temperature variability are inferred from tree ring widths of siberian pine at solongotyn davaa sol dav a timberline 2420 m site in mongolia this chronology can account for 33 of the temperature variance from 1882 1993 the warmest conditions over the past millennium are during the 20th century the 1999 ring width has the highest index value over the past millennium both warmer and colder intervals are inferred during the medieval warm epoch the most severe cold occurred in the 19th century unusual cold and frost in ad 536 545 coincide with extremes in other proxies and historical accounts confirming a widespread catastrophic event trends resemble those of other eurasian paleoseries and hemispheric scale reconstructions over the past millennium more chronologies such as sol dav are essential to improve coverage in the uncertain earlier centuries of these reconstructions and their estimates of natural variability relative to recent anthropogenic change

a 1052 year tree ring proxy for alpine summer temperatures a june august alpine temperature proxy series is developed back to ad 951 using 1 527 ring width measurements from living trees and relict wood the reconstruction is composed of larch data from four alpine valleys in switzerland and pine data from the western austrian alps these regions are situated in high elevation alpine environments where a spatially homogenous summer temperature signal exists in an attempt to capture the full frequency range of summer temperatures over the past millennium from inter annual to multi centennial scales the regional curve standardization technique is applied to the ring width measurements correlations of 0 65 and 0 86 after decadal smoothing with high elevation meteorological stations since 1864 indicate an optimal response of the rcs chronology to june august mean temperatures the proxy record reveals warm conditions from before ad 1000 into the thirteenth century followed by a prolonged cool period reaching minimum values in the 1820s and a warming trend into the twentieth century this latter trend and the higher frequency variations compare well with the actual high elevation temperature record the new central alpine proxy suggests that summer temperatures during the last decade are unprecedented over the past millennium it also reveals significant similarities at inter decadal to multi centennial frequencies with large scale temperature reconstructions however deviating during certain periods from h h lamb s european north atlantic temperature history

fluid immiscibility in volcanic environments volatile components enter the atmosphere oceans and other surface waters through pre syn and post eruptive volcanic processes that involve a variety of fluids aluminosilicate poor fluids include aqueous or carbonic to sulfide sulfate chloride fluoride carbonate and phosphate rich compositions in volcanic environments but other more complex combinations of these constituents may be involved giggenbach 1977 roedder 1984 1992 lowenstern 1995 other rare gases and dissolved constituents e g h2 n2 he ar h3bo3 hg ch4 and other hydrocarbon compounds metals and metalloids are also present but in general they are not sufficiently concentrated to form their own phases or control bulk fluid composition and volcanic processes as a result of their diverse compositions volcanic fluids ranging from vapor to liquid exhibit widely different densities and show strong distinctions in heat capacity dielectric behavior viscosity and other chemical and physical characteristics geiger et al 2006a the density of a saline liquid in the system nacl h2o at 50 mpa and 800 c for example is more than 7 times that of the coexisting aqueous vapor e g 0 2 gm cm3 henley and mcnabb 1978 multiple fluids move mix and or unmix in magma chambers and volcanic conduits in the root zone of fumaroles and in the convective hydrothermal systems that are ubiquitous to volcanic environments hydrothermal systems are typically centered on magmatic intrusions and may include crater lakes at the top of eruptive conduits hydrothermal processes involving two or more fluid phases operate through a variety of geologically relevant shallow crustal pressure temperature composition conditions volcanic fluids occur at supersolidus to subsolidus conditions meaning that the dense aluminosilicate melt that is common to volcanism may or may not be present this chapter addresses the evidence for and occurrence of two or more low density aluminosilicate poor fluids

ascent driven crystallisation of dacite magmas at mount st helens 1980 1986 we introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system qz ab or h2o in order to relate variations in volcanic glass chemistry to changing pressure p and temperature t conditions in the sub volcanic magma system by this means an important distinction can be made between ascent driven and cooling driven crystallisation under water saturated or undersaturated conditions in samples containing feldspar and a silica phase quartz or tridymite quantitative p t estimates of the conditions of last equilibrium between crystals and melt can be made formation of highly silicic melts i e 77 wt sio2 is a simple consequence of the contraction of the silica phase volume with decreasing pressure such that high silica glasses can only form by crystallisation at low pressure resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation groundmass and inclusion glasses in dacites from the 1980 1986 eruption of mount st helens volcano wa span a wide range in sio2 68 80 wt anhydrous the compositions of the least evolved sio2 poor inclusions in amphibole phenocrysts record entrapment of silicic liquids with 5 4 wt water corresponding to a water saturation pressure of 200 mpa at 900 c the compositions of more evolved higher sio2 plagioclase hosted inclusions and groundmass glasses are consistent with extensive ascent driven fractional crystallisation of plagioclase oxide and orthopyroxene phenocrysts and microlites to low pressures during this polybaric crystallisation plagioclase phenocrysts trapped melts with a wide range of dissolved water contents 3 5 5 7 wt magmas erupted during the plinian phase of the 18 may 1980 eruption were derived from a large reservoir at depths of 6 km subsequent magmas ascended to varying depths within the sub volcanic system prior to extraction from glass chemistry and groundmass texture two arrest levels have been identified at depths of 0 5 1 and 2 4 km a single dome sample from february 1983 contains groundmass plagioclase tridymite and quartz testifying to temperatures of at least 885 c at 11 mpa these shallow storage conditions are comparable to those in the cryptodome formed during spring 1980 the corresponding thermal gradient 0 2 c mpa 1 is consistent with near adiabatic magma ascent from 8 km we argue that the crystallisation history of mount st helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation this challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980 1986 eruption because the crystallisation process is both polybaric and fractional it cannot be simulated directly using isobaric equilibrium crystallisation experiments however calculation of the phase proportions in water saturated 910 15 c experiments by rutherford et al 1985 over the pressure range 220 125 mpa reproduces the crystallisation sequence and phenocryst modes of mount st helens dacites from 18 may 1980 by allowing for the effects of fractional versus equilibrium crystallisation entrained residual source material and small temperature differences between nature and experiment phase compositions can also be matched to the natural samples we conclude that decompression of water saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres

influence of volcanic eruptions on the climate of the asian monsoon region 1 several state of the art general circulation models gcms predict that large volcanic eruptions should result in anomalous dry conditions throughout much of monsoon asia here we use long and well validated proxy reconstructions of asian droughts and pluvials to detect the influence of volcanic radiative forcing on the hydroclimate of the region since the late medieval period superposed epoch analysis reveals significantly wetter conditions over mainland southeast asia in the year of an eruption with drier conditions in central asia our proxy and model comparison suggests that gcms may not yet capture all of the important ocean atmosphere dynamics responsible for the influence of explosive volcanism on the climate of asia

mineral disequilibrium in lavas explained by convective self mixing in open magma chambers characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features including reversely zoned crystals resorption surfaces wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix these features are often interpreted as evidence of the mixing of magmas of contrasting composition temperature and origin1 2 here however we propose that such features can also be caused by convection within a magma body with a single composition that is heated from below and cooled from above we describe petrological observations of andesite lava erupted at the soufriere hills volcano montserrat which indicate a heating event and the intermingling of crystals that have very different thermal histories we present experimental data on a representative groundmass composition of this lava which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature the concept of convective self mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions

basalt unveiling fluid filled fractures inducing sediment intra void transport ephemerally examples from katla 1918 abstract this article documents textures within basaltic katla 1918 pyroclasts where particle filled fractures and bubbles have been observed these features are analogous to tuffisite veins particle filled fractures which represent the preserved remains of transient degassing pathways in shallow conduits such fractures have long been considered restricted to high viscosity silicic melts however through bse images and compositional maps we have identified similar tuffisite like features in crystal poor basalt pyroclasts from the 1918 ce subglacial eruption of katla iceland k1918 clast textures record transient mobility of juvenile lithic particles melt droplets and gas through magmatic fractures and connected vesicles key evidence includes 1 the presence of variably sintered fine ash particles within variably healed fractures and vesicles present in 80 of clasts analysed 2 compositional maps that reveal the presence of foreign particles within preserved and healed permeable pathways and 3 lower vesicularities immediately surrounding fracture walls suggestive of diffusive volatile loss into a permeable network the 1918 ce eruption of katla occurred under a thick glacier however the ice was quickly breached owing partly to the explosive nature of the eruption we propose that the formation and preservation of these transient permeable networks have been facilitated by rapid decompression of a relatively volatile rich magma in a confined subglacial environment with combined magmatic and phreatomagmatic fragmentation followed by rapid quenching by meltwater tuffisite veins in rhyolite demonstrate repeated fracture healing cycles which drive incremental release of overpressured gas and help to defuse explosive eruptions interestingly the permeable network at katla failed to defuse the 1918 ce eruption which involved a particularly violent subglacial eruptive phase it is unclear whether this demonstrates an inability of mafic tuffisite like features to efficiently degas magma perhaps owing to the especially transient nature of permeable pathways in low viscosity magmas or an ability to enhance fragmentation by providing infiltration pathways for external water the latter scenario may explain the rapid melting of the overlying glacier as the large surface area to volume ratio of fractured magma would allow rapid heat transfer nevertheless we document a previously unrecognised texture in basaltic magmas it is intriguing why it has not to the best of our knowledge been documented elsewhere have these permeable pathways been overlooked in the past e g mistaken for bad sample preparation or not noticed without high magnification bse images and are in fact a widespread phenomenon in subglacial and other basalts or do our samples in fact represent a rarely preserved texture either way they offer a new insight into the degassing and fragmentation of subglacial basalt

mafic enclaves record syn eruptive basalt intrusion and mixing mafic enclaves hosted by andesite erupted at the soufriere hills volcano between 1995 and 2010 yield insights into syn eruptive mafic underplating of an andesite magma reservoir magma mixing and its role in sustaining eruptions that may be widely applicable in volcanic arc settings the mafic enclaves range in composition from basalt to andesite and are generated from a hybrid thermal boundary layer at the interface between the two magmas where the basalt quenches against the cooler andesite and the two magmas mix we show using an analytical model that the enclaves are generated when the hybrid layer just a few tens of centimetres thick becomes buoyant and forms plumes which rise up into the andesite mafic enclave geochemistry suggests that vapour saturated basalt was underplated quasi continuously throughout the first three eruptive phases of the eruption the end member basalt became more mg and v rich over time the andesite erupted during the final phases of the eruption contained more abundant and larger enclaves and the enclaves were more extensively hybridised with the andesite suggesting that at some time during the final few years of the eruption the intrusion of mafic magma at depth ceased allowing the hybrid layer to reach a greater thickness generating larger mafic enclaves the temporal trends in mafic enclave composition and abundance suggests that basalt recharge and underplating sustained the eruption by the transfer of heat and volatiles across the interface and when the recharge ceased the eruption waned our study has important implications for the petrological monitoring of long lived arc eruptions