stratigraphy of the 1902 and 1929 nuee ardente deposits mt pelee martinique mild nuee ardentes in 1902 1903 and 1929 1930 at mt pelee formed high aspect ratio h a r deposits i e deposits with a high thickness extent ratio in the riviere blanche channel several violent nuees ardentes from may to august 1902 yielded low aspect ratio l a r deposits unevenly blanketing an area of 58 km2 on the southwestern flank of the volcano three closely spaced nuees ardentes on august 30 additionally affected 56 km2 on the southeastern flank the l a r deposits are divided into eight stratigraphic units u1 u8 either compound or consisting of a single flow unit the deposits of the largest nuees ardentes on may 8 20 and august 30 1902 are identified as u1 u3 and u8 respectively and can be traced over most of the area individual l a r flow units are lenticular and show large erratic lateral variations of thickness and grain size most flow units consist of three distinct beds bed 1 is a few centimeters thick lenticular fines depleted layer bed 2 is the thickest layer and contains more silt sized ash than bed 1 it is normally graded as a whole though reverse grading may occur in the lower part bed 2 generally consists of two parts bed 2a nonbedded and bed 2b finer grained and laminated with cross bedding and duned structures bed 3 is a thin silty capping layer which contains accretionary lapilli the distribution of the l a r flow units especially of the coarse grained facies suggests that they were formed from overpressured blast flows expanding radially from the vent at a high rate in the same manner as the laterally directed blast of mount st helens on may 18 1980 the transport system of the major 1902 nuees ardentes was an inflated relatively low concentration turbulent cloud as suggested by several lines of evidence 1 the transported material was able to cross ridges up to 300 m high 2 many scour and fill and other erosional features can be observed in the deposits at any distance from the vent 3 survivors within the devastated area on may 8 did not experience a dense low profile cloud but instead a choking hot ash laden cloud 4 many human corpses in st pierre remained where they fell instead of having floated away if the current flow surge terminology were to be used the 1902 blast flows would be better considered as pyroclastic surges the normally graded multi layered structure of the flow units is also consistent with deposits primarily formed from relatively low concentration flows

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

the 1995 1998 eruption of the soufriere hills volcano montserrat wi eruption of the soufriere hills volcano began on 18 july 1995 after three years of elevated seismic activity four months of increasingly vigorous phreatic activity culminated in mid november 1995 with the initiation of dome growth growth rates increased unevenly through early march 1996 with fluctuations on time scales from hours to months since march 1996 gravitational collapse of the unstable dome flank has affected an ever increasing area with pyroclastic flows surges and ashfalls major collapse of the eastern flank on 17 september 1996 resulted in a sub plinian explosive eruption later that day by february 1997 the dome had outgrown the confines of the crater and begun to spill into the surrounding valleys a large collapse on 25 june 1997 caused pyroclastic flows and surges on the northern flanks and resulted in the only deaths of the eruption in august september and october 1997 vulcanian explosions followed further collapses on the western and northern flanks the largest event of the eruption occurred on 26 december 1997 with failure of the southwestern flank of the volcano producing a debris avalanche and large dome collapse pyroclastic flows dome growth ceased in early march 1998 but residual volcanic activity has continued and consists of ash venting mild explosions and dome collapse pyroclastic flows

diffusion of dissolved co2 and cl in hydrous silicic to intermediate magmas the bulk diffusivity of dissolved co sub 2 in obsidian and dacite melts containing 0 11 wt dissolved water at 800 1 100c and 1 gpa was characterized using a sup 14 c radiotracer technique five values were also obtained for cl in melts containing 8 h sub 2 o carbon dioxide diffusivity d sub co sub 2 in nominally dry molten obsidian is similar to previously reported values for a simple melt in the na sub 2 o al sub 2 o sub 3 sio sub 2 system and for dry basalt indicating little effect of melt composition on bulk co sub 2 diffusion three d sub co sub 2 values for hydrous dacite melt are consistent with this lack of a melt composition effect dissolved water however causes an increase of d sub co sub 2 in molten obsidian by about an order of magnitude for each 5 of added water the activation energy of approximately 18 kcal mol is lower by a factor of approximately 3 than that for diffusion in dry basalt in comparison with existing data for diffusion of water in silicic melts at their typical liquidus temperatures d sub co sub 2 is generally lower unless the melt contains more than about 6 dissolved h sub 2 o in which case d sub co sub 2 approximately d sub h sub 2 0 the few data obtained frommore chlorine diffusion reveal the d sub cl is lower by a factor of 5 10 than d sub co sub 2 in the same melt the overall systematics of the new data on dissolved volatile diffusion indicate that diffusional fractionation effects are likely to be most significant in silicic to intermediate magmas having water contents in the 0 4 range less

emission of bromine and iodine from mount etna volcano 1 constraining fluxes of volcanic bromine and iodine to the atmosphere is important given the significant role these species play in ozone depletion however very few such measurements have been made hitherto such that global volcanic fluxes are poorly constrained here we extend the data set of volcanic br and i degassing by reporting the first measurements of bromine and iodine emissions from mount etna these data were obtained using filter packs and contemporaneous ultraviolet spectroscopic so2 flux measurements resulting in time averaged emission rates of 0 7 kt yr 1 and 0 01 kt yr 1 for br and i respectively from april to october 2004 from which we estimate global br and i fluxes of order 13 range 3 40 and 0 11 range 0 04 6 6 kt yr 1 observed changes in plume composition highlight the coherent geochemical behavior of hcl hf hbr and hi during magmatic degassing and strong fractionation of these species with respect to so2

rainwater and ash leachate analysis as proxies for plume chemistry at soufriere hills volcano montserrat chloride and sulphate concentrations in rainwater and water soluble leachates from volcanic ash samples track the compositions of gas emissions at the soufriere hills volcano montserrat from 1996 to 2001 there are both systematic spatial and temporal variations in the chloride sulphate ratio expressed as the equivalent hcl so2 mass ratio in rainwater and ash leachates temporal variations reflect changes in eruption rate and eruptive style mass ratios of hcl so2 in ash leachates correspond closely with those obtained by open path fourier transform infrared op ftir spectroscopy and reflect changes in volatile emissions throughout the eruption both leachate and op ftir spectroscopic analyses show mass ratios of hcl so2 1 during dome growth and hcl so2 1 during non eruptive periods the hcl so2 mass ratios in rainwater samples from 1996 and 1997 show temporal variations that correlate with changes in extrusion rate the hcl so2 ratios in plume affected rainwater and ash leachates from june and july 2001 correlate positively with increasing rockfall energy and with increasing eruption rate prior to a dome collapse event the hcl so2 mass ratios in water soluble ash leachates and rainwater samples collected at the same time and from the same sites are linearly correlated with rainwater hcl so2 ratios systematically two to three times higher than ash leachate ratios spatial patterns of rainwater ph and hcl so2 in rainwater and ash leachates are principally influenced by the proximity of the sampling sites to the active dome and to the typical pattern of dispersion of the plume by tropospheric winds these results demonstrate that rainwater chemistry and ash leachate analysis provides a useful indicator of volcanic activity and represents a valuable supplement to volcano surveillance efforts

petrologic constraints on the decompression history of magma prior to vulcanian explosions at the soufriere hills volcano montserrat abstract a series of 88 vulcanian explosions occurred at the soufriere hills volcano montserrat between august and october 1997 conduit conditions conducive to creating these and other vulcanian explosions were explored via analysis of eruptive products and one dimensional numerical modeling of magma ascent through a cylindrical conduit the number densities and textures of plagioclase microlites were documented for twenty three samples from the events the natural samples all show very high number densities of microlites and 50 by number of microlites have areas 2 pre explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions our comparisons suggest quench pressures 13 75 mpa each during ascent from chamber to surface values are consistent with field studies of the same events and statistical analysis of explosion time series data the microlite volume number density trend with depth reveals an apparent transition from growth dominated crystallization to nucleation dominated crystallization at pressures of 7 mpa and lower a concurrent sharp increase in bulk density marks the onset of significant open system degassing apparently due to a large increase in system permeability above 70 vesicularity this open system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to vulcanian explosions the corresponding inferred depth of overpressure at 250 700 m near the base of the dense plug is consistent with depth to center of pressure estimated from deformation measurements here we also illustrate that one dimensional models representing ascent of a degassing crystal rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis the comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open system degassing through conduit walls

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

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

using a discrete element approach to model lava dome emplacement and collapse abstract lava dome collapses can lead to explosive activity and pyroclastic flow generation which makes them one of the most deadly consequences of volcanic activity the mechanisms linked to a collapse are however still poorly understood and very few numerical models exist that investigate the actual collapse of a lava dome after emplacement we use a discrete element method implemented in the modelling software particle flow code to investigate lava dome growth but also go further to test the stability of the dome under the following conditions increased internal pressure switch in extrusion direction caused by partial cooling of the dome and extrusion of lava onto variable underlying topography we initially show the morphology development of a growing lava dome and how the rheological boundary between core and talus evolves throughout the lifetime of a dome and with varied solidus pressures through visualisation of strain accumulation within the lava dome we show superficial rockfall development due to interaction with topography whereas large deep seated failures occur when the dome is exposed to internal overpressures we find that a switch in extrusion direction promotes a transition from endogenous to exogenous dome growth and leads to lava lobe formation we demonstrate that lava dome collapse exhibits many features similar to common landslides and by investigating strain patterns within the dome we can use numerical modelling to understand features that elude field observations

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

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

evidence for a new shallow magma intrusion at la soufriere of guadeloupe lesser antilles insights from long term geochemical monitoring of halogen rich hydrothermal fluids more than three decades of geochemical monitoring of hot springs and fumaroles of la soufriere of guadeloupe allows the construction of a working model of the shallow hydrothermal system this system is delimited by the nested caldera structures inherited from the repeated flank collapse events and the present dome built during the last magmatic eruption 1530 ad and which has been highly fractured by the subsequent phreatic or phreatomagmatic eruptions because it is confined into the low volume highly compartmented and partially sealed upper edifice structure the hydrothermal system is highly reactive to perturbations in the volcanic activity input of deep magmatic fluids the edifice structure sealing and fracturing and meteorology wet tropical regime the current unrest which began with a mild reactivation of fumarolic activity in 1990 increased markedly in 1992 with seismic swarms and an increase of degassing from the summit of the dome in 1997 seismic activity increased further and was accompanied by a sudden high flux hcl rich gas from summit fumaroles we focus on the interpretation of the time series of the chemistry and temperature of fumarolic gases and hot springs as well as the relative behaviours of halogens f cl br and i this extensive geochemical time series shows that the deep magmatic fluids have undergone large changes in composition due to condensation and chemical interaction with shallow groundwater scrubbing it is possible to trace back these processes and the potential contribution of a deep magmatic source using a limited set of geochemical time series t co2 and total s content in fumaroles t and cl in hot springs and the relative fractionations between f cl br and i in both fluids coupling 35 years of geochemical data with meteorological rainfall data and models of ion transport in the hydrothermal aquifers has allowed us to identify a series of magmatic gas pulses into the hydrothermal system since the 1976 1977 crisis the contrasting behaviours of s and cl bearing species in fumarolic gas and in thermal springs suggests that the current activity is the result of a new magma intrusion which was progressively emplaced at shallow depth since 1992 although it might still be evolving the characteristics of this new intrusion indicate that it hasalready reached a magnitude similar to the intrusion that was emplaced during the 1976 1977 eruptive crisis the assessment of potential hazards associated with evolution of the current unrest must consider the implications of recurrent intrusion and further pressurization of the hydrothermal system on the likelihood of renewed phreatic explosive activity moreover the role of hydrothermal pressurization on the basal friction along low strength layers within the upper part of the edifice must be evaluated with regards to partial flank collapse at this stage enhanced monitoring research and data analysis is required to quantify the uncertainties related to future scenarios of renewed eruptive activity and magmatic evolution

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

zn isotope compositions of the thermal spring waters of la soufriere volcano guadeloupe island abstract to trace the sources and pathways of zn in hydrothermal systems the zn isotope compositions of seventeen water samples from eight thermal springs and six gas samples from two fumaroles from la soufriere an active volcano on guadeloupe island french west indies fwi were analyzed using a method adapted for purifying zn from fe and so4 enriched thermal solutions the fumaroles are enriched in zn 100 to 8000 times compared to the local bedrock and have isotopic compositions 66zn values from 0 21 to 0 35 similar to or slightly higher than fresh andesite 0 21 the enrichment of zn in the thermal springs compared with the surface waters shows that zn behaves as a soluble element during hydrothermal alteration but is significantly less mobile than na the 66zn values of most of the spring waters are relatively constant approximately 0 70 indicating that the thermal springs from la soufriere are enriched in heavy isotopes i e 66zn compared to the host rocks from 0 14 to 0 42 only three thermal springs have lower 66zn values as low as 0 43 while the zn in the fumaroles is essentially derived from magma degassing which is consistent with a previous study on merapi volcano toutain et al 2008 we show that the zn in the thermal springs is mainly derived from water rock interactions the 66zn enriched isotopic signature in most of the spring waters can be explained qualitatively by the precipitation at depth of sulfide minerals that preferentially incorporate the light isotopes this agrees with the isotopic fractionation that was recently calculated for aqueous complexes of zn the few thermal springs with lower 66zn values also have low zn concentrations indicating the preferential scavenging of heavy zn isotopes in the hydrothermal conduits this study shows that unlike chemical weathering under surface conditions hydrothermal alteration at high temperatures significantly fractionates zn isotopes and enriches thermal waters in heavy zn isotopes e g 66zn continental hydrothermal systems therefore constitute a source of heavy zn isotopes to the oceans this should be taken into account in the global oceanic budget of zn

vapor phase cristobalite as a durable indicator of magmatic pore structure and halogen degassing an example from white island volcano new zealand vesicles in volcanic rocks are physical records of magmatic degassing however the interpretation of their textures is complicated by resorption coalescence and collapse we discuss the textural significance of vesicle hosted vapor phase cristobalite high t low p sio2 polymorph and its utility as a complement to textural assessments of magmatic degassing using a representative dacite bomb erupted from white island volcano new zealand in 1999 imaging in 2d sem and 3d ct shows the bomb to have 56 bulk porosity almost all of which is connected 99 and devoid of sio2 phases the remaining 1 of porosity is in isolated sub spherical vesicles that have corroded walls and contain small 30 m across prismatic vapor phase cristobalite crystals 98 4 0 4 wt sio2 with diagnostic laser raman spectra halogen degassing models show vapor phase cristobalite to be indicative of closed system chlorine and fluorine partitioning into h2o rich fluid in isolated pores at white island this occurred during shallow 100s of meters ascent and extensive 50 groundmass crystallization associated with slow cooling in a volcanic plug pristine textures in this white island bomb demonstrate the link between pore isolation and vapor phase cristobalite deposition we suggest that because these crystals have higher preservation potential than the bubbles in which they form they can serve as durable qualitative textural indicators of halogen degassing and pre quench bubble morphologies in slowly cooled volcanic rocks e g lava flows and domes even where emplacement mechanisms have overprinted original bubble textures

stable chlorine isotope behavior during volcanic degassing of h 2 o and co 2 at mono craters ca trends in co2 and h2o concentrations and d values of obsidian clasts from mono craters volcanic field california demonstrate clear chemical and isotopic evidence for eruptive degassing however neither cl concentrations nor stable isotopes 35cl and 37cl track the degassing process which is likely because of disequilibrium due to slow diffusion of cl in the cooling melt obsidian pyroclasts n 29 were collected from tuff layers representing a single eruptive sequence that occurred circa 1340 a d as well as rhyolitic obsidian samples n 12 were collected from three high silica 74 sio2 flows forming the domes and coulees in the region the cl h2o and co2 concentrations recorded by the eruptive pyroclastic obsidians track the chemical evolution of the magmatic system during eruption whereas the concentrations of the dome samples represent the final degassed product the h2o and co2 concentrations of the pyroclastic samples range from 0 49 to 2 13 wt and 2 to 35 ppm respectively whereas concentrations in the dome glasses range from 0 17 to 0 33 wt and 1 to 3 ppm respectively h2o and co2 concentrations in the pyroclastic fall and dome samples are strongly correlated and reflect the degassing trend of the eruptive sequence chlorine concentrations of the pyroclastic fall samples and the domes range from 609 to 833 ppm and 681 to 872 ppm respectively cl concentrations do not display a strong correlation with either h2o or co2 concentrations d values of the pyroclastic fall obsidians vary between 84 and 55 whereas the d values of the dome obsidians vary between 117 and 91 d h ratios decrease with total water content following a distillation trend controlled by both closed and open system degassing 37cl values of pyroclastic fall obsidians 1 9 to 0 1 overlap with those of dome samples 1 2 to 0 0 the similar cl concentrations between the pyroclastic fall and dome obsidians argue for lack of cl degassing despite h2o and co2 loss these observations can be explained by disequilibrium effects due to the slow diffusion rate of cl compared to h2o and co2 in silicate melt buffering by a separate brine phase or by fluxing of cl from a deeper magma source with the slow diffusion rate of cl being the preferred explanation the wide range in 37cl values may be indicative of isotopic compositional heterogeneities in the magma source due to assimilation of sedimentary material or fluxing of mantle derived cl to a crustal melt

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

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

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

volcanic degassing of bromine and iodine experimental fluid melt partitioning data and applications to stratospheric chemistry abstract in order to understand the degassing behavior of heavy halogens in volcanic processes we experimentally studied the distribution of cl br and i between albite melt and hydrous fluids containing 0 01 2 wt of nacl nabr or nai respectively all experiments were carried out at 2 kbar and 900 c in rapid quench cold seal autoclaves with a run duration of 7 days the major element compositions and cl contents of the glassy run products were measured by electron microprobe bromine and iodine were measured by proton induced x ray emission fluid compositions were obtained by mass balance all halogens investigated were found to partition strongly into the fluid phase over the range of concentrations studied the halogen contents in the melt are proportional to the concentrations in the fluid the fluid melt partition coefficients df m are 8 1 0 2 for cl 17 5 0 6 for br and 104 7 for i the logarithm of df m is linearly correlated with the ionic radius of the halogenide ion on the basis of our experimental data we estimate the amount of bromine injected into the stratosphere by major volcanic explosions for the 1991 mount pinatubo eruption we obtain br yields of 11 25 kt as minimum estimates these numbers are comparable to the total annual influx of bromine into the stratosphere from all other natural and anthropogenic sources about 100 kt year since bromine is much more efficient in destroying stratospheric ozone than chlorine it could at least be partially responsible for the massive ozone depletion observed after the 1991 mount pinatubo eruption

transition from dome forming to plinian eruptive styles controlled by h2o and cl degassing the transition from a plinian pumice to an effusive dome forming eruptive style is frequently observed in volcanic systems and is generally attributed to the progressive loss of volatiles from magma stored in a superficial reservoir this explosive effusive transition has been explained by the evolution from a closed to an open system of degassing1 2 3 4 but in this context an eruption at mt pelee martinique french west indies dated at 650 years ago which exhibited a rarely observed5 6 succession from dome forming to plinian activity in a short interval of time7 is at odds with such an explanation in this eruption near surface explosions of the dome produced two peleean turbulent pyroclastic flows whose deposits are similar to those of the effusive 1902 eruption and then plinian activity produced pumice fallouts and flows the reconstruction of the degassing paths of both eruptive regimes using the densities and the h2o and cl contents of the clasts shows that the interaction of rising magma with hydrothermal fluids at shallow depth may play a critical role in determining eruptive style

modelling reactive halogen formation and ozone depletion in volcanic plumes in this paper we present a purpose built model plumechem to simulate the reactive halogen chemistry in plumes from persistently degassing volcanoes the model includes photolytic gas phase and aqueous phase chemical reactions and dispersion of the plume into the background atmosphere we investigate how high temperature chemistry close to the vent believed to enhance plume nox hox brox and clox affects bro formation in the downwind plume plumechem simulations initialised using output from thermodynamic calculations at 1000 c with atmospheric magmatic gas ratio 0 1 above the compositional discontinuity are in good quantitative agreement with observations of bro so2 and also provide a mechanism whereby clo and oclo may be formed in the plume the model reproduces the observed magnitudes and rates of increase of bro so2 at several volcanoes the hbr content of the initial plume is shown to be an important control in determining the bro so2 ratio and ozone depletion in the downwind plume the model simulates conversion of nox to hno3 and destruction of ozone in the plume providing an explanation for observed elevated hno3 in volcanic plumes and in agreement with an in plume observation of depleted ozone the predicted ozone depletion was locally and regionally significant for three different volcanic plume compositions therefore we present a new model of volcanic plume chemistry supported by observations of volcanic bro and hno3 which identifies hbr emission as a key determinant in ozone depletion by volcanoes

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

lithium isotope fractionation during magma degassing constraints from silicic differentiates and natural gas condensates from piton de la fournaise volcano reunion island abstract recent volcanic products from the piton de la fournaise volcano reunion show pronounced depletion or enrichment in lithium and significant isotopic fractionation related to degassing 1 trachytic pumices from the april 2007 eruption show extreme li depletion 90 and isotopic fractionation 7 li of 21 the depletion of water and volatiles cl f b cs in these samples suggests that li loss occurred in response to degassing which most likely occurred as the small isolated volume of magma underwent extensive differentiation near the surface because the pre degassing composition is relatively well known the composition of the degassed pumice constrains the partition coefficient to 60 v m v m to 1 010 at magmatic temperatures unlike d v m v m does not depend on whether crystallization and degassing occurred successively or concomitantly 2 basaltic samples from the interior wall of the long lived 1998 piton kapor were extensively altered by acidic gas they also show extreme li depletion but barely significant isotopic fractionation 7 li 4 5 suggesting that high temperature leaching of li by volcanic gas does not significantly fractionate li isotopes 3 high temperature 400 325 c gas condensates formed during degassing of the thick lava flow of april 2007 display high li contents 50 100 ppm which are consistent with li being as volatile as zn and sn their isotopically light li signature average of 1 7 is consistent with their derivation from isotopically heavy vapor 13 5 if the factor of isotopic fractionation between condensate and vapor is less than 0 985 a degassing crystallization model accounts for the evolution of trace species which like lithium are volatile but also moderately incompatible

injection of vesicular magma into an andesitic dome at the effusive explosive transition article i nfo a single vesicular pyroclast collected from a major 11 10 6 m 3 dome collapse eruption of the soufriere hills volcano montserrat 21st september 1997 was found to contain textural and raman evidence for multiple decompression events and magma injection into the dome just prior to the onset of dome collapse quartz plagioclase and hornblende phenocrysts contain closed and burst melt inclusions mis the latter in the form of either foam or cylindrical melt extrusions from cavities on fracture surfaces fractures within ruptured quartz phenocrysts were infilled with glass whereas those within plagioclase are entirely free of glass indicatingrelativelyearlyfracturing decompression ofthequartz thisinterpretationissupportedbyslightly higher measured h2otin burst mis in quartz 0 44 wt 0 07 1 s d compared with plagioclase 0 31 wt 0 05 1 s d indicating equilibration on rupturing of the quartz at higher ph2o h2ot levels in glass from burst mis in plagioclase are consistent with plagioclase fragmentation at h2o saturation pressures b5 mpa at an estimated depth of b210 m within the dome the fragmentation occurred whilst the groundmass glass was still plastic as evident from the presence of stretched vesicles in glass at fracture terminations the fractures in plagioclase are empty of vapour phase precipitates mainly cristobalite that infills matrix vesicles which constrains the fracturing of plagioclase to the last depressurisation event recorded in the pyroclast which was the dome collapse which led to the pyroclast being ejected from the dome we suggest that our study provides the first direct i e non seismic evidence for the injection of gas charged magma into the dome at the onset of dome collapse marking the transition from effusive to explosive eruption and that the magma injection could have provided the triggering mechanism for dome collapse

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

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

water solubility and chlorine partitioning in cl rich granitic systems effects of melt composition at 2 kbar and 800 c abstract experiments have been conducted in the system na2o k2o al2o3 sio2 h2o cl2o 1 at 2 kbar and 800 c to determine the effects of melt composition on the solubility of water in haplogranite melt and on the distribution of cl between aqueous fluid and haplogranite melt the melts studied include peraluminous subaluminous and peralkaline haplogranites and quartz albite and quartz orthoclase compositions the solubility of water at 2 kbar and 800 c in water saturated haplogranite melts varies strongly as a function of melt composition water solubility in melt decreases in the order strongly peralkaline haplogranite quartz albite composition moderately peralkaline haplogranite peraluminous haplogranite subaluminous haplogranite quartz orthoclase composition d cl wt cl in fluid wt cl in melt is a strong function of system composition d cl increases as the concentration of cl in the system increases a comparison of the distribution of cl between haplogranite melts of subaluminous peraluminous and peralkaline compositions and aqueous fluids shows that with similar concentrations of cl in the system d cl is largest for subaluminous melts a nk 1 having relatively high k2o concentrations as the a nk of melt increases or decreases from 1 or as the n nk of the melt increases both d cl and the concentration of cl in the fluid decrease the experimental data imply that complexes involving cl and na in hydrous subaluminous granite melts are dominant furthermore the data imply that in hydrous peraluminous granite melts cl complexes with network modifying a13 and in hydrous peralkaline granite melts cl complexes with network modifying na k on a molar basis cl apparently complexes with network modifying a13 in peraluminous melts as strongly as it does with na and k in peralkaline melts experimental studies of cl partitioning in granitic systems suggest that the strongest enrichment of magmatic hydrothermal fluids in cl and ore metals complexed with cl will most likely occur in fluids exsolved from subaluminous granite magmas that are relatively enriched in k2o sio2 and cl and relatively depleted in f co2 and na2o

the relationship between degassing and ground deformation at soufriere hills volcano montserrat we examine the correlations between so2 emission rate seismicity and ground deformation in the month prior to the 25 june 1997 dome collapse of the soufriere hills volcano montserrat during this period the volcano exhibited a pattern of cyclic inflation and deflation with an 8 14 h period we find that so 2 emission rates measured by cospec correlate with the amplitude of these tilt cycles and that higher rates of so 2 emission were associated with stronger ground deformation and enhanced hybrid seismicity within tilt cycles degassing peaks coincide with maximum deformation gradients increases in the amount of gas in the magma conduit feeding the dome probably due to increases in volatile content of ascending magma volume can account for the observed increases in tilt amplitude hybrid seismicity and so2 emission rate q 2000 elsevier science b v all rights reserved

transitions between explosive and effusive phases during the cataclysmic 2010 eruption of merapi volcano java indonesia transitions between explosive and effusive activity are commonly observed during dome forming eruptions and may be linked to factors such as magma influx ascent rate and degassing however the interplay between these factors is complex and the resulting eruptive behaviour often unpredictable this paper focuses on the driving forces behind the explosive and effusive activity during the well documented 2010 eruption of merapi the volcano s largest eruption since 1872 time controlled samples were collected from the 2010 deposits linked to eruption stage and style of activity these include scoria and pumice from the initial explosions dense and scoriaceous dome samples formed via effusive activity as well as scoria and pumice samples deposited during subplinian column collapse quantitative textural analysis of groundmass feldspar microlites including measurements of areal number density mean microlite size crystal aspect ratio groundmass crystallinity and crystal size distribution analysis reveal that shallow pre and syn eruptive magmatic processes acted to govern the changing behaviour during the eruption high an up to 80 mol an microlites from early erupted samples reveal that the eruption was likely preceded by an influx of hotter or more mafic magma transitions between explosive and effusive activity in 2010 were driven primarily by the dynamics of magma ascent in the conduit with degassing and crystallisation acting via feedback mechanisms resulting in cycles of effusive and explosive activity explosivity during the 2010 eruption was enhanced by the presence of a plug of cooled magma within the shallow magma plumbing system which acted to hinder degassing leading to overpressure prior to initial explosive activity

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

volcanic ash leachates a review and recommendations for sampling methods tephra in plumes can scavenge and thereby rapidly deposit volatiles including sulphur halogen and metal species these may then be leached e g by rainfall potentially releasing heavy loadings to soils and water bodies several eruptions have resulted in contamination of pasture sometimes with serious impacts on livestock water quality has also been an issue in some areas affected by tephra fall this work synthesises the literature on volcanic ash leachates and considers the controls on volatile adsorption general trends emerge for basaltic intermediate and silicic tephra as well as for variable particle size and transport distance the applications of ash leachate data to plume gas geochemistry calculation of volatile budgets and environmental impact assessment are evaluated comparisons for different eruptions are hampered by disparities in leachate analysis techniques a standardised methodology is therefore proposed to facilitate future health impact assessment and volcanological interpretation of results from different sites

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

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

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

partitioning of light lithophile elements during basalt eruptions on earth and application to martian shergottites an enigmatic record of light lithophile element lle zoning in pyroxenes in basaltic shergottite meteorites whereby lle concentrations decrease dramatically from the cores to the rims has been interpreted as being due to partitioning of lle into a hydrous vapor during magma ascent to the surface on mars these trends are used as evidence that martian basaltic melts are water rich mcsween et al 2001 lithium and boron are light lithophile elements lle that partition into volcanic minerals and into vapor from silicate melts making them potential tracers of degassing processes during magma ascent to the surface of earth and of other planets while lle degassing behavior is relatively well understood for silica rich melts where water and lle concentrations are relatively high very little data exists for lle abundance heterogeneity and degassing in basaltic melts the lack of data hampers interpretation of the trends in the shergottite meteorites through a geochemical study of lle volatile and trace elements in olivine hosted melt inclusions from kilauea volcano hawaii it can be demonstrated that lithium behaves similarly to the light to middle rare earth elements during melting magma mixing and fractionation considerable heterogeneity in lithium and boron is inherited from mantle derived primary melts which is dominant over the fractionation and degassing signal lithium and boron are only very weakly volatile in basaltic melt erupted from kilauea volcano with vapor melt partition coefficients 4 wt the observed terrestrial lle partitioning behavior is extrapolated to martian primitive melts through modeling the zoning observed in the shergottite pyroxenes is only consistent with degassing of lle from a martian melt near its liquidus temperature if the vapor melt partition coefficient was an order of magnitude larger than observed on earth the range in lle and trace elements observed in shergottite pyroxenes are instead consistent with concurrent mixing and fractionation of heterogeneous melts from the mantle

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

degassing of cl f li and be during extrusion and crystallization of the rhyolite dome at volcan chaiten chile during 2008 and 2009 we investigated the distribution of cl f li and be in pumices obsidians and crystallized dome rocks at chaiten volcano in 2008 2009 in order to explore the behavior of these elements during explosive and effusive volcanic activity electron and ion microprobe analyses of matrix and inclusion glasses from pumice obsidian and microlite rich dome rock indicate that cl and other elements were lost primarily during crystallization of the rhyolitic dome after it had approached the surface glass in pumice and microlite free obsidian has 888 121 ppm cl whereas residual glass in evolved microlite rich dome rock generally retains less cl as low as 0 7 mt cl with a potential maximum of 1 8 mt for the entire 0 8 km3 dome elemental variations reflect an integrated bulk distribution ratio for cl 1 7 1 7 times more cl was degassed or incorporated into crystals than remained in the melt because cl is lost dominantly as the very last h2o is degassed and cl is minimally if at all partitioned into microlites the integrated vapor melt distribution ratio for cl exceeds 200 200 times more cl in the evolved vapor than in the melt cl is likely lost as hcl which is readily partitioned into magmatic vapor at low pressure cl loss is accelerated by the change in the composition of the residual melt due to microlite growth cl loss also may be affected by open system gas fluxing integrated vapor melt distribution ratios for li f and be all exceed 1 000 on degassing an unknown fraction of these volatiles could be immediately dissolved in rainwater

application of refraction seismics in alpine permafrost studies a review abstract refraction seismics is a common geophysical application to investigate permafrost in alpine studies this study reviews the physical basis of the use of refractions seismics and introduces the history of its application in alpine environments seismic properties are influenced by rock or soil properties and fluid or ice effects in laboratory measurements these influences are determined and linked to mechanical properties or used for calibration of field studies in field conditions supplementary environmental factors influence the seismic properties of landforms in this paper laboratory studies of rocks from different lithologies and with different properties as well as case studies on rock glaciers moraines talus slopes and debris covered slopes are collected and influencing factors quantitatively analyzed the data demonstrate how lithology porosity and anisotropy influence p wave velocity environmental factors result in a high variation of p wave velocities in landforms however p wave velocities enable the differentiation of active layer and permafrost in rock glaciers and moraines in talus slopes and debris covered slopes p wave velocity contrasts between active layer and permafrost layer might be insufficient and require the application of additional methods for a final differentiation

geochemical and geo electrical study of mud pools at the mutnovsky volcano south kamchatka russia behavior of elements structures of feeding channels and a model of origin this study presents data on the geochemical composition of boiling mud pools at the mutnovsky volcano the physicochemical characteristics of the pools and the concentrations of major minor and trace elements in pool solutions vary widely a comparison of the geochemical compositions of host rocks and solutions indicates that leaching from rocks is not the only source of chemicals in thermal solutions geophysical studies reveal the inner structure of thermal fields which reflect the shapes of the underground reservoirs and feed channels using geophysical methods electrical resistivity tomography and frequency domain investigations it was shown that the vertical structure and complex geochemical zonation of the feed channels leads to a high contrast in the compositions of the mud solutions these findings answer questions about the origin and composition of surface manifestations to elucidate the mechanisms of solution formation an attempt was made to describe the magmatic fluid evolution and the resulting mixing of waters by physical and mathematical models the model illustrates fluid migration from a magma chamber to the surface it is shown that the formation of brines corresponding to the mud pool composition is possible during secondary boiling

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

constraints on cooling and degassing of pumice during plinian volcanic eruptions based on model calculations during explosive volcanic eruptions pumice clasts are transported into the atmosphere and their thermal and degassing histories determine how much volatiles are lost syneruptively this process is studied by combining a steady state eruption column model with a model for the cooling and degassing of pumice in the model we investigate the impact of various parameters e g mass eruption rate biot number eruption temperature and geometry on the cooling and degassing of pumice and find that the biot number and the eruption temperature are the most influential during typical risetimes of pumices inside eruption columns 200 300 s those smaller than 0 5 cm in diameter are found to lose little of their volatiles syneruptively and remain in thermal equilibrium with the plume in the case of larger pumices the ratio of the cooling timescale r p 2 where rp is the pumice radius and is the thermal diffusivity to the degassing timescale w 2 d 0 where w is the vesicle wall thickness and d 0 is the species diffusivity controls degassing if this ratio is larger than 50 degassing of the pumice will be nearly complete with a few percent of the original volatiles left in the outer rind of the pumice for a ratio of less than 0 1 no volatiles escape the pumice typical values for thermal and species diffusivity as well as vesicle wall thickness indicate that in the case of degassing cl and h 2 o the transition from 0 1 to 50 will be in the pumice size range of 1 10 cm here volatile concentration is a strong function of position inside the pumice suggesting that values for volatile contents measured on picked matrix glass from crushed samples may not be the best way to estimate the volatile content of matrix glass a number which is frequently used to estimate the total volatile input of volcanic eruptions this effect is even more pronounced when looking at hydrogen isotopic fractionation with larger pumices tending toward d 80

causes and consequences of pressurisation in lava dome eruptions abstract high total and fluid pressures develop in the interior of high viscosity lava domes and in the uppermost parts of the feeding conduit system as a consequence of degassing two effects are recognised and are modelled quantitatively first large increases in magma viscosity result from degassing during magma ascent strong vertical gradients in viscosity result and large excess pressures and pressure gradients develop at the top of the conduit and in the dome calculations of conduit flow show that almost all the excess pressure drop from the chamber in an andesitic dome eruption occurs during the last several hundred metres of ascent second microlites grow in the melt phase as a consequence of undercooling caused by gas loss rapid microlite growth can cause large excess fluid pressures to develop at shallow levels theoretically closed system microlite crystallization can increase local pressure by a few tens of mpa although build up of pressure will be countered by gas loss through permeable flow and expansion by viscous flow microlite crystallization is most effective in causing excess gas pressures at depths of a few hundred metres in the uppermost parts of the conduit and dome interior some of the major phenomena of lava dome eruptions can be attributed to these pressurisation effects including spurts of growth cycles of dome growth and subsidence sudden onset of violent explosive activity and disintegration of lava during formation of pyroclastic flows the characteristic shallow level long period and hybrid seismicity characteristic of dome eruptions is attributed to the excess fluid pressures which are maintained close to the fracture strength of the dome and wallrock resulting in fluid movement during formation of tensile and shear fractures within the dome and upper conduit

volcanic sources of tropospheric ozone depleting trace gases 1 recent investigations report mixing ratios of bro gas reaching 1 ppb in plume 4 7 km downwind of the summit of soufriere hills volcano montserrat the detection of bro in volcanic plumes is potentially important evidence of halogen catalyzed tropospheric ozone destruction and suggests that volcanoes either directly emit bro or emit bromine species that are rapidly converted to reactive bromine in volcanic plumes species distribution models constrained by volcanic gas and condensate analytical data for arc rift and hot spot volcanoes demonstrate that shallow magma degassing generates volcanic gases with ppb to ppm levels of reactive radicals br cl h and ho but no significant bro or clo the conversion of volcanic br and cl by reaction with ozone in gas phase catalytic reaction cycles during plume transport can lead to secondary bro and clo at mixing ratios of a ppt and higher possibly approaching a ppb for especially halogen rich volcanic gases several kilometers downwind of degassing sources in general however ppb bro and clo levels in volcanic plumes several kilometers downwind probably require near vent high temperature reaction of magmatic gases with air and or in plume heterogeneous chemical processes involving aerosols during plume transport these processes oxidize bromine and chlorine in hbr and hcl giving rise to increased levels of reactive br cl and ho in addition to nox they can also produce significant amounts of the photochemically active precursors br2 brcl and cl2 which are photolyzable to reactive br and cl the precursors may build up in nighttime volcanic plumes and accelerate ozone destruction and bro and clo production after sunrise downwind conversion of reactive br cl h and ho from volcanic gas sources by catalytic reaction chains to hobr and hocl may trigger heterogeneous chemical processes which are probably essential for sustaining ozone destruction in volcanic plumes gas phase reactions of hbr and hcl with ho in volcanic plumes might also boost bro and clo downwind thus in plume processes that generate reactive halogens may contribute significantly to ozone destruction in volcanic plumes enhanced levels of hox and nox in volcanic emissions may trigger nonhalogen catalytic cycles depleting ozone in tropospheric volcanic plumes

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

crystallization history of obsidian dome inyo domes california samples obtained by u s department of energy research drilling at the 600 year old obsidian dome volcano provide the rare opportunity to examine the transition from volcanic dome to plutonic intrusion textures in a silicic magma system textures in the lavas from obsidian dome record multiple periods of crystallization initiated in response to changes in undercooling t related to variable degassing in the mag ma phenocr ysts formed first at low t a drastic increase in t related to loss of a vapor phase during initial stages of eruption caused nucleation of microlites all of the lavas thus contain phenocrysts and microlites extrusion and subsequent devitrification of the dry 0 1 wt h2o magma crystallized spherulites and fine grained rhyolite at high t a granophyric texture representing crystallization at a moderate t formed in the intrusions beneath obsidian dome textures in the intrusion apparently represent crystallization of hydrous 1 2 wt h2o rhyolitic magma at shallow depths

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

chemistry of hot springs along the eastern lau spreading center abstract the eastern lau spreading center elsc is the southernmost part of the back arc spreading axis in the lau basin west of the tonga trench and the active tofua volcanic arc over its 397 km length it exhibits large and systematic changes in spreading rate magmatic tectonic processes and proximity to the volcanic arc in 2005 we collected 81 samples of vent water from six hydrothermal fields along the elsc the chemistry of these waters varies both within and between vent fields in response to changes in substrate composition temperature and pressure ph water rock ratio and input from magmatic gases and subducted sediment hot spring temperatures range from 229 to 363 c at the five northernmost fields with a general decrease to the south that is reversed at the mariner field the southernmost field vai lili emitted water at up to 334 c in 1989 but had a maximum venting temperature of only 121 c in 2005 due to waning activity and admixture of bottom seawater into the subseafloor plumbing system chloride varies both within fields and from one field to another from a low of 528 mmol kg to a high of 656 mmol kg and may be enriched by phase separation and or leaching of cl from the rock concentrations of the soluble elements k rb cs and b likewise increase southward as the volcanic substrate becomes more silica rich especially on the valu fa ridge iodine and 7li increase southward and 11b decreases as b increases apparently in response to increased input from subducted sediment as the arc is approached species that decrease southward as temperature falls are si h2s li na cl fe mn and 87sr 86sr whereas ph alkalinity ca and sr increase oxygen isotopes indicate a higher water rock ratio in the three systems on valu fa ridge consistent with higher porosity in more felsic volcanic rocks vent waters at the mariner vent field on the valu fa ridge are significantly hotter more acid and metal rich less saline and richer in dissolved gases and other volatiles including h2s co2 and f than the other vent fields consistent with input of magmatic gases the large variations in geologic and geophysical parameters produced by back arc spreading along the elsc which exceed those along mid ocean ridge spreading axes produce similar large variations in the composition of vent waters and thus provide new insights into the processes that control the chemistry of submarine hot springs