viscosity of microlite bearing rhyolitic obsidians an experimental study to investigate the influence of microlites on lava flow rheology the viscosity of natural microlite bearing rhyolitic obsidians of calc alkaline and peralkaline compositions containing 0 1 0 4 wt water was measured at volcanologically relevant temperatures 650 950 c stresses 103 105 pa and strain rates 10 5 to 10 7 s 1 the glass transition temperatures t g were determined from scanning calorimetric measurements on the melts for a range of cooling heating rates based on the equivalence of enthalpic calorimetric and shear viscosity relaxation we calculated the viscosity of the melt in crystal bearing samples from the t g data the difference between the calculated viscosity of the melt phase and the measured viscosity for the crystal bearing samples is interpreted to be the physical effect of microlites on the measured viscosity the effect of 45 vol crystals for both of these samples a connectivity factor is apparent with for the latter a close packing framework of crystals which is interpreted to influence the apparent viscosity

explosive origin of silicic lava textural and δd h2o evidence for pyroclastic degassing during rhyolite effusion a long standing challenge in volcanology is to explain why explosive eruptions of silicic magma give way to lava a widely cited idea is that the explosive to effusive transition manifests a two stage degassing history whereby lava is the product of non explosive open system gas release following initial explosive closed system degassing direct observations of rhyolite eruptions indicate that effusive rhyolites are in fact highly explosive as they erupt simultaneously with violent volcanic blasts and pyroclastic fountains for months from a common vent this explosive and effusive overlap suggests that pyroclastic processes play a key role in rendering silicic magma sufficiently degassed to generate lava here we use precise h isotope and magmatic h2o measurements and textural evidence to demonstrate that effusion results from explosion s lavas are the direct product of brittle deformation that fosters batched degassing into transient pyroclastic channels tuffisites that repetitively and explosively vent from effusing lava our measurements show specifically that d h ratios and h2o contents of a broad suite of explosive and effusive samples from chaiten volcano hydrous bombs plinian pyroclasts tuffisite veins and lava define a single and continuous degassing trend that links wet explosive pyroclasts 1 6 wt h2o d 76 4 to dry obsidian lavas 0 13 wt h2o d 145 7 this geochemical pattern is best fit with batched degassing model that comprises small repeated closed system degassing steps followed by pulses of vapour extraction this degassing mechanism is made possible by the action of tuffisite veins which by tapping already vesicular or brecciated magma allow batches of exsolved gas to rapidly and explosively escape from relatively isolated closed system domains and large tracts of conduit magma by giving them long range connectivity even though tuffisite veins render magma degassed and capable of effusing they are nonetheless the avenues of violent gas and particle transport and thus have the potential to drive explosions when they become blocked or welded shut thus the effusion of silicic lava traditionally thought to be relatively benign process presents a particularly hazardous form of explosive volcanism

small fluctuations in the density and thickness of a dry firn column it is of practical importance to have a description on time scales of 1 100 years of the relationship between the mass imbalance of an ice sheet and its rate of change of thickness in this paper a linearized treatment off the relationship is described closed form expressions are derived that relate the time variant density in an isothermal firn layer to the fluctuations in accumulation rate and density that occur at the surface these expressions are used to provide a spectral description of the contribution of surface accumulation and surface density fluctuations to the rate of change of thickness of an ice sheet using these the contribution of firn densification to the variability of ice sheet thickness is examined as a function of the time interval over which the ice sheet is observed this contribution is illustrated for sites in antarctica and greenland it is concluded that it is important to give greater attention than hitherto to the spatial scale of accumulation fluctuations if satellite observations of ice sheet elevation change are to be used to estimate ice sheet imbalance over short time intervals

d h ratios and h2o contents record degassing and rehydration history of rhyolitic magma and pyroclasts abstract volcanic eruptions of rhyolitic magma often show shifts from powerful vulcanian to plinian explosive episodes to a more gentle effusion of viscous lava forming obsidian flows another prevailing characteristic of these eruptions is the presence of pyroclastic obsidians intermingled with the explosive tephra this dense juvenile product is similar to the tephra and obsidian flow in composition but is generally less degassed than its flow counterpart the formation mechanism s of pyroclastic obsidians and the information they can provide concerning the extent to which magma degassing modulates the eruptive style of rhyolitic eruptions are currently subject to active research porous tephra and pyroclastic and flow obsidians from the 1060ce glass mountain rhyolitic eruption at medicine lake volcano california were analyzed for their porosity water content h2o and hydrogen isotopic composition d h2o in porous pyroclasts is correlated negatively with d and positively with indicating that the samples were affected by post eruptive rehydration numerical modeling suggests that this rehydration occurred at an average rate of 10 23 5 0 5 m2 s 1 during the 960 years since the eruption causing some pyroclasts to gain up to 1 wt of meteoric water pyroclastic and flow obsidians were not affected by rehydration due to their very low porosity comparison between modeled d h2o relationships in degassing magma and values measured in the glass mountain samples supports the idea that rhyolitic magma degasses in closed system until its porosity reaches a value of about 65 5 beyond which degassing occurs in open system until quench during the explosive phase rapidly ascending magma fragments soon after it becomes permeable creating porous lapilli and ash that continue to degas in open system within an expanding gas phase as suggested by recent studies some ash may aggregate and sinter on the conduit sides at different depths above the fragmentation level partly equilibrating with the continuously fluxing heavier magmatic vapor explaining the wide range of h2o contents and high variability in d measured in the pyroclastic obsidians using only h2o and d it is impossible to rule out the possibility that pyroclastic obsidians may also form by permeable foam collapse either syn explosively near the conduit sides below the fragmentation level or during more effusive periods interspersed in the explosive phase during the final effusive phase of the eruption slowly ascending magma degasses in open system until it reaches the surface creating flows with low h2o and d this study shows that h2o measured in highly porous pyroclasts of a few hundred years or more cannot be used to infer syn eruptive magma degassing pathways unless careful assessment of post eruptive rehydration is first carried out if their mechanism of formation can be better understood detailed analysis of the variations in texture and volatile content of pyroclastic obsidians throughout the explosive phase may help decipher the reasons why rhyolitic eruptions commonly shift from explosive to effusive phases

magmatic volatiles in explosive rhyolitic eruptions obsidian clasts in rhyolitic tephra deposits preserve preeruption magmatic volatile contents providing a direct means for determining the volatile content of explosively erupted magmas small to moderate volume plinian eruptions 10 sup 3 to 10 sup 1 km sup 3 appear to be driven by 0 5 1 0 wt volatiles consisting dominantly of h sub 2 o with minor co sub 2 analysis of obsidian from eruptive sequences consisting of tephra and flows indicates that this hydrous magma abruptly overlies magma with only 0 1 0 2 wt h sub 2 o

dynamics of an unusual cone building trachyte eruption at pu u wa awa a hualālai volcano hawai i the pu u wa awa a pyroclastic cone and pu u anahulu lava flow are two prominent monogenetic eruptive features assumed to result from a single eruption during the trachyte dominated early post shield stage of hual lai volcano hawai i pu u wa awa a is composed of complex repetitions of crudely cross stratified units rich in dark dense clasts which reversely grade into coarser pumice rich units pyroclasts from the cone are extremely diverse texturally ranging from glassy obsidian to vesicular scoria or pumice in addition to fully crystalline end members the 100 m thick pu u anahulu flow is in contrast entirely holocrystalline using field observations coupled with whole rock analyses this study aimed to test whether the pu u wa awa a tephra and pu u anahulu lava flows originated from the same eruption as had been previously assumed crystal and vesicle textures are characterized along with the volatile contents of interstitial glasses to determine the origin of textural variability within pu u wa awa a trachytes e g magma mixing vs degassing origin we find that 1 the two eruptions likely originated from distinct vents and magma reservoirs despite their proximity and similar age 2 the textural diversity of pyroclasts forming pu u wa awa a can be fully explained by variable magma degassing and outgassing within the conduit 3 the pu u wa awa a cone was constructed during explosions transitional in style between violent strombolian and vulcanian involving the formation of a large cone and with repeated disruption of conduit plugs but without production of large pyroclastic density currents pdcs and 4 the contrasting eruption styles of hawaiian trachytes flow cone and pdc forming are probably related to differences in the outgassing capacity of the magmas prior to reaching the surface and not in intrinsic compositional or temperature properties these results further highlight that trachytes are kinetically faster magmas compared to dacites or rhyolites likely degassing and crystallizing more rapidly

bubble geobarometry a record of pressure changes degassing and regassing at mono craters california water concentration profiles around bubbles offer a new kind of geobarometer we measure h 2 o and co 2 concentrations in glass adjacent to bubbles in pyroclastic obsidian from mono craters california united states h 2 o and co 2 concentration gradients are preserved during the eruption and record nonequilibrium degassing a key result is that h 2 o is enriched in the glass surrounding the bubbles indicating that bubbles were resorbing into the melt just prior to the eruption the required pressure increase for the observed water enrichment is inferred to be the last in a series of pressure cycles with amplitude 5 30 mpa that are caused by repeated fragmentation and annealing co 2 concentrations vary substantially in individual obsidian clasts suggesting that slow diffusion of co 2 and nonequilibrium degassing contributes to high co 2 h 2 o ratios in pyroclastic obsidian from mono craters these data are direct evidence for vapor melt disequilibrium and demonstrate that degassing paths from a single parental melt need not be unidirectional hence volatile concentration gradients offer a tool for evaluating degassing models and inferring time scales of magmatic processes

microlite orientation in obsidian flow measured by synchrotron x ray diffraction clinopyroxene and plagioclase andesine microlites in an obsidian flow from glass mountain ne california usa display strong alignment synchrotron x ray diffraction coupled with rietveld analysis was used to quantify crystallographic preferred orientation cpo clinopyroxene with a rod shaped morphology shows a strong alignment of 001 in the flow direction and 010 aligned parallel to the inferred flow plane andesine with a platy morphology displays an alignment of 010 platelets in the flow plane some pole densities exceed 90 multiples of random distribution applying a model of rigid ellipsoidal inclusions in a viscous matrix the local pure shear strains are between 2 and 3

distribution of dissolved water in magmatic glass records growth and resorption of bubbles volcanic eruptions are driven by the growth of gas bubbles in magma bubbles grow when dissolved volatile species principally water diffuse through the silicate melt and exsolve at the bubble wall on rapid cooling the melt quenches to glass preserving the spatial distribution of water concentration around the bubbles now vesicles offering a window into pre eruptive conditions we measure the water distribution around vesicles in experimentally vesiculated samples with high spatial resolution we find that contrary to expectation water concentration increases towards vesicles indicating that water is resorbed from bubbles during cooling textural evidence suggests that resorption occurs largely before the melt solidifies speciation data indicate that the molecular water distribution records resorption whilst the hydroxyl distribution records earlier decompressive growth our results challenge the emerging paradigm that resorption indicates fluctuating pressure conditions and lay the foundations for a new tool for reconstructing the eruptive history of natural volcanic products

surface area porosity and water adsorption properties of fine volcanic ash particles our understanding on how ash particles in volcanic plumes react with coexisting gases and aerosols is still rudimentary despite the importance of these reactions in influencing the chemistry and dynamics of a plume in this study six samples of fine ash 500 a all the specimens had similar pore size distributions with a small peak centered around 50 a these findings suggest that fine ash particles have relatively undifferentiated surface textures irrespective of the chemical composition and eruption type adsorption isotherms for water vapour revealed that the capacity of the ash samples for water adsorption is systematically larger than predicted from the nitrogen adsorption a s values enhanced reactivity of the ash surface towards water may result from i hydration of bulk ash constituents ii hydration of surface compounds and or iii hydroxylation of the surface of the ash the later mechanism may lead to irreversible retention of water based on these experiments we predict that volcanic ash is covered by a complete monolayer of water under ambient atmospheric conditions in addition capillary condensation within ash pores should allow for deposition of condensed water on to ash particles before water reaches saturation in the plume the total mass of water vapour retained by 1 g of fine ash at 0 95 relative water vapour pressure is calculated to be 10 2 g some volcanic implications of this study are discussed

diffusive dehydration and bubble resorption during open system degassing of rhyolitic melts abstract degassing through open paths such as bubble and or fracture networks is considered to be the principal mode of degassing in silicic magmas however its detailed mechanisms remain unclear to investigate the behavior of bubbles in a hypothetical open system condition we performed a series of vesiculation experiments on natural rhyolitic obsidian using a newly designed semipermeable cell which artificially maintains a pressure difference between its inside and outside the thick wall cell maintains a constant volume within the sample chamber while allowing water vapor to escape the cell during the experimental runs the cells containing obsidian cores with ca 0 66 wt initial water content were externally heated to 1000 c for a period of 1 288 h the run charges generally showed a zonal structure composed of two contrasting regions a central region within which the bubbles were concentrated bubble rich core bc and a bubble free melt region surrounding the bc bubble free margin bfm with increased heating duration the thickness of the bfm increased via dissolution of the outermost bubbles in the bc the water content was nearly uniform throughout the bc whereas an outward decreasing gradient was observed in the bfm we found that diffusive dehydration occurred from the sample surface and the bubbles were resorbed into the melt thus the bfm bc boundary moved inwards these processes were modelled numerically and the calculation results were in good agreement with the experimental data if a lifetime of open paths is approximated as the relaxation time of a melt in a shallow volcanic environment then the paths have to be pinched off quickly 1 2 h at maximum pressure difference between the open path and the melt and thus the thickness of the bubble free layer reaches at most 0 1 mm for the formation of bubble free obsidian layers with a width of a few millimeters which are often observed in natural obsidian flows open paths should be maintained for at least a few hundred hours

analysis of local ice crystal growth in snow the structural evolution of snow under metamorphism is one of the key challenges in snow modeling the main driving forces for metamorphism are curvature differences and temperature gradients inducing water vapor transport and corresponding crystal growth which is detectable by the motion of the ice air interface to provide quantitative means for a microscopic validation of metamorphism models a vtk based image analysis method is developed to track the ice air interface in time lapse ct experiments to measure local interface velocities under both isothermal and temperature gradient conditions using estimates of local temperatures from microstructure based finite element simulations a quantitative comparison of measured interface velocities with theoretical expressions is facilitated for isothermal metamorphism the data are compared with a kinetics and a diffusion limited growth law in both cases the data are largely scattered but consistently show a mean curvature dependency of the interface velocity for temperature gradient metamorphism we confirm that the main contribution stems from the temperature gradient induced vapor flux accompanied by effects of mean curvature as a secondary process the scatter and uncertainties are discussed in view of the present theoretical understanding the experimental setup and complications such as mechanical deformations

thermal properties of vesicular rhyolite abstract thermal diffusivity of rhyolite melt and rhyolite foam 70 80 porosity has been measured using the radial heat transfer method cylindrical samples length 50 55 mm diameter 22 mm of rhyolite melt and foam have been derived by heating samples of little glass mountain obsidian using available data on heat capacity and density of rhyolite melt the thermal conductivity of samples has been determined the difference in thermal conductivity between rhyolite melt and foam at igneous temperatures 1000 c is about one order of magnitude the effect of thermal insulation of magmas due to vesiculation and foaming of the top layer is discussed in terms of the data obtained using a simple illustrative model of magma chamber convection

rheological change and degassing during a trachytic vulcanian eruption at kilian volcano chaine des puys france magma ascent during silicic dome forming eruptions is characterized by significant changes in magma viscosity permeability and gas overpressure in the conduit these changes depend on a set of parameters such as ascent rate outgassing and crystallization efficiency and magma viscosity which in turn may influence the prevailing conditions for effusive versus explosive activity here we combine chemical and textural analyses of tephra with viscosity models to provide a better understanding of the effusive explosive transitions during vulcanian phases of the 9 4 ka eruption of kilian volcano chaine des puys france our results suggest that effusive activity at the onset of vulcanian episodes at kilian volcano was promoted by i rapid ascent of initially crystal poor and volatile rich trachytic magma ii a substantial bulk and melt viscosity increase driven by extensive volatile loss and crystallization and iii efficient degassing outgassing in a crystal rich magma at shallow depths trachytic magma repeatedly replenished the upper conduit and variations in the amount of decompression and cooling caused vertical textural stratification leading to variable degrees of crystallization and outgassing outgassing promoted effusive dome growth and occurred via gas percolation through large interconnected vesicles fractures and tuffisite veins fostering the formation of cristobalite in the carapace and talus regions build up of overpressure was likely caused by closing of pore space bubbles and fractures in the dome through a combination of pore collapse cristobalite formation sintering in tuffisite veins and limited pre fragmentation coalescence in the dome or underlying hot vesicular magma sealing of the carapace may have caused a transition from open to closed system degassing and to renewed explosive activity we generalize our findings to propose that the broad spectrum of eruptive styles for trachytic magmas may be inherited from a combination of characteristics of trachytic melts that include high water solubility and diffusivity rapid microlite growth and low melt viscosity compared to their more evolved subalkaline dacitic and rhyolitic equivalents we show that trachytes may erupt with a similar style e g vulcanian but at significantly higher ascent rates than their andesitic dacitic and rhyolitic counterparts this suggests that the periodicity of effusive explosive transitions at trachytic volcanoes may differ from that observed at the well monitored andesitic dacitic and rhyolitic volcanoes which has implications for hazard assessment associated with trachytic eruptions

the effect of water on the viscosity of a haplogranitic melt under p t x conditions relevant to silicic volcanism the viscosities of hydrous haplogranitic melts synthesized by hydrothermal fusion at 2 kbar pressure and 800 to 1040 c have been measured at temperatures just above the glass transition and at a pressure of 1 bar using micropenetration techniques the micropenetration viscometry has been performed in the viscosity range of 109 pa s to 1012 pa s the samples ranged in water content from 0 4 to 3 5 wt for samples with up to 2 5 wt h2o the water contents have been determined using infrared spectroscopy obtained before and after each viscometry experiment to be constant over the duration of the measurements above this water content a measurable loss of water occurs during the viscometry the viscosity data illustrate an extremely nonlinear decrease in viscosity with added water the viscosity drops drastically with the addition of 0 5 wt of water and then shallows out at water contents of 2 wt an additional viscosity datum point obtained from the analysis of fluid inclusions in a water saturated hpg8 confirms a near invariance of the viscosity with the addition of water between 2 and 6 wt these measurements may be compared directly with the data of hess et al 1995 in press for the effects of excess alkali and alkaline earth oxides on the viscosity of hpg8 also obtained at 1 bar the viscosity of the melts compared on an equivalent molar basis increases in the order h2o li2o na2o k2o rb2o cs2o bao sro cao mgo beo the extraordinary decrease in melt viscosity with added water is poorly reproduced by the calculation scheme of shaw 1972 for the range of water contents investigated here the speciation of water in the quenched glasses can be used to quantify the dependence of the viscosity on hydroxyl content considering only the hydroxyl groups as active fluidizers in the hydrous melts the nonlinearity of the viscosity decrease and the difference with the effects of the alkali oxides becomes larger consequences for degassing calcalkaline rhyolite are discussed

early formation of preferential flow in a homogeneous snowpack observed by micro ct we performed x ray microtomographic observations of wet snow metamorphism during controlled continuous melting and melt freeze events in the laboratory three blocks of snow were sieved into boxes and subjected to cyclic superficial heating or heating cooling to reproduce vertical water infiltration patterns in snow similarly to natural conditions periodically samples were taken at different heights and scanned results suggest that wet snow metamorphism dynamics are highly heterogeneous even in an initially homogeneous snowpack consistent with previous work we observed an increase with time in the thickness of the ice structure which is a measure of grain size however this was coupled with large temporal scatter between consecutive measurements of the specific surface area and of the statistical moments of grain thickness distributions because of marked differences in the right tail grain thickness distributions did not show shape invariance with time contrary to previous analyses in our experiments wet snow metamorphism showed two strikingly different patterns homogeneous coarsening superimposed by faster heterogeneous coarsening in areas that were affected by preferential percolation of water liquid water movement in snow and fast structural evolution may be thus intrinsically coupled by early formation of preferential flow at local scale these observations suggest that further experiments are highly needed to fully understand wet snow metamorphism and infiltration patterns in a natural snowpack

gas transport and bubble collapse in rhyolitic magma an experimental approach a series of experiments was conducted to test concepts of porous flow degassing of rhyolitic magma during ascent and of the subsequent collapse of vesicles in degassed magma to form obsidian dense synthetically hydrated natural glasses were pressurized under water saturated conditions and then decompressed to achieve a range of porosities in the presence of a tracer vapor d2o rapid isotopic exchange indicative of vapor transport rather than of simple diffusion occurred at a porosity 60 vol in accord with earlier gas permeability measurements on cold natural samples in another series of experiments natural and synthetic pumices vesiculated by degassing to atmospheric pressure rapidly collapsed to dense glass on repressurization to the modest pressures prevailing in lava flows no relict bubble textures remained these results support the hypothesis that effusive eruptions result from the syneruptive escape of gas from permeable magmatic foam and that a process analogous to welding yields dense lavas when such foams are extruded

h2o diffusion models in rhyolitic melt with new high pressure data abstract water diffusion in silicate melts is important for understanding bubble growth in magma magma degassing and eruption dynamics of volcanos previous studies have made significant progress on water diffusion in silicate melts especially rhyolitic melt however the pressure dependence of h2o diffusion is not constrained satisfactorily we investigated h2o diffusion in rhyolitic melt at 0 95 1 9 gpa and 407 1629 c and 0 2 5 2 wt total water h2ot content with the diffusion couple method in a piston cylinder apparatus compared to previous data at 0 1 500 mpa h2o diffusivity is smaller at higher pressures indicating a negative pressure effect this pressure effect is more pronounced at low temperatures assuming h2o diffusion in rhyolitic melt is controlled by the mobility of molecular h2o h2om the diffusivity of h2om dh2om at h2ot 7 7 wt 403 1629 c and 1 9 gpa is given by d h 2 o m d 0 e x p a x with d 0 e x p 13 375 1 8875 p 12939 3625 6 p t and a 37 256 75884 t where d0 is in m2 s x is mole fraction of h2ot on a single oxygen basis t is temperature in k and p is pressure in gpa h2ot diffusivities dh2ot in m2 s can be calculated from h2om diffusivity or directly from the following expression l n d h 2 o t x 13 47 49 996 x 7 0827 x 1 8875 p 9532 3 91933 x 13403 x 3625 6 p t at low h2ot content up to 2 wt if an error of a factor of 2 is allowed h2ot diffusivity is approximately proportional to h2ot content d h 2 o t c c 0 e x p 9 5279 1 8875 p 9698 5 3625 6 p t where c is h2ot content in wt and c0 is 1 wt the new expressions for h2o diffusion not only reproduce our own data but also match data in literature from different laboratories and using different methods indicating good inter laboratory and multi method consistency the new expressions cover a wide range of geological conditions and can be applied to h2o diffusion in rhyolitic melts in various volcanic and magmatic processes

isotopic insights into the degassing and secondary hydration of volcanic glass from the 1980 eruptions of mount st helens abstract the magmatic degassing history of newly erupted volcanic glass is recorded in its remaining volatile content however this history is subsequently overprinted by post depositional secondary hydration the rates and origins of which are not yet adequately constrained here we present the results of a natural experiment using products of the 1980 eruptions of mount st helens we measured water concentration dglass and 18obsg 18o of the bulk silicate glass of samples collected during the dry summer months of 1980 and compared them with material resampled in 2015 from the same deposits samples collected from the subsurface near gas escape pipes show elevated water concentrations near 2 0 wt and these are associated with lower dglass 110 to 130 and 18obsg 6 0 to 6 6 values than the 1980 glass 70 to 100 and 6 8 to 6 9 respectively samples collected in 2015 from the surface to 10 cm subsurface of the 1980 summer deposits have a small increase in average water contents of 0 1 0 2 wt but similar 18obsg 6 8 6 9 values compared to the 1980 glass values these samples however show 15 higher dglass values exchange with meteoric water is expected to yield lower dglass values we attribute higher dglass values in the upper portion of the 1980 deposits collected in 2015 to rehydration by higher d waters that were degassed for several months to a year from the hot underlying deposits which hydrated the overlying deposits with relatively high d gases our data also contribute to magmatic degassing of crystal rich volcanoes using the 1980 samples our reconstructed d h2o trends for the dacitic mount st helens deposits with rhyolitic groundmass yield a trend that overlaps with the degassing trend for crystal poor rhyolitic eruptions studied previously elsewhere suggesting similar behavior of volatiles upon exsolution from magma furthermore our data support previous studies proposing that exsolved volatiles were trapped within a rapidly rising magma and started degassing only at shallow depths during the 1980 eruptions

vesiculation rates of obsidian domes inferred from h2o concentration profiles 1 explosivity of rhyolite domes depends on bubble nucleation and growth to understand these processes we measure millimeter scale variations in h2o by synchrotron fourier transform infrared ftir spectromicroscopy on interlayered obsidian pumice samples the h2o contents of all samples are above the 1 atm solubility value 0 10 wt and decrease systematically towards vesicular zones indicating that gas bubbles were growing and that degassing of melt to atmospheric pressure was incomplete h2o profiles are compared with models for water diffusion in order to constrain the temporal scale of vesiculation diffusion timescales range from 0 4 to 15 days and represent the time between bubble nucleation and quenching we use these results to estimate the duration of volatile exsolution and evaluate the timing of the formation of explosion pits frequently found on the surfaces of obsidian domes though never witnessed

unravelling textural heterogeneity in obsidian shear induced outgassing in the rocche rosse flow abstract obsidian flow emplacement is a complex and understudied aspect of silicic volcanism of particular importance is the question of how highly viscous magma can lose sufficient gas in order to erupt effusively as a lava flow using an array of methods we study the extreme textural heterogeneity of the rocche rosse obsidian flow in lipari a 2 km long 100 m thick 800 year old lava flow with respect to outgassing and emplacement mechanisms 2d and 3d vesicle analyses and density measurements are used to classify the lava into four textural types glassy obsidian 40 vesicles high aspect ratio shear banded lava 20 40 vesicles and low aspect ratio frothy obsidian with 30 60 vesicles textural heterogeneity is observed on all scales m to m and occurs as the result of strongly localised strain magnetic fabric described by oblate and prolate susceptibility ellipsoids records high and variable degrees of shearing throughout the flow total water contents are derived using both thermogravimetry and infrared spectroscopy to quantify primary magmatic and secondary meteoric water glass water contents are between 0 08 0 25 wt water analysis also reveals an increase in water content from glassy obsidian bands towards frothy bands of 0 06 0 08 wt reflecting preferential vesiculation of higher water bands and an extreme sensitivity of obsidian degassing to water content we present an outgassing model that reconciles textural volatile and magnetic data to indicate that obsidian is generated from multiple shear induced outgassing cycles whereby vesicular magma outgasses and densifies through bubble collapse and fracture healing to form obsidian which then re vesiculates to produce dry vesicular magma repetition of this cycle throughout magma ascent results in the low water contents of the rocche rosse lavas and the final stage in the degassing cycle determines final lava porosity heterogeneities in lava rheology vesicularity water content microlite content viscosity play a vital role in the structural evolution of an obsidian flow and overprint flow scale morphology post emplacement hydration also depends heavily on local strain whereby connectivity of vesicles as a result of shear deformation governs sample rehydration by meteoric water a process previously correlated to lava vesicularity alone

discriminating secondary from magmatic water in rhyolitic matrix glass of volcanic pyroclasts using thermogravimetric analysis abstract pyroclasts from explosive eruptions such as the 1060 ce explosive glass mountain eruption of medicine lake volcano california contain large amounts of water this may be the consequence of diffusive rehydration of the volcanic glass by meteoric secondary water after the eruption discriminating between magmatic and secondary water in the matrix glass of pyroclasts is important because the degassing of magmatic water affects the intensity of volcanic eruptions such discrimination has remained a challenging problem especially because some aspects of water diffusion in silicate glasses at low temperatures and atmospheric pressure remain poorly constrained we used thermogravimetry to analyze the loss of water from natural volcanic glasses and glasses that were hydrated in the laboratory at magmatic temperatures and pressures numerical modeling of diffusive water loss during thermogravimetric analyses accounted for the interconversion of molecular water h 2 o m and hydroxyls groups oh and indicates that glass mountain pumices contain 0 2 0 5 wt primary water but gained 1 2 wt of meteoric water by diffusive rehydration during the past 950 years these results confirm that the majority of magmatic water is lost from the magma during explosive eruptions furthermore the integration of thermogravimetric analysis and numerical modeling facilitates discrimination between the magmatic and secondary water content of volcanic glasses

hydrogen isotope determination by tc ea technique in application to volcanic glass as a window into secondary hydration abstract the use of volcanic glass as recorder of paleoenvironmental conditions has existed for 30 years in this paper we investigate the methodological aspects of the determination of water content isotopic composition and water speciation in volcanic glass using the high temperature conversion elemental analyzer tcea mass spectrometer system on milligram quantities of glass concentrates it is shown here that the precision and the reproducibility of this method is comparable to off line conventional methods that require 100 times greater amount of material d 3 h2o tot 10relative if 1 wt but is quicker and permits easy replication this method extracts 100 of the water as verified by ftir measurements finally this study confirms the interest of drift spectroscopy in the nir range for the study of porous samples such as volcanic pumices and tephra to determine the water speciation h2o oh it may complement conventional ftir transmission measurements in the mir or nir range that usually require homogeneous transparent sections or high degree of sample dilution in a non absorbing matrix using these methods we attempt to discriminate residual magmatic from secondary meteoric water in volcanic glass using mafic to differentiated samples from different geological settings and different climatic conditions we show that the h isotope composition and water content of volcanic glass alone are not always sufficient to provide clear distinction between magmatic and meteoric origin however if the magma is known to have a d between 90 and 40 60 for morb mantle source it is quite easy to resolve the d evolution during magmatic degassing from post depositional rehydration by meteoric water with d 20 water speciation measurements may provide additional information in most cases isotopic and total water measurements should be complemented by characterization of water speciation during magmatic degassing from 6 wt to 0 1 wt water the h2o oh is expected to decrease from 2 to close to 0 however our dataset suggests that during secondary glass hydration from 0 1 wt to 6 wt water the h2o oh ratio decreases from 5 to 2 which is the complete opposite overall our results support the use of h isotopes of volcanic glass to discuss the composition of meteoric waters and paleo climate within a specific region to this purpose the volcanic glass has to be almost fully rehydrated in order to fingerprint the isotopic composition of the ambient environmental water as rehydration is exponentially faster with increasing temperature efficient rehydration taking months to years may occur in a cooling volcanic deposits that are meters thick and thus can remain at a few hundred c for a years to hundreds of years after the eruption such deposits could then provide a snap shot view of climatic conditions at the time of the studied eruption

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

eruption dynamics and explosive effusive transitions during the 1400 cal bp eruption of opala volcano kamchatka russia abstract deposits and pumice from the 1400 cal bp eruption of opala volcano record activity that occurred at the explosive effusive transition resulting in intermittent or stop start behavior where explosive activity resumed following a pause the eruption deposited distinctive biotite bearing rhyolite tephra across much of kamchatka and its stratigraphy consists of a lithic rich pumice fall overlain by pumice falls and pyroclastic density deposits with the proportion of the latter increasing with height this sequence repeats such that the middle of the total deposit is marked by a lithic rich fall with abundant obsidian clasts notably the eruptive pumice are poorly vesiculated with vesicle textures that record fragmentation of a partially collapsed magmatic foam the eruption vent baranii amphitheater is filled with obsidian lavas of the same composition as the rhyolite tephra based upon the stratigraphic and compositional relations we divide the eruption into four phases phase i initiated with eruption of a lithic rich pumice fall followed by eruption of plinian falls and pyroclastic density currents during phase ii the eruption paused for at least 5 6 h in this time microlites nucleated and began to grow in the magma phase iii essentially repeated the phase i sequence obsidian lavas were emplaced during phase iv the pumice textures suggest that the magma ascended very near the threshold decompression rate for the transition between explosive fast and effusive slow behavior the pause during phase ii likely occurred as decompression slowed enough for the magma to develop sufficient permeability for gas to escape resulting in collapse of the magmatic foam stopping the eruption and temporarily sealing the conduit after about 5 6 h eruption resumed with once again magma decompressing very near the explosive effusive transition phase iii ended when the decompression rate slowed and lava dome emplacement began distributions of pumice and lithic clasts and inclusion of data from previous workers indicate minimum deposit volumes of 0 75 and 0 75 1 15 km3 dre and eruption column heights of 18 and 20 km for phases i and iii respectively phases i iii had a likely total duration of 60 80 h including a pause in activity of 5 6 h during phase ii this study demonstrates that analysis of vesicle textures from numerous pumice combined with stratigraphic data can reveal syn eruptive changes in and links between magma permeability decompression rate and eruption style

volcanic glass as a proxy for cenozoic elevation and climate in the cascade mountains oregon usa abstract after deposition volcanic glass hydrates with ambient water recording the average hydrogen isotope ratio d or 2h of local meteoric water during the hydration period previous researchers have used ancient glass d values to reconstruct paleotopography and paleoclimate while others have questioned the long term reliability of the proxy as a recorder of ancient meteoric water in this study we sampled volcanic glasses ranging in age 33 ma to 1 ma and local meteoric water converted to glass d values throughout the study area we propose a paleoenvironmental interpretation of volcanic glass results that resolves previously published isotopic data and agrees well with the petrologic structural and stratigraphic record namely the oregon cascades have been a significant topographic barrier since at least the mid miocene and likely as far back as the oligocene since reaching a topographic maximum during the eruption of columbia river flood basalts in the mid miocene surface elevations in oregon have decreased while the northern cascades in washington continue to rise

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

long term stability of hydrogen isotope ratios in hydrated volcanic glass abstract the advancement of conceptual and numerical geodynamic models necessitates quantitative orogen scale paleoelevation data felsic volcanic glasses which record the hydrogen isotope compositions d of meteoric water shortly after deposition provide several advantages as a paleoelevation proxy questions remain however about the reliability of this relatively new proxy including the effect of hydrofluoric hf acid abrasion in the preparation of glass shards for hydrogen isotope analysis and the stability of hydrogen isotope ratios in hydrated glass shards over geologic time 10 6 10 7 years hf acid abrasion of natural ancient glass shards results in systematic shifts in glass d values away from modern water d values to evaluate the effectiveness of hf acid abrasion we treated 70 150 m glass shards separated from various natural tephras with deuterium labeled water dlw d 18 205 for up to 400 days for all glasses this treatment resulted in elevated d values in comparison to untreated samples hf acid abrasion after dlw exposure however removed this effect and restored glass shards to their original untreated d values in samples older than 10 4 years hf acid abrasion removes hydrous alteration precipitates at the glass surface without measurably changing the d values of the underlying hydrated glass regardless of abrasion duration or glass composition additionally 45 34 ma glasses record d values that directly reflect their depositional environments as determined by stratigraphy glasses from tuffs deposited in demonstrably evaporative lacustrine environments have relatively high d values compared to glasses from contemporaneous tuffs deposited in nearby fluvial environments which have much lower d values the preservation of d values that systematically vary with original depositional environment despite 30 myr of post hydration exposure to the same meteoric water indicates that these volcanic glasses resisted measurable h isotope exchange on geologic timescales this research supports the use of natural felsic glass separates for paleoaltimetry and demonstrates the necessity of hf acid abrasion and glass shard isolation in sample preparation methods

fragmentation of magma during plinian volcanic eruptions the ratio of the volume of vesicles gas to that of glass liquid in pumice clasts v g v l reflects the degassing and dynamic history experienced by a magma during an explosive eruption v g v l in pumices from a large number of plinian eruption deposits is shown here to vary by two orders of magnitude even between pumices at a given level in a deposit these variations in v g v l do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities despite large ranges in these variables gas volume ratios of pumices do however vary systematically with magma viscosity estimated at the point of fragmentation and we infer that pumices do not quench at the level of fragmentation but undergo some post fragmentary evolution on the timescale of plinian eruptions pumices with viscosities 109 pa s can expand after fragmentation as long as their bubbles retain gas at a rate inversely proportional to their viscosity once the bubbles connect to form a permeable network and lose their gas expansion halts and pumices with viscosities 105 pa s can collapse under the action of surface tension textural evidence from bubble sizes and shapes in pumices indicates that both expansion and collapse have taken place the magnitudes of expansion and collapse therefore depend critically on the timing of bubble connectivity relative to the final moment of quenching we propose that bubbles in different pumices become connected at different times throughout the time span between fragmentation and quenching after accounting for these effects we derive new information on the fragmentation process from two characteristics of pumices the most important is a relatively constant minimum value of v g v l of 1 78 64 vol vesicularity in all samples with viscosities 105 pa s this value is independent of magma composition and thus reflects a property of the eruptive mechanism the other characteristic is that highly expanded pumices 85 vol vesicularities are common which argues against overpressure in bubbles as a mechanism for fragmenting magma we suggest that magma fragments when it reaches a vesicularity of 64 vol but only if sheared sufficiently strongly the intensity of shear varies as a function of velocity in the conduit which is related to overpressure in the chamber so that changes in overpressure with time are important in controlling the common progression from explosive to effusive activity at volcanoes

seismic monitoring of an alpine mountain river the canfranc underground laboratory lsc excavated under the central pyrenees is mainly devoted to the study of phenomena which needs cosmic silence it also hosts a geodynamical facility named geodyn which holds an accelerometer a broadband seismometer and two high resolution laser strainmeters during the routine processing of the seismic data we detected an unusual spectral signature in the 2 10 hz frequency band which does not correspond to the typical sources of seismic noise and which can also be recognized in the strain records after checking against meteorological and hydrological data we can relate those signals to variations in the discharge by the aragon river an alpine style river in the southern pyrenees located about 400 m from the lsc geodyn facility four main episodes have been identified since early 2011 each lasting 1 2 to 6 8 days additionally a limited number of shorter episodes have also been detected three types of river generated seismic events have been identified related respectively to moderate rainfall snowmelt and flooding events associated to severe storms each of those types has distinctive characteristics which allow monitoring the hydrological events from the analysis of seismic and deformation data a few previous studies have already described the seismic noise close to rivers with larger discharge or in small scale experimental settings and we are showing here that the so called fluvial seismology can be useful to study the hydrological evolution of alpine style streams and may have a potential interest for the civil authorities in charge of the management of hydrological basins

a novel apparatus for the simulation of eruptive gas rock interactions the chemical interactions between hot volcanic gases and co erupted magmatic and lithic particles within eruption plumes and pyroclastic flows are increasingly investigated for their relevance to the impacts of ash emission on natural and human environments laboratory experiments are critical to our understanding of high temperature gas ash interactions but previous studies are yet to replicate the chemical composition of the high temperature volcanic gases involved here we present a unique apparatus the advanced gas ash reactor capable of generating an atmosphere of h2o co2 so2 and hcl at temperatures ranging from 200 to 900 c under variable heating and cooling rates experiments utilising the reactor can inform investigations of a range of topics from subsurface gas rock interaction and in plume gas adsorption processes to the effect of ash surface chemistry on marine nutrient loadings and atmospheric chemistry our results demonstrate the differences in high temperature gas uptake by volcanic glass powders under both hydrous and anhydrous atmospheres and accordingly demonstrate the utility of the new reactor

firn model intercomparison experiment firnmice evolution of cold dry snow and firn plays important roles in glaciology however the physical formulation of a densification law is still an active research topic we forced eight firn densification models and one seasonal snow model in six different experiments by imposing step changes in temperature and accumulation rate boundary conditions all of the boundary conditions were chosen to simulate firn densification in cold dry environments while the intended application of the participating models varies they are describing the same physical system and should in principle yield the same solutions the firn models all produce plausible depth density profiles but the model outputs in both steady state and transient modes differ for quantities that are of interest in ice core and altimetry research these differences demonstrate that firn densification models are incorrectly or incompletely representing physical processes we quantitatively characterize the differences among the results from the various models for example we find depth integrated porosity is unlikely to be inferred with confidence from a firn model to better than 2 m in steady state at a specific site with known accumulation rate and temperature firn model intercomparison experiment can provide a benchmark of results for future models provide a basis to quantify model uncertainties and guide future directions of firn densification modeling

magma degassing during the plinian eruption of novarupta alaska 1912 1 we have modeled the nucleation and isothermal growth of bubbles in dacite from the 1912 plinian eruption of novarupta alaska bubble growth calculations account for the exsolution of h2o and co2 beginning with bubble nucleation and ending when bubble sizes reproduced the observed size distribution of vesicles in novarupta pumice clasts assuming classical nucleation theory bubbles nucleated with a diameter of the order of 10 8 m and grew to sizes ranging from 10 6 m to greater than 10 3 m the typical range of vesicle sizes found in novarupta pumice the smallest vesicles in novarupta pumices are also the most abundant and bubbles with radii of 10 6 m to 10 5 m comprise almost 90 of the entire bubble population we find that these bubbles must have nucleated and grown to their final size within a few 100 milliseconds despite these extremely fast growth rates the pressures of exsolved volatiles contained within the bubbles remained high up to about 107pa in excess of ambient pressure assuming a closed system the potential energy of these compressed volatiles was sufficient to cause magma fragmentation even though only a fraction of the pre eruptive volatiles had exsolved unless the matrix glasses of novarupta pyroclasts retains a large fraction of pre eruptive volatiles the majority of magmatic volatiles 80 90 was likely lost by open system degassing between magma fragmentation and quenching

telkibanya lava domes lithofacies architecture of a miocene rhyolite field tokaj mountains carpathian pannonian region hungary abstract this work focuses on the miocene telkibanya lava dome field tokaj mountains hungary proposed as an example of poorly investigated lithofacies architecture of old silicic lava dome fields a detailed fieldwork combined with petrographic and radiometric k ar dating analyses allows us the redefinition of the volcano stratigraphy of a 25 km2 area and the recognition of two temporary different generation of domes that grew in a subaerial environment 12 cohesive and autoclastic lithofacies are identified and grouped together to document the internal variability of the domes as well as the erosional phases that affected the lithofacies architecture furthermore stratigraphy lithofacies association and morphology allow recognizing the areal distribution of 5 lithosomes lava flows coulees or domes all these findings represent a major breakthrough in the identification and interpretation of the volcanic processes that control the growth of silicic lava domes and their products in late miocene environment of carpathian pannonian region

δd and δ18o variations of the magmatic system beneath deception island volcano antarctica implications for magma ascent and eruption forecasting abstract geochemistry of volatiles in active volcanoes provides insights into the magmatic processes and evolution at depth such as magma evolution and degassing which can be implemented into volcanic hazards assessment deception island is one of the most active volcanoes in antarctica with more than twenty explosive eruptions documented over the past two centuries hydrogen and oxygen isotopic variations in the volatiles trapped in the deception island rocks glass and melt inclusions in phenocrysts provide essential information on the mechanisms controlling the eruptive history in this volcanic suite thus understanding the petrological and related isotopic variations in the island has the potential to foresee the possible occurrence and its main eruptive features of a future eruption the new isotopic results of this study combined with exhaustive petrologic knowledge reveal in deception island i fast ascent and quenching of most magmas preserving pre eruptive magmatic signal of water contents and isotopic ratios with local modification by rehydration due to glass exposition to seawater and by meteoric and fumarolic waters ii a plumbing system s variable with time and currently dominated by closed system degassing leading to explosive eruptions iii control on the interactions of ascending magmas with the surface waters producing hydrovolcanic activity throughout the two main fault systems in deception island and at different magma stagnation depths these results can be considered in further studies of volcanic monitoring to improve the capability to interpret geophysical data and signals recorded during volcanic unrest episodes and hence forecast volcanic eruptions and related hazards

water in volcanic glass from volcanic degassing to secondary hydration abstract volcanic glass is deposited with trace amounts 0 1 0 6 wt of undegassed magmatic water dissolved in the glass after deposition meteoric water penetrates into the glass structure mostly as molecular h2o due to the lower d values of non tropical meteoric waters and the 30 offset between volcanic glass and environmental water during hydration secondary water imparts lighter hydrogen isotopic values during secondary hydration up to a saturation concentration of 3 4 wt h2o we analyzed compositionally and globally diverse volcanic glass from 0 to 10 ka for their d and h2ot across different climatic zones and thus different d of precipitation on a thermal conversion elemental analyzer tcea furnace attached to a mass spectrometer we find that tephrachronologically coeval rhyolite glass is hydrated faster than basaltic glass and in the majority of glasses an increase in age and total water content leads to a decrease in d while a few equatorial glasses have little change in d we compute a magmatic water correction based on our non hydrated glasses and calculate an average 103ln glass water for our hydrated felsic glasses of 33 which is similar to the 103ln glass water determined by friedman et al 1993a of 34 we also determine a smaller average 103ln glass water for all our mafic glasses of 23 we compare the d values of water extracted from our glasses to local meteoric waters following the inclusion of a 33 103ln glass water we find that following a correction for residual magmatic water based on an average d and wt h2ot of recently erupted ashes from our study the d value of water extracted from hydrated volcanic glass is on average within 4 of local meteoric water to better understand the difference in hydration rates of mafic and felsic glasses we imaged 6 tephra clasts ranging in age and chemical composition with bse by fei sem down to a submicron resolution mafic tephra have more bubbles per unit area 25 77 mm 2 than felsic tephra 736 mm 2 and thicker average bubble walls 0 07 mm than felsic tephra 0 02 mm we use a simplified diffusion model to quantify the hydration rate of vesicular glass as a function of the diffusivity of water and the average bubble wall thickness based on fits to our hydration rate data we estimate the initial low temperature diffusivity at 0 1 wt h2ot in volcanic glass mafic and felsic to be on the order of 10 3 to 10 4 m2 year and find that differences in hydration rates between mafic and felsic tephra can be attributed primarily to differences in vesicularity although slightly slower hydration of basalt cannot be precluded we also observe no consistent temporal difference in secondary meteoric water uptake in wet versus dry and hot versus cold climates

variations in hydration within perlitised rhyolitic lavas evidence from torfajokull iceland perlitic fractures form due to the hydration of glassy rhyolitic lavas perlitised lavas are also an important industrial commodity yet there has been little study on the mechanisms of outcrop perlitisation here the fracture populations perlitisation and volatile concentrations of subglacial rhyolitic glassy facies have been studied adding a quantitative dimension to previous qualitative studies samples include hyaloclastite perlitised and non perlitised obsidian and microcrystalline rhyolite which are all present in lava lobes at torfajokull iceland fractures formed through cooling increase in spacing with increased distance inwards from the margins of lobes the size distribution of perlitic beads is shown to follow a log normal distribution the degree of perlitisation and the total volatile content measured using thermogravimetric analysis tga of samples are shown to decrease with distance inwards from lobe margins in general increased perlitisation is accompanied by increased hydration although complexity exists within single outcrops the elevated total volatile content of perlite is shown to be due to hydration by environmental water with the structural location of the water changing as total volatile content increases

bubble nucleation growth and coalescence during the 1997 vulcanian explosions of soufriere hills volcano montserrat abstract soufriere hills volcano had two periods of repetitive vulcanian activity in 1997 each explosion discharged the contents of the upper 0 5 2 km of the conduit as pyroclastic flows and fallout frothy pumices from a deep gas rich zone lava and breadcrust bombs from a degassed lava plug and dense pumices from a transition zone vesicles constitute 1 66 vol of breadcrust bombs and 24 79 of pumices all those larger than a few tens of m being interconnected small vesicles few hundreds of m in pumices are interpreted as pre dating explosion implying pre explosive conduit porosities up to 55 about a sixth of large vesicles in pumices and all those in breadcrust bombs are angular voids formed by syn explosive fracturing of amphibole phenocrysts an intermediate sized vesicle population formed by coalescence of the small syn explosive bubbles bubble nucleation took place heterogeneously on titanomagnetite number densities of which greatly exceed those of vesicles and growth took place mainly by decompression development of pyroclast vesicle textures was controlled by the time interval between the onset of explosion decompression and surface quench in contact with air lava plug fragments entered the air quickly after fragmentation 10 s so the interiors continued to vesiculate once the rinds had quenched forming breadcrust bombs deeper gas rich magma took longer 50 s to reach the surface and vesiculation of resulting pumice clasts was essentially complete prior to surface quench this accounts for the absence of breadcrusting on pumice clasts and for the textural similarity between pyroclastic flow and fallout pumices despite different thermal histories after leaving the vent it also allowed syn explosive coalescence to proceed further in the pumices than in the breadcrust bombs uniaxial boudinage of amphibole phenocrysts in pumices implies significant syn explosive vesiculation even prior to magma fragmentation probably in a zone of steep pressure gradient beneath the descending fragmentation front syn explosive decompression rates estimated from vesicle number densities 0 3 6 5 mpa s 1 are consistent with those predicted by previously published numerical models

hydrogen isotope analysis of rehydrated silicic lavas implications for eruption mechanisms abstract bulk and step heating hydrogen isotope measurements of glassy and vesicular samples from three young rhyolite lavas demonstrate for the first time that magmatic h 2 o can be distinguished from meteoric h 2 o added during rehydration step heat extractions of gases obtained at five temperatures show that most meteoric h 2 o is released at around 400 c while magmatic h 2 o comes off during the fusion step at 1100 1200 c magmatic h 2 o contents from vertically arrayed lava samples show a downward increase consistent with the equilibrium pressure dependence of h 2 o solubility these gradients imply that volatile concentrations high enough to generate pyroclastic flows during front collapse may be found in the upper portions of silicic domes produced by relatively high eruption rates

low δd hydration rinds in yellowstone perlites record rapid syneruptive hydration during glacial and interglacial conditions hydration of silicic volcanic glass forms perlite a dusky porous form of altered glass characterized by abundant onion skin fractures the timing and temperature of perlite formation are enigmatic and could plausibly occur during eruption during post eruptive cooling or much later at ambient temperatures to learn more about the origin of natural perlite and to fingerprint the hydration waters we investigated perlitic glass from several synglacial and interglacial rhyolitic lavas and tuffs from the yellowstone volcanic system perlitic cores are surrounded by a series of conchoidal cracks that separate 30 to 100 m thick slivers likely formed in response to hydration induced stress h2o and d h profiles confirm that most d h exchange happens together with rapid h2o addition but some smoother d h variations may suggest separate minor exchange by deuterium atom interdiffusion following hydration the hydrated rinds 2 3 wt h2o transition rapidly within 30 m or by 1 wt h2o per 10 m to unhydrated glass cores this is consistent with quenched hydration fronts where h2o diffusion coefficients are strongly dependent on h2o concentrations the chemical 18o and d systematics of bulk glass records last equilibrium between 110 and 60 c without chemical exchange but with some 18o exchange similarly the 18o of water extracted from glass by rapid heating suggests that water was added to the glass during cooling at 400 c experimental data the thick hydration rinds in perlites measuring hundreds of microns preserve the original d h values of hydrating water as a recorder of paleoclimate conditions measured d values in perlitic lavas are 150 to 191 or 20 40 lower than glass hydrated by modern yellowstone waters this suggests that yellowstone perlites record the low d signature of glacial ice cooling calculations combined with the observed high water diffusion coefficients noted for 60 150 c suggest that if sufficient hot water or steam is available any rhyolite flow greater than 5 m thick can develop the observed 250 m hydration rinds within the expected timescale of cooling weeks years as the process of hydration involves shattering of 30 to 100 m thick slivers to expose unhydrated rhyolite glass the time required for hydration may be even shorter rapid hydration and formation of relatively thick walled glass shards allow perlites to provide a snapshot view of the meteoric water and thus climate at the time of initial alteration perlites retain their initial hydration d h signal better than thin walled ash which in contrast hydrates over many thousands of years with time averaged precipitation

interfacial and structural relaxations of snow under isothermal conditions we investigated the morphological evolution of laboratory snow under isothermal conditions at 3 9a nd 19 c using x ray tomography we employed a two point density correlation function to measure spatial fluctuations of the density of the bicontinuous ice vapor system at different length scales length scales were derived from the correlation function to distinguish between interfacial coarsening due to the minimization of surface energy on the smallest scales and anisotropic structural re arrangements due to gravity on larger scales on the smallest scales our data suggest a crossover between t 9a nd 19 c from evaporation condensation to surface diffusion as the dominant transport mechanism anomalous growth was found for the slope of the correlation function at the origin and it was similar to those reported for the coarsening of fractal clusters this is consistent with the observed persistence of dendritic structures throughout an entire year the dynamics of large scale morphology was characterized by the first zero crossing of the correlation function which displays a nonmonotonic evolution with a pronounced anisotropy between the direction of gravity and horizontal directions since the correlation function naturally emerges in problems of scattering of radiation in snow our results appear to be important for optical and remote sensing methods

pre explosive conduit conditions of the 1997 vulcanian explosions at soufriere hills volcano montserrat i pressure and vesicularity distributions an authoritative case of vulcanian eruptive dynamics is the series of 88 explosions that occurred between august and october 1997 at soufriere hills volcano on montserrat island the state of the magmatic column just before a vulcanian explosion is still poorly understood but conditions the eruptive style this study establishes such a pre explosive stratigraphy by 1 documenting the textures covering the range of the 1997 products 2 quantitative analysis of h2o content in interstitial glass measured by karl fischer titration and 3 combining these data with a simple model linking pre and post explosive vesicularities the model shows that syn explosive degassing affects greatly the way porosity evolves by decompression during an explosion the stratigraphy reconstruction shows a three part vertical layering of the conduit prior to explosion with overall denser values than those previously suggested a dense and strongly degassed plug caps the column it is underlain by a shallow transition zone featuring complex mingling between vesicular and dense magma up to 10 mpa at higher pressure up to 80 mpa lies a more homogeneous zone of relatively dense 10 20 vol magma which was emplaced under partly open system degassing this conduit stratigraphy gives the vision of a strongly heterogeneous magma column immediately prior to its disruption our analysis suggests that fragmenting such a composite magma cannot happen in a single coherent pulse but rather as stages the transition zone contains heterogeneous amounts of exsolved gas that could explain the pulsatory nature of the vulcanian jets at the beginning of the explosions this contrasts with the nearly constant vesicularities of the deeper part of the pre explosive magma column which are propitious to a general short lived disruption

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

triaxial tests on snow at low strain rate part ii constitutive behaviour fine grained dry snow with a density range of 190 435 kg m 3 was tested in triaxial compression at 12 c with confining pressures varying between 0 and 40 kpa the tests were strain rate controlled with strain rates ranging between 7 4 x 10 7 s 1 and 6 6 10 5 s 1 the analysis of the test results revealed that the relationship between yield stress and viscous strain rate is best given by a power law similar to polycrystalline ice however the power law exponent n is a function of density and varies between 1 8 low density snow p 320 kg in 3 the tests also showed that lower density snow displays a significant non linear stress strain response before yielding two further aspects of the constitutive behaviour of snow were identified 1 the strain rate independence of the post yield work hardening behaviour in compression and 2 the independence of the axial yield stress in relation to the confining pressure the experimental observations are discussed with respect to the mechanical properties of polycrystalline ice which is the constituent material of the load bearing ice skeleton

bubble nucleation in rhyolitic melts experiments at high pressure temperature and water content abstract we report the first measurement of bubble nucleation in hydrated rhyolitic melts in response to pressure release two rhyolitic obsidians one containing less than 1 of microlites of fe ti oxides and the other about 20 of various crystals were hydrated at 150 mpa and 780 850 c after saturation was reached 5 3 5 5 wt water pressure was lowered and the samples were allowed to nucleate and grow bubbles for various amounts of time before the final rapid quenching of the experiments the results demonstrate the importance of heterogeneous nucleation microlites of fe ti oxides are very efficient as sites for bubble nucleation in their presence modest nucleation was observed even after decompression by 6 10 8 bubbles cm 3 in the absence of microlites no nucleation occurred at p mpa at p 10 mpa bubbles also nucleated on crystals of biotite zircon and apatite modest nucleation 10 3 10 5 cm 3 took place even in crystal free samples but it was still heterogeneous when p exceeded 80 mpa nucleation in crystal free samples became extensive 10 5 10 7 cm 3 the lack of correlation of bubble density with either time or decompression suggests that nucleation was still heterogeneous nucleation rates were controlled mainly by the availability of sites rates were faster than 10 6 cm 3 s 1 when microlites were present and faster than 10 5 cm 3 s 1 in the absence of microlites at p 70 mpa narrow size distributions in most samples suggests that nucleation took place immediately after the pressure drop the experimental data we present here indicate that the presence or absence of efficient nucleation sites can lead to two distinct modes of bubble formation when a large number of efficient sites e g fe ti oxide are present bubble nucleation requires very little supersaturation and to a good approximation gas and magma are in equilibrium in magmas that are crystal free or contain crystals that are inefficient at nucleating bubbles very high degrees of supersaturation are required in order to initiate nucleation these two modes of exsolution may lead to contrasting styles of convection pressure build up and eruption

water and the viscosity of andesite melts the viscosity of a synthetic andesite like melt has been measured between 1010 and 1014 p for water contents in the range 0 3 5 wt the very slow kinetics of water exsolution over this viscosity range allowed the measurements to be made at 1 bar with a high precision after a steep viscosity decrease of 5 orders of magnitude for 1 wt h2o an additional 2 5 wt h2o causes a further viscosity decrease of only 2 orders of magnitude these viscosity decreases are qualitatively similar to those observed previously for more silica rich compositions the new data join smoothly with available high temperature measurements made at high pressures on water bearing andesite melts because the intrinsic effects of pressure are as small for water bearing as for water free samples the depressing effect of water on the viscosity of natural andesite melts can be estimated changes in water speciation as a function of either temperature or pressure do not seem to have marked effects on the viscosity although quantitative applications are not yet possible the configurational entropy theory accounts qualitatively for these features

experimental constraints on degassing of magma isothermal bubble growth during continuous decompression from high pressure numerical models predict that rapid ascent of hydrous magma can lead to supersaturation of dissolved volatile constituents possibly leading to explosive eruption we have performed controlled decompression experiments to investigate the ascent rates required to maintain bubble melt equilibrium high silica rhyolitic melts were saturated with water at 200 mpa and 825 c decompressed to lower pressures at constant rates of 0 025 0 25 0 5 and 1 0 mpa s 1 and then rapidly quenched isobarically other samples were saturated with water over the pressure range investigated to determine equilibrium water solubility in order to quantify degassing efficiency during decompression at a decompression rate of 0 025 mpa s 1 melt vapor equilibrium was maintained over the entire pressure range examined 200 to 17 5 mpa a single bubble nucleation event occurred in response to decompression and quenched bubble sizes can be modeled by a equilibrium bubble growth model that takes into account the number density of bubbles at decompression rates of 0 25 0 5 and 1 0 mpa s 1 rhyolitic melts could not degas in equilibrium when pressure decreased from 200 mpa to 140 mpa and water supersaturation p in the melt reached up to 60 mpa with higher values at faster decompression rates further pressure release resulted in near equilibrium degassing and p dropped significantly in each case p decreased when bubbles exceeded 10 vol a single heterogeneous bubble nucleation event occurred in each experiment when p 20 mpa no other bubbles nucleated despite p reaching 60 mpa which is probably too low to trigger homogeneous nucleation compared to estimates for magma decompression rates during lava dome eruptions our results indicate that magmas can degas efficiently throughout their ascent to the surface in explosive eruptions decompression rates may exceed those of this study and hence melts may become supersaturated with water such fast decompressions are expected however only when magma is highly vesicular which would aid approach to equilibrium degassing

an energy and mass model of snow cover suitable for operational avalanche forecasting a numerical model has been developed to simulate energy and mass evolution of snow cover at a given location as a function of meteorological conditions precipitation air temperature humidity wind velocity and incoming short wave and long wave radiation this model named crocus was first tested on a well instrumented field during a whole winter showing its ability to simulate the important phenomena affecting the evolution of the snow layers high temperature gradients wetting compaction and melting freezing cycles a second test was conducted at two locations in the french network used for operational avalanche forecasting though the weather observations are made there only twice daily the snow profiles calculated by the model were very close to those obtained once a week by a pit observation crocus proved itself sufficient to be considered now as a useful objective tool for operational avalanche forecasting

a re examination of the influence of composition on argon retentivity in metamorphic calcic amphiboles abstract the effect of composition on argon retention in metamorphic hornblende has been re evaluated by the 40 ar 39 ar step heating method utilizing the same samples from a k ar study in which a relationship between hornblende composition and argon retention was inferred incremental heating of the hornblendes yields 40 ar 39 ar age plateaus or near plateaus between 1081 3 and 1112 3 ma these data support a simple cooling history through argon closure following high grade metamorphism consistent with recent geochronological studies of the bamble sector of norway no clear evidence of extraneous argon is preserved in the 40 ar 39 ar data nor is there any sign of a thermal disturbance following argon closure contrary to the k ar results the 40 ar 39 ar data show no relationship between hornblende cooling age and composition the results of this investigation together with those from other studies suggest that the composition of metamorphic hornblende is not an important factor in controlling its argon closure temperature published studies inferring a compositional dependence on the argon closure temperature in metamorphic hornblende are commonly based on analytical data from impure retrogressed altered recrystallized or microstructurally complex hornblendes which are not representative of the diffusive properties of a pure hornblende such mineralogical and structural complexities may form both above and below the closure temperature of hornblende which may give rise to variable hornblende ages from rocks that have experienced the same geologic history such complexities forming at high temperature may contribute to a modified or variable characteristic grain size s of diffusion resulting in a modified complex hornblende closure temperature these complexities may contribute to low apparent hornblende ages if formed below its true closure temperature in addition variable hornblende ages may be obtained if fine grained mineral intergrowths are present and undergo recoil related phenomena during irradiation geologically meaningful hornblende ages are possible only when proper care is taken in sample selection and characterization

bubble growth in rhyolitic melts experimental and numerical investigation bubble growth controlled by mass transfer of water from hydrated rhyolitic melts at high pressures and temperatures was studied experimentally and simulated numerically rhyolitic melts were hydrated at 150 mpa 780 850 c to uniform water content of 5 5 5 3 wt the pressure was then dropped and held constant at 15 145 mpa upon the drop bubbles nucleated and were allowed to grow for various periods of time before final rapid quenching of the samples the size and number density of bubbles in the quenched glasses were recorded where number densities were low and run duration short bubble sizes were in accord with the growth model of scriven 1959 for solitary bubbles however most results did not fit this simple model because of interaction between neighboring bubbles hence the growth model of proussevitch et al 1993 which accounts for finite separation between bubbles was further developed and used to simulate bubble growth

kinetics of the reaction h2o o 2oh in rhyolitic and albitic glasses preliminary results the kinetics of homogeneous reactions are important in understanding the cooling history of rocks and in understanding experimental speciation data we have experimentally studied the kinetics of the interconversion reaction between h 2o molecules and oh groups in natural rhyolitic glasses 0 5 2 3 total water and a synthetic albitic glass l total water at 400 600 c the reaction rate increases with temperature and total water content equilibrium is not always approached monotonically the speciation may first depart from equilibrium and then come back to equilibrium experimental reaction rates agree with those inferred from previous speciation data of rhyolitic glasses quenched from 850 c the experimental data are modeled successfully by considering both the reaction and the diffusion of oh that brings oh groups together to react this study shows that species concentrations in glasses quenched from 600 c reflect those at experimental temperatures unless the water content is higher than that used in the present study species concentrations in glasses with total water contents 0 8 wt and which were rapidly quenched in water from 850 c do not represent their equilibrium concentrations in the melt at 850 c but record a lower apparent equilibrium temperature that depends on water content and quench rate natural rhyolitic glasses and glass inclusions do not record preeruptive melt speciation though total water content may be conserved the experimental data are used to infer cooling rates for natural obsidian glasses pyroclastic glass fragments from the bb site of mono craters have cooling rates similar to air cooled experimental charges 3 c s different types of glasses from the mono craters have different cooling rates which cover four orders of magnitude some natural obsidians appear to have had complex cooling histories the wide range of cooling rates and thermal histories is consistent with previous inferences that some obsidian clasts at the mono craters formed as glass selvages lining volcanic conduits or dikes that were subsequently caught up in the explosive eruption which led to variable degrees of transient heating followed by rapid cooling and deposition these experimental data reveal surprisingly rich detail in water speciation in volcanic glasses and show how at least in principle quantitative constraints on thermal histories can be extracted by experimentation and application of kinetic models

subglacial intermediate volcanism at kerlingarfjoll iceland magma water interactions beneath thick ice the products of andesite and dacite glaciovolcanism at kerlingarfjoll are unlike others previously described in the literature three sequences of lithofacies are described and interpreted here the andesitic deposits at campsite gully are divided into massive vitriclastic lapilli tuff with intrusions fluidal clast bearing vitriclastic lapilli tuff and stratified pumice rich vitriclastic lapilli tuff at tindur andesitic eruptions produced contorted pillow fragment breccia and the clast supported lithic breccia finally pillow lava with intrusions and crudely bedded vitriclastic lapilli tuff of dacitic composition are described at haraldur abundant vitriclasts the presence of pillow lavas hackly fracturing of lava bodies and lack of oxidation of clasts demonstrate that each of these lithofacies formed in the presence of abundant water this contrasts with all other descriptions of subglacially erupted intermediate magmas which are characterised by jointed and glassy lava flows and domes with a marked scarcity of fragmental material the kerlingarfjoll sequences therefore demonstrate that it is possible for intermediate magmas to generate and interact with significant volumes of water at the base of a glacier preliminary estimates of volatile contents in glassy clasts correspond to quenching pressures equivalent to 500 m water or 550 m ice this is consistent with eruption beneath an ice sheet that was thick enough to overwhelm the underlying topography and where meltwater drainage was controlled by the morphology of the glacier surface it is argued that the drainage of water due to steep topography and or thin and fractured ice as opposed to thermodynamic considerations is the most likely explanation for the absence of evidence for significant magma water interaction in previously described instances of intermediate glaciovolcanism the apparent low viscosity of the kerlingarfjoll magmas may relate to relatively high eruption temperatures and or the inhibition of degassing of the magma due to high ambient pressures and the consequent limitation of groundmass crystallisation

degassing driven crystallisation in basalts abstract syn eruptive crystallisation can drastically increase magma viscosity with profound implications for conduit dynamics lava emplacement and volcanic hazards there is growing evidence that crystallisation is not only cooling driven but can also occur almost isothermally during decompression induced degassing on ascent from depth here we review field and experimental evidence for degassing driven crystallisation in a range of magma compositions we then present new results showing for the first time experimental evidence for this process in basaltic magma our experiments use simultaneous thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry tga dsc ms to monitor degassing patterns and thermal events during heating and cooling of porphyritic basaltic samples from mt etna italy the partly degassed samples which contained 0 39 0 81 wt total volatiles in the glass fraction were subjected to two cycles of heating from ambient to 1250 c on the first heating tga data show that 30 60 of the total volatiles degassed slowly at

dynamics of a powerful deep submarine eruption recorded in h2o contents and speciation in rhyolitic glass the 2012 havre eruption constraining the syn eruptive volatile contents of magmatic melt is critical to understanding the intensities and styles of deep submarine volcanic eruptions for which direct observations are scarce quantifying residual magmatic water contents in volcanic glass is complicated by rehydration i e late stage addition of molecular water the 2012 deep submarine silicic eruption of havre volcano provides an unusual opportunity to quantify glass water contents from a recent well sampled stratigraphic sequence fourier transform infrared and microraman spectroscopy measurements of water concentration and water speciation across the havre 2012 eruptive sequence reveal an unanticipated range of excess molecular water within pumice this excess water requires rapid timescales of diffusion that are inconsistent with our current understanding of low temperature secondary rehydration in both subaerial and subaqueous eruptive products diffusion models applied to enrichment profiles at vesicle edges confirm that low temperature rehydration is an unlikely cause we instead support higher temperature syn eruptive pumice rehydration by condensed magmatic water and seawater in a submarine plume hydroxyl concentrations suggest shallow quenching depths of havre pumice hundreds of meters above the 900 meter deep main vent our data also support the presence of a vapor rich plume and consequent modification of ocean pressure above the vent we combine this novel volatile data with textural information and cooling rate calculations to explore the conditions that would cause slower shallow cooling of clasts from a deep submarine eruption by exploring the physical conditions for the interaction between pumice and submarine plumes we emphasize fundamental differences between subaerial and submarine clast producing eruptions

chemically driven growth and resorption of bubbles in a multivolatile magmatic system abstract a carbon dioxide rich fluid from a deep source fluxes in a water rich shallow stored magma and causes a volatile exchange with the melt we carried out hydrothermal experiments on both bubble free and bearing rhyolitic melts in an h 2 o co 2 system to explore the kinetics of chemical re equilibration between the fluid and the magma in the bubble free experiment a water saturated rhyolitic melt was first synthesized at 800 c and 100 mpa and this interacted with a carbon dioxide rich fluid under the same pt conditions for a period of 3 h the melt temporarily became undersaturated upon the volatile exchange as water quickly diffused out of the melt while dissolution of the carbon dioxide was much slower in the bubble bearing experiments the rhyolitic melt was saturated with h 2 o co 2 fluids at 800 c and 100 mpa decompressed to 50 mpa and then kept for up to 5 56 h water rich bubbles were generated by decompression and initially grew but then dissolved in the melt during the retention time this bubble resorption was attributed to water depletion of the melt upon re equilibration with the carbon dioxide rich surrounding fluid such water depletion and resulting bubble resorption are specific to multivolatile systems such as h 2 o co 2 in which the diffusivity and solubility differ among the end components the bubble resorption occurring in this mechanism may explain the genesis of carbon dioxide rich bubble poor obsidian pyroclasts which are considered to have undergone carbon dioxide rich fluid fluxing we also discuss possible consequences of this effect on the bulk magma density the dissolution of a small amount of carbon dioxide may cause expulsion of a much larger amount of water from the hydrous melt which may increase fluid fraction and thus decrease the bulk density by up to 188 and 104 kg m 3 at 100 and 200 mpa respectively for rhyolitic melts we suggest that such a process has the potential to trigger a volcanic eruption

snow replica method for three dimensional x ray microtomographic imaging visualization and quantification of snow structures at a scale of a few millimetres is important in understanding the mechanical thermal and electromagnetic properties of snow surface sections and to an even greater degree three dimensional 3 d reconstructions of cast snow samples are difficult to prepare and automatic image processing is notoriously difficult and often requires manual evaluation here we present a new method to measure the 3 d structure of cast snow samples snow samples cast with diethyl phthalate dep and frozen are cut to a sample size a few centimetres in diameter and up to 10 cm in height the ice of these samples is then sublimated in high vacuum and the remaining negative structure replica is imaged using x ray microtomography micro ct the accuracy of the method is demonstrated by comparing micro ct scans of the original snow structure and the replica the method described here allows easy transportation of samples requires little manual interaction has a very high spatial resolution of up to 10 m and is environmentally friendly