the cooling of frontal flow ramps a calorimetric study on the rocche rosse rhyolite flow lipari aeolian islands italy relaxation geospeedometry using differential scanning calorimetry dsc has been applied to quantify the cooling history across the glass transition of flow ramps at the front of the calcalkaline rhyolite rocche rosse flow of lipari aeolian islands italy modelled cooling rates for the obsidian retrieved from two profiles range between 0 2 and 0 03 k min 1 cooling at the flow front appears to be dominated by conductive heat loss of individual flow ramps forming individual cooling units cooling rates of tens of kelvins per day appear to have controlled the last stage of viscous deformation before the entire flow came to rest it is inferred that cooling rates slower than those modelled are required to sustain flow in highly viscous rhyolitic lavas

ganges basin geometry records a pre 15 ma isostatic rebound of himalaya the tertiary continental strata of the himalayan foreland basin are subdivided into two groups but the meaning of this subdivision was previously unclear from the analysis of drill holes seismic lines dated sections outcrops and balanced cross sections we find that the southward migration rate of the depositional pinch out of the younger group is 19 5 mm yr and equals the himalayan shortening rate this equality shows that the flexural foreland basin development is mainly controlled by the motion of the thrust load the long term pinch out migration rate was slower for the older synorogenic group erosion locally occurred at the end of its deposition due to tectonic reactivation of lineaments of the indian shield we suggest that this change in the basin development is linked to the detachment of the subducted indian lithosphere that decreased the slab pull and increased the mean compressive stress within the indian plate whereas the plate motion remained constant the most important implication of our work is that the associated isostatic rebound could have increased the himalayan elevation prior to 15 ma

shallow vent architecture during hybrid explosive effusive activity at cordon caulle chile 2011 12 evidence from direct observations and pyroclast textures in june 2011 an eruption of rhyolite magma began at the puyehue cordon caulle volcanic complex southern chile by january 2012 explosive activity had declined from sustained pyroclastic plinian to sub plinian fountaining to mixed gas and ash jetting punctuated by vulcanian blasts this explosive activity was accompanied by synchronous effusion of obsidian lava in a hybrid explosive effusive eruption fortuitous climatic conditions permitted ground based observation and video recording of transient vent dynamics as well as real time collection of proximal juvenile ash as it sedimented from the active plume the main eruptive vent complex and site of lava effusion was represented by two loci of vulcanian blasts within a single tephra cone containing a pancake shaped proto lava dome these blast loci each consisted of clusters of sub vents that expressed correlated shifts in eruption intensity indicating the presence of partially connected and or branching zones of high permeability within the upper conduit pyroclast textures were examined by x ray computed microtomography and their permeability was modeled by lattice boltzmann simulations the porosity 39 to 67 and darcian permeability 3 1x10 15 m2 perpendicular to fabric to 3 8x10 11 m2 parallel to fabric of fine ash emitted during ash jetting indicates that the permeable zones comprised highly sheared tube like bubbly magma and contrast with the low porosity 17 and nul permeability of bombs ejected to hundreds of metres from the vent in vulcanian blasts residual h2o content of ash 0 14 wt and two bombs 0 2 0 25 wt determined by karl fischer titration indicate degassing of this pyroclastic material to near atmospheric pressures ash textures and simple degassing vesiculation models indicate the onset of permeability by ductile processes of shear enhanced bubble coalescence in the upper 1 to 1 5 km of the conduit repeated ash jetting and vulcanian blasts indicate that such ductile processes were not sufficiently effective to accommodate all degassing requirements and additional brittle e g fragmentation events transiently assisted in maintaining an open system degassing regime that permitted long lived hybrid explosive effusive activity the eruption at cordon caulle has provided an excellent opportunity to closely observe key processes of silicic volcanism

an early pliocene thermal disturbance of the main central thrust central nepal implications for himalayan tectonics since the beginning of the collision between india and asia at about 50 ma the convergence in the himalaya has largely been taken up along major thrust zones in this study samples of the lesser himalaya formations up to 10 km below the main central thrust mct and the greater himalaya sequence up to 12 km above the mct have been analyzed by the 40ar 39ar and u pb methods to investigate the thermal history of the mct the ages can be summarized as follows 1 the youngest ages from muscovites 3 1 ma biotites 3 4 ma and hornblendes 4 1 ma all come from within 1 km of the mct 2 there is a marked asymmetry of ages between the footwall and the hanging wall of the mct the maximum mica age in the hanging wall greater himalaya sequence is 13 ma whereas a muscovite 5 km below the mct in the footwall has an age spectrum with a gradient from 400 to 1400 ma 3 five k feldspars from 6 2 to 11 9 km above the mct all give minimum ages in the range 3 0 6 4 ma and 4 a 206pb 238u age on a brannerite from the greater himalayan sequence is 4 8 ma structural and petrologic observations preclude the possibility that this age distance distribution reflects faulting within the greater himalaya sequence and production of the necessary thermal energy by shear heating requires unrealistically high shear stresses infiltration of hot fluids through the mct zone appears to be the best hypothesis to explain these data simple numerical simulations which account for heat transfer by advection within the fluid flow zone and by conduction outside it indicate that the observed age distribution could have been produced by infiltration of hot fluids through the mct zone at circa 5 to 4 ma within the following range of conditions the fluids heated rocks to temperatures in the range 470 to 510 c for less than 1 million years in a region narrower than the entire mct zone the temperature of the thermal disturbance inferred from the 40ar 39ar data is consistent with petrologic data from albitic alteration assemblages in the greater himalaya sequence and the u pb age of the brannerite is similar to the youngest 40ar 39ar hornblende age this scenario hydrothermal heating culminating at about 4 ma is similar to the model of le fort 1981 for the generation of the high himalayan leucogranites in the late oligocene early miocene by dehydration of the footwall rocks of an active thrust in the younger instance we interpret that fluids derived from the footwall rocks of the main boundary thrust migrated upward through the lesser himalaya formations and were subsequently channeled along the mct producing the thermal disturbance

slit dam design for debris flow mitigation debris flows cause the destruction of engineering facilities including buildings roads and culverts a slit dam may be used to control small volumes of debris that can damage or destroy small watershed infrastructure such as culverts the height and spacing of the dam posts and the location of the dam are the primary design factors the posts may be arranged in single or double rows with the spacing of the slits a function of the culvert diameter being protected the height of the dam is a function of the slope on which it is placed the advantages of slit dams include the simplicity of design and installation and the relatively low cost they are also stronger than other baffle systems and debris fences

conduit to localized scale degassing during plinian eruptions insights from major element and volatile cl and h2o analyses within vesuvius ad 79 pumice textural investigations of the ad 79 vesuvius pumice emphasize the complexity of magma degassing and crystallization during the eruption which emitted two types of pumice white and gray associated with different magma bodies of phonolitic and tephriphonolitic compositions respectively these studies proposed that velocity gradients caused spatial variations in degassing within the ascending magma column at both the conduit and the localized scale to validate this hypothesis analyses of volatiles cl h2o and major elements in pumice glasses and melt inclusions were performed using high spatial resolution tools microraman spectrometry and electron microprobe and combined with major element and volatile concentration profiles and maps the results indicate that the melt phase differentiated through degassing induced crystallization of leucite and that the gray pumice magma was efficiently homogenized prior to degassing because cl diffuses more slowly than h2o during fast ascent it behaves as an incompatible element and can be used as a tracer of crystallization and h2o degassing we emphasize the importance of strain localization in generating zones of preferential exsolution and permeable pathways for gases and establish degassing scenarios that incorporate the effects of shear zones

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

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

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

neogene foreland basin deposits erosional unroofing and the kinematic history of the himalayan fold thrust belt western nepal sedimentological and provenance data from the lower miocene pliocene dumri formation and siwalik group in western nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the himalaya and the cenozoic 87 sr 86 sr record of the ocean in western nepal the dumri formation is an 750 1300 m thick fluvial sandstone and overbank mudstone unit the siwalik group is 4200 m thick and consists of a lower member 850 m of 2 12 m thick fluvial channel sandstones and oxidized calcareous paleosols a middle member 2400 m of very thick 20 m channel sandstones and mainly organic rich histosols and an upper member 1000 m composed of gravelly braided river deposits paleocurrent data indicate that middle miocene pliocene rivers in western nepal flowed southward transverse to the thrust belt throughout deposition of the siwalik group no evidence was found for an axial fluvial trunk system i e the paleo ganges river in siwalik group sandstones a major increase in fluvial channel size is recorded by the transition from lower to middle siwalik members at 10 8 ma probably in response to an increase in seasonal discharge modal petrographic data from sandstones in the dumri formation and the siwalik group manifest an upsection enrichment in potassium feldspar carbonate lithic fragments and high grade metamorphic minerals modal petrographic analyses of modern river sands provide some control on potential source terranes for the miocene pliocene sandstones the dumri formation was most likely derived from erosion of sedimentary and low grade metasedimentary rocks in the tibetan tethyan himalayan zone during early miocene emplacement of the main central thrust the presence in dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the greater himalayan zone perhaps in response to tectonic unroofing by extensional detachment faults of the south tibetan detachment system during deposition of the lower siwalik group 15 11 ma emplacement of the dadeldhura thrust sheet one of the synformal crystalline thrust sheets of the southern himalaya on top of the dumri formation supplied abundant metasedimentary lithic fragments to the foreland basin a steady supply of plagioclase grains and high grade minerals was maintained by deeper erosion into the main central thrust sheet from 11 ma to the present k feldspar sand increased steadily suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower siwalik group this provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the dadeldhura thrust sheet above a large duplex in the lesser himalayan rocks since the onset of deposition of the conglomeratic upper siwalik group 4 5 ma fault slip in this duplex has been fed updip and southward into the main boundary and main frontal thrust systems we obtained 113 u pb ages on detrital zircons from modern rivers and siwalik group sandstones that cluster at 460 530 ma 850 1200 ma 1 8 2 0 ga and 2 5 ga an abundance of cambrian ordovician grains in the siwalik group suggests sources of siwalik detritus in the granites of the dadeldhura thrust sheet and possibly the greater himalayan orthogneisses the older ages are consistent with sources in the greater and lesser himalayan zones an overall upsection increase in zircons older than 1 7 ga suggests increasing aerial exposure of lesser himalayan rocks none of the detrital zircons even in the modern river samples yielded a cenozoic age that might suggest derivation from the cenozoic greater himalayan leucogranites but this may be attributable to the inheritance problems that characterize the u pb geochronology of the leucogranites when compared with recent studies of the 87 sr 86 sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the siwalik group the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the lesser himalayan zone and cambrian ordovician granitic rocks of the crystalline thrust sheets in central and eastern nepal may have played a significant role in elevating the 87 sr 86 sr ratio of middle miocene synorogenic sediments in the indo gangetic foreland basin and the bengal fan as well as global seawater

tectonometamorphic evolution of the himalayan metamorphic core between the annapurna and dhaulagiri central nepal the metamorphic core of the himalaya in the kali gandaki valley of central nepal corresponds to a 5 km thick sequence of upper amphibolite facies metasedimentary rocks this greater himalayan sequence ghs thrusts over the greenschist to lower amphibolite facies lesser himalayan sequence lhs along the lower miocene main central thrust mct and it is separated from the overlying low grade tethyan zone tz by the annapurna detachment structural petrographic geothermobaro metric and thermochronological data demonstrate that two major tectonometamorphic events charac terize the evolution of the ghs the first eohimalayan episode included prograde kyanite grade metamorphism during which the ghs was buried at depths greater than c 35 km a nappe structure in the lowermost tz suggests that the eohimalayan phase was associated with underthrusting of the ghs below the tz a c 37 ma 40ar 39ar hornblende date indicates a late eocene age for this phase the second neohimalayan event corresponded to a retrograde phase of kyanite grade recrystallization related to thrust emplacement of the ghs on the lhs prograde mineral assemblages in the mct zone equilibrated at average t 880 k 610 c and p 940 mpa 35 km probably close to peak of meta morphic conditions slightly higher in the ghs final equilibration of retrograde assemblages occurred at average t 810 k 540 c and p 650 mpa 24 km indicating re equilibration during exhumation controlled by thrusting along the mct and extension along the annapurna detachment these results suggest an earlier equilibration in the mct zone compared with higher levels as a consequence of a higher cooling rate in the basal part of the ghs during its thrusting on the colder lhs the annapurna detachment is considered to be a neohimalayan synmetamorphic structure representing extensional reactivation of the eohimalayan thrust along which the ghs initially underthrust the tz within the upper ghs a metamorphic discontinuity across a mylonitic shear zone testifies to significant late to post metamorphic out of sequence thrusting the entire ghs cooled homogeneously below 600 700 k 330 430 c between 15 and 13 ma middle miocene suggesting a rapid tectonic exhumation by movement on late extensional structures at higher structural levels

syn eruptive soft sediment deformation structures in a deep submarine caldera havre 2012 the 2012 rhyolitic eruption of havre submarine caldera in the kermadec arc generated an assemblage of new soft sediment deformation structures on the caldera floor these structures are the first documented examples of syn eruptive soft sediment deformation structures within a submarine caldera they are exquisitely portrayed on a high resolution 1 m bathymetric map made in 2015 their timing has been resolved by means of stratigraphic relationships with an overlying giant pumice deposit that correlates with the pumice raft first observed on 18th july 2012 utc weeks to months earlier in 2012 the largest lava was erupted from a vent on the southwestern caldera wall flowed down to the caldera floor and advanced almost 1 km farther a surficial layer of the caldera floor sediment surrounding the leading edge of the lava failed most likely in response to load and heat transfer from the lava failure produced a slump sheet comprising folds with arcuate crests at the toe an undeformed domain in the middle and a basin and ridge domain at the head adjacent to the lava subsequently 1 the lava ruptured on two main faults that propagated across the caldera floor and 2 strike slip on the faults moved the lava that was in between forward simultaneously offsetting segments of the slump sheet these two events were probably triggered by the earthquake swarm recorded on the 17th july 2012 utc a day before the pumice raft appeared the leading edge of the lava then sank slowly into the underlying sediment sinking was accommodated by sideways expulsion of sediment from beneath the lava modifying the topography around the edge of the lava the caldera floor sediment was also disturbed when part of the southwestern caldera wall collapsed generating a small debris avalanche which is represented by a cluster of hummocks on the caldera floor collapse was a response to shallow intrusion of a rhyolite dyke into the southwestern caldera wall

obsidian hydration a new paleothermometer the natural hydration of obsidian was first proposed as a dating technique for young geological and archaeological specimens by friedman and smith 1960 who noted that the thickness of the hydrated layer on obsidian artifacts increases with time this approach is however sensitive to temperature and humidity under earth surface conditions this has made obsidian hydration dating more difficult but potentially provides a unique tool for paleoclimatic reconstructions in this paper we present the first successful application of this approach based on combining laboratory based experimental calibrations with archaeological samples from the chalco site in the basin of mexico dated using stratigraphically correlated 14 c results and measuring hydration depths by secondary ion mass spectrometry the resultant data suggest first that this approach is viable even given the existing uncertainties and that a cooling trend occurred in the basin of mexico over the past 1450 yr a result corroborated by other paleoclimatic data

quenching of steam charged pumice implications for submarine pyroclastic volcanism abstract huge quantities of silicic pumice have been deposited in intra oceanic convergent margin settings throughout earth s history the association of submarine silicic calderas with thick proximal accumulations of pumice lapilli suggests that these pyroclasts were deposited as a direct result of submarine eruptions yet when first erupted these highly vesicular gas filled clasts had densities significantly less than seawater experiments carried out 1 atm on heated pumice samples whose vesicles were charged with steam the dominant component of magmatic volatiles show that buoyancy of freshly erupted submarine pumice is transient upon quenching the phase change of steam to liquid water creates strong negative pore pressures within the pumice vesicles that accelerate the absorption of surrounding water generating high density pumice and promoting rapid clast sinking variations in the physical properties of steam with temperature and pressure have important implications for submarine pyroclastic eruptions firstly highly vesicular pumice can be deposited on the seafloor at temperatures elevated significantly above ambient if they are erupted at sufficient depths to remain wholly submarine 200 m and either the fluid in which they cool contains heated water and or they only absorb sufficient water to sink secondly the rapid increase in density of the eruption column caused by condensation and the transition from buoyant gas filled to denser water saturated pumice lapilli together with turbulent mixing with the surrounding seawater favour collapse and transport of pyroclasts in water supported gravity currents finally this mixing of the ejecta with seawater and the ease of water ingestion into permeable pumice clasts suggest that water supported transport mechanisms can operate as primary dispersal processes in explosive submarine eruptions

cooling rates of plinian fall and pyroclastic flow deposits in the bishop tuff inferences from water speciation in quartz hosted glass inclusions cooling rates inferred from water speciation in rhyolitic glass inclusions in quartz phenocrysts from the bishop tuff california vary over more than 5 orders of magnitude from about 10 to 10 4 c s 1 many of the cooling rates are within the previously published experimentally calibrated range of this technique but the slowest rates 10 4 c s 1 are extrapolated such extrapolation suggests cooling rates as low as 10 8 c s 1 0 3 c year 1 for the interiors of thick pyroclastic flow deposits the temperatures at which oh and molecular h2o species would be in equilibrium range from 400 c for glass inclusions from plinian fall deposits to 250 c for the most slowly cooled glassy inclusions from pyroclastic flow deposits notwithstanding appreciable uncertainties cooling rates correlate with depositional process fall vs flow deposition and distance above the cold ground or below overlying hot pyroclastic flow deposits plinian inclusions within 2 5 m of the cold pre bishop ground cooled at 1 10 c s 1 consistent with thermal modeling of small pumice clasts falling through cold air very slow inferred cooling rates for pyroclastic flow deposits 10 6 to 10 8 c s 1 also agree with thermal conduction models faster cooling rates 10 1 to 10 4 c s 1 for inclusions in the lowermost 0 5 m of pyroclastic flow deposits reflect the effect of quenching against the cold ground at blind spring hill east of long valley caldera intermediate cooling rates 10 3 to 10 6 c s 1 are observed in plinian deposits overlain by and partially interbedded with pyroclastic flow deposits indicating that reheating of rapidly cooled plinian inclusions can reset water speciation melt inclusions in pyroclastic flow deposits are quenched to glass only if the deposit is relatively thin or has a relatively low emplacement temperature in relatively thick or hot deposits inclusions may be quenched to glass at the base of the deposit where it is more rapidly cooled by contact with the pre eruption ground

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

age of crustal melting and leucogranite formation from u pb zircon and monazite dating in the western himalaya zanskar india new u pb data from leucogranites in the high himalayan slab in the western himalaya of zanskar reveal that crustal melting occurred at 21 19 5 ma and that anatexis occurred along the himalaya at least from kashmir zanskar to eastern nepal at approx 24 19 5 ma high u monazites from leucogranitic melt pods in migmatites within the 35 km wide sillimanite k feldspar zone at the deepest structural levels exposed umasi la have u pb ages of 20 6 19 5 ma higher level migmatitic melt pods from shafat in the suru valley also contain magmatic monazites with u pb ages of 20 8 0 3 ma zircons from the leucogranites contain an approx 460 ma inherited component as do some umasi la monazites showing that the protoliths were at least ordovician in age

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

eocene early miocene foreland basin development and the history of himalayan thrusting western and central nepal sedimentologic petrographic and u pb detrital zircon ages from middle eocene through early miocene sedimentary rocks in the lesser himalayan zone of western and central nepal indicate that a peripheral foreland basin system had developed in the eastern himalayan collision zone by middle eocene time the shallow marine eocene bhainskati formation accumulated in a back bulge depozone between a southward migrating forebulge and the indian craton migration of the forebulge through this region during eocene oligocene time produced a regional unconformity that spans 15 20 myr by early miocene time the forebulge unconformity was onlapped by the distal fringes of the southward migrating foredeep depozone represented by fluvial deposits of the dumri formation continued southward migration of the foredeep during the neogene accommodated the fluvial siwalik group light mineral provenance data and u pb detrital zircon ages suggest that the bhainskati was derived partly from tethyan sedimentary rocks of the tibetan himalayan zone during initial growth of the himalayan fold thrust belt the dumri was derived from metasedimentary and crystalline rocks of the greater himalayan zone during emplacement of the main central thrust and contemporaneous tectonic unroofing by normal faulting along the south tibetan detachment system the lesser himalayan crystalline thrust sheets were emplaced soon after deposition of the dumri formation 15 10 ma paleocurrent and lithofacies data from the dumri formation indicate deposition by west southwestward flowing rivers that drained into the indus portion of the himalayan foreland basin system during the early miocene thick channel sandstones in the lower dumri may represent the early miocene counterpart of the modern ganges river eastward diversion of the ganges drainage system to near its present location had occurred by 15 ma as the high standing aravalli range on the northern indian shield approached the front of the fold thrust belt assuming reasonable values for the flexural rigidity of indian lithosphere the time span of the forebulge unconformity yields a velocity of 14 33 mm yr for the southward migration of the fold thrust belt relative to india this range of values is consistent with neogene and present day estimates and suggests that only one third to one half of india eurasia convergence has been accommodated by shortening in the himalayan fold thrust belt since the onset of collision

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

distributed deformation in southern and western tibet during and after the india asia collision field and radiometric data are used to describe and date strain and stress states in southern longitude 88 o to 91oe latitude 28 o to 30on and western tibet longitude 79 o to 82oe latitude 30 o to 34on we factorize deformation into syncollisional and postcollisional and we present stretching lineation and displacement orientation maps two sections across the indian shelf sequence and stress orientations calculated from mesoscale fault slip data in southern tibet syncollisional stretching and displacement directions trend 90 46 o and displacement is top to south synkinematic low grade metamorphism is dated at 50 ma at one locality in the indian shelf sequence underlying the main mantle thrust of the indus yarlung suture this implies paleocene onset of continental collision for the investigated section postcollisional structures comprise a backthrust group which includes foreland and hinterland directed thrusts reverse and strike slip faults and folds it dominates postcollisional deformation is concentrated along the indus yarlung suture and portrays n s compression otrend of 8o 17 o o2 of 97o 17o a strike slip group consists of conjugate strike slip faults is concentrated in east trending narrow highly deformed zones and indicates that n s compression is locally compensated by e w extension oof 15o 29 o o3 of 103o 30o synkinematic muscovite dates postcollisional deformation as late early miocene 17 5 ma at one locality at the suture strike slip and oblique normal 03 of 600 23 o oof 144o 21 o and normal o3 114o 16 o faulting dated between late miocene and recent and including active deformation represents dominant e w and minor n s extension due to e w stretching of southern tibet and oroclinal bending along the himalayan arc restoring syncollisional and postcollisional deformation yields a minimum of 67 258 km shortening across the indian shelf sequence incorporating recently published contraction estimates across the eastern himalaya yields minimum shortening between undeformed india and the indus yarlung suture of 66 536 km the himalaya tibet orogenic system south of the indus yarlung suture had an initial width of 811 km in the southern tibetan section in western tibet imbrication of an ophiolite sequence of the bangong nujiang suture is top to south stretching lineation trend of 15o 18o and o3 of active deformation trends ese faulting along the shiquanhe fault zone which transfers displacement from the northern part of the karakorum fault to a system of rifts in western central tibet indicates dextral strike slip alternating with sinistral oblique normal faulting and block rotations around vertical axes during a prolonged shearing history the indian shelf sequence south of mount kailas shows top to south imbrication stretching lineation trend of 52o 60o both indian shelf rocks and oligocene miocene kailas conglo merates record backthrusting and backfolding oof 33 o and recent e w extension 03 of 85o 28o

detection of inherited monazite in the manaslu leucogranite by 208pb232th ion microprobe dating crystallization age and tectonic implications abstract although leucogranites are among the least petrologically variable of all igneous rocks ironically they are among the most difficult to reliably date the high himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt for a variety of reasons linked to the minimum melt composition of these leucogranites these dating studies have not been entirely successful we report results of a new ion microprobe dating method based on the decay of 232 th to 208 pb in monazite that has directly revealed the presence of inherited pb in monazite from the manalsu granite casting doubt on its previously accepted age monazite ages obtained from this leucogranite yield two distinct populations a large number of ages with a normal distribution and mean age of 22 4 0 5 ma 2 s e that we interpret to be the crystallization age and a smaller inherited fraction with an age of ca 600 ma because formation of the granite is thought to be related to slip on the main central thrust both this date and a second less precise result from a structurally similar pluton near mt everest indicate that the main central thrust was active in the interval 24 22 ma but do not constrain its initiation together with crosscutting relationships these data require that movement on the north himalayan fault occurred prior to 22 ma at both locations

rapid ascent and emplacement of basaltic lava during the 2005 06 eruption of the east pacific rise at ca 9 51 n as inferred from co2 contents abstract eruption rates at the mid ocean ridges mors are believed to strongly control the morphology and length of lava flows emplaced along the ridge axis and thus the structure and porosity of the upper oceanic crust eruption rate also represents one of the few tools to gain insight into the driving pressures within sub ridge magmatic systems as eruption rate is inferred to vary systematically along the global mid ocean ridge system understanding of how to assess eruption rate in submarine systems and how it maps to observable features of the ridge axis would provide a powerful tool to understand earth s largest magmatic system eruption rates at mors are poorly constrained however because of a lack of direct observations preventing the duration of an eruption to be quantified this study uses decompression experiments of morb samples and numerical modeling of co 2 degassing to reconstruct the timescales for magma ascent and lava emplacement during the 2005 06 eruption of the east pacific rise at ca 9 51 n samples collected from the lava flow are all supersaturated in dissolved co 2 contents but co 2 decreases with distance from the vent presumably as a consequence of progressive co 2 diffusion into growing bubbles samples collected at the vent contain 10 5 vesicles per cm 3 pieces of these samples were experimentally heated to 1225 c at high pressure and then decompressed at controlled rates results plus those from numerical modeling of diffusive bubble growth indicate that magma rose from the axial magma chamber to the seafloor in 1 h and at a rate of 2 3 km h 1 our modeling as validated by experimental decompression of morb samples with 10 6 vesicles cm 3 also suggests that co 2 degassed from the melt within 10 100 min as the vesicular lava traveled 1 7 km along the seafloor implying a volumetric flow rate on order of 10 3 4 m 3 s 1 given an ascent rate of 0 2 m s 1 the width of a rectangular dike feeding the lava would have been 1 2 m wide morb samples from the pacific ridge are generally more supersaturated in dissolved co 2 than those from slower spreading atlantic and indian ridges our results suggest that pacific morbs ascend to the seafloor faster than atlantic or indian morbs

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

volcanology vulcan rule beneath the sea over 70 of the volcanism on earth occurs beneath an ocean veil now robotic and fibre optic based technologies are beginning to reveal this deep environment and identify subaqueous volcanoes as rich sources of sulfur carbon dioxide and life

quench rates in air water and liquid nitrogen and inference of temperature in volcanic eruption columns we applya geospeedometer previouslydeveloped in this lab to investigate cooling rate profiles of rhy olitic samples initiallyheld at 720 750 c and quenched in water liquid nitrogen and air for quench of mm size samples in liquid nitrogen and in air the cooling rate is uniform and is controlled byheat transfer in the quench medium instead of heat conduction in the sample the heat transfer coefficient in static air decreases with increasing sample size for quench of mm size samples in water heat transfer in water is rapid and the cooling rate is largelycontrolled byheat conduction in the sample our experimental results are roughlyconsistent with previous calculations for cooling in air and in water although constant heat transfer coefficients were used in these calculations but cooling rate in liquid nitrogen is only1 8 2 3 times that in static air and slower bya factor of 2 than calculated byprevious authors cooling rate in compressed airflow is about the same as that in liquid nitrogen the experimental results are applied to interpret cooling rates of pyroclasts in ash beds of the most recent eruptions of the mono craters cooling rates of pyroclasts are inversely correlated with sample size and slower than those in air the results indicate that the hydrous species concentrations of the pyroclasts were frozen in the eruption column rather than inside ash beds or in flight in ambient air from the cooling rates we infer eruption column temperature in a region where and at a time when hydrous species concentrations in a pyroclast were locked in the temperature ranges from 260 to 570 c for the most recent eruptions of mono craters these are the first estimates of temperatures in volcanic eruption columns the abilityto estimate cooling rates and eruption column temperatures from eruptive products will provide constraints to dynamic models for the eruption columns 2002 elsevier science b v all rights reserved

thermal evolution of the lesser himalaya central nepal insights from k white micas compositional variation abstract the lesser himalayan low to medium grade metamorphic rocks in central nepal are rich in k white micas occurring as porphyroclasts and in matrix defining s 1 and s 2 porphyroclasts are usually zoned with celadonite poor cores and celadonite rich rims the cores are the relics of igneous or high grade metamorphic muscovites and the rims were re equilibrated or overgrown under lower t metamorphic conditions the matrix k white micas defining s 1 pre dating the main central thrust activity are generally celadonite rich they show heterogeneous compositional zoning with celadonite rich cores and celadonite poor rims they were recrystallized at lower t condition prior to the main central thrust activity most probably prior to the india asia collision pre himalayan metamorphism the matrix k white micas along s 2 synchronous to the main central thrust activity neohimalayan metamorphism are relatively celadonite poor and were recrystallized under relatively higher t condition k white micas defining s 1 also were partially re equilibrated during the neohimalayan metamorphism the average compositions of recrystallized k white micas defining both s 1 and s 2 become gradually poor in fe mg and si contents and rich in al and ti contents from south to north showing an increase of metamorphic grade from structurally lower to higher parts in the lesser himalaya this shows that the metamorphism is inverted throughout the inner lesser himalaya the tectono metamorphic significance of the published k ar and 40 ar 39 ar k white micas ages from the lesser himalaya need re evaluation in the context of observed intrasample compositional variation and zoning and possible higher closure temperature 500 c for k ar system

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

highly vesicular pumice generated by buoyant detachment of magma in subaqueous volcanism many submarine caldera volcanoes are blanketed with deposits of highly vesicular pumice typically attributed to vigorous explosive activity1 2 3 4 however it is challenging to relate volcanic products to specific eruptive styles in submarine volcanism5 6 here we document vesicularity and textural characteristics of pumice clasts dredged from the submarine macauley volcano in the kermadec arc southwest pacific ocean we find that clasts show a bimodal distribution with corresponding differences in vesicle abundances and shapes specifically we find a sharp mode at 91 vesicularity and a broad mode at 65 80 subordinate clasts show gradients in vesicularity we attribute the bimodality to a previously undocumented eruptive style that is neither effusive nor explosive the eruption rate is insufficient to cause magma to fragment explosively yet too high to passively feed a lava dome instead the magma foam buoyantly detaches at the vent and rises as discrete magma parcels or blebs while continuing to vesiculate internally the blebs are widely distributed by ocean currents before they disintegrate or become waterlogged this disintegration creates individual clasts from interior and rim fragments yielding the bimodal vesicularity characteristics we conclude that the generation and widespread dispersal of highly vesicular pumice in the marine environment does not require highly explosive activity

the critical point and two phase boundary of seawater 200 500 c abstract the two phase boundary of seawater was determined by isothermal decompression of fully condensed seawater in the range of 200 500 c the pressure at which phase separation occurred for each isotherm was determined by a comparison of the refractive index of fluid removed from the top and bottom of the reaction vessel the critical point was determined to be in the range of 403 406 c 285 302 bar and was located by the inflection in the two phase boundary and by the relative volume of fluid and vapor as a function of temperature the two phase boundary of 3 2 nacl solution was found to coincide exactly with that of seawater over the range tested in the present study the boundary for both is described by a single seventh order polynomial equation the two phase boundary defines the maximum temperature of seawater circulating at depth in the oceanic crust thus the boundary puts a limit of about 390 c for seawater circulating near the seafloor at active ocean ridges 2 5 km water depth and about 465 c at the top of a magma chamber occurring at 2 km below the seafloor

contrasting pyroclast density spectra from subaerial and submarine silicic eruptions in the kermadec arc implications for eruption processes and dredge sampling pyroclastic deposits from four caldera volcanoes in the kermadec arc have been sampled from subaerial sections raoul and macauley and by dredging from the submerged volcano flanks macauley healy and the newly discovered raoul sw suites of 16 32 mm sized clasts have been analyzed for density and shape and larger clasts have been analyzed for major element compositions density spectra for subaerial dry type eruptions on raoul island have narrow unimodal distributions peaking at vesicularities of 80 85 whereas ingress of external water wet type eruption or extended timescales for degassing generate broader distributions including denser clasts submarine erupted pyroclasts show two different patterns healy and raoul sw dredge samples and macauley island subaerial emplaced samples are dominated by modes at 80 85 implying that submarine explosive volcanism at high eruption rates can generate clasts with similar vesicularities to their subaerial counterparts a minor proportion of healy and raoul sw clasts also show a pink oxidation color suggesting that hot clasts met air despite 0 5 to 1 km of intervening water in contrast macauley dredged samples have a bimodal density spectrum dominated by clasts formed in a submarine eruptive style that is not highly explosive macauley dredged pyroclasts are also the mixed products of multiple eruptions as shown by pumice major element chemistry and the sea floor deposits reflect complex volcanic and sedimentation histories the kermadec calderas are composite features and wide dispersal of pumice does not require large single eruptions when coupled with chemical constraints and textural observations density spectra are useful for interpreting both eruptive style and the diversity of samples collected from the submarine environment

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

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

note on the analysis of plunging of density flows this note is devoted to the correction of an error in a calculation contained in a classical published paper on the plunging of river flows as they enter lakes or reservoirs the correction of the error provides a felicitous result according to which not only the ratio of underflow thickness just after plunging to depth just before plunging but also the densimetric froude numbers both just before and just after plunging are all specified as a function of a single parameter this single parameter characterizes the tendency of plunging to entrain ambient water into the density underflow

nonequilibrium degassing regassing and vapor fluxing in magmatic feeder systems magma degassing models typically invoke volatile depletion of a single parental melt with permeable loss of exsolved gas having served for many years as the paradigm for the transition from volatile rich explosive eruptions to volatile depleted lava flows these degassing models are guided by measurements of h2o co2 and hydrogen isotope variations retained in melt that quenched to glass but the existing models are not uniquely constrained by the data there also remains uncertainty surrounding the origin and significance of volcanic glass fragments we show that individual obsidian pyroclasts from mono craters california usa are heterogeneous in dissolved h2o and co2 suggesting that clasts are assembled from juvenile melt and rewelded ash during magma ascent this is in contrast to the conventional view that clasts are chemically homogeneous and sample the chilled glassy margins of conduit walls the new measurements of dissolved h2o and co2 help reconcile existing open system degassing models used to explain elevated co2 h2o ratios provide time scales based on diffusion modeling for pyroclast formation and show that magma does not necessarily lose volatiles monotonically during ascent driven decompression

40ar 39ar age constraints on deformation and metamorphism in the main central thrust zone and tibetan slab eastern nepal himalaya in eastern nepal structural and petrologic observations suggest that movement on the main central thrust mct the development of an inverted metamorphic sequence and leucogranite plutonism were genetically related samples from across the mct zone and its hanging wall the tibetan slab were collected from four locations in the everest region for 40ar 39ar studies 1 the mct zone 2 the lower tibetan slab 3 the upper tibetan slab away from intrusive rocks and 4 the upper tibetan slab adjacent to and including leucogranitic material within the mct zone muscovite and hornblende yield cooling ages of 12 0 0 2 ma and 20 9 0 2 ma respectively k feldspar gives a minimum age of 8 0 0 2 ma closure temperature tc 220 15 c in the lower tibetan slab a sample from a sheared pegmatite yields muscovite and biotite isochron ages of 7 7 0 4 ma and 7 5 0 6 ma and a k feldspar minimum age of 6 4 0 8 ma tc 225 20 c an adjacent gneiss yields a 9 1 0 2 ma biotite isochron and a k feldspar minimum age of 3 6 0 2 ma tc 210 50 c in the upper tibetan slab samples collected 200 m from visible intrusives yield a biotite isochron age of 20 2 0 2 ma and a complex hornblende age spectrum suggestive of excess argon at the fourth location an augen gneiss a pegmatite and a tourmaline bearing leucogranite all in mutual contact yield indistinguishable mineral ages the average biotite and muscovite isochron ages for these samples are 17 0 1 4 and 16 6 0 2 ma respectively the minimum age for k feldspar from the leucogranite is 15 5 1 8 ma as thermobarometric data from the mct zone in this area suggest synmetamorphic deformation at temperatures of 500 550 c the mct hornblende tc 500 c dates this event at 21 ma diffusion experiments on hornblendes from the mct zone provide data which support a maximum duration of peak metamorphic temperatures of 2 ma biotite ages as old as 20 ma from the upper tibetan slab near leucogranites indicate that leucogranite intrusion was essentially coeval with deformation and metamorphism in the mct zone very young ages in a ductile shear zone in the lower tibetan slab suggest that there has been deformation within the tibetan slab that postdates major movement on the mct

dynamics of deep submarine silicic explosive eruptions in the kermadec arc as reflected in pumice vesicularity textures abstract despite increasing recognition of silicic pumice bearing deposits in the deep marine environment the processes involved in explosive silicic submarine eruptions remain in question here we present data on bubble sizes and number densities number of bubbles per unit of melt matrix for deep submarine erupted pumices from three volcanoes healy raoul sw and havre along the kermadec arc sw pacific to investigate the effects of a significant 1 km overlying water column and the associated increased hydrostatic pressure on magma vesiculation and fragmentation we compare these textural data with those from chemically similar subaerially erupted pyroclasts from nearby raoul volcano as well as submarine erupted tangaroan fragments derived by non explosive buoyant detachment of foaming magma from macauley volcano also along the kermadec arc deep submarine erupted pumices are macroscopically similar colour density texture to subaerial or shallow submarine erupted pumices but show contrasting microscopic bubble textures deep submarine erupted pyroclasts have fewer small

inverted metamorphism in the pre siwalik foreland basin sediments beneath the crystalline nappe western nepal himalaya abstract one of the pre siwalik foreland basin sedimentary units the dumri formation is tectonically covered by the lesser himalayan crystalline nappe and the kuncha naudanda thrust sheet it is narrowly distributed in the eastern margin of the karnali klippe along the nne ssw trending chakure fault the whole sequence of the fluvial dumri formation attaining 1500 m in thickness is weakly metamorphosed to muscovite phyllite and foliated phyllitic sandstone the metamorphic grade decreases stratigraphically downward and underlying nummulitic limestone of the middle eocene bhainskati formation is converted into a slaty limestone no metamorphic mica is detected from the late cretaceous to paleocene amile formation below the bhainskati formation these facts indicate that the tansen group has undergone inverted metamorphism a 40 ar 39 ar plateau age of 25 69 0 13 ma was obtained from garnetiferous biotite gneiss in the lower part of the crystalline nappe another 40 ar 39 ar age spectrum from muscovite phyllite of the dumri formation suggests that metamorphism occurred at 16 17 ma the origin of the inverted metamorphism limited to the uppermost part of the lesser himalayan autochthon can be attributed to heat from the hot crystalline nappe and shearing along the sole thrust of the kuncha naudanda thrust sheet the depositional age of the dumri formation is estimated to be 26 17 ma provenance of the dumri formation is considered to be from the naudanda quartzite the kuncha formation and the tibetan tethys sediments because the sandstone contains orthoquartzite pebbles phyllitic lithic fragments and a sparry calcite cement the sedimentary facies indicates deposition by meandering rivers on flood plains in the distal part of the foreland basin no proximal facies such as alluvial fan and pebbly braided river deposits could be detected from the formation though it is near the main central thrust mct the northern continuation of the foreland basin sediments must be concealed beneath the higher himalayan crystalline judging from the present distribution of the dumri formation from the south of the main boundary thrust mbt to near the mct and from the shortening of the lesser himalayan sediments by thrusts and folds the width of the foreland basin where the dumri formation was deposited is estimated to have been more than 300 km

nd isotopic data reveal the material and tectonic nature of the main central thrust zone in nepal himalaya abstract the main central thrust mct is a tectono metamorphic boundary between the higher himalayan crystallines hhc and lesser himalayan metasediments lhs reactivated in the tertiary but which had already formed as a collisional boundary in the early paleozoic to investigate the nature of the mct we analyzed whole rock nd isotopic ratios of rocks from the mct and surrounding zones in the taplejung ilam area of far eastern nepal annapurna galyang area of central nepal and maikot barekot area of western nepal we define the mct zone as a ductile brittle shear zone between the upper mct umct and lower mct lmct the protoliths of the mct zone may provide critical constraints on the tectonic evolution of the himalaya the lhs is lithostratigraphically divided into the upper and lower units in the taplejung ilam area different lithologic units and their e nd 0 values are as follows hhc 10 0 to 18 1 mct zone 18 5 to 26 2 upper lhs unit 17 2 and lower lhs unit 22 0 to 26 9 there is a distinct gap in the e nd 0 values across the umct except for the southern frontal edge of the ilam nappe in the annapurna galyang and maikot barekot areas different lithologic units and their e nd 0 values are as follows hhc 13 9 to 17 7 mct zone 23 8 to 26 2 except for an outlier of 12 4 upper lhs unit 15 6 to 26 8 and lower lhs unit 24 9 to 26 8 these isotopic data clearly distinguish the lower lhs unit from the hhc combining these data with the previously published data the lowest e nd 0 value in the hhc is 19 9 we regard rocks with e nd 0 values below 20 0 as the lhs in contrast rocks with those above 19 9 are not always the hhc and some parts of them may belong to the lhs due to the overlapping nd isotopic ratio between the hhc and lhs most rocks of the mct zone have nd isotopic ratios similar to those of the lhs but very different from those of the hhc the spatial patterns in the distribution of e nd 0 value around the umct suggest no substantial structural mixing of the hhc and lhs during the umct activities in the tertiary a discontinuity in the spatial distribution of e nd 0 values is laterally continuous along the umct throughout the himalayas these facts support the theory that the umct was originally a material boundary between the hhc and lhs suggesting the mct zone was mainly developed with undertaking a role of sliding planes during overthrusting of the hhc in the tertiary

propagation of the thrust system and erosion in the lesser himalaya geochemical and sedimentological evidence centre de recherches pe trographiques et ge ochimiques bp 20 54501 vandoeuvre les nancy franceabstractsedimentological and nd isotope data of two sections of the sub himalaya of westernnepal are used as new constraints for understanding the erosion history of the himalaya throughout the deposition of the middle and upper members of the siwalik group thelesser himalaya contribution to the total detrital input progressively increased from lessthan 20 to 40 the increasing proportion of lesser himalaya sediments started at ca 10 8 ma and is associated with a coarsening of the maximum grain size at both micro scopic and macroscopic scales thin skinned tectonics of the lesser himalaya thrust sys tem would have controlled the exhumation of the lesser himalaya rocks and would havebegun at 12 10 ma taking into account the delay for denudation together with otherstudies these data restrict the onset of movement on the lesser himalaya thrust systemto less than 3 m y along more than 1750 km this short time frame implies a ratio oflateral propagation rate to shortening rate far too high for the propagation of a singlecrustal thrust thus we suggest instead the simultaneous initiation of several thrusts aheadof the main central thrust at ca 12 ma we suggest that a rapid rise of the tibetanplateau at this time has transformed the himalaya to an overcritical thrust wedge thathas propagated forward to return to a stable state this regional rising could be the primecause of the increase of sediment in ux at ca 11 ma around the himalaya keywords siwalik formations neodymium grain size erosion thrust sheets himalaya

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

spherulites as in situ recorders of thermal history in lava flows spherulites in rhyolitic obsidian provide a record of the thermal history of their host lava during the interval of spherulite growth we use trace element concentration profiles across spherulites and into the obsidian host from yellowstone national park usa to reconstruct the conditions that existed during spherulite formation the measured transects reveal three behaviors expulsion of the most diffusively mobile elements from spherulites with no concentration gradients in the surrounding glass type 1 enrichment of slower diffusing elements around spherulites with concentration gradients extending outward into the glass type 2 and complete entrapment of the slowest diffusing elements by the spherulite type 3 we compare the concentration profiles measured by laser ablation inductively coupled plasma mass spectrometry and fourier transform infrared spectroscopy to the output of a spherulite growth model that incorporates known diffusion parameters the temperature interval of spherulite growth the cooling rate of the lava and data on the temporal evolution of spherulite radius our results constrain spherulite nucleation to the temperature interval 700 550 c and spherulite growth to 700 400 c in a portion of lava that cooled at 10 5 2 0 3 c s 1 which matches an independent experimental estimate of 10 5 3 c s 1 measured using differential scanning calorimetry maximum spherulite growth rates at nucleation are on the order of 1 m hr 1 and are inferred to decrease exponentially with time hence spherulites may serve as valuable in situ recorders of the thermal history of lava flows

tracking the growth of the himalayan fold and thrust belt from lower miocene foreland basin strata dumri formation western nepal u s national science foundation nsf tectonics ear 1140068 conoco fellowship scholarship 2016 chevron texaco geology fellowship 2016 university of arizona galileo circle scholarship 2017 u s national science foundation nsf nsf ear 1338583 ear1649254

numerical investigation of plunging density current when a buoyant inflow of higher density enters a reservoir it sinks below the ambient water and forms an underflow downstream of the plunge point the flow becomes progressively diluted due to the fluid entrainment the entrainment rate is strongly dependent on the richardson number and reaches a constant value well downstream of the plunge point this study is concerned with the analysis of the plunging phenomenon and the determination of the entrainment a k e model including buoyancy effects both in a sloping and a diverging channel is used to reproduce the main flow characteristics a relation between the depth at the plunge point in a channel of constant width and in a diverging channel is established and theoretical results for the calculation of the dense layer thickness are provided the latter indicates that the spreading rate of the dense layer in a diverging channel is a function of both the entrainment rate and the channel width the predictions of the plunge line location are in agreemen

non hydrostatic effects in layered shallow water flows this paper develops a one dimensional extension to classical layered hydraulics that incorporates non hydrostatic effects general results for a homogeneous layer in a multi layer steady flow are applied to single and two layer flow over a two dimensional sill the equation obtained for single layer flows is the same as that obtained by naghdi vongsarnpigoon 1986 using the direct theory of constrained fluid sheets and compares very well with the laboratory measurements of sivakumaran et al 1983 the new equation derived for two layer flows provides excellent agreement with the laboratory measurements of lawrence 1993 accurate solutions are obtained for a regime of two layer flow whose behaviour cannot be explained even qualitatively using classical hydraulic theory

glass transition temperatures of natural hydrous melts a relationship with shear viscosity and implications for the welding process abstract glass transition temperatures tg have been determined for natural multicomponent melts using differential scanning calorimetry trachytic dacitic phonolitic and basaltic base compositions have been analysed over a range of water contents up to 3 75 wt for each sample tg has been obtained over a range of cooling heating rates using the extrapolated onset and the peak temperatures in heat capacity temperature curves tg of all compositions are strongly reduced by increasing water content particularly for the first 1 wt added base composition also has an effect with the lowest tg occurring in the peralkaline phonolite suite for all samples a clear dependence on the cooling heating rate has been recorded these results have been compared with rheological investigations on the same samples on the basis of the equivalence of the shear and enthalpic relaxation process timescales we provide a method to predict the shear viscosity at the glass transition for all the melts investigated in this study both dry and hydrous our determinations of tg provide a lower limit for the time temperature envelope that gives rise to densely welded deposits and constraints on their emplacement temperature furthermore by using the viscosity values predicted at the glass transition we suggest that welding processes may occur over timescales on the order of tens of seconds to tens of minutes at tg

hyperquenched volcanic glass from loihi seamount hawaii abstract explosive submarine basaltic eruptions occurring in water depths of several kilometres commonly result in the formation of layered volcaniclastic deposits in order to quantify the cooling of such volcaniclastics we have identified and separated two types of glassy fragments from these deposits on loihi seamount 1 fine 30 80 m limu o pele bubble wall fragments and 2 coarser 0 8 1 2 mm dense angular fragments these pristine basaltic glasses have been subjected to differential scanning calorimetry dsc from room temperature up to temperatures above their glass transition heat capacity c p in j g 1 k 1 data reveal glassy and liquid regimes separated by clear hysteresis behaviour within the glass transition the transient c p in the glass transition interval exhibits a deep trough before the peak indicating very high cooling rates it is a classic feature of so called hyper quenching having been observed in dsc experiments on glasses produced using the splat quench d b dingwell p courtial d giordano and a r l nichols viscosity of peridotite liquid earth and planetary science letters 226 1 2 127 138 2004 and cascade fibre spinning y z yue j d christiansen and s l jensen determination of the fictive temperature for a hyperquenched glass chemical physics letters 357 1 2 20 24 2002 techniques the trough is deepest for the limu o pele fragments energy matching methods developed for the estimation of cooling rates for such glasses yield a rate of 10 5 31 k s 1 for the cooling of the fine sieve fraction of limu glass on the sea floor at loihi thus limu o pele volcaniclastics believed to be formed during mild submarine pyroclastic eruptions have experienced the fastest cooling of any natural volcanic glass measured to date such extreme cooling rates are likely to impact on many chemical and physical aspects of the stability of these glasses on the sea floor as well as their use as proxies for field variables of the earth s physical state and as monitors of the efficiency of chemical lithosphere hydrosphere exchange

modeling a plunging underflow a coupled three dimensional hydrodynamics and two dimensional underflow model is adapted to provide simulation of plunging inflows in reservoirs the new approach accounts for the effect of the barotropic term prior to the plunge point of the inflow simulations of plunging flows in constant width and constant slope channels are conducted and the resulting plunge depths are in agreement with prior empirical models simulation of a previously measured underflow in wellington reservoir australia demonstrates the model application to a plunging inflow in a natural water body and good agreement between field and model results

new constraints on the age of the manaslu leucogranite evidence for episodic tectonic denudation in the central himalayas the manaslu leucogranite of central nepal transacts one segment of the south tibetan detachment system a major extensional feature that helped to moderate large topographic gradients in the himalayan orogen in middle tertiary time 40 ar 39 ar ages for hornblendes from the northeastern contact aureole of the pluton indicate that intrusion occurred prior to 22 23 ma providing a minimum age for formation of the detachment system in the manaslu region that is at least 2 m y older than previous estimates from other parts of the orogen 40 ar 39 ar mica ages from the aureole similar to previously published mica ages from the upper part of the leucogranite indicate an episode of rapid cooling at 19 16 ma we attribute this cooling event to tectonic denudation of the pluton by movement on structurally higher exten sional faults and we suggest that gravitational collapse of the orogenic front occurred episodically over the late oligocene early miocene interval in the manaslu area

laser 40ar 39ar dating of single detrital muscovite grains from early foreland basin sedimentary deposits in india implications for early himalayan evolution in india the dagshai and overlying kasauli formations represent the oldest exposed continental foredeep sediments eroded from the himalayan orogen 40ar 39ar dating of individual detrital white micas from these sedimentary units has provided maximum depositional ages of 28 ma for the dagshai formation at one locality and 25 ma at a second locality whereas deposition of the kasauli formation occurred after 28 ma at two localities and after 22 ma at a third locality this timing suggests that in india the start of substantial exhumation and erosion from the rising himalayan orogen was delayed until 28 ma

energy dissipative plunging flows the analysis of plunging flows using energy dissipation hypothesis is presented in this paper the classic two control volume theory has recently been found questionable and needs to be corrected in this study we avoid the questionable part of that framework and use a single control volume for the analysis of plunging flows furthermore we hypothesize that the increase in kinetic energy in the underflow and entrained flow should be no greater than the loss in potential energy as the inflow plunges in the current framework the entrained flow rate does not exceed approximately 40 of the total inflow rate the densimetric froude number at the plunge point fdp q g hp3 where q inflow rate per unit width g reduced gravity of inflow and hp flow depth at the plunge point is typically in the range of 0 3 fdp 0 6 for plunging flows in a constant width channel

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

products of neptunian eruptions a common pyroclastic facies in subaqueous volcanic successions comprises massive to graded very thick several to tens of meters laterally extensive several kilometers beds of nonwelded pumice lapilli with volumes ranging to tens of cubic kilometers this facies may be overlain by laminated ash or bimodal ash and giant 1 m pumice clasts and underlain by coarse lithic breccia the association is inferred to be the typical product of sustained magmatic volatile driven explosive eruptions from vents at water depths of 1300 200 m we propose the term neptunian for such eruptions and their products the eruption column rapidly mixes with the surrounding water cools increases in density and collapses while remaining under water lithic clasts that are too heavy to be entrained in the column are deposited close to the source forming a neptunian lithic breccia pumice lapilli are rapidly waterlogged and form the dominant component in the collapsing column and in eruption fed water supported density currents neptunian density currents hot buoyant giant pumice clasts continue to rise and may reach the water surface before being waterlogged and settling along with temporarily suspended ash forming neptunian suspension deposits eruption magnitude fragmentation mechanisms and juvenile pyroclast characteristics especially vesicularity are very similar in neptunian and plinian style eruptions but column behavior differs primarily because of the contrasting physical properties of the ambient fluid water versus air

explosive submarine eruptions driven by volatile coupled degassing at lō ihi seamount hawai i abstract there is a growing body of information on submarine explosive eruptions but important questions remain about their causes and diversity magmatic degassing influences eruption dynamics but a paucity of appropriate samples from submarine pyroclastic deposits has limited thorough analysis of this process we present major element and volatile analyses of matrix glasses and olivine hosted glass inclusions from a submarine scoria cone the northern cone at 1060 m below sea level on l ihi seamount hawai i to quantitatively constrain ascent and degassing processes and infer the magmatic plumbing geometries that permit submarine explosive eruption of basalt in deep water olivine crystals form two populations the first consists of a single crystal fo86 8 88 6 bearing inclusions of high fe tholeiitic glass 1 18 to 1 28 wt h2o 182 to 505 ppm co2 major element concentrations indicate that these high fe inclusions represent parental melt that subsequently evolved by crystal fractionation to form the erupted bulk melt the second population consists of variably zoned crystals bearing low fe inclusions with lower volatile contents 0 63 to 0 74 wt h2o 29 to 176 ppm co2 low fe inclusions are not geochemically related to the bulk melt and their host crystals are interpreted as xenocrysts entrained from shallow storage the northern cone magma experienced strongly volatile coupled closed system where bubbles remain in physical contact with the melt from which they have grown ascent from at least 1 5 km deep in the conduit retention of co2 in bubbles facilitated strong exsolution of h2o and s exsolution of h2o dominated the vesiculation and acceleration of ascending melt accounting for the measured 47 to 65 vesicles in lapilli co2 played an essential chemical role but negligible physical role in driving magma ascent our results contradict the common interpretation that high hydrostatic pressures require submarine explosive eruptions to be strombolian driven by the accumulation and buoyant decoupling of co2 rich fluids comparison to volatile data from other explosive and effusive eruption products on l ihi indicates that explosivity is controlled primarily by the style of degassing rather than magma type or initial volatile content magmas that ascend directly from source to vent under volatile coupled conditions retain maximum explosive potential conversely magmas that degas under open system conditions or ascend from shallow reservoirs will have their explosive potential diminished as exsolved volatile fluids are lost to hydrothermal systems this latter path appears to be the most common at l ihi causing most magmas to erupt effusively