volatile characteristics of peralkaline rhyolites from kenya an ion microprobe infrared spectroscopic and hydrogen isotope study peralkaline rhyolites of the greater olkaria volcanic complex kenya rift valley were derived from separated though closely related magma chambers ion microprobe analyses of glass inclusions in quartz phenocrysts show pre eruptive water contents of up to 3 4 wt contrasting with previous estimates that the magmas were anhydrous the values approach predicted solubility levels corresponding to water saturation at low crustal pressures 1 kbar the glass matrices of the rhyolites have low water contents ranging from 0 07 to 0 46 wt suggesting significant degassing during or prior to eruption infrared measurements of the matrix glasses show variation in the relative proportions of the two hydrous species dissolved in the glasses the amount of molecular water determined semi quantitatively apparently increases with increased fluorine content and peralkalinity this suggests a competition between hydroxyl groups and fluoride ions for similar sites within the melt structure the mechanism of degassing has been investigated using hydrogen isotopes the range of d values in most rocks can be produced by varied degrees of open system degassing of rhyolite melt initially in equilibrium with water of a fixed or limited d value there is evidence to suggest that closed system degassing may also have been a significant component in some rhyolites the exact mechanisms of degassing remain uncertain particular problems include the relative contribution of open and closed system degassing during eruption and the initial vapour compositions and solubility relationships

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

tephra beds in deep sea cores off northern new zealand implications for the history of taupo volcanic zone mayor island and white island volcanoes twenty piston cores were collected from water depths of 650 3300 m from offshore bay of plenty north island new zealand they contain tephra from 11 okataina volcanic centre and 4 taupo volcanic centre rhyolite eruptions during the last 50 kyr that produce a tephrostratigraphic framework across the tectonically and volcanically complex region of southern havre trough alderman trough and the bay of plenty continental shelf this allows correlation between offshore and onshore sequences up to 200 km from source volcanoes covering much of marine isotope stages 3 and 2 the framework temporally constrains poorly dated and newly recognised volcanic events macroscopic tephra layers from the peralkaline mayor island volcano are documented for the first time at pre 50 post 50 40 5 37 4 22 2 and 14 2 ka in addition to the well known 7 ka tuhua event these macroscopic layers represent some of the most explosive events from this volcano and provide new marker horizons they are dispersed up to 90 km north east to east of the edifice minor tephra dispersal is also recorded at 17 8 25 5 35 and pre 50 ka the tephra contain high sio2 73 75 5 wt glass and subordinate basaltic components and each tephra represents a distinct magma batch that can be fingerprinted the mayor island tephra form two temporal trends toward less evolved magma compositions punctuated by a large caldera forming event at 36 ka the historically active andesitic white island volcano does not have widely dispersed tephra and the oldest primary deposit found is 21 ka five pre 50 ka rhyolite eruptions from an unknown taupo volcanic zone source provide evidence for explosive activity in a time interval poorly documented on land the cores demonstrate the patchy and uneven preservation of large magnitude tephra falls caused by local faulting bioturbation and ponding in bathymetrically complex regions reworked tephra layers are common and often lack indicative lithological features such units could easily be misinterpreted as primary events without micro beam geochemical analyses of glass shards

mobility of rare earth and other elements during crystallization of peralkaline silicic lavas abstract pantellerite lavas of the dormant caldera volcano mayor island new zealand have thick nonhydrated obsidian selvages at the tops and bases and crystalline microspherulitic interiors the flows were chemically homogeneous on emplacement but the crystalline portions have lost significant na f cl cs y and ree percentage losses of y and ree may exceed 60 ce yb ratios decrease by as much as 20 but eu eu ratios are unchanged loss of y and ree is attributed to the expulsion at near solidus temperatures of y ree 3 chloride complexes but other complexing agents may have contributed to this mobility na is also lost in the same manner groundwater has introduced mg and sr and leached or redistributed na cs and halogens analysis of hydrated glassy plinian pumice shows that secondary hydration has not affected y and ree concentrations the elements na mg f cl sr cs y and ree are highly mobile during the crystallization of peralkaline silicic melts and should not be used in petrogenetic evaluations these elements but not y and ree are also redistributed during secondary hydration processes magmas of undersaturated peralkaline composition will be similarly affected

discriminating hydrothermal and terrigenous clays in the okinawa trough east china sea evidences from mineralogy and geochemistry abstract the okinawa trough ot in the east asian continental margin is characterized by thick terrigenous sediment and ubiquitous volcanic hydrothermal activities in this study the clays collected during iodp expedition 331 to the middle ot iheya north knoll were analyzed for mineralogical and geochemical compositions by comparing with the clays from the east china sea shelf and surrounding rivers we examine different clay origins the hydrothermal field in the mid ot is dominated by mg rich chlorite while the recharge zone has clay mineral assemblages similar to the shelf and rivers showing high content of illite subordinate chlorite and kaolinite and scarce smectite compared to the terrigenous clays the hydrothermal clays in the ot have high concentrations of mg mn and zr but low fe na k ca ba sr p sc and ti while the hydrothermal clays in the mid ocean ridge are relatively enriched in fe and v and depleted in al mg zr sc and ti different fractionation patterns of rare earth elements also register in the terrigenous and hydrothermal clays diagnostic of variable clay origins we infer that the ot hydrothermal clay was primarily formed by the chemical alteration of detrital sediments subject to the hydrothermal fluids the remarkably different compositions of hydrothermal clays between the sediment rich back arc basin like ot and the sediment starved ocean ridge suggest different physical and chemical processes of hydrothermal fluids and fluid rock sediment reactions under various geologic settings

volatile emission during the eruption of baitoushan volcano china north korea ca 969 ad magma volume dre 24 5 km3 the main phase ca 95 vol is represented by comenditic tephra deposited dominantly as widespread fallout blankets and proximal ignimbrites the eruption column is estimated to have reached ca 25 km and thus entered the stratosphere a late phase 5 vol is represented by trachyte emplaced chiefly as moderately welded ignimbrites the comendites contain 3 and the trachytes 10 20 vol phenocrysts mainly anorthoclase hedenbergite and fayalite primary glassy melt inclusions with no signs of leakage were found only in phenocrysts in the comenditic tephra whereas those in phenocrysts in the trachytes are devitrified the comendite magma is interpreted to have been generated by fractional crystallization from a trachyte magma represented by melt inclusions in the phenocrysts in the comendite tephra the mass of volatiles emitted to the atmosphere during the eruption was estimated using the petrologic method the average h2o concentration of the comenditic matrix glass is 1 5 wt probably largely secondary and of the corresponding melt inclusions 5 2 wt melt inclusions in feldspar and quartz present the highest halogen concentrations with a calculated average for chlorine of 4762 ppm and for fluorine of 4294 ppm the comenditic matrix glasses are represented by a fluorine rich 3992 ppm f and fluorine poor group 2431 ppm f averaging 3853 ppm for chlorine only 20 of all sulfur analyses of the comenditic matrix glasses and melt inclusions are above the detection limit of 250 ppm s the difference between pre and post eruptive concentration of h2o is at least 3 7 0 6 wt h2o taking into consideration re hydration of the matrix glass and possible leakage of melt inclusions the difference between pre and post eruptive concentrations of the halogens amounts to 909 90 ppm cl and 1863 280 ppm and 302 40 ppm f the difference for s was estimated based on the average of the maximum s concentrations in the melt inclusions 455 ppm s and the detection limit resulting in 205 40 ppm s the calculated mass of volatiles injected into the atmosphere based on the erupted magma volume and volatile data is 1796 453 megatons for h2o 45 10 megatons for chlorine 42 11 megatons for fluorine and 2 0 6 megatons for sulfur the 969 20 ad eruption of baitoushan volcano one of the largest eruptions of the past 2000 years is thought to have had a substantial but possibly short lived effect on climate

chemical and isotopic investigations into the origin of clay minerals from the galapagos hydrothermal mounds field abstract a dark green authigenic nontronite is the major component of the galapagos hydrothermal mounds field sediments oxygen isotopic compositions of the chemically purified in situ mounds temperatures of up to 15 c assuming an authigenic origin the fe rich montmorillonite that dominates in the noncarbonate clay fraction of the surrounding pelagic ooze has isotopic formation temperatures of 27 to 39 c compared with measured in situ temperatures of ca 3 5 to 6 5 c the higher isotopic formation temperatures calculated for the hydrothermal nontronite suggest either complex patterns of fluid circulation and nontronite precipitation presently within the mounds or a higher thermal history associated with rapid and episodic periods of deposition during the holocene pleistocene the apparent high isotopic temperature of the fe rich montmorillonite may reflect 1 formation under hydrothermal conditions at spreading centers with subsequent dispersal by bottom currents 2 a detrital origin of the mineral or 3 a mixture of authigenic fe montmorillonite and detrital al montmorillonite in this region

pre eruptive melt composition and constraints on degassing of a water rich pantellerite magma fantale volcano ethiopia to determine the pre eruptive composition of peralkaline magma at frantale volcano ethiopia we have studied glass inclusions in phenocrysts from a lateceupting glassy pantelleritic lava flow matrix glass and crystal free glass inclusions in quartz were analyzed for all major and most minor elements by electron microprobe and for h2o and 15 lithophile trace elements by ion microprobe sims compositions of inclusions may have been slightly modified by post trapping quartz crystallization the average concentrations of all constituents but silica may be artificially high by 10 relative glass inclusions contain extreme enrichments in h2o mean of 4 6 to 4 9 wt and several lithophile trace elements which suggest that the lava erupted from a highly evolved water rich fraction of magma the pre eruptive concentration of water was much higher than that generally considered to occur in pantellerite magmas trends observed for lithophile elements in whole rock samples from pre syn and post caldera eruptive units are mimicked in glass inclusions from the studied pantellerite lava concentrations of rb y zr nb and ce cl increase with progressive differentiation with the exception of cl and h2o contents the composition of matrix glass is similar to that of glass inclusions suggesting that few constituents exsolved from magma or cooling glass eruption and quench of the lava occurred rapidly and the matrix glass is largely compositionally representative of melt higher average abundances of cl and h2o in glass inclusions suggest that these volatiles exsolved after melt entrapment degassing could have occurred as either an equilibrium or disequilibrium process

the lami pyroclastic succession lipari aeolian islands a clue for unravelling the eruptive dynamics of the monte pilato rhyolitic pumice cone abstract the lami succession piled up during the explosive activity of the monte pilato on lipari island central aeolian arc italy it comprises a repetitive alternation of fall and pyroclastic density current beds which define 13 units separated by fine ash beds despite the homogeneous rhyolitic composition the juveniles fragments show strong textural heterogeneities from highly vesicular pumice to dense black obsidians the pumice types are termed i tube which shows elongated tubular vesicles ii frothy highly vesicular with large sub rounded vesicles and iii agglomerated and welded two types of obsidian pyroclasts are recognized i aphyric black obsidian and ii grey obsidian showing spherulitic texture all these pyroclastic types occur throughout the stratigraphic succession and mixed in the same bed they account for changing physical conditions along the conduit which is hypothesised to be filled by a melt plug during the eruption of 12 out of 13 units the strong textural heterogeneities of the juvenile fragments testify for the existence of differential shear rates which can be explained though two shear mechanism i simple shear producing horizontal zonation of texture in the conduit with black obsidians along the conduit walls passing to tube and frothy pumice to the centre and ii pure shear producing a vertical zonation in the conduit with black obsidians at the top overlying frothy pumice agglomerated and welded pumice of unit 13 is thought to represent the portion of magma heated by a batch of hotter melt that entered the magma chamber and triggered the eruption the deposits of unit 13 represent a unique case of spatter accretion due to accumulation of hot low viscous rhyolitic pumice fragments emplaced from pyroclastic fountaining

massive sulfide deposition and trace element remobilization in the middle valley sediment hosted hydrothermal system northern juan de fuca rdge the bent hill massive sulfide deposit and odp mound deposit in middle valley at the northernmost end of the juan de fuca ridge are two of the largest modern seafloor hydrothermal deposits yet explored trace metal concentrations of sulfide minerals determined by laser ablation icp ms were used in conjunction with mineral paragenetic studies and thermodynamic calculations to deduce the history of fluid mineral reactions during sulfide deposition detailed analyses of the distribution of metals in sulfides indicate significant shifts in the physical and chemical conditions responsible for the trace element variability observed in these sulfide deposits trace elements mn co ni as se ag cd sb pb and bi analyzed in a representative suite of 10 thin sections from these deposits suggest differences in conditions and processes of hydrothermal alteration resulting in mass transfer of metals from the center of the deposits to the margins enrichments of some trace metals pb sb cd ag in sphalerite at the margins of the deposits are best explained by dissolution reprecipitation processes consistent with secondary remineralization results of reaction path models clarify mechanisms of mass transfer during remineralization of sulfide deposits due to mixing of hydrothermal fluids with seawater model results are consistent with patterns of observed mineral paragenesis and help to identify conditions ph redox temperature that may be responsible for variations in trace metal concentrations in primary and secondary minerals differences in trace metal distributions throughout a single deposit and between nearby deposits at middle valley can be linked to the history of metal mobilization within this active hydrothermal system that may have broad implications for sulfide ore formation in other sedimented and unsedimented ridge systems

low temperature fluid flow through sulfidic sediments from tag modification of fluid chemistry and alteration of mineral deposits low temperature fluid flow has been identified as an important component of the heat mass and chemical flux at sites of active venting one way to study the movement of water through hydrothermal deposits is to analyse the pore fluids extracted from sulfidic sediments the composition of diffuse flow fluids in one sediment core from the south of the active tag mound reflects the dilution of high temperature black smoker fluids with modification inside the mound during ongoing circulation fluid modification includes conductive cooling removal of si from solution and entrainment and removal of u from seawater which can represent significant sinks of these elements precipitation of clay minerals and fe oxides is currently occurring within this sediment core generating ochreous type material subsurface extensive low temperature alteration of basaltic material at the core top is apparent in conjunction with supergene enrichment of cu and mn

fluoride occurrence in publicly supplied drinking water in estonia a study was undertaken to examine the content and spatial distribution of fluoride in drinking water water samples 735 from public water systems covering all estonian territory were analysed using spadns method in order to specify the natural source of fluoride the chemistry data from five aquifer systems utilised for water supply were included into the study fluoride concentrations in tap water to a great extent ranged from 0 01 to 6 95 mg l drinking water in southern estonia where terrigenous middle devonian aquifer system is exploited has a fluoride concentration lower than recommended level 0 5 mg l thus promoting susceptibility to dental caries the western part of the country is supplied by water with excess fluoride content 1 5 6 9 mg l groundwater abstracted for drinking purposes originates from ordovician and silurian carbonate rocks the content of fluoride in silurian ordovician aquifer system is associated with the groundwater abstraction depth and the main controlling factors of dissolved fluoride are the ph value and the chemical type of water

the role of prokaryotes in subsurface weathering of hydrothermal sediments a combined geochemical and microbiological investigation a detailed geochemical and microbiological study of a 2 m sediment core from the inactive alvin mounds within the tag hydrothermal field was conducted to examine for the first time the role of prokaryotes in subsurface weathering of hydrothermal sediments results show that there has been substantial post depositional remobilisation of metal species and diagenetic overprinting of the original high temperature hydrothermal minerals and aspects have involved prokaryotic processes prokaryotic enumeration demonstrates the presence of a population smaller than the average for deep sea sediments probably due to the low organic carbon content but not inhibited by and hence adapted to the metal rich environment there was a small but significant increase in population size associated with the active redox boundary in an upper metal sulphide layer 50 70 cm around which active metal remobilisation was concentrated cu au cd ag u zn and zn hence subsurface prokaryotes were potentially obtaining energy from metal metabolism in this near surface zone close association of numbers of culturable mn and fe reducing prokaryotes with subsurface fe2 and mn2 pore water profiles suggested active prokaryotic metal reduction at depth in core cd102 43 to 175 cm in addition a prokaryotic mechanism which is associated with bacterial sulphate reduction is invoked to explain the u enrichment on pyrite surfaces and zn and pb remobilisation in the upper sediment although prokaryotic populations are present throughout this metalliferous sediment thermodynamic calculations indicated that the inferred low ph of pore waters and the suboxic anoxic conditions limits the potential energy available from fe ii oxidation which may restrict prokaryotic chemolithotrophic biomass this suggests that intense prokaryotic fe oxidation and weathering of seafloor massive sulphide deposits may be restricted to the upper portion of the deposit that is influenced by near neutral ph and oxic seawater unless there is significant subsurface fluid flow

post eocene volcanics of the abazar district qazvin iran mineralogical and geochemical evidence for a complex magmatic evolution abstract the style of volcanism of post eocene volcanism in the alborz zone of northern iran is different to that of eocene volcanism karaj formation indeed the volcanic succession of the abazar district located in a narrow volcanic strip within the alborz magmatic assemblage is characterized by distinct mineralogical and chemical compositions linked to a complex magmatic evolution the succession was produced by explosive eruptions followed by effusive eruptions two main volcanic events are recognized 1 a thin rhyolitic ignimbritic sheet underlain by a thicker lithic breccia and 2 lava flows including shoshonite latite and andesite that overlie the first event across a reddish soil horizon plagioclase in shoshonite an48 92 shows normal zoning whereas plagioclase in latite and andesite an48 75 has a similar composition but shows reverse and oscillatory zoning quilf temperature calculations for shoshonites and andesites yield temperatures of 1035 c and 1029 c respectively the geothermometers proposed by ridolfi et al 2010 and holland and blundy 1994 yield temperatures of 960 c and 944 c for latitic lava respectively the samples of volcanic rock show a typical geochemical signature of the continental arc regime but the andesites clearly differ from the shoshonites the latites and the rhyolites the mineralogical and chemical characteristics of these rocks are explained by the following petrogenesis 1 intrusion of a hot mantle depth mafic shoshonitic magma which differentiated in the magma chamber to produce a latitic and then a rhyolitic liquid 2 rhyolitic ignimbritic eruptions from the top of the magma chamber following by shoshonitic and then latitic extrusions 3 magma mingling between the latitic and andesitic magmas as indicated by the occurrence of andesite clasts within the latite and 4 andesitic effusions the youngest volcanic events in the alborz zone show a close chemical relationship with continental arc rocks indicating that they formed in a continental collision setting

the pre eruptive volatile contents of recent basaltic and pantelleritic magmas at pantelleria italy abstract pantelleria island located in the sicily channel rift zone italy is the type locality for the peralkaline rhyolitic rocks called pantellerites in the last 50 ka after the large green tuff caldera forming eruption volcanic activity at pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows lava domes and poorly dispersed pantelleritic pumice fall deposits basaltic cinder cones and lava flows are only present outside the caldera in the nw sector of the island the most recent basaltic cuddie rosse 20 ka and pantelleritic cuddia randazzo and cuddia del gallo 6 ka pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions the melt inclusions in pyroxene and olivine phenocrysts of cuddie rosse scoriae have an alkali basalt composition the dissolved volatiles comprise 0 9 1 6 wt h2o several hundred ppm of co2 1600 2000 ppm of sulphur and 500 900 ppm of chlorine the water carbon dioxide couple gives a confining pressure 2 kbar prior to the eruption this result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption the melt inclusions in feldspar fayalite and aenigmatite phenocrysts of cuddia del gallo and cuddia randazzo pumice have a pantelleritic composition agpaitic indices 1 3 2 1 up to 4 4 wt h2o 8700 ppm cl 6000 ppm f and co2 below the detection limit sulphur averaging 420 ppm has been measured in cuddia randazzo melt inclusions these data indicate relatively high volatile contents for these low energy strombolian type eruptions melt inclusions in cuddia del gallo pumice show the most evolved composition agpaitic indices 2 2 1 and the highest volatile content in agreement with fluid saturation conditions in the magma chamber prior to the eruption this implies a confining pressure of 1 kbar for the top of the pantelleritic reservoir the composition of melt inclusions and mineralogical assemblage of cuddia randazzo pumice indicate that it has a lower evolutionary degree agpaitic indices 1 3 1 8 and lower pre eruptive cl and h2o contents than cuddia del gallo pumice an increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the cuddia del gallo explosive eruption evidence of widespread pre eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the cuddia randazzo event

volatiles in pantellerite magmas a case study of the green tuff plinian eruption island of pantelleria italy abstract the green tuff gt plinian eruption the largest in magnitude at pantelleria erupted 3 to 7 km3 dre of pantellerite magma and a small volume of trachyte fifty nine anorthoclase hosted melt inclusions from the two basal pumice members were analyzed by ft ir spectroscopy in order to assess the pre eruptive h2o content in the pantellerite melt microanalytical methods were used to determine major element cl f and s contents melt inclusions and glassy groundmasses have a nearly homogeneous pantelleritic composition peralkaline index 1 9 2 2 and variable water contents ranging from 1 4 to as high as 4 2 wt i e much higher than the 1 4 wt of earlier published studies the chlorine content is constant at about 1 wt combined cl and h2o data were used to estimate a confining pressure of about 50 mpa depth around 2 3 km for the gt magma chamber the chamber was characterized by a compositional zoning with a dominant pantellerite overlying a trachyte magma soon after the gt eruption intra caldera volcanism was dominated by the eruption of voluminous trachyte lava flows while pantellerite melt production resumed after about 20 ka with numerous low volume mildly explosive strombolian to effusive eruptions comparison with data from the literature reveals that despite the different explosivity the post caldera strombolian eruptions and the gt plinian eruption were fed by pantelleritic magmas with similar water contents chlorine and co2 contents suggest that the young magma reservoirs feeding the strombolian to effusive activity were deeper h 4 5 km than the much larger based on erupted volumes magma chamber which fed the gt eruption

introduction to special section on measurements and monitoring at the tag hydrothermal field mid atlantic ridge 26 n 45 w the international oceanographic community carried out a coordinated series of cooperative cruises and dives with japanese russian and u s submersibles between 1993 and 1995 to perform baseline measurements and to establish a seafloor observatory to monitor the active sulfide mound in the tag hydrothermal field before during and after drilling by ocean drilling program leg 158 in october november 1994 the results of that effort and related work are reported herein as group of 23 papers comprising a special section in this issue of geophysical research letters

pre eruptive volatile content and degassing history of an evolving peralkaline volcano abstract the volatile contents of melt inclusions trapped within phenocrysts of quartz and feldspar in peralkaline rhyolites from mayor island new zealand have been studied using fourier transform infrared spectroscopy and ion microprobe analyses the glass inclusions analyzed span the 130 ka subaerial eruptive history of the island and come from volcanic deposits representing a wide range of eruptive styles explosive vs effusive magmatic vs phreatomagmatic low vs high discharge rates the water content of all inclusions analysed is uniformly high at 4 4 wt h2o whereas co2 contents are below the limits of detection 50 ppm chlorine in melt inclusions ranges from 2070 to 5200 ppm while coexisting matrix glasses have generally lower cl concentrations of 1700 4200 ppm the apparent bulk distribution coefficient describing cl partitioning between melt and vapor phase cl fluid cl melt during degassing is 5 15 fluorine appears to be less affected by eruptive degassing than is cl and melt inclusion and matrix glass f concentrations show significant overlap 1400 2520 ppm f in inclusions 1550 2890 ppm f in matrix glass the observed invariance of melt inclusion water content with sample age contrasts with ion microprobe data on incompatible trace elements e g zr nb which suggest 35 fractionation between the oldest and youngest samples this along with the cl partitioning behaviour suggests that at least the upper erupting portion of the mayor island magma chamber was water saturated throughout the volcano s 130 ka eruptive history furthermore the large range of eruptive styles observed on mayor island are not due to differences in water concentration of the erupting magma but instead must reflect differences in rates of magma ascent and supply which control the efficiency of non explosive degassing

evolution of peralkaline silicic complexes lessons from the extrusive rocks abstract young 4 wt and the rhyolitic varieties evolve to temperatures 4 10 6 pa s result in rapid growth of volcanic edifices highly efficient crystal melt separation and the ubiquitous development of compositionally zoned caps to reservoirs no peralkaline equivalent of the monotonous intermediate ignimbrites found in some calc alkaline systems has yet been found

boomerang seamount the active expression of the amsterdam st paul hotspot southeast indian ridge during a survey of the axis of the southeast indian ridge seir we discovered a 1100 m tall volcanically active submarine volcano boomerang seamount near the spreading center on the amsterdam st paul asp plateau the summit of the volcano is 650 m below sea level and has a 200 m deep 2 km wide circular caldera samples of very fresh volcanic glass dated by the 210po 210pb technique at 5 months old were collected from the floor of the caldera in march 1996 the volcano is 18 km northeast of amsterdam island near the intersection of a long spreading segment and the boomerang transform fault it is built on the stationary antarctic plate where widely scattered volcanic activity thickens the crust continuing to build the plateau water column profiles reveal a 1 7 c temperature anomaly and a 0 3 v stepped nephelometer water column turbidity anomaly within the caldera nearly an order of magnitude larger than other hydrothermal plume anomalies we measured these anomalies suggest hydrothermal activity within the caldera volcanic glass compositions at two sample sites on the volcano summit are similar to each other and to amsterdam and st paul island basalts but have some important differences as well k2o tio2 ratios in boomerang seamount glasses are similar to st paul island samples but differ significantly from amsterdam island samples rare earth element patterns in lavas from boomerang amsterdam and st paul are similar sr nd and pb isotope ratios in samples from the boomerang caldera floor are similar to samples from amsterdam island however another sample from boomerang seamount deviates from a seir st paul amsterdam mixing trend and shows evidence of mixing with a kerguelen like source component the geochemical complexity of these three closely spaced volcanic edifices on the asp plateau suggests that the boomerang seamount source is heterogeneous on a very small scale

diversity of eruptive styles inferred from the microlites of mt pelee andesite martinique lesser antilles abstract we have studied the compositional and textural characteristics of plagioclase microlites from several dome forming eruptions dome block and ash flows and surges and plinian fallouts of the recent period of activity of mt pelee in order to infer magma ascent dynamics dome forming and plinian samples display a wide range of plagioclase microlite compositions from an 35 to an 90 and an 55 to an 90 respectively microlites with compositions an 50 60 crystallized from a mafic melt that intruded the andesitic reservoir in proportion of 10 vol during magma ascent in the conduit the fraction number density and morphology of the decompression induced microlites compositions 50 60 suggest that the plinian magmas experienced a nucleation dominated crystallization regime with high undercooling in contrast magmas from domes and block and ash flows were dominated by a growth regime with low undercooling and the surges were influenced by both regimes according to these crystallization regimes and new experimental data on h 2 o content of the matrix glasses we propose magma relative ascent rates and fragmentation pressures for the different eruptive styles observed at mt pelee that may explain why surges are explosive events and why they may or may not be followed by a plinian event

taupo volcanic zone calc alkaline tephras on the peralkaline mayor island volcano new zealand identification and uses as marker horizons mayor island is a peralkaline rhyolitic caldera volcano characterised by numerous sector confined pyroclastic deposits together with lavas forming at least five composite shields correlation of sequences between sectors is difficult because of the scarcity of island wide marker beds however eight distal calc alkaline fall tephras ca 7 3 14c ka to 64 ka from okataina and taupo volcanic centres in the nearby taupo volcanic zone tvz have been identified on the island these foreign tvz tephras provide marker planes to correlate activity in different sectors of mayor island volcano and refine an eruptive chronology at least seventeen pyroclastic eruptions and fourteen lava producing events including multiple shield forming events have occurred in the past ca 64 ka age controls provided by the calc alkaline tephras confirm the extremely local dispersal characteristics of many of the mayor island eruptives and show that kar ages as young as 25 33 ka on obsidians with 4 2 4 4 k2o are reliable

ion microprobe analyses of trace elements in anorthoclase hedenbergite aenigmatite quartz apatite and glass in pantellerite evidence for high water contents in pantellerite melt

conductive heat flow at the tag active hydrothermal mound results from 1993 1995 submersible surveys we report 70 measurements of conductive heat flow at the 50 m high 200 m diameter tag active hydrothermal mound made during submersible surveys with alvin in 1993 and 1995 and shinkai 6500 in 1994 the stations were all measured with 5 thermistor 0 6 or 1 m long alvin heat flow probes which are capable of determining both gradient and thermal conductivity and were transponder navigated to an estimated accuracy of 5 10 m relative to the 10 m diameter central complex of black smokers within 20 m of this complex conductive heat flow values are extremely variable 0 1 100 w m which can only be due to local spatial and possible temporal variability in the immediate vicinity of the vigorous discharge sites a similar local variability is suggested in the kremlin area of white smokers to the southeast of the black smoker complex on the south and southeast side of the mound there is very high heat flow 3 7 25 w m on the sedimented terraces that slope down from the kremlin area heat flow is also high 0 3 3 w m in the pelagic carbonate sediments on the surrounding seafloor within a few tens of meters of the southwest northwest and northeast sides of the mound on the west side of the sulfide rubble plateau that surrounds the central black smoker peak there is a coherent belt of very low heat flow 20 mw m 20 50 m west of the smokers suggestive of local shallow recharge of bottom water the three submersible surveys spanned nearly two years but showed no indication of any temporal variability in conductive heat flow over this time scale whether natural or induced by odp drilling in 1994

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

monitoring shear deformations above compacting high pressure high temperature reservoirs with calliper surveys abstract approximately thirty multi finger calliper mfc surveys have been acquired in elgin and franklin development wells between 2003 and 2013 these surveys reveal around one hundred shear deformation features characterized by a kink in the pipe the calliper surveys were depth shifted against the open hole reference depth used in geoscience to accurately identify the interfaces along which the deformations occur the deformations have been found to be distributed at the top of the fulmar formation reservoir in the late jurassic shales in the early cretaceous cromer knoll group and the late cretaceous chalk group for each deformation feature the relative displacement of the well was estimated using the calliper tool eccentricity resulting from the movement of the toolstring through the kink in the pipe maximum relative displacements were estimated at 3 7 in in the 7 in liner and at 1 2 in in the 9 7 8 in production casing the analysis of the calliper surveys show that the deformation features result from slip displacements along various discontinuities such as bedding planes reservoir faults extending in the overburden and overburden faults it is interpreted that these interfaces have been reactivated because of the strong reservoir depletion this work clearly demonstrates that mfc surveys are an effective tool for monitoring shear deformations occurring above a compacting reservoir

phase equilibrium constraints on pre eruptive conditions of recent felsic explosive volcanism at pantelleria island italy we present experimental phase equilibria carried out on a pantelleritic bulk rock composition with a peralkalinity index pi molar na2o k2o al2o3 1 4 which is representative of the most energetic pumice fall eruption of the resurgent post caldera volcanism on pantelleria island for the explored conditions p 25 150 mpa t 680 800 c h2omelt up to 6 wt fo2 nno where nno is nickel nickel oxide buffer clinopyroxene is the liquidus phase followed by alkali feldspar and then quartz the crystallization of amphibole is limited to temperatures below 700 c aenigmatite crystallizes near the liquidus for p 100 mpa when clinopyroxene is the sole liquidus phase the coexisting melt is characterized by a decrease in peralkalinity index to 1 2 if compared with the starting material approaching comendite composition in contrast when alkali feldspar joins the liquidus phase assemblage residual melts become extremely peralkaline pi 3 2 with feo contents in excess of 14 wt comparison of our experiments with natural phase abundances and compositions constrains the depth of the fastuca pantellerite reservoir to be around 5 1 km or p 120 20 mpa at t 730 10 c with h2omelt around 4 wt and fo2 1 5 log unit below nno such conditions correspond to the dominant phase assemblage of feldspar clinopyroxene and aenigmatite amphibole occurrence requires both higher melt water contents and lower temperatures up to 6 5 wt at 680 c and may reflect provenance from a deeper and wetter portion of the tapped reservoir in contrast quartz is more probably related to crystallization during magma ascent which explains the lower melt water content of its melt inclusions overall the results confirm that pantellerites may be both water rich and reduced magmas these characteristics imply that any coexisting fluid phase in the reservoir will be dominated by water and hydrogen species the latter amounting to a few mole per cent magma unrest at pantelleria may thus be heralded by emissions of hydrous and reduced gases i e co2 poor and h2 rich if the present day reservoir is felsic in composition

the internal structure of an active sea floor massive sulphide deposit the hydrothermal circulation of sea water through permeable ocean crust results in rock water interactions that lead to the formation of massive sulphide deposits these are the modern analogues of many ancient ophiolite hosted deposits1 4 such as those exposed in cyprus here we report results obtained from drilling a series of holes into an actively forming sulphide deposit on the mid atlantic ridge a complex assemblage of sulphide anhydrite silica breccias provides striking evidence that such hydrothermal mounds do not grow simply by the accumulation of sulphides on the sea floor indeed the deposit grows largely as an in situ breccia pile as successive episodes of hydrothermal activity each form new hydrothermal precipitates and cement earlier deposits during inactive periods the collapse of sulphide chimneys dissolution of anhydrite and disruption by faulting cause brecciation of the deposit the abundance of anhydrite beneath the present region of focused hydrothermal venting reflects the high temperatures 150 c currently maintained within the mound and implies substantial entrainment of cold sea water into the interior of the deposit these observations demonstrate the important role of anhydrite in the growth of massive sulphide deposits despite its absence in those preserved on land

geochemical and thermal fluxes high temperature venting and diffuse flow from mid ocean ridge hydrothermal systems the tag hydrothermal field mid atlantic ridge 26 n

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

the origin of clay minerals in active and relict hydrothermal deposits samples of fe oxide rich hydrothermal sediments were collected from active and inactive portions of the transatlantic geotraverse tag hydrothermal field on the mid atlantic ridge clays separated from tag metalliferous sediments in this study all consist of al poor nontronite oxygen isotope thermometry of the clays yields formation temperatures of 54 67 c for samples from the inactive alvin mound compared with 81 96 c for samples from the active tag site the latter are the highest recorded temperatures for authigenic hydrothermal clays sr isotope analysis of the clays from the active mound suggests that they precipitated from seawater dominated fluids containing less than 15 hydrothermal end member fluid in contrast nontronite from the inactive alvin mound has 87sr 86sr values that closely resemble that of detrital north atlantic clays suggesting a dominantly continental source for the sr rare earth element data are consistent with a significant detrital input to the inactive site but also demonstrate the extent of hydrothermal input to the low temperature fluid crystallographic fractionation of the trivalent ree is apparent in the heavy ree enrichments for all nontronite samples the inferred formation mechanism for nontronite rich fe oxyhydroxide deposits at the surface of the active mound is by direct precipitation from low temperature fluids at the inactive alvin site in contrast the deposits form during alteration of pelagic sediments by diffuse fluids and replacement of biogenic carbonate with nontronite and fe oxyhydroxide these two modes of formation are both important in seafloor hydrothermal settings where clay minerals are a significant component of the hydrothermal deposit

homogeneous bubble nucleation in rhyolitic magmas an experimental study of the effect of h2o and co2 1 rapid decompression experiments were performed to study homogeneous bubble nucleation in a crystal free rhyolitic liquid at 800 c bubble nucleation was produced by lowering the pressure at 1 10 mpa s 1 from an initial value between 200 and 295 mpa to a final value below the volatile saturation pressure psat six volatile compositions with 4 1 7 7 wt h2o and 10 1200 ppm co2 were investigated for each composition we determined the critical pressure phon below which homogeneous nucleation can proceed the samples quenched below phon showed a nucleated core with a large number of uniformly spaced bubbles with decreasing pressure bubble number densities increased from 1015 m 3 the degree of supersaturation required for homogeneous nucleation phon psat phon increased with decreasing h2o content or increasing co2 content huge values of phon 135 to 310 mpa were measured in the h2o poor compositions 4 1 4 6 wt h2o 50 1100 ppm co2 much lower values from 60 to 160 mpa were obtained in the h2o rich compositions 7 0 7 7 wt h2o 10 630 ppm co2 the high phon in liquids with 4 5 wt h2o should result in the buildup of large degrees of supersaturation during magma ascent a very late nucleation event and a rapid explosive vesiculation by contrast rhyolitic liquids with much larger water contents have higher saturation pressures and much lower phon bubble nucleation may therefore occur at depth in the volcanic conduit favoring a subsequent near equilibrium degassing

gum rosin acetone system as an analogue to the degassing behaviour of hydrated magmas solutions of gum rosin and acetone reproduce the volatile and temperature dependent viscosity together with the phase behaviour of hydrated magmas a range of experimental exsolution conditions was investigated including the variation of supersaturation rate of decompression solution temperature and volatile content degassing processese were controlled by the formation of an exsolution interface above a supersaturated liquid the end products ranged from a mildly degassed liquid to a solid foam which preserved strained vesicles solutions of gum rosin and acetone are proposed as a suitable analogue system with which to study magma degassing processes

experimental constraints on pre eruption conditions of pantelleritic magmas evidence from the eburru complex kenya rift the phase relationships and compositions of a pantellerite from the eburru complex in the kenya rift valley have been determined at 150 mpa and under reducing conditions 2 log units below the ni nio solid buffer the effects of temperature and melt water content on phase relationships have been explored alkali feldspar and quartz crystallise alone at temperatures above 700 c irrespective of melt water content below 700 c sodic amphibole and clinopyroxene also crystallise the amphibole being the liquidus phase under water rich conditions the coexistence of amphibole phenocrysts with alkali feldspar and quartz in a crystal poor pantellerite implies temperatures below 700 c and melt water contents higher than 4 wt possibly up to 5 6 wt pantellerites have lower liquidus temperatures than associated comendites which supports a parent daughter relationship between the two magma types the melts produced in the experiments extend the compositional trend displayed by the natural rock series and reproduce some extreme compositions occasionally observed in alkaline volcanic series with feo contents above 12 wt and na2o contents approaching 10 wt pantellerites are therefore the true near minimum melt compositions of alkaline oversaturated magma series

rare earth element geochemistry of hydrothermal deposits from the active tag mound 26 n mid atlantic ridge the rare earth element ree geochemistry of various phases from the active tag hydrothermal mound has been examined and related to their mineralogy and fluid chemistry the mound deposits range from black and white smoker chimneys massive anhydrite sulphide mixtures oxides and ochres all phases except black smoker chimney anhydrite demonstrate a positive eu anomaly when normalised to chondrite ree values ree substitution into sulphide and sulphate phases appears to be strongly influenced by crystallographic control for all ree other than eu precipitation of anhydrite within the tag mound is the major mechanism for removal of ree during mound circulation and 0 15 0 35 g anhydrite is inferred to precipitate from every kg of fluid venting from the white smoker chimneys oxides from the mound fall into three different categories with distinct ree patterns oxide rims on sulphides atacamite bearing oxides and silica rich fe oxides and ochres the oxide rim phases contain sulphide and seawater derived rees whereas the atacamite bearing oxides and the ochreous material exhibit no seawater signature which suggests precipitation from or alteration by a modified hydrothermal fluid

decompression experiments as an insight into ascent rates of silicic magmas an experimental study of h2o exsolution bubble growth and microlite crystallisation during ascent decompression of silicic magmas in the volcanic conduit is presented isobaric and decompression experiments were performed on a rhyolitic melt at 860 c nno 1 h2o saturation and pressures between 15 and 170 mpa two sets of decompression experiments were performed with decompression rates varying between 0 001 and 960 mpa min 1 from 150 to 50 mpa high pressure decompression and 2 from 50 to 15 mpa low pressure decompression the experiments highlight incomplete h2o exsolution for decompression rates 100 mpa min incomplete bubble growth for decompression rates 0 1 mpa min crystal nucleation time lags and incomplete chemical re equilibration to final pressures the observed crystallisation process i e growth versus nucleation depends on the decompression range indeed decompression induced crystallisation during high pressure decompressions is dominated by growth of existing crystals whereas during low pressure decompressions crystal nucleation is the dominating process this study provides a means to infer magma ascent rates in eruptions of silicic magmas through a combined petrologic and experimental approach

uranium enrichment in metalliferous sediments from the mid atlantic ridge abstract previous studies of metal rich sediments at mid ocean ridge spreading centres have suggested that u fe ratios are relatively constant from site to site a suite of cores from the mid atlantic ridge has been analysed for u and fe content and most samples exhibit constant u fe ratios sulphide bearing parts of one core however contain excess u compared with that observed in other metalliferous sediments but apparently only during oxidative alteration of pyrite phases fission track registration has demonstrated that this excess u is intimately associated with sulphide grains in the sediment scanning electron microscope examination indicates that this u enrichment is limited to very small areas within the oxidation layer and is associated with an enhanced p concentration ion probe analyses allow semi quantitative analyses of various elements through a u rich grain the observed distribution of u coupled with its specific enrichment suggests that microbial mediation might be controlling this enrichment u fe ratios in fully oxidised tag sediments are constant and approximately an order of magnitude higher than in young plume particulates this discrepancy is attributed to a second type of post depositional u enrichment producing an homogeneous distribution of u throughout the sediment

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

new data on magmatic h 2 o contents of pantellerites with implications for petrogenesis and eruptive dynamics at pantelleria infrared spectroscopic analyses of melt inclusions in quartz phenocrysts from pantellerites erupted at pantelleria italy show that the magmas contained moderate pre eruptive h2o contents ranging from 1 4 to 2 1 wt melt h2o concentrations increase linearly with incompatible elements demonstrating that h2o contents were not buffered significantly during fractionation by any crystalline or vapor phase the relatively low h2o contents of pantellerites are consistent with an origin by partial melting of alkali gabbros rather than fractional crystallization of basalt preeruptive h2o concentrations do not correlate with the volume or explosivity of pantellerite eruptions decompression history is critical in determining the style of pantellerite and other eruptions

petrogenesis of coexisting high silica aluminous and peralkaline rhyolites from yunshan yongtai southeastern china abstract the late cretaceous bimodal yunshan yongtai volcanics in fujian province contain peralkaline rhyolites the only presence of such rhyolites in southeastern china whole rock and mineral chemical compositions are analyzed for the coexisting aluminous metaluminous to weakly peraluminous peralkaline high silica rhyolites from the yunshan volcanics they are sparsely porphyritic and contain k feldspar ferromagnesian minerals quartz magnetite and titanomagnetite phenocrysts as well as accessory minerals such as fayalite chevkinite apatite and zircon the mineral assemblage indicates an oxidizing pre eruption condition these rhyolites exhibit diagnostic geochemical features of a type granites such as elevated 10 4 ga al mostly greater than 2 6 and feo t feo t mgo ratios enrichment in high field strength elements hfse such as zr 400 ppm and nb and strong depletion in al 2 o 3 2 o poor meta granitic igneous rocks in the deep crust and cannot be fractionated from the coeval yunshan mafic magmas geochemical variations of major and trace elements indicate the possible fractionation of k feldspar calcium rich pyroxene fe ti oxides and minor chevkinite during the magma evolution in peralkaline rhyolites we found that the pre existing fe ti oxide and hedenbergite phenocrysts had been transformed into aegirine oxide and aegirine oxide fluorite assemblages respectively these mineral assemblages are the products of the subsolidus reaction of pre existing phenocrysts and extraneous na f rich fluids such na f rich fluids may be derived from the degassing of the subvolcanic rocks the reactions indicate that the yunshan peralkaline rhyolites could be generated through the reaction of highly fractionated aluminous silica magmas and na f rich fluids

effusive eruptions from a large silicic magma chamber the bearhead rhyolite jemez volcanic field nm abstract large continental silicic magma systems commonly produce voluminous ignimbrites and associated caldera collapse events less conspicuous and relatively poorly documented are cases in which silicic magma chambers of similar size to those associated with caldera forming events produce dominantly effusive eruptions of small volume rhyolite domes and flows the bearhead rhyolite and associated peralta tuff member in the jemez volcanic field new mexico represent small volume eruptions from a large silicic magma system in which no caldera forming event occurred and thus may have implications for the genesis and eruption of large volumes of silicic magma and the long term evolution of continental silicic magma systems 40ar 39ar dating reveals that most units mapped as bearhead rhyolite and peralta tuff the main group were erupted during an 540 ka interval between 7 06 and 6 52 ma these rocks define a chemically coherent group of high silica rhyolites that can be related by simple fractional crystallization models preceding the main group minor amounts of unrelated trachydacite and low silica rhyolite were erupted at 11 9 and 8 ma respectively whereas subsequent to the main group minor amounts of unrelated rhyolites were erupted at 6 1 and 1 5 ma the chemical coherency apparent fractional crystallization derived geochemical trends large areal distribution of rhyolite domes 200 km2 and presence of a major hydrothermal system support the hypothesis that main group magmas were derived from a single large shallow magma chamber the 540 ka eruptive interval demands input of heat into the system by replenishment with silicic melts or basaltic underplating to maintain the bearhead rhyolite magma chamber although the volatile content of main group magmas was within the range of rhyolites from major caldera forming eruptions such as the bandelier and bishop tuffs eruptions were smaller volume and dominantly effusive bearhead rhyolite domes occur at the intersection of faults and are cut by faults suggesting that the magma chamber was structurally vented preventing volatiles from accumulating to levels high enough to trigger a caldera forming eruption

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

magma ascent and eruptive processes from textural and compositional features of monte nuovo pyroclastic products campi flegrei italy geochemical and textural studies were carried out on alkaline products of the ad 1538 monte nuovo eruption due to the integration of the volcanological study with eyewitness reports the dynamics and timing of each phase of the eruption and the volume of emitted magmas are known in detail on this basis unique in campi flegrei the relations between magma chamber mechanisms eruptive styles magma ascent dynamics and volatile exsolution processes have been explored glass and phenocryst compositions indicate that the erupted magma has a homogeneous phono trachytic composition textures and compositions of phenocrysts indicate that they crystallised at equilibrium with the melt in the magma chamber likely as a mushy boundary layer along the chamber wall where the temperature was below the liquidus temperature of the crystal free chamber core the estimated crystallisation temperature is 850 40 c the magma phase relations in petrogeny s residua system suggest that phenocryst crystallisation occurred at text p text h 2 text o between 100 and 200 mpa corresponding to depths ranging from 3 to 8 km the microlite composition and their close genetic relations with vesicles indicate that groundmass crystallisation occurred during the eruption as a consequence of magma degassing and vesiculation induced by decompression during its ascent toward the surface crystal size distributions reveal that microlites grew in two stages of undercooling that we define as 1 magma migration onset upward from the chamber and 2 magma rising through the conduit to the surface possibly lasting tens of days and few days respectively these results provide information on the physical conditions that characterise pre and syn eruptive processes which may be useful in order to define eruptive scenarios and to evaluate short term precursors furthermore the collected data provide for the first time information on degassing induced crystallisation during the eruption of a highly evolved alkaline magma

discrete and diffuse heat transfer atashes vent field axial volcano juan de fuca ridge abstract heat fluxes of discrete and diffuse components of hydrothermal discharge are estimated from submersible measurement in the ashes vent field located in the caldera of axial volcano on the central juan de fuca ridge the discrete component comprises discharge from individual vents and the diffuse component comprises seepage from large areas of the seafloor the investigation centered on the high temperature portion of the field a 100 100 m area of fractured lobate and sheet lava flows encompassing discrete discharge 326 c from seven individual vents and diffuse flow from intervening areas a total of 4 4 2 10 6 w was estimated for the discrete heat flux based on measurements of effluent temperature flow rate and orifice diameter at all the known individual vents by d s v alvin in september 1987 a total of 15 75 10 6 w was calculated with a standard plume model for the diffuse flux based on a grid of temperature measurements at altitudes of 1 m and 20 m above the study area using a 1 m long vertical array of temperature sensors mounted at the front of alvin s instrument platform the study indicates that 1 the diffuse component rises separately from the discrete component of hydrothermal discharge and is laterally advected by a prevailing current below the discrete component and that 2 the diffuse component of convective heat flux is approximately an order of magnitude greater than the discrete component in the high temperature portion of the ashes vent field

gamma ray spectrometric analysis of hydrothermally altered dacite samples from the pacmanus hydrothermal field implications for the interpretation of gamma ray wireline measurements abstract spectral gamma ray wireline logging derives the concentrations of potassium thorium and uranium of the formation by measuring the gamma ray spectrum under the assumption of secular equilibrium these measurements were carried out in a number of boreholes drilled by ocean drilling program leg 193 at the pacmanus hydrothermal field spectral gamma ray logs from several of these holes show intervals of increased radioactivity associated with high uranium values of up to 60 ppm snowcap hydrothermal field and 25 ppm roman ruins hydrothermal field nine samples of hydrothermally altered dacite were analyzed using gamma spectrometry to examine the origin of elevated radioactivity and to test for distortion of secular equilibrium core spectrometry indicates that secular equilibrium is distorted distortion can be explained by either an uptake of nuclides of the lower part of the 238u decay series or by removal of 238u or 234th from the rocks in all cases wireline uranium logs and core spectrometry do not reflect true uranium concentrations of the formation but uranium is overestimated by a factor of 3 4 at roman ruins uranium values from wireline logging show higher uranium concentrations over large intervals than uranium values from core spectrometry and from published core geochemistry even when a possible leaching of uranium is taken into account wireline logs indicate that depth intervals of increased radioactivity are related to the occurrence of stockwork mineralization in these depth intervals core recovery was extremely low in addition to the distortion of secular equilibrium low core recovery and preferential loss of softer and more altered material cause a sampling bias between core samples and wireline data and may explain the observed difference in uranium concentrations between core and wireline logs

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

degassing processes during lava dome growth insights from santiaguito lava dome guatemala abstract eruptions of intermediate magma may be explosive or effusive the development of open system degassing has been proposed as a pre requisite for effusion of intermediate magma however processes leading to open system degassing are poorly understood to better understand degassing processes during lava dome extrusion we report high temporal resolution so2 emission rate measurements collected with an ultra violet imaging camera at santiaguito guatemala santiaguito is an ideal case study as the dome lava is compositionally very similar to products of the 1902 plinian eruption of the parental santa maria volcano we find that degassing is weak 0 4 1 kg s 1 but continuous and explosions are associated with small increases in emission rates up to 2 3 kg s 1 continuous repose degassing occurs through a shallow cap rock which likely represents a proto crust on the block lava flow which is extruded from the same vent the continual permeability of the upper conduit argues against a mechanism of explosion triggering in which gas pressure builds beneath a viscous cap rock or plug rather we consider degassing data better consistent with a model of shear fracturing at the conduit margins using field constraints we model the viscosity of santiaguito magma as a function of depth and show that conditions for shear fracturing are met from 150 600 m to the surface this is in line with independent estimates of explosion initiation depth we show that repose timescales are orders of magnitude longer than the timescale for shear fracture and suggest that explosions are triggered when a continuous network of smaller scale fractures develops at which point decompression occurs and an explosion is triggered fracture healing occurs by viscous relaxation however near to the surface where viscosity is highest an unconsolidated gouge layer may develop our model implies that the observed explosions are a by product of extrusion shear fracturing can drive open system degassing of crystal rich intermediate magma at shallow levels in the conduit as high magma viscosity is able to overcome the low strain rates associated with slow ascent of magma

history and geochemistry of a metalliferous sediment core from the mid atlantic ridge at 26 n fourteen thousand years of hydrothermal deposition are recorded in a metalliferous sediment core recovered from the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge at 26 n an estimated 26 to 96 of the sediment at this core site is hydrothermally derived as determined from the caco3 al and fe data layers of essentially pure vent precipitates contain 3 caco3 and 0 5 al with high concentrations of fe 43 cu 4 1 zn 1 2 mn 1 1 v 480 ppm pb 175 ppm cd 32 ppm and hg 3 8 ppm sediment accumulation rates vary from 1 to 30 g cm2 1000 y throughout the core a function of the intensity of hydrothermal inputs distinct hydrothermal events are recorded at 6000 and 8500 y b p in layers containing 90 vent derived material vertical metal profiles and interelement relationships in the core result from variable deposition of oxides and sulfides oxidation and dissolution of sulfide phases and scavenging of metals from seawater

geochemistry of a hydrothermal sediment core from the obs vent field 21 n east pacific rise abstract we report a geochemical investigation of near vent hydrothermal sediment from the east pacific rise collected in a short 11 cm core by submersible from the obs vent site at 21 n the sediment has high concentrations of fe 23 25 cu 10 22 zn 4 13 and s 22 30 but low mn concentrations 108 279 ppm indicating a much fresher sulfidic input than in previously reported near vent sediments from the mid atlantic ridge further studies are required to fully characterise near field hydrothermal sediments but the data in this study allow the first comparisons with similar near vent material collected previously from the mid atlantic ridge as well as ridge flank data from the epr and juan de fuca ridge compared to ridge flank sediments the near field sediments exhibit high ratios of fe mn cu fe and zn fe unlike cu and zn pb is not so significantly enriched pb isotope compositions are indistinguishable from local morb indicating a dominant hydrothermal source for this element in these sediments this may not necessarily be the case for ridge flank sediments however where scavenging from seawater may also be important ree distributions throughout the core exhibit low concentrations and characteristics typical of vent fluids with lree enrichments and pronounced positive eu anomalies these patterns are similar to those of relatively fresh sulfides but there is also evidence for progressive oxidation downward into the core by ingressing seawater

the fragmentation threshold of pyroclastic rocks in response to rapid decompression porous magma may fragment explosively this occurs when the melt can no longer withstand forces exerted upon it due to the overpressure in included bubbles this occurs at a critical pressure difference between the bubbles and the surrounding magma in this study we have investigated this pressure threshold necessary for the fragmentation of magma here we present the first comprehensive high temperature experimental quantification of the fragmentation threshold of volcanic rocks varying widely in porosity permeability crystallinity and chemical composition we exposed samples to increasing pressure differentials in a high temperature shock tube apparatus until fragmentation was initiated experimentally we define the fragmentation threshold as the minimum pressure differential that leads to complete fragmentation of the pressurized porous rock sample our results show that the fragmentation threshold is strongly dependent on porosity high porosity samples fragment at lower pressure differentials than low porosity samples the fragmentation threshold is inversely proportional to the porosity of the other factors permeability likely affects the fragmentation threshold at high porosity values whereas chemical composition crystallinity and bubble size distribution appear to have minor effects the relationship for fragmentation threshold presented here can be used to predict the minimum pressure differential necessary for the initiation or cessation of the explosive fragmentation of porous magma

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

mineralogy and geochemistry of hydrothermal sediments from the serpentinite hosted saldanha hydrothermal field 36 34 n 33 26 w at mar abstract the saldanha hydrothermal field is located at the top of a serpentinized massif mount saldanha ms at a non transform offset nto5 along the mid atlantic ridge mar south of the azores it is one of the rare known sites on a worldwide basis where direct evidence of low temperature 7 9 c hydrothermal activity has been provided by direct observation of hydrothermal fluid venting through small orifices in the ocean floor sedimentary cover this study focuses on the mineralogy and geochemistry of 14 sediment cores collected at ms for comparison four samples collected at the rainbow site nto6 were also studied mount saldanha hydrothermal sediments are highly diluted within a dominant foraminiferal nanofossiliferous ooze with small fragments of underlying rocks the mineral assemblage of the hydrothermal component is characterized by sulphides nontronite smectites poorly crystallized mn oxyhydroxides and amorphous material cu zn and fe sulphides mn mg oxy hydroxides and putative manganobrucite were also identified in one sample collected at an orifice vent in this sample micro chimneys conduits composed of isocubanite and sphalerite were also identified mount saldanha sediments show a clear enrichment in elements such as mn mg fe cu p and v derived from hydrothermal fluids and ni cr and co derived from ultramafic rocks the geochemical data together with the observed mineral assemblage suggest that the hydrothermal fluids are at a higher temperature than those measured at the escape orifices 7 9 c and a strong enrichment in mg mainly at the top of the mount agrees with extensive mixing of the hydrothermal fluid with unmodified seawater nevertheless the mineral assemblage of ms sediments is consistent with the precipitation from hydrothermal fluids at much lower temperatures than at rainbow the presence of serpentinized and steatitized talcshist ultramafic rocks and the occurrence of a strong methane anomaly within the overlying water column collectively suggest that the hydrothermal circulation at ms is driven by exothermic reactions closely associated with the serpentinization process rainbow sediments have a higher concentration in transition metals and consequently an enrichment in sulphides these differences are likely to be a consequence of the higher temperature of hydrothermal fluids reflected in the composition of hydrothermal solutions and of a stronger hydrothermal flux at the rainbow site

hydrothermal nontronite formation associated with microbes from low temperature diffuse hydrothermal vents at the south mid atlantic ridge oceanic nontronite deposits have been identified to be closely related to low temperature hydrothermal activities however their formation mechanisms associated with microbes in diffuse hydrothermal vents still remain largely unknown the friable deposits collected from the low temperature diffuse flow at the southern atlantic ridge display a layered structure scanning electron microscope sem and transmission electron microscope tem analyses reveal that abundant filamentous spherical and rod shaped mineralized forms are preserved in the yellowish green layer of the deposits these mineralized forms primarily consist of si and fe selected area electron diffraction saed patterns of the mineralized forms indicate that they are composed of nontronite high intensities of 12c and 12c14n signals derived from cellular structures determined by nano secondary ion mass spectrometry nanosims suggest the intimate relationship between nontronite and microbes the results of 454 pyrosequencing analyses provide insights into the microbial communities involved in the biologically induced mineralization in the yellowish green layer we propose an evolutionary model for establishing paragenetic sequences among nontronite mn oxide and fe oxyhydroxide in the deposits this paragenetic sequence could be widespread in modern and ancient low temperature hydrothermal fields

pre eruptive water content of rhyolitic magmas as determined by ion microprobe analyses of melt inclusions in phenocrysts abstract the ion microprobe was used to analyze trapped melt inclusions in phenocrysts from two rhyolitic eruptions for h2o f and incompatible trace elements eleven melt inclusions from phenocrysts in air fall tephra from obsidian dome near long valley usa gave an average of 4 1 1 2 wt h2o and showed large variations in f and incompatible trace elements reflecting a complex history reported to involve mixing of at least two magma types eight inclusions in phenocrysts from the taupo ultraplinian event new zealand gave 4 5 0 8 wt h2o with small variations in the other elements analyzed measured water contents are similar to earlier estimates of the pre eruptive water contents of these and similar rhyolites and major and trace element analyses of inclusions compare closely with bulk analyses suggesting that the analyses of inclusions represent the chemistry of the magma at the time of entrapment