evidence for perchlorates and the origin of chlorinated hydrocarbons detected by sam at the rocknest aeolian deposit in gale crater 1 a single scoop of the rocknest aeolian deposit was sieved 150 m and four separate sample portions each with a mass of 50mg were delivered to individual cups inside the sample analysis at mars sam instrument by the mars science laboratory rover ss ample acquisition system the samples were analyzed separately by the sam pyrolysis evolved gas and gas chromatograph mass spectrometer analysis modes several chlorinated hydrocarbons including chloromethane dichloromethane trichloromethane a chloromethylpropene and chlorobenzene were identified by sam above background levels with abundances of 0 01 to 2 3nmol the evolution of the chloromethanes observed during pyrolysis is coincident with the increase in o2 released from the rocknest sample and the decomposition of a product of n methyl n tert butyldimethylsilyl trifluoroacetamide mtbstfa a chemical whose vapors were released from a derivatization cup inside sam the best candidate for the oxychlorine compounds in rocknest is a hydrated calcium perchlorate ca clo4 2 nh2o based on the temperature release of o2 that correlates with the release of the chlorinated hydrocarbons measured by sam although other chlorine bearing phases are being considered laboratory analog experiments suggest that the reaction of martian chlorine from perchlorate decomposition with terrestrial organic carbon from mtbstfa during pyrolysis can explain the presence of three chloromethanes and a chloromethylpropene detected by sam chlorobenzene may be attributed to reactionsofmartian chlorine released during pyrolysiswith terrestrial benzene or toluene derived from 2 6 diphenylphenylene oxide tenax on the sam hydrocarbon trap at this time we do not have definitive evidence to support a nonterrestrial carbon source for these chlorinated hydrocarbons nor do we exclude the possibility that future sam analyses will reveal the presence of organic compounds native to the martian regolith

rhyolitic globular peperites from the aznalcollar mining district iberian pyrite belt spain physical and chemical controls abstract globular peperites are exposed along the aznalcollar mining district iberian pyrite belt ipb spain these rocks originated by mingling of a low viscosity rhyolitic sill with unlithified wet sediments highly amygdaloidal up to 80 vol millimeters to centimeters across rhyolite globules extend from the upper contact of the sill for several centimeters into fine grained volcaniclastic host rocks the sill is very similar to other rhyolites in the ipb in terms of petrography and chemistry it is aphyric to very sparsely quartz phyric with small quartz phenocrysts vesicularity has been measured in the sill as vesicle area fraction determined by point counting over a rectangular grid and number density per unit area determined by counting every vesicle within the grid area amygdale distribution measured within the sill indicates a process by which vesicles migrated vertically from the lower portions of the sill to the roof zone we suggest that this migration process caused local density changes large enough to allow the rhyolitic magma to become buoyant enough to move through the overlying sediments producing globular peperites globular structures are preserved when rhyolite globules entered the wet sediment

a miocene coarse volcaniclastic mass flow deposit in the shimane peninsula sw japan product of a deep submarine eruption a subaqueous volcaniclastic mass flow deposit in the miocene josoji formation shimane peninsula is 15 16 m thick and comprises mainly blocks and lapilli of rhyolite and andesite pumices and non to poorly vesiculated rhyolite it can be divided into four layers in ascending order layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0 3 6 m thick layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6 11 m thick this layer bifurcates laterally into minor depositional units individually composed of a massive lithic rich lower part and a diffusely stratified pumice rich upper part with inverse to normal grading of both lithic and pumice clasts layer 3 is 2 5 3 m thick and consists of interbedded fines depleted pumice rich and pumice poor layers a few centimeters thick layer 4 is a well stratified and well sorted coarse ash bed 1 5 2 m thick the volcaniclastic deposit shows internal features of high density turbidites and contains no evidence for emplacement at a high temperature the mass flow deposit is extremely coarse grained dominated by traction structures and is interpreted as the product of a deep submarine explosive eruption of vesicular magma or explosive collapse of lava

structure and emplacement of a rhyolitic obsidian flow little glass mountain medicine lake highland northern california many rhyolitic obsidian flows show consistent stratigraphic relations among textural units exposed in the flow fronts of undissected flows and in cross sections of older flows the stratigraphy of the holocene little glass mountain rhyolitic obsidian flow consists of from bottom to top air fall tephra deposits basal breccia coarsely vesicular pumice obsidian finely vesicular pumice and surface breccia slightly crystalline rhyolite occurs near the vent areas a model of obsidian flow emplacement based on these textural relations is presented this stratigraphy may reflect the distribution of volatiles within the magma source region with the interlayered contact between coarsely vesicular pumice and overlying obsidian indicating stratification of volatiles in the magma body the distribution of pumiceous and glassy zones along with the orientations of flow banding can be used to map the surface structure of rhyolitic obsidian flows this complex structure as mapped on the little glass mountain flow reflects both the initial flow stratigraphy and subsequent deformational processes during emplacement of the lava three processes disrupt the initial flow configuration the rise of coarse pumice diapirs from the base of the flow the inward propagation of fractures in areas of extension and surface folding in sites of flow parallel compression subvertical flow banding in vent areas indicates that fracturing accompanies the emergence of lava most of the observed upper surface of a dome originates as a fracture plane the structure of domes that form over vent areas may reflect the orientation of dike like conduits as well as the local state of stress during extrusion

the use of hornblende etching clast weathering and soils to date alpine glacial and periglacial deposits a study from southwestern montana many techniques for relative dating of surficial deposits suffer from 1 the need to make judgments and 2 poor reproducibility of data furthermore many of the techniques generate data that do not permit the use of statistical analysis in contrast assessing the etching of hornblende grains yields objectively obtained reproducible data that can be subjected to statistical analysis in a study of the deposits of the bear gulch basin tobacco root range southwestern montana the hornblende etching data proved superior to the clast weathering data and the soil profile data in making interpretations of age relationships the mean etching of hornblende grains decreases as a logarithmic function of depth hornblende grains from older deposits are more etched especially near the surfaces of the profiles the collective evidence supports a record of four first order glacial and periglacial events which have been correlated with 1 the youngest pre bull lake glaciation about 280 000 yr b p 2 the bull lake glaciation about 140 000 yr b p 3 the pinedale glaciation consisting of two second order advances the first and most extensive between 26 000 and 20 000 yr b p and the second about 12 000 yr b p and 4 a mid neoglacial episode of periglacial activity in addition a major landslide blocked the valley in pre bull lake time and an episode of flood erosion along the valley floor is believed to have truncated the late pinedale terminal moraine about 1500 yr b p in a study area for which numerical ages of the deposits are available hornblende etching can be expressed mathematically as a function of the time since deposition with the resulting relationship used to date deposits wherein ages are unknown

subaqueous rhyolite block lavas in the miocene ushikiri formation shimane peninsula sw japan abstract a rhyolite mass of the miocene ushikiri formation in the western part of the shimane peninsula sw japan is a small subaqueous edifice about 600 m high and 4 km wide formed at water depths between 200 and 1000 m it consists mainly of three relatively flat lava flow units 50 300 m in maximum thickness each of which includes lobes and their polyhedral fragments the lava lobes are poorly to well vesiculated glassy to microcrystalline and flow banded and folded compared with mafic pillows they are large having thick quenched and brecciated glassy crusts because of their high viscosity surface tension and thermal conductivity their surfaces disintegrate into polyhedral fragments and grade into massive volcanic breccia the massive volcanic breccia composed of the lobe fragments is poorly sorted and covered with stratified volcanic breccia of the same rock type the rhyolite lavas commonly bifurcate in a manner similar to mafic pillow lavas however they are highly silicic with 1 5 vol phenocrysts and have elongated vesicles and flow folds implying that they were visco plastic during flowage their surface features are similar to those of subaerial block lava with respect to rheological and morphological features they are subaqueous equivalents of block lava

substorm wave base felsic hydroclastic deposits in the archean lac des vents volcanic complex abitibi belt canada abstract volcaniclastic deposits of the 2 3 km thick archean lac des vents volcanic complex are an integral part of major submarine volcanic construction the volcanic edifice which formed on a subaqueous basalt plain is comparable to modern seamounts resting on the ocean floor the initial 770 m of the mafic felsic edifice subject of this study is composed of massive brecciated and pillowed basalts massive to brecciated felsic lava flows and abundant felsic fragmental rocks of hydroclastic origin four distinct volcaniclastic lithofacies constitute the latter 1 the pumice lapilli tuff lithofacies 2 the lapilli tuff breccia lithofacies characterized by two sublithofacies 3 the turbidite tuff and tuff breccia lithofacies and 4 the volcanic sandstone and breccia lithofacies these four volcaniclastic lithofacies are considered to be the result of explosive and non explosive hydrovolcanic fragmentation processes operating at depths below storm wave base 200 m primary deposition or limited remobilization of unconsolidated hydroclastic debris is shown by the preservation of delicate clasts and volcanic textures and heat retention structures the principal transport agents are high concentration sediment gravity flows occurring under laminar and turbulent flow conditions high and low density turbiditic tuffs and fine grained tuff fallout deposits are related to either the dissipating stages of volcanic eruptions or slumping of syneruptive volcanic debris on the flanks of a subaqueous volcanic edifice ubiquitous interstratification of volcaniclastic turbidites shale and pillowed basalt flows with the felsic lava flows and fragmental debris favours subaqueous deposition these features combined with the absence of wave induced sedimentary structures imply deposition in water depths in excess of 200 m viscous feldspar phyric massive and brecciated felsic flows and associated volcaniclastics cross cut by felsic dykes suggest vent proximity the abundance of breccia size hydroclastic debris is consistent with this interpretation collectively these criteria argue for subaqueous fragmentation and deposition of volcaniclastics of inferred hydroclastic origin close to the central vent area at depths below storm wave base

large scale rhyolite peperites jurassic southern chile development of a jurassic volcano tectonic rift basin in the southern andes created a setting in which thick rhyolitic volcaniclastic sequences accumulated in submarine environments and were penetrated by hypabyssal intrusions during or shortly after deposition in the ultima esperanza district of southern chile extensive masses of peperite were produced when rhyolite magma underwent quenching disruption and commingling with wet unconsolidated sediments during intrusion at shallow levels beneath the sea floor the peperite forms discordant intrusive masses with volumes of up to several cubic kilometers in which large widely spaced coherent rhyolite feeder pods are surrounded by and grade into closely packed and dispersed peperite closely packed peperite consists of tightly fitting clasts separated by sediment filled fractures in dispersed peperite the sediment forms a matrix surrounding large masses of fractured rhyolite and smaller more widely separated rhyolite clasts evidence of in situ quench fragmentation is well preserved on both outcrop and thin section scales thin sections show that clast margins and in some cases entire small clasts underwent cooling contraction granulation releasing shards of quenched rhyolite and fragments of phenocrysts into the adjacent sediment interaction between magma and wet sediment was non explosive and involved fluidization of the host sediments creating space for the intruding magma and causing pervasive injection of highly mobile sediment along thermal contraction cracks in quench fragmented rhyolite the ability of the magma to undergo complex intermixing with large volumes of sediment with widespread preservation of in situ fragmentation textures is interpreted to reflect a relatively low magma viscosity presumably caused by retention of volatiles in the magma at the ambient pressures involved beds of redeposited peperite within the rift basin fill indicate that some of the intrusive peperite masses reached the sea floor undergoing slumping and mass flow the peperites were thus an important local source of coarse grained debris during the evolution of the basin

a comparison of miocene and archean rhyolite hyaloclastites evidence for a hot and fluid rhyolite lava abstract in this paper we compare the geology and petrography of miocene and archean submarine rhyolite hyaloclastites the hyaloclastites are sparsely 10 or less plagioclase quartz and pyroxene phyric the hyaloclastites consist of a feeder dyke from which branch lava lobes and irregularly shaped lava pods the lava bodies consist of a holocrystalline core with microlitic texture grading outward into a flow layered rim zone and finally into obsidian the proportion of plagioclase and pyroxene microlites decreases outward some layers of the rim zone may be pumiceous vesicularity up to 50 vesicle size 1 mm or less but most of the lava has less than 5 vesicles one or a few cm long the obsidian shows perlitic fracture patterns the lava bodies grade through an in situ breccia into a hyaloclastite composed of angular obsidian granules and in many cases of fragments of lava lobes evidence for alteration at high temperature is as follows in the archean rhyolite hyaloclastites plagioclase microlites are overgrown by quartz albite spherulites furthermore parts of the miocene and archean hyaloclastite have been cemented and granules have been marginally replaced by quartz and albite hyaloclastite cemented at high temperature locally shows columnar joints at low temperatures obsidian has been hydrated and or has been replaced by clay minerals zeolites chlorite or prehnite chess board albite and fibroradial prehnite in archean hyaloclastite is possibly a pseudomorph after zeolites the sparsely porphyritic nature of the lava and the absence of microlites from the quenched glass suggests that the thyolite hyaloclastites extruded at high near liquidus temperature furthermore pumice is present only locally in the flow layered rim zone and in fragments derived from that zone these features suggest that vesiculation was inhibited by the weight of the water column high temperature and possibly the volatile h 2 o content explain the relatively low viscosity and shear strength of the lava and resulted in the flow morphology particular to this type of hyaloclastic rhyolite flows

lowermost mantle anisotropy and deformation along the boundary of the african llsvp shear wave splitting of sk k s phases is often used to examine upper mantle anisotropy in specific cases however splitting of these phases may reflect anisotropy in the lowermost mantle here we present sks and skks splitting measurements for 233 event station pairs at 34 seismic stations that sample d beneath africa of these 36 pairs show significantly different splitting between the two phases which likely reflects a contribution from lowermost mantle anisotropy the vast majority of discrepant pairs sample the boundary of the african large low shear velocity province llsvp which dominates the lower mantle structure beneath this region in general we observe little or no splitting of phases that have passed through the llsvp itself and significant splitting for phases that have sampled the boundary of the llsvp we infer that the d region just outside the llsvp boundary is strongly deformed while its interior remains undeformed or weakly deformed

geothermometry geochronology and mass transfer associated with hydrothermal alteration of a rhyolitic hyaloclastite from ponza island italy abstract a rhyolitic hyaloclastite from ponza island italy was hydrothermally altered producing four distinct alteration zones based on x ray diffraction mineralogy and field textures 1 nonpervasive argillic zone 2 propylitic zone 3 silicic zone and 4 sericitic zone the unaltered hyaloclastite is volcanic breccia with clasts of vesiculated obsidian in a matrix of predominantly pumice lapilli incomplete alteration of the hyaloclastite resulted in the nonpervasive argillic zone characterized by smectite and disordered opal ct the other three zones exhibit more complete alteration of the hyaloclastite the propylitic zone is characterized by mixed layer illite smectite i s with 10 to 85 i mordenite opal c and authigenic k feldspar akspar the silicic zone is characterized by i s with 90 i pure illite quartz akspar and occasional albite the sericitic zone consists primarily of i s with 66 i pure illite quartz and minor akspar and pyrite k ar dates of i s indicate hydrothermal alteration occurred at 3 38 0 08 ma oxygen isotope compositions of i s systematically decrease from zones 1 to 4 in the argillic zone smectite has 18o values of 21 7 to 22 0 and i s from the propylitic silicic and sericitic zones ranges from 14 5 to 16 3 12 5 to 14 0 and 8 6 to 11 9 respectively 18o values for quartz from the silicic and sericitic zones range from 12 6 to 15 9 by use of isotope fractionation equations and data from authigenic quartz hosted primary fluid inclusions alteration temperatures ranged from 50 to 65 c for the argillic zone 85 to 125 c for the propylitic zone 110 to 210 c for the silicic zone and 145 to 225 c for the sericitic zone fluid inclusion data and calculated 18owater values indicate that hydrothermal fluids were seawater dominated mass transfer calculations indicate that hydrothermal alteration proceeded in a relatively open chemical system and alteration in the sericitic zone involved the most extensive loss of chemical species especially si systematic gains in mg occur in all alteration zones as a result of i s clay mineral formation and systematic losses of na ca and k occur in most zones with the exception of ca calculations of mass transfer associated with hydrothermal alteration on ponza agree with chemical fluxes observed in laboratory experiments involving hydrothermal reactions of rhyolite and seawater the anomalous ca loss at ponza may be due to hydrothermal formation of anhydrite and later low temperature dissolution on the basis of mg enrichments derived from circulating seawater we estimate the following minimum water rock ratios 9 3 6 and 9 for the argillic propylitic silicic and sericitic zones respectively hydrothermal fluid ph for the propylitic and silicic zones was neutral to slightly basic and relatively acidic for the sericitic zone as a result of condensation of carbonic and perhaps other acids

facies architecture of a silicic intrusion dominated volcanic centre at highway reward queensland australia abstract the highway reward massive sulphide deposit is hosted by a silicic volcanic succession in the cambro ordovician seventy mile range group northeastern australia three principal lithofacies associations have been identified in the host succession the volcanogenic sedimentary facies association the primary volcanic facies association and the resedimented syn eruptive facies association the volcanogenic sedimentary facies association comprises volcanic and non volcanic siltstone and sandstone turbidites that indicate submarine settings below storm wave base lithofacies of the primary volcanic facies association include coherent rhyolite rhyodacite and dacite and associated non stratified breccia facies autoclastic breccia and peperite the resedimented volcaniclastic facies association contains clasts that were initially formed and deposited by volcanic processes but then redeposited by mass flow processes resedimentation was more or less syn eruptive so that the deposits are essentially monomictic and clast shapes are unmodified this facies association includes monomictic rhyolitic to dacitic breccia resedimented autoclastic facies siltstone matrix rhyolitic to dacitic breccia resedimented intrusive hyaloclastite or resedimented peperite and graded lithic crystal pumice breccia and sandstone pumiceous and crystal rich turbidites the graded lithic crystal pumice breccia and sandstone facies is the submarine record of a volcanic centre s that is not preserved or is located outside the study area pumice shards and crystals are pyroclasts that reflect the importance of explosive magmatic and or phreatomagmatic eruptions and suggest that the source vents were in shallow water or subaerial settings the lithofacies associations at highway reward collectively define a submarine shallow intrusion dominated volcanic centre contact relationships and phenocryst populations indicate the presence of more than 13 distinct porphyritic units with a collective volume of 0 5 km 3 single porphyritic units vary from the size and shape of the lavas and intrusions reflect a restricted supply of magma during eruption intrusion the style of emplacement and the subaqueous emplacement environment due to rapid quenching and mixing with unconsolidated clastic facies the sills and cryptodomes did not spread far from their conduits the shape and distribution of the lavas and intrusions were further influenced by the positions of previously or concurrently emplaced units magma preferentially invaded the sediment avoiding the older units or conforming to their margins large intrusions and their dewatered envelope may have formed a barrier to the lateral progression and ascent of subsequent batches of magma

a global view of martian surface compositions from mgs tes thermal emission spectrometer tes data from the mars global surveyor mgs are used to determine compositions and distributions of martian low albedo regions two surface spectral signatures are identified from low albedo regions comparisons with spectra of terrestrial rock samples and deconvolution results indicate that the two compositions are a basaltic composition dominated by plagioclase feldspar and clinopyroxene and an andesitic composition dominated by plagioclase feldspar and volcanic glass the distribution of the two compositions is split roughly along the planetary dichotomy the basaltic composition is confined to older surfaces and the more silicic composition is concentrated in the younger northern plains

melting of the glacier base during a small volume subglacial rhyolite eruption evidence from blahnukur iceland abstract although observations of recent volcanic eruptions beneath vatnajokull iceland have improved the understanding of ice deformation and meltwater drainage little is known about the processes that occur at the glacier base we present observations of the products of a small volume effusive subglacial rhyolite eruption at blahnukur torfajokull iceland lava bodies typically 7 m long have unusual conical morphologies and columnar joint orientations that suggest emplacement within cavities melted into the base of a glacier cavities appear to have been steep walled and randomly distributed these features can be explained by a simple model of conductive heat loss during the ascent of a lava body to the glacier base the released heat melts a cavity in the overlying ice the development of vapour escape pipes in the waterlogged permeable breccias surrounding the lava allows rapid heat transfer between lava and ice the formed meltwater percolates into the breccias recharging the cooling system and leaving a steam filled cavity the slow ascent rates of intrusive rhyolitic magma bodies provide ample time for a cavity to be melted in the ice above even during the final 10 m of ascent to the glacier base an equilibrium cavity size is calculated at which melting is balanced by creep closure this is dependent upon the heat input and the difference between glaciostatic and cavity pressure the cavity sizes inferred from blahnukur are consistent with a pressure differential of 2 4 mpa suggesting that the ice was at least 200 m thick this is consistent with the volcanic stratigraphy which indicates that the ice exceeded 350 m in thickness although this is the first time that a subglacial cavity system of this type has been reconstructed from an ancient volcanic sequence it shares many characteristics with the modern firn cave system formed by fumarolic melting within the summit crater of mount rainier washington at both localities it appears that localised heating at the glacier base has resulted in heterogeneous melting patterns despite the different rheological properties of ice and firn similar patterns of cavity roof deformation are inferred the development of low pressure subglacial cavities in regions of high heat flux may influence the trajectory of rising magma with manifold implications for eruptive mechanisms and resultant subglacial volcanic landforms

facies analyses and volcanic setting of the giant neves corvo massive sulfide deposit iberian pyrite belt portugal in the iberian pyrite belt volcanic rocks are relatively scarce accounting for approximately only 25 of the geologic record with the remaining 75 consisting of sedimentary units this association is very clear in the host succession to the neves corvo massive sulfide deposit in portugal the neves corvo host succession comprises the products of explosive and effusive rhyolitic eruptions intercalated with mudstone that records a submarine below wave base environment and provides precise biostratigraphic age constraints the first and second volcanic events involved eruptions at local intrabasinal vents the first event generated thick beds of fiamme breccia that are late famennian in age the fiamme were originally pumice clasts produced by explosive eruptions and were subsequently compacted the second event was the late strunian latest famennian effusion of rhyolitic lava that was pervasively quench fragmented the third and final event is younger than the massive sulfide deposits poorly represented in the mine area and minor compared with the two other events the integration of biostratigraphic data with the volcanic facies architecture indicates that the neves corvo ore deposits are similar in age to the late strunian rhyolitic lava although regionally the iberian pyrite belt is essentially a sedimentary succession ore formation at neves corvo can be closely linked to discrete volcanic events that produced a relatively narrow range of volcanic facies

subaqueous paleoproterozoic metarhyolite dome flow cone complex flin flon greenstone belt manitoba canada abstract two sparsely plagioclase phyric rhyolite domes 50 100 m thick and 125 250 m long as observed in 2 dimensional vertically dipping exposures are partly mantled by a rhyolite volcaniclastic complex that on the south side of the domes also forms a half cone like mound at least 60 m thick and 500 m long the domes and mantling volcaniclastic complex are overlain by three aphyric to sparsely phyric rhyolite flows that are 70 120 m thick and 550 1200 m long domes and flows have a lower columnar jointed highly fractured locally brecciated subfacies that grades upward into 40 95 crackled and disaggregated breccia crackled breccia is a highly fractured subfacies in which clast like areas are bounded by closely spaced joints and minor matrix but there is only limited rotation of clasts crackled breccia was produced by quench fracturing combined with hydraulic action of water converted to steam or supercritical fluid within the fractures overlying disaggregated breccia comprises rotated particles and 25 50 matrix it is most abundant near flow margins and is a crumble breccia produced by flow advance or expansion the volcaniclastic mound comprises lenticular interlayered facies of 1 resedimented phreatomagmatically generated heterolithic rhyolitic tuff and lapilli tuff 2 isolated to close packed rhyolite lobes that are 0 5 5 m thick and 0 5 50 m long the lobes are at least in part pillows and 3 units comprising small

can we use pyroxene weathering textures to interpret aqueous alteration conditions yes and no pyroxene minerals are a significant component of shergottite nakhlite chassignite snc meteorites e g velbel 2012 and detected across large areas of mars surface e g mustard et al 2005 these minerals are associated with chloride sulfate and perchlorate salts that may represent briny waters present in mars history previous textural analyses by velbel and losiak 2010 comparing pyroxenes and amphiboles from various natural weathering environments showed no correlation between apparent apical angles describing the morphology of denticular weathering textures and mineralogy or aqueous alteration history in relatively dilute solutions however high salinity brines preferentially dissolve surface species potentially leading to different textures dependent on the brine chemistry in this study we performed controlled pyroxene dissolution experiments in the laboratory on a well characterized diopside to determine if aqueous alteration in different high salinity brines representative of potential weathering fluids on mars produce unique textural signatures following two months of dissolution in batch reactors we observed denticles on etch pit margins and pyroxene chip boundaries in all of the solutions investigated ultrapure water 18 m cm 1 a h 2 o 1 low salinity solutions containing 0 35 m nacl a h 2 o 0 99 0 35 m na 2 so 4 a h 2 o 0 98 and 2 m naclo 4 a h 2 o 0 9 and near saturated brines containing 1 7 m na 2 so 4 a h 2 o 0 95 3 m nacl a h 2 o 0 75 and 4 5 m cacl 2 a h 2 o 0 35 no systematic change in denticle length or apical angle was observed between any of the solutions investigated even when altered in brines with significantly different salinity activity of water and anion composition based on these and previous results from natural systems apical angle measurements are not a useful proxy for determining the extent or nature of aqueous alteration however since denticles form relatively slowly during weathering at circum neutral ph denticle length may be a useful proxy for chemical weathering duration all of the experimental solutions produced median denticle lengths 1 m likely due to the brief weathering experiments however perchlorate brines produced a significantly wider range of denticle lengths than those observed in all the other experimental solutions tested since perchlorate is likely a common constituent in martian soils glotch et al 2016 denticle length measurements should be used cautiously as proxies for extent of aqueous alteration on mars particularly in samples that also contain perchlorate

an explosive intrusive subglacial rhyolite eruption at dalakvisl torfajokull iceland this paper describes unusual rhyolitic deposits at dalakvisl torfajokull iceland that were emplaced during a quaternary subglacial eruption despite its small volume 0 2 km3 the eruption mechanisms were highly variable and involved both explosive and intrusive phases the explosive phase involved vesiculation driven magma fragmentation at the glacier base and generated a pumiceous pyroclastic deposit containing deformed sheets of dense obsidian textures suggest that the obsidian was generated by the collapse of partly fragmented foam that was intruding the deposit and water contents indicate quenching at elevated pressures in contrast the intrusive phase of the eruption generated vesicle poor quench hyaloclastites associated with a variety of peperitic lava bodies the presence of juvenile rich fluvio lacustrine sediments is the first documented evidence that meltwater may pond close to the vent during subglacial rhyolite eruptions if the bedrock topography is favourable in order to explain the variable eruption mechanisms a conceptual model is presented in which the transition from an explosive to an intrusive eruption was controlled by the space available for fragmentation within the subglacial cavity melted above the vent when the cavity became completely filled by volcanic deposits the vent became blocked and rising magma was forced to intrude through poorly consolidated debris this led to arrested fragmentation and welding of foam domains to form vesicle poor obsidian lava the transition to an intrusive eruption has taken place although this vent blocking mechanism is particularly relevant to subglacial eruptions it may also apply to subaerial rhyolitic eruptions where patterns of explosive and effusive activity cannot be explained by shallow degassing processes alone meanwhile the variable style of a small volume subglacial rhyolite eruption further highlights the complex processes that mediate volcano ice interactions

spherulites quench fractures and relict perlite in a late devonian rhyolite dyke queensland australia abstract a late devonian rhyolite dyke displays perlitic and other fracture sets as well as textures generated by crystallisation of the glass the dyke is less than a metre wide and has sharp contacts with ignimbrite although originally glassy no glass is preserved aligned magnetite after pyroxene microlites and trains of small 0 5 1 mm spherical spherulites crystallised early at temperatures above the glass transition temperature and before formation of the fracture sets long subplanar fractures oriented perpendicular to the dyke walls extend almost the full dyke width and end by merging with adjacent long fractures short subplanar cross fractures are perpendicular to and terminate at the long fractures well defined perlitic fractures are present within the volumes of rock generally

facies architecture and palaeoenvironmental constraints of subaqueous felsic volcanism in the iberian pyrite belt the paymogo volcano sedimentary alignment abstract the paymogo volcano sedimentary alignment pvsa is an essentially igneous sequence associated with vhms deposits of the iberian pyrite belt unlike other areas of this iberian mining district where mafic igneous rocks are abundant in the pvsa these are essentially felsic two facies associations have been distinguished based on petrographic and chemical features a lower dacite facies association and an upper rhyolite facies association the main ore horizons appear related to volcanic breccias of the rhyolite facies association a detailed petrographic study of these breccias suggests they are resedimented hyaloclastite deposits the analysis of volcanic facies indicates that the volcanism developed at water depths deeper than 500 m consistent with the minimum depth at which vhms deposits can form we conclude that detailed volcanological studies provide effective constraints to palaeoenvironmental interpretations which are relevant in assessing regional stratigraphy and area selection for vhms exploration

constraints on the composition and particle size of chloride salt bearing deposits on mars chloride salt bearing deposits on mars were discovered using the mars odyssey thermal emission imaging system themis and have been characterized by both mid infrared mir and visible to near infrared vnir remote sensing instruments the chloride salt bearing deposits exhibit a blue slope at mir wavelengths and a featureless red slope at vnir wavelengths these deposits also lack strong 3 m bands in vnir spectra indicating that they are desiccated compared to the surrounding regolith the lack of vnir spectral features suggests that an anhydrous chloride salt the most likely of which is halite is responsible for the observed spectral slope in this work we use laboratory spectra and a hybrid t matrix hapke light scattering model to constrain the particle sizes and salt abundances of the martian chloride salt bearing deposits our work shows that the two broad spectral classes of these deposits observed by themis can be explained by a difference in the particle size of the admixed silicate regolith in all cases chloride salt abundances of 10 25 are required to match the themis data the chloride salt abundances determined in this work suggest deposition in a lacustrine playa setting or in association with late stage groundwater upwelling

jarosite dissolution rates and maximum lifetimes in high salinity brines implications for earth and mars abstract jarosite is a ferric sulfate salt k h na fe 3 so 4 2 oh 6 that forms in acidic oxidizing environments on earth and has also been observed in outcrops on mars high chloride concentrations within the outcrops at meridiani planum suggest that jarosite likely interacted with high salinity brines this study examines jarosite dissolution in h 2 o cacl 2 and h 2 o nacl brines activity of water a h 2 o 0 35 and 0 75 respectively to determine the effects of high salinity brines and a h 2 o on jarosite dissolution rates within brines with a h 2 o 0 75 and 0 35 initial k jarosite dissolution rates at 298 k decrease from log r 9 9 to 11 6 mol m 2 s 1 and na jarosite rates decrease from log r 10 6 to 11 2 mol m 2 s 1 respectively in addition k jarosite dissolution in nacl brine at 263 k yielded an average dissolution rate of log r 11 6 mol m 2 s 1 applying a shrinking sphere model to determine 1 mm jarosite particle lifetimes extends the maximum duration of fluid alteration from lifetimes of attack on residual fe 3 left on the surface of jarosite grains if brines on mars became highly concentrated in chlorine ions through sulfate precipitation the dissolution rate of jarosite and perhaps other minerals as well could accelerate with time shortening particle lifetimes and the inferred duration of aqueous diagenesis significantly

distinguishing peperite from other sediment matrix igneous breccias lessons from the iberian pyrite belt abstract breccias composed of coarse monomictic porphyritic igneous clasts and fine grained matrix are common in subaqueous volcanic successions we use the descriptive name sediment matrix igneous breccia for this facies and have recognized at least five different origins among examples in the iberian pyrite belt 1 peperite 2 sediment infill volcanic breccia 3 mud matrix resedimented hyaloclastite 4 mud rich water settled fiamme breccia and 5 apparent sediment matrix igneous breccia because the components and textures are similar discriminating among the different origins can be very difficult both peperite and sediment infill volcanic breccia can occur along top contacts of thick intervals of felsic coherent and monomictic breccia facies the presence of peperite indicates that the contact is intrusive whereas the presence of sediment infill volcanic breccia indicates that the contact is depositional hence correct distinction between peperite and sediment infill volcanic breccia is an important means of discriminating felsic intrusions from felsic lavas and domes the distinction underpins the reconstruction of volcanic centers and facies architecture and the ordering of volcanic intrusive sedimentary and mineralizing events in ancient submarine volcanic successions in addition in volcanic hosted massive sulfide districts such as the iberian pyrite belt paleoseafloor positions are considered highly prospective for massive sulfide ore bodies correct interpretation of sediment matrix igneous breccias is also important in this context because sediment infill volcanic breccia mud matrix resedimented hyaloclastite and mud rich water settled fiamme breccia all form or are deposited at the seafloor and hence delineate paleoseafloor positions in contrast peperite and apparent sediment matrix igneous breccias form subsurface and do not delineate paleoseafloor positions

olivine and pyroxene diversity in the crust of mars datafromthe observatoire pour la mineralogie l9eau les glaces et l9activite omega on the mars express spacecraft identify the distinct mafic rock forming minerals olivine low calcium pyroxene lcp and high calcium pyroxene hcp on the surface of mars olivine and hcp rich regions are found in deposits that span the age range of geologic units however lcp rich regions are found only in the ancient noachian aged units which suggests that melts for these deposits were derived from a mantle depleted in aluminum and calcium extended dark regions in the northern plains exhibit no evidence of strong mafic absorptions or absorptions due to hydrated materials

subaqueous calderas in the archean abitibi greenstone belt an overview and new ideas abstract the 300 700 km abitibi greenstone belt in canada contains numerous world class archean volcanogenic massive sulfide vms deposits yet documentation of archean subaqueous calderas hosting such deposits is lacking the modern sunrise deposit in the myojin knoll caldera of the izu bonin arc shows that submarine calderas are first order sites for vms the hunter mine and normetal calderas as well as the blake river megacaldera complex are reviewed with respect to geometry regional geology physical volcanology dyke emplacement and hydrothermal carbonate alteration these subaqueous calderas are placed into the geodynamic context of the abitibi greenstone belt the abitibi belt displays a complex history of oceanic arc formation evolution collision and fragmentation with numerous types of volcanic edifices and sedimentary basins controlled by oblique subduction and mantle plumes analogue models yielding new insights into the sequence of caldera forming events and their synvolcanic fault patterns are applied to explain archean examples underpressure experiments develop a consistent sequence of fault patterns that compare favourably with natural caldera events during subsidence two sets of faults propagate from the magma chamber margin 1 early reverse outward dipping faults and 2 normal inward dipping faults the largest massive sulfide deposits form preferentially at the caldera margin based on experimental studies and detailed facies mapping the hunter mine caldera is of the segmented piecemeal variety and the normetal caldera is akin to a piston structure the blake river megacaldera contains 1 the e w trending misema 2 the nw trending new senator and 3 the ne trending noranda calderas with the latter two being nested graben type calderas consistent with oblique archean subduction the poorly documented hydrothermal carbonate alteration is discussed and a new exploration model for calderas is presented the alteration study was evaluated at three levels microprobe analysis geochemical staining of drill core and whole rock analyses there were two principal stages of alteration related to vms formation an early silicification phase developed either at depth and or at the seawater interface whereby silicification was bedding parallel in fine grained volcaniclastic deposits near the water surface a pervasive and extensive semi conformable carbonate alteration zone with a discordant focused root zone along faults evolved subsequently three distinct carbonate pairings were observed 1 proximal siderite sideroplesite fe ankerite next to the vms deposit 2 an intermediate ankerite fe dolomite zone and 3 a distal calcite dolomite zone the carbonate phase is associated with the mineralizing event transitions between carbonate species are subtle and changes are indicated by mineral assemblage overlap the silica carbonate hydrothermal alteration pattern is a replacement product and is far more widespread than has been documented this alteration can be easily confused with primary carbonates and chemical precipitation deposits e g banded iron formation if detailed volcano sedimentary facies mapping with alteration has not been conducted and if outcrop is insufficient

forsterite dissolution rates in mg sulfate rich mars analog brines and implications of the aqueous history of mars high salinity brines although rare on earth s surface may have been important in the geologic history of mars increasing evidence suggests the importance of liquid brines in multiple locations on mars in order to interpret the effect of high ionic strength brines on olivine dissolution which is widely present on mars 47 new batch reactor experiments combined with 35 results from a previous study conducted at 25 c from 1 ph 4 in magnesium sulfate sodium sulfate magnesium nitrate and potassium nitrate solutions with ionic strengths as high as 12 m show that very high ionic strength brines have an inhibitory effect of forsterite dissolution rates multiple linear regression analysis of the data suggests that the inhibition in dissolution rates is due to decreased water activity at high ionic strengths regression models also show that mmg up to 4 m and mso4 up to 3 m have no effect on forsterite dissolution rates the effect of decreasing dissolution rates with decreasing ah2o is consistent with the idea that water acts as a ligand that participates in the dissolution process less available water to participate in the dissolution reaction results in a slower dissolution rate multiple linear regression analysis of the data produces the rate equation logr 6 81 0 52ph 3 26logah2o forsterite in dilute solutions with a water activity of one dissolves twice as fast as those in brines with a water activity of 0 8

a felsic dyke swarm formed under the sea the archean hunter mine group south central abitibi belt quebec canada an extensive rhyolitic dyke swarm has intruded subaqueous pyroclastic deposits iron formations hyaloclastite breccias and lava flows of the 2730 ma hunter mine group hmg in the south central part of the archean abitibi belt quebec the dyke swarm has a minimum width of 500 m and can be traced perpendicular to the section for 2 4 km based on crosscutting relationships chilled margins quartz content and colour five distinct dyke generations have been established each dyke generation has several magmatic pulses as indicated by parallel rows of columnar joints absence of brecciation between parallel rows suggests extremely brief intervals between magma pulses the central parts of most dykes display inverted v shaped patterns of columnar joint convergence inferred to indicate differential cooling during the late stages of dyke propagation the dykes commonly display delicate spherulites suggesting rapid cooling solidification temperatures between 400 and 600 c and penecontemporaneous devitrification quartz feldspar aggregates in the groundmass have locally developed microgranophyric textures large spherulites near the chilled margins probably formed at temperatures below 400 c percolation of abundant water throughout the dyke complex is suggested by ubiquitous prominent chilled dyke margins development of a chilled margin 500 m along one dyke suggests that water percolated at least 500 m below the water rock interface because the dykes intruded subaqueous pyroclastic deposits of similar composition dyke emplacement below the sea floor is inferred interstratification of pillowed flows and brecciated pillowed flows containing rhyolite fragments at the top of the 4 5 km thick sequence indicates that the central felsic complex probably never emerged during its evolutionary history supporting the contention that the felsic dyke complex was emplaced beneath the archean sea floor

denticles on chain silicate grain surfaces and their utility as indicators of weathering conditions on earth and mars abstract denticulated margins also known in older literature as sawtooth cockscomb or hacksaw terminations are a common feature of pyroxenes and amphiboles visible in optical and electron microscopes denticles are remnants of undissolved material that formerly constituted the walls between elongate etch pits the characteristic aqueous dissolution form of chain silicate minerals denticles occur widely in terrestrial near surface materials that have experienced low temperature aqueous alteration including chemically weathered regoliths soils in a variety of climatic and geomorphic settings sediments and sedimentary rocks ranges of dissolution forms and dimensions commonly tens of microns in length are identical on both pyroxenes and amphiboles in these materials denticles on terrestrial chain silicates are formed by low temperature aqueous alteration denticles on a soil mineral grain of unknown composition at the phoenix mars lander site and on terrestrial pyroxenes naturally weathered in terrestrial low temperature aqueous environments are broadly similar in size shape and orientation relationships between denticles however quantitative morphometric parameters vary among different terrestrially weathered pyroxenes and even between different images of compositionally identical pyroxenes from the same naturally weathered sample such variations are too large to allow simple identification of minerals of unknown composition from morphology alone nevertheless the overall morphological similarities are consistent with a low temperature aqueous corrosion origin of denticles at the phoenix mars lander site

alunite dissolution rates dissolution mechanisms and implications for mars abstract alunite kal 3 so 4 2 oh 6 is a hydrated aluminous sulfate mineral associated with acidic oxidizing aqueous environments on earth additionally orbiting spacecraft and rovers on mars have reported spectral data that indicate a range of mono and polyhydrated sulfate phases and hydroxysulfate phases suggesting such conditions also existed on mars in the past this study examines alunite dissolution rates in aqueous systems with varying ph temperature and solution chemistry conditions alunite dissolution rates in dilute solutions are 2 3 orders of magnitude slower than jarosite dissolution rates measured under analogous conditions similar to jarosite alunite dissolution rates vary as a function of activity of h and oh following the rate law log r mol m 2 s 1 0 133 0 02 ph 10 65 0 07 at ph r 0 194 0 04 ph 12 53 0 26 at ph 5 however minimum alunite dissolution rates are shifted to higher ph 5 5 5 likely due to differences in fe and al speciation alunite and jarosite rates converge in saturated nacl and cacl 2 brines as the activity of water decreases suggesting that differences in water exchange rates with fe 3 and al 3 control dissolution rates in dilute solutions while metal cl complexation occurs at similar rates within the brines particle lifetimes based on measured dissolution rates in dilute solutions show that alunite particles are expected to be preserved two orders of magnitude longer than jarosite particles over a range of ph and temperature conditions in particular alunite is more likely to be preserved in neutral to moderately alkaline systems compared to jarosite which is expected to be preserved in more acidic conditions alunite dissolution produced amorphous al rich alteration products at moderate to high ph unlike jarosite alunite dissolution does not show a clear trend as a function of temperature alunite dissolution rates do not increase with increasing temperature likely due to lower alunite solubility at higher temperature therefore alunite is expected to be more prevalent in hydrothermal systems compared to jarosite

spatial distributions of secondary minerals in the martian meteorite mil 03346 168 determined by raman spectroscopic imaging miller range mil 03346 is a nakhlite meteorite that has been extensively studied due to its unique complex secondary mineral phases and their potential implications for the hydrologic history of mars we conducted a set of raman spectroscopic and raman imaging studies of mil 03346 168 focusing on the secondary mineral phases and their spatial distributions with a goal to better understand the possible processes by which they were generated on mars this study revealed three types of calcium sulfates a solid solution of k na jarosite and two groups of hydrated species with low crystallinity hslc in the veins and or mesostasis areas of the meteorite the most abundant ca sulfate is bassanite that suggests two possible paths for its direct precipitation from a ca s h2o brine either having low water activity or with incomplete development producing bassanite with gypsum microcrystals on mars the second most abundant ca sulfate is soluble caso4 which raises a new question on the origins of this phase in the martian meteorite since caso4 readily hydrates in the laboratory but is apparently stable in atacama desert the close spatial relationship of k na jarosite solid solutions with rasvumite kfe2s3 magnetite hslc and fine grained low crystallinity alkali feldspar in mesostasis suggests a potential in situ formation of mesostasis jarosite from these fe k na s o h2o species

non explosive magma water interaction in a continental setting miocene examples from the eastern cordillera central andes nw argentina the middle upper miocene las burras almagro el toro bat igneous complex within the eastern cordillera of the central andes 24 s nw argentina has revealed evidence of non explosive interaction of andesitic magma with water or wet clastic sediments in a continental setting including peperite generation we describe and interpret lithofacies and emplacement mechanisms in three case studies the las cuevas member 11 8 ma comprises facies related to i andesite extruded in a subaqueous setting and generating lobe hyaloclastite lava and ii marginal parts of subaerial andesite lava dome s in contact with surface water comprising fluidal lava lobes hyaloclastite and juvenile clasts with glassy rims the lampazar member 7 8 ma is represented by a syn volcanic andesite intrusion and related peperite that formed within unconsolidated water saturated coarse grained volcaniclastic conglomerate and breccia the andesite intrusion is finger shaped and grades into intrusive pillows pillows are up to 2 m wide tightly packed near the intrusion fingers and gradually become dispersed in the host sediment 50 m from the parent intrusion the almagro a member 7 2 ma shows evidence of mingling between water saturated coarse grained volcaniclastic alluvial breccia and intruding andesite magma the resulting intrusive pillows are characterized by ellipsoidal and tubular shape and concentric structure the high level penetration of magma in this coarse sediment was unconfined and irregular magma was detached in apophyses and lobes with sharp contacts and fluidal shapes and without quench fragmentation and formation of a hyaloclastite envelope the presence of peperite and magma water contact facies in the bat volcanic sequence indicates the possible availability of water in the system between 11 7 ma and suggests a depositional setting in this part of the foreland basin of the central andes characterized by an overall topographically low coastal floodplain that included extensive wetlands

endogenous growth of a miocene submarine dacite cryptodome rebun island hokkaido japan momo iwa rebun island hokkaido japan is a dacite cryptodome 200 300 m across and 190 m high the dome is inferred to have intruded wet poorly consolidated sediment in a shallow marine environment the internal structure of the dome is concentric with a massive core banded rim and narrow brecciated border all of which are composed of compositionally uniform feldspar phyric dacite boundaries between each of the zones are distinct but gradational the massive core consists of homogeneous coherent unfractured dacite and is characterized by radial columnar joints 60 200 cm across the banded rim encircles the massive core and is 40 m wide it is characterized by large scale flow banding parallel to the dome surface the flow banding comprises alternating partly crystalline and more glassy bands 80 150 cm thick the outermost brecciated border is up to 80 cm thick and consists of in situ breccia and blocky peperite the in situ breccia comprises polyhedral dacite clasts 5 20 cm across and a cogenetic granular matrix the blocky peperite consists of polyhedral dacite clasts 0 5 2 cm across separated by the host sediment mudstone the internal structures of the dome suggest endogenous growth involving a continuous magma supply during a single intrusive phase and simple expansion from the interior although much larger the internal structures of momo iwa closely resemble those of lobes in subaqueous felsic lobe hyaloclastite lavas

morphology and growth style of a miocene submarine dacite lava dome at atsumi northeast japan abstract a miocene submarine lava dome at tate iwa atsumi yamagata japan displays well preserved primary morphological features the dome currently rises above a wave cut coastal platform and is 55 m high and 90 180 m across it comprises a massive core and a lava lobe hyaloclastite rim both of which are composed of compositionally uniform feldspar phyric dacite sio2 64 wt the boundary between these two zones is distinct but gradational the massive core consists of homogeneous coherent dacite and is characterized by flow banding along the margin and by columnar joints radiating from the centre to the margin the lava lobe hyaloclastite rim encircles the massive core and consists of a complex of 80 90 dacitic lava lobes and 10 20 hyaloclastite the lava lobes are 1 6 m thick 3 12 m wide and more than 5 m long each lobe consists of a radially columnar jointed core and a glassy rim 10 30 cm thick the hyaloclastite comprises polyhedral dacite clasts 5 100 cm across in a matrix of dacite fragments up to 5 mm across the lava lobes and hyaloclastite have gradational contacts in places paleobathymetric studies based on foraminifera in sediment beneath the dome and the k ar age of the dome suggest that it was extruded in a middle bathyal environment 1500 2000 m below sea level at 12 9 0 6 ma the internal structures of the dome suggest that it formed by a combination of exogenous growth involving extrusions of small dacite lobes and endogenous growth involving a continuous magma supply and simple expansion from the interior the presence of small lava lobes along the rim suggests that the magma had relatively low viscosity at the time of extrusion in spite of its high silica content magma temperatures calculated by two pyroxene and fe ti oxide geothermometers were 999 1042 and 957 1005 c respectively the inferred low viscosity may be attributed to the high temperature of the magma and or a high confining pressure resulting from the deep sea environment

facies architecture of the upper oligocene submarine smrekovec stratovolcano northern slovenia abstract the upper oligocene smrekovec volcanic complex svc represents the remains of a submarine stratovolcano which underwent early miocene tectonic dissection and large scale displacement along the periadriatic line according to the lithofacies architecture abundance and spatial distribution central medial and distal zones of the former volcano edifice have been recognized the zones extend at a distance of 0 2 km 2 5 km and 5 20 km south of the periadriatic line respectively lavas shallow intrusive bodies autobreccia peperite and hyaloclastite deposits occur mainly in the central zone which is extensively altered under hydrothermal conditions characterised by the development of zeolites mainly laumontite prehnite chlorite albite and epidote explosive volcanic activity intensified with the evolution of magma from basaltic andesitic to dacitic gas and water supported eruption fed density flows are assumed their deposits show lithofacies organisation in pyroclastic depositional units pdus similar to either volcaniclastic turbidites type 1 or thin subaqueous ignimbrites type 2 volcaniclastic debris flow deposits are developed as volcaniclastic breccia and sandy debris flow deposits the former occurs in the central and medial zones and the latter occurs in the medial zone only the most widespread lithofacies in the svc are volcaniclastic turbidite deposits three types of sedimentary unit tsu have been distinguished type 1 is 2 5 m thick and dominated by the massive division it mainly occurs in the central zone type 2 is 0 7 3 5 m thick and occurs in the central and medial zones it comprises the massive division bedded fining and thinning upward division and a horizontally wavy and cross laminated division topped by massive fine grained tuff type 3 is 0 1 1 5 m thick and mainly occurs in the distal zone the massive division is absent and thinly bedded and laminated divisions are topped by massive fine grained tuff volcaniclastic turbidites were settled from low density turbidity currents and related suspension clouds mixed volcaniclastic siliciclastic deposits are subordinate in occurrence and restricted to the distal zone they are commonly reworked by bottom currents

facies architecture and origin of a submarine rhyolitic lava flow dome complex ponza italy the islands of ponza and gavi western pontine archipelago italy preserve parts of a subaqueously emplaced high silica 75 77 sio2 rhyolitic lava flow that overlies the margins of three older domes exposures of the lava alone cover 24 km2 and with a thickness of 150 m exposed in the cliffs the preserved volume of the lava flow is 3 6 km3 and the original volume is likely to have exceeded 7 km3 the lava flow consists of coherent obsidian and several hyaloclastite facies including in situ and clast rotated breccias and sandstones which all exhibit gradational relationships the coherent obsidian occurs as separate domains and is often flow banded although flow folds occur the banding is generally subhorizontal dipping gently to the nne the hyaloclastite representing 90 of the lava by volume has a diffuse layering varying from 0 5 to 2 m in thickness this layering also dips gently to the nne and is marked in places by alternation of some coherent horizons coarse and finely fragmented in situ hyaloclastite pumiceous hyaloclastite and clast rotated hyaloclastite the layering is laterally continuous and consistent in orientation over most of the 13 km length of ponza the subhorizontal orientation of the flow banding and presence of resedimented deposits in the upper parts of the sequence indicate that the unit is a lava flow the resedimented deposits comprise both proximal and distal deposits of debris flows and turbidity currents initiated by gravitational instability of the upper and marginal parts of the lava flow and include stratified monomictic obsidian breccias and bedded vitric siltstones and sandstones the lava flow has been intruded and crosscut by rhyolitic bodies of various ages including discordant coherent to internally fractured contemporaneous rhyolite bodies and late penecontemporaneous dikes with glassy margins and alteration haloes

morphological and sedimentological characteristics of the neogene submarine coherent lavas and hyaloclastites in southwest hokkaido japan abstract the neogene formations in southwest hokkaido japan are mostly composed of submarine volcanic products such as submarine coherent lavas and hyaloclastites associated with feeder dikes this paper presents criteria for recognition of submarine effusion products through description of the morphological and sedimentological characteristics of coherent lavas such as pillow lobes sheet flow lava lobes and pseudo pillow lavas and hyaloclastites and feeder dikes and a small amount of associate pyroclastic rocks

three dimensional lithofacies variations in hyaloclastite deposits analysis of the spatial lithofacies variability within lava fed delta formations in southern iceland has revealed complex three dimensional volcanic architectures in hyaloclastite deposits in non glacial settings two depositional environments are studied a lava entering a marine embayment stori nupur and b lava advancing into a body of water of the flanks of a surtseyan cone hjorleifshof i interaction between environmental factors such as shoreline geomorphology water depth wave energy levels the nature of the lava transport system lava supply rate all affect the resulting lava deltas creating complex lithofacies arrangements and stacking patterns recognised here are two types of hyaloclastite deltas one of syn sedimentary origin hjorleifshof i and one derived from primary fragmentation processes stori nupur syn sedimentary systems are dominated by destabilisation of the hyaloclastite pile leading to reworking downslope and share similarities to alluvial delta systems conversely primary fragmented systems are controlled by waxing and waning cycles in volcanic effusivity whereby the hyaloclastite unit recorded is not the product of one lava flow rather than one eruptive event

subaqueous cryptodome eruption hydrothermal activity and related seafloor morphologies on the andesitic north su volcano abstract north su is a double peaked active andesite submarine volcano located in the eastern manus basin of the bismarck sea that reaches a depth of 1154 m it hosts a vigorous and varied hydrothermal system with black and white smoker vents along with several areas of diffuse venting and deposits of native sulfur geologic mapping based on rov observations from 2006 and 2011 combined with morphologic features identified from repeated bathymetric surveys in 2002 and 2011 documents the emplacement of a volcanic cryptodome between 2006 and 2011 we use our observations and rock analyses to interpret an eruption scenario where highly viscous crystal rich andesitic magma erupted slowly into the water saturated gravel dominated slope of north su an intense fragmentation process produced abundant blocky clasts of a heterogeneous magma olivine crystals within a rhyolitic groundmass that only rarely breached through the clastic cover onto the seafloor phreatic and phreatomagmatic explosions beneath the seafloor cause mixing of juvenile and pre existing lithic clasts and produce a volcaniclastic deposit this volcaniclastic deposit consists of blocky non altered clasts next variably 1 100 altered clasts hydrothermal precipitates and crystal fragments the usually applied parameters to identify juvenile subaqueous lava fragments i e fluidal shape or chilled margin were not applicable to distinguish between pre existing non altered clasts and juvenile clasts this deposit is updomed during further injection of magma and mechanical disruption gas propelled turbulent clast recycling causes clasts to develop variably rounded shapes an abundance of blocky clasts and the lack of clasts typical for the contact of liquid lava with water is interpreted to be the result of a cooled high viscosity crystal rich magma that failed as a brittle solid upon stress the high viscosity allows the lava to form blocky and short lobes the pervasive volcaniclastic cover on north su is partly cemented by hydrothermal precipitates these hydrothermally cemented breccias crusts and single pillars show that hydrothermal circulation through a thick layer of volcaniclastic deposits can temporarily increase slope stability through precipitation and cementation

microdenticles on naturally weathered hornblende abstract microdenticles with lengths in the micron submicron range rather than tens of microns are developed on the lateral surfaces of larger classic denticles on naturally weathered hornblende from weathered amphibolite of the carroll knob complex in western north carolina microdenticles share the shape and orientation of the larger more typical denticles producing arrays of microdenticles that give the larger host denticle the appearance of a surface covered with imbricate pointed or rounded scales the arrays of imbricate microdenticles are formed by low temperature aqueous alteration during weathering of the carroll knob complex hornblende they are later stage corrosion forms on already corroded surfaces of hornblende that show larger scale evidence of typical weathering in the carroll knob occurrence hornblende microdenticles are associated with dilute weathering solutions suggesting possible control by extreme undersaturation of solutions with respect to hornblende

exogenous and endogenous construction of the subaqueous glenwood felsic lava flow complex abitibi greenstone belt quebec canada abstract both intrusive and extrusive volcanic facies of a felsic lava flow complex are exposed as the outcrops of the glenwood rhyolite within the blake river group of the abitibi greenstone belt quebec canada sub vertically dipping volcanic facies allows for the identification of the internal architecture of an archean subaqueous felsic flow complex detailed facies analyses show volcanic facies and volcanic textures and structures atypical of subaqueous pyroclastic deposits or deposits of dome complexes additionally a complex network of mafic dykes cross cut the felsic facies when integrated with facies specific geochemical analyses a coherent model for all volcanic and intrusive facies and supplemental information regarding the host volcanic complex has been developed detailed mapping reveals extrusive or exogenous aphanitic felsic volcanic facies and intrusive or endogenous quartz and feldspar phyric felsic volcanic facies exogenous facies are comprised of massive lobate in situ brecciated flow breccia and flow front breccia facies whereas the only visible endogenous facies is a quartz and feldspar phyric unit that intrudes the brecciated facies on the eastern side of the complex all felsic facies demonstrate a west to east flow direction both exogenous and endogenous facies have similar chemical compositions ranging from dacite to rhyodacite with a transitional affinity mafic to intermediate dykes predominantly strike north south and east west with occasional east northeast trending units wispy terminations at some dyke margins and dismembered dykes indicate they were intruded into a still cooling viscoelastic medium geochemical analyses further defines that most east west trending dykes have a tholeiitic affinity whereas north south trending dykes are dominantly calc alkaline and east northeast trending dykes are transitional the subaqueous glenwood felsic flow complex was emplaced in a fault bounded depression and consists of stacked exogenous dacite rhyodacite lobes and intruded by a late quartz and feldspar phyric endogenous lobe the intrusion of mafic dykes occurred relatively soon after the cessation of felsic activity as evidenced by primary textures consistent with intrusion into a viscoelastic medium combined the extrusive and intrusive facies of the complex attests to the complexity of the host volcanic environment

aqueous corrosion of olivine in the mars meteorite miller range mil 03346 during antarctic weathering implications for water on mars abstract several nakhlites clinopyroxenite meteorites from mars contain olivine phenocrysts with corrosion features identical in size shape and distribution to the smaller etch pits of well characterized weathered terrestrial olivine miller range mil 03346 is an antarctic nakhlite find recovered after long exposure to antarctic conditions the distribution of discrete olivine etch pits almost exclusively within a few hundred microns of allocation mil 03346 171 s documentably exposed surface suggests that they formed by terrestrial weathering in antarctica the small size of olivine etch pits in mil 03346 171 relative to commonly much larger etch pits in even incipiently weathered terrestrial examples suggests that the duration of its exposure to weathering conditions was short or the weathering conditions to which it was exposed did not favor olivine corrosion in the form of etch pit formation or both time scales for the formation of etch pits estimated from experimentally determined dissolution rates of olivine over a range of phs are comparable to the measured terrestrial age of the meteorite and short relative to the time available for possible similar corrosion on mars etch pits of the observed size on mil 03346 olivine phenocrysts would be relatively easy to form supraglacially under brief episodic acidic antarctic conditions but the terrestrial age of mil 03346 is long enough that its olivine might have been weathered to the observed state by englacial films of alkaline antarctic water the paucity of similar etch pits in olivine from the interior of mil 03346 suggests that olivine in this mars meteorite was exposed to even less aqueous alteration after iddingsitization during its 1 3 billion years on mars than its exterior was subjected to during its pleistocene holocene exposure to antarctic weathering conditions

late orogenic basins in the archaean superior province canada characteristics and inferences abstract the late orogenic archaean duparquet kirkland and stormy basins of the canadian superior province are characterized by bounding crustal scale faults and abundant porphyry stock emplacement lava flows and pyroclastic deposits are restricted to the kirkland and stormy basins and coarse clastic detritus characterizes the duparquet basin seven distinct lithofacies are identified 1 mafic volcanic 2 felsic volcanic 3 pyroclastic 4 volcaniclastic 5 conglomerate sandstone 6 sandstone argillite conglomerate and 7 argillite sandstone tuffaceous sandstone the mafic and felsic volcanic lithofacies represent effusive lava flows the pyroclastic lithofacies is formed of subaerial surge and airfall deposits and the volcaniclastic lithofacies is composed of reworked volcanic debris the conglomerate sandstone lithofacies is interpreted as alluvial fan fan delta or proximal braided stream deposits whereas the sandstone argillite lithofacies is consistent with sandy dominated flood or braidplain deposits a dominantly shallow water lacustrine setting is inferred for the argillite sandstone lithofacies these different lithofacies record the basin history and can be used to identify basin forming processes lithofacies stacking and rapid lateral changes of lithological units in conjunction with interformational unconformities and basin margin faults suggest tectonically induced sedimentation volcanism can also influence basin evolution and the delicate balance between erosion sedimentation and prevalent transport processes is affected by volcanic input catastrophic influx of pyroclastic material facilitated mass wasting processes and formation of non confined hyperconcentrated flood flow deposits account for local congestion of alluvial or fluvial dispersal patterns confined stream flow processes govern sedimentation during intravolcanic phases or prominent tectonic uplift in addition climate which controls the weathering processes and vegetation which stabilizes unconsolidated material affects the transport and depositional process a co 2 rich aggressive weathering humid archaean atmosphere favours traction current deposits and an absence of vegetation promotes rapid denudation although tectonism is the prevalent long term controlling factor in restricted basins the effects of volcanism climate and lack of vegetation can also be detected

felsic fire fountaining beneath archean seas pyroclastic deposits of the 2730 ma hunter mine group quebec canada abstract the lapilli tuff breccias ltb 1 and ltb 2 of the archean hunter mine group in the south central part of the abitibi greenstone belt are inferred to be the product of subaqueous lava fountaining intercalated sub wave base iron formations interstratified turbiditic tuffs the absence of wave induced sedimentary structures and the stratigraphic position of lapilli tuff breccias beneath basaltic komatiites support this contention a complete eruptive sequence shows a tripartite division into a massive breccia b stratified lapilli tuff and c turbiditic tuff lapilli tuff division the massive breccia division is characterized by clusters of isolated and compressed irregular shaped clasts inferred to be deposited directly from the hot magmatic lava fountain abundant vesicular pyroclasts with a vesicle content of up to 60 exhibit locally coalescing vesicles indicating bubble nucleation prior to eruption the prevalence of irregular to amoeboid clast shapes suggests transport from the vent in a steamy rich high density current to the site under a self generated steam cupola ubiquitous subequant lapilli size pyroclasts of the stratified lapilli tuff division suggest that significant ingress of water into the fountain changed the prevalent fragmentation process from magmatic to hydrovolcanic the turbiditic tuff lapilli tuff division composed of pumice lithic fragments and vitric ash is envisaged to have formed by gravitational collapse of a subaqueous turbulent eruptive plume this type of eruptive mechanism constituted a minor but important process of volcanic construction on the ocean floor during the archean and possibly during incipient arc and backarc formation in modern day settings

water settling and resedimentation of submarine rhyolitic pumice at yali eastern aegean greece abstract the yali pumice breccia is a very thick 150 m quaternary succession of submarine pumice that has been uplifted and exposed in the southern part of yali island in the eastern aegean greece the pumice breccia comprises moderately to well sorted 0 03 3 m thick beds of loose pumice clasts that are poor in fine matrix predominantly the larger pumice clasts cobbles and boulders are prismatic with quenched margins and internal polyhedral joints in comparison the smaller pumice clasts are polyhedral angular to subrounded blocky pebbles and granules these smaller clasts lack quenched margins and have sharp curviplanar surfaces that cross cut vesicle boundaries we interpret the larger pumice clasts to be the products of spalling and explosive fragmentation of a small extrusion of submarine pumiceous lava the smaller pumice clasts were generated by a combination of 1 passive and explosive disintegration of the larger pumice clasts and 2 phreatomagmatic explosions phreatomagmatic explosions also generated a minor component of non or poorly vesicular juvenile and basement derived lithic clasts the yali pumice breccia includes four major facies cobble boulder pebble mixed cobble dominant and mixed pebble dominant the cobble boulder facies occurs in well sorted massive tabular beds of large 64 mm 1 5 m pumice up to 3 m thick that lack both lithic clasts and fine matrix this facies has textural and lithological features consistent with deposition of large pumice clasts by means of water settling from suspension the three remaining facies are comparatively less well sorted although still fines poor and include rare lithic clasts they exhibit massive or internally diffusely stratified wedging 0 1 2 m thick beds of coarser grained medium pebble to cobble pumice clasts that onlap or are separated by wedge shaped thinner

spherulitic aphyric pillow lobe metatholeiitic dacite lava of the timmins area ontario canada a new archean facies formed from superheated melts fragmental rocks of the v10 units of the vipond formation of the tisdale assemblage previously have been identified as pillow basalts but many samples are shown to be intermediate to felsic in character likely tholeiitic dacite in composition specifically the v10b unit is mapped as a pillow lobe dacite aside from being more geochemically evolved in terms of their immobile trace elements these rocks differ from typical pillow basalts in that they have more abundant primary breccia and hyaloclastite the pillow lobes are contorted having been folded in a plastic state and are zoned typically having a spherulite rich core moreover the flows are aphyric interpreted to mean that they were erupted in a superheated state this along with their pillow lobe nature demonstrates that they were erupted as relatively low viscosity melts for such silicic compositions interaction with water quenched the outer pillow lobe and contributed to the formation of the abundant breccia the fact that the melt was crystal and microlite free inhibited crystal growth such that the bulk of the lobes were quenched to crystal free glass nucleation occurred only in the cores where cooling rates were lower in comparison to the medial and exterior areas of the pillow lobes although in the cores crystal growth rates were high so that abundant spherulite formation took place the flows are exposed over a strike length of more than 10 km and are interpreted to be continuous and to have resulted from fissure eruptions the resulting porous permeable high fe mg high surface area glassy rocks may have been ideal for channeling and reacting with gold bearing hydrothermal solutions descriptions of most other archean subaqueous felsic lobes document that the lavas were phyric and that spherulites formed near the external lobe margin the distinctly different zoning of the v10b pillow lobes suggests that they constitute a new archean facies formed from superheated melts

evidence for silicate dissolution on mars from the nakhla meteorite veins containing carbonates hydrous silicates and sulfates that occur within and between grains of augite and olivine in the nakhla meteorite are good evidence for the former presence of liquid water in the martian crust aqueous solutions gained access to grain interiors via narrow fractures and those fractures within olivine whose walls were oriented close to 001 were preferentially widened by etching along 001 this orientation selective dissolution may have been due to the presence within olivine of shock formed 001 100 and 001 110 screw dislocations the duration of etching is likely to have been brief possibly less than a year and the solutions responsible were sufficiently cool and reducing that laihunite did not form and fe liberated from the olivine was not immediately oxidized the pores within olivine were mineralized in sequence by siderite nanocrystalline smectite a fe mg phyllosilicate and then gypsum whereas only the smectite occurs within augite the nanocrystalline smectite was deposited as submicrometer thick layers on etched vein walls and solution compositions varied substantially between and sometimes during precipitation of each layer together with microcrystalline gypsum the fe mg phyllosilicate crystallized as water briefly returned to some of the veins following desiccation fracturing of the smectite these results show that etching of olivine enhanced the porosity and permeability of the nakhlite parent rock and that dissolution and secondary mineralization took place within the same near static aqueous system

rates of hornblende etching in soils in glacial deposits of the northern rocky mountains wyoming montana u s a influence of climate and characteristics of the parent material abstract hornblende grains in the soil matrix of glacial deposits in the northern rocky mountains etch at rates that depend primarily upon the influences on soil moisture of the climate and the texture of the parent materials etching is a logarithmic function of age of the parent material and etching rates decline rapidly after a period of initial high rates over the first 10 15 ka after deposition in this study a comparison of etching rates was made among four chronosequences in the wind river range of western wyoming and the tobacco root range of southwestern montana which have differences in mean annual precipitation map and texture of the till parent materials using rates calculated for the first 12 ka after deposition etching is slowest 0 02 m ka 1 in coarse textured granitic parent materials where the map is 25 40 cm in contrast etching is faster by an order of magnitude 0 21 m ka 1 where the map is 110 150 cm and the parent material is finer textured because 15 sedimentary rock material has been mixed with the granitic component within individual chronosequences deposits at higher elevations have accelerated etching rates due to higher orographic precipitation or the influence of late lying snow

geologic setting of pacmanus hydrothermal area high resolution mapping and in situ observations abstract this study presents a systematic analysis and interpretation of autonomous underwater vehicle based microbathymetry combined with remotely operated vehicle rov video recordings rock analyses and temperature measurements within the pacmanus hydrothermal area located on pual ridge in the bismarck sea of eastern manus basin the data obtained during research cruise magellan 06 and so 216 provides a framework for understanding the relationship between the volcanism tectonism and hydrothermal activity pacmanus is a submarine felsic vocanically hosted hydrothermal area that hosts multiple vent fields located within several hundred meters of one another but with different fluid chemistries vent temperatures and morphologies the total area of hydrothermal activity is estimated to be 20 279 m2 the microbathymetry maps combined with the rov video observations allow for precise high resolution mapping estimates of the areal extents of hydrothermal activity we find the distribution of hydrothermal fields in the pacmanus area is primarily controlled by volcanic features that include lava domes thick and massive blocky lava flows breccias and feeder dykes spatial variation in the permeability of local volcanic facies appears to control the distribution of venting within a field we define a three stage chronological sequence for the volcanic evolution of the pacmanus based on lava flow morphology sediment cover and lava sio2 concentration in stage 1 sparsely to moderately porphyritic dacite lavas 68 69 8 wt sio2 erupted to form domes or cryptodomes in stage 2 aphyric lava with slightly lower sio2 concentrations 67 2 67 9 wt sio2 formed jumbled and pillowed lava flows in the most recent phase stage 3 massive blocky lavas with 69 to 72 5 wt sio2 were erupted through multiple vents constructing a volcanic ridge identified as the pacmanus neovolcanic zone the transition between these stages may be gradual and related to progressive heating of a silicic magma following a recharge event of hot mantle derived melts

distribution and significance of crystalline perlitic and vesicular textures in the ordovician garth tuff wales diverse spherulitic and granular crystalline fabrics perlitic textures and fabrics related to the growth and migration of vesicles occur in the garth tuff a largely welded ordovician ignimbrite defining the distribution of such textures helps to constrain the ignimbrite s cooling and degassing history suites of spherulitic and perlitic textures closely reflect variation in cooling rates seven facies are defined based on the style and intensity of crystallisation 1 a medium to coarsely crystalline equigranular facies 2 an intensely spherulitic facies 3 a sparsely spherulitic facies 4 a pectinate facies 5 a microcrystalline to cryptocrystalline equigranular facies 6 a lithophysal facies and 7 a transitional perlite pectinate facies textural changes from facies 1 to 5 reflect progressively higher cooling rates facies 1 occurs in proximal settings in the ignimbrite s core facies 2 to 5 successively envelop facies 1 with facies 2 becoming the dominant fabric in the ignimbrite s core in medial settings facies 5 is typically developed in the originally glassy perlitic zones at the ignimbrite s welded margins crystallisation under hydrous conditions is reflected by second boiling textures in the sporadically developed lithophysal facies the seventh facies reflects a subtle interplay between cooling hydration and crystallisation which locally prevented perlitic fracturing the distribution of amygdales reflects patterns of volatile migration and entrapment in the lower levels of the ignimbrite amygdales occur in irregular concentrations or rare subvertical pipe like structures pipe like structures attest to fumarolic activity while the ignimbrite was in a rheomorphic state amygdales are widespread and evenly distributed in the upper levels of the ignimbrite however the top of the welding profile is characterised by a thin poorly vesiculated originally vitrophyric horizon that abruptly caps an intense concentration of amygdales ductile and brittle fabrics developed during the upward migration of gas microscopic drag folds occur around some amygdales jigsaw fit to clast rotated breccias originated through both late stage pneumatic fracturing and autobrecciation vaporisation of water at the flow base provided a significant source of volatiles in addition to gas released during cooling and crystallisation secondary alteration has enhanced or modified some fabrics perlitic zones were susceptible to patchy chlorite sericite carbonate diagenetic alteration diagenesis and metamorphism have contributed to the infilling of vesicles

facies reconstruction of a hydrothermally altered dacite extrusive sequence evidence from geophysical downhole logging data odp leg 193 odp leg 193 drilled into the pacmanus hydrothermal field papua new guinea which is an activehydrothermal vent field associated with felsic magmatism in a convergent geodynamic setting thepacmanus hydrothermal field is part of the eastern manus basin and is located near the crest of pualridge a 500 to 700 meters high felsic neo volcanic ridge two sites snowcap site 1188 and romanruins site 1189 were drilled approximately 800 m apart although low core recovery limited theexamination of the subsurface geology geophysical downhole measurements provided continuous recordsof in situ physical properties with depth and were used to reconstruct lithostratigraphic profiles downholelogging operations included standard wire line logging and logging while drilling lwd measurements electrical resistivity images of the borehole wall were examined to find characteristic textures and threedifferent volcanic facies were distinguished coherent dacite volcaniclastic dacite and brecciated dacite inaddition intervals with sulfide mineralization were identified based on characteristic responses of thephotoelectric factor log a comparison of the reconstructed profiles from both sites shows considerabledifferences in the proportions of facies and the thickness of individual layers thick units of coherent faciesare predominant at snowcap and indicate a proximal position of the site within the volcanic system of pualridge at roman ruins thin individual layers with rapid changes in volcanic facies and a higherproportion of volcaniclastic and brecciated dacite suggest a medial position these differences in volcanicfacies show that the volcanic architecture at pual ridge is characterized by small scale facies changes emplacement of small volume individual lava flows and complex geometries of individual emplacementunits geophysical logging data suggest that subseafloor hydrothermal activity is focused to the areaaround one hole at roman ruins downhole logs from this hole show characteristic cyclic patterns in theelectrical resistivity and photoelectric factor logs indicating that hydrothermal sulfide mineralization isconcentrated at the tops of individual lava flows representing paleo seafloor positions

drill core based facies reconstruction of a deep marine felsic volcano hosting an active hydrothermal system pual ridge papau new guinea odp leg 193 abstract pual ridge is a deep marine felsic volcanic edifice in the eastern manus back arc basin papua new guinea with an estimated volume of 6 to 9 km 3 it is 1 1 5 km wide 20 km long and rises 500 600 m above the surrounding ocean floor the active pacmanus hydrothermal field on the crest of pual ridge at 1640 1690 m below sea level was the target of ocean drilling program leg 193 variably altered dacite lavas have been recovered from the subsurface of a low t discharge site snowcap and a high t black smoker site roman ruins reaching a maximum depth of 380 m below seafloor mbsf volcanic facies interpretation of these cores is difficult due to incomplete recovery and widespread pseudoclastic textures generated by fracturing and multi phase incomplete fluid dacite interaction however distinction of genuine volcaniclastic facies and facies with alteration related clastic appearance is important in order to define paleo seafloor positions within the volcanic stratigraphy that may be prospective for massive sulfide mineralization this has been accomplished using remnant primary characteristics indicative of transportation such as polymictic composition grading or textural evidence for differential movement of individual clasts three phases of volcanic activity can be distinguished and a proximal facies association dominated by coherent facies of dacite lavas exists below snowcap at roman ruins a medial facies association consists of lava flows with about equal proportions of coherent and volcaniclastic facies endogenous growth was an important process during lava flow emplacement and groundmass textures such as perlite flow banding and spherulites indicate that cooling rates were variable locally allowing for high temperature devitrification a tube pumice breccia unit is interpreted as the resedimented facies of a quench fragmented highly vesicular dacite lava carapace sulfide accumulations in the subsurface are restricted to roman ruins suggesting that the abundance of volcaniclastic facies favored ascent of metal bearing fluids due to the inherent high permeability evidence for mineralization at paleo seafloor positions at 110 and 195 mbsf indicate a complex interrelationship between volcanic and hydrothermal activity a facies model has been developed which may help to locate subaqueous felsic lava dominated volcanic centers and associated sulfide deposits in ancient volcano sedimentary successions

giant rafted pumice blocks from the most recent eruption of taupo volcano new zealand insights from palaeomagnetic and textural data abstract giant blocks of pumice lie strewn along a former shoreline of intracaldera lake taupo new zealand and are the sole subaerial evidence of the most recent volcanism at the taupo supervolcano geochemically they are identical to material erupted during the complex and multiphase 1 8 ka taupo eruption which they post date by one to two decades the blocks some of which are 10 m long show complex jointing patterns indicative of both surface chilling and continued interior expansion as well as heterogeneous vesicularity with dense rims mean density 917 kg m3 grading via an intervening transition zone mean density 844 kg m3 into a more highly vesicular interior mean density 815 kg m3 analysis of thermal demagnetisation data indicates significant reorientation of the blocks as they cooled through a series of blocking temperatures some parts of block rims cooled to below 580 c well before emplacement on the shore whereas other parts in the interior and transition zones which cooled more slowly acquired different orientations before stranding some block interiors cooled after blocks were finally deposited and record the direction of the 1 8 ka field the blocks are believed to be derived from one or both of a pair of rhyolitic lava domes that developed on the bed of lake taupo several decades after the climactic taupo eruption over the inferred vent area these and similar giant rafted pumice blocks in other marine and lacustrine settings raise a number of questions about how volatile rich felsic magma can be erupted underwater with only limited thermal fragmentation furthermore the prolonged flotation of out sized fragments of vesiculated magma formed during subaqueous dome growth contrasts with the rapid sinking of smaller pieces of hot plinian pumice under laboratory conditions the genesis of pumice forming the blocks is not entirely clear most simply the blocks may represent part of a vesiculated carapace of a growing lava dome broken loose as the dome grew and deformed then rising buoyantly to the surface parts of the carapace could also be released by local magma water explosions some textures of the pumice however suggest fresher magma released from beneath the carapace this may suggest that silicic dikes and pillows pods intruded into a growing mound of silicic hyaloclastite itself formed by quench fragmentation and thermal granulation of the dike margins this fragmental cover would have inhibited cooling of a still hot and actively vesiculating interior which was then released to float to the surface by gravitational destabilisation and collapse of the growing pile following their formation the large fragments of pumice floated to the lake s surface where they were blown ashore to become embedded in accumulating transgressive shoreface sediments and continue cooling

explosive rhyolite tuya formation classic examples from kerlingarfjoll iceland rhyolite eruptions in iceland mostly take place at long lived central volcanoes examples of which are found associated with each of the present day rift zone ice caps subglacial eruptions at kerlingarfjoll central volcano produced rhyolite tuyas that are notable for their exposures of early erupted pyroclastic material observations from a number of these edifices are synthesised into a general model for explosive rhyolite tuya formation eruptions begin with violent phreatomagmatic explosions that generate massive tuff mt but the influence of water quickly declines leading to the formation of massive lapilli tuffs mlt containing magmatically fragmented vesicular pumice and ash these are deposited rapidly near the vent probably by moist pyroclastic density currents confined by ice but not within a meltwater lake the explosive effusive transition is controlled by the ascent rate and gas content of the magma an unusual obsidian rich massive lapilli tuff lithofacies omlt is identified and interpreted as pyroclastic material that was intruded into gas fluidised deposits at the explosive effusive transition the effusive phase of eruption involves the emplacement of intrusions and lava caps intrusions of lava into the early erupted phreatomagmatic deposits are characterised by peperitic margins and the formation of hyaloclastite intrusions into stratigraphically higher levels of the pyroclastic material show more limited interaction with the host tephra and have microcrystalline cores large lava bodies with columnar jointed margins cap the tuyas and have intrusive basal contacts with the tephras the main influence of the ice is to confine the rhyolite eruptive products to immediately above the vent region this is in contrast to subglacial basaltic tuya forming eruptions which are characterised by the formation of meltwater lakes phreatomagmatic fragmentation and subaqueous deposition the lack of meltwater storage may reduce the potential for large jokulhlaups

hydrothermal alteration of a rhyolitic hyaloclastite from ponza island italy abstract a rhyolitic hyaloclastite from ponza island italy has been hydrothermally altered producing four distinct alteration zones based on xrd and field textures 1 non pervasive argillic zone 2 propylitic zone 3 silicic zone and 4 sericitic zone the unaltered hyaloclastite is a volcanic breccia with clasts of vesiculated obsidian in a matrix of predominantly pumice lapilli incomplete alteration of the hyaloclastite resulted in the non pervasive argillic zone characterized by smectite and disordered opal ct obsidian clasts some pumice lapilli and pyrogenic plagioclase and biotite are unaltered smectite has an irregular flakey morphology although euhedral particles are occasionally observed the propylitic zone is characterized by mixed layer illite smectite i s with 10 to 85 illite i mordenite opal c and authigenic k feldspar akspar the matrix of the hyaloclastite is completely altered and obsidian clasts are silicified however plagioclase and biotite phenocrysts remain unaltered flakey i s replaces pumice and mordenite akspar and silica line and fill pores i s particles are composed predominantly of subequant plates and euhedral laths the silicic zone is characterized by highly illitic i s with 90 i quartz akspar and occasional albite in this zone the matrix and clasts are completely altered and pyrogenic plagioclase shows significant alteration illitic i s has a euhedral lath like morphology in the sericitic zone the hyaloclastite altered primarily to illitic i s with 66 i quartz and minor akspar and pyrite clay minerals completely replace pyrogenic feldspars and little evidence remains of the original hyaloclastite texture unlike other zones illitic i s is fibrous and pure illite samples are composed of euhedral laths and hexagonal plates the temperatures of hydrothermal alteration likely ranged from 30 to 90 c for the argillic zone from 110 to 160 c for the propylitic zone from 160 to 270 c for the silicic zone and were possibly as high as 300 c for the sericitic zone the four zones occur as linear bands that increase in intensity north of the bentonite mine at cala dell acqua the alteration zones have two orientations and may be structurally controlled by e w and ne sw trending faulting which is consistent with the dominant structural trends of the pontine archipelago finally hydrothermal alteration most likely involved seawater based on the geologic evolution of ponza

fluidization of wet sediments during the emplacement and cooling of various igneous bodies during the emplacement and cooling of igneous bodies fluidization of wet sediment by water vapour may occur by heating or by pressure relief during the opening of fractures momentary fluidization causes sediment reconstitution and local transport continuous fluidization can result in substantial sediment displacement theoretical considerations show that fluidization due to heating is not likely to occur at depths where pressure is much greater than 312 bars studies of the emplacement of sills of andesite ayrshire fluid rhyolite ramsey island and viscous rhyolite moelwyn hills recognize a wide range of phenomena attributable to wet sediment fluidization and in places modify existing interpretations the transgressive bases of the subaqueously welded ash flow tuffs of the capel curig volcanic formation are consistent with emplacement attended by fluidization of the subjacent sediment evidently fluidization is a means by which large volumes of wet sediment can be replaced by igneous material with minimal disturbance of the remaining host also fluidization can result in exceptionally complex contact relationships between sedimentary host and intrusive rocks and can cause fractures formed during cooling to be thoroughly pervaded by sediment fluidization is commonly associated with the formation of peperites

spectral evidence for hydrated salts in recurring slope lineae on mars transient streaks that appear seasonally on martian slopes are consistent with brine flows but evidence of water or salts has been lacking analysis of spectral data reveals hydrated salts associated with the streaks confirming a briny origin

dissolution of nontronite in chloride brines and implications for the aqueous history of mars increasing evidence suggests the presence of recent liquid water including brines on mars brines have therefore likely impacted clay minerals such as the fe rich mineral nontronite found in martian ancient terrains to interpret these interactions we conducted batch experiments to measure the apparent dissolution rate constant of nontronite at 25 0 c at activities of water ah 2o of 1 00 0 01 m cacl 2 or nacl 0 75 saturated nacl or 3 00 mol kg 1 cacl 2 and 0 50 5 00 mol kg 1 cacl 2 experiments at ah 2o 1 00 0 01 m cacl 2 were also conducted at 4 0 c 25 0 c and 45 0 c to measure an apparent activation energy for the dissolution of nontronite apparent dissolution rate constants at 25 0 c in cacl 2 containing solutions decrease with decreasing activity of water as follows 1 18 10 12 9 10 14 mol mineral m 2 s 1 ah 2o 1 00 2 36 10 13 3 1 10 14 mol mineral m 2 s 1 ah 2o 0 75 2 05 10 14 2 9 10 15 mol mineral m 2 s 1 ah 2o 0 50 similar results were observed at 25 0 c in nacl containing solutions 1 89 10 12 1 10 13 mol mineral m 2 s 1 ah 2o 1 00 1 98 10 13 2 3 10 14 mol mineral m 2 s 1 ah 2o 0 75 this decrease in apparent dissolution rate constants with decreasing activity of water follows a relationship of the form log k diss 3 70 0 20 ah 2o 15 49 where k diss is the apparent dissolution rate constant and ah 2o is the activity of water the slope of this relationship 3 70 0 20 is within uncertainty of that of other minerals where the relationship between dissolution rates and activity of water has been tested including forsteritic olivine log r 3 27 0 91 ah 2o 11 00 olsen et al 2015 and jarosite log r 3 85 0 43 ah 2o 12 84 dixon et al 2015 where r is the mineral dissolution rate this result allows prediction of mineral dissolution as a function of activity of water and suggests that with decreasing activity of water mineral dissolution will decrease due to the role of water as a ligand in the reaction apparent dissolution rate constants in the dilute nacl solution 1 89 10 12 1 10 13 mol mineral m 2 s 1 are slightly greater than those in the dilute cacl 2 solutions 1 18 10 12 9 10 14 mol mineral m 2 s 1 we attribute this effect to the exchange of na with ca in the nontronite interlayer an apparent activation energy of 54 6 1 0 kj mol was calculated from apparent dissolution rate constants in dilute cacl 2 containing solutions at temperatures of 4 0 c 25 0 c and 45 0 c 2 33 10 13 1 3 10 14 mol mineral m 2 s 1 4 0 c 1 18 10 12 9 10 14 mol mineral m 2 s 1 25 0 c and 4 98 10 12 3 8 10 13 mol mineral m 2 s 1 45 0 c the greatly decreased dissolution of nontronite in brines and at low temperatures suggests that any martian nontronite found to be perceptibly weathered may have experienced very long periods of water rock interaction with brines at the low temperatures prevalent on mars with important implications for the paleoclimate and long term potential habitability of mars

calcium carbonate and sulfate of possible extraterrestrial origin in the eeta 79001 meteorite abstract two varieties of ca carbonate were found in a total of three interior 2 cm depth samples of glass inclusions from the shergottite meteorite elephant moraine antarctica a79001 two of the samples including the largest deposit around a vug near the center of the meteorite 8 cm depth contained veins of granular calcite with significant magnesium avg atomic mg mg ca 0 14 0 15 and phosphorus avg atomic p ca 0 04 either as mg calcite with dissolved p or as calcite with very finely intergrown mg bearing phosphate the second variety which occurred in a third sample with a previously documented high concentration of trapped gases consisted of disseminated 10 20 m anhedral grains of nearly pure caco3 and was intimately associated with laths and needles of ca sulfate possibly gypsum the coexisting carbonate and sulfate appeared to be partially decrepitated relict grains that were trapped during rapid solidification of quench textured pyroxene and glass for at least the latter occurrence textural relationships clearly indicate a pre terrestrial origin for the salts all evidence considered it is probable that both varieties of cacarbonate and the ca sulfate formed on a planetary body probably mars before the meteorite fell on earth

high pressure metamorphism and deep crustal seismicity evidence from contemporaneous formation of pseudotachylytes and eclogite facies coronas the metamorphic complex of the western gneiss region wgr norway constitutes the root of the caledonian mountain belt and experienced temperatures of 700 800 c and pressures in excess of 20 kbar during peak metamorphism mafic bodies surrounded by strongly banded felsic gneisses commonly exhibit variable reequilibration to granulite and eclogite facies conditions and locally preserve igneous minerals and textures the krakeneset gabbro located on the island of vagsoy in the mixed hp uhp zone of the western wgr display evidence for extensive metastability through the entire prograde and retrograde p t histories eclogite constitutes less than a few percent of the total volume of the body and high pressure assemblages typically form thin coronas around magmatic phases or occur along localized zones of brittle deformation and fluid infiltration the gabbro displays pseudotachylyte vein networks that define subparallel brittle fault zones 50 cm wide transecting the gabbro body the pseudotachylytes contain m to mm scale amoeboid and dendrite like textures of garnet and plagioclase with inclusions of the eclogite facies minerals orthopyroxene omphacite amphibole and dolomite suggesting rapid disequilibrium growth of minerals during high pressure conditions textural and petrological evidence from pseudotachylytes and corona structures show that the growth of these unusual textures occurred shortly after pseudotachylyte crystallization by a process of rapid solid state alteration of a microcrystalline pseudotachylyte matrix the pseudotachylyte lined fault zones are in close spatial association with numerous amphibole carbonate filled hydrofractures with conspicuous fracture parallel alteration zones defined by hydrous eclogite facies assemblages these eclogite facies hydrofractures testify to the existence of high fluid pressures and to fluid infiltration following brittle failure during high grade metamorphic conditions geothermobarometric estimates ca t 650 700 c p 20 kbar and petrological data imply that hydrofracturing pseudotachylyte crystallization and the subsequent pseudotachylyte alteration process must have occurred during high pressure metamorphism our observations are suggestive of a deep crustal earthquake scenario where a high pressurized fluid phase plays a double role by causing both seismic failure through the embrittlement effect and facilitating eclogitization of the metastable anhydrous gabbro metamorphic reaction along hydrofractures and fault planes led to the development of eclogite facies foliation fabrics and illustrate the rheological change from brittle to plastic behavior associated with the gabbro to eclogite transition the formation of weak deep crustal shear zones following brittle failure represents an arrested initiation of the physical breakup and metamorphic reequilibration of the krakeneset gabbro during its residence deep in the former caledonian collision zone

proterozoic deep marine volcanic eruptive product obsidian glass in chanda limestone penganga group india its implications for basin evolution and metallogenesis abstract subaqueous effusive to low explosive eruptive product composed of blocky and fluidal fragments of glass and sparsely crystalline glass of obsidian lava are present in fine grained volcaniclastic debris flow conglomerate and turbidite occurring in a deep marine sequence of thin and even bedded grey siliceous micritic limestone of the chanda limestone the siliceous limestone conformably encloses stratiform manganese deposit obsidian glass fragments consist of coherent and amorphous glassy groundmass containing patches of devitrified fibrous glass oriented microlites and microphenocrysts of quartz and k feldspars euhedral crystals resorbed rounded and embayed crystals spherical vesicles microlites and flow texture are distinctive volcanic features observed in these fragments chemically similar to obsidian lava the blocky fragments represent the products of hydroclastic fragmentation whereas the fludial one may indicate formation under steam explosion gravitational instability of the resulting slurry of glass fragments and the ambient sea water sediments at the site of eruption causes gravity flows that carries the fragments along with epiclasts away from vent to a distal site for final deposition this study records maiden evidence of volcanism in the chanda limestone of the neoproterozoic penganga group and infers rifting and partial melting of the sialic crust and silicic lava flow during deposition of limestone unusually rich in silica the inferred tectono sedimentary setting facies association and mode of occurrence of the manganese deposit suggest a hydrothermal source for the metal

development of etch pit size distributions on dissolving minerals we have applied the theory of crystal size distributions to the evolution of etch pit size distributions psd s on dissolving minerals population balance equations are derived based on nucleation growth and annihilation of etch pits psd plots of log n number of pits per size per area vs w etch pit width are used to compare etch pit populations we have used psd analysis to study the development of etch pits on calcite dissolved in a rotating disk experiment far from equilibrium 25 c initial ph 8 6 0 7 m kcl a maximum is present on the psd plot in the early stages of dissolution and appears to result from nucleation of pits at all available sites on the initial cleaved surface over a finite time interval as pits grow larger become flat bottomed and annihilate the peak moves to larger sizes and decreases in magnitude to become insignificant after 17 5 hr after 0 5 hr the psd plots become linear at small pit widths marking a population balance as new pits nucleate within flat pits grow larger become flat bottomed and finally annihilate this population of short lived small 100 m etch pits possibly results from nucleation at point defect or impurity clusters a linear psd for long lived larger pits appears at 17 5 hr possibly reflecting etch pits at dislocations the overall psd plot at 17 5 hr is kinked due to these two populations calcite dissolution rates calculated from the coefficients of the psd are within a factor of two of the dissolution rate measured from solution chemistry such estimates will be more accurate if the duration of dissolution does not greatly exceed the mean lifetime of etch pits psd plots for hornblende and microcline grains from a loess soil that has weathered for 12 500 yr show similarities to the psd s from our calcite experiments the hornblende psd shows a peak and the microcline psd is linear or possibly kinked bulk dissolution rates for the grains calculated from the psd s are slower than rates measured for these minerals in the laboratory dissolution rates estimated from the psd coefficients for the hornblende and microcline soil grains are 3 4 and 1 2 orders of magnitude slower respectively than the corresponding laboratory rates

facies architecture emplacement mechanisms and eruption style of the submarine andesite el barronal complex cabo de gata se spain abstract el barronal complex consists of a succession of andesite lavas and andesite volcaniclastic facies interbedded with carbonate and siliciclastic sedimentary rocks carbonate and siliciclastic rocks were deposited in a shallow marine environment during periods of volcanic quiescence lavas consist of an inner coherent core grading outward into hyaloclastite breccia made of dense clasts that in turn grade into hyaloclastite breccia made of vesicular clasts in massive to layered zones volcaniclastic facies contain clasts produced during explosive eruptions and reworked clasts from sources above wave base volcaniclastic facies were deposited from cold granular flows with different grain size populations stratigraphy and facies architecture at el barronal suggest that a succession of several discrete eruptive events occurred with a similar cyclic pattern made of an initial explosive phase followed by effusive emplacement of lavas in turn followed by a period of quiescence of volcanic activity hyaloclastic fragmentation of magma took place in the final stages of lava emplacement allowing only for local disorganization of the jigsaw fit texture

evolution of the south central part of the archean abitibi belt quebec part i stratigraphy and paleogeographic model the region comprises parts of the southern zone of the abitibi greenstone belt and of the bellecombe gneiss belt and the belleterre angliers greenstone belt the stratigraphic sequence of the abitibi belt consists of 1 a lower ultramafic division 2 a middle tholeiitic division and 3 an upper diverse division these divisions lap onto an older volcanic sequence to the north and west in the bellecombe belt sediments overlie the lower ultramafic division these sediments coarsen toward the abitibi belt and toward the stratigraphic top the volcanic rocks of the belleterre angliers belt overlie the sediments of the bellecombe belt five paleogeographic phases have been recognized during phase i a deep marine ultramafic lava plain covered the south part of the abitibi belt and the bellecombe belt faulting took place at the duparquet destor break during phase ii tholeiitic basalts formed a deep marine lava plain in the west of the abitibi belt whereas flows erupted mainly from central volcanic c

etch pit size and shape distribution on orthoclase and pyriboles in a loess catena abstract etch pits on 50 100 m orthoclase and pyribole grains from the e b and c horizons of a catena or drainage sequence of well drained somewhat poorly drained and poorly drained loessal soils were analyzed using sem and digital image analysis the objective was to determine the influence of soil drainage conditions and the associated geochemical environments on dissolution of primary silicates as determined from the distribution of etch pits on the grain surfaces etch pit size frequency distributions indicated that the poorly drained soil contained the smallest and most numerous etch pits for both minerals this indicated low growth rates for etch pits in the poorly drained soil analysis of etch pit shapes indicated that etch pits on pyriboles were more elongated and irregular in the poorly drained soil possibly resulting from a greater growth rate whereas etch pits on orthoclase grains were more elongated and irregular in the well drained soil the larger more elongated and irregularly shaped etch pits on orthoclase from the well drained soil indicative of a greater pit growth rate is consistent with a flushing model of mineral weathering in soils the etch pit size distribution on pyriboles is consistent with the flushing concept but the shape data indicates that other mechanisms of weathering may be involved

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

textural and mineralogical changes associated with the incipient hydrothermal alteration of glassy dacite at the submarine pacmanus hydrothermal system eastern manus basin variably altered dacite from the pacmanus vent field in the eastern manus back arc basin papua new guinea was studied to determine the textural and mineralogical characteristics of hydrothermal alteration taking place in the immediate subsurface of this modern seafloor hydrothermal system detailed textural investigations show that fluid flow through the glassy dacite has been strongly controlled by the primary volcanic textures quench fractures and networks of interconnected perlitic cracks linking vesicles provided pathways for hydrothermal fluids flowing up to the seafloor hydrothermal alteration along these pathways resulted in the formation of pseudoclastic textures textural evidence suggests that alteration of the glassy dacite has not been sustained the samples have been affected by incipient hydrothermal alteration that is typically not preserved in ancient volcanic rock hosted massive sulfide deposits interaction of the glassy dacite with hydrothermal fluids primarily resulted in the conversion of volcanic glass to dioctahedral smectite only minor amounts of trioctahedral smectite were formed destruction of the volcanic glass and the formation of smectite caused pronounced changes in the chemistry of the dacite samples in particular a decrease in the sio2 whole rock content and the na2o k2o ratio the two alkali elements behaved differently during hydrothermal alteration due to preferential incorporation of k into the interlayer position of the newly formed dioctahedral smectite smectite formation occurred under rock dominated conditions although the addition of mg was required to form trioctahedral smectite from the silicic volcanic glass primary plagioclase was resistant to hydrothermal alteration highlighting the fact that the destruction of volcanic glass and feldspar are not necessarily contemporaneous in massive sulfide forming hydrothermal systems incipient alteration of the glassy dacite close to the seafloor occurred at temperatures below 150 c in an environment that allowed the development of steep temperature gradients comparison of the new data to the findings of deep drilling during odp leg 193 suggests that the smectite rich alteration in the immediate subsurface of the pacmanus hydrothermal vent field represents the low temperature equivalent of illite and chlorite rich alteration associations forming in the upflow zones of the hydrothermal fluids in the deeper portion of the volcanic sequence

an upper pliocene coarse pumice breccia generated by a shallow submarine explosive eruption milos greece the filakopi pumice breccia fpb is a very well exposed pliocene volcaniclastic unit on milos greece and has a minimum bulk volume of 1 km3 it consists of three main units a basal lithic breccia 4 8 m mainly composed of angular to subangular andesitic and dacitic clasts up to 2 6 m in diameter b very thickly bedded poorly sorted pumice breccia 16 17 m and c very thick reversely graded grain supported coarse pumice breccia 6 5 20 m at the top the depositional setting is well constrained as shallow marine up to a few hundred metres by overlying fossiliferous and bioturbated mudstone this large volume of fine pumice clasts is interpreted to be the product of an explosive eruption from a submarine vent because 1 pumice clasts are the dominant component 2 the coarse pumice clasts 64 mm have complete quenched margins 3 very large 1 m pumice clasts are common 4 overall the formation shows good hydraulic sorting and 5 a significant volume of ash was deposited together with the coarsest pyroclasts

morphological features of miocene submarine coherent lavas from the green tuff basins examples from basaltic and andesitic rocks from the shimokita peninsula northern japan basaltic and andesitic volcanic rocks of miocene age exposed in the shimokita peninsula northern japan illustrate morphological features of typical submarine coherent lavas of the green tuff basins in japan they are pillow lobes with surface structures such as ropey wrinkles corrugations spreading cracks and tensional cracks and lava lobes composed of a lithic core and glassy border zone or rim with an in situ breccia zone grading outward into surrounding hyaloclastite in addition they include massive lavas with columnar joints and jointed dykes the submarine coherent lavas and dykes are commonly associated with hyaloclastic breccias such as pillow fragment breccia and angular fragment breccia the descriptions of the miocene volcanic rocks in the shimokita peninsula provide good criteria for recognition of submarine coherent lavas of basalt and andesite

age constraints and characteristics of subaqueous volcanic construction the archean hunter mine group abitibi greenstone belt the archean hunter mine group hmg is a 6 7 km thick south facing volcanic sequence at the southern limit of the northern volcanic zone abitibi greenstone belt the hmg is composed of 1 felsic lava flows and endogenous and exogenous domes with autoclastic breccia deposits 2 pyroclastic fire fountain products and high to low concentration volcaniclastic sediment gravity flows of autoclastic and pyroclastic origin interstratified with iron formation 3 mafic pillowed and brecciated flows 4 a prominent felsic dyke swarm and 5 abundant mafic intrusions the stratigraphy is complex as inferred by detailed volcano sedimentary facies mapping a 5 7 km wide n trending felsic dyke swarm coupled with abundant synvolcanic and synsedimentary faults chaotic breccias and mineralization suggest a central volcanic depression which is consistent with a caldera structure voluminous effusive activity and explosive fire fountaining eruptions contributed to caldera development rifting of this arc edifice is supported by the presence of felsic dykes and mafic tholeiitic sills the lithofacies and overlying komatiites flows are consistent with a deep water marine setting u pb age determinations help constrain the hmg caldera formation to 6 million years the sampled quartz feldspar phyric dykes yielded u pb zircon ages of 2728 3 4 3 4 and 2728 9 0 8 ma and together with a previously published dyke age of 2729 6 1 4 ma from the same dyke swarm a case is made that the swarm evolved over 1 million years 2728 6 2729 6 ma a lobate flow and flow breccia revealed similar ages of 2724 6 4 6 1 7 and 2727 6 4 2 2 0 ma whereas the up to 1 km thick gabbro diorite roquemaure sill precedes both felsic dykes and flows as indicated by the 2731 8 2 2 2 0 ma age the 2706 4 5 7 4 3 ma feldspar quartz phyric dykes occurred late in the history of the hmg and were not associated with subaqueous volcanic construction the 207pb 206pb ages of analyzed zircon fractions have an inherited component within error of the oldest crystallization ages obtained for the hmg complex collectively these age determinations and inheritance as well as stratigraphy support a tripartite division into a 2734 2730 ma lower formational stage a 2732 ma middle formational stage and a 2730 2728 ma upper formational stage the study shows that age constraints without detailed facies mapping may lead to erroneous conclusions on volcanic construction and that felsic volcanic dykes and flows may inherit an earlier component of their own volcanic edifice

products of an effusive subglacial rhyolite eruption blahnukur torfajokull iceland we present field observations from blahnukur a small volume 400 m thick lithofacies associations attest to complex patterns of volcano ice interaction erosive channels at the base of the subglacial sequence are filled by both eruption derived material and subglacial till which show evidence for deposition by flowing meltwater this suggests that meltwater was able to drain away from the vent area during the eruption much of the subglacial volcanic deposits consist of conical to irregularly shaped lava lobes typically 5 10 m long set in poorly sorted breccias with an ash grade matrix a gradational lavabreccia contact at the base of lava lobes represents a fossilised fragmentation interface driven by magma water interaction as the lava flowed over poorly consolidated waterlogged debris sets of columnar joints on the upper surfaces of lobes are interpreted as ice contact features the morphology of the lobes suggests that they chilled within conically shaped subglacial cavities 2 5 m high avalanche deposits mantling the flanks of blahnukur appear to have been generated by the collapse of lava lobes and surrounding breccia a variety of deposit characteristics suggests that this occurred both prior to and after quenching of the lava lobes collapse events may have occurred when the supporting ice walls were melted back from around the cooling lava lobes and breccias much larger lava flows were emplaced in the latter stages of the eruption columnar joint patterns suggest that these flowed and chilled within subglacial cavities 20 m high and 100 200 m in length there is little evidence for magma water interaction at lava flow margins which suggests that these larger cavities were drained of meltwater as rhyolite magma rose to the base of the glacier the nature of the subglacial cavity system played an important role in governing the style of eruption and the volcanic facies generated we present evidence that the cavity system evolved during the eruption reflecting variations in both melting rate and edifice growth that are best explained by a fluctuating eruption rate

a halite siderite anhydrite chlorapatite assemblage in nakhla mineralogical evidence for evaporites on mars we report the results of a study of a halite siderite anhydrite chlorapatite assemblage in the shergotty nakhla chassigny snc martian meteorite nakhla these minerals are found associated with each other in interstitial areas with halite often being adjacent to or enclosing siderite we suggest the halite and other minerals are martian in origin because 1 the conditions of fall preclude significant amounts of terrestrial contamination or weathering having taken place 2 textures indicate that the minerals within this assemblage crystallized at the same stage as some silicate and oxide minerals within the nakhla parent and 3 the association with siderite which previous studies have shown has c and o isotopic compositions incompatible with an origin on the earth siderite has the range of compositions caco3 0 1 5 7 mgco3 2 0 40 9 feco3 23 2 87 0 mnco3 1 0 39 9 mol there are two compositional groupings high mnco3 30 mol and low mnco3 high feco3 with a gap identified between the two this may be a miscibility gap or alternatively the two compositional groupings may mark separate generations of carbonate we have not found any textural evidence for the latter explanation but acceptance of the presence of a miscibility gap would require independent work on fe mn carbonates to verify its existence trace element abundances have been determined by ion microprobe analysis on three siderite and one anhydrite grains siderite has light rare earth elements 2 2 7 3 c1 greater than heavy rare earth elements 0 32 0 79 c1 without ce or eu anomalies and the anhydrite has a similar pattern these abundances reflect the source composition rather than partitioning or complexing controls they are not typical of hydrothermal signatures which generally do not have such smooth ree abundance patterns the nature of the mineral assemblage suggests that its source rocks on mars were evaporites these may be common in the craters and flood plains of the martian southern highlands two models are suggested in this paper to explain the incorporation of evaporitic material into the nakhla igneous parent it may have happened as a low temperature process 200 c by crystallisation from an aqueous fluid an origin at low temperature is compatible with the available experimental data on siderite stability alternatively we suggest evaporitic material may have been incorporated into the nakhla parent while melt was still present and crystallized 800 c the latter model can more readily explain the trace element abundances and also the siderite textures that imply intergrowth with residual interstitial melt both high and low temperature models are consistent with the presence of evaporite sediments on mars

hornblende etching and quartz feldspar ratios as weathering and soil development indicators in some michigan soils weathering can be used as a highly effective relative age indicator one such application involves etching of hornblende grains in soils etching increases with time duration and decreases with depth in soils and surficial sediments other variables related to intensity of weathering and soil formation are generally held as constant as possible so as to only minimally influence the time etching relationship our study focuses on one of the variables usually held constant climate by examining hornblende etching and quartz feldspar ratios in soils of similar age but varying degrees of development due to climatic factors we examined the assumption that the degree of etching varies as a function of soil development even in soils of similar age the spodosols we studied form a climate mediated development sequence on a 13 000 yr old outwash plain in michigan their pedogenic development was compared to weathering related data from the same soils in general soils data paralleled weathering data hornblende etching was most pronounced in the a and e horizons and decreased rapidly with depth quartz feldspar ratios showed similar but more variable trends in the two most weakly developed soils the q f ratio was nearly constant with depth implying that this ratio may not be as effective a measure as are etching data for minimally weathered soils our data indicate that hornblende etching should not be used as a stand alone relative age indicator especially in young soils and in contexts where the degree of pedogenic variability on the geomorphic surface is large d 2004 university of washington all rights reserved

extremely fast supercooling of water in the anti jovian hemisphere of europa a speculative model for the fracturing pattern and ascent of brines along cracks the anti jovian hemisphere of europa the hemisphere opposite the one facing jupiter displays a complex array of fractures including roughly concentric and arcuate ones the geometry of this intriguing pattern on the icy surface of europa resembles the textural features displayed by perlitic volcanic glass we argue that fast cooling of a rhyolitic magma may serve as an analogue for fast supercooling of water leading to formation of amorphous ice glassy water we envisage two possible scenarios for ice melting and subsequent extremely fast supercooling 1 massive subglacial volcanic activity or 2 a large impact on the anti jovian hemisphere we suggest that extremely fast formation of amorphous ice on europa would result in a fracture pattern geometrically equivalent to that observed in perlitic glass fast supercooling of water will initially lead to brine retention in the ice phase however after some time these brines would be laterally rejected toward fractures furthermore as solidification progresses downward a shrinking ocean would increase brine concentration these brines would eventually escape to the surface via the fracture network finally high pressure crystallization of salts within fractures may provide an efficient mechanism for differential plate separation horizontal movement and formation of new fractures