detailed tephrochronology and composition of major holocene eruptions from avachinsky kozelsky and koryaksky volcanoes in kamchatka abstract avachinsky kozelsky and koryaksky volcanoes form one of the most volcanically active clusters in the kamchatka volcanic arc and are located in close proximity of the cities of petropavlovsk kamchatsky and elizovo the most populated area in kamchatka in this paper we report a compilation of new and revised previously published data on the eruptive history of these volcanoes during the past 13 5 kyrs we identify 217 explosive eruptions of these volcanoes determine their ages using 207 radiocarbon dates and bayesian statistical modeling and characterize their tephra geochemically using major and trace element compositions of bulk samples 40 samples and volcanic glass 75 samples avachinsky has been the most active during the holocene time and had 150 explosive eruptions koryaksky produced 60 eruptions and kozelsky had only two eruptions in the early holocene our new data confirm the onset of the avachinsky postglacial activity at 11 3 cal ka bp and previously distinguished two major stages of avachinsky holocene eruptive history stage i 8 3 8 cal ka bp and stage ii 3 8 cal ka bp present during stage i eruptions were relatively rare but they included at least six large pumice eruptions with tephra volumes exceeding 0 5 km3 stage i tephras had low k andesitic bulk compositions and low k rhyolitic matrix glasses the andesites likely sampled volatile rich crystal mush from a long lived magma chamber under avachinsky volcano the stage ii started at 3 8 cal ka bp with a powerful eruption and was related to the construction of the young cone inside the avachinsky somma the subsequent late holocene eruptions were frequent but most of them did not exceed the volume of 0 3 km3 the stage ii tephras are mostly cindery basaltic andesites containing well crystallized groundmasses of andesitic composition these tephras originate from smaller perhaps more shallow magmatic reservoirs and their matrix glasses are likely products of in situ crystallization of relatively mafic magmas on their ascent to the surface koryaksky volcano was mostly active in the early holocene when avachinsky was quiet koryaksky tephras had a relatively constant bulk medium k andesitic composition during the holocene thanks to characteristic compositions high frequency and well constrained ages tephras of avachinsky and koryaksky volcanoes can be used for high resolution dating of local sediments some eruptions of avachinsky volcano reached volcanic explosivity index vei 5 and produced widely dispersed tephras these eruptions could have had global environmental effects and their tephras can be used for the correlation of disparate sedimentary archives some avachinsky and koryaksky eruptions were closely spaced in time however their tephras are easily distinguished by respective low k and medium k compositions and by different trace element patterns which imply compositionally different sources in the mantle wedge we interpret these differences to reflect the increasing slab surface temperature and transition of slab component from a relatively low temperature fluid like phase under avachinsky to more high temperature and solute rich supercritical fluid or melt under koryaksky the transition appears to be very sharp in kamchatka causing a large compositional shift in magmas just behind the volcanic front

an experimental study of the kinetics of decompression induced crystallization in silicic melt 1 experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression pinatubo dacite was held at 780 c 220 mpa fo2 nno 2 h2o saturated conditions for an equilibration period decompressed to final pressures pf ranging from 175 to 5 mpa and then held for 0 3 931 hours according to the plagioclase liquidus curve in ph2o t space for the relevant melt composition these decompressions impose effective undercoolings teff of 34 266 c growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 pf 150 mpa teff 34 93 c and equilibrium crystal modes are achieved in 168 hours microlite nucleation is the dominant transformation process for 10 pf 50 mpa teff 125 241 c and chemical equilibrium is not attained by 168 hours under these conditions slow steady decompressions typically produced normally zoned euhedral and planar faceted feldspar crystals although anhedral morphologies were produced at very low pf contrary to expectation slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure although counterintuitive these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth controlled by teff and melt viscosity experienced during each decompression path analysis of liquid to solid transformation kinetics using ttt style diagrams shows that crystallization occurs most rapidly at 100 mpa by a crystal growth mechanism the next most efficient crystallization conditions are at 25 mpa in a crystal nucleation dominated regime

eruptive history of earth s largest quaternary caldera toba indonesia clarified single grain laser fusion sup 40 ar sup 39 ar analyses of individual sanidine phenocrysts from the two youngest toba indonesia tuffs yield mean ages of 73 plus minus 4 and 501 plus minus 5 ka in addition glass shards from toba ash deposited in malaysia were dated at 68 plus minus 7 ka by the isothermal plateau fission track technique these new determinations in conjunction with previous ages for the two oldest tuffs at toba establish the chronology of four eruptive events from the toba caldera complex over the past 1 2 m y ash flow tuffs were erupted from the complex every 0 34 to 0 43 m y culminating with the enormous 2500 3000 km sup 3 youngest toba tuff eruption caldera formation and subsequent resurgence of samosir island timing of this last eruption at toba is coincident with the early wisconsin glacial advance the high precision sup 40 ar sup 39 ar age eruption of such magnitude may provide an important marker horizon useful as a baseline for research and modeling of the worldwide climatic impact of exceptionally large explosive eruptions

variable parent magmas and recharge regimes of the parinacota magma system n chile revealed by fe mg and sr zoning in plagioclase abstract zoning patterns of an content and fe mg and sr concentrations in plagioclase phenocrysts in andesites from parinacota volcano n chile reflect alternating recharge events with two chemically distinct mafic magmas these magmas are characterized by low and high sr contents similar to two recent mafic flank eruptions one end member basaltic andesite shows large sr enrichment and heavy rare earth depletions and thus equilibrated with lower crustal rocks at depth where plagioclase high sr is unstable and garnet high hree y is stable a second end member magma is lower in sr ba contents and has ree patterns typical for parent magmas elsewhere in the central andes the number of recorded recharge events increases after a catastrophic sector collapse and during the subsequent rebuilding of the stratocone variations of an fe and mg contents and morphology of zones suggest also changes in water pressure including decompression under water under saturated and water saturated conditions evidence for decompression is more present in post collapse samples suggesting that the change in the volcano dynamics involves changes in magma chamber location this shows the importance of the cone collapse event in the volcano s magmatic evolution we propose that both end members are only seen at the surface of parinacota volcano because of the particular dynamics of this volcanic system and that similar processes might occur in other volcanic centres of the central volcanic zone

potential atmospheric impact of the toba mega eruption 71 000 years ago an approx 6 year long period of volcanic sulfate recorded in the gisp2 ice core about 71 000 5000 years ago may provide detailed information on the atmospheric and climate impact in the toba mega eruption deposition of these aerosols occur beginning of an approx 1000 year long stadial event but not immediately before the longer glacial period beginning approx 67 500 years ago total stratospheric loading estimates over this approx 6 year period range from 2200 to 4400 mt of h sub 2 so sub 4 aerosols the range in values is given to compensate for uncertainties in aerosol transport magnitude and longevity of the atmospheric loading may have led directly to enhanced cooling during the initial two centuries of this approx 1000 year cooling event 25 refs 2 fig 1 tab

a melt inclusion study of the toba tuffs sumatra indonesia the toba caldera in northern sumatra indonesia is the site of earth s largest quaternary volcanic eruption this eruption dated at 74 ka produced the 2800 km3 youngest toba tuff ytt and ash fall quartz bearing silicic pumices ranging from 68 to 77 wt sio2 indicate that the ytt magma was zoned compositionally and mineralogically prior to the ytt eruption two other silicic quartz bearing tuffs known as the middle toba tuff mtt and the oldest toba tuff ott were erupted from toba at 0 501 ma and 0 840 ma respectively although the volatile contents of the toba magmas are poorly constrained aerosols generated by the ytt eruption are generally thought to have caused a global volcanic winter an evaluation of the pre eruptive dissolved gas contents of the ytt magma is fundamental to understanding the global effects of this eruption we used melt inclusions in quartz crystals to determine the dissolved h2o co2 s cl and f contents of the ytt mtt and ott magmas quartz crystals selected from pumice blocks and welded tuffs that spanned the compositional ranges of the three units were chosen for study major and trace element analyses of melt inclusions were also conducted to characterize melt evolution and provide context to the volatile data melt inclusions from the ytt mtt and ott are rhyolitic in composition 73 77 wt sio2 and have overlapping indistinguishable major element trends inclusions from the large eruptions ytt and ott have similar geochemical relationships where the most evolved melt inclusions occur in the least silicic bulk rock samples and are more evolved than their matrix glasses in contrast the least evolved melt inclusions occur in the most silicic samples and either overlap with or are less evolved than their matrix glasses these patterns can be explained if quartz crystals in the least silicic magmas were inherited from more evolved melt while those in the most silicic melt formed in situ several lines of evidence suggest that crystal settling of early formed quartz crystals in long lived magma bodies was an important process in the ytt and ott magmas trace element data from the ytt melt inclusions have wide ranges rb 250 830 ppm ba 10 485 ppm and mostly support these models however incompatible trace elements are enriched 2 3 over their matrix glass compositions in all samples in the smaller mtt melt inclusions overlapped with the compositional range of the bulk rock samples and are consistent with in situ crystallization ftir analyses indicate that the ytt mtt and ott melts contained about 4 0 5 5 2 0 5 5 and 2 0 5 5 wt h2o respectively co2 contents in all units are generally 100 ppm and cl is 2000 ppm water gradients occur in all 3 tuffs cl gradients are evident only in the ytt and the least silicic samples from all units had no detectable co2 sulfur contents in melt inclusions are low in all samples 32 ppm and overlap with matrix glass s contents applying broad degassing constraints about 1014 g of h2so4 aerosols were likely loaded into the atmosphere during the ytt eruption 2 orders of magnitude less than previous petrologic estimates considerably more cl and f about 1015 g of each may have been part of the ytt aerosol cloud

temperatures of entering magma formation and dimensions of magma chambers of volcanoes a mechanism of formation of magma chambers that feed volcanoes is discussed heat conditions and dimensions of magma chambers which have existed for more than several thousand years may become stable the approximate equations of heat balance of these chambers are derived by calculating the temperaturet1 of the magma entering chambers and the radiia of chambers calculations show that the radius of the shallow peripheral chambers of the avachinsky volcano is less than 3 3 5 km possible maximum radii of peripheral magma chambers were estimated for the kamchatkan volcanoes of medial size the temperature difference in their chambers may reach 100 200 c this method can be applied to the calculations of roots of central type volcanoes

an experimental study of dissolution and precipitation of forsterite in a thermal gradient implications for cellular growth of olivine phenocrysts in basalt and melt inclusion formation the morphology of crystals in magmas strongly depends on the temperature regime of the system in particular the degree of undercooling and the cooling rate to simulate low degrees of undercooling we developed a new experimental setup based on thermal migration in which large cylinders of forsterite single crystals immersed in haplobasaltic melt were subjected to a temperature gradient as forsterite solubility is sensitive to temperature the forsterite on the high temperature side undergoes dissolution and the dissolved components are transported toward the low temperature side where a layer of newly grown forsterite forms up to 340 m thick after 101 h a striking feature is that the precipitation process does not produce a planar front of forsterite advancing at the expense of liquid the growth front shows a fingered outline in planar section with solid lobes separated by glass tubes that are perpendicular to the growth front we ascribe this texture to cellular growth a type of growth that had not been experimentally produced so far in silicate systems we find that the development of cellular growth requires low degrees of undercooling a few c and large crystal liquid interfaces 1 mm across or more and that it occurs at a growth rate of the order of 10 9 m s we found natural occurrences of cellular growth on the rims of olivines from basanites but otherwise cellular textures are poorly documented in natural volcanic rocks melt inclusions were produced in our experiments showing that they can form in olivine at relatively slow rates of growth 10 9 m s or lower

dissolution kinetics of plagioclase in the melt of the system diopside albite anorthite and origin of dusty plagioclase in andesites the textures and kinetics of reaction between plagioclase and melts have been investigated experimentally and origin of dusty plagioclase in andesites has been discussed in the experiments plagioclase of different compositions an96 an61 an54 an23 and an22 surrounded by glasses of six different compositions in the system diopside albite anorthite was heated at temperatures ranging from 1 200 to 1 410 c for 30 min to 88 h textures were closely related to temperature and chemical compositions a crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt glass and remained its original euhedral shape below the liquidus whatever the temperature the crystal melt interface became rough and often more complicated sieve like texture composed of plagioclase melt mixture in the scale of a few m was developed from the surface of the crystal inward formation of mantled plagioclase if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase from these results the following two types of dissolution have been recognized 1 a crystal simply dissolves in the melt which is undersaturated with respect to the phase simple dissolution and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt partial dissolution the amount of a crystal dissolved and reacted increased proportional to the square root of time this suggests that these processes are controlled by diffusion probably in the crystal mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so called resorbed plagioclase in igneous rocks chemical compositions and textures of plagioclase phenocrysts in island arc andesites of magma mixing origin have been examined cores of clear and dusty plagioclase were clacic about an90 and sodic about an50 respectively this result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing

the role of microphysical and chemical processes in prolonging the climate forcing of the toba eruption the mega eruption of toba sumatra occurred around 73 ka ago during the onset of a glaciation of the late quaternary this coincidence combined with the unprecedented amount of sulphur released by this volcano has led to the hypothesis that toba sulphate aerosols caused a transient surface cooling which may have contributed to a shift of the climate system because of the self limiting effect of gravitational sedimentation the climatic impact of extremely large sulphur injections into the stratosphere are thought to be rather limited here we present model calculations combining microphysical and chemical feedbacks which show that the eruption could instead have led to the formation of a long lasting volcanic aerosol layer although the concentrations of radiatively active species such as o3 or so2 could also have been considerably perturbed the resulting forcings should have only slightly moderated the aerosol cooling effect during the first few years following the eruption according to our results extremely high stratospheric sulphur loading could lead to a more prolonged effect on the climate than previously assumed

comment on environmental impact of the 73ka toba super eruption in south asia by m a j williams s h ambrose s van der kaars c ruehlemann u chattopadhyaya j pal and p r chauhan palaeogeography palaeoclimatology palaeoecology 284 2009 295 314 abstract williams et al williams m ambrose s h van der kaars s ruehlemann c chattopadhyaya u pal j n chauhan p 2009 environmental impact of the 73 ka toba super eruption in south asia palaeogeography palaeoclimatology palaeoecology 284 295 314 recently reported late pleistocene palaeoenvironment indicators from terrestrial sites in central india and a marine core from the bay of bengal they correlate isotopic data from soil carbonates and pollen from the marine core using occurrences of the youngest toba tuff ytt a widespread tephra erupted from the sumatran toba caldera 74 000 years ago they conclude that the toba eruption caused both climatic cooling and prolonged deforestation in south asia however consideration of the bases for these claims reveals that the ytt eruption is used as a chronological marker of cooling at the end of dansgaard oeschger interstadial 20 and no evidence is provided that the eruption caused or prolonged this process furthermore while the presented data add to the south asian palaeoenvironmental literature the applicability of the results to questions of impact on human and other populations is overstated

high resolution quantitative imaging of plagioclase composition using accumulated backscattered electron images new constraints on oscillatory zoning oscillatory zoning in plagioclase is investigated at small scale 10 m using accumulated backscattered electron bse images as high resolution imaging method combined with electron microprobe quantitative analysis gray value profiles across these images can be calibrated for an content with a resolution of 0 5 mol an applied to oscillatory zoned crystals this new application of bse imaging allows better characterization of zoning patterns along a profile and quantification of wavelength amplitude and shape of the oscillations we also obtain high resolution information on the morphology of growth zones boundaries this approach allows us to better classify the different types of oscillations and concentric zoning dissolution is more frequent than usually recognized major resorption surfaces crosscut several growth zones irregular 5 10 m saw tooth zones are delimited by faint wavy dissolution surfaces and must be distinguished from small scale oscillations 1 3 m with straight boundaries this suggests at least two mechanisms for the formation of these zoning patterns faint oscillations are probably caused by local kinetic control whereas wavy dissolution surfaces involve magma chamber dynamics

crystalline placer gold from the rio neuquen argentina implications for the gold budget in placer gold formation recent fluvial sediments of the rio neuquen argentina contain variable amounts of placer gold in addition to rounded detrital placer gold the sediments also contain pristine crystalline gold type 1 gold which occurs as overgrowths on detrital placer gold cores type 1 gold is locally rimmed by gold with a wormlike or crystalline texture type 2 gold the chemistry of the primary cores indicates variable intragrain and intergrain compositional heterogeneity this is interpreted to indicate a multisource origin type 1 gold is composed of virtually pure gold 98 at au and type 2 gold is an au hg amalgam 4 9 at hg both varieties are authigenic in origin the formation of the virtually pure type 1 gold is interpreted to be due to the addition of au from an external source rather than by the removal of ag and other metals the auhgag type 2 gold is interpreted to be due to hg contamination from historic mining type 1 gold is interpreted to be analogous to the ag poor rims present on many placer gold grains the evidence presented in this study supports the view that the ag poor rims on placer gold grains are the result of au addition and not ag removal this study also has implications for both quantitative morphological and paragenetic studies of placer gold grains

local and regional scale impacts of the 74 ka toba supervolcanic eruption on hominin populations and habitats in india abstract the toba supervolcanic eruption of 74 000 years ago is argued to have devastated homo sapiens populations causing a human population bottleneck through a combination of rapid global climatic deterioration and ecological disruption following the widespread ash fall this dramatic eruption the largest in at least the last 2 million years is hypothesized to have shaped the genetic structure and diversity of human populations today past assessments of toba s human impacts however have been predominantly theoretical using archaeological and geological evidence from sites in india this study addresses toba s local scale impacts on hominin technology behavior demography and habitats in two valleys in india the jurreru and middle son valleys using paleoclimatological and volcanological data regional scale impacts throughout india are inferred a refugia hypothesis is presented that argues for variation in the scale of toba s impact throughout india driven by regional differences in monsoonal dynamics geography and topography areas of north west india the indo gangetic plain and parts of the deccan to the east of the western ghats are argued to have been the most affected whereas areas of southern and eastern india may have preserved the largest refugia for hominin populations

preliminary evidence for the involvement of budding bacteria in the origin of alaskan placer gold lacelike networks of micrometre size filiform gold associated with alaskan placer gold particles are interpreted as low temperature pseudomorphs of a pedomicrobium like budding bacterium submicron reproductive structures hyphae and other morphological features similar to those of pedomicrobium manganicum occur as detailed three dimensional facsimiles in high purity gold in and on placer gold particles from lillian creek alaska in a scanning electron microscope survey the majority of gold particles at nine alaskan placer deposits appear to include gold that has accumulated chemically at low temperatures in and on the cells of p manganicum similar bacterioform gold from a paleozoic deposit in china and from the precambrian witwatersrand deposit in south africa may indicate that bacterioform gold is widespread

can biological toxicity drive the contrasting behavior of platinum and gold in surface environments abstract in spite of the similar chemical properties of gold au and platinum pt au is highly mobile in surface environments whereas pt appears to be far less so in this study we assess if geomicrobial processes are likely to cause these differences as the mobility of pt and au should differ little based on thermodynamic solubility alone to achieve an accurate comparison it is important that both metals occur in the same environment mineral and groundwater samples were obtained from the fifield pt au field in new south wales australia where pt and au nuggets occur in a series of tertiary eluvial and alluvial paleo placers in particular we studied the m scale dispersion of au and pt within an extraordinary 10 mm sized fragment of ferruginous paleochannel material which contained abundant native au and isoferroplatinum grains gold grains displayed complex secondary morphologies indicative of biogeochemical transformations whereas isoferroplatinum grains appeared smooth and well rounded and showed no signs of supergene transformation gold grains were surrounded by a dusting of highly pure metallic au particles 10 m in diameter whereas no metallic pt particles were detected a search for ionic pt was also unsuccessful a series of biotic and abiotic incubation experiments was conducted to investigate the hypothesis that these differences in mobility are driven by interactions with microbiota biofilms consisting of metallophilic bacteria formed on ultraflat au surfaces and caused significant surface transformations in contrast only subtle changes were observed on pt surfaces incubated under similar conditions minimal inhibitory concentrations for au complexes are more than an order of magnitude lower than those measured for pt complexes in cupriavidus metallidurans cells this higher cell toxicity of mobile au compared to pt complexes can lead to toxic levels of mobile au in the vicinity of au grain surfaces the elevated toxicity drives the formation of au detoxifying biofilms that catalyze the biomineralization of spheroidal nano particulate and bacteriomorphic au in contrast this does not occur on isoferroplatinum grains due to the lower toxicity of pt in conclusion these results are consistent with microbial adaptation to element toxicity driving the cycling of precious metals in surface environments

the application of electron microscopy and microanalysis in conjunction with organic petrology to further the understanding of organic mineral association examples from mount isa and mcarthur basins australia organic petrology supported by electron microscopical and micro analytical techniques was applied to organic matter in proterozoic sediments to better understand hydrothermal processes responsible for ore grade mineralisation it was shown that organic maturation was not only closely linked to the geological history of the sediments but also highlighted heat transfer by convection as differentiated from conduction solely through sediment burial and step wise subsidence water rock ratios effect organic maturation in hydrothermal systems and erratic reflectance profiles are indicators of convective heat transfer identification and characterisation of organic materials in terms of source rock and migrated hydrocarbons was shown to be a powerful tool in reconstructing the thermal history of sediments identifying hydrothermal episodes fluid pathways and heat source in the northern australian proterozoic basins higher reflectance of organic matter towards the central parts of the mount isa basin and some of the most northerly parts point to proximity to higher heat flow at times in contrast to relatively low temperatures prevailing in the western parts of the basin next to the murphy inlier a close correlation shown between peak organic reflectance values and super sequence boundaries farther highlighted the valuable information to be gained from organic petrology by allowing the separation of processes responsible for metal dissolution and transportation from those inducing precipitation c 2001 elsevier science b v all rights reserved

observations of plagioclase zoning using interference imaging abstract interference imaging techniques laser interference microscopy in transmitted light and nomarski differential interference contrast in reflected light have been used to study different types of zoning in magmatic plagioclase crystals samples studied come from a wide range of rock types and geological environments analysis of zoning profiles for individual unit zones shows that a saw tooth pattern in which an initially high an decreases as growth proceeds is the most common type of zoning configuration we recognize 2 major types of zoning type 1 small zones usually less than 10 microns thick with compositional amplitudes up to 10 an and type 2 zones up to50 microns thick with large compositional amplitudes of up to 35 an detailed analysis of zoning patterns shows that oscillatory zoning is not actually oscillatory in the sense of a sine wave it is discontinuous unit zones are characteristically bounded by discontinuities this common pattern is consistent with alternating periods of growth and periods of dissolution reaction and lack of growth or possibly very slow growth our observations firmly establish that dissolution and magmatic reaction involving both growth and dissolution are common during growth of the magmatic plagioclase which we have studied type 1 zonation appears consistent with perturbations of local conditions in a diffusion controlled boundary layer around the growing crystal type 2 zoning is probably due to large scale conditions faceted planar growth surfaces appear to be the rule in magmatic crystallization although in some instances rounded or irregular convolute zonation can occur the repetitive nature of many of the features we have observed is consistent with and suggestive of some form of self organization

timescales of pre eruptive magmatic processes at vulcano aeolian islands italy during the last 1000 years abstract an integrated petrological and geochemical study on plagioclase crystals erupted at vulcano island aeolian islands southern italy over the past 1000 years allowed us to draw a detailed model of the internal structure of the volcanic system in which modes of magma interaction and timescales of storage at crustal depth are shown the integration of compositional textural and temporal record preserved in plagioclase crystals provides evidence for an articulated plumbing system constituted by several reservoirs connected at the crust mantle boundary here a basaltic shoshonitic magma resides and is thought to feed the shallow magma reservoir of both la fossa and vulcanello centers finally triggering eruptions throughout injections from depth textural and micro compositional data on plagioclase crystals suggest the presence of three main magma levels located between ca 17 and 2 km of depth beneath la fossa cone which were intermittently reactivated over the whole period of activity considered plagioclase textures and compositional zoning indicate that the shallow

holocene key marker tephra layers in kamchatka russia detailed tephrochronological studies in kamchatka peninsula russia permitted documentation of 24 holocene key marker tephra layers related to the largest explosive eruptions from 11 volcanic centers each layer was traced for tens to hundreds of kilometers away from the source volcano its stratigraphic position area of dispersal age characteristic features of grain size distribution and chemical and mineral composition confirmed its identification the most important marker tephra horizons covering a large part of the peninsula are from north to south ages given in14c yr b p sh2 1000 yr b p and sh3 1400 yr b p from shiveluch volcano kz 7500 yr b p from kizimen volcano krm 7900 yr b p from karymsky caldera khg 7000 yr b p from khangar volcano av1 3500 yr b p av2 4000 yr b p av4 5500 yr b p and av5 5600 yr b p from avachinsky volcano op 1500 yr b p from the baraniy amfiteatr crater at opala volcano khd 2800 yr b p from the maar at khodutka volcano ks1 1800 yr b p and ks2 6000 yr b p from the ksudach calderas ksht3 a d 1907 from shtyubel cone in ksudach volcanic massif and ko 7700 yr b p from the kuril lake iliinsky caldera tephra layers sh5 2600 yr b p from shiveluch volcano av3 4500 yr b p from avachinsky volcano optr 4600 yr b p from opala volcano ks3 6100 yr b p and ks4 8800 yr b p from ksudach calderas ksht1 1100 yr b p from shtyubel cone and zlt 4600 yr b p from iliinsky volcano cover smaller areas and have local stratigraphic value as do the ash layers from the historically recorded eruptions of shiveluch sh1964 and bezymianny b1956 volcanoes the dated tephra layers provide a record of the most voluminous explosive events in kamchatka during the holocene and form a tephrochronological timescale for dating and correlating various deposits

the in vitro growth of gold grains gold colloids octahedral platelets and foils directly and indirectly formed from the reduction of soluble au i thiosulfate and au iii chloride complexes by iron oxidizing bacteria cyanobacteria and sulfate reducing bacteria were combined in an experimental system this system represented simplified biogeochemical conditions occurring within a fluvial environment in which placer au could occur in this study biofilm formation and physical aggregation i e sedimentation processes were critical for the accumulation of nanometer to micrometer sized au particles into grains 4 5 mm in size characterization of grain surface textures by scanning electron microscopy in association with monitoring soluble au concentrations over time suggested that dissolution and reprecipitation processes were occurring at the au grain fluid interface this laboratory model demonstrates that the biogeochemical cycling of au can contribute to the formation of anomalous enrichments such as placer au deposits

unusual magma storage conditions at mt etna southern italy as evidenced by plagioclase megacryst bearing lavas implications for the plumbing system geometry and summit caldera collapse at mt etna volcano the emission of plagioclase megacryst bearing lavas known locally as cicirara has occurred rarely and generally in association with unusual volcanological phenomena in this work we interpret the magma chamber processes and the structural features of the plumbing system that led to the production of these peculiar volcanic rocks based on a detailed study of plagioclase megacrysts including their oscillatory zoning sieve textures and fluid inclusions patchy zoning suggests limited ascent in the deep levels of the plumbing system based on the plagioclase nucleation threshold and the volatile saturation depth at intermediate water undersaturated levels of the plumbing system ascent is faster as indicated by crystals with coarse sieve textures storage at shallow water saturated levels less than 6 km deep is associated with oscillatory zoning with very small changes in an slightly larger an variations coupled with different wavelengths provide evidence of convection of crystals across distinct zones of the chamber stripes of melt inclusions formed at steps of magma ascent and volatile loss whereas layers of fluid inclusions may be related to episodes of volatile flushing into the magma chamber in contrast strongly sieve textured envelopes with an increase and constant feo may be related to mixing with more volatile rich magmas of similar composition we interpret the repeated occurrence of cicirara lavas as evidence that the shallow portion of the plumbing system underwent a progressive coalescence of a complex network of dykes and sills in response to increasing rates of magma supply from depth major magma withdrawals from this larger reservoir may be linked to episodes of summit instability associated with major caldera collapses

the deposition of elemental gold from gold i thiosulfate complexes mediated by sulfate reducing bacterial conditions the role of sulfate reducing bacteria in the precipitation of elemental gold through a geologic stratum was investigated using column experiments the mixed culture dominated by sulfate reducing bacteria was isolated from the driefontein consolidated gold mine witwatersrand basin republic of south africa bacterially mediated gold precipitation from the gold i thiosulfate complex was more efficient than the corresponding abiotic experiments in the bacterial systems sulfate reducing bacteria i e desulfovibrio sp deposited gold inside the cells as spherical nanoparticles over time these nanoparticles of gold were released from the cells and deposited at cell surfaces and in the bulk solutions ultimately these nanoparticles of gold contributed to the formation of micrometer scale octahedral gold crystals occasional framboidlike structures 1 5 m diam and millimeter scale gold foils typically surrounding the silicate grains spherical gold particles 1 m diam were precipitated in the abiotic experiments and the formation of octahedral gold framboidlike structures and gold foils were not observed in these experiments the reduction and enrichment of elemental gold by sulfate reducing bacteria occurred without the concomitant death of the bacteria suggesting that in natural systems this process could go on over geologic time scales as long as nutrients and gold i thiosulfate were provided to the system

the 74 ka toba super eruption and southern indian hominins archaeology lithic technology and environments at jwalapuram locality 3 hominins living in southern india 74 000 years ago faced a deteriorating environment as the global climate moved from interglacial into full glacial conditions at the same time south asian populations witnessed the widespread deposition of tephra from the sumatran toba super eruption the largest explosive volcanic event of the past two million years here we report new data on the lithic technology and environmental context for a southern indian site with hominin occupation in association with toba tephra deposits jwalapuram locality 3 in the jurreru valley sedimentological and isotopic studies demonstrate that a cooling trend was in effect in this part of southern india prior to the eruption and that thick deposits of ash in the jurreru valley supported grassland communities before more wooded conditions were re established detailed technological analyses of an expanded lithic sample from locality 3 suggest cultural continuity after the eruptive event and comparisons with lithic core technologies elsewhere indicate that homo sapiens cannot be ruled out as the creator of these middle palaeolithic assemblages

dispersion of the volcanic sulfate cloud from a mount pinatubo like eruption 1 we use the geos 5 general circulation model to simulate the transport of the volcanic cloud from an eruption similar to the 1991 eruption of mount pinatubo the simulated aerosol optical thickness and transport of the volcanic cloud are in good agreement with observations of the actual pinatubo eruption from the stratospheric aerosol and gas experiment ii sage ii and the advanced very high resolution radiometer avhrr and with vertical profiles of sulfur dioxide observed by the microwave limb sounder mls we tested the importance of initial conditions corresponding to the specific meteorological situation at the time of the eruption by comparing results when geos 5 is initialized using modern era retrospective analyses for research and applications merra reanalysis fields with results when it is initialized from an existing model run we found no significant difference in the transport of the cloud we show how the inclusion of the interaction between volcanic sulfate aerosol and radiation is essential for a reliable simulation of the transport of the volcanic cloud the absorption of longwave radiation by the volcanic sulfate largely induces the rising of the volcanic cloud up to the middle stratosphere and the divergent motion from the latitude of the eruption to the tropics our simulations indicate that the cloud is transported to the northern hemisphere through a lower stratospheric pathway and to middle and high latitudes of the southern hemisphere through a middle stratospheric pathway centered at about 30 hpa the direction of the middle stratospheric pathway depends on the season of the eruption

diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase residence times of plagioclase crystals in magma reservoirs can be determined by modeling the compositional zoning of trace elements in these crystals we present a formulation to model diffusion of trace elements in plagioclase paying special attention to certain thermodynamic and kinetic aspects in particular we account for the compositional dependence on anorthite content of the chemical potential and diffusion coefficients of trace elements e g mg the choice of suitable boundary conditions and potential effects of diffusion in more than one dimension we show that contrary to intuition diffusive fluxes of trace elements may be coupled to major element concentration gradients and ignoring such coupling can lead to incorrect estimates of timescales we illustrate application of the model using plagioclase crystals from a suite of gabbroic xenoliths from a holocene dacitic lava flow of volcan san pedro chilean andes 36 s the inferred timescale for metasomatism of the xenoliths by evolved liquids is on the order of 100 30 to 148 yr and serves to illustrate how trace element zoning in plagioclase provides a window into timescales of magmatic processes inaccessible by isotopic or other methods

seismicity and structure of the kamchatka subduction zone the configuration of the pacific plate subducted beneath the kamchatka peninsula and the stress distribution in the kamchatka subduction zone ksz were studied using the catalog of the kamchatka regional seismic network focal mechanism solutions estimated from p wave first motions the formal inversion of long period waveforms and centroid moment tensor solutions to the south of 55 n the slab shows an approximately constant dip angle of 55 to the north of 55 n the dip of the slab becomes shallower reaching 35 the maximum depth of seismicity dm varies from 500 km depth near 50 n to 300 km depth at 55 n the volcanic front is almost linear along the main part of the ksz whereas it is sharply shifted landward to the north of 55 n the variation of dm is apparently consistent with the standard empirical relation dm where is the thermal parameter of the subducted slab to the north of 55 n the slab is offset toward the northwest and it is sharply deformed in a narrow contorted zone which is 30 km wide 56 n 161 e to the north of this contortion dm decreases to 100 km the landward shift of the northern part of the slab is reflected by a sharp deviation of the volcanic front to the northwest which follows the 90 160 km isodepth range of the subducted slab the observed value of dm in the northern segment significantly diverges from the global relation dm we interpret this as an effective decrease of the thermal thickness of the subducted lithosphere

gas burst vs gas rich magma recharge a multidisciplinary study to reveal factors controlling triggering of the recent paroxysmal eruptions at mt etna abstract since january 2011 mt etna volcano has been affected by more than forty paroxysmal eruptions at the summit new south east crater nsec on the basis of their very variable duration seven eruptions have been selected among the twenty five of 2011 2012 in order to decipher potential differences in their triggering mechanism paroxysms have been investigated through a multidisciplinary approach that integrates data from volcanic tremor and petrology textures and micro analysis on plagioclase crystals our results lead to the conclusion that close relationships exist between the duration of the eruptions and the temporal evolution of the volcanic tremor amplitude especially during the strombolian phase preceding the paroxysmal activity in this regard we distinguished 1 paroxysms preceded by long lasting initial strombolian phases characterized by low rate of volcanic tremor amplitude increase and 2 eruptions preceded by short initial strombolian phases showing high rate of volcanic tremor amplitude increase based on the pattern of volcanic tremor amplitude increase the former mainly showed a ramp shaped morphology while the latter a bell shaped trend location of the volcanic tremor centroid during the quiescent intervals between the paroxysmal eruptions has highlighted the presence of a magmatic volume at 1 2 km a s l beneath the north east crater nec during the syn eruptive strombolian and lava fountaining phases the centroid of volcanic tremor migrates below the nsec this leads to the consideration that the magma batch residing beneath nec played an important role in the volcanic activity at nsec during the considered period also the textures and compositional zoning anorthite and iron variations in selected plagioclase crystals of the analyzed lavas suggest relations between duration of the paroxysms and dynamics of pre eruptive magmatic processes at depth particularly two mechanisms have been accounted for triggering of eruptions at the nsec on the basis of the concordant or discordant behavior of anorthite and iron in plagioclase coupled with disequilibrium textures at the rim concordant anorthite and iron increases in plagioclase crystals with sieve textured rims indicate recharge by more mafic gas rich magma this textural compositional behavior has been related to long lasting eruptions whose volcanic tremor amplitude evolution produced ramp shaped increase of the volcanic tremor amplitude before the paroxysmal phase on the contrary crystals with sieve textures at the rim characterized by increasing iron at rather constant or decreasing anorthite suggest the prominent role of gas injections into the residing system in this instance the compositional behavior has been linked with short lasting eruptions whose volcanic tremor amplitude evolution led to a sudden increase of the seismic amplitude before the climax of the eruption thus our work put forward the idea that the evolution and duration of the strombolian phase preceding the paroxysmal eruptions of 2011 2012 at mt etna are strongly controlled by the eruption triggering mechanism which can be either gas burst or gas rich magma recharge

calcic cores of plagioclase phenocrysts in andesite from karymsky volcano evidence for rapid introduction by basaltic replenishment calcic cores in plagioclase of karymsky andesite of the 1996 2000 eruptive cycle texturally and compositionally both trace and major elements mimic the plagioclase phenocrysts of basalt erupted 6 km away at the onset of the cycle these observations support the view that simultaneous eruption of andesite and basalt at karymsky in the beginning of the cycle represents an example of replenishment and eruption triggering of an andesitic reservoir homogeneity of andesitic output occurred within two months this suggests to us that blending of injected basalt into reservoir magma was thorough and rapid

magma evolution and open system processes at shiveluch volcano insights from phenocryst zoning phenocryst zoning patterns are used to identify open system magmatic processes in the products of the 2001 eruption of shiveluch volcano kamchatka the lavas and pumices studied are hornblende plagioclase andesites with average pre eruptive temperatures of 840c and fo2 of 1 5 2 1 log units above nickel nickel oxide nno plagioclase zoning includes oscillatory and patchy zonation and sieve textures hornblendes are commonly unzoned but some show simple multiple or patchy zoning apatite microphenocrysts display normal and reverse zoning of sulphur the textural similarity of patchy hornblende and plagioclase together with ba sr systematics in patchy plagioclase indicate that the cores of these crystals were derived from cumulate material plagioclase liquid equilibria suggest that the patchy texture develops by resorption during h2o undersaturated decompression when h2o saturated crystallization recommences at lower pressure reduced ph2o results in lower xan in plagioclase causing more al rich hornblende to crystallize plagioclase cores with diffuse oscillatory zoning and unzoned hornblende crystals probably represent a population of crystals resident in the magma chamber for long periods of time in contrast oscillatory zoning in the rims of plagioclase phenocrysts may reflect eruption dynamics during decompression crystallization increasing fe al in oscillatory zoned rims suggests oxidation as a result of degassing of h2o during decompression a general lack of textural overlap between phenocryst types suggests that different phenocryst populations were spatially or temporally isolated during crystallization we present evidence that the host andesite has mixed with both more felsic and more mafic magmas olivine and orthopyroxene xenocrysts with reaction or overgrowth rims and strong normal zoning indicate mixing with basalt sieve textured plagioclase resulted from mixing of a more felsic magma with the host andesite the mineralogy and mineral compositions of a mafic andesite enclave are identical to those of the host magma which implies efficient thermal quenching and thus small volumes of intruding magma mixing of this magma with the host andesite results in phenocryst zoning because of differences in dissolved volatile contents we suggest that small magma pulses differentiated at depth and ascended intermittently into the growing magma chamber producing incremental variations in whole rock compositions

volcanic winter and accelerated glaciation following the toba super eruption the eruption of toba in sumatra 73 500 years ago was the largest known explosive volcanic event in the late quaternary1 it could have lofted about 1015 g each of fine ash and sulphur gases to heights of 27 37 km creating dense stratospheric dust and aerosol clouds here we present model calculations that investigate the possible climatic effects of the volcanic cloud the increase in atmospheric opacity might have produced a volcanic winter 2 a brief pronounced regional and perhaps hemispheric cooling caused by the volcanic dust followed by a few years with maximum estimated annual hemispheric surface temperature decreases of 3 5 c the eruption occurred during the stage 5a 4 transition of the oxygen isotope record a time of rapid ice growth and falling sea level3 we suggest that the toba eruption may have greatly accelerated the shift to glacial conditions that was already underway by inducing perennial snow cover and increased sea ice extent at sensitive northern latitudes as the onset of climate change may have helped to trigger the eruption itself4 we propose that the toba event may exemplify a more general climate volcano feedback mechanism

variable regimes in magma systems documented in plagioclase zoning patterns el misti stratovolcano and andahua monogenetic cones abstract magmas may experience closed or open system evolution during their ascent and evolution in the upper crust chemically closed system processes can involve heat transfer from underplating mafic magmas or decompression due to magma ascent on the other hand newly injected magmas can mix with the residing magma and hybridize plagioclase zonation patterns and electron microprobe major and trace element analysis constrain these contrasting scenarios for andesites from the 80 km3 el misti stratovolcano 5822 m and a field of relatively small monogenetic andahua scoria cones both in southern peru in particular fe concentration in plagioclase reveals valuable information within a magmatic system to explore the dominance of closed vs open system processes large and correlated anorthite fe contrasts at resorption zones identify compositional mixing related to recharge subhorizontal an fe variations record thermal or water induced effects el misti volcano a large but still relatively youthful stratovolcano is dominated during most of its approx 120 ka history by closed system effects during its evolution recharge and mixing mostly occurred cryptically as it is not recorded in drastic compositional changes in magmas or their phenocrysts a small volume erupted at el misti however is affected by strong open system behaviour as reflected in strongly correlated variations in an and fe contents of plagioclase compositions its crystal population is texturally and chemically very similar to lavas of the monogenetic cinder cones of the andahua volcanic field for the el misti magma reservoir resorption events caused by thermal effects or magma ascent are more frequent in comparison to the actual eruption frequencies many cycles of closed system evolution heat transfer and decompression therefore occurred without resulting in an eruption plagioclase zonations from lavas of the monogenetic andahua field indicate that any mafic batch that intruded the much smaller reservoirs immediately triggered an eruption by contrast a large magma system that feeds el misti volcano can easily accommodate a new batch without causing eruption

aerosol size confines climate response to volcanic super eruptions extremely large volcanic eruptions have been linked to global climate change biotic turnover and for the younger toba tuff ytt eruption 74 000 years ago near extinction of modern humans one of the largest uncertainties of the climate effects involves evolution and growth of aerosol particles a huge atmospheric concentration of sulfate causes higher collision rates larger particle sizes and rapid fall out which in turn greatly affects radiative feedbacks we address this key process by incorporating the effects of aerosol microphysical processes into an earth system model the temperature response is shorter 9 10 years and three times weaker 3 5 k at maximum globally than estimated before although cooling could still have reached 12 k in some midlatitude continental regions after one year the smaller response plus its geographic patchiness suggests that most biota may have escaped threshold extinction pressures from the eruption

the relationship between placer gold particle shape rimming and distance of fluvial transport as exemplified by gold from the klondike district yukon territory canada a semiquantitative study of various physical features of approximately 2 700 gold particles from 21 lode deposits and 36 placer deposits in the klondike district yukon territory canada was undertaken the shape parameters of roundness and flatness were determined using a classification method there appears to be a systematic variation between these parameters and between the parameters and the distance of transport of the particle these relationships were confirmed by quantitative measurements on critical samples rim characteristics were also estimated they show a systematic relationship between one another with the particle shape and less clearly with the distance of transport gold particle shape data shows a smooth well defined relationship to distance of transport from the lode source of the gold both roundness and flatness show a rapid increase within the first 5 km from the lode source and a slower more linear increase beyond this relationship seems to apply worldwide the relationship between the shape of a gold particle and the rim characteristic supports the conclusion that rims are formed by the removal of ag hg and cu and not by the precipitation of au thus rim thickness and the associated high fineness is probably the result of the dynamic process of ag removal in stagnant sediments to create the rim and abrasion in active sediments to remove the rim the composition of the rims represents the stable composition of gold in normal surficial environments together these observations place limits on the occurrence of new gold hammering is the main cause of shape change in fluvially transported gold particles its principal effect is to flatten particles abrasion has its most marked effect on surface texture hammering and abrasion both decrease the mass size and increase the roundness of the gold particles it is therefore possible to make a connection between hydraulic conditions during gold particle transport and its shape change

characteristics and origin of coarse gold in late pleistocene sediments of the cariboo placer mining district british columbia canada the cariboo placer mining district 1000 km2 sited in the interior plateau of central british columbia canada is the premier placer gold mining district of the province gold is recovered from three late pleistocene sedimentary facies postglacial fluvial gravels 25 ka this study reports the morphology size roundness sphericity of 1636 gold grains ranging in size from 0 25 to 17 mm recovered from 19 placer mines older gravels contain the smallest gold grains mean grani size 1 53 mm grains of intermediate size occur in till 2 23 mm and the coarsest gold occurs in postglacial gravels 2 34 mm with a mean of 1 93 mm for the mining district as a whole the most common grain shapes are sub rounded discoidal 14 73 of the grain population sub angular discoidal 10 88 and sub rounded sub discoidal 9 59 the most angular grains occur in postglacial gravels in situ growth of coarse angular grains is indicated by a composite grain structure consisting of aggregates of gold particles welded together by high grade ag 2 filamentous gold in situ coarsening may be reliant on organic complexing agents produced below a dense forest cover an evolutionary sequence of grain form from angular aggregates to rounded pumpkin seed grains is suggested rounded grains commonly show a crystalline structure which may result from the cold hammering of gold during transport fracturing along crystal boundaries is common gold grains may undergo cycles of coarsening rounding diagenesis and breakup in response to repeated recycling through pleistocene sedimentary environments

morphogenesis and composition of native gold in the chelopech volcanic hosted au cu epithermal deposit srednogorie zone bulgaria native gold is an important economic component of the complex ores of the chelopech high sulphidation volcanic hosted epithermal au cu deposit bulgaria the ore consists of pyrite chalcedonic silica chalcopyrite enargite luzonite tennantite bornite sphalerite galena and numerous other sulphide arsenide and telluride minerals gold is paragenetically associated with most of the arsenic bearing and base metal sulphide minerals the chemistry and morphology of gold grains which were separated from disintegrated ores were systematically studied by scanning electron microscopy sem and electron probe microanalysis epma the gold is characterised by high fineness 950 and a mean composition of 94 1 wt au 5 27 wt ag 0 53 wt cu and 0 10 wt fe gold grains show variable morphology subhedral flakes irregular grains euhedral isometric crystals elongated rods wires and fine fibrous crystals 111 twins fine dendrite like formations spongy gold and polycrystalline grains the densest faces 111 are morphologically the most important morphometric measurements show a pronounced flatness of the gold particles the mean value of the corey factor a measure of flatness from 0 to 1 spherical is 0 14 and the mean length to width value is 1 64 it is suggested that crystal growth of gold took place in small voids fine cracks and intergranular space the geometry of which have controlled the highly variable grain shapes no direct correlation between the size and composition of gold grains exists but larger grains tend to be of higher fineness

plagioclase zoning as an indicator of magma processes at bezymianny volcano kamchatka back scattered electron bse derived zoning patterns of plagioclase phenocrysts are used to identify magma processes at bezymianny volcano kamchatka based on the 2000 2007 sequence of eruptive products the erupted magmas are two pyroxene andesites which last equilibrated at 915 c temperature 77 87 mpa pressure and a water content of 1 4 wt textural and compositional zoning of individual plagioclase phenocrysts typically includes a repeated core to rim sequence of oscillatory zoning an50 60 truncated by a dissolution surface followed by an abrupt increase in an content up to an85 which then gradually decreases rimward this zoning pattern is interpreted to be the result of frequent replenishments of the magma chamber which cause both thermal and chemical interaction between resident and recharge magmas the outermost 70 to 150 m wide zoning patterns of plagioclase phenocrysts are composed of dissolution surface with a subsequent increase in an and fe contents zoning patterns of the rims exhibit correlation among plagioclase phenocrysts within one eruption rims are interpreted as a result of crystallization of a batch of magma in the conduit after recharge event

melting kinetics of a plagioclase feldspar partial melting experiments on plagioclase feldspar have been carried out to investigate textures and kinetics of the melting a labradorite single crystal was heated at one atmosphere pressure and temperatures within its melting interval as a function of time so called honeycomb fingerprint or sieve textures were produced except for the runs just below the liquidus the melting was initiated by heterogeneous nucleation of melt at the surface and or interior cracks and possively dislocations of the crystal the pattern of the melt is dendritic with a few m arm spacing after the melt develops throughout the crystal the volumes of melt and residual crystal become larger and smaller respectively without changing the arm spacings the melt is homogeneous and has the approximate temperature dependent liquidus composition irrespective of the time there are compositional gradients in the residual crystal after short periods of melting the an content of the crystals increases with increasing time until it finally reaches equilibrium with the melt after several thousands minutes of heating it is concluded that the enlargement of the melt the main process of the melting is controlled by diffusion in the crystal the fact that partial melts have the composition of the equilibrium liquidus even from the first several minutes strongly suggests that the local equilibrium at the crystal liquid interface is satisfied during the melting some of the honeycomb fingerprint and sieve textures found in xenoliths and phenocrysts of sodic plagioclase in volcanic rocks would be caused by heating events such as magma mixing during which temperatures of magmas were temporarily higher than the solidus of some of the minerals

tomographic imaging of the p wave velocity structure beneath the kamchatka peninsula summary a total of 5270 shallow and intermediate depth earthquakes recorded by the 32 stations of the regional seismic network of the geophysical service of russia are used to assess the p wave velocity structure beneath the kamchatka peninsula in the western pacific the tomographic inversion is carried out in three steps first a 1 d tomographic problem is solved in order to obtain an initial velocity model based on the 1 d velocity model 3 d tomographic inversions with homogeneous and heterogeneous starting models are obtained the conrad 15 km depth and moho 35 km depth discontinuities determined from the 1 d tomographic inversion and the upper boundary of the subducting slab are taken into account in the heterogeneous starting model for the traveltimes and ray path determinations both velocity structure and hypocentral locations are determined simultaneously in the inversion the spacing of the grid nodes is a half degree in the horizontal direction and 20 50 km in the vertical direction a detailed p wave tomographic image is determined down to a depth of 200 km the resulting tomographic image has a prominent low velocity anomaly that shows a maximum decrease in p wave velocity of approximately 6 per cent at 30 km depth beneath a chain of active volcanoes at depth low velocity anomalies are also observed in the mantle wedge extending down to a depth of approximately 150 km these anomalies are apparently associated with the volcanic activity the sedimentary basin of the central kamchatsky graben to the west of the volcanic front and the accretionary prism at the trench correlate with shallow low velocity anomalies highvelocity anomalies observed at a depth of 10 km may be associated with the location of metamorphic basements in the ganalsky valaginskoe uplift and upper crust of shipunsky cape the results also suggest that the subducted pacific plate has p wave velocities approximately 2 7 per cent higher than those of the surrounding mantle and a thickness of approximately 70 km

water solubility and d h fractionation in the system basaltic andesite h2o at 1250 c and between 0 5 and 3 kbars abstract water solubilities in melt of basaltic andesite composition have been measured in the range 500 to 3000 bars 1200 1250 c the saturation curve obey an empirical relationship very close to the square root of pressure law of hamilton et al hamilton d l burnham c w osborn e f 1964 the solubility of water and effects of oxygen fugacity and water content on crystallisation in mafic magmas j petrol 5 21 39 the best fit is obtained by a power law h 2 o wt ap n p in b where a 0 05 0 01 and n 0 63 the departure from n 0 5 is mainly due to the increasing importance of dissolved molecular water at high pressures hydrogen isotopic fractionation between water vapor and total water dissolved in the melt d v m has been measured at 0 5 2 and 3 kb d v m decreases with pressure from 32 to 20 2 this variation is related to the increase in molecular water proportions relative to hydroxyl group when water solubility increases the variation of the d h ratios in water dissolved in such melts thus appears as a possible indirect measurement of the original oh h 2 o ratio of total dissolved water not subject to variation during quenching as the later is this correlation shows that the d v m isotopic fractionation at very high water vapor pressure tend toward a limit of 16 3 for basic magmas

magma storage ascent and recharge history prior to the 1991 eruption at avachinsky volcano kamchatka russia inferences on the plumbing system geometry article i nfo textural and compositional features of plagioclase phenocrysts of the 1991 eruption lavas at avachinsky volcano kamchatka russia were used to investigate the feeding system processes volcanics are porphyritic basaltic andesites and andesites with low k affinity a fractionation modeling for both major and trace elements was per formed to justify the development of these evolved compositions the occurrence of other magma chamber pro cesses was verified through high contrast bse images and core to rim compositional profiles an and feo wt on plagioclase crystals textural types include small and large scale oscillation patterns disequilibrium textures at the crystal core patchy zoning coarse sieve textures dissolved cores disequilibrium textures at the crystal rim sieve textures melt inclusion alignments at the rim disequilibrium textures at the cores may testify episodes of destabilization at various decompression rates under water undersaturated conditions which suggests different pathways of magma ascent at depth at shallower water saturated conditions plagioclase crystallization continues in a system not affected by important chemical physical perturbations oscillatory zoning develops strongly sieve textured rims along with an increase at rather constant feo are evidence of mixing before the 1991 eruption between a residing magma and a hotter and volatile richer one the textural evidence implies that crystals underwent common histories at shallow levels supporting the existence of a large magma reservoir whose top is at 5 5 km of depth distinct textures at the outer rims in a hand size sample are evidence that crys

mineralogical and geochemical evidence for coupled bacterial uranium mineralization and hydrocarbon oxidation in the shashagetai deposit nw china unusual mineral structures have recently been found in a sandstone hosted roll type uranium deposit in the middle jurassic zhiluo formation in the shashagetai deposit the northern ordos basin nw china the structures possess a chemical composition and crystal structure characteristic of mineral coffinite usio4 1 x oh 4x which occurs as nanoparticles with size ranging from 5 to 25 nm these structures are interpreted to be fossilized microorganisms based on mineralogical biosignatures including morphology size occurrence of biogenic coffinite as nano crystals and biological elements such as p the intimate intergrowth of coffinite with secondary pyrite of bacterial origin as defined by low 34 s values and calcite cements with petroleum derived carbon supports its biogenic origin oil inclusions in the host sandstone are characterized by abundant n alkanes slightly increased pr nc17 and ph nc18 ratios significant amounts of demethylated hopanes and tricyclic terpanes and the existence of unresolved complex mixtures these characteristics are interpreted to be a result of mixing of an earlier heavily degraded oil with a later charged fresh oil subsequently the oils were slightly degraded these lines of evidence lead to the proposal that the reduction of sulfate and oxidization of petroleum are likely synchronous with reduction of hexavalent u vi to tetravalent u iv uranium by sulfate reducing bacteria srb the discovery of a natural association of microorganism like structures a uranium mineral and biodegraded petroleum has implications for uranium biomineralization and fossil fuel exploration

sedimentology and gravel bar morphology in an archaean braided river sequence the witpan conglomerate member witwatersrand supergroup in the welkom goldfield south africa

coupled caal nasi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry abstract the rate of caal nasi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400 to 1000 c in calcic plagioclase an80 81 by homogenizing coherent exsolution lamellae the dependence of the average interdiffusion coefficient on temperature is given by the expression d 10 99 cm 2 sec exp 123 4 kcal mol rt t in k this value is for diffusion perpendicular to the 03 1 interface of the lamellae caal nasi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400 to 1200 c and possibly 10 orders of magnitude slower at subsolidus temperatures the large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions e g the skaergaard which retain delicate ca na compositional zoning profiles on the micron scale but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks caal nasi diffusion is slow the closure temperature is high within the solidus liquidus interval and ca na zoning is preserved oxygen diffusion is faster the closure temperature is lower 350 400 c and the feldspars exchange oxygen with the low temperature hydrothermal fluids the complex micron scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase cooling histories typical of large mafic intrusions e g the stillwater are slow begin at high initial temperatures 1200 c and result in homogenization of oscillatory zones on the scale of 10 microns the oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature 1000 c limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion

dissolution origin and syn entrapment compositional change of melt inclusion in plagioclase abstract in the system di ab an h 2 o melt inclusions in plagioclase were reproducibly formed in a series of partial dissolution experiments and the compositional differences from the surrounding matrix melt were evaluated at 200 mpa and 1050 1150 c a seed crystal of an 59 was partially dissolved into the matrix melt which was in equilibrium with more calcic plagioclase than the seed the reaction proceeded inward from the crystal surface by a dissolution and recrystallization process to form a reaction zone composed of melt channels and recrystallized calcic plagioclase the melt channels were choked necked off by the recrystallized plagioclase at the crystal surface to create melt inclusions the experimentally formed inclusions having square and zigzag shapes up to a few tens of micrometers in size are similar in texture to some common types of natural melt inclusion since the partial dissolution of phenocrysts can occur during various magma mixing processes it should be taken into consideration as one of the inclusion formation mechanisms when trying to interpret information from natural melt inclusions especially for those in hydrous arc magmas the chemical composition of the reproduced melt inclusions deviates from the matrix melt basically along the plagioclase liquidus isotherm the molar ratio of ca ca na and the mgo concentration in the synthesized melt inclusion are up to 17 and 61 relative lower than those in the matrix melt respectively this compositional difference is caused by a combination of factors 1 supersaturation with dissolved plagioclase component 2 insufficient diffusive homogenization in the melt channels due to rapid dissolution of plagioclase and 3 the choking off effect on the basis of the experimental results a model for diffusion in a growing melt channel in dissolving plagioclase was constructed which revealed that the concentration of incompatible elements with low diffusivities can be significantly lower in the melt inclusions than that in the matrix melt these results demonstrate the importance of petrography for interpretation of inclusion data and of more experimental studies on melt inclusion origin

laser interference and nomarski interference imaging of zoning profiles in plagioclase phenocrysts from the may 18 1980 eruption of mount st helens washington

recharge in volcanic systems evidence from isotope profiles of phenocrysts strontium isotope ratios measured from core to rim across plagioclase feldspar crystals can be used to monitor changes in the isotope composition of the magma from which they grew in samples from three magma systems from convergent margin volcanoes sudden changes in major element composition petrographic features and strontium isotope composition were found to correspond to discrete magmatic events most likely repeated recharge of more mafic magma with lower ratios of strontium 87 to strontium 86 into a crustally contaminated magma

three years 2011 2013 of eruptive activity at mt etna working modes and timescales of the modern volcano plumbing system from micro analytical studies of crystals abstract tracking magma dynamics at active volcanoes by using the compositional and textural records of minerals is becoming an important aspect of modern volcanology to solve key issues such as temporal relationships between magma recharge and eruption duration of magma storage and final ascent upward to the surface these issues are relevant at mt etna where the style of volcanic activity during the last years showed drastic variations in duration and intensity the paroxysmal activity between 2011 and 2013 at the volcano summit has given the opportunity to investigate the chemical physical processes driving the activity and to fix their spatial temporal relationships into the plumbing system an extensive compositional dataset of plagioclase and olivine crystals from lavas emitted between the 2011 and 2013 at mt etna has been used to constrain modes and timescales of magma storage and transfer to the surface plagioclase crystals display either near equilibrium or disequilibrium textures at the core and rim that indicate complex histories of magma crystallization under variable chemical and physical conditions crystals with different textures have been characterized for major an minor fe and mg and trace elements sr ba textural relationships at the plagioclase cores support the idea that crystals undergo variable decompression rates in the deep parts of the plumbing system fe and mg zoning vs anorthite in correspondence of plagioclase sieve textures at the rim suggest that processes of gas flushing have a dominant role in triggering paroxysmal eruptions determining a sudden intensification of the eruptive activity up to the fountaining phase the sr ba ratio in oscillatory zoned plagioclase revealed the dominant presence into the plumbing system of high sr magma volumes coexisting with low sr magma batches that preserve a geochemical signature similar to that of magmas feeding the pre 1971 activity through sr diffusion modeling in plagioclase the maximum time of magma storage during the considered eruptive period has been evaluated timescales of crystal residence in the plumbing system are short five years to three decades suggesting limited magma storage and faster transfer dynamics to the surface chemical zoning of olivine crystals highlights processes of multi step magma transfer and residence at different levels of the plumbing system the upward migration of magmas occurred primarily through multiple episodes of injection and mixing between five compositionally distinct magmatic environments m i whose p t o 2 characteristics and concentrations in dissolved volatiles were constrained by thermodynamic modeling on the basis of the forsterite contents found at the olivine cores from a deepest reservoir located at depth of 600 mpa the most primitive magma m00 fo 84 moved along dominant pathways intercepting the m0 fo 80 82 at 390 mpa and or m1a fo 78 250 mpa m1b fo 75 140 mpa and finally the shallow m2 fo 70 73 40 mpa storage zones for some eruptive episodes olivine zoning highlights a preferential route of transfer connecting the m00 and m2 storage zones that facilitated the fast migration of primitive magma at shallow depth fe mg diffusion modeling on olivine normal and reverse zoning defines the timescales of magma transfer and storage across these magmatic environments which vary from 1 to 18 months whereas intrusion and mixing by more basic magma into the shallowest reservoir occurred always within 5 months before the eruption relevance of this study mainly relies on the quantification of volcanic processes at depth that may have considerable consequences in development of unusual high energy eruptions at basaltic volcanoes generally acknowledged for their weak to mild explosive activity

mineral textures and zoning as evidence for open system processes investigations of mineral textures and zoning as evidence for open system processes during magmatic evolution have always been a centerpiece of petrological studies and have provided some of the best evidence for magma mixing and crustal contamination for many decades e g milch 1905 kuno 1936 eichelberger 1975 sato 1975 anderson 1976 in fact evidence from mineral studies was instrumental in the acceptance of magma mixing as important petrological process ever since it was initially proposed by bunsen 1851 in recent years mineral studies are invigorated by the development of high precision high resolution analytical instruments and techniques through which textural information and compositional data are combined e g jerram and davidson 2007 as minerals respond texturally and compositionally to changing magmatic environments they preserve in their crystal growth stratigraphy a wealth of information regarding their past history of magmatic processes and compositions cf ginibre et al 2007 on the other hand magmatic liquids melts are snapshots of current magmatic states and provide less direct evidence of the processes responsible for their evolution in addition liquids may crystallize thereby destroying the direct evidence they provide with an appropriate set of observations and measurements we can correlate textures with mineral compositions and thus produce a richer composite picture of magmatic evolution than compositional data of minerals alone this contribution reviews the zoning and textures of minerals frequently encountered in volcanic rocks many of which apply equally to plutonic rocks and the interpretations ascribed to these features in terms of open system magmatic processes first i will address the analytical tools that are used to acquire textural and compositional data followed by a brief review of open system processes and a summary of the relevant textures from that point on i will address some mineral specific aspects followed by a discussion

noble gases k u th and pb in native gold we present determinations of the noble gas and pb isotopic abundances and of k th and u concentrations of native gold our results demonstrate that gold is an excellent carrier for crustal volatiles but direct dating of gold using the u th 4he 40k 40ar and u fission xe methods was not successful for various reasons the main significance of this work is the great sensitivity of gold for trapped gases as well as for gases that were produced in situ which gives the prospects of using gold and its fluid and solid inclusions for the study of paleogas composition numerous nuclear effects characterize the noble gas inventory of placer gold from switzerland and italy vein gold from italy south africa and venezuela and lode gold from south africa the degassing patterns obtained by mass spectrometry show a low temperature release of volatiles around 500 c from fluid inclusions mainly in vein gold and a high temperature release from solid inclusions and the gold itself the low temperature volatiles represent species that were trapped when the gold crystallized we investigated the following trapped species the isotopes of he ne ar kr xe and pb and the abundances of k u th h2o and co2 the crustal gases trapped by gold comprise 3he from 6li n 3h 3he 4he and 40ar from the u th and k decay and xe from 238u fission we observe 4he 40ar 3 9 for the radiogenic trapped gases of tertiary gold and a ratio of 1 4 for archean gold these ratios are consistent with the production ratios from u and k at the respective times and demonstrate that gold can be used as a sampler of ancient atmospheric gases the concentrations of u and th range from a few parts per billion to a few parts per million and those of k and pb range up to some tens of parts per million the antiquity of trapped pb is indicated by the pb pb model age of about 3000 ma for the lead extracted from vein gold and quartz of the lily gold mine south africa gold also contains noble gases produced in situ by u th and k decay we obtained 4he up to the extremely high concentration of 6 8 10 2 cm3 stp g stp atmospheric pressure and 0 c for gold separated from a specimen of the witwatersrand west rand gold fields indicating that this gold has a large u content

the response of ozone and nitrogen dioxide to the eruption of mt pinatubo at southern and northern midlatitudes observations have shown that the mass of nitrogen dioxide decreased at both southern and northern midlatitudes in the year following the eruption of mt pinatubo indicating that the volcanic aerosol had enhanced nitrogen dioxide depletion via heterogeneous chemistry in contrast the observed ozone response showed a northern midlatitude decrease and a small southern midlatitude increase previous simulations that included an enhancement of heterogeneous chemistry by the volcanic aerosol but no other effect of this aerosol produce ozone decreases in both hemispheres contrary to observations the authors simulations show that the heating due to the volcanic aerosol enhanced both the tropical upwelling and southern hemisphere extratropical downwelling this enhanced extratropical downwelling combined with the time of the eruption relative to the phase of the brewer dobson circulation increased southern hemisphere ozone via advection counteracting the ozone depletion due to heterogeneous chemistry on the pinatubo aerosol

nanoforms of gold from abandoned placer deposits of wądroze wielkie lower silesia poland the evidence of authigenic gold mineralization abstract being the most recently exposed gold grains from historical tailings of underground and surface workings offer the opportunity to study supergene transformations occurring over historic periods in this study gold grains were collected from the abandoned historically mined deposits of w droze wielkie at lower silesia all three distinct gold sub types of detrital gold display surface morphotypes and internal textures of au and ag dissolution indicative of supergene gold modification as well as authigenic au nanoparticle formation and aggregation the indicative morphotypes include nano particulate semispherical or bubble like gold and aggregates of bacteriomorphic gold that could be evidence for microbial gold biomineralization sem investigation of the microtopography and internal texture of gold grains revealed indications of post depositional dissolution of au ag alloys and redistribution of these components within the sediment some of seed and bubble like growths are also embedded in clayey kaolinite masses within crevices suggesting that dissolution re precipitation processes occur at the gold grain interface authigenic gold occurs as overgrowths and aggregates of nearly pure gold on a detrital au ag alloy that are readily removed during transport but are replaced by re precipitation occurring over historic rather than geological periods

volcanological evolution of the rivi capo volcanic complex at salina aeolian islands magma storage processes and ascent dynamics lava flows and pyroclastic deposits from strombolian fallout related to the activity of the rivi and capo volcanoes which are representative of early subaerial volcanoes on salina aeolian islands have been investigated through a geological petrological approach our geological field survey shows that rivi and capo volcanoes are part of a single n50 e aligned volcanic complex here named rivi capo volcanic complex rcvc stratigraphically specific rock sampling has allowed reconstruction of the magma feeding processes through time whole rock major element compositions together with core to rim profiles of plagioclase and clinopyroxene crystals show a general evolution toward more basic compositions through the three formations constituting the capo volcano and within the rivi center melts simulations and mass balance modeling suggest that the rcvc rocks are the result of fractional crystallization of plagioclase clinopyroxene and olivine ca 45 of solid removed from a primary magma in addition to fractional crystallization continuous recharge and mixing with more basic magma coming from deeper parts of the magmatic plumbing system contributed to the final volcanic rock compositions our textural and microanalytical data on plagioclase and clinopyroxene crystals allow the definition of a multilevel magmatic storage system with reservoirs at 20 and 3 km below sea level when processes of magma differentiation ascent and storage are considered together with the stratigraphic position of each sample a history of continuous modification of the rcvc plumbing system can be constructed volcanism may have been characterized by fissure type eruptions during the early stages lower capo lower rivi and middle capo formations gradually changing later to central type volcanism upper capo and upper rivi formations

a complex magma mixing origin for rocks erupted in 1915 lassen peak california the eruption of lassen peak in may 1915 produced four volcanic introduction rock types within 3 days and in the following order 1 hybrid magma mixing is an important process among interblack dacite lava containing 2 undercooled andesitic inclusions mediate magmas in volcanic arcs e g turner 3 compositionally banded pumice with dark andesite and light campbell 1986 philpotts 1990 and references therein dacite bands and 4 unbanded light dacite all types represent laboratory studies of synthetic analogs of silicate magma stages of a complex mixing process between basaltic andesite systems and mathematical modeling of mixing viscous and dacite that was interrupted by the eruption they contain liquids have suggested a range of possible mixing mechdisequilibrium phenocryst assemblages characterized by the coanisms e g cashman bergantz 1991 however use existence of magnesian olivine and quartz and by reacted and of these results to interpret magmatic systems is limited unreacted phenocrysts derived from the dacite the petrography and by the applicability of synthetic analogs to magmatic crystal chemistry of the phenocrysts and the variation in rock conditions and by the imprecise knowledge of the effective compositions indicate that basaltic andesite intruded dacite magma viscosity of silicate liquid crystal mixtures at magmatic and partially hybridized with it phenocrysts from the dacite magma temperatures and pressures if magmas mix incompletely were reacted cooling crystallization and vesiculation of the hybrid i e mingle features such as compositional banding or andesite magma converted it to a layer of mafic foam the decreased undercooled inclusions are usually apparent but if they density of the andesite magma destabilized and disrupted the foam mix completely mineralogical disequilibrium may be the blobs of foam rose into and were further cooled by the overlying dacite magma forming the andesitic inclusions disaggregation of only direct evidence for a mixing origin andesitic inclusions in the host dacite produced the black dacite and if the thermal and compositional contrasts between light dacite magmas formation of foam was a dynamic process two magmas are great and the ratio of silicic to mafic removal of foam propagated the foam layer downward into the magma is large there is little interaction between them hybrid andesite magma eventually the thermal and compositional and the mafic magma is undercooled e g bacon 1986 contrasts between the hybrid andesite and black dacite magmas were many undercooled inclusions contain reacted phereduced then they mixed directly forming the dark andesite magma nocrysts inherited from their host silicic magma heiken about 40 50 andesitic inclusions were disaggregated into the eichelberger 1980 and bacon 1986 pointed out host dacite to produce the hybrid black dacite thus disaggregation that undercooled inclusions are typically formed from of inclusions into small fragments and individual crystals can be hybrid magmas in general the formation of undercooled an efficient magma mixing process disaggregation of undercooled inclusions retards further mixing sakuyama 1984 inclusions carrying reacted host magma phenocrysts produces cothompson dungan 1985 sparks marshall 1986 existing reacted and unreacted phenocryst populations koyaguchi blake 1991 however fragmentation and or disaggregation of undercooled inclusions plays an important role in hybridization in some magma systems thompson dungan 1985 coulon et al 1986 clynne christiansen 1987 clynne 1989 linneman myers

ages of calderas large explosive craters and active volcanoes in the kuril kamchatka region russia the ages of most of calderas large explosive craters and active volcanoes in the kuril kamchatka region have been determined by extensive geological geomorphological tephrochronological and isotopic geochronological studies including more than 600 14c dates eight krakatoa type and three hawaiian type calderas and no less than three large explosive craters formed here during the holocene most of the late pleistocene krakatoa type calderas were established around 30 000 40 000 years ago the active volcanoes are geologically very young with maximum ages of about 40 000 50 000 years the overwhelming majority of recently active volcanic cones originated at the very end of the late pleistocene or in the holocene these studies show that all holocene stratovolcanoes in kamchatka were emplaced in the holocene only in the eastern volcanic belt periods of synchronous intensified holocene volcanic activity occurred within the time intervals of 7500 7800 and 1300 1800 14c years bp

modeling postbreakup landscape development and denudational history across the southeast african drakensberg escarpment margin 1 we employ a numerical surface processes model to study the controls on postbreakup landscape development and denudational history of the southeast african margin apatite fission track data presented in the companion paper suggest that the drakensberg escarpment formed by rapid postbreakup river incision seaward of a preexisting drainage divide located close to its present position and subsequently retreated at rates of only 100 m m y 1 numerical modeling results support such a scenario and show that the prebreakup topography of the margin has exerted a fundamental control on subsequent margin evolution the rheology of the lithosphere lithological variations in the eroding upper crust and inland base level falls provided secondary controls a relatively low flexural rigidity of the lithosphere te 10 km is required to explain the observed pattern of denudation as well as the observed geological structure of the southeast african margin lithological variations have contributed to the formation of flat topped ridges buttressing the main escarpment as well as major fluvial knickpoints both these features have previously been interpreted as supporting significant cenozoic uplift of the margin an inland base level fall possibly related to back cutting of the orange river drainage system and occurring 40 50 m y after breakup explains the observed denudation inland of the escarpment as well as the development of inland drainage parallel to the escarpment our model results suggest that in contrast to widely accepted inferences from classical geomorphic studies the southeast african margin has remained tectonically stable since breakup and escarpment retreat has been minimal 25 km

gold nugget growth during tectonically induced sedimentary recycling otago new zealand abstract an alluvial fan 5 km wide and 3 5 km long has been derived over the past 65 000 years from an actively rising schist mountain range dunstan range in central otago new zealand the fan can be locally subdivided into four distinctive consecutively formed gravel sequences the earliest of which has been deformed along the range margin these gravels are pre dated by miocene mature fluvial sediments manuherikia group which have been recycled into the alluvial fan gravels during uplift all these sedimentary sequences contain gold which has been mined sporadically manuherikia gold grains up to 13 wt ag are well rounded and typically 300 m across typical grain size of gold becomes progressively coarser with each gravel sequence in the alluvial fan and nuggets up to 2 cm occur in younger gravel sequences gold grain size increase is due to addition of authigenic gold on to pre existing grains authigenic gold occurs as a cement between earlier detrital grains 1 4 wt ag as amorphous coatings and protrusions 0 1 wt ag and as overgrowths of lattice intergrowths 0 1 wt ag gold nuggets formed in the gravels have overgrown detrital minerals and authigenic clay minerals chemical accretion of nuggets may be still continuing today in parts of the alluvial fan

noise and oscillatory zoning of minerals oscillatory mineral zonation is usually associated with crystal growth in an open system either a hydrothermal system or a melt after a period of magma mixing or degassing such systems may be driven sufficiently far from thermodynamic equilibrium to produce autonomous patterns by geochemical self organization the resulting zonation patterns will be the result of coupling between the generally nonlinear crystal growth dynamics and the boundary conditions imposed by externally controlled fluctuations we examine the effects of noisy boundary conditions on four different crystal growth models these are models for plagioclase growth in magmatic systems l heureux and fowler 1996 for carbonates in sedimentary systems wang and merino 1992 for garnets in hydrothermal systems jamtveit 1991 and for silicate growth from a melt wang and wu 1995 the plagioclase model is sensitive to noise even to low amplitude noise implying that an observed zonation pattern will be significantly affected by processes other than local growth and transport processes for the garnet model fluctuations in the external environment may cause synchronization so that different crystals develop similar zonation patterns even if the formation of zonation patterns is a consequence of nonlinear local dynamics this implies that similarity in intracrystalline zonation does not necessarily imply that the zonation pattern was produced by changes in the external environmental conditions the formation of a zonation pattern may be a consequence of local nonlinearities in the growth process but the pattern details may be strongly affected by subtle changes in the external environment copyright 2000 elsevier science ltd

nanoparticle factories biofilms hold the key to gold dispersion and nugget formation biofilms living on gold au grains play a key role in the biogeochemical cycle of au by promoting the dispersion of au via the formation of au nanoparticles as well as the formation of secondary biomorphic au gold grains from queensland australia are covered by a polymorphic organic inorganic layer that is up to 40 m thick it consists of a bacterial biofilm containing au nanoparticles associated with extracellular polymeric substances as well as bacterioform au focused ion beam fib sectioning through the biofilm revealed that aggregates of nanoparticulate au line open spaces beneath the active biofilm layer these aggregates bacterioform au type 1 resulted from the reprecipitation of dissolved au and their internal growth structures provide direct evidence for coarsening of the au grains at the contact between the polymorphic layer and the primary au bacterioform au type 2 is present it consists of solid rounded forms into which crystal boundaries of underlying primary au extend and is the result of dealloying and ag dissolution from the primary au this study demonstrates that 1 microbially driven dissolution precipitation and aggregation lead to the formation of bacterioform au and contribute to the growth of au grains under supergene conditions and 2 the microbially driven mobilization of coarse au into nanoparticles plays a key role in mediating the mobility of au in surface environments because the release of nanoparticulate au upon biofilm disintegration greatly enhances environmental mobility compared to au complexes only

iridium and other platinum group elements as geochemical markers in sedimentary environments iridium anomalies are commonly used as indicators of extraterrestrial processes which caused global extinctions however platinum group element pge anomalies may result from several different processes both syn and postdepositional acting separately or jointly the sources and processes of enrichment involved are extraterrestrial asteroid comet cosmic dust impact ejecta volcanogenic pge rich condensates precipitation from seawater low sedimentation rate anoxic conditions microbial concentration dissolution reprecipitation exhalative hydrothermal leaching transport and precipitation at a redox boundary geochemical characteristics of pge and accompanying elements in sediments can be used to identify the contributions of some specific sources and processes 1 extraterrestrial chondritic ru ir and os ir ratios low 187os 186os ratio high ir compared to pt pd and au 2 volcanogenic high iridium and se as sb and in contents 3 exhalative hydrothermal high au pd and pt relatively low ir contents 4 precipitation from seawater high pt and low pd contents also high cd and ag contents under anoxic conditions and 5 reduction from intermediate and low temperature solutions high pge and cu or mo u ni contents

weathering profile of the cerro de maimon vms deposit dominican republic textures mineralogy gossan evolution and mobility of gold and silver abstract cerro de maimon in the central dominican republic is currently the only vms deposit under production in the caribbean region it is hosted in the maimon formation of early cretaceous age which is part of the oldest and chemically most primitive island arc in the caribbean from bottom to top this deposit can be divided into i a primary sulfide zone ii a supergene enrichment zone and iii an oxidized zone this study reports new data on the textural and mineralogical characteristics of the oxidized zone gossan leached capping zone with emphasis in the au ag bearing phases the mineral paragenesis of the oxidized zone is essentially composed of goethite hematite quartz and barite botryoidal cellular and brecciated textures can be distinguished botryoidal and brecciated textures dominate in the upper parts of the oxidized zone whereas cellular textures are more common in the intermediate and lower parts however the weathering profile is very heterogeneous the leached capping profile shows evidence of both transported and indigenous gossans gold in the oxide paragenesis is extremely pure 99 au suggesting that chemical refining took place silver occurs mainly as iodargyrite agi and minor agbr agcl in botryoidal aggregates particles of au ag alloy in the primary mineralization exposed to a weathering environment can be leached and transported by various agents chemical and biochemical that may exist simultaneously in the presence of halides gold and silver can be leached and transported in a wide range of ph eh conditions especially if iodine is present silver is leached more rapidly and over a broader range of ph eh conditions and is preferentially transported as iodine complexes than other halides the presence of iodargyrite in the cerro de maimon gossan fully developed under tropical conditions suggests that this mineral cannot be considered an indicator of extremely arid environments as typically claimed by many authors iodargyrite occurrence would rather point to the presence of saline halide rich groundwater in oxide deposits

environmental impact of the 73 ka toba super eruption in south asia abstract the cooling effects of historic volcanic eruptions on world climate are well known but the impacts of even bigger prehistoric eruptions are still shrouded in mystery the eruption of toba volcano in northern sumatra some 73 000 years ago was the largest explosive eruption of the past two million years with a volcanic explosivity index of magnitude 8 but its impact on climate has been controversial in order to resolve this issue we have analysed pollen from a marine core in the bay of bengal with stratified toba ash and the carbon isotopic composition of soil carbonates directly above and below the ash in three sites on a 400 km transect across central india pollen evidence shows that the eruption was followed by initial cooling and prolonged desiccation reflected in a decline in tree cover in india and the adjacent region carbon isotopes show that c 3 forest was replaced by wooded to open c 4 grassland in central india our results demonstrate that the toba eruption caused climatic cooling and prolonged deforestation in south asia and challenge claims of minimal impact on tropical ecosystems and human populations

modeling of gas driven magmatic overturn tracking of phenocryst dispersal and gathering during magma mixing 1 we present a combined multiphase numerical and crystal tracking approach that provides a framework to investigate the transport and zoning of crystals associated with a gas driven mixing event mixing in compositionally intermediate to silicic magmatic systems is often initiated by gas exsolution in the recharging magma causing a density inversion and subsequent overturn the overturn is simulated for a range of bubble volume fractions bubbles and therefore indirectly for a range of reynolds numbers re all simulations show chaotic flow dynamics with fast overturn timescales of minutes to hours the large scale mixing is inefficient during a single overturn resulting in a continuously stratified system with respect to bubble volume fraction the crystal tracking algorithm provides us with information on the small scales i e 10 cm on this length scale we observe gathering of different crystals during the overturn that typically ranges on the order of tens of meters thus a complex crystal population may arise within a single overturn this gathering and dispersal of crystals is strongest and most uniform for high re for low re crystal populations are characterized by less gathering of crystals that originated from distal portions of the magma body during the overturn the crystals pass through environments of changing chemical potential we apply the damkohler number da which compares the crystallization or dissolution to the advection timescale results show an asymmetry between crystallization and dissolution crystallization times are too slow during gas driven overturn to record transient changes in chemical potential crystals most likely only record their initial as well as their final chemical environment in contrast dissolution and advection rates are of similar order suggesting potential dissolution during the overturn on the basis of the results for gas driven overturn we expect that slower physical mixing processes may be continuously recorded in the zonation pattern of the crystal phases as long as the changes in chemical potential produce crystal growth

an analytical and experimental study of zoning in plagioclase a detailed electron microprobe study has been conducted on natural and experimentally grown zoned plagioclase feldspars discontinuous sector and oscillatory chemical zoning are observed superimposed on continuous normal or reverse zoning trends the relative accuracy of 3 element na ca k microprobe traverses was found statistically to be 2 mole percent comparison of microprobe data on natural zoned plagioclase with zoned plagioclase grown in controlled experiments has shown that it may be possible to distinguish zonal development resulting from physio chemical changes to the bulk magma from zoning related to local kinetic control on the growth of individual crystals

limited global change due to the largest known quaternary eruption toba 74 kyr bp abstract the 74 kyr bp super eruption of toba volcano in sumatra is the largest known quaternary eruption on the basis of preserved deposits the eruption magnitude has been estimated at 7 10 15 kg 2800 km 3 of dense magma the largest sulphate anomaly in the greenland ice sheet project 2 core has been identified as fallout from toba s stratospheric aerosol veil correlation of the sulphate and oxygen isotope stratigraphy of the ice core suggests that the toba eruption might have played a role in triggering a millennium of cool climate prior to dansgaard oeschger event 19 although a comparable stadial preceded event 20 a possible 6 yr duration volcanic winter immediately following the eruption has also been proposed as the cause of a putative bottleneck in human population supporting in a general way the garden of eden model for the origin of modern humans however along with counter arguments regarding the timing of any demographic crash there remain major gaps in our understanding of the 74 kyr bp toba eruption that hinder attempts to model its global atmospheric and climatic and hence human consequences the tephra record reveals basic aspects of the eruption style but calculations of the duration and hence intensity and plume height of the event are poorly constrained furthermore estimates of the sulphur yield of the erupting magma central to predictions of its atmospheric and climatic impacts vary by two orders of magnitude 3 5 330 10 10 kg previous estimates of globally averaged surface cooling of 3 5 c after the eruption are probably too high a figure closer to 1 c appears more realistic the volcanological uncertainties need to be appreciated before accepting arguments for catastrophic consequences of the toba super eruption

sector collapses and large landslides on late pleistocene holocene volcanoes in kamchatka russia on kamchatka detailed geologic and geomorphologic mapping of young volcanic terrains and observations on historical eruptions reveal that landslides of various scales from small 0 001 km3 to catastrophic up to 20 30 km3 are widespread moreover these processes are among the most effective and most rapid geomorphic agents of 30 recently active kamchatka volcanoes at least 18 have experienced sector collapses some of them repetitively the largest sector collapses identified so far on kamchatka volcanoes with volumes of 20 30 km3 of resulting debris avalanche deposits occurred at shiveluch and avachinsky volcanoes in the late pleistocene during the last 10 000 yr the most voluminous sector collapses have occurred on extinct kamen 4 6 km3 and active kambalny 5 10 km3 volcanoes the largest number of repetitive debris avalanches 10 during just the holocene has occurred at shiveluch volcano landslides from the volcanoes cut by ring faults of the large collapse calderas were ubiquitous large failures have happened on both mafic and silicic volcanoes mostly related to volcanic activity orientation of collapse craters is controlled by local tectonic stress fields rather than regional fault systems specific features of some debris avalanche deposits are toreva blocks huge almost intact fragments of volcanic edifices involved in the failure some have been erroneously mapped as individual volcanoes one of the largest toreva blocks is mt monastyr a 2 km3 piece of avachinsky somma involved in a major sector collapse 30 40 ka bp long term forecast of sector collapses on kliuchevskoi koriaksky young cone of avachinsky and some other volcanoes highlights the importance of closer studies of their structure and stability

isolating the roles of different forcing agents in global stratospheric temperature changes using model integrations with incrementally added single forcings satellite instruments show a cooling of global stratospheric temperatures over the whole data record 1979 2014 this cooling is not linear and includes two descending steps in the early 1980s and mid 1990s the 1979 1995 period is characterized by increasing concentrations of ozone depleting substances odss and by the two major volcanic eruptions of el chichon 1982 and mount pinatubo 1991 the 1995 present period is characterized by decreasing ods concentrations and by the absence of major volcanic eruptions greenhouse gas ghg concentrations increase over the whole time period in order to isolate the roles of different forcing agents in the global stratospheric temperature changes we performed a set of simulations using the nasa goddard earth observing system chemistry climate model with prescribed sea surface temperatures we find that in our model simulations the cooling of the stratosphere from 1979 to present is mostly driven by changes in ghg concentrations in the middle and upper stratosphere and by ghg and ods changes in the lower stratosphere while the cooling trend caused by increasing ghgs is roughly constant over the satellite era changing ods concentrations cause a significant stratospheric cooling only up to the mid 1990s when they start to decrease because of the implementation of the montreal protocol sporadic volcanic events and the solar cycle have a distinct signature in the time series of stratospheric temperature anomalies but do not play a statistically significant role in the long term trends from 1979 to 2014 several factors combine to produce the step like behavior in the stratospheric temperatures in the lower stratosphere the flattening starting in the mid 1990s is due to the decrease in ozone depleting substances mount pinatubo and the solar cycle cause the abrupt steps through the aerosol associated warming and the volcanically induced ozone depletion in the middle and upper stratosphere changes in solar irradiance are largely responsible for the step like behavior of global temperature anomalies together with volcanically induced ozone depletion and water vapor increases in the post pinatubo years

spatial and temporal patterns in arctic river ice breakup revealed by automated ice detection from modis imagery abstract the annual spring breakup of river ice has important consequences for northern ecosystems and significant economic implications for arctic industry and transportation river ice breakup research is restricted by the sparse distribution of hydrological stations in the arctic where limited available data suggests a trend towards earlier ice breakup the specific climatic mechanisms driving this trend however are complex and can vary both regionally and within river systems consequently understanding the response of river ice processes to a warming arctic requires simultaneous examination of spatial and temporal patterns in breakup timing in this paper we describe an automated algorithm for river ice breakup detection using modis satellite imagery that enables identification of spatial and temporal breakup patterns at large scales we examine breakup timing on the mackenzie lena ob and yenisey rivers for the period 2000 2014 by dividing the rivers into 10 km segments and classifying each river pixel in each segment as snow ice mixed ice water or open water based on modis reflectance we determine breakup dates with a mean uncertainty of 1 3 days all statistically significant temporal trends are negative indicating an overall shift towards earlier breakup considerable variability in the statistical significance and magnitude of trends along each river suggests that different climatic and physiographic drivers are impacting spatial patterns in breakup trends detected on the lower mackenzie corroborate recent studies indicating weakening ice resistance and earlier breakup timing near the mackenzie delta in siberia the increased magnitude of trends upstream and strong correlation between breakup initiation and whole river breakup patterns suggest that earlier onset of upstream discharge may play the dominant role in determining breakup timing exploratory analysis demonstrates that modis imagery may also be used to differentiate thermal and mechanical breakup events

ascent driven crystallisation of dacite magmas at mount st helens 1980 1986 we introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system qz ab or h2o in order to relate variations in volcanic glass chemistry to changing pressure p and temperature t conditions in the sub volcanic magma system by this means an important distinction can be made between ascent driven and cooling driven crystallisation under water saturated or undersaturated conditions in samples containing feldspar and a silica phase quartz or tridymite quantitative p t estimates of the conditions of last equilibrium between crystals and melt can be made formation of highly silicic melts i e 77 wt sio2 is a simple consequence of the contraction of the silica phase volume with decreasing pressure such that high silica glasses can only form by crystallisation at low pressure resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation groundmass and inclusion glasses in dacites from the 1980 1986 eruption of mount st helens volcano wa span a wide range in sio2 68 80 wt anhydrous the compositions of the least evolved sio2 poor inclusions in amphibole phenocrysts record entrapment of silicic liquids with 5 4 wt water corresponding to a water saturation pressure of 200 mpa at 900 c the compositions of more evolved higher sio2 plagioclase hosted inclusions and groundmass glasses are consistent with extensive ascent driven fractional crystallisation of plagioclase oxide and orthopyroxene phenocrysts and microlites to low pressures during this polybaric crystallisation plagioclase phenocrysts trapped melts with a wide range of dissolved water contents 3 5 5 7 wt magmas erupted during the plinian phase of the 18 may 1980 eruption were derived from a large reservoir at depths of 6 km subsequent magmas ascended to varying depths within the sub volcanic system prior to extraction from glass chemistry and groundmass texture two arrest levels have been identified at depths of 0 5 1 and 2 4 km a single dome sample from february 1983 contains groundmass plagioclase tridymite and quartz testifying to temperatures of at least 885 c at 11 mpa these shallow storage conditions are comparable to those in the cryptodome formed during spring 1980 the corresponding thermal gradient 0 2 c mpa 1 is consistent with near adiabatic magma ascent from 8 km we argue that the crystallisation history of mount st helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation this challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980 1986 eruption because the crystallisation process is both polybaric and fractional it cannot be simulated directly using isobaric equilibrium crystallisation experiments however calculation of the phase proportions in water saturated 910 15 c experiments by rutherford et al 1985 over the pressure range 220 125 mpa reproduces the crystallisation sequence and phenocryst modes of mount st helens dacites from 18 may 1980 by allowing for the effects of fractional versus equilibrium crystallisation entrained residual source material and small temperature differences between nature and experiment phase compositions can also be matched to the natural samples we conclude that decompression of water saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres

crystal residence times from trace element zoning in plagioclase reveal changes in magma transfer dynamics at mt etna during the last 400 years abstract trace element zoning in plagioclase of selected alkaline lavas from the historic 1607 1892 ad and recent 1983 2013 ad activity of mt etna volcano has been used to explore the possible role that volcano tectonics exert on magma transfer dynamics the observed textural characteristics of crystals include near equilibrium textures i e oscillatory zoning and textures with variable extent of disequilibrium patchy zoning coarse sieve textures and dissolved cores historic crystals exhibit lower k concentrations at lower anorthite contents a feature in agreement with the general more potassic character of the recent lavas if compared to the historic products historic plagioclases have statistically higher ba and lower sr concentrations than the recent crystals which result in different sr ba ratios for the two suites of plagioclase variations in the anorthite content along core to rim profiles obtained on crystals with different types of textures for both the historic and recent eruptive periods were evaluated particularly versus sr ba at comparable average an contents crystals characterized by oscillatory zoning which are representative of near equilibrium crystallization from the magma display distinct sr ba ratios we suggest that these features are primarily related to recharge of a new geochemically distinct magma into the storage and transport system of the volcano in addition to distinct trace element and textural characteristics of plagioclase sr diffusion modeling for plagioclase suggests that residence times are generally shorter for crystals found in recently erupted lavas 25 77 years average 43 years compared to those of the historic products 43 163 years average 99 years shorter residences times correlate with gradual increases in eruption volume and eruption frequency rates through time we attribute these features to an increasing influence since the 17th century of extensional tectonic structures within the upper 10 km of the etnean crust which have promoted shorter residence times and higher eruption frequency

minor and trace element zoning in plagioclase implications for magma chamber processes at parinacota volcano northern chile textural and compositional zoning in plagioclase phenocrysts in a sample from parinacota volcano chile was investigated using backscattered electron images and electron microprobe analysis of major and trace elements large 2 mm oscillatory zoned crystals type i with resorption surfaces of moderate an discontinuities 10 an and decreasing trace element contents sr mg ti towards the rim reflect melt differentiation and turbulent convection in the main magma body early recharge with a low sr mafic magma is seen in the core small scale sr variations in the core indicate limited diffusion and thus residence and differentiation times of the magma shorter than a few thousand years smaller crystals type ii with low trace element an ratio reflect the influence of an h2o rich melt probably from a differentiated boundary layer closed system in situ crystallisation mafic magma recharge and the role of a water rich differentiated boundary layer can be distinguished from the an trace element relationships crystals apparently move relatively freely between different parts and regimes in the magma chamber evidence for convective crystal dispersion high sr type ii crystals indicate an earlier input of sr rich mafic magma recharge of two distinct mafic magma types is thus identified high sr and low sr which must have been present at increasing recharge rates with time in the plumbing system throughout the volcano s history

textural constraints on the kinetics of crystallization of igneous rocks natural systems provide time controlled experiments on scales unreachable in the laboratory and with complexities unapproachable by computer experiments since temperature undercooling cannot be directly scaled with time in situ crystallization experiments provide information unavailable by other means the kinetics of crystallization in simple silicate systems have been studied extensively for glass ceramic systems e g uhlmann 1982 james 1982 but predictive models for nucleation behavior in particular remain inadequate part of the problem in creating models of nucleation lies in the difficulty of experimentally isolating the nucleation process crystal growth measurements are more accessible experimentally and for this reason crystal growth is better understood e g dowty 1980 kirkpatrick 1981 james 1982 baronnet 1984 however predictive models of crystal growth e g lasaga 1982 require accurate models for the temperature and compositional dependence of both the thermodynamic and kinetic driving forces for crystallization information that is currently limited the cse parameterization of the position of ymax as a function of t tl and t suggested by dearnley 1983 provides a potentially useful model that requires further testing similarly application of the adiabatic nucleation model of meyer 1986 to simple systems suggests that the position of tmax relative to tl has an approximately constant value of 0 55 0 60 exceptions are low dsf minerals such as albite and quartz additionally while experimental crystallization rate data is a necessary link between processes in natural melts and numerical models experimental growth rate data is confined to simple systems and conditions of moderately large undercooling while growth rate estimates in complex natural systems suggest that under most conditions undercoolings remain very small quantitative textural studies of dikes suggest a pronounced dependence of growth rate on total crystallization time e g ikeda 1977 which is in turn related to effective undercooling a function of both total dike width and distance from the dike margin by analogy minimum crystallization rates may be estimated from programmed cooling experiments if a temperature interval of crystallization is assumed growth rates determined in this way show the same relationship between growth rate and total crystallization time seen in the dike studies reflecting a systematic relationship between growth rate and cooling rate effective undercooling for plagioclase in basaltic systems while the data are not available for other mineral systems there is no reason to believe that similar patterns would not exist for other silicate minerals i e the similarity in dendritic olivine growth rates estimated by fowler et al 1989 using fractal analysis to the growth rates of plagioclase under conditions of rapid cooling estimated from programmed cooling rate experiments analysis of measured crystal size frequencies provides a means of extracting effective bulk kinetic information from geologic systems that can help to constrain the dynamic feedback between physical models of magmatic systems and rates of chemical change

interaction of arc magmas with subvolcanic hydrothermal systems insights from compositions and metasomatic textures of olivine crystals in fresh basalts of daisen and mengameyama western honshu japan