upper mantle electrical resistivity structure beneath the central mariana subduction system this paper reports on a magnetotelluric mt survey across the central mariana subduction system providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back arc spreading ridge after calculation of mt response functions and their correction for topographic distortion two dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab the resultant isotropic electrical resistivity structure contains several key features there is an uppermost resistive layer with a thickness of up to 150 km beneath the pacific ocean basin 80 100 km beneath the mariana trough and 60 km beneath the parece vela basin along with a conductive mantle beneath the resistive layer a resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge there is no evidence for a conductive feature beneath the back arc spreading center sensitivity tests were applied to these features through inversion of synthetic data the uppermost resistive layer is the cool dry residual from the plate accretion process its thickness beneath the pacific ocean basin is controlled mainly by temperature whereas the roughly constant thickness beneath the mariana trough and beneath the parece vela basin regardless of seafloor age is controlled by composition the conductive mantle beneath the uppermost resistive layer requires hydration of olivine and or melting of the mantle the resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three dimensional and of limited size in contrast the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab the resistive region beneath the back arc spreading center can be explained by dry mantle with typical temperatures suggesting that any melt present is either poorly connected or distributed discontinuously along the strike of the ridge evidence for electrical anisotropy in the central mariana upper mantle is weak

petrological modifications in granitic rocks from the siljan impact structure evidence from cathodoluminescence abstract cathodoluminescence cl and microprobe analyses were performed on quartz plagioclase and k feldspar from granitic rocks in the siljan impact area central sweden in the upper 2200 m and in the vicinity of the gravberg 1 well secondary reddish luminescing quartz and brownish luminescing albite ab 98 and k feldspar or 95 are the dominant minerals primary magmatic quartz plagioclases oligoclase andesine and k feldspar show blue yellowish and bluish luminescence colours respectively the reddish luminescence colour of quartz is interpreted to have been induced by the shock heating episode of the impact and subsequent elevated temperature in the granites the impact shock pressure effects as shown by shatter cones and planar features were retained within granite dominated by blue luminescing quartz most alteration of the feldspar took place during the cooling episode that followed the impact heating post impact fracturing facilitated the infiltration of meteoric water which has further enhanced alteration of feldspars

constraints on the evolution of crustal flow beneath northern tibet crustal flow is an important tectonic process active in continent continent collisions and which may be significant in the development of convergent plate boundaries in this study the results from multidimensional electrical conductivity modeling have been combined with laboratory studies of the rheology of partially molten rocks to characterize the rheological behavior of the middle to lower crust of both the songpan ganzi and kunlun terranes in the northern tibetan plateau two different methods are adopted to develop constraints on melt fraction temperature and crustal flow velocity in the study area the estimates of these parameters are then used to evaluate whether crustal flow can occur on the northern margin of the tibetan plateau in the songpan ganzi crust all conditions are satisfied for topography driven channel flow to be dominant with partial melt not being required for flow at temperature above 1000 c further north the kunlun fault defines the southern boundary of a transition zone between the tibetan plateau and the qaidam basin constrained by the estimated melt fractions it is shown that channel injection across the fault requires temperatures close to 900 c the composition of igneous rocks found at the surface confirm those conditions are met for the southern kunlun ranges to the north the qaidam basin is characterized by colder crust that may reflect an earlier stage in the channel injection process in the study area at least 10 of the eastward directed tibetan crustal flow could be deflected northward across the kunlun fault and injected into the transition zone defining the northern margin of the tibetan plateau

outburst floods from glacial lake missoula abstract the pleistocene outburst floods from glacial lake missoula known as the spokane floods released as much as 2184 km3 of water and produced the greatest known floods of the geologic past a computer simulation model for these floods that is based on physical equations governing the enlargement by water flow of the tunnel penetrating the ice dam is described the predicted maximum flood discharge lies in the range 2 74 106 13 7 106 m3 sec 1 lending independent glaciological support to paleohydrologic estimates of maximum discharge

magmatic processes revealed by textural and compositional trends in merapi dome lavas syn eruptive degassing of volcanoes may lead to syn eruptive crystallization of groundmass phases we have investigated this process using textural and compositional analysis of dome material from merapi volcano central java indonesia samples included dome lavas from the 1986 88 1992 93 1994 and 1995 effusive periods as well as pyroclastic material deposited by the november 1994 dome collapse with total crystallinities commonly in excess of 70 phenocrysts 1 microlites the liquids present in merapi andesites are highly evolved rhyolitic at the time of eruption feldspar microlites in dome rocks consist of plagioclase cores ab63an29or8 surrounded by alkali feldspar rims ab53an5or42 compositional pairs which are not in equilibrium a change in the phase relations of the ternary feldspar system caused by degassing best explains the observed transition in feldspar composition a small proportion of highly vesicular airfall tephra grains from the 1994 collapse have less evolved glass compositions than typical dome material and contain rimless plagioclase microlites suggesting that the 1994 collapse event incorporated less degassed partially liquid magma in addition to fully solidified dome rock as decompression drives volatile exsolution rates of degassing and resultant microlite crystallization may be governed by magma ascent rate microlite crystallinity is nearly identical among the 1995 dome samples an indication that similar microlite growth conditions ph2 o and temperature were achieved throughout this extrusive period however microlite number density varied by more than a factor of four in these samples and generally increased with distance from the vent low vent ward microlite number densities and greater microlite concentrations down flow probably reflect progressively decreasing rates of undercooling at the time of crystal nucleation during extrusion of the 1995 dome comparison between dome extrusion episodes indicates a correlation between lava effusion rate and microlite number density suggesting that extrusion slowed during 1995 crystal textures and compositions in the 1992 93 and 1994 domes share the range exhibited by the 1995 dome suggesting that transitions in crystallization conditions i e rates of undercooling determined by effusion rate are cyclic q 2000 elsevier science b v all rights reserved

laboratory measurements of electrical conductivity of hydrous and dry silicic melts under pressure an interpretation of the electrical signature of molten rocks within the earth s interior in terms of nature and temperature conditions of magma requires additional laboratory data on the electrical conductivity of silicate melts this paper describes an experimental setup and presents measurements of the electrical impedance of dry and hydrous 1 3 wt h2o metaluminous obsidians determined in an internally heated pressure vessel in the range of 50 400 mpa and 350 1325 c it is shown that the temperature and pressure dependences of electrical conductivity of both hydrous and dry obsidian can be fitted by an arrhenius law applying in the melt and the glass regions this suggests that a similar transport mechanism operates in both melt and glass the determined activation energies are 70 kj mol for the dry obsidian and 65 and 61 kj mol for the 1 and 3 wt h2o melts respectively the activation volume is 20 cm3 mol combination of tracer diffusion and electrical conductivity reveals that sodium is the dominant charge carrier in the hydrous and dry obsidians the temperature and pressure effects on conductivity are therefore interpreted in terms of activation energy and activation volume for na mobility in dry and hydrous rhyolites an increase in conductivity associated with addition of water was observed and is shown to reflect the effect of water incorporation in melts on the mobility of sodium as a prospective it is anticipated that both the mobility and content of sodium could control the electrical conductivity of most terrestrial silicate melts magma under differentiation becomes more conductive due to sodium and water enrichment associated with fractional crystallization therefore its electrical signature must reveal its nature and maturity

electrical conductivity of clinopyroxene nacl h2o system at high temperatures and pressures implications for high conductivity anomalies in the deep crust and subduction zone

the sequence and timing of large late pleistocene floods from glacial lake missoula abstract glacial lake missoula formed when the purcell trench lobe of the cordilleran ice sheet dammed clark fork river in montana during the fraser glaciation marine oxygen isotope stage 2 over a period of several thousand years the lake repeatedly filled and drained through its ice dam and floodwaters coursed across the landscape in eastern washington in this paper we describe the stratigraphy and sedimentology of a significant new section of fine grained glacial lake missoula sediment and compare this section to a similar previously described sequence of sediments at ninemile creek 26 km to the northwest the new exposure which we informally term the rail line section is located near missoula montana and exposes 29 units each of which consists of many silt and clay couplets that we interpret to be varves the deposits are similar to other fine grained sediments attributed to glacial lake missoula similar varved sediments overlie gravelly flood deposits elsewhere in the glacial lake missoula basin each of the 29 units represents a period when the lake was deepening and all units show evidence for substantial draining of glacial lake missoula that repeatedly exposed the lake floor the evidence includes erosion and deformation of glaciolacustrine sediment that we interpret happened during draining of the lake desiccation cracks that formed during exposure of the lake bottom and fluvial sand deposited as the lake began to refill the floods date to between approximately 21 4 and 13 4 cal ka ago based on regional chronological data the total number of varves at the rail line and ninemile sites are respectively 732 and 583 depending on lake refilling times each exposure probably records 1350 1500 years of time we present three new optical ages from the rail line and ninemile sites that further limit the age of the floods these ages in calendar years are 15 1 0 6 ka at the base of the ninemile exposure and 14 8 0 7 and 12 6 0 6 ka midway through the rail line exposure the sediment at the two sections was deposited during later stages of glacial lake missoula after the largest outburst events

isotopically depleted late pleistocene groundwater in columbia river basalt aquifers evidence for recharge of glacial lake missoula floodwaters 1 late pleistocene outburst flooding of ice dammed glacial lake missoula and possible discharge from the cordilleran ice sheet cis catastrophically altered the northwestern united states landscape yet little is known about potential infiltration of flood waters into the subsurface this study provides compelling evidence for the presence of late pleistocene cis related recharge waters in the columbia river basalt aquifers crbas in central washington crba groundwaters with corrected 14c ages from 15 7 and 33 3 k yrs bp during periods of flood events have anomalously low 18o values 18 9 to 17 6 compared to late pleistocene soil waters 16 1 to 13 4 and modern precipitation in the region average 15 9 consistent with cis related meltwater recharge these results have implications for our understanding of megaflood phenomena on earth and mars

a modified archie s law for two conducting phases abstract many types of mixing model are used widely within the earth sciences to determine the electrical properties of porous media consisting of solid and fluid phases of known conductivities volume fractions and distributions i e phase connectivities most models are valid for two or more conducting phases however of the simple models only archie s law includes a variable term the archie cementation exponent m that describes the connectivity of the phases unfortunately archie s law is only valid for one conducting phase distributed within a non conducting phase which makes it inapplicable in instances where the rock matrix has a significant conductivity such as for clay rich rocks and in calculations involving partial melting more complex models exist which account for multiple conducting phases and control over phase conductivity we have adapted the conventional archie s law to produce a simple modified archie s law that can be used with two conducting phases of any conductivity and any volume fraction while retaining the ability to model variable connectivities within those phases that result from variations in their distribution the modified model has two exponents m and p that describe the connectivity of each of the two phases the exponents are related through an equation that depends also on the volume fractions of the two phases the modified and the conventional versions of archie s law have been tested on a granular analogue porous medium with a conducting matrix and a pore space saturated with a range of saline fluids with different salinities and conductivities the new model describes the experimentally determined electrical behaviour of the system extremely well improving greatly on the conventional archie s law

pleistocene megafloods in the northeast pacific massive discharges of freshwater from the glacial lake missoula to the northeast pacific ocean are thought to have sculpted the channeled scablands of eastern washington and debouched via the columbia river near 46 n the dynamics and timing of these events and their impact on northeast pacific circulation remain uncertain here we date marine records of anomalous freshwater inputs to the ocean based on freshwater diatoms oxygen isotopes in foraminifera and radiocarbon data low salinity plumes from the columbia river reduced sea surface salinities by as much as 6 psu practical salinity units more than 400 km away between 16 and 31 cal calendar ka b p anomalously high abundances of freshwater diatoms in marine sediments from the region precede generally accepted dates for the existence of glacial lake missoula implying that large flooding or freshwater routing events were common during the advance of the cordilleran ice sheet and that such events require multiple sources

a paleotemperature record derived from dissolved noble gases in groundwater of the aquia aquifer maryland usa low 14c activities in groundwater of the confined part of the aquia aquifer in southeastern maryland suggest that most of this water infiltrated at least 30 000 years ago however radiocarbon contents of the dissolved inorganic carbon seem to be affected by isotopic exchange possibly with secondary calcite deposits in the formation leading to overestimated 14c ages whereas the geochemistry of the aquia aquifer complicates the application of the widely used 14c dating method the accumulation of radiogenic he seems to provide a viable alternative for establishing a chronology the quasi linear increase of he concentrations with flow distance observed in the aquia aquifer can be explained entirely by accumulation of in situ produced radiogenic he u and th concentrations in aquia sand were measured in order to determine the accumulation rate of 4he with sufficient confidence to establish a he time scale concentrations of dissolved atmospheric noble gases were used to derive mean annual ground temperatures at the time of infiltration these noble gas temperatures ngts clearly show the presence of water that infiltrated under much cooler conditions than at present ngts are correlated with chloride concentrations corroborating the hypothesis that chloride variations in this aquifer constitute a climate signal in contrast the stable isotope ratios 180 and d do not provide a clear record of past climatic changes in the aquia aquifer and the correlation between ngts and stable isotope ratios is weak the ngt record suggests that mean annual temperatures in this midlatitude coastal site during the last glacial maximum lgm were 9 0 0 6 c colder than during the holocene this difference is slightly lower than estimates derived from pollen data for this region but considerably larger than the rather uniform cooling of about 5 c indicated by noble gas studies in more southern locations of north america the larger cooling is ascribed to the influence of the laurentide ice sheet which at its maximum extension came as close as 250 km to our study site

magnetic fabric and 3d geometry of the hombreiro sta eulaliapluton implications for the variscan structures of eastern galicia nw spain abstract in this paper the internal structure and 3d geometry of the hombreiro sta eulalia granitic pluton hercynian belt nwspain are described characterization of the internal structure was performed from surface data based on microstructural analysis and anisotropy of magnetic susceptibility studies the geometrical features of the pluton morphology at depth were derived from gravity data the pluton is made up of paramagnetic granites and leucogranites k values ranging between 1 1 and 13 3 10 5 si with well developed magnetic fabrics anisotropy degree p 1 05 for 78 of the sampling localities the results obtained enable us to depict a flat lying magnetic structure with associated n120 e trending magnetic lineations the gravity survey corroborates the subhorizontal laminar geometry of the pluton which has an estimated thickness of 2 km and an almost planar floor the 3d morphology is elongate at depth parallel to the linear internal fabric this would imply that magma emplacement took place along this preferential direction whereas the deepest zones of the pluton trend wnw ese elongate parallel to regional 1 it is almost rootless we present data showing that the hombreiro sta eulalia pluton emplacement was closely related spatially and chronologically along with other granitoid plutons from the lugo dome with two major structures of eastern galicia the basal shear zone of the mondonedo nappe and the extensional vivero fault

magma recharging beneath the weishan volcano of the intraplate wudalianchi volcanic field northeast china implied from 3 d magnetotelluric imaging

carbon 14 dating of ground water in the palouse basin of the columbia river basalts summary delineation of the spatial distribution of ground water that is several thousands of years old can provide an important piece of the puzzle in the evaluation of long term ground water resource sustainability under pumping conditions ground water for municipal and local water supplies within the palouse basin of eastern washington and northern idaho are derived almost entirely from two basalt aquifer systems decades of continual water level declines in the deeper aquifer system in response to interstate pumping have suggested that this high transmissivity low storativity aquifer system is being mined average water level declines in the deeper aquifer system have been on the order of 0 46 m 1 5 ft per year for the past 40 plus years carbon isotopic measurements on dissolved inorganic carbon were used to provide information on the relative ages of the ground water pumped from various locations within the basin mass balance modeling of hydrochemical reactions together with steep downward vertical gradients throughout the basin suggest that the ground water being withdrawn currently required several thousand years to traverse spatially variable sequences of loess fluvial sediments and basalt flows with associated sedimentary interbeds that overlie the primary producing zones in the basin restrictions to the vertical movement of recharge water such as low permeability overburden deposits well developed fragipans and thick low permeability sedimentary interbeds increase in the eastern portion of the palouse basin compared to the western portion this has resulted in accumulation of the oldest ground water in the topographically highest areas of the basin adjacent to the basin divide

number and size of last glacial missoula floods in the columbia river valley between the pasco basin washington and portland oregon field evidence and radiocarbon age dating combined with hydraulic flow modeling provide new information on the magnitude frequency and chronology of late pleistocene missoula floods in the columbia river valley between the pasco basin washington and portland oregon more than 25 floods had discharges of 1 0 106 m3 s at least 15 floods had discharges of 3 0 106 m3 s at least six or seven had peak discharges of 6 5 106 m3 s and at least one flood had a peak discharge of 10 106 m3 s a value consistent with earlier results from near wallula gap but better defined because of the strong hydraulic controls imposed by critical flow at constrictions near crown and mitchell points in the columbia river gorge stratigraphy and geomorphic position combined with 25 radiocarbon ages and the widespread occurrence of the ca 13 ka radiocarbon years mount st helens set s tephra show that most if not all the missoula flood deposits exposed in the study area were emplaced after 19 ka radiocarbon years and many were emplaced after 15 ka more than 13 floods perhaps postdate ca 13 ka including at least two with discharges of 6 106 m3 s from discharge and stratigraphic relationships upstream we hypothesize that the largest flood in the study reach resulted from a missoula flood that predated blockage of the columbia river valley by the cordilleran ice sheet multiple later floods probably including the majority of floods recorded by fine and coarse grained deposits in the study area resulted from multiple releases of glacial lake missoula that spilled into a blocked and inundated columbia river valley upstream of the okanogan lobe and were shunted south across the channeled scabland

properties of magmatic liquids by molecular dynamics simulation the example of a morb melt abstract a new atom atom interaction potential is introduced for describing by classical molecular dynamics md simulation the physical properties of natural silicate melts the equation of state the microscopic structure the viscosity the electrical conductivity and the self diffusion coefficients of ions in a mid oceanic ridge basalt morb melt are evaluated by md over a large range of temperature and pressure 1673 3273 k and 0 60 gpa a detailed comparison with experimental data shows that the model reproduces the thermodynamic structural and transport properties of a morb with an unprecedented accuracy in particular it is shown that the morb melt crystallizes at lower mantle conditions into a perovskite phase whose the equation of state eos is compatible with those proposed in the experimental literature moreover in accordance with experimental findings the simulation predicts not only that the morb viscosity exhibits a slight minimum with the pressure but also that the viscosity at high temperature remains very low 2273 k even at high pressure up to 40 gpa however the evolution of the electrical conductivity with temperature and pressure is not always the symmetrical of that of the viscosity in fact the relationship between viscosity and electrical conductivity shows a crossover at around 2073 k

integrated two dimensional lithospheric conductivity modelling in the pyrenees using field scale and laboratory measurements abstract recent magnetotelluric mt studies have shown that the lower crust in the pyrenees contains a high conductivity zone consistent with a subducting continental slab whose conductivity is 0 33 s m partial melting has been interpreted to be the most plausible explanation for this high conductivity here we report a two dimensional conductivity model of the lithosphere by integrating field scale and laboratory determinations of the conductivity of continental crustal and mantle rocks the laboratory data provide empirical formulas which allow us to determine the fluid saturated rock and melt conductivity when temperature pressure and lithology are known consequently we have also calculated the density lithostatic pressure and several alternative temperature profiles for use in the model from gravity seismic and thermal field data these can be used with a prescribed melt fraction to predict the electrical conductivity at depth which can be compared with the mt conductivity data alternatively the laboratory data can be combined with the mt conductivity data to predict the melt fraction at depth the primary outputs of the modelling are conductivity and melt fraction prediction profiles for six mixing models i waff s model hashin shtrikman hs upper bound ii hs lower bound iii parallel layers iv perpendicular layers v random melt areas and vi a modified archie s law that takes account of the presence of two conducting phases the modelling results indicate that a good match to the mt data can be obtained along the whole profile by the influence of pressure temperature and the fluid phase with the only exception being the subducted slab where a minimum of 4 7 melt fraction is necessary to explain the data

the channeled scabland back to bretz the channeled scabland washington state united states is only partly the result of erosion by the catastrophic drainage of glacial lake missoula there were other sources of meltwater recent sedimentary investigations of some sites in the missoula basin and in the channeled scabland support a single large late wisconsin flood as opposed to multiple floods proposed for this time period sediment in the glacial lake missoula basin records rapid infill by jokulhlaups draining into lake missoula from upstream punctuating a long period of normal varve sedimentation this was independent of sedimentation in the main scabland tract where proximal and distal rhythmic beds are explained as resulting from multiple pulses or surges within a single flood geomorphic and sedimentary evidence supports the conclusion that drainage from the cordilleran trunk valleys was important and pulses were probably related to the drainage of these valleys

oxygen isotope evidence for the late cenozoic development of an orographic rain shadow in eastern washington usa topographic development of the southern washington cascade range and its influence on regional climate on the leeward side of the range for the past 15 6 m y are evaluated using oxygen isotope ratios d 18 o of ancient meteoric water recorded in authigenic smec tites the d 18 o values of authigenic smectites from paleosols and altered tuffs on the east side of the range exhibit a temporal and continuous decrease of 3 4 from 15 6 ma to the present taking into account a regional temperature change in eastern washington since the middle miocene the calculated d 18 o values of regional meteoric water show a negative shift of 3 5 4 5 over the same interval such a decrease is similar to the change in the d 18 o values of modern precipitation from the coastal side to a region down wind of the washington cascades this negative shift of the calculated d 18 o values on the east side of the range is best explained by the development of a rain shadow due to the tectonic rock uplift of the cascades during the late cenozoic based on the empirically calculated relationship between change in elevation and change in the d 18 o value of pre cipitation paleorelief of the southern washington cascades since 15 6 ma is estimated to be 1 2 1 7 km

the influence of oxygen isotope exchange between co2 and h2o in natural co2 rich spring waters implications for geothermometry abstract oxygen isotope ratio 18o value deviations from the meteoric water line with no significant change in the hydrogen isotope 2h composition have been reported in naturally occurring co2 rich waters from around the world here we review the effects of oxygen isotope exchange with co2 high temperature equilibration with bedrock minerals and mineral dissolution and precipitation reactions on the co2 rich water isotopic composition we present two case studies from daylesford australia and pah tempe utah usa mineral springs where we use a numerical geochemical modelling approach to resolve the influence of low temperature water rock interactions and co2 equilibration to the oxygen isotope ranges observed in the mineral waters in both cases we find that mineral dissolution precipitation reactions are unlikely to have a significant effect on the groundwater isotopic compositions and that the observed 18o values in natural co2 springs can be simply explained by equilibrium fractionation between water and free phase co2 traditionally the interaction of co2 and water in a natural co2 rich groundwater setting has only been associated with water 18o depletion and this is the first study to consider 18o enrichment we establish that in a natural setting co2 and water equilibration can result in water 18o depletion or enrichment and that the change in the oxygen isotope composition ultimately depends on the initial co2 and water 18o values our new conceptual model therefore provides a mechanism to explain water 18o enrichment at ambient temperatures this finding is critical for the use of 18o in groundwater geothermometry and for the interpretation of natural water circulation depths we argue that in some cases natural waters previously interpreted as geothermal based on their oxygen isotope composition may actually have acquired their isotopic signature through interaction with co2 at ambient temperatures

magnitudes and implications of peak discharges from glacial lake missoula new field evidence and discharge calculation procedures provide new estimates of maximum late pleistocene glacial lake missoula flood discharges for two important reaches along the flood route within the spokane valley near the point of release the peak discharge probably exceeded 17 3 million m3 sec 1 downstream at wallula gap a major point of flow convergence peak discharge was about 10 2 5 million m3sec 1 flow duration was on the order of several days these are the largest known terrestrial fresh water flows consideration of these discharge values constrains models for the failure of glacial lake missoula the maximum discharges estimated here are larger than theoretical and empirical predictions of maximum subglacial jokulhlaup style releases for lake missoula we postulate consistent with geological relations in the glacial lake missoula basin and in the channeled scabland that the largest late wisconsinan missoula flood resulted from a cataclysmic failure of the impounding ice dam of glacial lake missoula this large release may have been the result of a complete rupture of the ice dam subsequent multiple flows of lesser magnitude may have resulted from repeated subglacial releases from the lake

imaging the deep source of the rotorua and waimangu geothermal fields taupo volcanic zone new zealand abstract magnetotelluric data were recorded in a 45 10 km band crossing the rotorua and waimangu geothermal fields in the northern part of the taupo volcanic zone in the central north island of new zealand 3 d inverse modelling of these data show that beneath the low resistivity areas marking the near surface geothermal fields localised electrically conductive zones are present in the crust below about 2 5 and 3 5 km depth at rotorua and waimangu respectively at increasing depth these conductive zones broaden and appear to merge with a larger conductive zone at 8 km depth situated between the geothermal systems at rotorua the top of the conductive zone is situated directly beneath the area of greatest surface heat and gas discharge at waimangu the uppermost part of the deeper conductive zone is situated beneath the western part of lake rotomahana also an area of intense surface thermal activity and high heat flux the localised conductive zones are interpreted to be high temperature quasi magmatic fluids rising from a broader zone of partial melt at deeper levels

the roles of magmatic and hydrothermal processes in pge mineralization ferguson lake deposit nunavut canada the ferguson lake ni cu co platinum group element pge deposit in nunavut canada occurs near the structural hanging wall of a metamorphosed gabbroic sill that is concordant with the enclosing country rock gneisses and amphibolites massive to semi massive sulfide occurs toward the structural hanging wall of the metagabbro and a low sulfide high pge style of mineralization sulfide veins and disseminations locally occurs 30 50 m below the main massive sulfide water rock interaction in the ferguson lake ni cu co pge deposit is manifested mostly as widespread post metamorphic epidote chlorite calcite veins and replacement assemblages that contain variable amounts of sulfides and platinum group minerals pgm pgm occur as inclusions in magmatic pyrrhotite and chalcopyrite in both the massive sulfide and high pge zones at the contact between sulfides and hornblende or magnetite inclusions in the massive sulfide in undeformed sulfide veins and adjacent chlorite and or epidote halos in hornblende adjacent to hydrothermal veins and in plagioclase chlorite aggregates replacing garnet cemented by sulfide the pgm are mostly represented by the kotulskite pdte sobolevskite pdbi solid solution but also include michenerite pdbite froodite pdbi2 merenskyite pdte2 mertieite ii pd8 sb as 3 and sperrylite ptas2 and occur in variety of textural settings those that occur in massive and interstitial sulfides interpreted to be of magmatic origin and formed through exsolution from base metal sulfides at temperatures 600 c are dominantly bi rich i e te bearing sobolevskite whereas those that occur in late stage hydrothermal sulfide silicate veins and their epidote chlorite alteration halos tend to be more te rich i e bi bearing kotulskite the chemistry and textural setting of the various pgm supports a genetic model that links the magmatic and hydrothermal end members of the sulfide pgm mineralization the association of pgm with magmatic sulfides in the massive sulfide and high pge zones has been interpreted to indicate that pge mineralization was initially formed through exsolution from base metal sulfides which formed by magmatic sulfide liquid segregation and crystallization however the occurrence of pgm in undeformed sulfide bearing veins and in their chlorite epidote halos and differences in pgm chemistry indicate that hydrothermal fluids were responsible for post metamorphic redistribution and dispersion of pge

a geochemical study of the impact of irrigation and aquifer lithology on groundwater in the upper yakima river basin washington usa the yakima river a major tributary of the columbia river is currently overallocated in its surface water usage in part because of large agricultural water use as a result groundwater availability and surface water groundwater interactions have become an important issue in this area in several sub basins the yakima river water is diverted and applied liberally to fields in the summer creating artificial recharge of shallow groundwater major ion trace element and stable isotope geochemistry of samples from 26 groundwater wells from a transect across the yakima river and 24 surface waters in the kittitas sub basin were used to delineate waters with similar geochemical signatures and to identify surface water influence on groundwater major ion chemistry and stable isotope signatures combined with principal component analysis revealed four major hydrochemical groups one of these groups collected from shallow wells within the sedimentary basin fill displays temporal variations in no3 and so4 along with high 18o and d values indicating significant contribution from yakima river and or irrigation water two other major hydrochemical groups reflect interaction with the main aquifer lithologies in the basin the columbia river basalts high na groundwaters and the volcaniclastic rocks of the ellensburg formation ca mg hco3 type waters the fourth major group has interacted with the volcaniclastic rocks and is influenced to a lesser degree by surface waters the geochemical groupings constrain a conceptual model for groundwater flow that includes movement of water between underlying columbia river basalt and deeper sedimentary basin fill and seasonal input of irrigation water

an experimental study of the influence of oxygen fugacity on fe ti oxide stability phase relations and mineral melt equilibria in ferro basaltic systems

evidence for dilution of deep confined ground water by vertical recharge of isotopically heavy pleistocene water new analyses of the isotopic composition of water sup 14 c dating of dissolved inorganic carbon and order of magnitude darcy calculations suggest that a dilute body of water trending north south in the cambrian ordovician aquifer of iowa was emplaced as vertical recharge of pleistocene age water from the base of the des moines lobe of late wisconsin time the recharge occurred through more than 300 m of overlaying silurian to mississippian age rocks the delta sup 18 o values range from minus 10 per thousand to minus 9 per thousand for the dilute water body and are consistent with a mixture of des moines lobe meltwater and precipitation found today in the north central us these results suggest that 1 the climate at the end of the last glaciation was mild and 2 a ground water stable isotope signature similar to that of modern precipitation in an aquifers recharge area is not a priori evidence for relatively recent recharge

a high electrical conductive zone at lower crustal depth beneath the betic chain spain abstract magnetotelluric data with periods of up to 2000 s were collected and the conductivity structure of the crust and upper mantle was imaged in the central betic chain interpretation of the impedance tensor components and the geomagnetic transfer functions was made in terms of a two dimensional resistivity model along a nw se profile from the guadalquivir foreland basin and across the boundary between the external and the internal betics elongated high conductivity zones in the upper and middle crust related to fluid circulation along both sediments and faults were detected the conductors in the upper crust coincide with the areal distribution of the cenozoic and mesozoic sedimentary rocks from the guadalquivir basin and the external betics and the conductors in the middle crust with the major miocene extensional detachments affecting the basement rocks of the internal betic zone a high conductivity zone at lower crustal levels beneath the outer internal betics was also detected next to an area where earlier seismic reflection data had imaged a duplication of the reflective moho this conductor was interpreted as partial melting of a southeast dipping subducted iberian lower crust the geodynamic significance of this subduction depends mainly on whether the overthrusting lower crust belongs to the alboran domain or to the iberian plate in the former case it would correspond to a major plate boundary and in the latter to an intraplate feature

electrical conductivity of magma in the course of crystallization controlled by their residual liquid composition 1 the electrical conductivity of a magma in the course of crystallization was experimentally investigated in the temperature range of 1350 1018 c large samples of basaltic composition with a homogeneous crystal content were synthesized in a gas mixing furnace at 1 atm pressure the samples were analyzed by electron microprobe the relative proportions of the phases as a function of temperature were determined depending on temperature the phase assemblies included quenched silicate liquid plagioclase pyroxene fe ti oxides the crystal content varied from 0 to 80 wt in response to partial crystallization the residual liquid changed composition from basalt to andesite to dacite liquid the electrical conductivity of the partially crystallized basaltic samples was measured in addition above liquidus conductivity measurements were conducted on compositions matching the residual liquid at different temperature these supplemental electrical measurements allowed us to discriminate the effect of crystal content from the effect of changing liquid composition associated with partial crystallization combining with the modified archie s law a set of constraints describing the conductivity of the residual liquid versus chemical composition and temperature we propose an equation to calculate changes in conductivity associated with partial magma crystallization we showed how the composition of the residual liquid is critical on the electrical behavior of crystal liquid system the model overcomes the previous difficulties in finding a robust model for describing the electrical behavior of crystal liquid systems the effect of liquid composition on the electrical conductivity is related to diffusion mechanisms and transport properties in molten silicate combining known constraints on na tracer diffusion and our conductivity results confirms the statements that sodium is the dominant charge carrier silicate liquids from basalt to rhyolite these findings revealed that we need a comprehensive model that can predict the conductivity of molten silicate as a function of chemical composition

electrical conductivity of a phonotephrite from mt vesuvius the importance of chemical composition on the electrical conductivity of silicate melts the bulk electrical conductivity of the phonotephritic lava from the 1944 eruption of mt vesuvius was measured using complex impedance spectroscopy in a multianvil apparatus at 1 gpa and temperatures up to 700 c melting experiments prior to the electrical measurements were also performed on this sample in a piston cylinder apparatus in order to gauge how bulk conductivity varies as a function of its melt fraction unlike the behaviour found in basaltic rocks in which conductivity increases with increasing melt fraction we observe a conductivity decrease of the order of a factor of ten for samples at 700 c ranging in melt fraction from 32 vol to completely molten we attribute this anomalous behaviour to the progressive loss of highly conductive leucite upon melting the addition of potassium to the melt phase however does not result in an increase of the total alkali concentration due to the melting of other mineral components we also present an empirical model to predict the electrical conductivity of fully molten silicate liquids as a function of temperature and chemical composition based on conductivity data for natural silicate liquids found in the literature the inclusion of compositional terms reduces the error by more than a factor of four with respect to a composition independent temperature only parameterization

paleoclimate simulations for north america over the past 21 000 years features of the simulated climate and comparisons with paleoenvironmental data maps of upper level and surface winds and of surface temperature and precipitation illustrate the results of a sequence of global paleoclimatic simulations spanning the past 21 000 years for north america we review a the large scale features of circulation temperature and precipitation that appear in the simulations from the ncar community climate model version 1 ccm 1 b the implications of the simulated climate for the past continental scale distributions of three plant taxa picea spp pseudotsuga menziesii and artemisia tridentata which are broadly representative of the vegetation across the continent and c the potential explanations in terms of atmospheric circulation or surface energy and water balance processes for mismatches between the simulations and observations most of the broad scale features of previous paleoclimatic simulations with the ncar ccm 0 for north america are present in the current simulations many of the elements of a conceptual model based on previous climate simulations that describes the controls of paleoclimatic variations across north america during the past 21 000 years are found in simulations reviewed here these include 1 displacement of the jet stream by the laurentide ice sheet to the south of its present position in both winter and summer 2 generation of a glacial anticyclone over the ice sheet at the lgm and the consequent induction of large scale sinking motions induced over eastern north america 3 changes in the strength of surface atmospheric circulation features through time including weakening of the aleutian low in winter and strengthening of the eastern pacific and bermuda high pressure systems in summer as the ice sheet decreased in size 4 development of a heat low at the surface and a strengthened ridge in the upper atmosphere over the continent at the time of the maximum summer insolation anomaly 5 increases in summer temperature earlier in regions remote from the ice sheet these increases appear earlier in the present ccm 1 simulations than in the previous ccm 0 ones however and 6 continuation of negative winter temperature anomalies into the middle holocene in general simulated surface conditions that are discordant with paleoenvironmental observations can be attributed to the simulation of particular atmospheric circulation patterns e g those that suppress precipitation or advect warm air into a region with these mismatches amplified in beringia and the southeastern united states by surface energy and water balance processes

isotopic evidence for temporal variation in proportion of seasonal precipitation since the last glacial time in the inland pacific northwest of the usa abstract large scale atmospheric circulation patterns determine the quantity and seasonality of precipitation the major source of water in most terrestrial ecosystems oxygen isotope 18o dynamics of the present day hydrologic system in the palouse region of the northwestern u s a indicate a seasonal correlation between the 18o values of precipitation and temperature but no seasonal trends of 18o records in soil water and shallow groundwater their isotope values are close to those of winter precipitation because the palouse receives 75 of its precipitation during winter palouse loess deposits contain late pleistocene pedogenic carbonate having ca 2 to 3 higher 18o values and up to 5 higher carbon isotope 13c values than holocene and modern carbonates the late pleistocene 18o values are best explained by a decrease in isotopically light winter precipitation relative to the modern winter dominated infiltration the 13c values are attributed to a proportional increase of atmospheric co2 in soil co2 due to a decrease in soil respiration rate and 13c discrimination in plants under much drier paleoclimate conditions than today the regional climate difference was likely related to anticyclonic circulation over the pleistocene laurentide and ice sheet

electrical conductivity of molten basalt and andesite to 25 kilobars pressure geophysical significance and implications for charge transport and melt structure electrical conductivities of molten hawaiian tholeiite and crater lake andesite were measured between 1200 c and 1400 c at atmospheric pressure and at pressures up to 17and 25 kbar respectively isobaric plots of log versus 1 t is electrical conductivity are linear with the exception of the zero pressure tholeiite melt data conductivities decrease with increasing pressure in both melts with the andesitic melt exhibiting a greater pressure dependence between 5 and 10 kbar abrupt decreases in the slopes of isothermal log versus p plots i e decreases in activation volume are observed for both rock melts this discontinuity probably reflects changes in melt structure as opposed to changes in conduction mechanism in each pressure range the data for each rock melt can be described reasonably well by an equation of the form 0 exp ea p v kt where 0 is a preexponential constant ea is the activation energy and v is the activation volume a qualitative model involving depolymerization of the melt with increased pressure leading to increased efficiency of packing can explain the observed discontinuity in activation volume as well as the observed pressure dependences of other melt physical properties such as viscosity and density conductivity versus melt fraction curves for partially molten peridotite are reevaluated using high pressure tholeiitic melt conductivities and crystalline conductivity values recently determined by other workers minimum melt fraction estimates of 5 10 are required to explain upper mantle regions of anomalously high electrical conductivity in terms of a partial melting hypothesis

oxygen isotope composition of ground water and secondary minerals in columbia plateau basalts implications for the paleohydrology of the pasco basin concentrations of sup 18 o and deuterium in ground waters beneath the handford reservation washington state suggest that the meteoric waters recharging the basalt aquifers have been progressively depleted in these isotopes since at least pleistocene time this conclusion is supported by oxygen isotope analyses of low temperature secondary minerals filing vugs and fractures in the basalts which are used to approximate the sup 18 o content of ground water at the time the mineral assemblage formed a fossil profile of delta sup 18 o values projected for ground water in a 1500 m vertical section beneath the reservation suggest that the vertical mixing of shallow and deep ground water indicated by present day hydrochemical data was also occurring during neogene time these data also suggest that a unidirectional depletion of sup 18 o and deuterium recorded in pleistocene ground waters may have extended considerably further back in time this shift is tentatively attributed to the orographic depletion of sup 18 o associated with the progressive uplift of the cascade range since the middle miocene

cathodoluminescence microscopy of myrmekite rim wartlike and isolated myrmekite are the three types of myrmekite that occur in the rubidoux mountain leucogranite combined cathodoluminescence scanning electron microscopy and electron microprobe investigations suggest that myrmekite is the product of k feldspar and plagioclase replacement clusters of optically continuous quartz vermicules are interconnected exhibiting a dendroid shape the surrounding myrmekitic plagioclase is chemically undistinguishable from the host bluish luminescing plagioclase with a mean composition of ab 0 81 an 0 18 kf 0 01 only the fractured cores of some plagioclase grains which display yellowish luminescence represent a plagioclase with a higher an content an 0 59 ab 0 39 kf 0 2 petrographic evidence shows that myrmekite is a complex reaction where the main phase formed prior to exsolution of albite from k feldspar and crystallization of albitic rims the formation of myrmekite is therefore a reaction occurring during late stage magmatic crystallization

extensive deposits on the pacific plate from late pleistocene north american glacial lake outbursts abstract one of the major unresolved issues of the late pleistocene catastrophic flood events in the northwestern united states e g from glacial lake missoula has been what happened when the flood discharge reached the ocean this study compiles available 3 5 khz high resolution and airgun seismic reflection data long range sidescan sonar images and sediment core data to define the distribution of flood sediment in deepwater areas of the pacific ocean upon reaching the ocean at the mouth of the columbia river near the present day upper continental slope sediment from the catastrophic floods continued flowing downslope as hyperpycnally generated turbidity currents the turbidity currents resulting from the lake missoula and other latest pleistocene floods followed the cascadia channel into and through the blanco fracture zone and then flowed west to the tufts abyssal plain a small part of the flood sediment which was stripped off the main flow at a bend in the cascadia channel at its exit point f

paleoclimatic investigations using isotopic signatures of the late pleistocene holocene groundwater of the stratified aquifers in kuwait abstract the study attempts to determine paleo recharge sources for two major aquifer units of kuwait namely kuwait group aquifer kga and dammam formation dfm were collected and analyzed for major ions and isotopes the obtained results of 14c ages indicated that the groundwater samples ranged from 31 9 ky to 3 9 ky in dfm and 23 3 ky to 0 8 ky in kga the variations in 18o signatures with respect to age reflected that there were two major sources of rainwater such as enriched southern indian ocean monsoon and the depleted northwesterly mediterranean vapor source the samples found to show variations during the three marine isotopic stages mis 1 3 heinrich events bolling allerod and dryas time periods in both aquifers a shift in the intertropical convergence zone itcz was witnessed in the groundwater of both aquifers from 7 8 ky to 4 9 ky the variation in the holocene climate is mainly due to the influence of tropospheric easterly jet governed by the swing in itcz as a result of holocene volcanism the mean 18o values of dfm samples 3 81 reflect depletion than kga samples 2 31 and the d excess values were 0 95 and 2 05 respectively thus it could be inferred that the dfm was predominantly recharged during late pleistocene period and it was in a relatively cooler climate than recharge environment of kga the study infers that kga was recharged between 26on and 26 8on latitudes and that of the dfm was noted to be recharged south west of kuwait in two different patches between 25 8on and 26 2on and 26 7on 27 4on latitudes during late pleistocene the kga was concluded to be recharged between 25 5on and 27 5on latitudes south of kuwait during holocene

magnetotelluric and geomagnetic modelling reveals zones of very high electrical conductivity in the upper crust of central java abstract we present results of two and three dimensional modelling of magnetotelluric and geomagnetic data from central java indonesia we adopt a hypothesis testing approach where we examine the influence of individual structural elements of competing models on the total model responses the models show that ocean anomalies do not overwhelm the signal due to strong electrical conductivity anomalies within java these highly conducting regions with resistivities of below 1 m are located in the central and northern parts of the island fitting of the geomagnetic data requires that the innermost parts of both regions consist of material with extremely low resistivity 0 2 m young sediments tephra and their alteration products can be responsible for some of the low resistivity regions at shallow depths the two zones of extremely high conductivity however are best explained as consequences of geothermal activity in the vicinity of active volcanism mt merapi in central java and by fluid or graphite enrichment along shear zones in northern java subduction zone related processes in a depth range of 70 400 km cannot be resolved because the highly conductive regions at shallow depths effectively screen deeper structures

magmatic hydrothermal system inferred from the resistivity structure of kusatsu shirane volcano abstract the kusatsu shirane volcano is a quaternary andesitic to dacitic active volcano located on the central honshu arc the volcano is known to have had repeated phreatic eruptions in last 200 years a number of geochemical and geophysical studies have been conducted around the yugama crater lake the focus of current volcanic activity however the 2018 unexpected phreatic eruption occurred at mt motoshirane a different pyroclastic cone from that which hosts yugama there were also frequent magmatic eruptions until 1500 years ago at this cone and the magma produced at that time has not yet cooled and solidified a better understanding of the magmatic hydrothermal system of this volcano requires structural information gathered over a larger area and to greater depths than has been achieved to date here we describe the three dimensional 3 d electrical resistivity structure around mt motoshirane down to a depth of 10 km using broadband magnetotelluric mt data 320 0 0005 hz collected in 2015 and 2016 the 3 d resistivity structure model shows an extensive low resistivity layer at depths of 1 3 5 km beneath the summit however structures characteristic of the presence of magma are not observed beneath this layer it could be too deep or too small to be detected combining the new data with results of previous geochemical and geophysical studies we interpret the conductor as a hydrothermal fluid reservoir that supplies fluids to the crater lake and to hot springs on the eastern and western flanks of the volcano the existence of a fluid reservoir that extends from the vicinity of the yugama crater lake to the subsurface beneath an inactive pyroclastic cone that has produced repeated magmatic eruptions in the past suggests that a large scale magmatic hydrothermal system is developing beneath the volcano

high pressure and high temperature measurements of electrical conductivity in basaltic rocks from mount etna sicily italy 1 we have investigated the electrical properties of etnean rocks by in situ complex impedance spectroscopy using a multianvil apparatus from these measurements we determined the electrical conductivity of a lava flow sample whose basaltic composition can be considered close to that of the parent magma and also that of a mafic nodule representative of the high density cumulates interpreted as responsible for the main high velocity anomaly observed beneath the volcano the electrical conductivities of the two samples were measured at pressures of 0 9 and 1 5 gpa and temperatures from 400 to 1100 c at frequencies from 0 1 to 105 hz to investigate the electrical properties of the etnean products as a function of partial melting a few experiments were performed in a piston cylinder apparatus prior to the electrical measurements the obtained data were approximated using an equivalent circuit fitting technique for the lava flow sample electrical conductivity displays arrhenian behavior over the entire investigated temperature range with an activation energy of 0 8 ev within the uncertainties of our measurements we do not observe any effect of pressure on conductivity between 0 9 and 1 5 gpa on the contrary experiments performed on the series of partially molten samples indicate that conductivity increases with increasing quantity of glass while conductivities of samples with minor amounts of glass are comparable to that of the lava flow starting material with increased melting conductivity can increase by as much as a factor of 3 the mafic nodule was observed to have a conductivity higher than the lava flow sample e g at 800 c and 0 9 gpa a factor of 4 higher however scatter in the data is somewhat higher in comparison to the lava flow sample most likely because of the coarse texture of the samples and a consequence to their chemical and structural heterogeneity using our results we illustrate the effects that a layer of hot magma surrounded by a cooler wall rock has on apparent resistivities determined by one dimensional forward calculations our modeling demonstrates that both size and depth of magmatic intrusions strongly influence apparent resistivity and that these parameters can be extracted from field data if the electrical properties of the rocks below the surface are well understood

turbidite megabeds in an oceanic rift valley recording jokulhlaups of late pleistocene glacial lakes of the western united states abstract escanaba trough is the southernmost segment of the gorda ridge and is filled by sandy turbidites locally exceeding 500 m in thickness new results from ocean drilling program odp sites 1037 and 1038 that include accelerator mass spectrometry ams 14c dates and revised petrographic evaluation of the sediment provenance combined with high resolution seismic reflection profiles provide a lithostratigraphic framework for the turbidite deposits three fining upward units of sandy turbidites from the upper 365 m at odp site 1037 can be correlated with sediment recovered at odp site 1038 and deep sea drilling program dsdp site 35 six ams 14c ages in the upper 317 m of the sequence at site 1037 indicate that average deposition rates exceeded 10 m k yr between 32 and 11 ka with nearly instantaneous deposition of one 60 m interval of sand petrography of the sand beds is consistent with a columbia river source for the entire sedimentary sequence in escanaba trough high resolution acoustic strat

can in situ measurements of mantle electrical conductivity be used to infer properties of partial melts 1 constraints on bulk conductivity from magnetotelluric measurements and petrological analyses of late quaternary peridotite xenoliths from the southern sierra nevada allow evaluation of models commonly used to relate electrical conductivity to the physical and chemical state of the upper mantle in these models two conductive melts basalt and sulfide are embedded in a resistive matrix bounds on the amount of sulfide 0 06 0 4 and the bulk conductivity 0 03 0 1 s m place constraints on the degree of interconnection between the melts because the sulfide melt is very conductive even a small fraction of well connected melt results in a bulk conductivity larger than 0 1 s m similarly completely disconnected melts result in bulk conductivities much less than 0 03 s m the only models which matched both the bulk conductivity and sulfide bounds consisted of a small fraction 1 interconnected basalt melt with a discontinuous sulfide phase such a texture is observed in laboratory experiments with much larger sulfide melt fractions but has not been reported for small melt fractions a variant of the hashin shtrikman model and a hybrid model consisting of cascaded hashin shtrikman calculations were successful in matching the magnetotelluric and petrologic constraints with a model that appropriately simulates the melt interconnectivity we suggest that electrical conductivity may be used to infer in situ melt properties in the mantle

a late pleistocene holocene noble gas paleotemperature record in southern michigan 1 noble gas temperatures ngts and 14c derived ages in groundwaters of the michigan basin reveal a ground temperature of 1 c toward the end of the last glacial maximum lgm suggesting that groundwater recharge occurred under the laurentide ice sheet lis cover in addition to the general warming observed since the lgm the ngt record indicates an abrupt warming event between 12 8 and 11 1kyrs bp correlative to the bolling allerod boa warm phases ice sheet linked changes in freshwater delivery to the north atlantic together with changes in the north atlantic deep water nadw circulation are possible causes of such abrupt climate shifts in northeastern us pleistocene waters yielding the lowest ngts have the highest 18o and d values suggesting an atmospheric circulation pattern distinct from today with a stronger moisture component from the gulf of mexico possibly due to the presence of the lis which weakened the pacific westerly flow

giant magmatic water reservoirs at mid crustal depth inferred from electrical conductivity and the growth of the continental crust the formation of the continental crust at subduction zones involves the differentiation of hydrous mantle derived magmas through a combination of crystallization and crustal melting however understanding the mechanisms by which differentiation occurs at depth is hampered by the inaccessibility of the deep crust in active continental arcs here we report new high pressure electrical conductivity and petrological experiments on hydrated andesitic melt from uturuncu volcano on the bolivian altiplano by applying our results to regional magnetotelluric data we show that giant conductive anomalies at mid crustal levels in several arcs are characterized by relatively low amounts of intergranular andesitic partial melts with unusually high dissolved water contents 8 wt h2o below uturuncu the altiplano puna magma body apmb displays an electrical conductivity that requires high water content up to 10 wt dissolved in the melt based on crystal liquid equilibria and melt h2o solubility experiments such a super hydrous andesitic melt must constitute about 10 of the apmb the remaining 90 being a combination of magmatic cumulates and older crustal rocks the crustal ponding level of these andesites at around 6 kbar pressure implies that on ascent through the crust hydrous magmas reach their water saturation pressure in the mid crust resulting in decompression induced crystallization that increases magma viscosity and in turn leads to preferential stalling and differentiation similar high conductivity features are observed beneath the cascades volcanic arc and taupo volcanic zone this suggests that large amounts of water in super hydrous andesitic magmas could be a common feature of active continental arcs and may illustrate a key step in the structure and growth of the continental crust one sentence summary geophysical laboratory conductivity and petrological experiments reveal that deep electrical conductivity anomalies beneath the central andes cascades and taupo volcanic zone image the ponding of super hydrous andesitic melts which contributes to the growth of continental crust

an exsolution silica pump model for the origin of myrmekite myrmekite as defined here is the microscopic intergrowth between vermicular quartz and modestly anorthitic plagioclase calcic albite oligoclase intimately associated with potassium feldspar in plutonic rocks of granitic composition hypotheses previously invoked in explanation of myrmekite include 1 direct crystallization 2 replacement 3 exsolution the occurrence of myrmekite in paragneisses and its absence in rocks devold of discrete grains of potassium feldspar challenge those hypotheses based on direct crystallization or replacement however several lines of evidence indicate that myrmekite may in fact originate in response to kinetic effects associated with the exsolution of calcic alkali feldspar into discrete potassium feldspar and plagioclase phases exsolution of potassium feldspar system projected from alsi2o8 involves the exchange caalk 1si 1 in which the alsi 1 tetrahedral couple is resistant to intracrystalline diffusion by contrast diffusion of octahedral k proceeds relatively easily where it remains uncoupled to the tetrahedral exchange we suggest here that where the ternary feldspar system is open to excess silica the exchange reaction that produces potassium feldspar in the ternary plane is aided by the net transfer reaction k si orthoclase leaving behind indigenous si that reports as modal quartz in the evolving plagioclase as the caal component is concomitantly incorporated in this same phase thus silica is pumped into the reaction volume from a silica reservoir a process that enhances redistribution of both si and al through the exsolving ternary feldspar

the bolivian altiplano conductivity anomaly 1 a long period magnetotelluric study was carried out in the central andes between latitudes 19 5 s and 21 s along two almost parallel profiles of 220 and 380 km length respectively the investigation area extends from the pacific coast to the southern altiplano plateau in the back arc of the south american subduction zone the main geoelectrical structure resolved is a broad and probably deep reaching highly conductive zone in the middle and deeper crust beneath the high plateau although the data show deviations from two dimensionality a two dimensional approach is justified for large parts of the profiles sensitivity studies were carried out in order to constrain the depth extent another electrically conductive structure was resolved in the middle crust of the chilean forearc thought to be connected with the precordillera fault system the andean continental research program ancorp seismic reflection profile carried out along the same line at 21 s revealed highly reflective zones below the altiplano in good correlation with the upper boundary of the altiplano conductor this highly conductive domain also coincides with low seismic velocities and a zone of an elevated vp vs ratio and although not well resolved with low qp seismic quality factors taking into account the enhanced heat flow and a derived temperature model the most probable explanation lies in the assumption of granitic partial melts the good conductor below the volcanic arc which was found in regions farther south at 22 s gradually vanishes toward the north this is consistent with the results of seismic tomography concerning qp values and a gap of recent volcanism

three dimensional resistivity structure and magma plumbing system of the kirishima volcanoes as inferred from broadband magnetotelluric data broadband magnetotelluric mt measurements were conducted in 2010 and 2011 in the vicinity of shinmoe dake volcano in the kirishima volcano group japan where sub plinian eruptions took place 3 times during 26 27 january 2011 by combining the new observations with previous mt data it is found that an anomalous phase in excess of 90 is commonly observed in the northern sector of the kirishima volcano group because the anomalous phase is not explained by 1 d or 2 d structure with isotropic resistivity media 3 d inversions were performed by applying small errors to the anomalous phase we successfully estimated a 3 d resistivity structure that explains not only the normal data but also the anomalous phase data the final model shows a vertical conductor that is located between a deep seated conductive body at a depth greater than 10 km and a shallow conductive layer by applying the findings of geophysical and petrological studies of the 2011 sub plinian eruptions we infer that the subvertical conductor represents a zone of hydrothermal aqueous fluids at temperatures over 400 c in which a magma pathway interconnected melt is partially and occasionally formed before magmatic eruptions to the north of the deep conductor earthquake swarms occurred from 1968 to 1969 suggesting that these earthquakes were caused by volcanic fluids

the columbia river basalts between 17 million and 6 million years ago 200 000 square kilometers of the american northwest were flooded by basaltic lava that erupted through fissures in the crust up to 150 kilometers long larger individual eruptions covered over a third of the columbia plateau in a few days the lavas represent partial melts of the earth s mantle that were only slightly modified by near surface upper crustal processes the abundant chemical and mineralogical data now available offer an opportunity to study mantle composition and the processes involved in the evolution of the earth s crust

glacial meltwater input to the alaska coastal current evidence from oxygen isotope measurements results of a study of the oxygen isotopic composition of coastal pelagic and fresh waters from the northern gulf of alaska region are presented this study was undertaken to investigate whether isotopic tracers could be of use in determining the important freshwater inputs to the alaska coastal current acc and whether they could confirm the presence of the acc in coastal waters west of kodiak island the alaska coastal current the major coastal circulation feature of the northern gulf of alaska can be distinguished from oceanic waters on the basis of its lower salinity at least as far west as kodiak island this study adds significantly to the small amount of oxygen isotopic information available for the waters of this region the isotopic results suggest that in late summer glacial meltwater may provide a substantial portion of the total freshwater runoff into the acc and that the acc does extend as far to the west as unimak pass

evidence for a persistent magma reservoir with large melt content beneath an apparently extinct volcano abstract most active volcanoes display eruption frequencies of 10 1000s years but a class of volcanic systems has extremely long repose time 10 s kyr and are deemed as extinct yet some reawaken posing a particular threat because little is known about the way they endure and stir back to life reawakening primarily depends on the nature of the subvolcanic magma reservoir especially the presence and distribution of melt here we integrate petrology thermobarometry thermomechanical models geophysics and in situ electrical conductivity measurements to show that the magma storage beneath ciomadul a seemingly inactive volcano in eastern central europe that last erupted 30 ka may still hold 20 to 58 km3 of water rich silicic melt constituting up to 20 58 in parts of the upper crustal crystal mush body such a melt volume exceeds the volume of erupted lava over the entire history of the volcano this illustrates the important longevity of a magmatic reservoir at temperature above the solidus which implies that there is still a potential for rapid mush rejuvenation that a seemingly dead volcano like ciomadul is actually capable of erupting in the future calls for renewed attention to inactive volcanoes worldwide and perhaps for a redefinition of their activity inactivity status

electrical conductivity of dry and hydrous naalsi 3 o 8 glasses and liquids at high pressures we report the first study of electrical conductivities of silicate melts at very high pressures up to 10 gpa and temperatures up to 2 173 k impedance spectroscopy was applied to dry and hydrous albite naalsi3o8 glasses and liquids with 0 02 5 7 wt h2o at 473 1 773 k and 0 9 1 8 gpa in a piston cylinder apparatus using a coaxial cylindrical setup measurements were also taken at 473 2 173 k and 6 10 gpa in two multianvil presses using simple plate geometry the electrical conductivity of albite melts is found to increase with temperature and water content but to decrease with pressure however at 6 gpa conductivity increases rapidly with temperature above 1 773 k so that at temperatures beyond 2 200 k conductivity may actually increase with pressure moreover the effect of water in enhancing conductivity appears to be more pronounced at 6 gpa than at 1 8 gpa these observations suggest that smaller fractions of partial melt than previously assumed may be sufficient to explain anomalously high conductivities such as in the asthenosphere for dry melt at 1 8 gpa the activation energy at t 1 073 k is higher than that at t 1 073 k and the inflection point coincides with the rheological glass transition upon heating at 6 10 gpa dry albite glass often shows a conductivity depression starting from 1 173 k due to crystallization followed by rapid conductivity enhancement when temperature approaches the albite liquidus for hydrous melts at 0 9 1 8 gpa the activation energies for conductivity at 1 373 k are lower than those at 973 k with a complex transition pattern in between electrical conductivity and previously reported na diffusivity in albite melt are consistent with the nernst einstein relation suggesting the dominance of na transport for electrical conduction in albite melts

the influence of partial melting on the electrical behavior of crustal rocks laboratory examinations model calculations and geological interpretations abstract the complex electrical impedance of a granulite has been determined over the frequency range 10 1 to 105 hz at temperatures between 600 and 1200 c at normal pressure and different oxygen fugacities in the stability field of magnetite the impedance spectroscopic is measurements are compared with the results of melting experiments performed under the same experimental conditions and with the same rock sample as the is measurements a strong increase in the electrical conductivity of about one and a half orders of magnitude is observed during partial melting this increase is explained by the formation of an interconnected network of melt the wetting of the grain surfaces starts slightly above the solidus however complete interconnection of the network is detected at higher temperatures 1070 c and hence higher melt fractions 10 vol this is explained by deviations from the equilibrium fabric of the partially molten sample with a modified brick layer model mblm the electrical conductivity is calculated as a function of the observed melt portion this model is valid if the melt forms a complete interconnected network at the grain boundaries the electrical conductivity at different temperatures was modeled using the results of the melting experiments and the mblm melt 10 s m with ea melt 1 ev solid 0 01 s m with ea solid 1 4 ev the results of the laboratory examinations were applied to different crustal regions where partially molten rocks are assumed to exist the same mblm is used to estimate the melt fraction that is necessary to explain the high conducting zones hczs observed in the crust beneath the andean orogen the pyrenees and the tibetan plateau at least 14 4 and 4 vol of interconnected melt are necessary to produce the observed conductivity respectively the actual amount of melt should be higher due to perturbation problems and melt trapped in pockets

geophysical characterization of the northwest geysers geothermal field california abstract the clear lake volcanic field in northern california is the youngest and northern most part of a long lived volcanic system that has produced recent 10 ka eruptions adjacent to the clear lake volcanic field is the world s largest energy producing geothermal field the geysers the hottest part of the geyser s geothermal field is in the northwest where temperatures reach 400 c at 3 km depth low permeability high thermal gradients and low steam saturation prescribed development of an enhanced geothermal system egs in the northwest geysers to increase energy producing capacity though the northwest geysers is known to be the hottest part of the field geophysical methods have failed to adequately image any inferred heat source this project aims to image the heat source of the northwest geysers using newly collected gravity and magnetotelluric mt measurements gravity data were jointly modeled with existing magnetic data along a two dimensional profile aligned with an existing geologic cross section the key feature of the potential field model is a low density low susceptibility body at 5 km depth bmsl under the egs mt data were modeled in three dimensions to characterize subsurface resistivity structure where the upper 3 km of the resistivity model agrees well with existing data lithologic and steam saturation are estimated from modeled resistivity values using existing geophysical data below 3 km depth bmsl the resistivity model images a possible young intrusion under the egs a possible zone of partial melt

electrical conductivity of hydrous andesitic melts pertinent to subduction zones andesitic magmatism and rocks are widespread at convergent plate boundaries electrically conductive bodies beneath subduction zone arc volcanoes such as the uturuncu volcano bolivia may correspond to active reservoirs of h2o bearing andesitic magma laboratory measurements of electrical conductivity of hydrous andesitic melts are required to constrain the physicochemical conditions of these magma reservoirs in combination with magnetotelluric data this experimental study investigates electrical conductivity of andesitic melts with 0 01 5 9 wt of h2o at 1164 1573 k and 0 5 1 0 gpa in a piston cylinder apparatus using sweeping frequency impedance spectroscopy electrical conductivity of andesitic melt increases with increasing temperature and h2o concentration but decreases with pressure across the investigated range of h2o concentration electrical conductivity varies by 1 2 2 4 log units indicating stronger influence of h2o for andesitic melt than for rhyolitic and dacitic melts using the nernst einstein equation the principal charge carrier is inferred to be na in anhydrous melt but divalent cations in hydrous andesitic melts the experimental data are regressed into a general electrical conductivity model for andesitic melt accounting for the pressure temperature h2o dependences altogether modeling results show that the conductive layer at 20 km depths beneath the surface of the uturuncu volcano could be interpreted by the presence of less than 20 vol of h2o rich andesitic melt with 6 9 wt h2o

partial melting below the magmatic arc in the central andes deduced from geoelectromagnetic field experiments and laboratory data abstract magnetotelluric and geomagnetic deep soudings in northern chile revealed a pronounced high conductivity zone hcz below the western cordillera which constitutes the present magmatic arc with active volcanism of the south american continental margin conductivities in the range of 1 s m are observed the anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km are interpreted in terms of partial melting other geophysical observations such as a zone of low seismic velocities lvz at similar depths high heat flow values 100 mw m2 and a pronounced negative anomaly in the residual gravity field are also considered impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities at sub solidus temperatures electrical behavior is described by defect electrons with an activation energy of 1 34 ev and a conductivity of 2 5 ms m at 900 c model calculations using a modified brick layer model mbl were compared with experimental observations a good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 s m at 1250 c assuming an activation energy of 1 ev the same mbl model is used to calculate melt proportions beneath the western cordillera between 14 and 27 vol of interconnected melt are necessary to explain the observed hcz the stability of the melt rich crust is explained by a dynamic melting crystallisation behavior during crustal anatexis and by magma filled dikes

spatio temporal evolution of temperature and fluid flow through a new thermo lithological boundary the case of a pit crater of karthala volcano comoros archipelago refilled on january 13th 2007 by a lava flow on january 13 th 2007 the effusive eruption of karthala volcano in the comoros islands emitted a lava flow which has been perfectly constrained inside the cylindrical shape of the choungou chagnoumeni pit crater 225m in diameter and with a thickness of about 7m the good knowledge of the geometry of this lava flow allowed modeling its cooling with the rheolef code with a good accuracy the results of the thermal modeling were compared with other measurements performed on the field 1 a high resolution electrical resistivity tomography profile crossing the boundary of the crater and 2 the thermal monitoring of temperature with eleven sensors spaced 1 meter apart at 30 cm depth perpendicularly to the boundary of the crater thanks to this multidisciplinary approach it has been possible to display that in the first order the resistivity distribution in the soil 2 years and 5 months after the eruption is directly influenced by the residual temperature and not by the structural boundary during the first three years of cooling an inversion in the shallow lateral temperature gradient was observed in temperature data set collected at 30 cm depth

isotopic evidence for shifts in atmospheric circulation patterns during the late quaternary in mid north america wyoming is now at the eastern margin of westerlies originating in the pacific but in the pleistocene appears to have received moisture from elsewhere possibly the gulf of mexico oxygen isotope ratios of pedogenic carbonate in postglacial terraces correspond to ratios in equilibrium with present meteoric waters which show a strong relation to precipitation seasonality and storm sources in contrast the oxygen isotope ratios of all pre holocene soils are significantly more positive even though the carbon isotope composition of coexisting organic matter suggests that the carbonate formed in temperatures cooler than today the oxygen isotope ratios of paleowaters in mid north america appear to be more useful for identifying past storm sources than for estimating paleotemperatures

electrical conductivity of partial molten carbonate peridotite abstract in order to investigate the effect of carbonate content in partial melt on bulk conductivity under high pressure electrical conductivity measurements were performed on carbonate melt bearing peridotites using a kawai type multi anvil apparatus the starting materials were composed of spinel lherzolite klb1 with small amounts of dolomite 1 and 3 wt to obtain various melt fractions annealing experiments were performed at different temperatures above 1400 k at 3 gpa at low temperatures 1500 k the conductivity was distinctly higher than that of carbonate free peridotite and close to that of the carbonatite melt bearing olivine aggregates although the sample conductivity increased with increasing temperature the rate at which the conductivity increases was small and the conductivity approached that of silicate melt bearing peridotite co 2 concentration in the partial melt decreased with increasing annealing temperature thus the small increase of the conductivity with annealing temperature is attributed to a decrease of the melt conductivity due to a decrease in carbonate content in the partial melt as the carbonate concentration in the melt decreases the estimated melt conductivity approaches that of the basaltic melt therefore conductivity enhancement by the carbonate bearing melt is very effective at temperature just above that of the carbonate peridotite solidus

glacial anticyclone recorded in palouse loess of northwestern united states climate simulations for the last glacial maximum using general circulation models typically show a glacial anticyclone that produced an easterly wind anomaly south of the north american ice sheets evidence of this phenomenon has not been found in eolian sedimentary deposits that record surface wind patterns luminescence ages of loess and accompanying paleoecologic records from opal phytoliths and paleosol units across the columbia plateau washington state united states document a decrease up to fivefold in regional dust production and accumulation from 35 ka to 15 ka this interval corresponds to simulations indicating that a glacial anticyclone weakened prevailing south southwesterly winds that have shaped the eolian landscape of the columbia plateau since at least 75 ka at the same time that the glacial anticyclone suppressed dust production and accumulation enhanced cold and dry conditions resulted in soil formation dominated by intense bioturbation in periglacial steppe accompanied by shallow calcium carbonate precipitation this is the first evidence from eolian deposits in north america verifying that the glacial anticyclone altered surface wind patterns and affected eolian systems during the last glacial maximum

the channeled scabland a retrospective the channeled scabland of east central washington in the united states is a complex of anastomosing rock cut fluvial channels cataracts loess islands rock basins broad gravel deposits and immense gravel bars in the 1920s j harlen bretz demonstrated that the channeled scabland formed by cataclysmic erosion and deposition from pleistocene megaflooding derived from the margins of the cordilleran ice sheet particularly glacial lake missoula in western montana and northern idaho studies of this region and the high energy flood processes that generated it are stimulating a discoveries of similar megaflood related landscapes around the world and on mars b enhanced understanding of the processes involved in the fluvial erosion of bedrock and c the use of paleoflood indicators for understanding the magnitudes and frequency of flooding

evidence for present day leucogranite pluton growth in tibet several geophysical surveys carried out in southern tibet have revealed the occurrences of bright spots of high electrical conductivity at 15 20 km depth corresponding to zones of seismic attenuation and correlated to high heat flow at the surface such zones have been variably interpreted as intruding magmas partially melted middle crust or concentrations of free fluids here we show that experimentally derived electrical conductivities of hydrous granite melts under pressure temperature h2o conditions appropriate for crust derived magmas are in perfect agreement with those inferred from magnetotelluric data strongly suggesting that the tibetan bright spots image present day pluton assembly in the middle crust in a manner analogous to what happened during the miocene to form the leucogranite plutons now exposed southward in the high himalaya range

prediction of silicate melt viscosity from electrical conductivity a model and its geophysical implications 1 our knowledge of magma dynamics would be improved if geophysical data could be used to infer rheological constraints in melt bearing zones geophysical images of the earth s interior provide frozen snapshots of a dynamical system however knowledge of a rheological parameter such as viscosity would constrain the time dependent dynamics of melt bearing zones we propose a model that relates melt viscosity to electrical conductivity for naturally occurring melt compositions including h2o and temperature based on laboratory measurements of melt conductivity and viscosity our model provides a rheological dimension to the interpretation of electromagnetic anomalies caused by melt and partially molten rocks melt fraction 0 7