three dimensional electrical resistivity image of magma beneath an active continental rift taupo volcanic zone new zealand 1 magmatic activity in regions of continental extension may result in huge 400 km 3 explosive eruptions of viscous gas rich silicic magma geochemical and geological data suggest that the large volumes of magma erupted are produced by extracting interstitial liquid from a long lived mush zone a mixture of solid crystals and liquid melt that accumulates in liquid dominated lenses at the top of a much thicker region of lower melt fraction mush such lenses will be highly electrically conductive compared with normal mid crustal rocks here we use results of 220 magnetotelluric mt soundings to construct a 3 d electrical resistivity image of the northern silicic part of new zealand s taupo volcanic zone a young continental rift associated with very high heat flow and intense silicic volcanism the electrical resistivity image shows a plume like structure of high conductivity interpreted to be a zone of interconnected melt rising from depths 35 km beneath the axis of extension

magnetotelluric imaging of upper crustal convection plumes beneath the taupo volcanic zone new zealand 1 broadband mt magnetotelluric data were recorded that form an array of measurements at the south eastern margin of the tvz taupo volcanic zone in the central north island of new zealand these array data are used to investigate mechanisms by which the tvz s extraordinarily high heat flux is transported to the surface taken together with seismological data these mt data show compelling evidence that support a model of hydrothermal convection within the brittle upper 6 7 km part of the crust both 2 d and 3 d inversion models of these mt data show vertical low resistivity zones that connect surface geothermal fields to an inferred magmatic heat source that lies below the brittle ductile transition

three dimensional inversion of large scale earthscope magnetotelluric data based on the integral equation method geoelectrical imaging of the yellowstone conductive mantle plume 1 interpretation of the earthscope mt magnetotelluric data requires the development of a large scale inversion method which can address two common problems of 3d mt inversion computational time and memory requirements we have developed an efficient method of 3d mt inversion based on an ie integral equation formulation of the mt forward modeling problem and a receiver footprint approach implemented as a massively parallel algorithm this method is applied to the mt data collected in the western united states as a part of the earthscope project as a result we present one of the first 3d geoelectrical images of the upper mantle beneath yellowstone revealed by this large scale 3d inversion of the earthscope mt data these images show a highly conductive body associated with the tomographically imaged mantle plume like layer of hot material rising from the upper mantle toward the yellowstone volcano the conductive body identified in these images is west dipping in a similar way to a p wave low velocity body

compartmentalization of the coso east flank geothermal field imaged by 3 d full tensor mt inversion author s lindsey nj kaven jo davatzes n newman ga abstract the authors 2016 published by oxford university press on behalf of the royal astronomical society previous magnetotelluric mt studies of the high temperature coso geothermal system in california identified a subvertical feature of lowresistivity 2 5ohmm and appreciable lateral extent g1 km in the producing zone of the east flank field however these models could not reproduce gross 3 d effects in the recorded data we perform 3 d full tensor inversion and retrieve a resistivity model that out performs previous 2 d and 3 d off diagonal models in terms of its fit to the complete 3 d mt data set as well as the degree of modelling bias inclusion of secondary zxx and zyy data components leads to a robust east dip 60 to the previously identified conductive east flank reservoir feature which correlates strongly with recently mapped surface faults downhole well temperatures 3 d seismic reflection data and local microseismicity we perform synthetic forward modelling to test the best fit dip of this conductor using the response at a nearby mt station we interpret the dipping conductor as a fractured and fluidized compartment which is structurally controlled by an unmapped blind east flank fault zone

40ar 39ar dating of supergene mn oxides from the zunyi mn deposit guizhou plateau sw china implications for chemical weathering and paleoclimatic evolution since the late miocene 40ar 39ar laser incremental heating analyses of potassium bearing mn oxides from a weathering profile at the zunyi manganese deposit northern guizhou plateau sw china were applied to constrain the timing and rate of chemical weathering in critical zone and to provide insights into paleoclimatic conditions under which the weathering profile and associated mn oxides formed eighteen mn oxide grains from nine samples at three sites in saprolite zone of a single weathering profile yield well defined plateau or pseudo plateau ages ranging from 13 1 0 3 to 0 1 0 4 ma 2 the mn oxide ages are progressively younger toward lower elevations 13 1 0 3 to 11 0 1 8 ma at 951 m 3 69 0 07 to 2 16 0 16 ma at 922 m 1 3 0 2 ma at 914 m yielding an average weathering front propagation rate at 3 3 m ma for the past 13 1 ma the calculated weathering front propagation rate corresponds to a chemical denudation rate of 4 6 0 2 t km2 yr which is consistent with values deduced from mass balance calculation for suspended and dissolved loads of river discharge across the guizhou plateau the mn oxide 40ar 39ar ages suggest that humid climate conducive to intense weathering has prevailed over the guizhou plateau at least since the late miocene the climatic conditions inferred from the weathering geochronology are consistent with the continuous sedimentary and paleontological records from many cenozoic basins over the guizhou plateau confirming that supergene mn oxides can serve as a paleoclimate proxy for the past climatic condition the present 40ar 39ar dating results when combined with previous geochronological data of weathering profiles in the yunnan guizhou plateau and coastal areas of south china document progressively intensive erosion toward areas proximal to the eastern margin of the tibet plateau

petrological differentiation patterns and geomorphic distribution of ferricretes in central africa geomorphic distribution and petrological differentiation patterns of ferricretes widespread on landsurfaces were studied in the dembia zemio area southeastern central african republic four types of ferricretes are distributed on high plateaux hillslopes and low plateaux the main contrast corresponds to the differentiation between ferricretes of high plateaux rich in poorly hydrated minerals hematite and kaolinite and those covering hillslopes and low plateaux richer in hydrated minerals goethite and gibbsite a principal component analysis of geochemical and mineralogical data was used to characterize the main differentiation patterns of ferricretes in relation with their petrographic facies and the geomorphic features of the landscape the distribution of secondary minerals and their relationships to trace elements within and between the ferricrete types reflects differences in weathering and erosion processes our results document that the petrological patterns and the distribution of ferricretes are related to geomorphic features depending on hydroclimatic variations that govern the landscape evolution of many tropical shields

contemporary rates of chemical denudation and atmospheric co2 sequestration in glacier basins an arctic perspective this paper presents new estimates of solute fluxes from five high arctic glacier basins in svalbard these estimates are combined with data from two other glacier basins to assess the effectiveness of chemical denudation on svalbard and to estimate rates of temporary or transient co2 drawdown we use a solute provenance model to partition solutes into marine aerosol atmospheric and crustal components and to estimate their annual fluxes crustally derived solute fluxes are equivalent to a mean chemical denudation rate of 350 meq m 2 a 1 for svalbard range 160 560 meq m 2 a 1 which lies within the global range of 94 4200 meq m 2 a 1 for 21 glacier basins in the northern hemisphere and is close to the continental average of 390 meq m 2 a 1 specific annual discharge is the most significant control upon chemical denudation in the glacierized basins and basin lithology is an important secondary control with carbonate rich and basaltic lithologies currently showing the greatest chemical denudation rates estimates of transient co2 drawdown are also directly associated with specific annual discharge and rock type on svalbard transient co2 drawdown lies in the range 110 3000 kg c km 2 a 1 whilst the range is 110 13000 kg c km 2 a 1 for the northern hemisphere glacial data set transient co2 drawdown is therefore usually low in the svalbard basins unless carbonate or basalt rocks are abundant the analysis shows that a large area of uncertainty in the transient co2 drawdown estimates exists due to the non stoichiometric release of solute during silicate hydrolysis silicate hydrolysis is particularly non stoichiometric in basins where the extent of glacierization is high which is most probably an artefact of high flushing rates through ice marginal and subglacial environments where k feldspars are undergoing mechanical comminution copyright 2000 john wiley sons ltd

electrical image of passive mantle upwelling beneath the northern east pacific rise sea floor magnetotelluric soundings along a portion of the east pacific rise reveal a symmetric high conductivity triangular region at depths of 20 90 kilometres that is consistent with partial melting of passively upwelling mantle the east pacific rise is a fast spreading mid ocean ridge where sea floor volcanoes and hydrothermal vents result from a robust mantle melt supply to explore how this magma is supplied to the surface kerry key and colleagues use sea floor magnetotelluric soundings to image mantle electrical structure to a depth of about 160 km along the northern east pacific rise the images reveal a symmetrical high conductivity anomaly beneath the ridge consistent with partial melting of passively upwelling mantle they explain a deeper conductor observed east of the ridge by asymmetric upwelling due to viscous coupling across two nearby transform faults these findings suggest that plate driven passive upwelling dominates this ridge segment while dynamic forces are negligible melt generated by mantle upwelling is fundamental to the production of new oceanic crust at mid ocean ridges yet the forces controlling this process are debated1 2 passive flow models predict symmetric upwelling due to viscous drag from the diverging tectonic plates but have been challenged by geophysical observations of asymmetric upwelling3 4 5 that suggest anomalous mantle pressure and temperature gradients2 6 7 and by observations of concentrated upwelling centres8 consistent with active models where buoyancy forces give rise to focused convective flow2 here we use sea floor magnetotelluric soundings at the fast spreading northern east pacific rise to image mantle electrical structure to a depth of about 160 kilometres our data reveal a symmetric high conductivity region at depths of 20 90 kilometres that is consistent with partial melting of passively upwelling mantle9 10 11 the triangular region of conductive partial melt matches passive flow predictions suggesting that melt focusing to the ridge occurs in the porous melting region rather than along the shallower base of the thermal lithosphere a deeper conductor observed east of the ridge at a depth of more than 100 kilometres is explained by asymmetric upwelling due to viscous coupling across two nearby transform faults significant electrical anisotropy occurs only in the shallowest mantle east of the ridge axis where high vertical conductivity at depths of 10 20 kilometres indicates localized porous conduits this suggests that a coincident seismic velocity anomaly12 is evidence of shallow magma transport channels13 14 rather than deeper off axis upwelling we interpret the mantle electrical structure as evidence that plate driven passive upwelling dominates this ridge segment with dynamic forces being negligible

transtensional model for the sierra nevada frontal fault system eastern california active strike slip and normal faults along the eastern margin of the sierra nevada primarily accommodate northwestward translation of the sierra nevada central valley i e sierran microplate with respect to stable north america strike slip faults bordering the eastern sierran microplate are subparallel to small circles about the sierra nevada north american euler pole normal faults of the sierra nevada frontal fault system strike 45 clockwise of the small circle trajectories and exhibit well defined left stepping en echelon patterns consistent with formation in dextral transcurrent regime major graben bordering the northeastern sierran microplate are located in regions where the locus of range front deformation steps abruptly eastward in a releasing geometry relative to sierra nevada north american motion crustal shortening occurs at the northern end of the sierran microplate where a component of northwest dextral shear steps westward in a left restraining geometry across the sierran crest to the sacramento valley kinematic inversions of earthquake focal mechanisms from the walker lane belt bordering the eastern sierra nevada indicate that seismogenic deformation primarily is characterized by horizontal shearing and oblique crustal thinning directions of macroscopic dextral shear inferred from the inversions are subparallel to the trajectories of small circles about the sierra nevada north american euler pole normal faulting along most of the eastern sierran range front thus appears to primarily accommodate microplate translation rather than sierran uplift or regional basin and range extension

chemical composition of snow and ice from chhota shigri glacier central himalaya abstract a systematic study of the chemical composition of snow firn from a shallow core surface ice and of meltwater samples collected from the chhota shigri glacier central himalaya is reported here the average concentrations of na and cl in snow are 402 and 753 g l 1 respectively whereas those in old surface ice are relatively low 185 g l 1 and 375 g l 1 respectively the average cl na weight ratio in these samples is 1 9 quite similar to that in the sea salts suggesting that both cl and na are predominantly of marine origin the concentrations of other major ions such as k mg ca no3 and so4 in snow and ice samples are generally less than 1 mg l 1 the major ion concentrations when normalized to na indicate that the abundances of k and ca are dominated by terrestrial sources whereas mg appears to have both marine and terrestrial sources based on the seasonal variations of na and cl concentrations observed in the ice core samples an approximate accumulation rate at the core site has been estimated to be 1 6 m year 1 640 kg m 2 year 1

wide band magnetotelluric measurements across the taupo volcanic zone new zealand preliminary results the taupo volcanic zone tvz of new zealand is characterised by intensive geothermal activity and frequent rhyolitic volcanism sixteen wide band 0 01 1 800 s magnetotelluric soundings were measured along a 110 km long profile approximately perpendicular to the strike of the tvz a model obtained from 2d inversion of the soundings shows two near surface regions of high conductance which correspond to low density volcaniclastic sediments up to 3 km thick which infill a sequence of collapse calderas at deeper levels approximately 5 10 km a resistive layer underlies the entire tvz modelling shows other conductive zones occur beneath the tvz with the shallowest lying below the central part at a depth of 10 15 km given the high heat flux and volcanic history of the tvz the high conductivity at depth may indicate the presence of connected melt at greater depth 20 30 km the upper mantle beneath the tvz appears to be anomalously conductive consistent with observed high seismic attenuation

cryptomelane a tool to determine the age and the physical chemical regime of a plio pleistocene weathering zone in a granitic terrain hagendorf se germany abstract cryptomelane a k mn bearing oxide is widespread in the lowermost part of the weathering supergene alteration zone of the hagendorf pegmatite se germany this supergene mn mineral has been used in the present study for k ar dating of weathering and paleohydraulic processes as young as plio pleistocene different cryptomelane age data found in the same weathering and paleohydraulic system resulted from continuous hydraulic processes in the weathering system rather than from a stepwise uplift that shaped the landscape in the granitic area of the ne bavarian basement an almost pure cryptomelane mineralization is indicative of the basal zone of supergene alteration because a complete separation of mn from fe can favorably be achieved only in the infiltration zone under slightly reducing conditions or in the percolation zone under slightly oxidizing conditions cryptomelane is precipitated under slightly oxidizing conditions from alkaline solutions in the percolation zone and cast as a marker for fluctuating paleo water levels manganese oxides uranyl phosphates and silicates were found at different depths in the weathering zone of this granitic terrain they were dated by different dating methods k ar and u pb and yielded the same age of formation consequently the 4 ma signal of both groups of supergene minerals has geomorphological and paleohydraulic connotation and is related to a unique geomorphological process the pliocene peneplanation gradual hydraulic adjustments gave rise to the younger pleistocene age found at shallow depth in the weathering zone

weathering history and landscape evolution of western ghats india from 40 ar 39 ar dating of supergene k mn oxides laterites preserved on both sides of the western ghats escarpment of peninsular india have formed by long term lateritic weathering essentially after india seychelles continental break up following deccan traps emplacement c 63 ma ago supergene manganese ores of the western ghats were formed on late archean manganese protores among mn oxides composing the ores cryptomelane k rich mn oxide was characterized and dated by 40ar 39ar geochronology measured ages complement those previously obtained in other south indian manganese ores from the hinterland plateau bonnet et al 2016 and further document three major weathering periods c 53 44 ma c 39 22 ma and c 14 10 ma the later being documented for the first time in india these periods coincide with global paleoclimatic proxies and date the lateritic weathering of three successive paleolandscapes of the western ghats that evolved under slow denudation c 8 m myr over the last 44 myr and were mostly incised during the neogene 22 ma that indicates the western ghats are a relict of a south indian plateau preserved at the headwaters of very long east flowing river systems and above the western ghats escarpment topography and denudation history of this landscape do not require neogene tilt of the peninsula as recently proposed

organic rich and manganese sedimentation during the cenomanian turonian boundary event in the outer carpathian basins a new record from the skole nappe poland abstract upper cenomanian lower turonian strata in the splawa section of the skole nappe outer carpathians poland provide a detailed palaeoenvironmental record of the oceanic anoxic event oae 2 from a deep water basin below the ccd at the northern margin of the western tethys the oae 2 succession consists of black parallel laminated radiolarian shales enriched in organic matter of mainly marine origin toc up to 8 organic carbon isotope variations together with microfossil datum events allowed to correlate the studied organic rich facies with other black shale successions along the outer carpathians and other areas of the western tethys the mean values of sedimentation rate 3 5 5 5 mm kyr and organic matter accumulation rate 0 5 g m2 year calculated for the oae 2 interval in the outer carpathian basins are comparable to the values from other deep water marginal basins in the western tethys the average redox sensitive trace metal contents and their enrichment factors indicate water column anoxia in the outer carpathian basins intensified during the interval of the largest shift in 13c values the periods of bottom anoxia were interrupted by intervals of variable eh conditions with sedimentation of hemipelagic green shales including poor agglutinated foraminiferal assemblages an increase in productivity of organic and siliceous plankton during the oae 2 is documented based on increased values of silica and barium enrichments this closely corresponds to an interval of the largest positive stable carbon isotope excursion related to the beginning of the largest sea level rise linked with intensive coastal upwelling extremely low sedimentation rate or even hiatuses and the increase in deep water circulation causing basin oxygenation resulted in precipitation of two ferromanganese layers their formation was related to changes in hydrodynamic conditions at the basin floor expressed by precipitation of fe mn carbonates some chemical indices seem to suggest that the primary metal enrichments could be related to hydrothermal activity however the concentrations of some elements show also on hydrogenic to diagenetic influences in these metalliferous sediments the precipitation of these fe mn sediments was probably synchronous with the manganese enrichment across the c t transition recorded in other western tethyan basins and in the adjacent epicontinental seas

regional conductivity structure of cascadia preliminary results from 3d inversion of usarray transportable array magnetotelluric data 1 in conjunction with the usarray component of earthscope long period magnetotelluric mt data are being acquired in a series of arrays across the continental us initial deployments in 2006 and 2007 acquired data 10 10 000 s at 110 sites covering the us pacific northwest distributed with the same nominal spacing as the usarray seismic transportable array 75 km the most striking and robust features revealed by initial three dimensional inversion of this dataset are extensive areas of high conductivity in the lower crust beneath all of southeastern oregon and beneath the cascade mountains contrasting with very resistive crust in siletzia and the columbia embayment significant variations in upper mantle conductivity are also revealed by the inversions with the most conductive mantle beneath the washington backarc and the most resistive corresponding to subducting oceanic mantle citation patro p k and g d egbert 2008 regional conductivity structure of cascadia preliminary results from 3d inversion of usarray transportable array magnetotelluric data geophys res lett 35 l20311 doi 10 1029 2008gl035326

mineralogical characterization of the nkamouna co mn laterite ore southeast cameroon the nkamouna property is an oxide laterite deposit developed on serpentinized peridotite in southeast cameroon it is enriched in co and mn has sub economic ni grades and will be mined primarily for co the ore zone is ca 10 m thick and comprises the lower breccia 3 m thick and ferralite 7 8 m thick units sandwiched between an 8 m thick ferricrete overburden and a barren hydrated mg silicate saprolite the ore mineral assemblage includes mn oxyhydroxides magnetite maghemite ferritchromite goethite hematite kaolinite and gibbsite lithiophorite is the most common mn mineral and is the main host of co mn and a significant proportion of ni it occurs as coatings in pores and on other mineral grains and as concretions and impregnations in the matrix it is invariably associated with gibbsite in the lower breccia and with magnetite and ferritchromite in the ferralite although ore in the lower breccia is volumetrically less important than the ferralite it has the highest grade and co ni ratio the lithiophorite in the ore zone is authigenic and its formation was enhanced by influx of al3 from the overlying ferricrete magnetite and ferritchromite in the ferralite are relicts and contributed to mineralization by enhancing the permeability of the ferralite and providing substrates for the precipitation of the mn oxyhydroxides the structure and mode of occurrence of the lithiophorite makes nkamouna ore amenable to physical beneficiation producing a concentrate with co grades 2 3 4 5 times higher than the run of mine ore

geochemical balance of lateritization processes and climatic signatures in weathering profiles overlain by ferricretes in central africa a simple geochemical balance of lateritization processes governing the development of several tens of meters of weathering profiles overlain by ferricretes is estimated on the basis of detailed mineralogical and geochemical data the lateritic weathering mantle of the haut mbomou area in central africa is composed of different weathering layers described from the base to the top of vertical profiles as a saprolite a mottled clay layer a soft nodular layer a soft ferricrete and a ferricrete in which kaolinite gibbsite goethite and hematite occur in various quantities incongruent dissolution of kaolinite leads to the formation of gibbsite in the upper saprolite whereas the hematite does not clearly replace the kaolinite according to an epigene process in the upper ferruginous layers of the profiles instead that kaolinite is also transformed into gibbsite according to an incongruent dissolution under hydrated and reducing conditions induced by a relatively humid climatic pattern the respective relations of the silica iron and aluminum balances and the al substitution rate of the hematite on the one hand and of rhg rhg 100 hematite hematite goethite and the kaolinite on the other hand to the consumption or the release of protons h permit differentiation of aggrading ferruginization and degradation processes operating in the different lateritic weathering profiles the al substitution rate of the fe oxyhydroxides varies according to the nature of lateritization processes e g saprolitic weathering and aggrading ferruginization vs degradation the observations and results indicate that the ferruginization process of the weathering materials of parent rocks is not a simple ongoing process as often thought this suggests that the actual lateritic weathering mantle of the haut mbomou area may result from different stages of weathering and erosion during climatic changes

contemporary ground deformation in the taupo rift and okataina volcanic centre from 1998 to 2011 measured using gps the taupo volcanic zone tvz is one of the world s most productive regions of rhyolitic volcanism and contains the highly active okataina volcanic centre ovc within the tvz intra arc extension is expressed as normal faulting within a zone known as the taupo rift the ovc is located within a complex part of the rift where volcanism and deformation is considered influenced by rift structure and kinematics there has been significant research on the structural volcanic and geophysical properties of the rift and ovc but less focus on deformation using geodetic data the limited studies that have utilized geodetic data do not clearly resolve the distribution of deformation and strain rates within the rift and ovc this is essential to ensure that deformation signals from volcanic processes at the ovc are correctly identified and distinguished from those related to regional tectonic or local hydrothermal processes within the rift in this paper we present a picture of contemporary deformation at the ovc and within the surrounding rift in detail using existing and new gps campaign and continuous gps cgps data collected between 1998 and 2011 the results show a highly heterogeneous deformation and strain rate field both extension and shortening through the study area partitioned into different parts of the rift our results agree well with earlier geodetic studies as well as identify new features but some deformation patterns conflict with long term geological observations in the ovc we observe a locally rotated horizontal velocity field significant vertical deformation and variable strain rates across the caldera in the tarawera rift we identify elevated extension and shear rates which may have significant implications for volcanism there a shortening pattern is identified through the central rift which is unexpected in an intra arc rifting environment we attempt to explain the source s of shortening and extension and discuss their implications for geodetic monitoring efforts in the ovc

goethite u th he geochronology and precipitation mechanisms during weathering of basalts u th he geochronology of 33 goethite grains from in situ ferruginous duricrusts overlying the parana flood basalt in the guarapuava region parana brazil reveals ages ranging from 3 6 0 4 to 0 4 0 1 ma thirty one grains from detrital fragments of ferruginous duricrust yield ages in the 6 2 0 6 to 0 7 0 1 ma range the results show that goethites from detrital blocks are generally older than those from the in situ ferruginous layers and that all the goethites from the guarapuava sites are relatively young 6 2 0 6 ma goethites show variable u and th contents depending on the mode of precipitation goethite that forms by progressive ferrugization and pseudomorphic replacement of basalts are rich in th u poor contain various mineral inclusions e g kaolinite rutile ilmenite and display a distinct checkered appearance colloform goethite precipitated within cavities and desiccation cracks contain higher u show low th u values and are free of mineral contaminants suggesting direct precipitation from solution the geochronological results reveal a dynamic evolution for the soils overlying the parana basalt where recurrent duricrust formation and dismantling lead to the evolution of regional colluvial soil profiles while simultaneously preventing the evolution of chemically stratified lateritic weathering profiles the modes of formation and u th and he contents of goethite at guarapuava provide useful constraints on the expected compositions and modes of formation of goethite formed on basaltic soils on mars

a numerical evaluation of electromagnetic methods in geothermal exploration the size and low resistivity of the clay cap associated with a geothermal system create a target well suited for electromagnetic em methods and also make electrical detection of the underlying geothermal reservoir a challenge using 3 d numerical models we evaluate four em techniques for use in geothermal exploration magnetotellurics mt controlled source audio magnetotellurics csamt long offset time domain em lotem and short offset time domain em tem our results show that all of these techniques can delineate the clay cap but none can be said to unequivocally detect the reservoir we do find however that the em anomaly from a deep conductive reservoir overlain by a larger more conductive clay cap is caused by the presence of the electric charge at conductivity boundaries rather than electromagnetic induction this means that for detection of the reservoir methods such as mt which rely on electric field measurements are superior to those where only the magnetic field is measured t

application of the 3d magnetotelluric inversion code in a geologically complex area the wsinv3dmt code makes the implementation of 3d inversion of magnetotelluric data feasible using a single pc audio magnetotelluric data were collected along two profiles in a cu ni mining area in xinjiang china where the apparent resistivity and phase curves the phase tensors and the magnetic induction vectors indicate a complex 3d conductivity structure 3d inversions were carried out to reveal the electrical structure of the area the final 3d model is selected from the inversion results using different initial lagrange values and steps the relatively low root mean square rms misfit and model norm indicate a reliable electrical model the final model includes four types of low resistivity areas the first ones coincide with the known location of an orebody and further forward modelling indicates that they are not in full connectivity to form a low resistivity zone the second ones are not controlled by magnetotelluric sites and embody little information of the observed data they are considered as tedious structures the third one is near to the regional kangguer fault and should be treated carefully considering the effect of the fault the last ones are isolated and existing at a limited level as the first ones they should be paid more attention to

seismotectonics of the coso range indian wells valley region california transtensional deformation along the southeastern margin of the sierran microplate

a joint geophysical analysis of the coso geothermal field south eastern california abstract three dimensional density models derived from gravity data and two dimensional resistivity models derived from magnetotelluric data collected in the vicinity of the coso geothermal field are analyzed in order to determine the source region of the geothermal field the derived models show zones of both low resistivity and low density at and below 6 km depth in the devils kitchen and the coso hot springs areas these zones agree with seismic reflection and tomography results which found a high amplitude reflector at 5 km and low velocities zones below 5 km we interpret the density and resistivity zones to indicate the presence of cooling magmatic material that provides the heat for the shallower geothermal system in these regions a zone marked by high resistivity and low density was found to lie directly above the interpreted partially melted region extending to within 1 km depth below the surface in the reservoir region where it is capped by a low resistivity clay zone in addition the density models indicate that the high density bodies occurring under volcanic outcrops may be mafic intrusions

exploration for unconformity type uranium deposits with audiomagnetotelluric data a case study from the mcarthur river mine saskatchewan canada unconformity type deposits supply a significant amount of the world s uranium and consist of uranium that is generally codeposited with graphite in a fault zone the low resistivity of the graphite produces a significant contrast in electrical resistivity which can be located with electromagnetic em methods the athabasca basin in western canada hosts significant uranium deposits and exploration in deeper parts of the basin has required the application of new em methods this paper presents an evaluation of the audiomagnetotelluric amt exploration method at the mcarthur river mine in the athabasca basin amt data were collected at 132 stations on a grid and two dimensional 2d and three dimensional 3d inversions were used to generate resistivity models these models showed two major results 1 a significant conductor coincident with a major basement fault p2 and the uranium deposits this conductor begins at the unconformity at a depth of 550 m and extends to a depth of at least three km an

a geochronological 40ar 39ar and 87rb 87sr study of k mn oxides from the weathering sequence of azul brazil k mn oxides of hollandite group minerals such as cryptomelane k1 2 mn3 mn4 8o16nh2o are often precipitated authigenically in weathering profiles the presence of structural k allows these minerals to be dated by the k ar and 40ar 39ar methods making it possible to study the progression of oxidation fronts during weathering processes within the context of a recent 40ar 39ar study of cryptomelane from the azul mn deposit in the carajas region amazonia brazil vasconcelos et al 1994 defined three age clusters 65 69 51 56 and 40 43 ma and proposed that they correspond to the episodic precipitation of the three generations of mn oxide that have been identified in the deposit beauvais et al 1987 we performed a laser probe 40ar 39ar and 87rb 87sr study on new samples from the same mn deposit our 40ar 39ar data confirm that cryptomelane is a suitable mineral for 40ar 39ar dating although in some cases we clearly identify the existence of 39ar recoil effects although the corresponding age spectra are generally strongly disturbed our results also confirm that the earliest cryptomelane generation is of upper cretaceous paleocene age we obtained good plateau ages from veins and concretions of the second cryptomelane generation some of these results allow definition of a well constrained age cluster at 46 7 48 1 ma not observed by vasconcelos et al 1994 a petrographic study confirms that none of the samples analyzed in the present study contained material associated with the third generation of cryptomelane we propose that these new results support the idea of a more or less continuous crystallization of k mn oxides mainly constrained by local factors rather than the model advanced by vasconcelos et al 1994 which suggests that each cryptomelane generation corresponds to distinct weathering events related to global climatic changes 87sr 86sr data show large variations clearly inherited from the 2 1 ga parent rock of the mn protore the rb sr results demonstrate that minimum fractionation occurs during cryptomelane crystallization except for the latest generation which is depleted in sr this precludes use of the rb sr radiochronometer for dating secondary mn oxides in laterites

present day kinematics of the eastern california shear zone from a geodetically constrained block model we use global positioning system gps data from 1993 2000 to determine horizontal velocities of 65 stations in eastern california and western nevada between 35 and 37 n we relate the geodetic velocities to fault slip rates using a block model that enforces path integral constraints over geologic and geodetic time scales and that includes the effects of elastic strain accumulation on faults locked to a depth of 15 km the velocity of the sierra nevada block with respect to nevada is 11 1 0 3 mm yr with slip partitioned across the death valley 2 8 0 5 mm yr panamint valley 2 5 0 8 mm yr and airport lake owens valley 5 3 0 7 4 6 0 5 mm yr faults the western mojave block rotates at 2 1 0 8 my clockwise with 3 7 0 7 mm yr of left lateral motion across the western garlock fault we infer 11 2 mm yr of right lateral motion across the mojave region of the eastern california shear zone

glacier meltwater hydrochemistry abstract glacierised areas present an ideal environment in which to study water rock interaction since chemical weathering rates are high and anthropogenic impacts are often minimal detailed investigations of meltwater quality variations have suggested the importance of these environments in estimates of terrestrial chemical erosion and global biogeochemical cycles most notably the role of meltwaters in co2 sequestration during episodes of deglaciation has attracted considerable attention since this may impact on climate at glacial interglacial timescales however there is still considerable uncertainty surrounding estimates of co2 drawdown by meltwaters which remains to be resolved water flow through glaciers exerts an important control on ice mass dynamics and influences the quantity and quality of water delivered to environments downstream of glacierised basins thus the study of the configuration and dynamics of subglacial drainage systems is important not only to enhance scientific understanding but also to allow effective water resource utilisation in glacierised headwater catchments bulk meltwater quality characteristics draining terrestrial ice masses also offer the potential to provide unique information on hydrological and hydrochemical processes operating in the inaccessible subglacial environment here significant advances have been made in understanding the controls on chemical weathering reactions based on the identification of key dissolved indicator species this has allowed water quality variations to be exploited as a tool for both subglacial hydrochemical and hydrological investigations as a result this area of glaciology has received considerable attention in recent years utilising an increasing range of dissolved ions and integrating field and laboratory studies however uncertainty still surrounds certain areas of meltwater quality science including the role of microorganisms in a system which to date has largely been viewed as abiotic a better understanding of the processes and rates of chemical weathering in glacierised environments has the potential to enhance our understanding of the environment and to facilitate the exploitation of water quality variations for both scientific and applied objectives in this paper the development and current state of meltwater quality science as a tool for investigating subglacial hydrology and geochemistry is detailed and problems and future directions identified this will hopefully stimulate wider interest in an important area of aquatic chemistry with significant applied implications

three dimensional conductivity model of the tendaho high enthalpy geothermal field ne ethiopia abstract tendaho is one of the high enthalpy geothermal fields at advanced stage of exploration which is located in the afar depression in north eastern ethiopia six deep and shallow geothermal wells were drilled in the field between 1993 and 1998 here we present the first 3d conductivity model of the tendaho high enthalpy geothermal field obtained from 3d inversion of magnetotelluric mt data mt data from 116 sites at 24 selected periods in the period range from 0 003 s to 1000 s were used for the 3d inversion the 3d conductivity model reveals three main resistivity structures to a depth of 20 km the surface conductive structure 10 m and 1 km thick is interpreted as sediments geothermal fluids or hydrothermally altered clay cap the underlying high resistivity structure in the afar stratiod basalts is associated with the deep geothermal reservoir at a depth 5 km a high conductivity is observed across the whole of the tendaho geothermal field this structure is inferred to be the partial melt heat source of the geothermal system the most striking feature in the 3d model is a fracture zone upflow zone in the afar stratoid basalts at the dubti area which acts as a pathway for geothermal fluids targeting the inferred fracture zone by directional drilling will likely increase the permeability and temperature of the deep reservoir in the basalts hence the inferred presence of a fracture zone and shallow magma reservoir suggest that there is a huge potential with temperature exceeding 270 c at 2 km depth at tendaho for conventional hydrothermal geothermal energy development

chemical denudation and silicate weathering in himalayan glacier basins batura glacier pakistan original article can be found at http www sciencedirect com science journal 00221694 copyright elsevier b v doi 10 1016 s0022 1694 02 00036 7 full text of this article is not available in the uhra

trace element mobility in mine waters from granitic pegmatite u th ree deposits bancroft area ontario abstract small low grade granitic pegmatite u th ree deposits are found throughout the grenville geological province of eastern canada groundwater quality at historical mining properties in the bancroft area was investigated in order to better understand the mobility of trace elements that may pose health risks if there is renewed development of this class of mineral deposit groundwater samples were obtained from diamond drill holes flowing adits and flooded mine shafts uranium occurs almost entirely in the dissolved

differentiating induced and natural seismicity using space time magnitude statistics applied to the coso geothermal field a remarkable characteristic of earthquakes is their clustering in time and space displaying their self similarity it remains to be tested if natural and induced earthquakes share the same behavior we study natural and induced earthquakes comparatively in the same tectonic setting at the coso geothermal field covering the preproduction and coproduction periods from 1981 to 2013 we analyze interevent times spatial dimension and frequency size distributions for natural and induced earthquakes individually these distributions are statistically indistinguishable determining the distribution of nearest neighbor distances in a combined space time magnitude metric lets us identify clear differences between both kinds of seismicity compared to natural earthquakes induced earthquakes feature a larger population of background seismicity and nearest neighbors at large magnitude rescaled times and small magnitude rescaled distances local stress perturbations induced by field operations appear to be strong enough to drive local faults through several seismic cycles and reactivate them after time periods on the order of a year

estimated solubility products and fields of stability for cryptomelane nsutite birnessite and lithiophorite based on natural lateritic weathering sequences

chemical and physical erosion rhythms of the west african cenozoic morphogenesis the 39 ar 40 ar dating of supergene k mn oxides chemical weathering and mechanical erosion are first order processes of long term tropical morphogenesis which is still poorly deciphered for lack of time constraints we address this issue by laser probe 39ar 40ar dating of generations of cryptomelane k1 2mn8o16 nh2o from the manganese ore deposit of tambao in northern burkina faso this mn deposit results from the supergene weathering of carbonate and silicate mn protores underneath lateritic palaeolandsurfaces it consists of an upper cryptomelane rich domain and a lower domain where pyrolusite b mno2 is the dominant mn oxide the oldest 39ar 40ar ages 59 45 ma are obtained on surface outcrops while the youngest ones characterize deep oxidation fronts 3 4 2 9 ma apparent correlations of 39ar 40ar age groups with d18o and eustatic curves allow definition of the different stages of morphogenesis paleocene eocene ages 59 45 ma bracket a greenhouse period propitious to bauxitic weathering the lack of significant ages between 45 and 29 ma characterizes a period dominated by mechanical erosion during which detrital sediments including lateritic materials were accumulated in intracratonic basins allowing the exhumation of a new lateritic landsurface two major weathering periods separated by a second erosion episode 24 18 ma are also depicted at the end of oligocene 29 24 ma and lower to mid miocene 18 11 5 ma in the upper domain during which newly shaped land surfaces conspicuously weathered the shorter weathering and erosion episodes recorded in the lower domain from 18 to 2 9 ma led to the final geomorphic changes that were conducive to the formation of glacis the preservation of old cryptomelane 59 45 ma in the upper part of the ore deposit indicates a cenozoic denudation limited to the erosion of previous bauxites and partly of ferricretes

melt distribution beneath a young continental rift the taupo volcanic zone new zealand 1 taupo volcanic zone tvz is a zone of intense volcanism and rifting associated with the subduction of the pacific plate beneath the continental crust of new zealand s north island an image of the conductivity structure beneath the central part of the tvz has been constructed using 2 d inverse modeling of magnetotelluric data a rapid increase in conductivity at a depth of 10 km beneath the tvz 3 km beneath the base of the seismogenic zone but well above the base of the quartzo feldspathic crust 16 km is interpreted to mark the presence of an interconnected melt fraction 4 within the lower crust beneath the quartzo feldspathic crust the model shows a zone of increased conductivity on the eastern side of the tvz consistent with an increased concentration of melt at deeper levels the pacific plate is resistive compared with the overlying mantle

partial melt or aqueous fluid in the mid crust of southern tibet constraints from indepth magnetotelluric data summary the indepth project has applied modern geophysical techniques to the study of the crustal structure and tectonic evolution of the tibetan plateau in the lhasa block seismic reflection surveys in 1994 detected a number of bright spots at 15 20 km depths that indicate zones of crustal fluids aqueous fluids or partial melt coincident magnetotelluric mt data collected in 1995 detected a major zone of high electrical conductivity at the same depth as the brightspots using constrained inversion the mt data require a minimum crustal conductance of 6000 s this abnormally high electrical conductance can be best explained by a layered model with fluids partial melt aqueous fluids or a combination of partial melt and aqueous fluids the non uniqueness of the mt method means that a wide range of melt fraction thickness combinations for the above models could all explain the 6000 s conductance to distinguish between these three models other geophysical and geological data are required reflection seismic data suggest that a high fluid content 15 per cent is present at the top of the layer the amplitude versus offset data suggest that the top of this layer may be aqueous fluids rather than partial melt passive seismic data imaged a 20 km thick layer of lower fluid content that is probably partial melt petrological studies suggest that concentrations of aqueous fluids above 0 1 per cent at mid crustal depth cannot be sustained taken together these data show that the high conductivity in southern tibet is most probably the result of a relatively thin layer of aqueous fluids 100 200 m overlying a thicker zone of partial melt 10 km

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

assessment of solute and suspended sediments acquisition processes in the bara shigri glacier meltwater western himalaya india various physico chemical parameters of meltwater samples from bara shigri glacier collected during 2012 2013 were analysed to understand the dissolved ion chemistry and solute acquisition processes regulating major ion chemistry of the study area ionic dominance of anions followed the trend as hco3 so4 2 cl no3 whereas ionic dominancy of cations varied as ca2 mg2 k na for both the years high share of ca mg in the total cations and also dominancy of ca mg over na k for both the years demonstrate that solute acquisition processes in the meltwater of bara shigri glacier are mainly governed by the weathering of carbonates the piper plot identified ca hco3 as the dominant water type followed by caso4 type of water in the study area the average value of partial pressure of carbon dioxide pco2 for meltwater of the bara shigri glacier was 10 2 6 atm in 2012 and 10 2 3 atm in 2013 and were higher than the pco2 value of atmosphere 10 3 5 atm it indicates that the meltwater is in disequilibrium with respect to the atmosphere contents of suspended sediment in the meltwater were 1432 mg l in july and 596 mg l in september 2012 whereas in 2013 suspended sediment contents were 1093 mg l in august and 425 mg l in september following discharge pattern of the bara shigri glacier we found that bara shigri and gangotri glaciers demonstrate the higher rate of regional melting

chemical characteristics and suspended sediment load of meltwaters from a himalayan glacier in india abstract meltwaters draining from the snout of the chhota shigri glacier eastern lahul valley himachal pradesh india were analysed for major cations and anions the glacier lies on the southern slope of the himalayas at an elevation ranging between 4000 and 5000 m the drainage area of the basin is 40 km 2 while the glacier area is 10 km 2 water quality was determined during the summer ablation period from 17 july to 17 august 1987 meltwaters from the glacial surface site 1 and 2 have low solute content electrical conductivity 5 0 5 92 s cm 1 ca 2 0 73 mgl 1 mg 2 0 08mgl 1 na 0 36mgl 1 k 0 34mgl 1 whereas after passing through the glacier waters are chemically enriched electrical conductivity 23 2 29 0 s cm 1 ca 2 0 9 9 9mgl 1 mg 2 0 5 0 35mgl 1 na 0 7 0 8mgl 1 k 0 4 1 4 mgl 1 the variations of solute concentration in meltwaters at the snout is related to its discharge the water movement within the chhota shiggi glacier appears to be through the subglacial and englacial channels

magnetotelluric imaging of upper crustal partial melt at tendaho graben in afar ethiopia we report on a recent magnetotelluric mt survey across the manda hararo magmatic segment mhms within the tendaho graben in the afar depression in northeastern ethiopia twenty two broadband mt sites with 1 km station spacing were deployed along a profile with the recorded data covering a period range from 0 003 s to 1000 s a two dimensional 2 d resistivity model reveals an upper crustal fracture zone fault and partial melt with resistivity of 1 10 m at a depth of 1 km the partial melt has a maximum horizontal width of 15 km and extends to a depth of 15 km within the afar stratoid series basalts we estimate a melt fraction of about 13 based on geochemical and borehole data and bulk resistivity from the 2 d mt inversion model the interpreted upper crustal partial melt may have been formed by either a magma intrusion from mantle sources or a large volume of continental crust that has been fluxed by a small amount of mantle melt and heat within the mhms and tendaho graben a magma intrusion is a plausible explanation for the upper crustal conductor the inferred presence of a conductive fracture zone or fault with hydrothermal fluid and shallow heat sourcing magma reservoir also makes the tendaho graben a promising prospect for the development of conventional hydrothermal geothermal energy

lateritic concentrations of manganese oxyhydroxides and oxides latentes result from the weathering of a variety of rocks under humid tropical conditions when developed at the expenses of mn2 bearing parent rocks laterite consists of a thick mantle made up of manganese oxides and hydroxides two main examples are described from tropical africa the first example displays a lateritic profile on metamorphic rock the second example deals with lateritic profiles developed on a precambrian sedimentary rock from the base to the top of the studied profiles different mn hydroxide and oxide sequences have differentiated their composition depends on i the constitutive parent minerals with vary ing weathering rates ii the intensity of weathering which progresses towards the profile top permitting pseudomorphic replacement of mn4 oxides for early formed mn3 hydroxides most of the manganese ore result from a relative accumulation however mn can also be mobilized in different places of weathering profiles leading to accumulations in other places mainly at the base of weathering profiles that absolute accumulation can occur in non mn bearing weathering horizons kaolinitic horizons to form mn nodules in which kaolinite is dissolved this process provides al which combines to mn3 forming al mn hydroxides lithiophorite

formation and transformation processes of iron duricrust systems in tropical humid environment abstract formation and transformation processes of iron duricrust developed under tropical humid conditions in the southeastern part of the central african republic have been investigated in terms of geomorphological petrological and geochemical pathways the study of five typical weathering profiles of the most extensive iron duricrust system now known in the world indicates that saprolitization and ferruginization are sequentially involved in such processes as reflected by kaolinite gibbsite and hematite goethite replacements alkali earths rare earths metals and elements belonging to chemically inert minerals segregate during saprolite and iron duricrust formation and degradation mass balance calculations performed on a typical weathering system enable us to quantify net mass element transfers deriving from lateritic weathering processes volumetric change collapse or dilation as well as al and ti clearly discriminate between the saprolitization and ferruginization processes saprolite development preserves the geochemical inheritance of parent rocks whereas ferruginization depletes it and mostly reflects through mineralogical pathways pedoclimatic conditions and global climatic changes the global iron duricrust system of the southern part of central africa derived from previous hematite rich crust and not from ancient bauxites as it is generally believed for iron crusts formed on west african shields

first 40ar 39ar dating of intense late palaeogene lateritic weathering in peninsular india lateritic surface processes have shaped large platform and cratons of the tropical belt constraining the timing of such processes is crucial to decipher their role in cratonic morphogenesis and their response to long term climatic change and lithospheric deformation weathering histories have been documented for south america africa and australia but precise time constraints of the lateritic weathering processes in south india are still lacking we present 40ar 39ar ages of supergene cryptomelane k mn oxide formed in the sandur mn ore deposits exposed on the highest lateritic paleolandsurface that once covered the mysore plateau and the adjacent deccan traps significant 40ar 39ar ages are estimated between 36 and 26 ma from well defined plateaus in step heating 39ar release spectra and from best fitted inverse isochrones these ages constitute firm time constraints that document intense late eocene oligocene lateritic weathering over peninsular india under the influence of warm and wet climate comparable to that prevailing in tropical humid forests these results imply that southern india was weathered between 36 and 26 ma and may have been dissected mostly in the neogene

spatial variations in chemical weathering and co2 consumption in nepalese high himalayan catchments during the monsoon season the major ion chemistry of the marsyandi basin and six of its tributaries in the nepalese himalaya have been investigated during the monsoon months of 2002 weekly water samples taken at 10 river monitoring stations in the annapurna watershed over the course of 4 months provide chemical weathering data for the region at an unprecedented temporal and spatial resolution the river chemistry of all but one basin is heavily dominated by carbonate weathering which compared to silicate weathering contributes 80 to 97 of the total solute load this prevalence is due to a combination of a intrinsically faster dissolution kinetics of carbonates b relatively high runoff and c glacial meltwater and low temperatures at high altitudes resulting in enhanced carbonate solubilities monitoring stations with headwaters in the tethyan sedimentary series tss are particularly carbonate rich and slightly supersaturated with respect to calcite through half of the monsoon season silicate weathering in the tss is driven largely by sulfuric acid and therefore does not contribute significantly to the drawdown of atmospheric co2 with respect to the tributaries in the greater himalayan sequence ghs carbonate weathering is practically as predominant as for the tss in spite of the largely felsic lithology of the ghs relative to the tss the primary proton source in the ghs has shifted with at least 80 of the protons derived from carbonic acid averaged over the whole field area the co2 fluxes based on silicate derived ca and mg are considerably lower than the global average assuming that this study area is representative of the entire range we conclude that in situ weathering of the high himalayas does not represent a significant sink of atmospheric carbon dioxide despite the presence of a watershed south of the ghs that is characterized by a four times higher co2 consumption rate than the global average silicate weathering rates of all basins appear to be climate controlled displaying a tight correlation with runoff and temperature given the extremely low chemical weathering under transport limited conditions in high altitude crystalline terrains outside of the monsoon season this would result in virtually no chemical exhumation for 2 3 of the year in such a cold and arid climate north of the rain shadow cast by the high himalayas 2009 elsevier ltd all rights reserved

application of 3d inversion to magnetotelluric profile data from the deccan volcanic province of western india abstract using synthetic data siripunvaraporn et al 2005b demonstrated possible advantages of interpreting single profile mt data with a three dimensional 3d inversion program here we explore this idea further using real mt data from two profiles on the indian subcontinent the first profile 330 km long cuts across the deccan volcanic province of peninsular india the second 130 km long is in the narmada son lineament zone approximately 100 km further north using the data space occam inversion code of siripunvaraporn et al 2005a 3d inversion is carried out on each of these profiles independently and results are compared with previously published two dimensional 2d interpretations in addition to inversion of the full impedance tensor we consider 3d inversion of only the off diagonal components we also experiment with variants on the model covariance in particular allowing for longer smoothing length scales along the geoelectric strike not surprisingly the 3d inversion finds models that fit the data better than had been possible with the 2d programs many of the features inferred from these previous 2d interpretations are also present in the 3d inverse solutions but the positions and amplitudes of individual conductive features are in some cases changed the 3d models suggest substantial non uniqueness in the single profile data even without explicit or special treatment we find that the relatively modest near surface distortion effects in these datasets were well fit by the 3d inversion by inserting small scale conductive and resistive features in surface layers mostly off profile

magnetotelluric images of magma distribution beneath volcan uturuncu bolivia implications for magma dynamics the altiplano puna volcanic complex in the central andes records a history of major caldera forming eruptions over the past 10 m y geophysical and geodetic data indicate that magma is still present and perhaps mobile in the crust broadband magnetotelluric data were used to generate two dimensional and three dimensional electrical resistivity models of the altiplano puna magma body apmb with a focus on the zone of inflation around volcan uturuncu in southern bolivia low electrical resistivities 20 the upper surface of the apmb is shallowest beneath uturuncu and the geometry is consistent with geodynamic models that require the upward movement of a melt layer at this location the shallower resistivity structure is characterized by discrete electrically conductive bodies oriented east west near sea level depth of 5 km which are interpreted as a combination of partial melt and fluids

deformation and seismicity in the coso geothermal area inyo county california observations and modeling using satellite radar interferometry interferometric synthetic aperture radar insar data collected in the coso geothermal area eastern california during 1993 1999 indicate ground subsidence over a 50 km 2 region that approximately coincides with the production area of the coso geothermal plant the maximum subsidence rate in the peak of the anomaly is 3 5 cm yr 1 and the average volumetric rate of subsidence is of the order of 10 6 m 3 yr 1 the radar interferograms reveal a complex deformation pattern with at least two irregular subsidence peaks in the northern part of the anomaly and a region of relative uplift on the south we invert the insar displacement data for the positions geometry and relative strengths of the deformation sources at depth using a nonlinear least squares minimization algorithm we use elastic solutions for a prolate uniformly pressurized spheroidal cavity in a semi infinite body as basis functions for our inversions source depths inferred from our simulations range from 1 to 3 km which corresponds to the production depths of the coso geothermal plant underpressures in the geothermal reservoir inferred from the inversion are of the order of 0 1 1 mpa except a few abnormally high underpressures that are apparently biased toward the small source dimensions analysis of the insar data covering consecutive time intervals indicates that the depths and or horizontal extent of the deformation sources may increase with time this increase presumably reflects increasing volumes of the subsurface reservoir affected by the geothermal exploitation we show that clusters of microearthquakes associated with the geothermal power operation may result from perturbations in the pore fluid pressure as well as normal and shear stresses caused by the deflation of the geothermal reservoir

morphological chemical and mineralogical properties of some manganese rich oxisols derived from dolomite in mpumalanga province south africa abstract we investigated the morphological chemical and mineralogical properties of unusual black oxisols rich in mn oxides derived from intense weathering of dolomite near graskop in mpumalanga province south africa soil mapping of a small catchment was carried out and seven horizons from two profiles were sampled for detailed analysis a consistent pattern was evident in which black lower subsurface horizons with weak nodule development grade upwards into redder or browner horizons containing abundant pisolitic occasionally magnetic nodules surface horizons are humus rich 3 c and acidic 50 60 acid saturation of effective cec subsurface horizons have low ecec 2 mm fraction only very weak lithiophorite peaks could be identified in x ray powder diffractograms of the nodular material aluminium in the form of gibbsite was slightly enriched in the nodules but not fe it is proposed that the nodules have formed via the reprecipitation of mn reductively dissolved from the fine earth fraction during periods of low redox potential we envisage that this has given rise to progressive sequestration of mn in hard nodules and removal of the black matrix colour unveiling the colours of iron oxides and humus the presence of al in lithiophorite a higher point of zero charge than for other mn oxides and redox inertia associated with the exceptionally large concentrations of mn ranging from about 10 to 18 mno2 in the whole soil including nodules are all thought to contribute to the accumulation and persistence of mn in these soils some trace metals showed marked enrichment in the nodules up to 800 500 and 1000 mg kg 1 of co cu and ni respectively

seasonal variation of the solute and suspended sediment load in gangotri glacier meltwater central himalaya india abstract a systematic study on the seasonal variation of major cations and anions was carried out to understand the source of dissolved ions as well as the geochemical weathering processes controlling the meltwater chemistry of gangotri glacier calcium and magnesium are the major cations while sulphate is the dominant anion followed by bicarbonate the high ratios of ca mg na k ca na mg na hco3 na and low ratio of na k tz for pre monsoon monsoon and post monsoon seasons indicate the dominance of carbonate weathering which is a major source of the dissolved ions in the meltwater of gangotri glacier followed by silicate weathering high equivalent ratios of na cl and k cl as compared to sea water indicate relatively lesser contribution from atmospheric input to the chemical composition of meltwater correlation matrix and factor analysis were used to identify various factors controlling the major ion chemistry marked seasonal and diurnal variations were observed in the dissolved ions and suspended sediment concentration daily mean suspended sediment concentration for pre monsoon monsoon and post monsoon was observed as 1719 3281 and 445 mgl 1 respectively highest suspended sediment load was observed in monsoon season followed by pre monsoon and post monsoon seasons the cation denudation rates of gangotri glacier meltwater were calculated to be 42 2 46 5 and 15 9 t km 2 y 1 for pre monsoon june only monsoon and post monsoon respectively these values are higher than that of other himalayan glaciers whereas physical weathering rate of the gangotri glacier catchment was observed to be 7056 15 344 and 588 t km 2 y 1 for pre monsoon june only monsoon and post monsoon respectively much higher than the indian and world averages of river

lithospheric structure of an active backarc basin the taupo volcanic zone new zealand seismic data from both explosive and earthquake sources have been used to model the crustal and upper mantle velocity structure beneath the taupo volcanic zone tvz an active backarc basin in central north island new zealand volcanic sediments with p wave velocities of 2 0 3 5 km s 1 reach a maximum thickness of 3 km beneath the central tvz underlying these sediments to 16 km depth is material with velocities of 5 0 6 5 km s 1 interpreted as quartzo feldspathic crust east and west of the tvz crust with similar velocities is found to depths of 30 and 25 km respectively beneath the tvz material with p wave velocities of 6 9 7 3 km s 1 is found from 16 to 30 km depth and is interpreted as heavily intruded or underplated lower crust the base of the crust at 30 km depth under the tvz is marked by a strong seismic reflector interpreted as the moho modelling of arrivals from deep 40 km earthquakes near the top of the underlying subducting pacific plate reveals a region with low mantle velocities of 7 4 7 8 km s 1 beneath the crust of the tvz this region of low mantle velocities is best explained by the presence of partially hydrated upper mantle resulting from dehydration of hydrous minerals e g serpentinite carried down by the underlying subducting plate within the lower crust beneath the tvz a region of high 0 34 poisson s ratio is observed indicating the presence of at least 1 per cent partial melt this melt probably fractionates and assimilates crustal material before some of it migrates into the upper crust where it provides a source for the voluminous rhyolitic magmas of the tvz

geochronology of the baye mn oxide deposit southern yunnan plateau implications for the late miocene to pleistocene paleoclimatic conditions and topographic evolution 40ar 39ar laser incremental heating analyses of supergene k mn oxides from weathering profiles at the baye mn deposit southern yunnan plateau sw china were carried out to place constraints on the timing of weathering and derive insights into local paleoclimatic and landscape evolution weathering profiles in the baye mn mine are dominated by 20 30 m thick saprolites which are locally covered by unconsolidated alluvial deposits we analyzed 70 grains from 35 hand samples collected from four sites located at distinct elevations in most cases different grains from the same hand sample and different samples from the same site have reproducible 40ar 39ar plateau ages the plateau ages of individual grains are also consistent with the respective isochron integrated and ideogram ages this age consistency attests to the reliability of the geochronological results as numerical constraints on the formation and evolution of these weathering profiles sixty four grains from 32 hand samples collected from weathering profiles at four sites a to d yield well defined plateau or pseudoplateau ages ranging from 2 98 0 07 to 0 16 ma 2 the age results show that mn oxides become progressively younger towards lower elevations samples collected from the highest elevation site 1855 m a s l have the oldest ages at 2 98 0 07 to 2 83 0 13 ma samples from intermediate elevations 1821 to 1815 m a s l yield ages ranging from 2 91 0 10 to 2 08 0 11 ma and those from the lowest sites 1768 to 1753 m a s l are younger than 1 25 0 08 ma the age versus elevation relationship indicates fast downward propagation of weathering front since the late pliocene and permits estimating the weathering rates at 3 3 3 8 6 6 1 2 and 11 1 1 9 m myr 1 during the time periods of 2 98 2 83 2 91 2 08 and 1 25 to 0 16 ma respectively the average river incision rates estimated from the surface exposed ages range from 235 10 to 416 19 m myr 1 during the past 3 million years the incision rate calculated from minimum exposed ages at sites b d is 33 2 m myr 1 for the time period of 2 91 1 25 ma which is consistent with the slow incision of streams on the ridges over the yunnan plateau since the early pleistocene three mn oxide pebbles contained in the alluvial deposits overlying the saprolite dominated weathering profiles at the two highest sites provided additional information on the timing of weathering four grains from two pebbles yield plateau ages of 6 32 0 19 to 5 27 0 10 ma whereas the other two grains from the third pebble indicate the minimum formation ages of 8 2 0 4 and 9 3 0 3 ma these ages confirm the existence of older weathering profiles now dismantled in the region manganese oxide 40ar 39ar ages of the baye deposit when combined with results from other localities indicate that lateritic weathering and supergene mn enrichment and by inference warm and humid climates conducive to intense weathering have prevailed over the plateau since the middle to late miocene the climatic conditions inferred from the weathering geochronology are consistent with multiple independent marine and terrestrial sedimentary and paleontological records confirming that supergene mn oxides can be used as a useful proxy for past climate age clusters of mn oxides at 2 9 2 4 1 2 0 8 and 0 6 0 4 ma are broadly coincident with and thus likely reflect intensification events of the indian summer monsoon that brings moisture and abundant precipitation to the yunnan plateau these clusters also coincide with the periods of significant surface uplift in the yunnan plateau demonstrating a causal link between topographic evolution plateau uplift and intensification of the monsoonal climate

seismic studies of the crust under the hydrothermal areas of the taupo volcanic zone new zealand abstract observations of relative p wave travel time residuals for local mantle earthquakes and p s amplitude ratios for more distant events have been used to investigate the nature of the crust under the hydrothermal areas of the central taupo volcanic zone in contrast to some other regions of similar activity such as the yellowstone and geysers geothermal areas u s a there is no evidence in the data for an extensive region of semi molten or molten rock in the crust thus the large amounts of mafic intrusive rock and associated crustal melt inferred to be present on geological grounds must be now largely at temperatures below the appropriate solidus the data do provide further evidence for previously suspected lateral inhomogeneity in the underlying mantle

crust and upper mantle electrical conductivity beneath the yellowstone hotspot track combining long period magnetotelluric data from the spatially uniform earthscope usarray and higher resolution profiles we obtain a regional three dimensional electrical resistivity model in the snake river plain and yellowstone areas idaho and wyoming united states and provide new constraints on the large scale distribution of melt and fluids beneath the yellowstone hotspot track contrary to what would be expected from standard mantle plume models the electromagnetic data suggest that there is little or no melt in the lower crust and upper mantle directly beneath yellowstone caldera instead low mantle resistivities 10 m and below which we infer to result from 1 3 partial melt are found 40 80 km beneath the eastern snake river plain extending at least 200 km southwest of the caldera beneath the area of modern basaltic magmatism the reduced resistivities extend upward into the mid crust primarily around the edges of the snake river plain suggesting upward migration of melt and or fluid is concentrated in these areas the anomaly also shallows toward yellowstone where higher temperatures enhance permeability and allow melts to ascend into the crust the top of the conductive layer is at its shallowest in the upper crust directly beneath the modern yellowstone supervolcano

stratigraphy and structure of the ngatamariki geothermal system from new zircon u pb geochronology implications for taupo volcanic zone evolution abstract recent drilling at the ngatamariki geothermal field taupo volcanic zone new zealand has provided new constraints on the stratigraphy and volcanic evolution of the region over 2800 m thickness of volcanic products are present at ngatamariki mainly comprised of rhyolitic ignimbrites linked to large caldera forming events at sources outside the field area but locally sourced andesite and rhyolite lavas and domes are also encountered most of the rocks are allocated to the pre 0 35 ma tahorakuri formation crystallisation age spectra and consequent best estimates of eruption age have been obtained by u pb dating on zircons from otherwise severely hydrothermally altered magmatic rocks by secondary ion mass spectrometry techniques using a shrimp rg instrument the oldest rock dated is an ignimbrite which yields an eruption age estimate of 1 85 0 06 ma this ignimbrite plus comparable aged units dated at the adjacent rotokawa and ohaaki geothermal fields are interpreted to represent the oldest silicic volcanic deposits in the area and onlap the basal andesite lava pile that is best developed at rotokawa to the south other pyroclastic units and associated volcaniclastic sediments with another intercalated andesite lava unit return age estimates between 1 85 0 06 and 0 701 0 039 ma between 0 7 and 0 35 ma contemporaneous surface lithologies in the ngatamariki area are dominated by sediments with subordinate lava domes between 0 716 0 017 and 0 655 0 016 ma at least three shallow to 5 km 3 the intrusions generated a large alteration halo 25 km 3 minimum and intense silicification of the wall rocks at 0 35 and 0 34 ma the area was buried by two ignimbrite packages of the whakamaru group erupted from sources just west and well north respectively of the field ignimbrites of the waiora formation and several rhyolite lava domes were then emplaced over a period bracketed by domes dated at 0 282 0 011 and 0 257 0 011 ma coeval with more extensive volcanic activity in the maroa dome complex west of the field sediments of the huka falls formation and deposits of the 25 4 0 2 ka oruanui eruption then cap and seal the system the new u pb data coupled with detailed petrographical studies allow us to build the history of the area encompassed by the ngatamariki geothermal field the field despite 2 8 km of subsidence does not lie in a caldera and is the only one known to date to have a plutonic intrusive complex of quaternary age two chemically and temporally distinct hydrothermal events are located at the ngatamariki field with no evidence of continuity between the two

rare earth element trends and cerium uranium manganese associations in weathered rock from koongarra northern territory australia at koongarra australia three drill cores form the cahill schist formation containing u ore and regolith above it containing secondary u ore were studied to ascertain the distribution of rare earth elements rees and u the unaltered schist has a ree trend similar to the post archaean australian shale paas which is therefore used as a normalising standard unweathered rock from the zone of primary u mineralisation contained strong enrichment of the heavy rees and this pattern is retained during most of the weathering strongly bleached and very weathered shallow samples from this zone do not show enrichment of hrees in general however weathering appears to have little effect on the pattern of rees in the bulk rock at this site zones rich in mn oxy hydroxides occurring as coatings on fissure surfaces contain large concentrations of ce and u with a clear ce anomaly ce ce 3 8 the ce and u apparently occur together as microcrystalline oxides within 1 2 mu m diameter globules on mineral surfaces in the mn rich zones 46 refs 10 figs 2 tabs

the coso geothermal field a nascent metamorphic core complex investigation of the coso range using seismicity gravity and geochemistry of rocks and fluids supports the interpretation that the structure hosting the geothermal resource is a nascent metamorphic core complex the structural setting is a releasing bend in a dextral strike slip system that extends from the indian wells valley northward into the owens valley this tectonic setting results in nw directed transtension which is accommodated by normal and strike slip faulting of the brittle upper 4 6 km of the crust and shearing and ductile stretching below this depth accompanied by shallow igneous intrusions focal mechanisms of some small earthquakes that have occurred from 1996 to the present beneath the coso range exhibit depth dependent rotation of seismic p and t axes indicating that the local orientations of the principal stresses likely favor resolved shear stress on low angle faults these small earthquakes occur near the base of seismicity which we interpret as coincident with the brittle ductile transition geochemical results show a significant asthenospheric influence in the isotopic composition of rocks and fluids indicating that the crust is thinned within the coso structure thinned upper crust is underlain by a more dense mafic body at depths of 10 km or less this is consistent with observed gravity anomalies and models the mafic body may represent cumulates left over from the fractional crystallization of rhyolite which occurs as endogenous domes at coso or it could be a sheeted dike complex in the upper mid crustal area transtension began at 2 3 ma and continues today using a long term crustal deformation rate of 2 mm yr we infer that the basal detachment fault commonly observed in fully exhumed metamorphic core complexes will reach the surface in two to four million years