solar wind electron precipitation into the dayside martian upper atmosphere through the cusps of strong crustal fields measurements made by the magnetometer electron reflectometer on board the mars global surveyor spacecraft have shown spatially localized enhancements in electron fluxes over the strong crustal fields on both the dayside and night which are used to identify the cusps in between the closed magnetic fields this paper provides a comprehensive statistical study on the occurrence rate of dayside solar wind magnetosheath precipitation over the strong crustal fields also the occurrence rate s dependence on the magnetic elevation angles and the solar zenith angle is presented a seasonal variation of the precipitation is also expected and found due to both the tilt and the orbital eccentricity of mars the maximum occurrence rate is 40 when the solar zenith angles are small and the magnetic fields are nearly vertical finally the energy flux deposition of the solar wind electrons is calculated as well which is 0 1 2 of solar euv flux input

the origin and evolution of canadian shield brines evaporation or freezing of seawater new lithium isotope and geochemical evidence from the slave craton abstract new chemical and isotopic data for deep seated calcium chloride brine from the miramar con gold mine yellowknife n w t strongly suggest that the brine salinity is of marine origin diagnostic marine properties include uniformly elevated br cl ratios typical of seawater concentrated beyond halite saturation and li br ratios 0 0254 0 0325 and 6 li compositions 32 1 to 36 3 similar to seawater 32 3 the mean 6 li for all mine water samples of 35 1 may reflect minor uptake of li by secondary minerals this interpretation is supported by analyses of altered metabasalt from fault zones which is enriched in li but depleted in 6 li 14 7 to 15 6 relative to the unaltered metabasalt 5 4 the mechanism responsible for concentrating the hypersaline brine end member is not unequivocal as evidence exists to support both evaporative and cryogenic processes on the one hand the devonian sedimentary record in the great slave lake region in conjunction with yellowknife brine isotopic compositions 2 h and 34 s so 4 that are similar to various devonian fluids support an evaporative origin on the other hand the na cl br cl relationship in the brine strongly suggests a cryogenic mechanism regardless of the concentrative mechanism the chemical data indicate that the yellowknife parent brine was concentrated 28 to 30 fold relative to seawater the extreme depletion of mg and enrichment of ca in the brines accompanied by sr ca ratios similar to that of seawater are accounted for by dolomitization of an aragonite rich marine sediment by the brine before infiltration into the crystalline basement rocks subsequent alteration of silicate minerals in the shield added additional ca and sr to the brine as indicated by their radiogenic 87 sr 86 sr ratios up to 0 7147 based on mineral balance calculations the major mineral products of the cryogenic and evaporitic concentration and evolution paths are significantly different the cryogenic evolution results in some 15 mirabilite 60 hydrohalite and 18 dolomite whereas the major minerals formed from the evaporitic evolutionary sequence are 36 halite 8 gypsum 17 dolomite and 30 albite the great similarity between the calcium chloride brine from yellowknife and other such canadian shield brines indicates that they may share a common marine origin

exhumation of eclogites insights from depth time path analysis abstract literature data on geochronology and pressure temperature estimates have been used to assess the depth time paths for high pressure units in the alps and the variscan belt these depth time paths consistently show that exhumation rates decrease through time the first stages of the exhumation are rapid 1 mm yr 1 and last less than a few million to a few tens of million years in contrast the final 10 20 km ascent to the surface is at least one order of magnitude slower for the deepest and most important part of the exhumation trajectory the chronology of the retrogressive metamorphism excludes gravitational collapse as the major driving force material balance and structural arguments also exclude isostatically assisted erosion processes prism dynamics and tectonically assisted erosion are the most likely mechanisms accounting for the kinetics of exhumation isostasy assisted erosion and shallow level extension are nevertheless required to eventually bring the high pressure rocks to exposure

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

geochemistry and isotope systematics of small to medium volume neogene quaternary ignimbrites in the southern central andes evidence for derivation from andesitic magma sources the geochemical and isotopic compositions of small to medium volume 10 km3 50 km3 neogene quaternary ignimbrite sheets of the southern central andes 25 00 26 45 s 67 30 69 30 w have been analysed in order to assess the petrogenesis of this type of calc alkaline magma the ignimbrites are derived from the active volcanic zone around salar de la isla sis in chile and from behind the active arc in the area of the salar de antofalla saf argentina the ignimbrites in both areas are characterized by 87sr 86sr t ratios 0 7066 0 7090 that are lower and end t values 2 4 to 5 1 that are higher than in the large volume caldera sourced ignimbrites of cerro galan 100 km further east and those of ignimbrites from other megacalderas of the central andes this difference reflects smaller amounts of crustal material in the sis and saf ignimbrites rather than a change in basement composition as indicated by pb provinciality the isotope signature of the sis and saf ignimbrites is similar to that of late to post miocene composite calc alkalic andesite dacite stratovolcanoes of the frontal arc in this region this finding combined with major and trace element modelling lends support to a petrogenetic model for the sis and saf ignimbrites involving fractional crystallization from a lil depleted andesitic parental magma which was beforehand modified by crustal contamination

the origin of solutes within the groundwaters of a high andean aquifer abstract this paper investigates the origin of solutes within the groundwaters of the monturaqui negrillar tilopozo mnt aquifer system within the high andes of the atacama desert that discharges into the salar de atacama key questions include the relative significance of volcanic hydrothermal processes and evaporitic brine recycling over solute supply as well as the pathways of solute ingress to the mnt aquifer system groundwaters were analysed for elemental major minor and trace and isotopic 18o 2h 13c dic 34s so4 87sr 86sr constituents to which various hydrochemical and multivariate statistical methods have been applied groundwaters are all classified as thermal and show increasing temperatures 27 35 c and concentrations of hco3 4 4 10 4 mmol l 1 dissolved inorganic carbon dic with increasing proximity to volcano socompa resulting from an increasing mass flux of steam and magmatic co2 pco2 0 016 to 0 10 atm 13c co2 9 3 to 3 6 v pdb boiled off a deep hydrothermal reservoir superimposed upon this gradational and relatively smooth spatial increase in heat and mass flow is a sharp structurally controlled increase in tds 826 3632 mg l 1 and a concomitant change in 34s so4 0 79 to 4 9 v cdt and 87sr 86sr values 0 707375 0 706859 associated with the inflow of evaporitic solutes evaporitic inputs are chemically and isotopically distinct from localised secondary hydrothermally derived solutes with major minor and trace element data suggesting an origin within a highly oxidising alkaline evaporitic lake receiving dilute inflows enriched in volcanic fumarolic sulfur mineralisation probably from volcanoes socompa salin or pular that delimit the eastern topographic extent of the aquifer system the conceptual model presented in this paper proposes that basal leakage of evaporitic brines from active salar s within the high altiplano volcanic arc are actively entrained by sub regional groundwater flow and conveyed to the mnt aquifer system where they mix with solutes derived from localised secondary hydrothermal gas water rock interaction this work provides detail on the origin and processes controlling the solute composition of groundwater inflows to the salar de atacama within the volcanically active and hyperarid atacama desert and may be of significance to conceptual models of evaporitic brine evolution recycling of evaporitic brines and hydrothermalism in arid regions

isotopic evidence for the source of ca and s in soil gypsum anhydrite and calcite in the atacama desert chile abstract the origin of pedogenic salts in the atacama desert has long been debated possible salt sources include in situ weathering at the soil site local sources such as aerosols from the adjacent pacific ocean or salt encrusted playas salars and extra local atmospheric dust to identify the origin of ca and s in atacama soil salts we determined 34 s and 87 sr 86 sr values of soil gypsum anhydrite and 87 sr 86 sr values of soil calcite along three east west trending transects our results demonstrate the strong influence of marine aerosols on soil gypsum anhydrite development in areas where marine fog penetrates inland results from an east west transect located along a breach in the coastal cordillera show that most soils within 90 km of the coast and below 1300 m in elevation are influenced by marine aerosols and that soils within 50 km and below 800 m in elevation receive 50 of ca and s from marine aerosols 34 s values 14 and 87 sr 86 sr values 0 7083 in areas where the coastal cordillera is 1200 m in elevation however coastal fog cannot penetrate inland and the contribution of marine aerosols to soils is greatly reduced most pedogenic salts from inland soils have 34 s values between 5 0 to 8 0 and 87 sr 86 sr ratios between 0 7070 and 0 7076 these values are similar to average 34 s and 87 sr 86 sr values of salts from local streams lakes and salars 5 4 2 34 s and 0 70749 0 00045 87 sr 86 sr in the andes and atacama suggesting extensive eolian reworking of salar salts onto the surrounding landscape ultimately salar salts are precipitated from evaporated ground water which has acquired its dissolved solutes from water rock interactions both high and low temperature along flowpaths from recharge areas in the andes therefore the main source for ca and s in gypsum anhydrite in non coastal soils is indirect and involves bedrock alteration not surficially on the hyperarid landscape but in the subsurface by ground water followed by eolian redistribution of ground water derived salar salts to soils the spatial distribution of high grade nitrate deposits appears to correspond with areas that receive the lowest fluxes of local marine and salar salt supporting arguments for tropospheric nitrogen as the main source for soil nitrate

observations of the latitude dependence of the location of the martian magnetic pileup boundary 1 we report observations that show the dependence of the altitude of the magnetic pileup boundary mpb at mars on planetary latitude as seen by the mars global surveyor magnetometer electron reflectometer instrument the mpb is further away from mars on average at southern latitudes than at northern latitudes the data are consistent with a mpb distance mapped to the terminator plane that does not vary with latitude in the northern hemisphere but increases with increasing southern latitude in the southern hemisphere we also report increased variability in the mpb distance within the longitude range 90 270 e longitude in the southern hemisphere which is the region that contains the strongest crustal magnetic fields these trends are most obvious in a planet fixed coordinate system indicating a planet fixed driver of the mpb location the proposed mechanism is the local diversion of shocked solar wind flow by crustal magnetic fields

hydrochemistry mineralogy and chemical fractionation of mine and processing wastes associated with porphyry copper mines a case study from the sarcheshmeh mine se iran abstract the sarcheshmeh is one of the largest oligo miocene porphyry cu deposits in the world comparative hydrochemical mineralogical and chemical fractionation associated with mining efflorescence salts and processing wastes of this mine are discussed hydrochemical results showed that rock waste dumps reject wastes and old impoundments of tailings are the main sources of acid mine drainage waters amd that contain potentially toxic metals such as cd co cu mn ni and zn as well as al episodic fluxes of highly contaminated acidic waters were produced in a tailings dam over a short period of time secondary soluble minerals provide important controls on the quality of amd produced especially in old dry tailings impoundments secondary sulfate minerals such as gypsum magnesiocopiapite hydronium jarosite kornelite and coquimbite were found in rock waste drainages and in old weathered reject wastes highly soluble secondary minerals such as gypsum eriochalcite and bonattite are also observed in an evaporative layer on old tailings impoundments chemical fractionation patterns of potentially toxic elements showed that the geochemical behavior of metals is primarily controlled by the mineralogical composition of waste samples elements such as co cr cu mn ni and zn are readily released into the water soluble fraction from efflorescence salts associated with rock waste drainages as well as from the evaporative layer of old tailings potentially toxic elements such as as mo and pb are principally adsorbed or co precipitated with amorphous and crystalline fe oxides but they may also be associated with oxidizing primary sulfides and residual fractions following the development of the dammed tailings pond the secondary minerals were dissolved producing acidic waters contaminated by al 154 mg l 1 cu 150 mg l 1 cd 0 31 m gl 1 co 2 13 mg l 1 mn 73 7 mg l 1 ni 1 74 mg l 1 zn 20 3 mg l 1 and cl 1690 mg l 1 therefore the potential use of recycled water from the sarcheshmenh dammed tailings pond is diminished by the presence of corrosive ions like cl in highly acidic fluids that promote corrosion of pipes and pumps in the water recycling system

water chemistry and isotope study of streams and springs in northern chile abstract the relationship between the stable isotopic and chemical composition of rain spring and stream water was studied in the high andes of northern chile the isotopic pattern of the springs is mainly a reflection of the altitude of their recharge areas however processes that occur during snow melting seem to play a major role in the high altitude springs the streams show patterns similar to the springs in the higher part of the basins but their isotopic composition is modified along the river course due mainly to evaporation the difference in chemical composition between the springs at high and intermediate elevations reflects the duration of water rock interaction the best parameter to trace this length of travelling time is the na k ratio which increases as a result of removal of k into alteration minerals k silicates the na cl ratio increases with salinity in springs and decreases in stream water this reflects the different sources of salt whereas the main source of salts in spring water is the dissolution of silicate and sulfide minerals halite accumulations close to the surface is the main salt source in the streams during floods the stream water chemical characteristics change na cl increases with decreased salinity and na k decreases

multiple zircon growth during fast exhumation of diamondiferous deeply subducted continental crust kokchetav massif kazakhstan diamondiferous rocks from the kokchetav massif kazakhstan represent deeply subducted continental crust in order to constrain the age of ultra high pressure uhp metamorphism and subsequent retrogression during exhumation zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study four different zircon domains were distinguished on the basis of transmitted light microscopy cathodoluminescence trace element contents and mineral inclusions mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions domain 1 consists of rounded cores and lacks evidence of uhp metamorphism domain 2 contains diamond coesite omphacite and titanian phengite inclusions providing evidence that it formed at uhp metamorphic conditions p 43 kbar t 950 c domain 3 is characterised by low pressure mineral inclusions such as garnet biotite and plagioclase which are common minerals in the granulite facies overprint of the gneisses p 10 kbar t 800 c this multi stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from zr saturated partial melts present in the gneisses domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet most probably at amphibolite facies conditions p 5 kbar t 600 c the metamorphic zircon domains were dated by shrimp ion microprobe and yielded ages of 527 5 528 8 and 526 5 ma for domains 2 3 and 4 respectively these indistinguishable ages provide evidence for a fast exhumation beyond the resolution of shrimp dating the mean age of all zircons formed between uhp metamorphic conditions and granulite facies metamorphism is 528 3 ma indicating that decompression took place in less than 6 ma hence the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1 8 cm year we propose a two stage exhumation model to explain the obtained p t t path fast exhumation on top of the subducted slab from depth 140 to 35 km was driven by buoyancy and facilitated by the presence of partial melts a period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision

the origin of ch4 rich fluids in reduced porphyry skarn cu mo au systems abstract although most of the methane on earth is of biogenic origin significant abiogenic methane has been identified in a variety of geological environments moreover the presence of methane has been reported recently in fluid inclusions from a number of reduced porphyry deposits the genesis of this methane however remains poorly understood here we present results of a study of magma exsolved ch4 rich aqueous fluid inclusions in the seleteguole reduced porphyry skarn cu mo au deposit nw china four types of fluid inclusions namely intermediate density two phase aqueous inclusions low density vapor inclusions brine inclusions and aqueous liquid inclusions have been distinguished on the basis of the phases present at room temperature the intermediate density inclusions were trapped as a single phase and contain 3 8 10 4 mol kg ch4 corresponding to an oxygen fugacity between fmq 1 and fmq 0 5 this attests to the remarkably reduced nature of the fluid the 13c value of the ch4 hosted fluid inclusions ranges from 29 4 to 19 1 pdb which distinguishes this methane from biogenic methane indeed the high homogenization temperature of the ch4 rich inclusions 400 c implies that the ch4 was exsolved directly from the magma given the environment in which the magma was likely generated i e at or immediately above a subducting plate we propose that the methane was produced by reactions involving organic matter bearing carbonate rocks in the subduction zone although cu mo and au in most porphyry systems are interpreted to have been transported under oxidizing conditions our results indicate that reduced aqueous liquids and vapors are also capable of transporting appreciable concentrations of these metals

composition of soils and ground waters at the pampa del tamarugal chile anatomy of a fossil geochemical anomaly derived from a distant porphyry copper deposit the pampa del tamarugal in northern chile is a gravel plain part of the hyperarid atacama desert soil geochemical surveys over the pampa using partial extraction analyses identified a large 100 km2 geochemical anomaly for cu subsequent analyses showed unusually high concentrations of elements that are associated with copper porphyry deposits mo as and se the soils are saline nacl and contain high concentrations of br and i which are abundant in ground waters from northern chile the anomaly lies west of one of the world s most important copper porphyry districts which includes the chuquicamata radomiro tomic and el abra deposits follow up drilling in the center of the anomaly through 300 m of piedmont gravel cover revealed barren andesitic basement rocks the soil anomalies were caused by ground water reaching the surface then evaporating both ground waters and soils share a common element assemblage with higher concentrations of mo as and se than cu the former dissolve as anions in ground water which can travel far from their source whereas dispersion of cu2 is restricted by adsorption the soil anomaly is interpreted to be a fossil anomaly preserved by the hyperarid climate to the south the anomaly is cut by an alluvial fan another nearby fan has been dated at 7 ma to the west the anomaly was eroded in the flood plain of the rio loa prior to the present 50 m deep incision of the rio loa the water table in the area of the anomaly may have been higher than the present depth of 33 to 55 m facilitating capillary rise of water to the surface we suggest that the source for the anomaly was the el abra deposit 75 km east and updrainage from the anomaly

large 13c enrichment in primary carbonates from andean altiplano lakes northwest argentina abstract we report here extreme 13 c enrichments up to 13 pdb in primary calcite and aragonite precipitates in saline well oxygenated waters from high altitude lakes in the southern andean altiplano northwestern argentina biological effects as well as variations in carbon source inputs and in the exchange rate with atmospheric co 2 are commonly considered the main controls on the carbon isotope values of authigenic lacustrine carbonate we present sedimentological and geochemical evidence that favors physical processes evaporation effects and co 2 degassing as major controls on 13 c enrichment we propose that large enrichments may result from the non equilibrium gas transfer isotope fractionation during co 2 degassing from thermal springs and evaporation effects in arid environments the dilution effect by large quantities of 14 c free co 2 hinders accurate 14 c chronology of these lake records based on lacustrine organic matter and aquatic plants our results indicate that geothermal and volcanic co 2 sources in lake basins located in volcanic settings and physical fractionation may have a greater significance than commonly accepted to explain lacustrine carbon isotope records

ionospheric characteristics above martian crustal magnetic anomalies 1 we studied several thousand electron density profiles from the mars global surveyor radio science experiment electron densities in some of these profiles change significantly over vertical distances as short as 1 2 km often causing localized bite outs these anomalous profiles are preferentially located above crustal magnetic anomalies anomalous features and their association with strong magnetic fields are also seen in profiles from mariner 9 the probability of a profile being anomalous depends on the orientation of the magnetic field suggesting localized electrodynamic effects upon otherwise global photochemical ionospheric processes on mars unlike most other planets the magnetic field has a short characteristic lengthscale so its effects on the ionosphere will vary over short horizontal distances we discuss possible ionospheric dynamo processes and encourage additional ionospheric observations over regions of strong crustal magnetic field

iodine rich waters involved in supergene enrichment of the mantos de la luna argentiferous copper deposit atacama desert chile recent studies have suggested the involvement of highly saline deep formation waters that modified preexisting cu assemblages to form atacamite during supergene oxidation of cu deposits in the atacama region in this report we document the occurrence of ag i inclusions hosted by supergene chalcocite from mantos de la luna an argentiferous upper jurassic stratabound cu deposit in the coastal range of northern chile the presence of this unusual mineral assemblage indicates that iodargyrite precipitated from reducing iodine rich waters suggesting that the fluids involved in supergene enrichment of cu deposits in the coastal range were more complex than previously thought this suggests the prevalence of hyperarid conditions during the latest stages of supergene enrichment of the mantos de la luna cu deposit in the atacama region supporting the notion that supergene enrichment processes in hyperarid areas are dynamic in nature and do not exclusively require the presence of meteoric water

geochemistry of bolivian salars lipez southern altiplano origin of solutes and brine evolution abstract the southern bolivian altiplano is a volcanic area which contains numerous undrained basins occupied by playas and saline lakes locally named salars solutes carried by springs and rivers into the salars originate mostly from the alteration of the volcanic rocks and the re solution of ancient buried evaporites both weathering and hydrothermal alteration lead to na hco 3 inflow waters of similar composition high contents of lithium and boron are not specifically related to hydrothermal activity evaporite leaching leads to na cl inflow waters of higher salinity atmospheric precipitation contributes only a small amount of ca and so 4 and very little na or cl the calculated evaporative evolution of the inflow waters shows that about half of them should become highly alkaline brines and the other half should end as near neutral brines however alkaline soda lakes are rare in this region most lake brines are of the na cl and na cl so 4 types to understand this anomaly one characteristic sequence of evaporating waters was studied in detail precipitation of calcite and mg smectites accounts only partially for the reduction of alkalinity a further loss of alkalinity is probably related to the oxidation of wind blown native sulfur in peripheral ponds wind blown sulfur is also deposited around the lakes on the drainage basins in the rainy season ephemeral streams carry along sulfur particles in permanent pools of the preconcentration areas a hydrologic control of brine chemistry is suspected inflow waters that should become alkaline appear to evolve to neutral brines if they remain in contact with atmospheric dust and peripheral colluvium for a long time on the contrary inflow waters to soda lakes are generally close to the shore and have only minimal contact with atmospheric dust and peripheral sediments presently all lakes are at relatively low levels and inflow waters often drain an important area of the drainage basin before reaching the topographic low in the past lakes were at higher levels and less interaction occurred between inflow waters atmospheric dust and peripheral sediments as a result the chemistry of some lakes may have changed during the quaternary

source transport and fate of rhenium selenium molybdenum arsenic and copper in groundwater associated with porphyry cu deposits atacama desert chile we collected groundwaters in and around a large 313 mt at 1 08 cu and 0 3 cutoff undisturbed porphyry copper deposit spence in the hyperarid atacama desert of northern chile which is buried beneath 30 180 m of miocene piedmont gravels groundwaters within and down flow of the spence deposit have elevated se up to 800 g l re up to 31 g l mo up to 475 g l and as up to 278 g l concentrations compared to up flow waters interpreted to represent regional groundwater flow in contrast cu is only elevated up to 2036 g l in groundwaters recovered from within the deposit cu concentrations are low down gradient of the deposit the differential behavior of the metals metalloids occurs because the former group dissolves as anions enhancing their mobility whereas the base metals dissolve as cations and are lost from solution most likely through adsorption to clay surface exchange sites and through formation of secondary copper chlorides carbonates and oxides most groundwaters within and down flow of the deposit have eh ph values around the feii feiii phase boundary limiting the impact of fe oxyhydroxides on oxyanions mobility se re mo and as are all mobile with filtered unfiltered samples 1 to the limit of sampling 2 km down gradient from the deposit the increase in ore related metals metalloids and sulfate and decrease in sulfate s isotope ratios from values similar to regional salars 4 to 8 34scdt to lower values closer to hypogene sulfides 1 to 4 34scdt is consistent with active water rock reactions between saline groundwaters and the spence deposit it is likely that hypogene and or supergene sulfides are being oxidized under the present groundwater conditions and mineral saturation calculations suggest that secondary copper minerals antlerite atacamite malachite may also be actively forming suggesting that supergene and exotic copper mineralization is possible even under the present hyperarid climate of the atacama desert

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

continental subduction and exhumation of high pressure rocks insights from thermo mechanical laboratory modelling abstract thermo mechanical physical modelling of continental subduction is performed to investigate the exhumation of deeply subducted continental crust the model consists of two lithospheric plates made of new temperature sensitive analogue materials the lithosphere is underlain by liquid asthenosphere the continental lithosphere contains three layers the weak sedimentary layer the crust made of a stronger material and of a still stronger lithospheric mantle the whole model is subjected to a constant vertical thermal gradient causing the strength reduction with depth in each lithospheric layer subduction is driven by both push force and pull force during subduction the subducting lithosphere is heating and the strength of its layers reduces the weakening continental crust reaches maximal depth of about 120 km and cannot subduct deeper because its frontal part starts to flow up the subducted crust undergoes complex deformation including indicated upward ductile flow of the most deeply subducted portions and localised failure of the subducted upper crust at about 50 km depth this failure results in the formation of the first crustal slice which rises up between the plates under the buoyancy force this process is accompanied by the delamination of the crustal and mantle layers of the subducting lithosphere the delamination front propagates upwards into the interplate zone resulting in the formation of two other crustal slices that also rise up between the plates average equivalent exhumation rate of the crustal material during delamination is about 1 cm year the crust asthenosphere boundary near the interplate zone is uplifted the subducted mantle layer then breaks off removing the pull force and thereby stopping the delamination and increasing horizontal compression of the lithosphere the latter produces shortening of the formed orogen and the growth of relief the modelling reveals an interesting burial exhumation evolution of the sedimentary cover during initial stages of continental subduction the sediments of the continental margin are dragged to the overriding plate base and are partially accreted at the deep part of the interplate zone at 60 70 km depth these sediments remain there until the beginning of delamination during which the pressure between the subducted crust and the overriding plate increases this results in squeezing the underplated sediments out part of them is extruded upwards along the interplate zone to about 30 km depth at an equivalent rate of 5 10 cm year

tectonic implications of mars crustal magnetism mars currently has no global magnetic field of internal origin but must have had one in the past when the crust acquired intense magnetization presumably by cooling in the presence of an earth like magnetic field thermoremanent magnetization a new map of the magnetic field of mars compiled by using measurements acquired at an 400 km mapping altitude by the mars global surveyor spacecraft is presented here the increased spatial resolution and sensitivity of this map provide new insight into the origin and evolution of the mars crust variations in the crustal magnetic field appear in association with major faults some previously identified in imagery and topography cerberus rupes and valles marineris two parallel great faults are identified in terra meridiani by offset magnetic field contours they appear similar to transform faults that occur in oceanic crust on earth and support the notion that the mars crust formed during an early era of plate tectonics

atacamite in the oxide zone of copper deposits in northern chile involvement of deep formation waters atacamite a copper hydroxychloride is an important constituent of supergene oxide zones of copper deposits in northern chile whereas in similar deposits elsewhere it is rare in chile it has generally been assumed to be a primary constituent of the supergene zones there are two difficulties with this supposition the first is that atacamite requires saline water for its formation whereas supergene oxidation was caused by percolating oxygenated meteoric water mainly rainwater the second is that atacamite dissolves rapidly or undergoes phase change when exposed to fresh water supergene enrichment of copper deposits in northern chile extended over a long period 44 to 9 ma being terminated by the onset of hyperaridity during this period there was at least intermittent rainfall exposing previously formed atacamite to dissolution or phase change furthermore atacamite bearing oxide zones in several deposits are directly overlain by thick miocene alluvial gravels the stream waters that transported these gravels would have permeated the oxide zones in some deposits atacamite bearing assemblages occur both in the oxide zones and in contiguous gravels we suggest that atacamite bearing oxide assemblages are more likely to have been a replacement of preexisting oxide phases after the onset of hyperaridity at about 9 ma a hyperarid climate made possible evaporation and concentration of chloride in meteoric waters in this paper we discuss another source of saline waters to modify oxide zones dewatering of the domeyko basin expelled brines along faults some of which had earlier guided the location of porphyry deposits at the spence porphyry copper deposit saline waters which d vs 18o isotope analyses identify as basinal brines are presently rising through the deposit then flowing away along the base of the covering gravels compositions of these waters lie within the stability fields of atacamite and brochantite the two minerals that comprise the oxide zone evidence is presented for other porphyry deposits radomiro tomic and gaby sur that basinal brines may have been involved in the late formation of atacamite

u series dating of co seismic gypsum and submarine paleoseismology of active faults in northern chile 23 s abstract the convergence of the nazca and south american plates along the subduction margin of the central andes results in large subduction earthquakes and tectonic activity along major fault systems despite its relevance the paleoseismic record of this region is scarce hampering our understanding about the relationship between the andes building and earthquake occurrence in this study we used the u series disequilibrium method to obtain absolute ages of paleoearthquake events associated with normal displacements along the active mejillones and salar del carmen faults in the coastal range of the atacama desert of northern chile the 230th 234u disequilibrium ages in co seismic gypsum salts sampled along the fault traces together with marine evidences indicate that earthquakes occurred at ca 29 7 1 7 ka 11 4 ka and 2 4 0 8 ka when coupled with paleoseismic marine and radiocarbon 14c records in the nearby mejillones bay evidencing large dislocations along the mejillones fault the geochronological dataset presented here is consistent with the notion that gypsum salts formed during large earthquakes as a result of co seismic dilatancy pumping of saline waters along the major faults based on maximum observed cumulative vertical offsets in the studied faults this phenomena could have occurred episodically at a rate in the order of 1 40 to 1 50 with respect to the very large subduction earthquakes during the latest pleistocene holocene period the results presented here reveal that the u series disequilibrium method can be successfully applied to date the gypsum salts deposited along faults during seismic events and therefore directly constrain the age of large paleoearthquakes in hyperarid and seismically active zones

stable isotopic composition of soil calcite o c and gypsum s overlying cu deposits in the atacama desert chile implications for mineral exploration salt sources and paleoenvironmental reconstruction abstract soils overlying two porphyry cu deposits spence gaby sur and the pampa del tamarugal atacama desert northern chile were collected in order to investigate the extent to which saline groundwaters influence soil chemistry in regions with thick miocene and younger sediment cover soil carbonate calcite was analyzed for c and o isotopes and pedogenic gypsum for s isotopes soil calcite is present in all soils at the spence deposit but increases volumetrically above two fracture zones that cut the miocene gravels including gravels that overlie the deposit the c isotope composition of carbonate from the soils overlying fracture zones is indistinguishable from pedogenic carbonate elsewhere at the spence deposit all 13 c vpdb values fall within a narrow range 1 40 4 23 consistent with the carbonate having formed in equilibrium with atmospheric co 2 however 18 o vpdb for carbonate over both fracture zones is statistically different from carbonate elsewhere average 18 o vpdb 0 82 vs 2 23 respectively suggesting involvement of groundwater in their formation the composition of soils at the tamarugal anomaly has been most strongly affected by earthquake related surface flooding and evaporation of groundwater 13 c vpdb values 4 28 to 2 04 are interpreted to be a mixture of dissolved inorganic c dic from groundwater and atmospheric co 2 at the spence deposit soils only rarely contain sufficient so 4 for s isotope analysis the so 4 bearing soils occur only above the fracture zones in the gravel results are uniform 3 7 4 9 34 s cdt which is near the middle of the range for so 4 in groundwater 0 9 7 3 sulfur in soils at the gaby sur deposit 3 8 6 1 34 s cdt is dominated by gypsum which primarily occurs on the flanks and tops of hills suggesting deposition from so 4 rich fogs sulfate in gaby sur deposit gypsum is possibly derived by condensation of airborne so 4 from volcanic so 2 from the nearby andes at the gaby sur deposit and tamarugal anomaly pedogenic stable isotopes cannot distinguish between s from porphyry or redeposited so 4 from interior salars the three sites studied have had different histories of salt accumulation and display variable influence of groundwater which is interpreted to have been forced to the surface during earthquakes the clear accumulation of salts associated with fractures at the spence deposit and shifts in the isotopic composition of carbonate and sulfate in the fractures despite clear evidence of relatively recent removal of salts indicates that transfer from groundwater is an ongoing process the interpretation that groundwaters can influence the isotopic composition of pedogenic calcrete and gypsum has important implications for previous studies that have not considered this mechanism

evolutionary model for the taiwan collision based on physical modelling abstract 2 d and 3 d physical modelling of lithospheric convergence in the luzon taiwan ryukyu region is performed with properly scaled laboratory models the lithospheric model consists of two parts continental the asian plate ap and oceanic the philippine sea plate psp the oceanic lithosphere has one layer while the continental lithosphere includes both mantle and crustal layers the continental margin is covered by sediments a low viscosity asthenosphere underlies the lithosphere the opposing luzon and ryukyu subduction zones are initiated by inclined cuts made within the psp the subduction collision is driven by a piston pre collisional intraoceanic subduction along the luzon and ryukyu boundaries results in the formation of a transform zone between them with two tear faults at the ends the psp undergoes strong compression along this zone subduction of the chinese margin under the luzon boundary further increases the compression compressive stresses reach the yield limit of the psp in the arc area which is a weak zone in the experiments the plate fails at the western side of the arc along an eastward dipping fault the longitudinal valley fault underthrusting of the frontal wedge of the psp along this fault results in the closure of the fore arc basin and is then blocked the psp fails at the opposite side of the luzon arc along the westward dipping fault the failure releases lithospheric compression in this region and results in the initiation of southward propagating subduction of the psp under northeastern taiwan the incipient subduction zone becomes part of the southeastward retreating ryukyu subduction zone which allows the okinawa back arc rift to propagate into taiwan the taiwan collision thus includes the following succession of major processes over time or from south to north 1 an e w shortening of the psp in the luzon arc 2 a failure of this plate at the western side of the arc and the formation of the eastward dipping longitudinal valley fault the transient plate boundary 3 a closure of the fore arc basin and a rapid uplift of the orogen 4 a failure of the psp at the eastern side of the luzon arc partly overthrusting the orogen and the initiation of westward wn ward subduction of the psp 5 and finally back arc rifting in the rear of this incipient subduction zone i e in northern taiwan all these processes commence with some delay with respect to the preceding ones and propagate southwards

isotope hydrology of groundwaters in the pampa del tamarugal chile the water resources in northern chile are extremely scarce and development is limited by this this paper discusses the isotope hydrology of groundwaters in the pampa del tamarugal an area of extreme aridity rainfall 1 mm yr whose groundwater resources are mined for the town of iquique as well as for industrial and agricultural purposes the aim of the project was to obtain information on modern recharge and to delineate if possible recharge environments to obtain the necessary background information a precipitation survey in the high andes as well as spring and surface water studies were carried out the results show that a well defined meteoric water line exists where 2h 7 8 18o 10 3 and altitude effects depend on air mass movements and cannot be defined without a broader regional and detailed sampling programme however it is still possible to assign maximum altitudes of recharge to springs in the andes and at the eastern border of the pampa del tamarugal comparison of these data with groundwater compositions show that these groundwaters originate from infiltrating surface water rather than directly infiltrated precipitation a dependence of individual groundwater systems on specific quebradas river valleys is recognized however low 14c activities indicate that most of the waters pumped today are fossil and at least several hundreds if not thousands of years old some minor subsurface recharge does occur at the foot of the andes especially at pica where high altitude waters discharge but even there groundwater appears to be a diminishing resource

evolutionary model of the himalaya tibet system geopoem based on new modelling geological and geophysical data a two dimensional thermo mechanical laboratory modelling of continental subduction was performed the subducting continental lithosphere includes a strong brittle upper crust a weak ductile lower crust and a strong upper mantle the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is denser than the asthenosphere the lithospheric layers are composed of material whose strength is sensitive to and inversely proportional to temperature throughout the experiment the model surface was maintained under relatively low temperature and the model base at higher temperature the subduction rate satisfied the peclet criterion modelling confirms that the continental crust can be deeply subducted and shows that slab break off delamination and tectonic underplating are fundamental events with drastic consequences on the subsequent evolution of the convergent system combining these results with previous purely mechanical modelling we elaborate a new evolutionary model for the himalaya tibet convergent system the principal successive stages are 1 subduction of the indian continental lithosphere to 200 250 km depth following subduction of the tethys oceanic lithosphere 2 failure and rapid buoyancy driven uplift of the subducted continental crust from ca 100 km depth to some depth that varies along the mountain belt 20 30 km on average 3 break off of the indian subducted lithospheric mantle with the attached oceanic lithosphere 4 subduction underplating of the indian lithosphere under asia over a few to several hundred kilometers 5 delamination roll back and break off of the indian lithospheric mantle 6 failure of the indian crust in front of the mountain belt formation of the main central thrust and underthrusting of the next portion of indian lithosphere beneath tibet for a few hundred kilometers at the beginning of stage 6 the crustal slice corresponding to the crystalline himalayas undergoes erosion activated uplift and exhumation fl 2000 elsevier science b v all rights reserved

chemical and isotopic compositions of fluids at cumbal volcano colombia evidence for magmatic contribution we present chemical and isotopic data for fumarolic vapor and thermal spring discharges from cumbal volcano sw colombia in 1988 cumbal showed signs of apparent reactivation gases and steam condensates were sampled from summit fumaroles 83 375 c of cumbal in 1988 1996 and discharges from thermal springs 15 37 c on its flanks in 1995 1996 based on relative co2 total s h2s so2 and hcl contents fumarolic discharges are principally magmatic in composition fumarolic steam condensates 1993 1996 have 18o values of 11 4 to 2 5 and d values of 91 to 43 18o and d compositions indicate mixing between local meteoric and magmatic waters 3he 4he ratios in 1993 1996 samples 5 3 7 9 rcor are consistent with addition of mantle derived helium 13cco2 values for 1996 samples 6 7 to 5 0 likely indicate contribution of marine carbonate organic sediment and mantle derived co2 34sstotal compositions 4 6 to 5 6 of 1988 1996 fumarolic discharges have magmatic signatures and may reflect cycles of deposition and remobilization of native sulfur thermal waters are acid sulfate or bicarbonate in composition relative concentrations of chemical constituents of thermal waters imply that the composition of waters is controlled by absorption of magmatic volatiles into shallow ground and surface waters dilution with meteoric waters along flow paths and dissolution of host rocks 18o and d compositions are consistent with a meteoric origin of waters 34sstotal values for thermal spring gas discharges 9 6 10 5 suggest deposition of 34s depleted sulfur minerals along flow paths chemical and isotopic compositions of 1988 1995 fumarolic discharges provide evidence for input of magmatic volatiles into the cumbal hydrothermal system from 1995 to 1996 geochemical data show increasing hydrothermal signatures suggesting a decline in magmatic volatile input

impact of arc continent collision on the conditions of burial and exhumation of uhp lt rocks experimental and numerical modelling abstract a 2 d physical and finite element numerical modelling of arc continent collision was performed to study the deformation and failure of the overriding lithosphere the experimental technique allowed us to model the whole subduction collision process from oceanic subduction to deep subduction of the continental crust with the numerical approach we have modelled the deformation of the overriding plate only through initial stages of its failure and studied the influence of different parameters on this process the results obtained by both techniques are coherent and mutually complementary they show that the failure of the overriding plate is physically quite plausible or even inevitable during subduction the conditions for such a failure the weakening of this plate are prepared during oceanic subduction the weakening occurs due to the interaction between the subducting lithosphere and the asthenosphere in the mantle corner between the two plates and due to back arc spreading in oceanic subduction zones with a compressional regime no back arc opening thick and strong back arc lithosphere the weakest zone is volcanic arc area when weakening becomes sufficient during subduction the lithosphere fails in this area the failure occurs along a fault dipping under the arc in either of two possible directions and results either in subduction reversal or subduction of the fore arc almost half of the presently active subduction zones are characterised by a tensional subduction regime with back arc spreading in such subduction zones the weakest place is not the volcanic arc but the back arc spreading centre when a subduction regime changes from tensional to compressional failure occurs in the vicinity of the extinct spreading centre this process can occur during oceanic subduction again along either a trench vergent or trenchward dipping fault but the formation of a trench verging fault is most likely in this latter case which is a principal subject of our study the failure is followed by partial subduction of the arc plate complete subduction occurs during arc continent collision subduction of the continental margin when tectonic compression of the lithosphere increases rapidly and becomes sufficient to push the arc plate into the mantle the arc itself can be subducted completely or be partially or completely scraped off and accreted a deeply subducted material including continental margin is preserved at relatively low temperatures between the lithospheric mantle and the cold subducted arc plate to about 150 km depth subduction of the arc plate is a major phenomenon which affects all processes associated with continental subduction from deep burial and hp lt metamorphism to exhumation of subducted material does this process occur in nature future investigations will allow us to answer this question in this paper we analyse the conditions of emplacement of a very young oceanic lithosphere samail ophiolite on the continental crust in oman in the late cretaceous and argue that this lithosphere formed in a back arc basin it reached and overthrust the arabian continent after complete subduction of the arc plate

composition of groundwaters associated with porphyry cu deposits atacama desert chile elemental and isotopic constraints on water sources and water rock reactions abstract groundwaters were collected around the spence porphyry copper deposit atacama desert northern chile to study water porphyry copper ore bodies interaction and test hypotheses regarding transport of metals through thick overburden leading to the formation of soil geochemical anomalies the deposit contains 400 mt of 1 cu and is completely buried by piedmont gravels of miocene age groundwaters were recovered from the eastern up hydraulic gradient upflow margin of the spence deposit from within the deposit and for two kilometers down flow from the deposit water table depths decrease from 90 m at the upflow margin to 30 m 1 5 km down flow groundwaters at the spence deposit are compositionally variable with those upflow of the deposit characterized by relatively low salinities 900 7000 mg l and na so42 type compositions these waters have compositions and stable isotope values similar to regional groundwaters recovered elsewhere in the atacama desert of northern chile in contrast groundwaters recovered within and down flow of the deposit range in salinity from 10 000 to 55 000 mg l one groundwater at 145 000 mg l and are dominantly na cl type waters dissolved sulfate values are however elevated compared to upflow waters and 34scdt decreases into the deposit from 4 to 2 consistent with increasing influence of sulfur derived from oxidation of sulfide minerals within the deposit the increase in salinity and conservative tracers cl br li and na and the relationship between oxygen and hydrogen isotopes suggests that in addition to water rock reactions within the deposit most of the compositional variation can be explained by groundwater mixing with perhaps a minor role for evaporation a groundwater mixing scenario implies a deeper more saline groundwater source mixing with the less saline regional groundwater flow system flow of deeper more saline groundwater along pre existing structures has important implications for geochemical exploration and metal transport models

solar wind interaction with the ionosphere atmosphere and crustal magnetic fields at mars mars global surveyor magnetometer electron reflectometer radio science and accelerometer data 1 the solar wind sw mainly interacts with the martian ionosphere atmosphere however the intense crustal magnetization creates localized minimagnetospheres above the surface the interplanetary magnetic field imf can reconnect with these crustal fields and this allows sw and more energetic particles to precipitate into and heat the neutral atmosphere in localized regions especially in the southern hemisphere a study of 907 electron density profiles obtained by the mgs radio science rs experiment at latitudes from 63 to 77 and 219 profiles at latitudes 69 to 64 5 has been conducted the role of the magnetic field as a driving force of the cross terminator convection is discussed longitudinal variations of neutral atmosphere scale height hpeak derived from the mgs rs data have been analyzed ionospheric parameters are found to be more variable in the southern than in the northern hemisphere this correlates well with the variability of the magnetic field direction from local zenith the scale height of neutral atmosphere density hacc derived from the mgs accelerometer data is also found to be more variable in the southern than in the northern hemisphere

the origin of brines and salts in chilean salars a hydrochemical review abstract northern chile is characterized by a succession of north south trending ranges and basins occupied by numerous saline lakes and salt crusts collectively called salars fossil salt crusts are found to the west in the extremely arid central valley while active salars receiving permanent inflows fill many intravolcanic basins to the east in the semiarid cordillera sea salts and desert dust are blown eastward over the cordillera where they constitute an appreciable fraction of the solute load of very dilute waters salt content evaporative concentration of inflow waters leads to sulfate rich or calcium rich near neutral brines alkaline brines are almost completely lacking the alkalinity calcium ratio of inflow waters is lowered by the oxidation of native sulfur reducing alkalinity and the deposition of eolian gypsum increasing ca concentration theoretically so4 rich inflow waters and their derived so4 rich brines should be found in the intravolcanic basins of the cordillera because of the ubiquity of native sulfur while ca rich brines should prevail in sedimentary basins where ca rich minerals are abundant this relation is perfectly observed in the salar de atacama the largest in chile however several salars located within the volcanic cordillera belong to the ca rich group inflows and brines may have acquired their ca rich composition in pleistocene time when their drainage basins were mainly sedimentary later on recent lava flows and ignimbrites covered the sedimentary formations underground waters may have kept their early sedimentary signature by continuous recycling however the weathering of volcanic rocks tend to slowly shift the water compositions from the ca rich to the so4 rich type

atacamite formation by deep saline waters in copper deposits from the atacama desert chile evidence from fluid inclusions groundwater geochemistry tem and 36 cl data the presence of large amounts of atacamite in oxide zones from ore deposits in the atacama desert of northern chile requires saline solutions for its formation and hyperarid climate conditions for its preservation we investigated the nature and origin of atacamite forming solutions by means of coupling groundwater geochemical analyses with fluid inclusion data high resolution mineralogical observations and chlorine 36 36cl data in atacamite from the mantos blancos and spence cu deposits in both deposits the salinities of fluid inclusions in atacamite are comparable to those measured in saline groundwaters sampled from drill holes the average salinity of fluid inclusions in atacamite for the mantos blancos and spence deposits 7 9 and 2 3 wt nacleq respectively are strongly correlated to the salinities at which gypsum supersaturates from groundwaters in both deposits total dissolved solids 5 9 and 1 3 wt nacleq respectively this correlation is confirmed by transmission electron microscopy observations of atacamite bearing samples revealing an intimate association between atacamite and gypsum that can be traced down to the nanometer scale 36cl data in atacamite provide new lines of evidence concerning the origin and age of the saline waters that formed atacamite in various stratabound and porphyry cu deposits from the atacama desert all atacamite samples show very low 36cl to cl ratios 11 10 15 to 28 10 15 at at 1 comparable to previously reported 36cl to cl ratios of deep formation waters and old groundwaters in addition 36cl to cl ratios in atacamite correlate with u and th concentration in the host rocks but are independent from distance to the ocean this trend supports an interpretation of the low 36cl to cl ratios in atacamite as representing subsurface production of fissiogenic 36cl in secular equilibrium with the solutions involved in atacamite formation therefore 36cl in atacamite strongly suggest that the chlorine in saline waters related to atacamite formation is old 1 5 ma but that atacamite formation occurred more recently 1 5 ma than suggested in previous interpretations our data provide new constraints on the origin of atacamite in cu deposits from the atacama desert and support the recent notion that the formation of atacamite in hyperarid climates such as the atacama desert is an ongoing process that has occurred intermittently since the onset of hyperaridity

nightside electron precipitation at mars geographic variability and dependence on solar wind conditions 1 electron precipitation is usually the dominant source of energy input to the nightside martian atmosphere with consequences for ionospheric densities chemistry electrodynamics communications and navigation we examine downward traveling superthermal electron flux on the martian nightside from may 1999 to november 2006 at 400 km altitude and 2 a m local time electron precipitation is geographically organized by crustal magnetic field strength and elevation angle with higher fluxes occurring in regions of weak and or primarily vertical crustal fields while stronger and more horizontal fields retard electron access to the atmosphere we investigate how these crustal field organized precipitation patterns vary with proxies for solar wind sw pressure and interplanetary magnetic field imf direction generally higher precipitating fluxes accompany higher sw pressures specifically we identify four characteristic spectral behaviors 1 stable regions where fluxes increase mildly with sw pressure 2 high flux regions where accelerated peaked spectra are more common and where fluxes below 500 ev are largely independent of sw pressure 3 permanent plasma voids and 4 intermittent plasma voids where fluxes depend strongly on sw pressure the locations sizes shapes and absence existence of these plasma voids vary significantly with solar wind pressure proxy and moderately with imf proxy direction average precipitating fluxes are 40 lower in strong crustal field regions and 15 lower globally for approximately southwest proxy directions compared with approximately northeast directions this variation of the strength and geographic pattern of the shielding effect of mars crustal fields exemplifies the complex interaction between those fields and the solar wind

detecting induced correlations in hydrochemistry abstract plots containing common variables are prone to induced correlations these plots are commonly used in hydrochemistry and other branches of geochemistry with no discussion of the potential influence of the common variables on the observed trends some examples from the literature include 1 comparison of a sum to a component of the sum 2 comparison of a ratio to its own numerator or denominator 3 comparison of two ratios with the same numerator or denominator these types of common variable plots may increase or decrease the correlation that exists between two independent variables this paper presents a randomisation method for distinguishing between the real and induced relationships that can occur in plots of type 1 2 or 3 above thereby enabling a distinction between hydrochemical processes and induced correlation s if the slope and correlation coefficient of the actual data are similar to the randomised data the relationship cannot be attributed to a hydrochemical process alone if the slope and correlation coefficient are different it is likely that the data trend is the result of a real process but we recommend this be confirmed using additional evidence such as relationships between other element ion concentrations or independent isotopic evidence common variable plots are useful tools for identifying and describing hydrochemical processes but the data trends need to be tested to ensure that correlations are a result of real processes rather than an artefact caused by a common variable

magnetic field and plasma observations at mars initial results of the mars global surveyor mission the magnetometer and electron reflectometer investigation mag er on the mars global surveyor spacecraft has obtained magnetic field and plasma observations throughout the near mars environment from beyond the influence of mars to just above the surface at an altitude of 100 kilometers the solar wind interaction with mars is in many ways similar to that at venus and at an active comet that is primarily an ionospheric atmospheric interaction no significant planetary magnetic field of global scale has been detected to date 2 1021 gauss cubic centimeter but here the discovery of multiple magnetic anomalies of small spatial scale in the crust of mars is reported

supergene enrichment of copper deposits since the onset of modern hyperaridity in the atacama desert chile supergene enrichment of cu deposits in the atacama desert has played a critical role in making this the prime cu producing province of the world previously this has been believed to have occurred exclusively over a long period from the middle eocene to the late miocene which ended when climatic conditions changed from arid to hyperarid here we report u series disequilibrium ages in atacamite bearing supergene assemblages that provide a new conceptualization on both the supergene enrichment process and the onset of extreme hyperaridity in the atacama desert 230th 234u ages of gypsum intergrown with atacamite in supergene veins from cu deposits cluster at 240 ka chuquicamata 130 ka mantos blancos spence and 80 ka mantos de la luna michilla when coupled with previous data these results indicate that supergene enrichment of cu deposits did not cease after the onset of hyperaridity we propose that supergene enrichment in the atacama region developed in two main stages the main phase caused by downward circulation of meteoric waters in a semi arid setting was active from 45 until 9 ma with a last pulse ca 5 ma in the southern atacama desert during this phase atacamite bearing supergene assemblages were not preserved because atacamite requires saline water for its formation and rapidly dissolves when contacted by meteoric water this was followed by a second stage starting at 2 1 5 ma and continuing until at least the late pleistocene when deep formation waters derived from the basement passed up through and modified the pre existing supergene cu oxide minerals atacamite has then been preserved in the prevailing hyperarid climate

a 106ka paleoclimate record from drill core of the salar de atacama northern chile abstract a 100 m long salt core sqm 2005 from the salar de atacama northern chile 23 s 68 w a dry lake bed contains a 106 kyr paleoclimate record of hydrologic balances on the western slopes of the central andes of south america six u series disequilibrium dates range sequentially from 106 1 6 4 to 5 4 2 7 ka based on sedimentary structures and petrographic textures of salts and associated siliciclastic sediments interpretations of paleoenvironments and net hydrologic balance suggest that relatively wet periods saline lakes and expanded mudflats existed in the salar de atacama from 75 7 to 60 7 ka and from 53 4 to 15 3 ka with the wettest perennial lake interval from 26 7 to 16 5 ka short relatively wet periods also occurred in the holocene from 11 4 to 10 2 ka and from 6 2 to 3 5 ka these wet periods at salar de atacama correspond well with other late pleistocene climate records from the central andes that are a function of the net hydrologic budget the minchin tauca lake sequence from salar de uyuni bolivia is synchronous with the 53 4 15 3 ka saline lake wet period at salar de atacama the tauca phase coincided with the wettest perennial lake interval at salar de atacama from 26 7 to 16 5 ka the coipasa lake phase from uyuni was probably contemporaneous with the early holocene wet period at salar de atacama from 11 4 to 10 2 ka the early holocene wet interval at salar de atacama was also synchronous with the maximum holocene lake levels of the chilean altiplano lakes to the east and with grass rich rodent middens between 11 8 and 10 5 ka in age from the atacama basin the lake period at salar de atacama between 26 7 and 15 3 ka compares closely with low 18o values low anion concentrations and high accumulation rates from the sajama glacial ice core southwestern bolivia between 25 and 15 5 ka it is concluded that wet and dry phases were synchronous in the central andean plateau of southwestern bolivia and along the western flank of the andes of northern chile over the past 50 kyr this region which today receives its moisture from the amazon basin to the east probably also did so in the past the wet periods at salar de atacama are in phase with january insolation maxima at 15 s at 70 45 and 20 ka which suggests that past changes in summer insolation influenced precipitation patterns in the central andes however there are no major wet periods linked to the january insolation maximum at 93 ka and the present day extremely dry climate in the atacama desert is associated with a peak in summer insolation therefore there must be other important influences over precipitation in the central andes which modify the first order insolation controls

hydrogeochemistry groundwater ages and sources of salts in a granitic batholith on the canadian shield southeastern manitoba abstract the hydrogeochemistry of the lac du bonnet granitic batholith has been determined for the region of the whiteshell research area wra in southeastern manitoba canada this work forms part of the geosciences studies performed for the canadian nuclear fuel waste management program over the period 1980 1995 by atomic energy of canada limited aecl knowledge of the variation of groundwater chemistry and its causes is useful in assessing the performance and safety of a nuclear fuel waste vault located at depths of up to 1000 m in a crystalline rock formation of the canadian shield groundwaters and matrix pore fluids have been obtained by standard sampling methods from shallow piezometers in clay rich overburden from packer isolated borehole zones intersecting fractures or fault zones in the bedrock and from boreholes in unfractured rock in aecl s underground research laboratory url eighty six individual fracture groundwaters have been sampled and analysed from permeable zones in 53 boreholes drilled to depths of up to 1000 m in the lac du bonnet batholith in addition 28 groundwaters from piezometers in a large wetland area near the url have been sampled and analysed to determine the influence of clay rich overburden on the bedrock hydrogeochemistry analyses have been made for major and minor ions ph eh trace metals and stable and radioactive isotopes to characterise these groundwaters and relate them to their hydrogeologic regimes shallow groundwaters in the fractured bedrock are generally dilute tds 500 m na ca cl so4 waters of increasing salinity up to 50 g l with depth are found and in some fractures the waters have evolved to a ca na cl composition isotopic data indicate that these waters are warm climate and pre glacial in origin with residence times of over 1 ma pore fluids observed to drain from the unfractured rock matrix in the url facility are almost pure ca cl in composition 90 g l salinity and have a 2h 18o composition displaced well to the left of the global meteoric water line about which all other wra groundwaters lie this information indicates that these pore fluids have undergone prolonged water rock interaction and have residence times of 101 103 ma most of the deeper fracture groundwaters and pore fluids have low br cl ratios and moderate to high 34s values of dissolved so4 which indicates that their salinity could be derived from a marine source such as the basinal sedimentary brines and evaporites to the west of the batholith these fluids may have entered the batholith during early paleozoic times when sedimentary rocks were deposited over the granite and were driven by a hydraulic gradient resulting from higher ground in western canada the hydrogeochemical data and interpretations show that below 500 m in the wra fracture hosted groundwaters are very saline reducing and old and are therefore indicative of stagnant conditions over the period of concern for nuclear waste disposal 1 ma the intact rock matrix at these depths is extremely impermeable as indicated by the presence of pore fluids with unusual geochemical and isotopic characteristics the pore fluids may represent basinal brines that have evolved geochemically and isotopically to their current composition over periods as long as 103 ma

exploring for deeply covered mineral deposits formation of geochemical anomalies in northern chile by earthquake induced surface flooding of mineralized groundwaters northern chile is a major producer of copper from porphyry deposits most ores worked to date were exposed near the surface exploration is now focused on deposits along a basement unconformity buried under piedmont gravels deposited by ca 10 ma two recent discoveries the spence and gaby sur deposits are covered by gravel soils above these deposits have anomalies for elements present in saline mineralized groundwaters the anomalies occur above fracture zones in the gravels believed to represent the upward propagation of basement faults that guided emplacement of the porphyries we propose that during earthquakes in this seismically active region there was pumping of saline basement waters up the faults and fracture zones entraining mineralized groundwaters from the deposits after surface flooding and evaporation elements were redistributed by infrequent rains sampling shows that surface active cations such as cu 2 1 were adsorbed and retained in the top few centimeters of soil whereas nacl and porphyry indicator elements that dissolve as anions se re and as are not adsorbed and are removed to 40 cm depth given the 10 m y interval since the gravels were deposited and the hyperarid climate of the atacama desert anomalies may have formed by repeated episodes of seismically induced flooding company geochemical surveys show that such features are numerous in northern chile producing linear zones of saline soils plus other constituents of local groundwaters e g b and i where faults controlling these features intersect mineralization elements specific to the mineralization are also present saline zones may be outlined in the field by measuring the conductivity of soil plus water slurries sampling for full geochemical analyses can be limited to areas so defined anomalies may be generated along fault lines in other seismically active wetter climates but only for elements that are adsorbed on soil minerals although the context of the study is mineral exploration the process may be relevant to the formation of geochemical features of environmental interest above major faults

using iodine isotopes to constrain supergene fluid sources in arid regions insights from the chuquicamata oxide blanket although iodine is rare in crustal settings previous studies have documented its occurrence in the supergene zones of base and precious metal ore deposits in arid environments in this report we present a novel application of the iodine 129 129i isotope tracer to iodine rich samples from the world s largest supergene cu profile at the chuquicamata deposit in northern chile where anomalous concentrations of this element have been reported all supergene marshite cui samples from chuquicamata and iodine rich soil above the mansa mina deposit have 129i i isotope ratios 190 560 10 15 that are significantly lower than those of surface waters i e 1 500 10 15 indicating that iodine was most likely derived from a reservoir of marine origin rather than a meteoric and or atmospheric source geochemical modeling shows that the long term dilution of iodinerich deep formation waters by meteoric water during the main stage of supergene alteration 40 10 ma resulted in fluid 129i i ratios that are within the range of mineral soil 129i i ratios observed at chuquicamata strongly suggesting that iodine was remobilized from the lower jurassic to mid cretaceous marine basement results from this study show that iodine isotopes can be successfully applied to trace the origin and nature of supergene fluids in iodine rich oxide blankets leach caps and soils above buried cu deposits and also to constrain the timescales of fluid circulation during supergene enrichment

oxide mineralization at the radomiro tomic porphyry copper deposit northern chile the main copper oxide minerals in the radomiro tomic deposit are atacamite copper clays chrysocolla and copper wad their relative abundance provides the basis for dividing the oxide zone into two major geologic units the upper oxide unit and the lower oxide unit furthermore the study of mineral abundances in the oxide zones allowed the definition of an oxide mineral zonation that consists of five mineral assemblages based on the relative proportion of the three main copper oxide components atacamite copper clays and chrysocolla these classifications allow both an understanding of the distribution of copper minerals throughout the zone and an interpretation of the supergene processes that formed it the upper oxide unit is mineralogically heterogeneous the copper content is furnished on average by 40 vol percent atacamite 31 vol percent copper clays with minor chysocolla and copper wad the lower oxide unit is more homogeneous atacamite accounts for 70 vol percent of the copper ore minerals with minor amounts of copper clays and chrysocolla copper and chlorine concentrations and porosity are greater in the upper oxide unit whereas hematite chalcocite and molybdenite concentrations and specific gravity are greater in the lower oxide unit the vertical distribution of the cl cu ratio shows a chlorine deficient zone located between elevations of 2 780 and 2 870 m between these elevations the proportion of atacamite is lower and the proportion of copper clays chrysocolla and copper wad is higher than in other portions of either the upper oxide or lower oxide units the oxide zone at radomiro tomic was originated through the in situ oxidation of a preexisting secondary sulfide enrichment blanket oxidation produced an extensive zone of atacamite accompanied by weakly developed argillic alteration the upper part of the deposit was further enriched by lateral migration of copper rich solutions to the north and east where copper precipitated as chrysocolla copper wad and copper in clays in these areas strong argillic alteration characterized by the presence of montmorillonite was developed this second alteration phase impacted only the uppermost portion of the oxide zone and the base of overlying gravels

stratigraphy and structure of the indus suture in the lower swat pakistan nw himalaya abstract we present a geological map detailed structural description and new subdivisions of the indus suture zone lithologies in northeast pakistan lower swat region the indus suture zone is a dominantly fore arc related assemblage obducted onto the indian plate initial southward thrusting was followed by the reactivation of the indus suture as a ductile brittle normal fault significant strike slip faulting may have taken place at this stage later brittle reverse faults demonstrate that shortening outlasted normal faulting the comparison of the structural evolution of the indus suture zone and the indian plate shows that both terrains were sharing the deformational history possibly since the earliest stage of collision the lower swat area records a full orogenic cycle from early rifting through continent arc collision to orogenic collapse we present a geodynamic model that integrates all these elements

variability of the altitude of the martian sheath received 31 march 2005 revised 18 august 2005 accepted 25 august 2005 published 24 september 2005 1 using electron energy spectra we identify time periods when the mars global surveyor mgs spacecraft is in or above the martian magnetic pileup boundary mpb we use more than five years of data to develop a statistical picture of the location of the mpb relative to the mgs mapping altitude near 400 km we show for the first time that the mpb location is sensitive to interplanetary magnetic field imf orientation and to martian season and confirm a dependence upon solar wind pressure we confirm that crustal magnetic sources raise the altitude of the mpb and demonstrate that sheath electrons populate magnetic cusp regions in the southern hemisphere during southern summer strong crustal fields near the subsolar point raise the altitude of the mpb over the entire dayside implying that martian crustal fields modify the solar wind interaction globally citation brain d a j s halekas r lillis d l mitchell r p lin and d h crider 2005 variability of the altitude of the martian sheath geophys res lett 32 l18203

a model of palaeozoic obduction and exhumation of high pressure low temperature rocks in the southern urals abstract a new evolutionary geodynamic model of devonian continental subduction and exhumation of high pressure low temperature metamorphic rocks in the southern urals is proposed based on the results of physical modelling geological and geophysical data and on the comparison of this belt with other orogens the model includes the following principal phases 1 closure of the western uralian ocean by eastward intraoceanic subduction associated with the magnitogorsk volcanic arc 2 subduction of the european continental margin causing failure of the overriding plate in the arc area along a fault dipping eastward under the arc 3 subduction into the mantle of the fore arc block together with the underlying continental crust and sedimentary layer the sedimentary cover as well as the crustal and mantle fragments of this block are scraped off and accreted in front of and under the arc the continental crust shielded from the hot mantle by the fore arc block subducts to depths of more than 150 km remaining at a relatively low temperature the crust then fails at depths of several tens of kilometres 4 the subducted crustal slice starts to rapidly rise and intrude the interplate zone scraping and pushing up the sediments and slivers of the oceanic fore arc block previously dragged down to different depths 5 exhumation of the rising high pressure low temperature rocks within the uraltau dome separating the subduction accretionary complex from the magnitogorsk arc the eastern border of the uraltau dome coincides with the main uralian fault this fault represents a complex suture zone which at the stage of deep crustal subduction was associated with a major thrust subduction fault and at the exhumation stage with a major normal fault corresponding to the upper surface of the rising crustal sedimentary body

effect of crustal magnetic fields on the near terminator ionosphere at mars comparison of in situ magnetic field measurements with the data of radio science experiments on board mars global surveyor 1 electron density profiles of the ionosphere of mars derived from radio occultation data obtained by the radio science mars global surveyor mgs experiment have been compared with the crustal magnetic fields measured by the mgs magnetometer electron reflectometer mag er experiment in the northern hemisphere the crustal fields are rather weak and usually do not prevent direct interaction between the solar wind sw and the martian ionosphere atmosphere exceptions do occur in the isolated mini magnetospheres formed by the crustal anomalies much stronger crustal fields are located in limited regions in the southern hemisphere and lead to the formation of large scale mini magnetospheres a study of the electron density profiles obtained when the mini magnetosphere regions are near the terminator has been conducted using the magnetic field measurements at altitudes 170 180 km and 400 km the magnitude of the electron density peak and the effective scale height of the electron density for two altitude ranges 145 165 km and 165 185 km have been derived for each of the 326 selected profiles studied in each hemisphere the longitudinal variations of these derived parameters have been studied a significant difference between the large scale mini magnetospheres and regions outside of them has been found the neutral atmosphere is cooler inside the large scale mini magnetospheres the variations of the magnitude of the electron density peak indicate that the electrons are usually hotter inside a large scale mini magnetosphere than outside it appears that the mini magnetospheres formed by strong crustal magnetic fields prevent additional heating of the neutral atmosphere due to direct interaction of the sw and also confine the hotter electrons created by photo ionization

a revised late eocene age for porphyry cu magmatism in the escondida area northern chile new u pb zircon and 40 ar 39 ar single crystal incremental heating biotite dates are reported for porphyritic rocks from three mineralized centers in the escondida region of northern chile all dates are reported with 2sigma errors at the chimborazo porphyry cu prospect a weighted mean 40 ar 39 ar plateau age of 38 09 or 0 30 ma has been obtained for igneous biotite from a syn or postmineralization feldspar biotite quartz porphyry intrusion at the zaldivar porphyry cu mine zircon and biotite have been analyzed from a similar synmineralization feldspar biotite quartz porphyry the llamo porphyry the zircon u pb age is 38 7 or 1 3 ma whereas the weighted mean 40 ar 39 ar plateau age of igneous biotite is slightly younger at 37 40 or 0 18 ma u pb dating of two other rhyolitic quartz feldspar porphyry bodies that host much of the ore at zaldivar shows that these rocks are paleozoic in age antigua porphyry 290 or 4 ma zaldivar porphyry broadly coeval with antigua but age poorly defined at the escondida deposit the reportedly synmineralization escondida porphyry is dated by u pb at 37 9 or 1 1 ma whereas the reportedly postmineralization rhyolitic porphyry returned a u pb age of 34 7 or 1 7 ma these results indicate a single pulse of synmineralization magmatism in the escondida district at approximately 38 ma followed by minor largely nonmineralizing intrusive events the timing of main stage porphyry mineralization at escondida is apparently bracketed by the ages of the escondida 37 9 or 1 1 ma and rhyolitic porphyries 34 7 or 1 7 ma previously published k ar dates for intrusive rocks and alteration minerals at escondida which range from 39 to 31 ma are interpreted to reflect variable degrees of disturbance of the k ar system by later intrusive events and or by supergene weathering processes an age of approximately 38 ma for ore forming porphyry magmatism in the escondida region is significantly older than previously accepted best estimates based on k ar and u pb dating 32 34 ma but still falls within the age range of other large porphyry systems within the north south trending west fissure zone of northern chile 31 42 ma a common feature of the porphyry intrusions at zaldivar and escondida and also at chuquicamata and el salvador is the presence of zircons with inherited upper paleozoic cores these cores are interpreted to reflect interaction between arc magmas and crustal rocks during melting assimilation storage and homogenization mash and assimilation and fractional crystallization afc processes at various crustal levels

middle miocene climatic change in the atacama desert northern chile evidence from supergene mineralization at la escondida geochronology and paleotopographic reconstruction of the porphyry copper deposit at la escondida chile are used to calculate long term erosion rates and to deduce the timing of tertiary climatic change for a portion of the atacama desert region hypogene hydrothermal alteration and protore mineralization at la escondida took place between 33 7 1 4 and 31 0 1 4 ma based on k ar dating of hydrothermal biotite and sericite supergene weathering and copper sulfide en richment processes were active from 18 0 0 7 to 14 7 0 6 ma based on k ar dating of supergene alunite distinguished from hypogene alunite by grain size color and sulfur isotopic composition reworked lenses of volcanic ash in the vicinity of la escondida provide useful time stratigraphic markers at 8 7 0 4 6 5 0 2 and 4 2 0 2 ma within present soil profiles long term average rates of erosion are determined by these age dates and quantitative calculation of eroded leached capping thickness at la escondida using mass balance analysis of geochemical profiles coupled with an estimate of unmineralized lithocap thickness based on alteration petrology and fluid inclusion geobarometry at similar deposits the observed trend of decreasing long term average erosion rates with time is consistent with arid to semiarid conditions in the early miocene changing to hyperarid conditions during the middle miocene this climatic desiccation caused termination of significant supergene copper suffide enrichment at la escondida and elsewhere in the atacama region and preservation of surficial features including the ash horizons and the leached capping middle miocene climatic desiccation in northern chile and southern peru was probably related to a pronounced decrease in temperature of coastal waters supplied by an ancestral humboldt current and an increase in upwelling intensity as the antarctic ice cap became established at approximately 15 to 13 ma the central andes cordillera which now provides a rain shadow protecting the atacama region from precipitation from the east must have attained at least half its present elevation prior to about 15 ma to have played a similar role in the middle miocene

baseline and premining geochemical characterization of mined sites a rational goal for environmental restoration of new active or inactive mine sites would be natural background or the environmental conditions that existed before any mining activities or other related anthropogenic activities in a strictly technical sense there is no such thing as natural background or entirely non anthropogenic existing today because there is no part of the planet earth that has not had at least some chemical disturbance from anthropogenic activities hence the terms baseline and pre mining are preferred to describe these conditions baseline conditions are those that existed at the time of the characterization which could be pre mining during mining or post mining protocols for geochemically characterizing pre mining conditions are not well documented for sites already mined but there are two approaches that seem most direct and least ambiguous one is characterization of analog sites along with judicious application of geochemical modeling the other is reactive transport modeling based on careful synoptic sampling with tracer injection and subtracting inputs from known mining and mineral processing several examples of acidic drainage are described from around the world documenting the range of water compositions produced from pyrite oxidation in the absence of mining these analog sites provide insight to the processes forming mineralized waters in areas untouched by mining natural analog water chemistry data is compared with the higher metal concentrations metal fluxes and weathering rates found in mined areas in the few places where comparisons are possible the differences are generally 1 3 orders of magnitude higher for acid mine drainage

regional exploration for channel and playa uranium deposits in western australia using groundwater abstract shallow calcrete aquifers in the central north of the yilgarn craton in western australia are the host to numerous secondary carnotite u deposits sampling and analysis of approximately 1400 shallow aquifer groundwaters were conducted to test if u mineralisation of this type may be found using a 5 km sample spacing results show this can be achieved all the economic deposits and most of the minor deposits and occurrences are associated with groundwater that has carnotite kuo2vo4 approaching or exceeding saturated conditions soluble u concentrations alone identified the largest deposit yeelirrie and several smaller deposits but this parameter was not as successful as the mineral saturation indices palaeodrainage distribution and thickness of cover combined with surface drainage and catchment boundaries provided background information of u primary sources and for areas with the highest exploration potential for channel and playa u deposits granites in the se of the study area are less prospective with regard to secondary u deposits groundwater geochemistry in conjunction with palaeodrainage mapping may greatly improve exploration through cover where radiometric geophysics is not effective the study of regional shallow groundwater for u shows multiple benefits for mineral exploration the economy and potable water quality

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

geochemical challenges of diverse regolith covered terrains for mineral exploration in china abstract in recent years mineral exploration has concentrated on concealed deposits in regolith covered terrains in china the regolith covered landscapes mainly include desert windblown sand basins desert peneplains semi arid grassland loess plateaus forestry land alluvial plains and laterite terrains these diverse regolith covered areas represent geochemical challenges for mineral exploration in china this paper provides an overview of recent progress on mechanisms of metal dispersion from the buried ore deposits through the transported cover to the surface and penetrating geochemical methods to detect the anomalies case studies show that in arid and semi arid desert sand covered terrains sampling of fine fraction 120 mesh

more than ten million years of hyper aridity recorded in the atacama gravels abstract the atacama desert s hyper aridity is closely linked to the development of world class copper and nitrate iodine ores and to regional tectonics and global paleoclimate changes in the cenozoic era the timing when the hyper aridity commenced remains controversial with proposed ages ranging from late oligocene to pleistocene in this study we provide an independent constraint on the initiation of atacama hyper aridity utilizing a 100 m deep profile within the atacama gravels and underneath porphyry copper deposit in spence northern chile the overall high concentration of sulfate up to 10 wt and a multimodal distribution of water soluble salt sulfates chlorides and nitrates indicate multiple generations of sedimentation and salt accumulation events under semi arid to hyper arid climate conditions the multiple sulfate isotope compositions 17o 18o 34s of the upper section 15 0 to 34 5 m are close to those of modern hyperarid surface sulfates while the lower section 34 5 to 65 m displays a depth dependent isotope trend that is best interpreted as marking a period of climate change from semi arid to hyper arid when these data are combined with new chronological 40ar 39ar dates obtained from a volcanic ash layer at depth of 28 0 m our results show that hyper arid condition in the atacama desert was prevailing at least prior to 9 47 ma and may go back as old as the middle miocene

isotopic evolution of river water in the northern chile region two main northern chilean rivers the loa and tarapaca were investigated regarding their isotopic characteristics groundwater associated with various recharge zones and the input of tributaries along their courses mainly control their 180 2h isotopic composition of the rivers in the loa river carbon isotopic exchange between dissolved inorganic carbon dic and atmospheric coplays a minor role in the inorganic carbon cycle of the loa river the carbon isotopic composition 4 c 13 c of this river is probably controlled by carbon source associated with volcanic and geothermal activities and by the deposition of travertines for the tarapaca river the carbon isotope content of the dic reflects the input of recirculated water from irrigated areas along the river course and carbon isotopic exchange these findings imply that the input of volcanic and or geothermal co2 into the dic pool has to be evaluated in order to use carbon isotopes as a dating tool for groundwater in the loa basin and that modern conditions are not analogous to the paleohydrology of the tarapaca river

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

an experimental study into the rheology of synthetic polycrystalline coesite aggregates coesite has been found as a relic in ultrahigh pressure metamorphic uhpm crust worldwide and is expected to play a major role in the mechanical behavior of continental crust at uhpm conditions we performed triaxial compression tests on synthetic polycrystalline coesitite in a solid medium apparatus at confining pressures of 3 1 to 3 7 gpa temperatures of 700 to 1160 c and strain rates betweeen 6 10 7 and 1 10 3 s 1 the problem of the limited stress resolution of the solid medium apparatus was addressed by applying two extreme friction corrections that yield lower and upper bounds to the differential stress the correlation between the mechanical data and the microstructural record of the deformed samples as a function of temperature and imposed strain rate is consistent with deformation by dislocation creep we deduced parameters of a power law as n 3 1 and q 275 50 kj mol 1 extrapolation of the experimental data to natural conditions cannot be constrained by comparison with natural microstructures due to the lack of preserved coesite other than as single crystal inclusions nevertheless the extrapolation indicates a low strength of order 10 mpa for natural strain rates at typical uhpm conditions absent deformation of the uhpm brossasco granite dora maira massif western alps thus implies low stresses deformation must have been localized in very weak shear zones during burial and exhumation

chlorine isotope fractionation recorded in atacamite during supergene copper oxidation abstract in the atacama desert of northern chile large amounts of the copper hydroxy chloride mineral atacamite cu2cl oh 3 are formed in the supergene oxidation zone of cu deposits since atacamite requires saline water to form and is commonly preserved under hyperarid conditions it has been proposed as a climate sensitive mineral marker that can also provide relevant geochemical information regarding halogen in particular chlorine fluid sources during supergene cu oxidation however chlorine stable isotope data for atacamite in cu deposits are scarce and no experimental data for chlorine isotope fractionation between atacamite and water are currently available that could provide constraints on the possible mechanisms of fractionation in this study we report 37cl values of atacamite along a thick 100 m and well developed supergene enrichment profile at the barreal seco iron oxide copper gold iocg deposit in the atacama desert the 37cl values of atacamite along this profile range from 2 1 to 0 6 mean 0 4 0 7 median 0 3 and show a distinct trend with depth characterized by higher 37cl values towards the top of the profile with lower values at the bottom in addition the chlorine isotope compositions of experimentally synthetized atacamite crystals and coexisting cucl2 solutions were determined at room temperature 23 c the per mil fractionation factor 37clatacamite cl was determined at 0 75 showing that the heavy isotope 37cl is preferentially incorporated into atacamite we used numerical models to test two possible scenarios that might explain the observed 37cl profile at barreal seco a first scenario involves lowering of the water table with concomitant precipitation of atacamite which removes cl from solution following a rayleigh fractionation process this model qualitatively fits our observations but does not explain the full magnitude of the observed 37cl shift along the supergene profile in contrast a model that simulates cl diffusion following the upward injection of a deep brine that overlies fresh groundwater reproduces well the observed 37cl data suggesting that seismic pumping of basinal brines followed by cl diffusion is a feasible mechanism to explain the formation of atacamite at barreal seco these data are a first step towards interpreting 37cl profiles of atacamite in cu deposits in the atacama desert and exploring the potential use of stable chlorine isotopes to monitor cu weathering and enrichment in supergene environments

pleistocene recycling of copper at a porphyry system atacama desert chile cu isotope evidence we present cu isotope data of hypogene and supergene minerals from the late paleocene spence cu mo porphyry in the atacama desert of northern chile chalcopyrite displays a restricted range of 65cu values within the values reported for primary porphyry cu sulfides 0 28 to 0 34 n 6 supergene chalcocite samples show heavier and remarkably homogeneous 65cu values between 3 91 and 3 95 n 6 consistent with previous models of cu leaching and enrichment in porphyry systems secondary cu minerals from the oxide zone show a wider range of composition varying from 1 28 and 1 37 for chrysocolla n 6 to very light cu isotope signatures reported for atacamite between 5 72 to 6 77 n 17 these data suggest redox cycling of cu during supergene enrichment of the spence cu deposit characterized by a first stage of supergene chalcocite formation from acidic isotopically heavy leach fluids of meteoric origin down flowing in a semi arid climate 44 to 15 9 ma reworking of the initial supergene copper assemblage during the pleistocene by rising neutral and chlorine rich deep formation waters under well established hyper arid climate conditions lead to the formation of atacamite with extremely fractionated cu compositions essentially coeval chrysocolla formed by dissolution of atacamite during short episodes of wetter climatic conditions occurring in the latest pleistocene

neogene evaporites in desert volcanic environments atacama desert northern chile the upper miocene and pliocene evaporite deposits of the atacama desert of northern chile hilaricos and soledad formations are among the few nonmarine evaporites in which aridity not only formed the deposits but has also preserved them almost unaltered under near surface conditions these deposits are largely composed of displacive ca sulphate and halite together with minor amounts of glauberite thenardite and polyhalite however at the base and top of these deposits there are also beds of gypsum crystal pseudomorphs that originally formed as free growth forms within shallow brine bodies rather than as displacive sediments the halite is present as interstitial cement displacive cubes and shallow water bottom growth chevron crusts most of the calcium sulphate is presently anhydrite pseudomorphous after gypsum that was the primary depositional sulphate mineral the secondary anhydrite formed under early diagenetic conditions after slight burial some metres resulting from the effect of strongly evolved pore brines the anhydrite has been preserved without rehydration during late diagenetic and exhumation stages on account of the arid environment of the atacama desert both the hilaricos and the soledad formations contain geochemical markers indicating that these neogene evaporites had a largely non marine origin bromine content in the halite is very low few p p m indicating neither a sedimentological relation with sea water nor the likelihood of direct recycling of prior marine halites moreover the d 34 s of sulphates 4ae5 to 9 also reflects a non marine origin with a strong volcanic influence although some recycling of mesozoic marine sulphates cannot be ruled out d 34 s of dissolved sulphate from hot springs and streams in the area commonly displays positive values 2 to 10 leaching of oxidized sulphur and chlorine compounds from volcanoes and epithermal ore bodies very common in the associated drainage areas have been the main contribution to the accumulation of evaporites the sedimentary and diagenetic evolution of the hilaricos and soledad evaporites based on lithofacies analysis provides information about the palaeohydrological conditions in the central depression of northern chile during the neogene in addition the diagenesis and exhumation history of these evaporites confirms the persistence of strongly arid conditions from late miocene until the present a final phase of tectonism took place permitting the internal drainage to change and open to the sea resulting in dissolution and removal of a significant portion of these deposits despite the extensive dissolution the remaining evaporites have undergone little late exhumational hydration

large scale carbon isotope fractionation in evaporites and the generation of extremely 13c enriched methane petrographic fluid inclusion geochemical and gas stable isotope data are reported here for a permian zechstein evaporite sequence this deposit is a geochemically unaltered sequence bromine concentrations show a continuous evaporation profile with little postdepositional alteration in halite chemistry bacterial fermentation gases identified in primary inclusions change from an n 2 h 2 s composition in the lower middle halite series to a ch 4 h 2 composition in the upper halite and potash series carbon isotope results for ch 4 show a 1 3 c enrichment up sequence from typical biogenic values of 45 to 50 to extremely unusual 1 3 c enriched values as high as 21 the d values for these 1 3 c enriched ch 4 gases range from 240 to 377 a model is proposed for the formation of the ch 4 gases whereby the dominant isotopic fractionation process controlling the system was evaporation of the brines this generated a progressive 1 3 c enrichment in the carbon in the residual brines due to preferential loss of 1 2 co 2 to the atmosphere the resulting ch 4 generated in the sediments as evaporation and precipitation advanced recorded this 1 3 c enrichment in the carbon reservoir therefore the isotopic profile observed in this sequence today represents a primary feature with little evidence for postdepositional migration

sources sinks and long term cycling of iodine in the hyperarid atacama continental margin abstract the atacama region in northern chile hosts the driest desert on earth and is the world s premier iodine production province the origin of iodine enrichment in atacama is controversial and fundamentally different processes have been invoked over the years that involve marine eolian and more recently deep sedimentary fluid and groundwater sources as a result of the very limited geochemical iodine data in atacama and the western south american margin the origin of iodine enrichment in this region still remains elusive in this study we present a comprehensive survey of iodine concentrations and isotopic ratios 129i i of different reservoirs in the atacama desert of northern chile including nitrate soils supergene copper deposits marine sedimentary rocks geothermal fluids groundwater and meteoric water nitrate soils along the eastern slope of the coastal cordillera are found to have mean iodine concentrations of at least three orders of magnitude higher than the mean crustal abundances of 0 12 ppm with a mean concentration of 700 ppm soils above giant copper deposits in the central depression are also highly enriched in iodine 100 s of ppm range and cu iodide and iodate minerals occur in the supergene enrichment zones of some of these deposits further east in the precordillera jurassic sedimentary shales and limestones show above background iodine concentrations the latter averaging 50 ppm in the southern portion of the study area the highest iodine concentrations in fluids were measured in groundwater below nitrate soils in the coastal range 3 5 10 ppm and in geothermal waters 1 3 ppm along the volcanic arc although highly variable the iodine isotopic ratios 129i i of jurassic marine sedimentary rocks 300 600 10 15 nitrate soils 150 1500 10 15 and waters 215 10 15 are consistently low

hydrogeochemistry and stable isotopes of ground and surface waters from two adjacent closed basins atacama desert northern chile the geochemistry and stable isotopes of groundwaters surface waters and precipitation indicate different sources of some dissolved constituents but a common source of recharge and other constituents in two adjacent closed basins in the atacama desert region of northern chile 24 15 24 45 s waters from artesian wells trenches and ephemeral streams in the punta negra basin are characterized by concentrations of na ca mg and cl so4 with tds mg ca and so4 cl with tds also mg ca and so4 cl but with tds up to 40 g l the deep mine waters have ph between 3 2 and 3 9 and are high in dissolved co2 13c 4 8 pdb indicating probable interaction with oxidizing sulfides the deep mine waters have 18o values of 1 8 compared with values 3 5 for other hamburgo basin waters thus the mine waters may represent a mixture of meteoric waters with deeper metamorphic waters which had interacted with rocks and exchanged oxygen isotopes at elevated temperatures alternatively the deep mine waters may represent fossil meteoric waters which evolved isotopically along an evaporative trend starting from values quite depleted in 18o and dd relative to either precipitation or shallow groundwaters high i br ratios in the hamburgo basin waters and la escondida mine waters are consistent with regionally high i in surficial deposits in the atacama desert region and may represent dissolution of a wind blown evaporite component rain and snow collected during june 1984 indicate systematic 18o and d fractionation with increasing elevation between 3150 and 4180 m a s l 0 21 18o and 1 7 d per 100 m excluding the deep mine waters from la escondida the waters from the hamburgo and punta negra basins have similar d and 18o values and together show a distinct evaporative trend d 5 0 18o 20 2 snowmelt from the central andes cordillera to the east is the most likely source of recharge to both basins some of the waters in the hamburgo basin may have been recharged during late pleistocene when the climate was wetter and a lake filled the intervening punta negra basin as suggested by recent archaeological and geomorphological studies

the role of the antofagasta calama lineament in ore deposit deformation in the andes of northern chile during the late jurassic early oligocene interval widespread hydrothermal copper mineralization events occurred in association with the geological evolution of the southern segment of the central andes giving rise to four ns trending metallogenic belts of eastward decreasing age late jurassic early cretaceous late paleocene early eocene and late eocene early oligocene the antofagasta calama lineament acl consists of an important dextral strike slip ne trending fault system deformation along the acl system is evidenced by a right lateral displacement of the late paleocene early eocene metallogenic belts furthermore clockwise rotation of the early cretaceous mantos blancos copper deposit and the late paleocene lomas bayas porphyry copper occurred in the late eocene early oligocene metallogenic belt a sigmoidal deflection and a clockwise rotation is observed in the acl the acl is thought to have controlled the emplacement of early oligocene porphyry copper deposits 34 37 ma toki genoveva quetena and opache whereas it deflected the late eocene porphyry copper belt 41 44 ma esperanza telegrafo centinela and polo sur ore deposits these observations suggest that right lateral displacement of the acl was active during the early oligocene we propose that the described structural features need to be considered in future exploration programs within this extensively gravel covered region of northern chile