time scales of sedimentary transfer and weathering processes from u series nuclides clues from the himalayan rivers abstract in order to define time scales of erosion and sedimentary transfer in the himalaya 238 u 234 u 230 th disequilibria have been analysed in river bank sediments and in bedloads collected along the kali gandaki river one of the main nepalese rivers and in the ghaghara and gandak rivers two major plain tributaries of the ganges the th activity ratios and u th ratios in river sediments of the two ganges tributaries constantly decrease from upstream to downstream this is related to the maturation of sediments by weathering during their transfer to the plain the u series data allow to calculate a transfer time for the sediments in the alluvial gangetic plain from the chain front to the confluence with the ganges of about 100 kyr for both rivers the kali gandaki river sediment data highlight a decrease of both the th isotopic and u th ratios which is explained by a mixing between two sources with similar u th ratios but having suffered a different u th fractionation history interpretation of the u series data in the frame of this scenario gives long time scales of weathering of several 100 s kyr for the himalayan terranes the results imply that himalayan bedrocks are submitted to a long in situ stage of weathering before their erosion and transfer into the rivers in addition occurrence of similar u sr signatures in dissolved i e

geochemistry of laterites stability of al goethite al hematite and fe super 3 kaolinite in bauxites and ferricretes an approach to the mechanism of concretion formation

formation and evolution of lateritic profiles in the middle amazon basin insights from radiation induced defects in kaolinite abstract the content of radiation induced defects rids in kaolinite samples originating from lateritic soils and continental detritic sediments of the middle amazon basin brazil is investigated using electron paramagnetic resonance the paleodose registered by kaolinites ranges from 80 to 900 kgy present day dose rates of radiation determined from the whole rock u and th content range between 4000 and 40 000 mgy ka in most samples u and th concentrations are correlated suggesting that u has not been remobilized by lateritization this observation is consistent with the fact that 80 of the total u content is incorporated in resistant minerals such as zircon and ti oxides the heterogeneous distribution of u observed by induced fission tracks mapping makes it possible to neglect the radiation contribution of the u decay chains in the dose rate calculation the interpretation of the measured content of rids in kaolinite is then performed using the calculated present day dose rate and assuming equilibrium in the radioactive decay chains for the sedimentary samples the amount of rids is broadly correlated to the dose rate and provides apparent absolute ages older than 20 ma the rid contents in kaolinites from the lateritic soils provide apparent ages ranging from 10 to 6 ma the high rid content of these lateritic kaolinites shows that their chemical isotopic and crystallographic properties are not representative of present day weathering conditions models assuming the dynamical equilibrium of kaolinites with local physical chemical conditions prevailing in lateritic soils are thus questionable alternatively our findings bring strong support for the use of the isotopic composition of kaolinites to decipher continental paleo climates

determination of recent 238u 234u and 230th mobility in granitic rocks application of a natural analogue to the high level waste repository environment abstract the isotope activity ratios 234u 238u and 230th 234u have been investigated in drill core samples up to 1300 m depth in three granitoids from australia to identify the nature of possible recent 1 0 which has generated excess 230th these enriched ground waters do not appear to have penetrated below 530 m depth and 230th excess indicates bulk loss of u the observed disequilibria indicate that significant radionuclide mobilisation has occurred in these rocks during the recent past

time scale and conditions of weathering under tropical climate study of the amazon basin with u series the rio solimoes amazonas amazon river and its major tributaries have been analyzed for u series nuclides 238 u 234 u 230 th 226 ra disequilibria have been measured in the dissolved 0 2 lm as well as bed sands u series disequilibria are closely related to major and trace element compositions and therefore reflect elemental fractionation during chemical weathering moreover while the dissolved load records present day weathering suspended particles integrate the erosion his tory over much longer time scales 100 ka lowland rivers are characterized by long time scales of chemical erosion p100 ka result ing in a high weathering intensity moreover exchange between suspended particles and the dissolved load may explain the u series signature for these rivers by combining u series and pb isotopes in suspended particles we show that erosion in the rio madeira basin occurred as a multi step process whereby the pristine continental crust was eroded several hundreds of ma ago to produce sediments that have then been integrated in the cordillera by crustal shortening and are currently eroded in contrast recent erosion of a pristine crust is more likely for the rio solimoes amazonas 10 ka the suspended particles of the rivers draining the andes solimoes ama zonas madeira suggest time scales of weathering ranging between 4 and 20 ka this indicates that suspended particles transported by those rivers are not stored for long periods in the andean foreland basin and the tropical plain the sediments delivered to the ocean have resided only a few ka in the amazon basin 6 3 1 ka for the rio amazonas at obidos nevertheless a large fraction of the sediments coming out from the andes are trapped in the foreland basin and may never reach the ocean erosion in the andes is not operating in steady state u series systematics shows unambiguously that rivers are exporting a lot more sediments than predicted by steady state erosion and that is a consequence of soil destruction greater than production by relating this observation to the short time scales of weathering inferred for the andes a few ka it appears that the erosion regime has been recently perturbed resulting in high denudation rates a possible explanation would be the increase in precipitation less than 5 ka proposed by recent paleoclimatic studies our results indicate that erosion responds rapidly to high frequency climatic fluctuations

fixation of radionuclides in the 238u decay series in the vicinity of mineralized zones 1 the austatom uranium prospect northern territory australia abstract the minimum age of a zone of secondary uranium mineralization located at the austatom prospect in the alligator rivers region of australia is estimated to be 3 6 10 5 y this is derived from a geochronological model based on retarded leaching of 234 u with respect to 238 u and on ratios within the ore of these members of the 238 u decay series although kaolinite is a dominant mineral in the weathered schist host rocks retarded dissolution of 234 u occurs only in the presence of the clay minerals illite and montmorillonite in their absence the reverse occurs a model is proposed to explain the results ratios of 230 th to 238 u indicate that the mineralization has probably remained stationary within the weathered schist for at least 1 to 2 10 5 y future use of clay minerals as buffers in radioactive waste repositories is supported by the excellent long term retention obtained for oxidized uranium probably due in part to isomorphic substitution into the clay crystal lattice

reply to comment from zafar leng and chen on sulfide bearing cumulates in deep continental arcs the missing copper reservoir by chen et al earth planet sci lett 531 2020 115971

the delicate balance between soil production and erosion and its role on landscape evolution abstract the diversity in landscapes at the earth s surface is the result amongst other things of the balance or imbalance between soil production and erosion while erosion rates are well constrained it is only recently that we have been able to quantify rates of soil production uranium series isotopes have been useful to provide such estimates independently of erosion rates in this study new u series isotope are presented data from weathering profiles developed over andesitic parent rock in puerto rico and granitic bedrock in southeastern australia the site in australia is located on a highland plateau neighbouring a retreating escarpment where soil production rates between 10 and 50 mm kyr have been determined the results show that production rates are invariant in these two regions of australia with values between 15 and 25 mm kyr for the new site andesitic soils show much faster rates about 200 mm kyr overall soil production rates determined with u series isotopes range between 10 and 200 mm kyr this is comparable to erosion rates in soil mantled landscapes but faster than erosion in cratonic areas and slower than in alpine regions and cultivated areas this suggests that soil mantled landscapes maintain soil because they can there is a balance between production and erosion similarly thick weathering profiles develop in cratonic areas because despite slow erosion rates soil production is still significant bare landscapes in alpine regions are probably the result of the inability of soil production to catch up with fast erosion rates although this needs testing by u series isotope studies of these regions finally the range of production rates is up to several orders of magnitude lower than erosion rates in cultivated areas demonstrating quantitatively the fast depletion of soil resources with common agricultural practices

textural morphological and compositional varieties of modern arc sulfides a case study of the tolbachik volcano kamchatka abstract magma unmixing into separate sulfide and silicate melts is a key process in the formation of magmatic sulfide ore deposits sulfide inclusions in olivine preserve the original chemical composition of the segregated sulfide melt whereas the phase compositions of the solidified sulfide droplets and their textures record the final stages of sulfide evolution sulfide globules hosted in olivine phenocrysts of magnesian basalts from the tolbachik volcano kamchatka display a variety of textures ranging from homogeneous to fine grained to coarse grained and lamellar porous textures and textures associated with the oxidation and dissolution of sulfides were identified as separate categories the outer surfaces of sulfides vary significantly in shape which includes normal spherical or elliptical globules gravitationally layered inclusions thin sulfide foils and concave deformed sulfides in contact with fluid bubbles the estimated cooling rate of olivine phenocrysts responsible for observed crystallization textures varies by five orders of magnitude the fe ni cu ratio also exerts an influence on the resulting texture whereas the size of the globules does not have any significant effect the morphology and textural patterns of individual grains indicates that fine grained textures resulted from the rapid breakdown of a homogeneous solid phase which the sulfide melt solidified into during extremely rapid quenching the presence of large or abundant small pores in sulfides along with channels at the globule periphery indicates the separation of an appreciable amount of dissolved fluid from the sulfide melt during crystallization sulfides in the form of thin foils and planar swarms in healed cracks indicate cyclic pressure changes in the olivine environment which resulted in phenocrysts rupturing and healing the sulfide mineralogy is represented by high temperature sulfides mss monosulfide solid solution and iss intermediate solid solution as well as pyrrhotite pentlandite chalcopyrite cubanite bornite magnetite assemblages which formed at lower temperatures

the evolution of weathering profiles through time new insights from uranium series isotopes the sustainability of soil is a major issue for society in principle the evolution of soil resources can be constrained by comparing the rates of soil erosion and production cosmogenic isotopes provide one measurement of soil erosion rates they can also be used to estimate soil production rates but only if erosion is assumed to be balanced by production this implies that the evolution of soil resources thinning thickening or constant since soil thickness is assumed to be constant with time here we utilise an independent method to estimate soil production rates using uranium series u series isotopes the study of a site in temperate australia undisturbed by human activity shows that soil production rates inferred from u series isotopes are similar to erosion rates derived from beryllium 10 10be measurements implying that at this site there is no net accumulation or loss of soil saprolite production rates the migration rate of the weathering front into the bedrock are also similar to erosion rates so the thickness of the entire weathering profile is effectively in steady state this study demonstrates that the combination of u series and cosmogenic isotopes can be used to quantitatively assess soil evolution and the development of weathering profiles preliminary observations suggest that the rate of bedrock weathering i e saprolite production in temperate australia is of the same order of magnitude as that inferred for laterites in tropical climates this may suggest that for thick weathering profiles although the extent of weathering strongly differs between temperate and tropical climates the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present day climatic conditions

improved constraints on seismic source parameters of the 2011 tohoku earthquake from grace gravity and gravity gradient changes a new approach of using only the north component of gravity change from gravity recovery and climate experiment grace data reveals that a substantially higher spatial resolution of the observed seismic deformation following the 2011 tohoku earthquake is achievable at 333 km or longer here we show that grace observed north component of gravity change 17 6 1 1 gal and the corresponding gravity gradient change e g txz at 1 25 0 09 meotvos agree well with seismic gps model predictions localized slepian spectrum analysis further confirms that the grace gravity and gravity gradient changes agree well with seismic model spectra and have powers up to the limit of the grace solution complete to spherical harmonic degree 60 using the gravity observations for the fault parameter inversion via simulated annealing algorithm we show that the estimated slip orientation and centroid location are different from gps seismic solutions and potentially due to the additional offshore constraint from grace data

short lived u and th isotope distribution in a tropical laterite derived from granite pitinga river basin amazonia brazil application to assessment of weathering rate we have analyzed samples of a 15 m thick profile weathered from the madeira granite located in the pitinga basin river north of manaus in the state of amazonia brazil this profile consists essentially of a yellow red saprolite covered by a soil u and th concentrations are particularly high in the granite 20 and 80 g g respectively normalized element to th concentrations indicate that th is most resistant to chemical weathering except to some extent in the top soil higher concentrations in the saprolite compared to the granite comprise a relative enrichment resulting from a loss of mass the saprolites are initially generated by a descending weathering front which alters the granite to a yellow red saprolite a second front close to the top turns the saprolite into a soil weathering has led to leaching of u the 234u 238u and 230th 238u isotopic ratios are in radioactive disequilibrium numerous nodules are present and apparently started to form at the base of the saprolite these nodules achieve more developed form during their relative ascent until they are reached by the descending top front where they undergo dissolution the th and pb are concentrated in the nodules close to the top front the u being more mobile is strongly leached by the first front and most of the remainder freed by the second engages in a descending flux which supplies the underlying saprolite using the data an attempt is made to model the isotopic distribution in the profile we conclude that the first front has descended at a rate of 5 cm 1000 yt and that the time needed to create the saprolite must have been around 300 000 yr

rare earth and trace element geochemistry of termite mounds in central and northeastern namibia mechanisms for micro nutrient accumulation abstract rare earth element ree and trace element concentrations of ten termite mounds and adjacent topsoil from central and northeastern namibia were used to investigate the processes underlying the alteration of soil chemical and physical properties by termites macrotermes spp accumulation of micro nutrients in the mounds was of particular interest because of the ecological implications of the enhanced availability of these scarce elements the absolute concentrations of ree and trace elements including nine micro nutrients b fe mn ni cu zn se mo and cd were greater in termite mounds compared to topsoil suggesting a possible external supply of enriched materials or accumulation of in situ weathering products of the underlying bedrock similarities between the chondrite normalized ree patterns of the samples and those of the average upper continental crust ucc and post archaean average australian shale compositions indicated that the mounds and topsoil at each site originated from the same host bedrock coherent behavior between two incompatible elements th and la in both topsoil and mound samples suggested that the two sets of samples were exposed to two different weathering processes the relatively low zr sc ratios fine particle sizes and the persistence of positive eu anomalies of the mounds relative to ucc were interpreted as reflecting 1 a lack of grain sorting effects and 2 active transport of materials rich in primary clay minerals and trace elements from the deeper regolith to the surface by termites lower permeability of mound materials may also prevent leaching of primary minerals and trace elements titanium and zr enrichment in most mound samples 8 samples was attributed to the decomposition of biotite by macrotermes with the concomitant release of ti and zr into the mounds suggesting the termites ability to transform soil minerals and enhance chemical weathering this was further corroborated by a substantial increase in th u ratios in most mounds relative to topsoil because of the oxidizing conditions of the mounds ti and zr are likely to be adsorbed onto fe oxides positive ce anomalies relative to ucc and the high ph values in the mounds indicate calcrete aggradation or mound aeration as a result of burrowing activities of the termites and evaporation from mounds the impacts of the changes in environmental conditions and the termite activities on the mound geochemistry are highlighted by heavy ree hree as well as fe and mn retentions in the mounds heavy ree and light ree lree are likely to coprecipitate with carbonate complexes and fe and mn oxyhydroxides and in the process scavenge a series of micro nutrients such as zn cu cd and co a substantial enrichment of se in mounds despite the oxidizing conditions of the mounds could be attributed to active transport of se enriched materials from deep soil horizons the study demonstrated the suitability of ree and trace element geochemistry for assessing the influence of macrotermes on the physico chemical properties of semi arid soils

234u238u and 230th234u activity ratios in mineral phases of a lateritic weathered zone abstract a selective phase extraction procedure was developed for the identification of the significant phases of a typical deep soil profile sampled in the vicinity of the ranger no 1 uranium ore body alligator rivers region n t australia the significant phases were identified as amorphous iron oxide crystalline iron oxide and a clay quartz resistate the distribution of 238 u 234 u 230 th and 226 ra between the phases was measured the results indicated that the amorphous iron oxide phase is in adsorption desorption equilibrium with the ground water the crystalline iron oxide phase contains a chemical control the kinetics of which are commensurate with or less than the half life of 230 th 7 52 10 4 y the clay quartz resistate is enriched in 238 u descendants in such a way that they are not readily accessible to the ground water

oxygen isotope geochemistry of kaolinite in laterite forming processes manaus amazonas brazil abstract mineralogic crystallographic and oxygen isotopic analyses were made of kaolinites from a 20 m thick lateritic profile developed on the detrital sands and clays of the mid cretaceous to tertiary alter do chao formation central amazon basin brazil a detailed 18o profile of soil water from the upper 4 m of the same section was also measured kaolinite separates were prepared from different pedogenic facies of small hand specimens most of these kaolinite separates exhibit polymodal size distributions the separates were further divided into individual particle size modes for subsequent isotopic and crystallographic analysis in general hinckley index and 18o values of different size fractions of the same sample differ hinckley index values decrease strongly and fe content increases in the upward direction in the profile we conclude that 1 the kaolinite in the profile has progressively evolved in response to changing microenvironments as weathering fronts have moved progressively downward 2 different kaolinite fractions even within a single facies of a single sample a few cm3 in size typically formed under somewhat different conditions 3 the bulk of the fine grained kaolinite in the lateritic profile does not constitute a continuous inverted time record that is lower levels do not represent more recent kaolinite formation and higher levels do not represent older kaolinite formation and 4 isotopic compositions of kaolinites from lateritic paleosols should provide an indication of mean climatic conditions and some measure of seasonality during the interval preceding the removal of the laterite from the active weathering environment we speculate that the different conditions under which the different kaolinite fractions have formed may be related at least in part to variations in soil microenvironments that are ultimately derived from seasonal and year to year variations in the chemistry and isotopic composition of infiltrating rainfall

thorium uranium disequilibrium dating of late quaternary ferruginous concretions and rinds radioactive ferruginous nodules from the alligator rivers uranium province northern territory australia were systematically examined by optical microscopy xrd alpha and fission track autoradiography and analysed for u th ra and major stable elements correlation of autoradiographs with microscopic structure and analyses of selective extractions confirmed that u and th were strongly associated with fe oxides fission track autoradiography showed no concentration gradient evidence for postdepositional leaching of uranium strong acid leaching studies showed that irreversibly adsorbed u and authigenic 234u and 230th but not 226ra are quantitatively retained by accumulated oxide oxyhydroxide correlation of the groundwater activity ratio with oxidic 234u 238u activity ratios indicated the latter was radiogenically consistent with oxidic 230th 234u activity ratios 1 dense accumulated fe mn oxide matrices are apparently capable of forming radiogenic closed systems in respect of the 230u 234u 230th decay set 234u 238u and 230th 234u activity ratios were used to derive similar mean ages of rinds for four nodules in the same horizon th u ages were also determined using a th index detrital correction method previously used for impure carbonates of pedogenic fe mn accumulations having background concentrations of u and th from other locations in australia these ages were compared with tl and 14c ages of the host sediments the comparisions suggest that th u dating could be used to age indurated layers of fe mn oxides in soil horizons 350 ka old

226ra 238u and 228th 228ra disequilibrium as weathering indices in beach sand sediments associated with granitoids from cyclades greece abstract radioactive secular disequilibrium measured as the absolute difference from 1 of the 226ra 238u and 228ra 228th ratios have been used to determine weathering extent twenty eight samples from beach sands of cyclades islands all of them being the weathering products of the local granitic plutons are studied for their secular disequilibrium in both 238u and 232th series 226ra 238u and 228th 228ra respectively the results are compared to weathering indices already in use worldwide and suggest a significant linear correlation of u series disequilibrium with wip alkaline and alkaline earth element leaching and pwi silica leaching for the beach sands studied and significant non linear correlations with cia chemical index of alteration and pia plagioclase index of alteration considering only the heavy mineral enriched samples significant correlations exist between u series disequilibrium and cia ciw chemical index of weathering and pia therefore with an appropriate choice of the weathering index and samples both u series and weathering index can recover the extent of chemical weathering on the other hand the use of th series disequilibrium as a weathering index is limited due to 228th and 228ra geochemistry and especially due to their short radionuclide half lives u series disequilibrium can therefore be used as a relatively sensitive weathering index in sediments consistent with cia ciw and pia indices however the mineralogical composition must be considered more specifically the mineral content of the sediments should involve considerable amounts of sensitive to weathering minerals e g fe mg minerals as well as feldspars

chemical mobilizations in laterites evidence from trace elements and 238u 234u 230th disequilibria geochemical and mineralogical investigations including measurements of major and trace elements sr isotope ratios and 238u 234u 230th activity ratios were made on an old african laterite to reconstruct its formation steps and assess recent chemical mobilization the present data support a scenario of discontinuous formation for the laterite with different bedrock weathering conditions during the formation of each unit rather than a scenario of continuous formation absolute accumulation of fe u and lanthanides in the uppermost ferruginous unit suggests an autochthonous origin of this iron cap by leaching of an older overlying profile present chemical distributions of lanthanides as well as of rb k ba and sr within the profile cannot be linked to the mineralogical distribution of both relictual primary and authigenic secondary phases complementary lanthanide patterns indicate that these elements were primarily accumulated in the uppermost ferruginous unit before further remobilization and accumulation in the underlying horizons these redistribution processes may be related to the chemical instability of the ferruginous cap the 238u 234u 230th disequilibria indicate that recent u mobilization occurs in the whole profile and that as for lanthanides there is a vertical redistribution of u from the uppermost ferruginous unit to the underlying horizons moreover these data show that both u losses and gains exist at each level of the profile a simple modeling of this double u mobilization process is proposed to interpret the 238u 234u 230th data differences in the mobilization and fractionation intensities of the u input and removal processes can account for the two evolution trends which distinguish the ferruginous unit from the underlying ones furthermore on the basis of this modeling the profile appears to be in a transient state because of recent changes in the u mobilization conditions which could correspond to major pleistocene climatic variations

major element geochemistry and geomorphic relationships in brazilian cerrado soils abstract a thorough understanding of major and trace element geochemistry is important in assessing the impacts of rapid agriculturalization and increased human occupation of the cerrado region of central brazil a region with some of the oldest soils on earth the objectives of this paper are to summarize the major element composition of cerrado soils in the context of their morphology and geomorphology to describe impacts of biogeochemical cycling on the chemical properties of cerrado soils and to use elemental data to provide insights on the possible origin of soil parent materials on the south american erosion surface and on the serra geral basalts we sampled three major subregions selected to be representative of the main areas of the cerrado namely i eastern goias ii northwestern minas gerais and iii the tri ngulo mineiro area of western minas gerais five soils were sampled in each subregion each represented by three pedons sampled at 0 0 2 and 0 8 1 0 m depths geomorphologically the soils were on the south american velhas i and velhas ii erosion surfaces all sites were carefully selected to minimize potential anthropogenic contamination general chemical characterization was by standard procedures and total major and trace elemental contents were quantified using wavelength dispersive x ray fluorescence spectroscopy the soils ranged from loamy sands to clays with si 142 947 g kg 1 sio 2 al 14 439 g kg 1 al 2 o 3 fe 10 331 g kg 1 fe 2 o 3 and ti 3 101 g kg 1 tio 2 contents closely following the clay contents organic c averaged 17 g kg 1 for the surface and 7 g kg 1 for the subsurface horizons the na ca mg and k contents were very low tended to be higher in surface than in subsurface horizons because of recycling by vegetation and addition due to atmospheric deposition and were higher in soils containing hard plinthite nodules the th zr ratios suggest that the parent materials of the clayey soils on the south american erosion surface have a common origin in sediments from rocks in the central plateau of brazil that were transported eastwards to cover the underlying sandstone trace element signatures indicate that basalt derived soils occurring at different elevations in the tri ngulo mineiro area formed from distinctly different magmas that may have been deposited in different basalt flow events

trace element geochemistry in brazilian cerrado soils the cerrado region on the central plateau of brazil contains some of the oldest soils on earth with some soils weathering in place since the late cretaceous the region is currently undergoing rapid agriculturalization and industrialization the objectives of this paper are 1 to inventory the native trace element contents of pristine cerrado soils and 2 to better understand which trace elements accumulate and which are depleted in well drained soils over extremely long periods of pedogenic weathering we sampled the 0 20 and 80 100 cm layers of 45 pedons located in three widely separated areas on three different geomorphic surfaces and determined total elemental contents by x ray fluorescence analysis in comparing the contents of cerrado soils to soils worldwide the contents of sc v cr ga zr nb ce and th are 1 2 times the world average y la and u are about the same and mn co ni cu zn rb sr ba and pb are 0 4 times the world average trace elements that accumulated during weathering are either pentavalent nb5 tetravalent ti4 zr4 u4 th4 or trivalent sc3 v3 cr3 ga3 y3 la3 ce3 at the acid ph of these soils while trace elements that were depleted are either divalent or monovalent rb mn2 co2 ni2 cu2 zn2 sr2 ba2 and pb2 at acid ph thus trace element geochemistry over long periods of pedogenic weathering at acid ph mirrors major element geochemistry in that mono and divalent cations are leached and depleted while tri tetra and pentavalent elements accumulate

sulfide bearing cumulates in deep continental arcs the missing copper reservoir abstract resolving the geochemical discrepancies between the bulk continental crust and its building blocks basaltic arc magmas can provide insights into the processes by which the continental crust is formed one of the discrepancies is that the bulk continental crust is depleted in cu and has a lower cu ag ratio 500 than basaltic arc magmas 3500 how arc magmas become cu depleted and where the missing cu resides remain unclear here we report cu and ag concentrations in arizona garnet pyroxenite xenoliths which represent cumulates formed in a deep continental arc many of these cumulates are cu enriched with cu concentrations as high as 1000 ppm furthermore for the first time we show that these cumulates have higher cu ag ratios up to 10 000 than basaltic arc magmas complementary to the low cu ag in felsic arc magmas and the continental crust 500 in these cumulates cu and ag concentrations are initially low but begin to rise with progressive differentiation due to sulfide saturation when cumulate mg atomic ratio of mg mg fe decreases to 0 7 the onset of sulfide saturation in the cumulates is accompanied by a cu decrease in continental arc magmas fractional crystallization modelling shows that the early sulfide saturation during the differentiation of continental arc magmas was facilitated principally by cooling and iron depletion in the melt our findings suggest that most of the cu extracted from the mantle in continental arc settings is sequestered in sulfides in deep arc cumulates during magma differentiation the sulfide bearing cumulates represent the missing reservoir with both high cu concentrations and high cu ag ratios the low cu abundance and low cu ag ratio in the average continental crust requires the formation of sulfide bearing cumulate layer at arc roots followed by its delamination into the mantle moreover we confirm that sulfide saturation in continental arc magmas occurs earlier at higher mg than in island arc settings resulting in lower cu concentrations in the felsic upper crust of continental arcs because the bulk continental crust is strongly depleted in cu these observations together point to syn magmatic crustal thickening as a key process in making continents

soil formation rates on silicate parent material in alpine environments different approaches different results abstract high mountain soils develop in particularly sensitive environments consequently deciphering and predicting what drives the rates of soil formation in such environments are a major challenge in terms of soil production or formation from chemical weathering the predominating perception for high mountain soils and cold environments is often that the chemical weathering portion of soil development is temperature inhibited often to the point of non occurrence several concepts exist to determine long term rates of soil formation and development we present three different approaches 1 quantification of soil formation from minimally eroded soils of known age using chronosequences known surface age and soil thickness sast 2 determination of soil residence times srt and production rates through chemical weathering using un stable isotopes e g 230 th 234 u activity ratios and 3 a steady state approach using cosmogenic isotopes e g 10 be for each method data from different climate zones and particularly from high mountains alpine environment are compared the sast and steady state approach give quite similar results for alpine environments european alps and the wind river range rocky mountains usa using the srt approach soil formation rates in mountain areas but having a temperate climate do not differ greatly from the sast and steady state approaches independent of the chosen approach the results seem moderately comparable soil formation rates in high mountain areas alpine climate range from very low to extremely high values and show a clear decreasing tendency with time very young soils have up to 3 4 orders of magnitude higher rates of development than old soils 10 5 to 10 6 yr this apparently is a result of kinetic limits on weathering in regions having young surfaces and supply limits to weathering on old surfaces due to the requirement for chemical weathering to occur soil production rates cannot be infinitely high consequently a speed limit must exist in the literature this limit has been set at about 320 to 450 t km 2 a our results from the sast approach show however that in alpine areas soil formation easily reaches rates of up to 800 2000 t km 2 a these data are consistent with previous studies in mountain regions demonstrating that particularly young soils intensively weather even under continuous seasonal snowpack and thus that the concept of temperature controlled soil development soil forming intervals in alpine regions must be reconsidered