melt mantle interaction and melt evolution in the sartohay high al chromite deposits of the dalabute ophiolite nw china the sartohay block of the dalabute ophiolite consists chiefly of mantle harzburgite and lherzolite with minor dunite these rocks host voluminous chromite deposits with lenticular or vein like shapes the podiform chromitites are associated with and cross cut by numerous troctolite dykes chromite in the chromitites has al2o3 23 31 wt tio2 0 29 0 44 wt and cr2o3 contents 45 wt with cr s 100cr cr al 60 typical of high al chromite deposits the host peridotites in sartohay have been texturally and geochemically modified by magmas from which the high al chromitites and mafic dykes formed dunites commonly envelop the podiform chromite bodies and show transitional contacts with the peridotites some of the peridotites and chromitites contain plagioclase that crystallized from impregnated melts the dunite locally grades into troctolite with increasing plagioclase contents as a result of melt impregnation peridotites and dunites show variable ca and al contents and lree enrichment the parental magma of the chromitites was likely tholeiitic in composition derived from partial melting of the asthenospheric mantle in a rising diapir the interaction between this magma and pre existing lithospheric mantle composed of depleted lherzolite would have formed a more silicic tholeiitic magma from which high al chromitites crystallized during this interaction harzburgite and dunite were depleted in modal pyroxene and enriched in some incompatible elements such as al ca and lree due to melt impregnation

contrasting arc and morb like assemblages in the paleoproterozoic flin flon belt manitoba and the role of intra arc extension in localizing volcanic hosted massive sulphide deposits the flin flon belt trans hudson orogen manitoba and saskatchewan is the largest paleoproterozoic volcanic hosted massive sulphide vms district in the world with 118 7 million tonnes mt of z

chromium isotopes in carbonates a tracer for climate change and for reconstructing the redox state of ancient seawater abstract strontium and carbon isotopes of marine carbonates are routinely applied for chemostratigraphic cross correlations of time equivalent sedimentary sequences and for calibration of the compositional evolution of seawater throughout earth s history mainly for the purpose of reconstructing ancient climatic changes we here present results of a new isotopic tracer system stable chromium isotopes applied to a late ediacaran marine carbonate sequence exposed in the calera de recalde syncline arroyo del soldado group uruguay the aim was to compare cr isotope signatures directly to 13 c 87 sr 86 sr and 143 nd 144 nd fluctuations in a well defined stratigraphic profile comprising sediments that were deposited during cold warm periods accompanied by sea level changes in response to glaciation deglaciation at higher latitudes the studied section is characterized by a pronounced negative down to 3 3 13 c excursion in carbonates paralleled by a decrease of 87 sr 86 sr values chromium isotope signatures over this section also show a correlated decrease in 53 cr 53 cr 53 cr 52 cr sample 53 cr 52 cr srm979 1 1000 values from 0 29 to 0 17 which mirrors a decrease in positively fractioned seawater signatures to slightly negative values characteristic of high temperature magmatic sources linear correlations between 53 cr and end 570 ma 87 sr 86 sr and cr concentrations can be explained by mixing between two major input sources of cr nd and sr into the shallow seawater 1 a source characterized by negative 53 cr values of 0 2 low 87 sr 86 sr values of 0 707 and elevated 147 sm 144 nd values of 0 13 recognized as a subaqueous hydrothermal dominated input source and 2 a source characterized by positively fractionated 53 cr values of 0 2 higher 87 sr 86 sr values of 0 708 and lower 147 sm 144 nd values of 0 11 a source which is strongly affected by continentally derived input chromium isotopes provide a powerful tool for reconstructing the redox state of ancient seawater since positive values indicate that at least locally neoproterozoic shallow ocean waters were sufficiently oxidized to fractionate chromium and or that oxygen levels of the atmosphere were sufficient to transform cr iii into the more mobile hexavalent cr vi formed during weathering processes on land the fact that 87 sr 86 sr values despite 13 c fluctuations remain low indicative of a strong hydrothermal input into the basin at his time implies that co 2 limitation was the cause of negative 13 c and 53 cr excursions in otherwise nutrient rich late neoproterozoic basins and that glaciation is only one more consequence of a tectonically driven biologically mediated system in such a scenario glaciation acts as an amplifier of 53 cr signals these signals in marine carbonates are a sensitive tracer for redox processes in the ocean and or on land and have the potential to contribute significantly in combination with the other commonly used isotopic tracers to the reconstruction of climatic changes particularly those that are associated with major glaciation periods in earth s history

high temperature chromium isotope fractionation and its implications constraints from the kizildag ophiolite se turkey abstract chromium isotope data were obtained from olivine orthopyroxene and chromite separates of the kizildag ophiolite se turkey to investigate the effects of high temperature magma processes on cr isotope fractionation harzburgite in the kizildag ophiolite has the 53cr values of 0 14 to 0 12 in chromite and 0 08 to 0 01 in bulk rocks these cr isotope fractionations could be driven by partial melting and metasomatism the dunite and chromitite samples from the mantle crust transition zone of the kizildag ophiolite are cumulates and their chromite and olivine have 53cr values of 0 29 to 0 06 and 0 11 to 0 41 respectively the 53cr values of chromite are negatively correlated with the chemical indices of fractional crystallization e g fe of chromite suggesting that cr isotopes were fractionated during fractional crystallization in the chromitite the degree of cr isotope fractionation increases with fractional crystallization with 0 23 cr isotope fractionation occurring at mineral scale during fractional crystallization solid phases preferentially incorporate cr particularly heavy cr isotopes driving the depletion of cr and enrichment of light cr isotopes in the evolved melts the 53cr values of olivine are higher than the coexisting chromite which could be explained by the fact that the olivine grains were probably formed earlier than the chromite the cr isotopic features of chromite in the podiform chromitite 0 22 to 0 04 from the mantle sequence are consistent with the modelled fractional crystallization trend confirming the magmatic origin of the podiform chromitite therefore a significant cr isotope fractionation at chromite from the kizildag ophiolite could be induced by the high temperature fractional crystallization

does reactive surface area depend on grain size results from ph 3 25 c far from equilibrium flow through dissolution experiments on anorthite and biotite abstract laboratory determined mineral weathering rates need to be normalised to allow their extrapolation to natural systems the principle normalisation terms used in the literature are mass and geometric and bet specific surface area ssa the purpose of this study was to determine how dissolution rates normalised to these terms vary with grain size different size fractions of anorthite and biotite ranging from 180 150 to 20 10 m were dissolved in ph 3 hcl at 25 c in flow through reactors under far from equilibrium conditions steady state dissolution rates after 5376 h anorthite and 4992 h biotite were calculated from si concentrations and were normalised to initial and final mass and geometric geometric edge biotite and bet ssa for anorthite rates normalised to initial and final bet ssa ranged from 0 33 to 2 77 10 10 mol feldspar m 2 s 1 rates normalised to initial and final geometric ssa ranged from 5 74 to 8 88 10 10 mol feldspar m 2 s 1 and rates normalised to initial and final mass ranged from 0 11 to 1 65 mol feldspar g 1 s 1 for biotite rates normalised to initial and final bet ssa ranged from 1 02 to 2 03 10 12 mol biotite m 2 s 1 rates normalised to initial and final geometric ssa ranged from 3 26 to 16 21 10 12 mol biotite m 2 s 1 rates normalised to initial and final geometric edge ssa ranged from 59 46 to 111 32 10 12 mol biotite m 2 s 1 and rates normalised to initial and final mass ranged from 0 81 to 6 93 10 12 mol biotite g 1 s 1 for all normalising terms rates varied significantly p 0 05 with grain size the normalising terms which gave least variation in dissolution rate between grain sizes for anorthite were initial bet ssa and initial and final geometric ssa this is consistent with 1 dissolution being dominated by the slower dissolving but area dominant non etched surfaces of the grains and 2 the walls of etch pits and other dissolution features being relatively unreactive these steady state normalised dissolution rates are likely to be constant with time normalisation to final bet ssa did not give constant ratios across grain size due to a non uniform distribution of dissolution features after dissolution coarser grains had a greater density of dissolution features with bet measurable but unreactive wall surface area than the finer grains the normalising term which gave the least variation in dissolution rates between grain sizes for biotite was initial bet ssa initial and final geometric edge ssa and final bet ssa gave the next least varied rates the basal surfaces dissolved sufficiently rapidly to influence bulk dissolution rate and prevent geometric edge ssa normalised dissolution rates showing the least variation simple modelling indicated that biotite grain edges dissolved 71 132 times faster than basal surfaces in this experiment initial bet ssa best integrated the different areas and reactivities of the edge and basal surfaces of biotite steady state dissolution rates are likely to vary with time as dissolution alters the ratio of edge to basal surface area therefore they would be more properly termed pseudo steady state rates only appearing constant because the time period over which they were measured 1512 h was less than the time period over which they would change significantly

magnesium isotopic systematics of mafic rocks during continental subduction abstract magnesium isotopic compositions of a set of prograde metamorphosed rocks greenschists amphibolites and eclogites from east china were measured in order to understand the behavior of mg isotopes during continental subduction the 26mg values of the greenschists vary from 0 269 0 057 to 0 133 0 042 2sd and correlate negatively with the mgo contents and nb u ratios possibly due to the assimilation of isotopicallly heavy felsic schists 26mg up to 0 099 during the genesis of morb like protoliths the two subgroups of amphibolites have slightly different mg isotopic compositions the group i amphibolites with cumulate origin have 26mg values ranging from 0 243 0 061 to 0 192 0 050 which reflects clinopyroxene accumulation nature of the protolith the group ii amphibolites have relatively light 26mg values varying from 0 358 0 061 to 0 224 0 056 which may result from either source contamination or crustal assimilation of isotopically light components e g carbonates eclogitic garnet and clinopyroxene display large inter mineral mg isotope fractionations 26mgcpx grt 1 097 0 056 to 1 645 0 081 equilibrated at peak metamorphic temperatures the 26mg values of the eclogites vary from 0 348 0 041 to 0 137 0 063 overlapping the ranges of greenschists and amphibolites such mg isotopic variations are not related to unrepresentative sampling nor host eclogite chemical interactions instead they reflect the protolith heterogeneities collectively the similarity of mg isotopic compositions among the greenschists 0 196 0 044 amphibolites 0 271 0 042 and eclogites 0 226 0 044 suggests that mg isotope fractionation during continental subduction if any is limited

inter mineral fe isotope variations in mantle derived rocks and implications for the fe geochemical cycle iron isotope and major and minor element compositions of coexisting olivine clinopyroxene and orthopyroxene from eight spinel peridotite mantle xenoliths olivine magnetite amphibole and biotite from four andesitic volcanic rocks and garnet and clinopyroxene from seven garnet peridotite and eclogites have been measured to evaluate if inter mineral fe isotope fractionation occurs in high temperature igneous and metamorphic minerals and if isotopic fractionation is related to equilibrium fe isotope partitioning or a result of open system behavior there is no measurable fractionation between silicate minerals and magnetite in andesitic volcanic rocks nor between olivine and orthopyroxene in spinel peridotite mantle xenoliths there are some inter mineral differences up to 0 2 in 56fe 54fe in the fe isotope composition of coexisting olivine and clinopyroxene in spinel peridotites the fe isotope fractionation observed between clinopyroxene and olivine appears to be a result of open system behavior based on a positive correlation between the 56feclinopyroxene olivine fractionation and the 56fe value of clinopyroxene and olivine there is also a significant difference in the isotopic compositions of garnet and clinopyroxene in garnet peridotites and eclogites where the average 56feclinopyroxene garnet fractionation is 0 32 0 07 for six of the seven samples the one sample that has a lower 56feclinopyroxene garnet fractionation of 0 08 has a low ca content in garnet which may reflect some crystal chemical control on fe isotope fractionation the fe isotope variability in mantle derived minerals is interpreted to reflect subduction of isotopically variable oceanic crust followed by transport through metasomatic fluids isotopic variability in the mantle might also occur during crystal fractionation of basaltic magmas within the mantle if garnet is a liquidus phase the isotopic variations in the mantle are apparently homogenized during melting processes producing homogenous fe isotope compositions during crust formation

predictive mapping of prospectivity and quantitative estimation of undiscovered vms deposits in skellefte district sweden mapping of mineral prospectivity and assessment of undiscovered mineral deposits both aim to delineate prospective ground for mineral exploration but the latter is usually carried out exclusive of the former we propose that the spatial distribution of known mineral deposits of the type sought is the key to link mapping of mineral prospectivity and assessment of undiscovered deposits we demonstrate this proposition in regional scale mapping of prospectivity for volcanogenic massive sulphides vms deposits and estimation of undiscovered vms endowment in the skellefte district sweden the results of analyses of the spatial distribution of known vms deposits and their spatial associations with geological features are consistent with existing knowledge of geological controls on vms mineralization in the district and we used them to define spatial recognition criteria of regional scale vms prospectivity integration of layers of evidence representing spatial recognition criteria of vms prospectivity via application of data driven evidential belief functions results in a regional scale map of prospective areas occupying 15 of the district and having fitting and prediction rates of 100 we used the map of prospective areas and proxy measures for degrees of exploration based on the spatial distribution of known vms deposits in one level prediction of undiscovered mineral endowment we obtained estimates of 709 kt undiscovered cu endowment 3190 kt undiscovered zn endowment 95 mt undiscovered ore tonnage and 48 undiscovered vms deposits these estimates are slightly ca 5 on average lower than and thus corroborated by estimates obtained via radial density fractal analysis of the spatial distribution of known vms deposits therefore mineral prospectivity mapping can be a part of mineral resource assessment if the spatial distribution of discovered deposits of the type sought is considered in both predictive modeling processes

variation in element release rate from different mineral size fractions from the b horizon of a granitic podzol far from equilibrium batch dissolution experiments were carried out on the 2000 500 500 250 250 53 and 53 2 m size fractions of the mineral component of the b horizon of a granitic iron humus podzol after removal of organic matter and secondary precipitates the different size fractions were mineralogically and chemically similar the main minerals present being quartz alkali and plagioclase feldspar biotite and chlorite specific surface area increased with decreasing grain size the measured element release rates decreased in the order 53 2 2000 500 500 250 250 53 m surface area normalised element release rates from the 2000 500 500 250 and 250 53 m size fractions 0 6 77 10 14 mol m2 s were intermediate between literature reported surface area normalised dissolution rates for monomineralic powders of feldspar 0 1 0 01 10 14 mol m2 s and sheet silicates 100 10 14 mol m2 s dissolving under similar conditions element release rates from the 53 2 m fraction 400 3000 10 14 mol m2 s were a factor of 4 30 larger than literature reported values for sheet silicates the large element release rate of the 53 2 m fraction means that despite the small mass fraction of 53 2 m sized particles present in the soil dissolution of this fraction is the most important for element release into the soil a theoretical model predicted similar within a factor of 2 bulk element release rates for all the mineral powders if observed thicknesses of sheet silicate grains were used as input parameters decreasing element release rates with decreasing grain size were only predicted if the thickness of sheet silicates in the powders was held constant a significantly larger release rate for the 53 2 m fraction relative to the other size fractions was only predicted if either surface roughness was set several orders of magnitude higher for sheet silicates and several orders of magnitude lower for quartz and feldspars in the 53 2 m fraction compared to the other size fractions or if the sheet silicate thickness input in the 53 2 m fraction was set unrealistically low it is therefore hypothesised that the reason for the unpredicted large release rate from the 52 3 m size fraction is due to one or more of the following reasons 1 the greater reactivity of the smaller particles due to surface free energy effects 2 the lack of proportionality between the bet surface area used to normalise the release rates and the actual reactive surface area of the grains and 3 the presence of traces quantities of reactive minerals which were undetected in the 53 2 m fraction but were entirely absent in the coarser fractions

silurian devonian provenance changes of south qinling basins implications for accretion of the yangtze south china to the north china cratons abstract the north china and yangtze south china cratons are the most important tectonic units of china the two cratons finally coalesced through collision with the formation of the e w trending qinling mountains that are considered to be the eastern extreme of the paleotethys the north qinling and south qinling belts constitute the southern active margin of the north china craton and the northern passive margin of the yangtze craton respectively we have studied chemical compositions of 573 fine grained clastic sedimentary rocks of all the early proterozoic to neogene ages from the north china craton the yangtze craton and the north and south qinling orogenic belts as well as their ten tectonostratigraphic subunits the purpose of this study is to investigate possible provenance changes that are expected to have been recorded in the phanerozoic clastic sediments of the south qinling basins if north and south china had approached and accreted because the north china craton north qinling belt and yangtze craton have distinct upper crustal and provenance compositions both the north china and yangtze cratons and the north qinling belt are characterized by a relatively uniform non evolutionary composition of clastic sediments in la th sc co systematics during post archean times they can be however distinguished from each other in provenance in terms of the la th sc th th co as well as k2o na2o ratios in contrast there is a significant provenance change in the silurian devonian clastic sediments from the baiyun zhashui liuba yunxian and western xichuan subunits of the south qinling belt the late sinian 700 600 ma cambrian and ordovician pelites show characteristics that are consistent with the yangtze craton being high in la th 3 5 10 and sc th 1 4 2 4 and low in th co 0 2 1 0 and lacking la th ratios 3 5 these are accompanied by the increasing immaturity of the silurian devonian clastic sediments illustrated by the decreasing k2o na2o ratio which is attributed to the near source of the north qinling belt dominated by low k2o island arc igneous suites and immature metagraywackes we propose that the south qinling basins except the eastern xichuan subunit had been in a close proximity and accreted to the north qinling belt in silurian devonian times the result agrees well with recent studies of benthic faunas granite distribution age of granulite and clastic provenance directions of the south qinling devonian sediments but it is apparently inconsistent with the age of coesite and diamond bearing ecologite in the dabie mountains to the east and with paleomagnetic data both suggesting an important triassic jurassic collision event between north and south china it is likely that the silurian devonian and the mesozoic represent two different episodes phases of accretion and collision between north and south china new models regarding the phanerozoic tectonic evolution of east china are needed to reconcile the contrasting evidence

cr isotope systematics in the connecticut river estuary abstract stable chromium cr isotopes expressed in 53cr as the deviation of 53cr 52cr in samples from the international standard nist srm 979 are an emerging paleoredox proxy that has been used to track earth s redox evolution over a range of timescales there have been several major advances in our understanding of the cr isotope system but we are still developing a basic knowledge of the global cr isotope mass balance and there are key cr fluxes with poorly constrained isotopic compositions the cr isotope behavior in estuaries is one such poorly constrained aspect of the modern cr cycle therefore we collected water and suspended particulate samples from the estuary of the connecticut river and present the first cr isotope dataset from a salinity gradient as ph and salinity increases from brackish to salt water dissolved cr increases from 118 1 25 7 ng kg 1 n 6 to 225 1 26 0 ng kg 1 n 8 while particulate cr decreases from 341 3 319 8 ng kg 1 n 6 to 208 4 364 5 ng kg 1 n 8 dissolved 53cr decreases from 1 30 0 27 1 n 6 to 0 64 0 16 1 n 8 whereas particulate 53cr remained stable at 0 11 0 07 1 n 9 these data suggest that cr is lost from particles to solution when transitioning from brackish to salt water in the connecticut river estuary if these observations can be confirmed in other estuaries estuary systems can potentially enhance riverine dissolved cr fluxes and thus lower the cr oceanic residence time which has important implications for the cr isotope system as a paleoceanographic redox proxy there is a clear inverse correlation between dissolved cr and 53cr in the connecticut river estuary which yields a cr isotope fractionation factor of 0 89 that is indistinguishable from previous estimates based on open seawater samples large 53cr variation in oxygenated open and estuary seawater suggests that marine 53cr variations need not necessarily be exclusively tied to cr vi reduction in low oxygen zones therefore interpreting 53cr in the sedimentary record requires taking multiple processes into consideration

geochemical and nd isotopic evolution of paleoproterozoic arc type granitoid magmatism in the flin flon belt trans hudson orogen canada granitoid magmatism spans three flin flon belt evolutionary stages i evolved arc 1920 ma plus early juvenile arc 1904 1880 ma plutonism during intraoceanic arc back arc formation ii early 1878 1860 ma and middle 1860 1844 ma successor arc plutonism following accretion and successor arc s development and iii late 1843 1826 ma successor arc plutonism accompanying successor basin formation and waning arc magmatism amphibole bearing mineralogy metaluminous compositions and igneous microgranitoid enclaves indicate derivation from infracrustal sources predominance of intermediate calc alkaline compositions and negative nb anomalies on normalized patterns over a 46 77 wt silica range indicate an arc setting basaltic end members indicate important contributions directly from the mantle e nd t values are predominately in the range 0 to 4 3 reflecting mixing between depleted mantle melts and an archean crustal component preserved in evolved arc plutons 3 9 to 6 temporal variations include the following i early juvenile arc plutons are low k high field strength element hfse depleted with relatively flat rare earth element ree patterns and negative eu anomalies indicative of low pressure partial melting fractionation in the mantle wedge with residual pyroxene and plagioclase ii early and middle successor arc plutonism is medium k with steep ree patterns and no eu anomalies indicative of input from melting of basaltic sources likely subducted back arc oceanic crust under high pressure conditions with residual garnet and or amphibole and no plagioclase iii late successor arc plutons are high k more hfse enriched with both variable ree pattern slopes and eu anomalies indicative of a significant petrogenetic role of recycling of preexisting juvenile arc accretionary complex crust

observations and controls on the occurrence of inherited zircon in concord type granitoids new hampshire u pb analyses of zircons separated from two concord type plutons near sunapee and dixville notch new hampshire reveal differences in the pattern and magnitude of zircon inheritance which are related to differences in melt chemistry the sunapee pluton contains only slightly more zr than required to saturate the melt at the peak temperature of 700 30 sup 0 c traces of inherited zircon in this separate are inferred to be present as small largely resorbed grains in contrast the long mountain pluton near dixville notch contains about 240 more zr than required to saturate the melt thus more than half of the zr existed as stable inherited zircon crystals during the partial fusion event consistent with the observation of substantial inheritance in all grain size fractions ion probe intra grain analyses of zircon from the long mountain pluton indicate a complex pattern of inheritance with contributions from at least two proterozoic terranes and caution against simple interpretations of upper and lower intercepts of chords containing an inherited component ion probe analyses of zircons from the sunapee pluton reveal clear evidence of u loss which results in incorrect apparent conventional u pb ages ages of crystallization for the long mountain and sunapee pluton are approx more 350 and 354 5 ma respectively a sm nd measurement for the long mountain pluton yields a depleted mantle model age of 1 5 ga consistent with the observed inheritance pattern in contrast a sm nd model age for the sunapee pluton is improbably old due to minor monazite fractionation less

constraints on timing of peak and retrograde metamorphism in the dabie shan ultrahigh pressure metamorphic belt east central china using u th pb dating of zircon and monazite abstract the dabie shan ultrahigh pressure metamorphic uhpm belt occupies the suture between the yangtze and sino korean blocks in east central china the timing of uhpm in the dabie belt is controversial and most recent data come from dating of zircons monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones and monazite growth has been documented to occur during prograde peak and retrograde metamorphism zircons and monazites from uhp mafic rocks from maowu and a uhp jadeite quartzite from shuanghe were imaged in thin sections and in mineral separates and dated using a high resolution ion microprobe maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite maowu eclogites and garnet pyroxenites contain zircons with mean 206 pb 238 u age of 230 4 ma and monazites from a clinopyroxenite have 208 pb 232 th ages of 209 4 ma multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages i e they do not correspond to the cessation of diffusional pb loss as rocks cooled below the closure temperature during exhumation shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921 22 ma and lower intercept age of 236 32 ma interpreted to represent the ages of the source rocks and of peak metamorphism respectively zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1 5 gpa their pooled age of 238 3 ma agrees well with the lower intercept defined by cores suggesting that pb loss from cores and growth of new rims occurred during uhpm at 235 240 ma monazite records multiple events during retrograde metamorphism maowu clinopyroxenite and shuanghe jadeite quartzite experienced monazite growth at 209 ma interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration only the jadeite quartzite records growth at 223 1 ma estimated exhumation rates for maowu are 7 8 km ma

a chromate contaminated site in southern switzerland part 1 site characterization and the use of cr isotopes to delineate fate and transport abstract the risk of groundwater contamination by chromate at a former chromite ore processing industrial site in rivera switzerland was assessed by determining subsoil cr vi concentrations and tracking naturally occurring cr vi reduction with cr isotopes using a hot alkaline extraction procedure a total cr vi contamination of several 1000 kg was estimated jarosite kfe 3 so 4 x cro 4 1 x 2 oh 6 and chromatite cacro 4 were identified as cr vi bearing mineral phases using xrd both limiting groundwater cr vi concentrations to track assumed cr vi reduction at field scale 53 cr values of contaminated subsoil samples in addition to groundwater 53 cr data are used for the first time the measurements showed a fractionation of groundwater 53 cr values towards positive values and subsoil 53 cr towards negative values confirming reduction of soluble cr vi to insoluble cr iii using a rayleigh fractionation model a current cr vi reduction efficiency of approximately 31 along a 120 m long flow path was estimated at an average linear groundwater velocity of 3 3 m d groundwater and subsoil 53 cr values were compared with a site specific rayleigh fractionation model proposing that subsoil 53 cr values can possibly be used to track previous higher cr vi reduction efficiency during the period of industrial activity the findings strongly favor monitored natural attenuation to be part of the required site remediation measures

regional geochemistry of tertiary igneous rocks in central chile implications for the geodynamic environment of giant porphyry copper and epithermal gold mineralization the giant porphyry copper molybdenum deposits of central chile formed within a thick sequence of cretaceous to pliocene volcanic rocks the predominantly calc alkaline basaltic andesites of the cretaceous las chilcas formation are characterized by la smn ratios of 1 8 to 2 5 and sm ybn of 1 8 to 2 8 the upper oligocene to lower miocene abanico formation variously defined as the los pelambres abanico or coya machali formations ranges from basalt to rhyolite in composition and exhibits a broad southward transition from calc alkaline to tholeiitic all samples from this formation are characterized by lree enrichment and moderately or locally strongly fractionated hree la smn 1 3 4 1 sm ybn 1 5 5 8 the basaltic andesites and andesites of the middle miocene salamanca formation have ree chemistry similar to that of the upper cretaceous strata la smn 1 5 2 7 sm ybn 1 6 2 7 the overlying middle miocene farellones formation ranges from tholeiitic to calc alkaline and from basalt to andesitic and has a similar lree enrichment and fractionated hree la smn 1 7 2 5 sm ybn 1 7 3 3 the pliocene la copa rhyolite complex however is strongly lree enriched and hree depleted la smn 3 8 3 9 sm ybn 4 2 4 7 overall the trace element geochemistry of the cretaceous to middle miocene volcanic rocks is characterized by enriched lree and negative nb anomalies consistent with an arc setting with only minor differences in the abundance of most elements crustal thickening during the miocene in central chile has been suggested to have been responsible for a transition from an amphibole to garnet dominated residual mineralogy resulting in the release of fluids that enabled the formation of giant copper porphyry deposits however the gradual increase in la yb through the early middle and late miocene reported in earlier studies and interpreted to be a response to crustal thickening is not observed in the regional data instead a rapid change in the geochemical signature between the end of the eruption of the farellones formation and the eruption of the high la yb la copa rhyolite complex implies a more abrupt change in the tectonic environment isotopic data broadly support this although lower epsilon nd values in the farellones formation imply a greater role for crustal contamination in younger suites in the absence of gradual crustal thickening it is suggested that the subduction of the juan fernandez ridge may have been the key geodynamic process responsible for the genesis of the three middle miocene to lower pliocene giant porphyry copper deposits in central chile possibly by promoting crustal scale faulting and even acting as a source of metals

evaluating trace elements as paleoclimate indicators multivariate statistical analysis of late mississippian pennington formation paleosols kentucky u s a abstract the temporal and spatial distributions of trace elements in paleosols in relation to soil forming processes and climate have received little attention primarily due to their generally low concentrations 100 ppm and a fundamental lack of knowledge of their behavior in soil systems trace element concentrations of pennington formation paleosols spanning an 8 ma interval in the late mississippian chesterian were analyzed using linear and multivariate statistics of whole rock elemental data linear statistics of the elemental data set show that ti zr nb cs la hf ta w ce and th have the highest correlation through time with r values 0 75 nb served as the proxy trace element for comparison temporal trends in nb closely match trends in lessivage clay formation and accumulation by feldspar weathering mean annual precipitation map and chemical weathering map effectively controls soil hydrology and the accumulation of organic matter and clay map in conjunction with chemical we

internal and external fluid sources for eclogite facies veins in the monviso meta ophiolite western alps implications for fluid flow in subduction zones to contribute to our understanding of the mechanisms and pathways of fluid movement through deeply subducted crust we investigate high pressure veins cutting eclogite facies 2 0g pa and 6008c metagabbros of the monviso ophiolite italian western alps the veins consist mainly of omphacite with minor garnet rutile talc and accessory zircon most of the vein minerals have major and trace element compositions that are comparable with the host rock minerals and vein and host rock zircons have similar hf isotopic compositions these observations support the conclusions of previous studies that these veins largely formed from a locally sourced hydrous fluid during prograde or peak metamorphism however the bulk rock cr and ni contents of the veins are significantly higher than those of the surrounding host eclogites we also document distinct cr rich up to weight per cent levels zones in omphacite garnet and rutile in some vein samples vein garnet and talc also have relatively high mgo and ni contents x ray maps of vein garnet and rutile grains reveal complex internal zoning features which are largely defined by micrometre scale variations in cr content some grains have concentric and oscillatory zoning in cr whereas others feature a chaotic fracture like pattern these cr rich zones are associated with high concentrations of ni b as sb nb zr and high ratios of light rare earth elements lree to middle ree mree compared with low cr vein and host rock minerals petrological and mass balance constraints verify that the cr rich zones in the veins were not derived from internally sourced fluids but represent precipitates from an external fluid the external source that is consistent with the distinctive trace element characteristics of the vein components is antigorite serpentinite which forms the structural basement of the high pressure metagabbros we propose at least two separate growth mechanisms for the monviso veins most vein infillings were formed during progressive prograde metamorphism from locally derived fluid influx of the serpentinitederived or other external fluid was transient and episodic and was probably achieved via brittle fractures which preferentially formed along the pre existing vein structures the dehydration of serpentinite at high pressures in subduction zones may provide crucial volatiles and trace elements for arc magmas our results indicate that the movement of these fluids through subducted oceanic crust is likely to be highly channeled and transient so the progressive development of vein systems in mafic rocks may also be crucial for forming channelways for long distance fluid flow at depth in subduction zones

composite synvolcanic intrusions associated with precambrian vms related hydrothermal systems large subvolcanic intrusions are recognized within most precambrian vms camps of these 80 are quartz diorite tonalite trondhjemite composite intrusions the vms camps spatially associated with composite intrusions account for 90 of the aggregate sulfide tonnage of all the precambrian intrusion related vms camps these low alumina low k and high na composite intrusions contain early phases of quartz diorite and tonalite followed by more voluminous trondhjemite they have a high proportion of high silica 74 sio2 trondhjemite which is compositionally similar to the vms hosting rhyolites within the volcanic host rock successions the quartz diorite and possibly tonalite phases follow tholeiitic fractionation trends whereas the trondhjemites fall within the composition field for primitive crustal melts these transitional m i type primitive intrusive suites are associated with extensional regimes within oceanic arc environments subvolcanic composite intrusions related to the archean sturgeon lake and noranda and paleoproterozoic snow lake vms camps range in volume from 300 to 1 000 km3 three have a sill morphology with strike lengths between 15 and 22 km and an average thickness between 1 500 and 2 000 m the fourth has a gross stock like shape the vms deposits are principally restricted to the volcanic strata above the strike length of the intrusions as are areally extensive thin exhalite units the composite intrusions contain numerous internal phases which are commonly clustered within certain parts of the composite intrusion these clusters underlie eruptive centers surrounded by areas of hydrothermal alteration and which contain most of the vms deposits early quartz diorite and tonalite phases appear to have intruded in rapid succession evidence includes gradational contacts magma mixing and disequilibrium textures they appear to have been emplaced as sill dike swarms these early phases are present as pendants and xenoliths within later trondhjemite phases the trondhjemite phases contain numerous internal contacts indicating emplacement as composite sills common structural features of the composite intrusions include early xenolith phases abundant small comagmatic dikes fractures and veins and in places columnar jointing internal phases may differ greatly in texture from fine to coarse grained aphyric and granophyric through seriate to porphyritic mineralogical and isotopic evidence indicates that early phases of each composite intrusion are affected by pervasive to fracture controlled high temperature 350 450 c alteration reflecting seawater rock interaction trondhjemite phases contain hydrothermal magmatic alteration assemblages within miarolitic cavities hydrothermal breccias and veins this hydrothermal magmatic alteration may in part be inherited from previously altered wall rocks two of the four intrusions are host to cu mo rich intrusive breccias and porphyry type mineralization which formed as much as 14 ma after the main subvolcanic magmatic activity the recognition of these precambrian subvolcanic composite intrusions is important for greenfields vms exploration as they define the location of thermal corridors within extensional oceanic arc regimes which have the greatest potential for significant vms mineralization the vms mineralization may occur for 2 000 m above the intrusions in some cases vms mineralization has been truncated or enveloped by late trondhjemite phases of the composite intrusions evidence that much of the trondhjemitic magmatism postdates the principal vms activity is a critical factor when developing heat and fluid flow models for these subseafloor magmatic hydrothermal systems

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

evaluation of elemental depletion weathering rate estimation methods on acid sensitive soils of north eastern alberta canada abstract immobile element based weathering estimation methods assume that zr or ti is an immobile element and that weathering rates of other elements can be estimated according to the enrichment of zr in weathered horizons relative to an unweathered parent material this approach was used to estimate base cation weathering rates for 33 soil profiles on acid sensitive terrain in north eastern alberta zirconium generally showed enrichment within the rooting zone but the deepest subsoil samples were not always associated with the lowest zr concentrations weathering rates estimated with the zr depletion and pedological mass balance pmb methods were generally low ranges 0 51 and 0 58 mmol c m 2 yr 1 respectively however low base cation oxide concentrations and heterogeneity within soil profiles complicated weathering rate calculations and net base cation gains were calculated for several six sites evaluation of the zr depletion and pmb weathering estimates against those calculated with the process oriented profile model at a subset n 9 of the sites indicated the estimates were poorly related with profile rates typically being higher the effects based emissions management strategy for acid precursors in this region requires spatial coverage of soil properties including weathering rates across a large area but the apparent limitations associated with the immobile element based methods in this region identifying representative parent soils and deriving weathering rate estimates comparable to more robust methods are arguments against their candidacy for future use

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

zircon u pb dating of water rock interaction during neoproterozoic rift magmatism in south china abstract a combined study of zircon u pb dating and mineral o isotope analysis was carried out for neoproterozoic granite gabbro and gabbrodiorite from the beihuaiyang zone that marks a rifted margin between the south china block and the supercontinent rodinia the results provide precise constraints on timing of water rock interaction resulting in unusual 18 o depletion in igneous minerals cl imaging reveals that zircons mostly show clear oscillatory and or sector zoning with or without thin alteration rims no metamorphic zircon occurs in the igneous intrusives although they suffered greenschist facies metamorphism during triassic continental collision la icpms zircon u pb dating yields consistent ages of 739 8 to 754 5 ma which are interpreted as timing of magma crystallization mineral separates have highly variable 18 o values of 0 6 to 6 8 for zircon 1 2 to 12 5 for quartz and 0 3 to 8 4 for plagioclase oxygen isotope heterogeneities exist within single intrusions and between different intrusions magmatic zircons with low 18 o values and a weighted mean u pb age of 748 6 ma crystallized from low 18 o magmas that were derived from melting of hydrothermally altered low 18 o rocks both low and normal 18 o intrusives in the beihuaiyang zone are shown to form at the same time contemporaneous with the rodinia breakup thus tectonic evolution from supercontinental rift to breakup is proposed to create conditions for alternating processes of high t hydrothermal alteration and low 18 o magmatism such processes are a characteristic feature of high t water rock interaction in volcanic rift margins associated with localized water saturated melting while the occurrence of low 18 o magmas demonstrates penetration of meteoric water into the rift tectonic zone of crustal melting the existence of normal 18 o magmas indicates that their emplacement may serve as a thermal engine to drive the second stage of meteoric water rock interaction resulting in the unusual 18 o depletion in adjacent rocks the concurrent occurrence of both low and normal 18 o magmas appears to indicate highly heterogeneous high t water rock interaction during rift magmatism

contributors to chromium isotope variation of meteorites we report the results of a comprehensive high precision survey of the cr isotopic compositions of primitive chondrites along with some differentiated meteorites to ensure complete dissolution of our samples they were first fused with lithium borate tetraborate at 1050 1000 c relative to the nist cr standard srm 3112a carbonaceous chondrites exhibit excesses in 54cr 52cr from 0 4 to 1 6e 1e 1 part in 10 000 and ordinary chondrites display a common 54cr 52cr deficit of 0 4e analyses of acid digestion residues of chondrites show that carbonaceous and ordinary chondrites share a common 54cr enriched carrier which is characterized by a large excess in 54cr 52cr up to 200e associated with a very small deficit in 53cr 52cr 2e we did not find 54cr anomalies in either bulk enstatite chondrites or in leachates of their acid digestion residues this either requires that the enstatite chondrite parent bodies did not incorporate the 54cr anomaly carrier phase during their accretion or the phase was destroyed by parent body metamorphism chromium in the terrestrial rocks and lunar samples analyzed here show no deviation from the nist srm 3112a cr standard the eucrite and martian meteorites studied exhibit small deficits in 54cr 52cr the 54cr 52cr variations among different meteorite classes suggest that there was a spatial and or temporal heterogeneity in the distribution of a 54cr rich component in the inner solar system we confirm the correlated excesses in 54cr 52cr and 53cr 52cr for bulk carbonaceous chondrites but the new data yield a steeper slope 6 6 than that reported in shukolyukov and lugmair 2006 the correlated excesses may affect the use of the mn cr chronometer in carbonaceous chondrites we could not confirm the bulk carbonaceous chondrite mn cr isochron reported by shukolyukov and lugmair 2006 and moynier et al 2007 mostly because we find much smaller total variations in e53cr 0 2 all bulk chondrites have small e53cr excesses up to 0 3 relative to the earth most likely reflecting the sub chondritic mn cr ratio of the earth the e53cr variations in chondrites do seem to grossly correlate with mn cr and yield an initial solar system 53mn 55mn value of 5 4 2 4 10 6 corresponding to an absolute age of 4566 4 2 2 ma nuclear interactions with cosmic rays result in coupled excesses in e54cr and e53cr with a 4 1 ratio in phases with high fe cr these are most dramatically demonstrated in the iron meteorite carbo showing excesses in e54cr of up to 140e these new results show that the mn cr chronometer should be used with caution in samples minerals with high fe cr and long cosmic ray exposure ages

coesite bearing eclogite from the dabie mountains in central china coesite and coesite pseudomorphs are recognized in eclogite from the dabie mountains a collision zone between the sino korean and yangtze cratons in central china this third known occurrence of coesite in deep crustal rocks is within an archean gneiss terrane and is the first locality where coesite has been identified as an inclusion in both omphacite and garnet crystals at the same location coesite bearing eclogite is estimated to form at pressures of 28 kbar and temperatures from 600 to 710 c the dabie mountains eclogite underwent retrograde metamorphism to amphibolite facies after its formation the preservation of coesite in eclogite is found to be strongly controlled by the pressure temperature time path that coesite bearing eclogite followed during its uplift and retrograde metamorphism

chromium isotope signature during continental crust subduction recorded in metamorphic rocks the chromium isotope compositions of 27 metamorphic mafic rocks with varying metamorphic degrees from eastern china were systematically measured to investigate the cr isotope behavior during continental crust subduction the cr isotope compositions of all samples studied were bulk silicate earth bse like with 53crnist979 of greenschists amphibolites and eclogites ranging from 0 06 to 0 17 0 05 to 0 27 and 0 01 to 0 24 respectively the lack of resolvable isotopic variability among the metamorphic rocks from different metamorphic zones indicated that no systematic cr isotope fractionation was associated with the degree of metamorphism however the cr isotopic variability among homologous samples may have reflected effects induced by metamorphic dehydration with a change of redox state rather than protolith heterogeneity i e magma differentiation in addition the differences in 53cr 53crcpx gt between coexisting clinopyroxene cpx and garnet gt from two garnet pyroxenites were 0 06 and 0 34 respectively indicating that significant inter mineral cr isotope disequilibria could occur during metamorphism to provide a basis for comparison with metamorphic rocks and to provide further constraints on the potential cr isotope heterogeneity in the mantle and in the protolith of some metamorphic rocks we analyzed mantle derived chromites and the associated peridotites from luobusa and we obtained the following general order chromite free peridotites 0 21 to 0 11 chromite bearing peridotite 0 07 chromite 0 06 these findings imply potential mantle heterogeneity as a result of partial melting or fractional crystallization associated with chromite

fluctuations in late neoproterozoic atmospheric oxidation cr isotope chemostratigraphy and iron speciation of the late ediacaran lower arroyo del soldado group uruguay oxygenation of the earth s atmosphere occurred in two major steps near the beginning and near the end of the proterozoic eon 2500 to 542 ma ago but the details of this history are unclear chromium isotopes in iron rich chemical sediments offer a potential to highlight fine scale fluctuations in the oxygenation of the oceans and atmosphere and to add a further dimension in the use of redox sensitive tracers to solve the question regarding fluctuations of atmospheric oxygen levels and their consequences for earth s climate we observe strong positive fractionations in cr isotopes 53 cr up to 5 0 in iron rich cherts and banded iron formation horizons within the arroyo del soldado group ediacaran uruguay that can be explained by rapid effective oxidation of fe ii rich surface waters these fluctuations are correlated with variations in ratios of highly reactive iron fe hr to total iron fe tot which indicate a predominance of anoxic water columns fe hr fe tot 0 38 during the onset of oxidation pulses we favor the scenario by which isotopically heavy cr vi entered the basin after pulses of oxidative weathering on land and in which fe ii accumulated in the water column neodymium isotopes reveal that these oxygenation pulses were followed by increased influxes to the basin of continental crust derived detrital components of paleoproterozoic nd t dm model ages 2 1 2 2 ga provenance typical of the rio de la plata craton the association of positive 53 cr ferruginous fe hr fe tot 0 38 stratigraphic intervals with low diversity acritarch assemblages dominated by bavlinella faveolata strongly support models postulating a stratified eutrophic neoproterozoic ocean thus even within a few million years of the precambrian cambrian boundary paleoceanographic conditions resembled more those of paleoproterozoic oceans than phanerozoic and present oceans this highlights the sheer magnitude of ecological changes at the precambrian cambrian transition changes which ultimately led to the demise of the precambrian world and the birth of the metazoan dominated phanerozoic

sm nd and rb sr isotopic chronology and cooling history of ultrahigh pressure metamorphic rocks and their country rocks at shuanghe in the dabie mountains central china abstract ultrahigh pressure metamorphic uhpm rocks at shuanghe area in the dabie mountains occur as a uhpm block within the regional granitic gneiss a sm nd isochron defined by garnet omphacite rutile from coesite bearing eclogite yield an age of 226 3 3 2 ma another sm nd isochron defined by garnet two phengites form uhp gneiss yield a similar age of 226 5 2 3 ma the two consistent sm nd ages defined by three uhpm minerals suggest that nd isotopic equilibrium between uhpm minerals in these rocks during uhp metamorphism has been achieved this may correspond to the age of peak metamorphism with an average metamorphic temperature of 800 c it is demonstrated that the retrograde metamorphism of uhpm rocks occurred in an open chemical system whereas the nd and sr isotopic systematics of uhpm minerals may have remained closed nd and sr isotopic disequilibrium between uhpm minerals and retrograde metamorphic minerals has been observed therefore the tie line of garnet or phengite and whole rock containing retrograde metamorphic minerals gives an old sm nd age and a young rb sr age respectively with no geologic significance however a rb sr ages of 219 0 6 6 ma defined by phengite garnet from a uhpm gneiss indicates that the uhpm rocks at shuanghe cooled down to 500 c at that time it suggests that the uhpm rocks at shuanghe experienced the first rapid cooling during 226 219 ma on the other hand the rb sr ages of 174 7 8 to 169 2 3 3 ma defined by retrograde minerals amphibole or biotite with closure temperature ranging from 450 to 300 c and intensely retrograded metamorphic rocks reflect a second rapid cooling during this time interval this is consistent with the rapid cooling time of 190 170 ma of the orthogneiss in the dabie su lu uhpm belt obtained by ar ar dating method in contrast to uhpm rocks the nd isotopic composition of the garnet in granitic gneiss has been reset during retrograde metamorphism and is in equilibrium with those of retrograde epidote and biotite which yields a sm nd isochron age of 213 5 ma indicating the retrometamorphic time corresponding to amphibolite facies in addition the biotites from the granitic gneiss yield rb sr ages of 171 173 ma similar to those of the uhpm rocks these data suggest that the country rocks granitic gneiss may have a similar cooling history to the uhpm rocks at shuanghe two stages of rapid cooling of uhpm rocks at shuanghe may correspond two stages of fast uplift the initial rapid uplifting and cooling of uhpm rocks during 226 219 ma may be caused by compression tectonics during subducting time of the continental crust whereas the later rapid cooling may reflect the exhumation of the entire subducted continental crust by extension during the early middle jurassic

chromitites in ophiolites how where when why part i a review and new ideas on the origin and significance of platinum group minerals abstract platinum group minerals are the dominant control on the budget of platinum group elements in ophiolitic chromitites they vary widely in distribution morphology chemistry and os isotope compositions their distribution in different types of chromitites and different microstructural sites within them may reflect the role of chromite as a physical collector of the platinum group elements during crystallisation of both phases from melts the platinum group minerals show significant os isotope heterogeneity on the scale of single micrometric grains enclosed within individual chromite grains this micro scale isotopic heterogeneity and their random spatial distribution within the chromites argue strongly against an origin of the platinum group minerals via exsolution from the chromite lattice during post magmatic cooling a re interpretation of their microstructures and os isotopes analysed in situ had led us to review the traditional models and to propose several origins for the platinum group minerals in chromitites 1 the suites of platinum group minerals mainly record crystallisation during mixing of multiple different melts and physical entrapment in chromite 2 some platinum group minerals in chromitites may have been scavenged from wall rock peridotite during migration of the parental melts of the chromitite 3 other platinum group minerals may have precipitated from metasomatic fluid melts that infiltrated existing chromitites the recrystallization of pre existing platinum group minerals during polyphase metamorphism or recycling of the chromitite into deeper mantle levels can explain the presence of micrometric platinum group minerals with distinct os isotope compositions

mineral inclusions in diamonds from the sputnik kimberlite pipe yakutia abstract the sputnik kimberlite pipe is a small satellite of the larger mir pipe in central yakutia sakha russia study of 38 large diamonds 0 7 4 9 carats showed that nine contain inclusions of the eclogitic paragenesis while the remainder contain inclusions of the peridotitic paragenesis or of uncertain paragenesis the peridotitic inclusion suite comprises olivine enstatite cr diopside chromite cr pyrope garnet both lherzolitic and harzburgitic ilmenite ni rich sulfide and a ti cr fe mg sr k phase of the lindsleyite mathiasite lima series the eclogitic inclusion suite comprises omphacite garnet ni poor sulfide phlogopite and rutile peridotitic ilmenite inclusions have high mg cr and ni contents and high nb zr ratios they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite eclogitic phlogopite is intergrown with omphacite coexists with garnet and has an unusually high tio 2 content comparison with inclusions in diamonds from mir shows general similarities but differences in details of trace element patterns large compositional variations among inclusions of one phase olivine garnet chromite within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases p t conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions the geotherm at the time of diamond formation was near a 35 mw m 2 conductive model that is indistinguishable from the paleozoic geotherm derived by studies of xenoliths and concentrate minerals from mir a range of ni temperatures between garnet inclusions in single diamonds from both mir and sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere within the diamond stability field the minor differences between inclusions in mir and sputnik may reflect lateral heterogeneity in the upper mantle

the effect of land plants on weathering rates of silicate minerals abstract land plants and their associated microbiota directly affect silicate mineral weathering in several ways by generation of chelating ligands by modifying ph through production of co2 or organic acids and by altering the physical properties of a soil particularly the exposed surface areas of minerals and the residence time of water in laboratory experiments far from equilibrium 1 mm oxalate a strong chelator of al has a negligible effect on the dissolution rate of alkali feldspars but some effect on calcic feldspars and olivine by analogy to oxalate the overall effect of organic ligands on the weathering rate of silicate minerals in nature is likely to be small except perhaps in microenvironments adjacent to roots and fungal hyphae the effect of ph on silicate mineral dissolution rate depends on ph below ph 4 5 the rate increases with decreasing ph in the circumneutral region the rate is ph independent and at ph values above around 8 the rate increases with increasing ph vegetation should thus cause an increase in weathering rate through the ph effect only where the ph is below 4 5 as an overall generalization the effect of plants on weathering rate through changes in soil solution chemistry is probably small for granitic rocks it may be greater for more mafic rocks it is the release of ca and mg from mafic rocks that has the greatest influence on the global co2 budget the effect of changes in soil physical properties on weathering rate can be major by binding fine particles plants can greatly increase weathering rates in areas of high physical erosion where erosion rates are lower the effect of plants is less clear on long timescales plants may decrease chemical weathering by binding secondary products and isolating unweathered minerals from meteoric water a major unknown in estimating the effect of the advent of land plants on weathering rates is the nature thickness particle size distribution permeability of the regolith on the pre silurian continents the indirect effect of vegetation through changing the regional distribution of precipitation may be as important as the direct effects

the wathaman batholith largest known precambrian pluton the early proterozoic wathaman batholith in northern saskatchewan and manitoba is a 900 km long megacrystic granite granodiorite intrusion that straddles the junction between ensialic miogeoclinal and probably ensimatic eugeoclinal island arc terranes of the trans hudson orogen of the western churchill province although the largest precambrian batholith known it is apart from marginal complexities remarkably homogeneous throughout and unlike comparably sized and situated phanerozoic batholiths shows no evidence of multiple intrusion nor does it have comagmatic early mafic phases however it may be considered as just one phase of a larger batholithic belt that also includes numerous smaller plutons taken as a whole the composite batholithic belt is similar in many aspects to mesozoic pacific rim batholithic belts and like them probably was emplaced during plate collision the batholith is affected by pervasive internal deformation is bounded on the northwest by major blastomylonite zones a

cr isotopic composition of the laobao cherts during the ediacaran cambrian transition in south china abstract in this study we present the cr isotopic compositions from the cherts in the laobao formation 551 522 ma that were deposited in deep water in south china the cr concentrations and isotopic compositions in the laobao cherts were dominated by the contemporaneous seawater and detrital phases the lower part of the laobao cherts show that the 53cr of contemporaneous deep seawater in the huanan basin was approximately 0 1 and the 53cr of detrital materials was approximately 0 2 these findings imply that surface 53cr enriched cr vi species did not affect the deep ocean until the late ediacaran because of the strong redox stratification in the ocean although the oxidative cr cycle in terrestrial settings had emerged before that time a slight rise in 53cr in the deep ocean to approximately 0 08 was estimated for the upper part of the laobao cherts if the 53cr rise and its amplitude can be further determined this will imply that shallow 53cr enriched cr vi species had an impact on the deep ocean and will suggest an oxidation of the ocean in the huanan basin around the ediacaran cambrian boundary 542 ma

microtextural controls of weathering of perthitic alkali feldspars the relationship between the microtexture and dissolution behaviour of fresh hf acid etched and naturally weathered alkali feldspar phenocrysts from the lower devonian shap granite has been investigated by sem and tem a novel resin impregnation technique has revealed the three dimensional shape and interconnectivity of etch pits beneath the weathered crystal surface further electron microscope work suggests that shap phenocrysts are representative of the alkali feldspar in the protolith of many soils fresh and unweathered shap feldspars have a complex microtexture comprising areas of pristine cryptoperthite and lamellar microperthite cross cut by volumes of microporous altered feldspar or patch perthites cryptoperthites are made up of 75 nm wide albite films the platelets are coherent but albite films have numerous edge dislocations along their interface with orthoclase 001 and 010 cleavage surfaces intersect 2 3 edge dislocations m2 in three dimensions these edge dislocations form an orthogonal net in the murchison plane of easy fracture close to 601 patch perthites are irregular semicoherent to incoherent intergrowths of albite and irregular microcline subgrains with 0 65 0 70 sub m to m sized pores m2 microporous patch perthites form by dissolution reprecipitation reactions with magmatic or hydrothermal fluids and pores are present before the alkali feldspars enter the weathering regime dissolution of shap feldspars during natural weathering and laboratory acid etching is controlled by their microtexture especially by dislocations and exsolution lamellae the core and strain energy associated with dislocation outcrops on 001 and 010 cleavage surfaces promotes rapid dissolution at those sites and formation of nn sized etch pits after 30 s of laboratory etching with hf acid vapour with progressive hf etching crystallographically controlled differences in the reactivity of etch pit walls cause them to expand more rapidly into orthoclase than albite naturally weathered feldspars were collected from the glacial erratic boulders fine gravels surrounding exposed granite surfaces and from peat soil overlying the granite during natural weathering etch pits on microperthites enlarge almost exclusively by dissolution of albite and resin casts demonstrate that they can penetrate 15 m below the cleavage surface forming an interconnecting ladder like grid of submicrometer wide channels in the murchison plane coherent albite platelets and volumes of albite between dislocations in films dissolve uniformly but faster than orthoclase this is probably because the albite lamellae have significant elastic coherency strain but this is much less per unit volume of albite than the core and strain energy associated with edge dislocations patch perthites etch in hf vapour and weather rapidly in nature to produce a honeycomb like texture of interconnecting nanometer to micrometer sized pits which nucleate at preexisting micropores or incoherent subgrain boundaries the size and density of etch pits on microperthite surfaces which is determined by rates of growth and coalescence may be a useful progress variable for natural and experimental dissolution all alkali feldspars are highly heterogeneous materials whose chemical composition and microtexture can vary on a submicrometer scale these microtextures are critical variables with regard to the origin of surface roughness of fresh and weathered grains the controls on absolute dissolution rates and why they commonly change over time the nonlinear variation of dissolution rate with grain size the ratio of alkali ions released into solution and disparities between laboratory dissolution rates and those observed in the field

evolution of the snow lake portion of the palaeoproterozoic flin flon and kisseynew belts trans hudson orogen manitoba canada abstract understanding growth of early precambrian crust requires knowledge of the evolution of granite greenstone and metasedimentary belts two of the fundamental building blocks of early precambrian cratons u pb geochronologic data from the snow lake area which straddles the flin flon granite greenstone and kisseynew metasedimentary belts of the trans hudson orogen documents a 60 70 ma period of palaeoproterozoic crust formation from inception of these belts in ocean basins through their eventual structural telescoping during ocean basin consumption and collision with bounding archaean cratons earliest volcanic and plutonic rocks of the flin flon granite greenstone terrane at snow lake formed 1 89 ga mainly in a juvenile oceanic arc tectonic setting sedimentary rocks of the kisseynew belt previously thought to be contemporaneous with snow lake area arc volcanism are up to 30 50 ma younger 1 86 1 84 ga and postdate deformation associated with 1 88 1 87 ga arc arc collisions and 1 87 1 84 ga calc alkaline plutonism that affected much of the western flin flon belt deformation at snow lake occurred during two widely spaced episodes both of which effect the post 1 86 1 84 ga turbidite sequence and thus postdate 1 88 1 87 ga intraoceanic accretion in the western flin flon belt the older deformational episode involved isoclinal folding formed during 1 84 ga convergence of the kisseynew and flin flon belts and thrusting of kisseynew sedimentary lithologies southwards onto the flin flon volcanic rocks the younger deformational episode occurred 15 25 ma later and involved further folding and southwest thrusting accompanied by a 1 81 ga high grade thermal metamorphic episode centred in the kisseynew belt only minor increase of crustal material in the form of late intrusions were added following this episode

limited lithium isotopic fractionation during progressive metamorphic dehydration in metapelites a case study from the onawa contact aureole maine the major trace element and li isotopic compositions of granodiorite and metapelites from the onawa pluton and surrounding contact aureole have been measured to document the behavior of li and its isotopes during progressive metamorphic dehydration major andtraceelementconcentrationsinmetapelitesofthecontactaureolechangelittle whilelossonignition loi decreaseswithincreasing metamorphic grade which ranges from regional chlorite zone metamorphism far removed from the pluton to partially melted rocks adjacent to the pluton lithium concentrations in metapelites from all zones correlate with loi and abundances of the main li bearing minerals chlorite biotite muscovite decreasing by a factor of two from 130 ppm far removed from the pluton to 64 ppm adjacent to thepluton incontrast liisotopiccompositionsremainrelativelyunchangedacrosstheaureole 7 li 3 5to 1 0 andarecomparable to the range observed in schists and unmetamorphosed shales compared to the surrounding metasediments the granodiorite has a lower li content 45 ppm and a comparable 7 li value of 0 2 similar to those of other granites and the average upper crust 0 2 these observations are consistent with the removal of li from metapelites via rayleigh distillation during progressive metamorphic dehydration with isotopic fractionation factors between fluids and rocks ranging between 1 001 and 1 004 these values are similar to thoseofarecentexperimentalstudyandproduceisotopicfractionationthatisbarelybeyondanalyticaluncertainty 1 fortheobserved degree ofli depletion the good correlationsbetweenli concentrations and loiwithmineralogicalabundancesreflecttheimportanceof mineralogyincontrollingliconcentrationsandisotopiccompositionsofmetamorphicrocks comparisonoftheseresultswiththosefrom a study of a contact halo around the tin mountain pegmatite suggests that the nature of the intrusion plays a critical role in controlling the behavior of li in country rocks large diffusive driven isotopic fractionation over a small scale is expected to occur when rocks are infiltrated by li rich magmatic fluids while metamorphic devolatilization results in minor isotopic fractionation on the aureole scale published by elsevier b v

petrogenetic evolution of felsic volcanic sequences associated with phanerozoic volcanic hosted massive sulphide systems the role of extensional geodynamics abstract felsic volcanic hosted massive sulphide deposits form in many different geotectonic environments usually associated with episodes of extension however empirical geochemical links between felsic volcanism geotectonic environment and extensional geodynamics have not been adequately documented so as to distill the fundamental relationships between these features and formation of volcanic hosted massive sulphide vhms mineralization which is important considering most vhms deposits are hosted in felsic volcanic bearing sequences using felsic geochemical data from six different vhms mineralized phanerozoic geotectonic settings it seems that the felsic geochemical data are consistent with the overall geotectonic interpretations for these areas which in turn helps in stratigraphic and geotectonic interpretations for detailed vhms exploration in extensional geodynamic settings bimodal volcanism is common compared to the continuum of compositions observed under compressional to neutral high stress regimes in calc alkaline arc building environments the empirical geochemical metallogenic vhms relationships evident in the archean felsic volcanic rocks in particular the low zr y n yb n 2 o conditions in different source areas and geotectonic environments they seem to be the predominant controls on the compositional evolution of felsic magmas in extensional settings because of the limited degree of assimilation and fractional crystallization afc affecting their compositions with rapid intrusion into the uppermost crust to positions were they achieve neutral buoyancy in the volcanic to subvolcanic environment within plate felsic or a type anorogenic type and anomalous ocean ridge or type felsic magmas that have tholeiitic affinities relatively low alkali elements are common within parts of the volcanic sequences hosting massive sulphide deposits reflecting in part a relationship to extension the high contents of high field strength elements zr ga lree hree y nb etc and other lines of evidence indicate they are relatively high temperature relatively reduced magmas probably derived from previously dehydrated granulitic or melted oceanic simatic or continental sialic crust low p granulites or amphibolites but also in some cases possibly mantle rocks such that they are significantly water undersaturated during melting i e a h 2 o 1 reduced viscosities typical of high temperature melts enable rapid intrusion to higher levels diking vs diapirism higher rates of heat advection from mantle mafic magmas into the thickened oceanic and or continental crust during lithospheric extension are needed to facilitate melting of these rock to form a type and or or type partial melts tholeiitic felsic this usually results in bimodal magmatism as magma mixing is not favored in extensional settings especially at high magma production intrusion and extrusion rates i e high heat flow this would inhibit ferromagnesian phase in particular hornblende and magnetite fractionation especially at low f o 2 in the felsic magmas that would deplete the melt in y hree as well as ore related elements particularly zn which is 2 to 3 times higher in a type and or type rocks compared to other magma types therefore the empirical felsic igneous geochemical metallogenic relationships observed by others low zr y n yb n

the role of disseminated calcite in the chemical weathering of granitoid rocks accessory calcite present at concentrations between 300 and 3000 mg kg 1 occurs in fresh granitoid rocks sampled from the merced watershed in yosemite national park ca usa loch vale in rocky mountain national park co usa the panola watershed ga usa and the rio icacos puerto rico calcite occurs as fillings in microfractures as disseminated grains within the silicate matrix and as replacement of calcic cores in plagioclase flow through column experiments using de ionized water saturated with 0 05 atm co2 produced effluents from the fresh granitoid rocks that were dominated by ca and bicarbonate and thermodynamically saturated with calcite during reactions up to 1 7 yr calcite dissolution progressively decreased and was superceded by steady state dissolution of silicates principally biotite mass balance calculations indicate that most calcite had been removed during this time and accounted for 57 98 of the total ca released from these rocks experimental effluents from surfically weathered granitoids from the same watersheds were consistently dominated by silicate dissolution the lack of excess ca and alkalinity indicated that calcite had been previously removed by natural weathering the extent of ca enrichment in watershed discharge fluxes corresponds to the amounts of calcite exposed in granitoid rocks high ca na ratios relative to plagioclase stoichiometries indicate excess ca in the yosemite loch vale and other alpine watersheds in the sierra nevada and rocky mountains of the western united states this ca enrichment correlates with strong preferential weathering of calcite relative to plagioclase in exfoliated granitoids in glaciated terrains in contrast ca na flux ratios are comparable to or less than the ca na ratios for plagioclase in the subtropical panola and tropical rio icacos watersheds in which deeply weathered regoliths exhibit concurrent losses of calcite and much larger masses of plagioclase during transport limited weathering these results indicate that the weathering of accessory calcite may strongly influence ca and alkalinity fluxes from silicate rocks during and following periods of glaciation and tectonism but is much less important for older stable geomorphic surfaces

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

pressure temperature and fluid conditions during emerald precipitation southeastern yukon canada fluid inclusion and stable isotope evidence abstract the crown emerald veins are somewhat enigmatic displaying characteristics that are common to emerald deposits of tectonic hydrothermal origin and of igneous origin the veins cut the fire lake mafic meta volcanic rocks occurring within 600 m of an outcrop of cretaceous s type granite field work and vein petrography are consistent with a polythermal origin for the veins the primary vein mineralogy is quartz and tourmaline with variable sized alteration haloes consisting of tourmaline quartz muscovite chlorite and emerald the veins weather a buff brown colour due to jarosite scheelite and minor lepidocrocite which were precipitated during the waning stages of vein formation microthermometic studies of primary fluid inclusions within emerald growth zones are consistent with emerald precipitation from h 2 o co 2 ch 4 n 2 h 2 s bearing saline brines the estimated fluid composition is approximately 0 9391 mol h 2 o 0 0473 mol co 2 0 0077 mol ch 4 and 0 0059 mol nacl 2 wt nacl eq fluid inclusion and stable isotope studies are consistent with vein formation in the temperature range 365 498 c with corresponding pressures along fluid inclusion isochore paths ranging from 700 to 2250 bars these data correlate with a very slow uplift rate for the region of 0 02 0 07 mm year emerald deposits are generally formed when geological conditions bring together cr v and be cr and v are presumed to have been derived locally from the mafic and ultramafic rocks during hydrothermal alteration the be is most likely derived from the nearby cretaceous granite intrusion

the role of intragranular microtextures and microstructures in chemical and mechanical weathering direct comparisons of experimentally and naturally weathered alkali feldspars abstract electron microscopic observations of alkali feldspars from soils show that intragranular microtextures such as exsolution lamellae and microstructures primarily dislocations are both highly significant determinants of the weathering behaviour of these minerals in particular strained structure around intersecting edge dislocations in the plane of exsolution lamellae 6 01 dissolves at a rate which is orders of magnitude greater than unstrained feldspar producing a mesh of intersecting etch tubes extending 5 10 3 cm into the crystal as a result dissolution at dislocations is the major source of solutes during initial stages of chemical weathering in the field with progressive chemical weathering the most highly reactive feldspar is consumed by growth and coalescence of etch tubes but outer parts of the grain are physically weakened leading to mechanical flaking that increases available surface area and exposes further reactive sites in contrast previous dissolution experiments and microscopy of reacted surfaces have shown little or no correlation between dissolution rate and dislocation density and few visible signs of dissolution at particularly reactive sites to resolve the apparent discrepancy between field and laboratory behaviour we have carried out flow through dissolution experiments using ph 2 hcl at 25 c on three alkali feldspars with carefully characterized intragranular microtextures and microstructures these alkali feldspars were 1 eifel sanidine an alkali feldspar that has no microtextures at the tem scale and a low dislocation density 2 3 108 cm 2 and 3 naturally weathered alkali feldspars also from the shap granite which have the same microtextures as unweathered shap granite alkali feldspars but because they have been weathered have a lower density of dissolution reactive dislocations exposed on grain surfaces

chronology of crustal growth and recycling in the paleoproterozoic amisk collage flin flon belt trans hudson orogen canada u pb zircon ages have been compiled for magmatic and sedimentary rocks from the low metamorphic grade portion of the flin flon greenstone belt now recognized as a paleoproterozoic tectonic collage

quantification of mineral dissolution rates and applicability of rate laws laboratory studies of mill tailings abstract reliable quantification of mineral weathering rates is a key to assess many environmental problems in this study the authors address the applicability of pure mineral laboratory rate laws for dissolution of mill tailings samples mass normalised sulfide and aluminosilicate mineral dissolution rates determined in oxygenated batch experiments were found to be different between two samples from the same 50 year old carbonate depleted mill tailings deposit consideration of difference in particle surface area and mineralogy between the samples resolved most of this discrepancy in rates while the mineral surface area normalised dissolution rates of pyrite in a freshly crushed pure pyrite specimen and a sulfide concentrate derived from the tailings were within the range of abiotic literature rates of oxidation by dissolved molecular o 2 as were rates of sphalerite and chalcopyrite dissolution in the tailings dissolution rates of pyrite and aluminosilicates in the tailings generally differed from literature values this discrepancy obtained using a consistent experimental method and scale is suggested to be related to difficulties in quantifying individual mineral reactive surface area in a mixture of minerals of greatly varying particle size possibly due to factors such as dependence of surface area normalised mineral dissolution rates on particle size and time or to non proportionality between rates and bet surface area

iron isotope fractionation during magmatic differentiation in kilauea iki lava lake magmatic differentiation helps produce the chemical and petrographic diversity of terrestrial rocks the extent to which magmatic differentiation fractionates nonradiogenic isotopes is uncertain for some elements we report analyses of iron isotopes in basalts from kilauea iki lava lake hawaii the iron isotopic compositions 56fe 54fe of late stagemeltveins are 0 2 permil per thousand greater than values for olivine cumulates olivine phenocrysts are up to 1 2 per thousand lighter than those of whole rocks these results demonstrate that iron isotopes fractionate during magmatic differentiation at both whole rock and crystal scales this characteristic of iron relative to the characteristics of magnesium and lithium for which no fractionation has been found may be related to its complex redox chemistry in magmatic systems and makes iron a potential tool for studying planetary differentiation

geochemistry of 1 9 ga morb and oib like basalts from the amisk collage flin flon belt canada evidence for an intra oceanic origin subaqueously erupted basalts that occur in kilometre scale allochthons within the 1 9 ga flin flon belt canada appear to have been generated at oceanic ridges and possibly oceanic plateaus remote from archean cratons the ocean floor basalts fall into two categories 1 n type resembling n morbs and mariana type back arc basin basalts depleted to flat ree patterns high zrnb variable thnb and initial nd 3 3 to 5 4 2 e type resembling transitional and plume morbs slightly enriched ree patterns lower zrnb initial nd 3 1 to 4 5 in the largest and best studied allochthon the elbow athapapuskow assemblage mixing between depleted n morb and enriched oib sources or melts coupled with variable addition of a subduction lile component can explain the chemical variations in the basalts zircon u pb dates of 1904 ma for a syn volcanic diabase sill and 1901 6 5 ma for a gabbro peridotite cumulate complex demonstrate that crystallization of the ocean floor basalts overlapped with in part eruption of the tectonically juxtaposed 1 90 1 88 ga arc volcanic rocks the elbow athapapuskow allochthon is interpreted as back arc basin crust that developed simultaneously with flin flon arc magmatism subaerially erupted basalts that chemically resemble tholeiitic oibs 8 14 mgo relative hree depletion initial nd 2 2 to 3 4 occur in tectonic contact with the elbow athapapuskow assemblage the oibs may have been generated by deeper garnet residue melting of enriched mantle tapped during extension in the elbow athapapuskow back arc basin and were possibly erupted onto a remnant arc deeper mantle melting is also indicated by the presence of the lree enriched oceanic plateau like basalts of the sandy bay assemblage the back arc 01b and plateau volcanic assemblages were jux taposed against ca 1 9 ga arc assemblages in a philippines like intraoceanic accretionary complex by 1 87 ga

continental subduction and exhumation of uhp rocks structural and geochronological insights from the dabieshan east china in the dabieshan the available models for exhumation of ultrahigh pressure uhp rocks are poorly constrained by structural data a comprehensive structural and kinematic map and a general cross section of the dabieshan including its foreland fold belt and the northern dabieshan domain foziling and luzenguang groups are presented here south dabieshan consists from bottom to top of stacked allochtons 1 an amphibolite facies gneissic unit devoid of uhp rocks interpreted here as the relative autochton 2 an uhp allochton 3 a hp rock unit susong group mostly retrogressed into greenschist facies micaschists 4 a weakly metamorphosed proterozoic slate and sandstone unit and 5 an unmetamorphosed cambrian to early triassic sedimentary sequence unconformably covered by jurassic sandstone all these units exhibit a polyphase ductile deformation characterized by i a nw se lineation with a top to the nw shearing and ii a southward refolding of early ductile fabrics the central dabieshan is a 100 km scale migmatitic dome newly discovered eclogite xenoliths in a cretaceous granitoid dated at 102 ma by the u pb method on titanite demonstrate that migmatization post dates hp uhp metamorphism ductile faults formed in the subsolidus state coeval to migmatization allow us to characterize the structural pattern of doming along the dome margins migmatite is gneissified under post solidus conditions and mylonitic ultramylonitic fabrics commonly develop the north and west boundaries of the central dabieshan metamorphics i e the xiaotian mozitan and macheng faults are ductile normal faults formed before late jurassic early cretaceous a cretaceous reworking is recorded by synkinematic plutons north of the xiaotian mozitan fault the north dabieshan domain consists of metasediments and orthogneiss foziling and luzenguang groups metamorphosed under greenschist to amphibolite facies which never experienced uhp metamorphism a rare n s trending lineation with top to the south shearing is dated at 260 ma by the 40ar 39ar method on muscovite this early structure related to compressional tectonics is reworked by top to the north extensional shear bands the main deformation of the dabieshan consists of a nw se stretching lineation which wraps around the migmatitic dome but exhibits a consistently top to the nw sense of shear the central dabieshan is interpreted as an extensional migmatitic dome bounded by an arched top to the nw detachment fault this structure may account for a part of the uhp rock exhumation however the abundance of amphibolite restites in the central dabieshan migmatites and the scarcity of eclogites found only in a few places argue for an early stage of exhumation and retrogression of uhp rocks before migmatization this event is coeval to the n s extensional structures described in the north dabieshan domain recent radiometric dates suggest that early exhumation and subsequent migmatization occurred in triassic liassic times the main foliation is deformed by north verging recumbent folds coeval to the south verging folds of the south dabieshan domain an intense cretaceous magmatism accounts for thermal resetting of most of the 40ar 39ar dates a lithosphere scale exhumation model involving continental subduction synconvergence extension with inversion of southward thrusts into nw ward normal faults and crustal melting is presented

chromium isotope variations δ53 52cr in mantle derived sources and their weathering products implications for environmental studies and the evolution of δ53 52cr in the earth s mantle over geologic time here we report chromium isotope compositions expressed as delta cr 53 52 in per mil relative to nist 979 measured in selected cr rich minerals and rocks formed by the primary magmatic as well as the secondary metamorphic and weathering processes the main objectives of this study were i to further constrain the isotope composition of the earth s mantle cr inventory and its possible variation during geological history based on the analysis of globally distributed and stratigraphically constrained mantle derived chromites and ii to investigate the magnitude and systematics of cr isotope fractionation during oxidative weathering and secondary alteration i e hydration serpentinization of the magmatic cr sources specifically we analyzed delta cr 53 52 in a set of globally distributed mantle derived chromites femgcr2o4 n 30 collected from various locations in europe asia africa and south america and our results confirm that a chromite hosted earth s mantle cr inventory is uniform at 0 079 0 129 2sd which we named here as a canonical mantle d 53 52 cr signature furthermore our dataset of stratigraphically constrained chromites whose crystallization ages cover most of the earth s geological history indicate that the bulk cr isotope composition of the chromite hosted mantle inventory has remained uniform within about 0 100 since at least the early archean times similar to 3500 million years ago ma to investigate the systematics of cr isotope fractionation associated with alteration processes we analyzed a number of secondary cr rich minerals and variably altered ultramafic rocks i e serpentinized harzburgites lherzolites that revealed large positive delta cr 53 52 anomalies that are systematically shifted to higher values with an increasing degree of alteration and serpentinization the degree of aqueous alteration and serpentinization was quantified by the abundances of fluid mobile rb k elements and by the loss on ignition loi parameter which determines the amount of structurally bound water oh h2o present in secondary hydrated minerals like serpentine overall we observed that altered ultramafic rocks that yielded the highest loi values and the lowest amounts of fluid mobile elements also yielded the heaviest delta cr 53 52 signatures therefore we conclude that secondary alteration i e hydration serpentinization of ultramafic rocks in near surface oxidative environments tend to shift the bulk cr isotope composition of the weathered products to isotopically heavier values pointing to a dynamic redox cycling of cr in the earth s crustal and near surface environments hence if validated by future

subvolcanic and deep crustal tonalite genesis beneath the mexican peninsular ranges abstract cretaceous 115 103 ma tonalites throughout the western peninsular ranges batholith of baja california can be explained as hybrids between andesitic fractionates 65 and dehydration melts of amphibolite 35 mixing occurred between these two magmas at shallow 0 23 gpa crustal levels where heterogeneous tonalite trondhjemite with low sr y ratios 3 formed in ring complexes pluton scale hybridization between adakitic sr y 92 114 la lucn 12 16 end members also occurred at 0 7 1 0 gpa depths in the underlying basement during a tectonic collision event 6 11 m yr later voluminous tonalite genesis throughout northwestern mexico involved hybridization between dual intermediate magmas interacting in both the shallow and deep crust

the geochemical characteristics of granitoids in contrasting arcs and comments on magma sources geochemical variations in modern subduction related igneous suites with respect to arc maturity in time and space are illustrated using data for both volcanic suites basalt andesite and dacite and intrusive granitoid suites diorite tonalite granodiorite and granite from circum pacific arcs using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites a subduction zone enrichment of lithospheric mantle locally coupled with crustal assimilation allied with fractional crystallization afc in zones of thickened crust all of which yield magmas with enhanced concentrations of the large ion lithophile lil elements k rb th u lrees etc b with increasing distance from the active trench contributions from within plate sub continental lithosphere producing mantle derived magmas with enhanced levels of high field strength hfs elements among which nb ta hf and y are particularly distinctive thus even for the evolved granitoids of intrusive arc series ratios of hfs lil elements not significantly affected by crystal fractionation such as ta nb k rb la may throw some light on the origin of mafic intermediate precursor magmas in terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites 1 primitive calcic arc granitoids with low lil and hfs element abundances 2 normal calc alkaline continental arc granitoids with enhanced lil element abundances and low hfs lil ratios 3 mature alkali calcic arc granitoids with high levels of lil and hfs elements and higher hfs lil ratios 4 back arc anorogenic alkaline granitoids with the highest levels of hfs elements

the stable cr isotope inventory of solid earth reservoirs determined by double spike mc icp ms abstract we present the first comprehensive set of stable cr isotope data for the major igneous silicate earth reservoirs cr iii rich ores and minerals and hydrothermal chromates these were determined by mc icp ms using a double spike technique mantle xenoliths ultramafic rocks cumulates as well as oceanic and continental basalts share a common cr isotope composition with an average 53 52 cr value of 0 124 0 101 2 sd relative to the isotopically certified chromium standard nist srm 979 an isotopic difference between mantle xenoliths and basalts as was reported for iron was not observed for chromium thus the change in oxidation state that is observed when solid mantle rocks containing only trivalent chromium partially melt to form basaltic melts which predominantly contain bivalent chromium does not cause any measurable cr isotope fractionation chromite separates from major chromitite seams of the bushveld and great dyke layered igneous complexes are invariable in their cr isotope compositions and reproduce within uncertainties the average 53 52 cr srm 979 value of igneous silicate earth reservoirs this is important for environmental stable cr isotope studies because it labels the approximate isotope composition of industrial cr vi pollutants the cr isotope compositions of hydrothermal lead chromates crocoites pbcr vi o 4 from various localities yield 53 52 cr srm 979 values between 0 640 and 1 037 these cr isotope compositions are substantially heavier than those of igneous silicate rocks from which the chromium was leached precipitation experiments revealed an isotope fractionation of 53 52 cr crocoite cr vi aq of only ca 0 10 0 05 at a temperature of 20 c thus the heavy cr isotope signature of crocoites is most likely the result of repeated redox cycling of chromium in hydrothermal processes

theoretical estimates of equilibrium chromium isotope fractionations equilibrium chlorine isotope 37 cl 35 cl fractionations have been determined by using published vibrational spectra and force field modeling to calculate reduced partition function ratios for cl isotope exchange ab initio force fields calculated at the hf 6 31g d level are used to estimate unknown vibrational frequencies of 37 cl bearing molecules whereas crystalline phases are modeled by published lattice dynamics models calculated fractionations are principally controlled by the oxidation state of cl and its bond partners molecular mass or the absence of c h bonds also appears to play a role in determining relative fractionations among simple cl bearing organic species molecules and complexes with oxidized cl i e cl 0 cl etc will concentrate 37 cl relative to chlorides substances with cl at 298 k clo 2 containing cl 4 and clo 4 containing cl 7 will concentrate 37 cl relative to chlorides by as much as 27 and 73 respectively in rough agreement with earlier calculations among chlorides 37 cl will be concentrated in substances where cl is bonded to 2 cations i e fecl 2 mncl 2 micas and amphiboles relative to substances where cl is bonded to 1 cations such as nacl by 2 to 3 at 298 k organic molecules with c cl bonds will be even richer in 37 cl 5 to 9 at 298 k precipitation experiments in combination with our results provide an estimate for cl isotope partitioning in brines and suggest that silicates to the extent that their cl atoms are associated with nearest neighbor 2 cations analogous with fecl 2 and mncl 2 will have higher 37 cl 35 cl ratios than coexisting brine by 2 to 3 at room temperature calculated fractionations between hcl and cl 2 and between brines and such alteration minerals are in qualitative agreement with both experimental results and systematics observed in natural samples our results suggest that cl bearing organic molecules will have markedly higher 37 cl 35 cl ratios by 5 8 to 8 5 at 295 k than coexisting aqueous solutions at equilibrium predicted fractionations are consistent with the presence of an isotopically heavy reservoir of hcl that is in exchange equilibrium with cl aq in large marine aerosols

significance of pre 1860 ma granitoid magmatism for crustal evolution and exploration targeting in the flin flon assemblage trans hudson orogen canada granitoid plutonism a significant contributor to orogenic evolution is a commonly overlooked vector in exploration targeting using the flin flon glennie complex of the trans hudson orogen as a template we show that the geochemistry of granitoid rocks can be a useful parameter in differentiating prospective and nonprospective areas for volcanic hosted massive sulfide vhms deposits the flin flon glennie complex includes 1910 to 1860 ma plutonic rocks that can be subdivided into three suites 1 juvenile arc related rocks of ferroan and magnesian subtypes 2 contaminated arc related rocks and 3 successor arc related rocks of both low and high sr y ratio petrogenesis of ferroan juvenile and low sr y successor arc plutons indicates they formed in regions of thinner crust and hence would have been associated with higher heat flow and arc rifting conditions that favor vhms endowment regions of the orogen with both ferroan and low sr y plutonic subtypes were long lived zones of thin crust and preserve vhms mineralization related to both juvenile and successor arc magmatism in contrast volcanic belts dominated by contaminated arc or high sr y successor arc plutons formed on thicker substrates with contaminated arc rocks built on archean paleoproterozoic pericratonic slivers and high sr y successor arc rocks associated with submarine to subaerial arc systems these regions of thicker crust demonstrably lack vhms deposits and were also the locus for late unroofing our results suggest that pluton geochemistry regionally correlates with mineral occurrence and could represent a useful regional exploration tool in buried or poorly exposed parts of the flin flon glennie complex and other paleoproterozoic orogens