reliable late pleistocene stratigraphic ages and shorter groundwater travel times from14c in fossil snails from the southern great basin both aquatic and land snails are common in the geologic record but their utility in dating is greatly restricted by their well documented tendency to yield14c dates inconsistent with true14c ages in this study we examine the use of14c ages from 1 small previously unstudied terrestrial snails to date hosting spring deposits and from 2 cooccuring aquatic snails to constrain groundwater travel times during the last glacial period our study area in the southern great basin encompasses yucca mountain site of the proposed high level nuclear waste repository where information on the age and extent of past high water tables and on groundwater flow times is crucial to several licensing issues our results show that shells of small terrestrial snails belonging tovalloniasp yield14c dates consistent with14c ages of associated carbonized wood these results imply that these taxa can provide reliable14c age control on the broadly distributed deposits in which they have been described in contrast cooccurring aquatic snails from fossil spring deposits yield14c ages generally greater than the control age this is because the aquatic shells often formed in spring waters that had an initial14c deficiency however the magnitude of the deficiency is much less than that observed in nearby modern springs arguing for much higher average14c contents in late pleistocene groundwaters in these basins if representative this implies shorter groundwater travel times through aquifers in southern nevada during late glacial time

rutile solubility in albite h2o and na2si3o7 h2o at high temperatures and pressures by in situ synchrotron radiation micro xrf abstract rutile is an important mineral host for high field strength elements so its solubility in geologic fluids at high pressure and temperature plays an important role in the crustal and mantle processes that control the terrestrial cycling of these elements however experimental measurements of rutile solubility are in conflict by a factor of more than 100 at most studied conditions we performed new measurements of rutile solubility in h 2 o albite and h 2 o na 2 si 3 o 7 ns3 fluids by in situ synchrotron radiation x ray fluorescence spectroscopy using modified bassett type hydrothermal diamond anvil cells minimum detection limits were 1 9 and 2 3 ppm ti by weight for the two cells three albite h 2 o experiments at starting bulk compositions of 2 7 6 7 and 10 3 wt albite involved spectral acquisition at rutile saturation in the presence of albite crystals melt or a single homogeneous fluid phase after accounting for the additional phases corrected fluid compositions were 0 6 to 7 5 wt dissolved silicate over the run conditions at 2 7 wt albite rutile dissolution rate was slow and steady state was not achieved at 600 800 c however at higher dissolved albite contents constant solubility with time was observed rutile solubilities in the presence of a single fluid phase at 700 c 0 79 gpa 5 4 wt albite and at 800 c 1 10 gpa 6 7 wt albite were 37 2 and 156 6 ppm respectively these data agree with results acquired using hydrothermal piston cylinder methods with long run times and suppression of new crystal growth but not with data derived from visual observation in hydrothermal diamond anvil cells this discrepancy is likely due to lack of equilibrium in the latter approach two experiments in 10 and 30 wt ns3 at 660 800 c 0 5 0 1 gpa show extreme concentration dependent enhancement of rutile solubility to 4500 ppm the data indicate a strong positive correlation between rutile solubility and na al because na k al is likely to be greater than unity in aqueous fluids at high pressure and temperature due to incongruent dissolution of albite and micas the increase in rutile solubility along the albite ns3 join points to the possibility of significant ti transport by silicate bearing aqueous fluids in the lower crust and upper mantle

zircon u pb ages ree concentrations and hf isotope compositions of granitic leucosome and pegmatite from the north sulu uhp terrane in china constraints on the timing and nature of partial melting granitic leucosome and pegmatite are widely distributed within biotite bearing orthogneiss in the northern part of the sulu ultrahigh pressure uhp metamorphic terrane eastern china a combined study of mineral inclusions cathodoluminescence cl images u pb shrimp dates and in situ trace element and lu hf isotope analyses of zircons provided insight into the nature and timing of partial melting in these rocks zircon grains separated from biotite bearing orthogneiss typically have three distinct domains 1 pre metamorphic magmatic cores with qtz kfs pl ap inclusions which record a neoproterozoic protolith age of 790 ma 2 mantles with coe phe ap inclusions that record triassic uhp age at 227 3 ma and 3 narrow rims with quartz inclusions that record hp granulite facies retrograde metamorphism at 210 3 ma in contrast zircons separated from granitic leucosome have only two distinct domains 1 the central uhp areas with coe phe ap inclusions record triassic uhp age of 227 3 ma and 2 outer magmatic areas with qtz kfs ab ap inclusions that record partial melting time of 212 2 ma zircons separated from pegmatite contain mineral inclusions of qtz kfs ap and show regular magmatic zoning from centre to edge the centres record partial melting time of 212 2 ma in line with the outer domains of granitic leucosome whereas the edges give a younger age of 201 2 ma related to pb loss and partial recrystallization during late triassic regional amphibolite facies retrogression these data indicate that partial melting in the north sulu uhp gneissic rocks took place during post uhp retrograde hp granulite facies metamorphism pre metamorphic magmatic zircon cores from biotite bearing orthogneiss give uniform 176hf 177hf of 0 28187 0 00003 2 sd standard deviation corresponding to ehf 790 and hf model ages tdm2 of about 16 3 and 2 41 ga respectively this is consistent with the generation of its protolith by reworking of paleoproterozoic to late archean crust in contrast uhp zircon domains from biotite bearing orthogneiss and granitic leucosome are characterized by distinct trace element composition with low lu hf 0 006 low th u 0 1 and considerably higher 176hf 177hf 0 28233 0 00002 2 sd than the pre metamorphic cores the uniform but significantly different hf isotope composition between the uhp ehf 227 14 6 0 8 2 sd and pre metamorphic ehf 227 27 7 domains indicates equilibration of the lu hf isotope system only within the uhp metamorphic mineral assemblage the disequilibrium between whole rock and uhp zircon suggests that about two thirds of the whole rock hf retained in the pre metamorphic zircon domains zircon domains crystallized during partial melting at 212 ma in granitic leucosome and pegmatites have a hf isotope composition indistinguishable from that of the uhp zircon domains this suggests that only hf and zr equilibrated during uhp metamorphism was remobilized during partial melting while pre metamorphic zircon remained stable or was not accessible in contrast the magmatic zircon edges from pegmatite have somewhat lower 176hf 177hf 0 28216 and ehf t 17 6 1 2 2 sd indicating some release of less radiogenic hf for instance by dissolution of pre metamorphic zircon during late regional amphibolite facies retrogression

partial equilibrium of radiogenic and stable isotope systems in garnet peridotite during ultrahigh pressure metamorphism a critical question in radiometric dating of metamorphic rocks is whether minerals used to define internal isochrons have achieved isotopic equilibrium during a given metamorphic event followed by preservation of the equilibrium afterward a garnet peridotite at zhimafang in the sulu terrane of eastern china shows incongruent u pb rb sr and sm nd ages zircon shrimp u pb dating yielded an isochron age of 224 8 ma pointing to zircon growth during triassic uhp metamorphism two mineral assemblages gave consistent rb sr internal isochron ages of 201 4 and 205 4 ma but much older sm nd internal isochron ages of 376 16 and 378 24 ma apparently the sm nd radiometric system fails to reequilibrate among the dated minerals during the triassic uhp metamorphic event for the garnet peridotite this result is confirmed by the state of oxygen isotope equilibrium or disequilibrium among the constituent minerals in the garnet peridotite in particular garnet is not in oxygen isotope equilibrium with any other of the analyzed minerals the degree of oxygen isotope disequilibria among the other minerals varies from pair to pair oxygen isotope equilibrium is observed only between orthopyroxene and olivine for both samples and between phlogopite and clinopyroxene for one sample it appears that the u pb rb sr sm nd and o isotope systems in minerals of the garnet peridotite are in partial equilibrium during uhp metamorphism i e equilibrium with respect to some isotopes and minerals but not all based on available experimental diffusion data our study of the combined u pb rb sr sm nd and o isotope systems in the garnet peridotite demonstrates that the time scale for the uhp metamorphism and subsequent hp eclogite facies recrystallization is possibly in the range of about 12 to 26 ma which was not long enough for the reequilibration of the sm nd and o isotope systems but just long enough for the reequilibration of the rb sr isotope systems therefore the rate of sr diffusion in phlogopite thus sr isotope reequilibration between the mafic minerals is constrained to be faster than rates of nd and o diffusion in garnet thus sm nd and o isotope reequilibration between the same minerals under conditions of subduction zone metamorphism

the europium anomalies in banded iron formations and the thermal history of the oceanic crust abstract the ree rare earth element patterns in banded iron formations bif s from various sites in north america two sites in finland and three drillcores of the hamersley basin in western australia have been investigated using icp ms inductively coupled plasma mass spectrometry and inaa instrumental neutron activation analysis the results of this study as well as existing data show trends in the eu anomalies bif s older than 2 5 ga exhibit positive eu anomalies while younger ones exhibit none in chemical sediments between 2 5 and 2 3 ga both trends can be found because of the worldwide distribution of bif s this feature can be considered as a global change in the conditions controlling ree mobilization consideration of the redox conditions of bif formation in comparison with the calculated and measured eu redox equilibria suggests that a eu anomaly points to a dominantly hydrothermal source of ree s with solution temperatures above 175 c we suggest that the disappearance of the eu anomaly towards the proterozoic marks a change from mainly high temperature hydrothermal to dominantly low temperature alteration

very high pressure orogenic garnet peridotites mantle derived garnet peridotites are a minor component in many very high pressure metamorphic terranes that formed during continental subduction and collision some of these mantle rocks contain trace amounts of zircon and micrometer sized inclusions the constituent minerals exhibit pre and postsubduction microstructures including polymorphic transformation and mineral exsolution experimental mineralogical petrochemical and geochronological characterizations using novel techniques with high spatial temporal and energy resolutions are resulting in unexpected discoveries of new phases providing better constraints on deep mantle processes

prograde metamorphism decompressional partial melting and subsequent melt fractional crystallization in the weihai migmatitic gneisses sulu uhp terrane eastern china abstract in order to decipher fluid melt activity during subduction and subsequent exhumation of felsic continental crust we carried out a combined study of zircon internal structure zircon u pb age zircon trace element composition and whole rock geochemistry on migmatitic gneisses from the weihai region in the sulu ultra high pressure uhp metamorphic terrane eastern china the weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase pl rich leucosome and k feldspar kfs rich pegmatite veins whole rock geochemistry suggests that mgo fe2o3t cao tio2 p2o5 rare earth elements ree and large ion lithophile elements hfse are nearly completely retained in the melanosome whereas significant amounts of lile e g rb ba k sr pb and u are partitioned into the pl rich leucosome and the kfs rich pegmatite veins the kfs rich pegmatite vein has much higher k2o u pb and rb but lower cao and na2o contents than those of the pl rich leucosome layer protolith magmatic zircon domains with an upper intercept age of about 780 ma are common in the cores of zircon grains from the melanosome and the pl rich leucosome layers indicating that the protolith of the migmatitic gneiss is mid neoproterozoic magmatic rock metamorphic zircons with concordant ages ranging from 243 to 256 ma occur as overgrowth mantles on the protolith magmatic zircon cores they are characterized by remarkably flat heavy rare earth element hree patterns without obviously negative eu anomalies and have low y and th contents low th u and high hf y ratios indicating a prograde metamorphism in the hp eclogite facies during subduction recrystallized rims of zircons 228 2 ma in the pl rich leucosome layer are similar to magmatic zircons in terms of their hree enriched patterns with negative eu anomalies high y and low th contents and low hf y and th u ratios this indicates that the gneiss has experienced decompressional partial melting and the pl rich leucosome layers were formed by fractional crystallization of the primary melt during the early stage hot exhumation zircon grains 219 2 ma in the kfs rich veins are characterized by aqueous fluid melt related signatures with strongly hree enriched patterns high u pb and low th contents and low th u and high lu hf ratios indicating that the pegmatite vein was formed by crystallization of residual aqueous melt after the crystallization of pl rich leucosome layers during later stage cooling during exhumation our results suggest that the migmatitic gneiss successively recorded information concerning the protolith prograde hp metamorphism during subduction fractional crystallization of primary melt and crystallization of the residual melt during exhumation

petrology and trace element budgets of high pressure peridotites indicate subduction dehydration of pristine serpentinites cima di gagnone central alps switzerland

low δ18o zircons u pb dating and the age of the qinglongshan oxygen and hydrogen isotope anomaly near donghai in jiangsu province china zircons from metamorphosed granites exposed near qinglongshan have 18ovsmow values of 7 to 0 in both grain rims and cores the concordant 238u 206pb ages of zircon cores are 684 to 754 ma with rims at 221 ma discordant 238u 206pb ages range from 242 to 632 ma results demonstrate a neoproterozoic age for the origin of the qinglongshan oxygen and hydrogen isotope anomaly the low 18o values were imprinted on the rocks by a hydrothermal system charged with meteoric water from a cold climate groundwater circulation was driven by heat from cooling granitic magma the geologic age of the hydrothermal system correlates with that of the nantuo tillite in the sinian strata of the south china block suggesting that qinglongshan s cold climate may be a manifestation of neoproterozoic snowball earth

carbon and isotopic mass balance models of oasis valley fortymile canyon groundwater basin southern nevada environmental isotopes and carbon chemistry provide means of differentiating various recharge areas flow paths and ages of ground water in portions of the nevada test site and vicinity regional delta d delta sup 18 o trends are offset from the present day meteoric line by a deuterium depletion of 5 per thousandth suggesting paleoclimatic changes partial pressures of co sub 2 and the sup 18 o and sup 13 c data indicate solubility and isotopic equilibrium between the gas and water in the soil zone with progressive exchange with underlying ground water in the shallow alluvium of oasis valley application of a closed system co sub 2 model using the eq3nr eq6 reaction path simulator successfully reproduces chemical compositions observed in the alluvium in the amargosa desert and in the deep tuff aquifer beneath pahute mesa and yucca mountain initial p sub co sub 2 input to the soil zone during recharge was calculated to range from 0 03 to 0 10 atm which is comparable to measured soil co sub 2 pressures in oasis valley results are compared for sup 14 c ages using the delta sup 13 c dilution correction and a mass action correction term relating predicted and calculated ionic activity products of caco sub 3 results are generally comparable with discrepancies attributed to anomalous delta supmore 13 c values less

source to sink system reconstruction in the northern jiaolai basin eastern china by multiproxy provenance methods and implications for exhumation of the sulu orogen abstract the early cretaceous sedimentary successions in the jiaodong laiyang jiaolai basin provide valuable information about sediment provenances and tectonic processes a clear understanding of the provenance development of the jiaolai basin along the northern margin of the sulu orogen and would elucidate the exhumation history of the sulu orogen during the post orogenic period however the precise age and provenance evolution of the stratigraphy remain ambiguous here we employ multiple provenance proxies detailed paleocurrent conglomerate clast counts detrital composition point counts of sandstone and detrital zircon u pb geochronology to reconstruct the source to sink system of the northern part of the jiaolai basin visual and statistical analyses of the different proxy results and comparison with the signatures of potential source regions reveal that 1 wawukuang formation fm ca 123 ma was mainly sourced from the guocheng uplift in both the haiyang and laiyang regions 2 linsishan to zhifengzhuang fm ca 121 ma the laiyang region was mainly fed by the jiaobei terrane while in the haiyang region the guocheng uplift and sulu orogen were primary sources 3 shuinan to longwangzhuang fm the haiyang and laiyang regions were linked as a single basin fed by the jiaobei terrane and sulu orogen and 4 qugezhuang fm ca 114 ma was mainly sourced from early cretaceous plutons in the sulu orogen and jiaobei terrane based on the inflection point in the proportion of ages of the ultrahigh pressure metamorphic uhp rocks between the wawukuang and linsishan fms we conclude that the sulu belt experienced a rapid uplift that resulted in the exposure and erosion of the jurassic plutons and uhp rocks during ca 123 121 ma in addition our results show that the denudation processes of the mesozoic plutons in the sulu orogen have an obvious temporal sequence jurassic plutons first followed by cretaceous plutons

multiphase solid inclusions in uhp rocks su lu china remnants of supercritical silicate rich aqueous fluids released during continental subduction abstract primary multiphase solid ms inclusions without preserved fluid are found within peak minerals in kyanite quartzite topaz and kyanite phengite epidote eclogite from donghai area su lu terrane typical mineral association in inclusions is paragonite muscovite anhydrite corundum alunite type sulphate zircon calcite chlorite sio 2 barite pyrite apatite in quartzites and paragonite rutile apatite amphibole zn staurolite magnetite plagioclase zircon pyrite alunite type sulphate zn mg fe al ti spinel in eclogites on the basis of the fluid inclusion textures and of the daughter phase assemblage calculated fluid composition is as follows in quartzites 24 wt sio 2 30 wt al 2 o 3 9 wt cao 5 wt k 2 o 3 wt na 2 o 11 wt so 3 18 wt h 2 o with traces of tio 2 fe 2 o 3 feo mgo bao p 2 o 5 cl f and co 3 2 and in eclogites 26 wt sio 2 21 wt tio 2 20 wt al 2 o 3 2 wt mgo 4 wt feo 6 wt fe 2 o 3 7 wt cao 3 wt na 2 o 3 wt p 2 o 5 7 wt h 2 o 1 wt cl and traces of zno mno k 2 o so 3 and f originally trapped chloride poor aqueous fluids contained very high amounts in the order of tens of wt of si 4 al 3 and ti 4 solute species reflect the chemical composition of the host rocks mg 2 fe 2 ti 4 p 5 and na are abundant in the fluids present in eclogites while fluids are enriched in al 3 k na so 4 2 co 3 2 in quartzites we propose that multiphase solid inclusions represent remnants of high density supercritical silicate rich aqueous fluids that were in equilibrium with peak minerals at uhp conditions these fluids show characters which are transitional between aqueous fluids and silicate melts and were probably produced by dehydration reactions of the host rocks during the latest stages of subduction

conservative behavior of arsenic and other oxyanion forming trace elements in an oxic groundwater flow system summary groundwater samples were collected along a flow path in a shallow fractured tuffaceous aquifer from the oasis valley beatty wash region of southern nevada usa and analyzed for a number of oxyanion forming trace elements including arsenic as antimony sb selenium se molybdenum mo and tungsten w in addition ancillary geochemical parameters including ph major solute compositions dissolved silica dissolved oxygen and iron and manganese concentrations were quantified in the groundwaters arsenic concentrations range from 70 nmol kg up to 316 nmol kg in groundwaters of the oasis valley beatty wash flow system and generally exhibit increasing concentrations with flow down gradient along the flow path antimony w and to a lesser extent mo exhibit similar increasing concentration trends with flow down gradient in the aquifer albeit at lower concentrations levels e g mean sd for sb w and mo are 2 3 0 9 nmol kg 7 4 3 7 nmol kg and 101 19 nmol kg respectively selenium concentration which range between 4 and 11 nmol kg generally decrease in groundwaters with flow down gradients in the oasis valley beatty wash groundwater flow systems inverse modeling of groundwater chemistry evolution from the lower reaches of the oasis valley flow path using phreeqc indicate that the groundwater composition is consistent with mixing of nearly equal proportions of groundwater from upper reaches of oasis valley and beatty wash groundwater along with dissolution of volcanic glass potassium feldspar and gypsum followed by calcite precipitation and formation of secondary zeolites analcime clay minerals ca montmorillonite and cristobalite the geochemical modeling indicates that the concentrations of as and the other oxyanion forming trace elements are controlled by dissolution of volcanic glass water rock interaction with mineralized zones within the aquifer i e sulfide oxidation desorption from aquifer surface sites and mixing of oasis valley and beatty wash groundwaters

geochemical and isotopic evidence of a groundwater source in the corral canyon meadow complex central nevada usa major inorganic ions and stable carbon and oxygen isotopes in stream water groundwater groundwater seeps and springs were measured in the corral canyon meadow complex and watershed in the toquima mountains of central nevada usa the purpose of the study was to determine whether stream water or groundwater was the source of water that supports vegetation in the meadow complex water samples from the watershed and meadow complex were mixed cation hco3 type stream water sampled at different locations in the meadow complex showed variations in temperature ph and specific conductance the cation anion proportions for stream water were similar to groundwater groundwater seeps and runoff from the meadow complex stable oxygen isotope ratios for stream water 17 1 to 17 6 versus vsmow and groundwater and groundwater seeps in the meadow site 17 0 to 17 7 versus vsmow were similar and consistent with a local meteoric origin dissolved inorganic carbon dic and the 13cdic for stream water 12 1 to 15 0 versus vpdb were different from that of groundwater from the meadow complex 15 3 to 19 9 versus vpdb suggesting different carbon evolution pathways however a simple model based on cation 13cdic suggests that stream water was being recharged by shallow groundwater groundwater seeps and runoff from the meadow complex this leads to the conclusion that the source of water that supports vegetation in the meadow complex was primarily groundwater the results of this study suggest that multiple chemical and stable carbon isotope tracers are useful in determining the source of water that supports vegetation in meadow complexes in small alpine watersheds copyright 2004 john wiley sons ltd

the origin and significance of crustal minerals in ophiolitic chromitites and peridotites abstract various combinations of zircon quartz corundum k feldspar plagioclase apatite amphibole rutile titanite almandine garnet kyanite andalusite and coesite have been recovered from podiform chromitites of the luobusa and dongqiao ophiolites in tibet the oman ophiolite and the ray iz ophiolite in the polar urals russia chromitites in all four ophiolites also contain moissanite and the luobusa and ray iz ophiolites contain in situ diamonds most of the recovered zircons are sub rounded grains with complex internal structures indicating polyphase growth trace element contents and a low pressure inclusion assemblage quartz muscovite k feldspar apatite ilmenite rutile indicate a continental crustal origin for the zircons they have sims u pb ages that are generally much older than the host body total range 90 to 2500 ma the presence of numerous crustal minerals particularly zircon suggests derivation from metasedimentary rocks subducted into the mantle their preservation in chromitites and peridotites implies effective isolation from the mafic melts that formed the ophiolites we suggest that most of these minerals were derived from the crustal parts of subducted slabs and were encapsulated into chromite grains precipitated from rising asthenospheric and suprasubduction magmas the chromite grains were carried in melt channels to shallow levels in suprasubduction mantle wedges and then deposited as podiform chromitites near the moho the rise of asthenospheric peridotites into suprasubduction zones was facilitated by subduction initiation and possibly by slab tear allowing mixing of the uhp highly reduced minerals and crustal minerals now found in ophiolitic chromitites and peridotites

stable isotope characteristics of eclogites from the ultra high pressure metamorphic terrain east central china abstract a stable isotope study was made for the ultra high pressure uhp metamorphic rocks in the dabie and sulu terrains east central china 18 o values of minerals in eclogites cover a wide range from 4 to 6 the o isotope fractionations between minerals i e omphacite phengite and or amphibole and garnet in eclogites are mostly not in isotopic equilibrium the large range of o isotope composition of garnet may approximate the 18 o variations of eclogites during uhp metamorphism and must have been largely inherited from their pre subduction basaltic precursors which might have interacted with meteoric waters under variable temperature conditions and water rock ratios before uhp metamorphism the o isotope disequilibrium between minerals and garnet in eclogites was caused by retrograde hydration reactions the fluids required for these hydration reactions must have low or even negative o isotope compositions it is postulated that such fluids were either originated from meteoric deep infiltration or were derived from low 18 o para gneisses

recycling in the subduction factory archaean to permian zircons in the oceanic cretaceous caribbean island arc hispaniola abstract little mineralogical evidence is left of the recycling of continental and oceanic crust into the mantle at subduction zones zircon because of its exceptional robustness is probably the only surviving phase and the best mineral tracer of this global scale process this article combines new in situ u pb dating and o and hf isotope analyses on cretaceous co magmatic and pre cretaceous inherited zircons separated from albian aptian arc related igneous rocks from the dominican republic the o and hf systematics of cretaceous zircons reflect derivation from predominantly juvenile sources and variable mixing with evolved melts as expected for an oceanic island arc inherited zircons yield u pb ages between 256 and 2923 ma n 219 most studied inherited zircons are permian carboniferous peaking at ca 300 ma and formed in continental crust to judge from mineral inclusions including quartz orthoclase muscovite and apatite 18o 6 16 to 12 67 and ehf t 11 9 to 1 12 values ordovician to proterozoic zircons yield 18o between 4 93 and 9 21 and ehf t between 19 9 and 8 5 and hence crystallized in equilibrium with either mantle derived juvenile magmas or magmas derived from melting of supracrustal rocks for archean zircons 18o between 6 98 and 7 94 and ehf t between 18 5 and 1 6 might reflect contribution of supracrustal materials in the parent magmas we suggest that arc magmas picked up the inherited zircon cargo from their mantle sources accounting paleogeographic and paleo tectonic constraints mexican and colombian terranes are identified as the potential primary magmatic and secondary sedimentary sources for the studied inherited zircons we envisage that inherited zircons were transported downward to the mantle beneath the caribbean plate as detrital grains on top of the subducting proto caribbean and or pacific slabs

re os systematics of the raobazhai peridotite massifs from the dabie orogenic zone eastern china abstract the raobazhai ultramafic massif of the ultrahigh pressure sulu dabie orogenic belt central china is thought to be a segment of subcontinental lithospheric mantle that was subducted and exhumed during the triassic collision of the north china and yangtze cratons we performed a re os isotopic study of peridotites from the massif associated with major and trace element analysis and textural examination os 1 02 to 6 28 ppb and re 0 004 to 0 376 ppb concentrations are typical of orogenic lherzolite values and 187os 188os ratios 0 1157 to 0 1283 are all similar to or lower than the proposed primitive upper mantle value 187os 188os is roughly correlated with 187re 188os and strongly correlated with al2o3 these correlations can be explained by radiogenic ingrowth of 187os since an ancient partial melting event tma model ages 1 7 to 2 0 ga of refractory peridotites from the lower massif are consistent with the model age 1 8 ga obtained from the 187os 188os vs al2o3 correlation at 1 al2o3 this age cannot distinguish the cratonic provenance of the raobazhai massif since similar re os model ages have been obtained from both the north china and the yangtze cratons the poor quality of the 187os 188os vs 187re 188os correlation indicates that the re os ratios were disturbed perhaps during triassic subduction the mainly lherzolitic samples of the upper massif which were most strongly affected by this process have porphyroclastic textures with fine grained olivine pyroxene and amphibole neoblasts suggesting re mobility during recrystallization in the presence of fluids previous studies of ultramafic xenoliths from arc volcanics demonstrate that slab derived melts or fluids can both scavenge mantle os and add substantial amounts of radiogenic os to the suprasubduction mantle in raobazhai both trace element patterns and the abundance of hydrous phases provide evidence for extensive interaction with fluids during subduction and or exhumation nevertheless the strong correlation between 187os 188os and al2o3 and the high os concentrations of these rocks indicate that os isotopic ratios and probably even os concentrations were essentially unaffected by this process assuming that the arguments favoring a suprasubduction setting for the raobazhai massif are valid these data provide evidence that os systematics are sometimes surprisingly robust even above subduction zones

isotopic and trace element constraints on the origin and evolution of saline groundwaters from central missouri na ca cl groundwaters with salinities of 1 to 30 discharge from natural springs and artesian wells in mississippian carbonates and ordovician sandstones and carbonates in central missouri carbonate saturation and quartz supersaturation are maintained throughout the salinity range major and trace element and isotopic variations in the waters are used to place constraints on models for rock water interaction and regional hydrology the groundwaters have d values that range from 108 to 45 and 18 o values that range from 14 7 to 6 5 smow these data lie approximately along the meteoric water line ranging from values similar to local recharge in missouri to significantly lower values that are similar to 18 o and d for meteoric recharge in high altitude and high latitude regions of north america the more saline samples have the lowest 18 o and d contents a trend that is opposite to that observed in most other studies of saline waters the h o isotopic correlation and the range in salinity and several major and trace element concentrations in central missouri groundwaters are readily explained by a saline dilute water mixing model 87sr 86sr ratios for the groundwaters range from 0 7155 to 0 7161 these values are significantly higher than previously published values for 1 estimates of 87 sr 86 sr in phanerozoic seawater 0 7068 0 7092 and 2 carbonates of the mississippian burlington keokuk fm 0 7075 0 7105 through which some of the waters migrate as they pass to the surface nd 0 values in the groundwaters range from 10 9 to 8 1 and 147 sm 144 nd ratios range from 0 108 to 0 128 these values are similar to or lower than previously published analyses of carbonates and other authigenic phases from the burlington keokuk fm mean nd 0 7 8 0 8 1 mean 147 sm 144 nd 0 141 the h o sr and nd isotopic data and the results of model calculations preclude 1 models involving the modification of ancient seawater for the origin of the saline waters and 2 extensive interaction between the groundwaters and their host carbonates the waters apparently acquired their 18 o and d values as meteoric recharge and their 87sr 86sr and rare earth element signatures from extraformational crustal sources high 87 sr 86 sr low nd 0 low 147sm 144nd and largely preserved these isotopic signatures during subsequent migration through the carbonates chemical exchange via rock water interaction is required for isotopic exchange to occur therefore the present state of chemical equilibrium between the groundwaters and carbonate aquifer minerals may serve to limit rock water interaction and hence preserve the extraformational isotopic signatures in the waters the integration of geochemical isotopic and hydrologic data on a local and regional scale suggests a history for the central missouri groundwaters involving 1 meteoric recharge in the front range of colorado 2 dissolution of permian halite in the subsurface of kansas 3 interaction with predominantly silicate mineral assemblages in paleozoic strata and possibly precambrian basement with aquisition of crustal sr and ree signatures 4 dilution and migration to shallow aquifer levels in central missouri and 5 mixing with local meteoric recharge through mississippian carbonates with no significant change of the isotopic signatures acquired in stage 3

fluid regime in continental subduction zones petrological insights from ultrahigh pressure metamorphic rocks abstract intensive studies of ultrahigh pressure uhp metamorphic rocks in continental collision zones have provided petrological insights into the fluid regime during subduction and exhumation recent studies in the dabie sulu orogen of china have made important contributions to understanding fluid action in such systems hence these rocks are the main focus of this review sources of metamorphic fluid can be deduced from study of hydrous minerals structural hydroxyl and molecular water in nominally anhydrous minerals fluid and solid inclusions of uhp metamorphic minerals are also a record of fluid action in deep subduction zones the presence or absence of metamorphic fluid during subduction and exhumation dictates zircon growth and recrystallization making available materials for u pb dating isotopic dating demonstrates that after peak uhp metamorphism retrograde fluid with deuteric origin became active the decomposition of hydrous minerals and the exsolution of molecular water and structural hydroxyl during exhumation can provide sufficient amounts of aqueous fluid for quartz veining and amphibolite facies retrogression within uhp slabs aqueous fluid released by the exsolution of molecular water and structural hydroxyl from uhp nominally anhydrous minerals is characterized by low salinity and light hydrogen and oxygen isotope compositions therefore one part of the uhp slab is a source of aqueous fluid whereas another part is a sink in both cases the fluid action during subduction and exhumation is episodic rather than continuous supplementary material a description of the dabie sulu uhp metamorphic rocks analytical methods la icpms zircon u pb isotope data and rare earth element compositions are available at http www geolsoc org uk sup18355

rare earth element geochemistry of late devonian reefal carbonates canning basin western australia confirmation of a seawater ree proxy in ancient limestones rare earth element and yttrium ree y concentrations were determined in 49 late devonian reefal carbonates from the lennard shelf canning basin western australia shale normalized sn ree y patterns of the late devonian samples display features consistent with the geochemistry of well oxygenated shallow seawater a variety of different ancient limestone components including microbialites some skeletal carbonates stromatoporoids and cements record seawater like ree y signatures contamination associated with phosphate fe oxides and shale was tested quantitatively and can be discounted as the source of the ree y patterns co occurring carbonate components that presumably precipitated from the same seawater have different relative ree concentrations but consistent ree y patterns clean devonian early marine cements n 3 display ree y signatures most like that of modern open ocean seawater and the highest y ho ratios e g 59 and greatest light ree lree depletion average nd sn yb sn 0 413 sd 0 076 however synsedimentary cements have the lowest ree concentrations e g 405 ppb non contaminated devonian microbialite samples containing a mixture of the calcimicrobe renalcis and micritic thrombolite aggregates in early marine cement n 11 have the highest relative ree concentrations of tested carbonates average total ree 11 3 ppm stromatoporoid skeletons unlike modern corals algae and molluscs also contain well developed seawater like ree patterns samples from an estuarine fringing reef have very different ree y patterns with lree enrichment nd sn yb sn 1 possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged lree from a nearby riverine input source hence devonian limestones provide a proxy for marine ree geochemistry and allow the differentiation of co occurring water masses on the ancient lennard shelf although appropriate partition coefficients for quantification of devonian seawater ree concentrations from out data are unknown hypothetical devonian canning basin seawater ree patterns were obtained with coefficients derived from modern natural proxies and experimental values resulting devonian seawater patterns are slightly enriched in lree compared to most modem seawaters and suggest higher overall ree concentrations but are very similar to seawaters from regions with high terrigenous inputs our results suggest that most limestones should record important aspects of the ree geochemistry of the waters in which they precipitated provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high non seawater like ree contents hence we expect that many other ancient limestones will serve as seawater ree proxies and thereby provide information on paleoceanography paleogeography and geochemical evolution of the oceans copyright c 2004 elsevier ltd

metamorphic growth and recrystallization of zircons in extremely 18o depleted rocks during eclogite facies metamorphism evidence from u pb ages trace elements and o hf isotopes abstract a combined in situ sims and la mc icpms study of u pb ages trace elements o and lu hf isotopes was conducted for zircon from eclogite facies metamorphic rocks in the sulu orogen the two microbeam techniques sampled various depths of zircon domains revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique this enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization magmatic cores with u pb ages of 769 9 ma have positive 18o values of 0 1 10 1 high th u and 176lu 177hf ratios high ree contents and steep mree hree patterns with negative eu anomalies they are interpreted as crystallizing from positive 18o magmas during protolith emplacement in contrast newly grown domains have concordant u pb ages of 204 4 to 252 7 ma and show negative 18o values of 10 0 to 2 2 low th u and 176lu 177hf ratios low ree contents and flat hree patterns with weak to no eu anomalies they are interpreted as growing from negative 18o fluids that were produced by metamorphic dehydration of high t glacial hydrothermally altered rocks during continental subduction zone metamorphism differences in 18o between different domains within single grains vary from 0 8 to 12 5 suggesting different degrees of o isotope exchange between the positive 18o magmatic core and the negative 18o metamorphic fluid during the metamorphism the magmatic zircons underwent three subtypes of metamorphic recrystallization depending on their accessibility to negative 18o fluids the zircons recrystallized in solid state maintained positive 18o values and ree and lu hf isotopes of protolith zircon but their u pb ages are lowered the zircons recrystallized through dissolution exhibit negative 18o values similar to the metamorphic growths almost completely reset u pb ages and partially reset ree systems the zircons recrystallized through replacement show variably negative 18o values and partially reset ree and u pb and lu hf isotopic systems therefore this study places robust constraints on the origin of metamorphic zircons in eclogite facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision

introduction to tectonics of china abstract the continental crust of china is a mosaic of cratonic blocks and orogenic belts containing small cratons and terranes with various tectonic settings they have diverse origins and complex histories of amalgamation and often suffered repeated reworking after multiple episodes of amalgamation in the last three decades extensive geological geochemical and geophysical investigations have been carried out on these cratonic blocks and intervening orogenic belts producing an abundant amount of new data and competing interpretations this provides important insights into understanding the formation and evolution of the chinese continents the papers assembled in this volume present a timely and comprehensive overview on major advancements and controversial issues related to the formation and evolution of continental crust in china complex tectonic histories were experienced not only by the large scale cratonic blocks and orogenic belts but also by small scale terranes and orogens between and inside these blocks nevertheless our understanding of lithotectonic units and geological processes has been greatly advanced by recent studies of zirconology and geochemistry for various rock types from major petrotectonic units in china it has been further advanced from integrated interpretations of geochemical and petrological data for petrogenesis of magmatic rocks an overview of these observations and interpretations provides new insights into understanding the continental plate tectonics and the chemical geodynamics of subduction zones

exploration of groundwater flowpaths and effective recharge in the amargosa desert nevada using multivariate statistical analysis and elevation dependent chloride mass balance method estimation of groundwater flowpaths and effective recharge was performed in the vicinity of amargosa desert nevada usa using multivariate statistical methods msms and a simple elevation dependent chloride mass balance method modified by estimating a minimum effective recharge elevation mere msms were applied to major ion data to explain the relationships among different groundwater chemical species define hydrochemical facies needed to determine groundwater flowpaths and evolution and identify the influence of surface and geologic features on groundwater for each defined watershed an average groundwater chloride concentration is used and the chloride deposition rate is assessed in the subject watersheds over all land elevations above the estimated mere by assuming two constant spatial and temporal concentrations of chloride in precipitation which is a function of elevation the mere for the amargosa desert was appraised as 1200 m above mean sea level m amsl based upon the elevation of the orifice of cane spring 1237 m amsl one of the lowest elevation permanent springs on the nevada national security site this minimum elevation is the cutoff altitude for our calculations at which precipitation is considered to make a significant contribution to recharge elevation dependent precipitation was evaluated relating elevation data from the online mapping software acme labs mapper 2 1 and precipitation data from the prism climate group s prism these analyses provide further insight into the hydrology of the region and demonstrate the strong impact surface and geologic features can have on surface water infiltration and groundwater in arid regions specifically evidence is presented of past focused recharge around the ephemeral fortymile wash and interaction between geologic faults and groundwater infiltration and flowpaths in addition groundwater flow based upon water chemistry appears to be segmented by major faults groundwater recharge in the vicinity of the amargosa desert has been estimated around 15 280 acre ft year

vanadium geochemistry along groundwater flow paths in contrasting aquifers of the united states carrizo sand texas and oasis valley nevada aquifers abstract vanadium v concentrations were measured along the flow paths in two contrasting aquifer systems to study how changing solution composition and redox conditions affect v cycling and transport in groundwater flow systems the oasis valley groundwater system in nevada is an unconfined system where groundwaters generally remain oxic along the flow path whereas the carrizo sand aquifer is a deep confined coastal plain aquifer in texas that contains well defined fe iii and so 4 2 reduction zones along the flow path speciation modeling predicts that in groundwaters from the oasis valley v chiefly occurs as the vanadate oxyanion e g h 2 vo 4 hvo 4 2 which is mobile along the entire flow path because of vanadate s solubility in oxic and alkaline waters furthermore the data indicate that v concentrations increase in groundwaters along the flow path suggesting that mineral dissolution and or desorption reactions progressively contribute v to groundwaters of the oasis valley system geochemical modeling is consistent with dissolution of mafic phenocrysts that are common in the local tuffaceous aquifer rocks as a probable source of v to oasis valley groundwaters within the carrizo sand aquifer groundwater v concentrations decrease with flow beyond the recharge zone and into the confined portion of the aquifer thereafter total v concentrations remain low and relatively constant along the remainder of the studied flow path exhibiting no systematic variations across the fe iii and so 4 2 reduction zones groundwaters from the carrizo sand aquifer are generally reducing and as such geochemical modeling suggests that both v v and v iv species occur in carrizo sand groundwaters and furthermore that the proportion of v iv species increases concurrent with the decrease in total v concentrations along the flow path results from sequential extraction experiments on carrizo sand aquifer sediment samples suggest that v adsorption onto fe iii mn iv oxides oxyhydroxides acts to decrease the aqueous v concentrations in carrizo sand groundwater furthermore geochemical modeling is consistent with complexation of v iv with dissolved organic matter dom within the carrizo sand aquifer groundwaters which owing to the low dom concentrations appears to buffer v at low but relatively constant concentrations within the confined and reducing carrizo sand groundwaters these results support previous reports of v iv dom binding and imply that aquifer material maintains an essential role in not only determining the availability of v to enter aqueous solution but also in influencing partitioning between solid and aqueous phases

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

metasomatism and the crystallization of zircon megacrysts in archaean peridotites from the lewisian complex nw scotland zircon megacrysts are locally abundant in 1 40 cm thick orthopyroxenite veins within peridotite host rocks in the archaean lewisian gneiss complex from nw scotland the veins formed by metasomatic interaction between the ultramafic host and si rich melts are derived from partial melting of the adjacent granulite facies orthogneisses the interaction produced abundant orthopyroxene and within the thicker veins phlogopite pargasite and feldspathic bearing assemblages two generations of zircon are present with up to 1 cm megacrystic zircon and a later smaller equant population located around the megacryst margins patterns of zoning rare earth element abundance and oxygen isotopic compositions indicate that the megacrysts crystallized from crustal melts whereas the equant zircon represents new neocryst growth and partial replacement of the megacryst zircon within the ultramafic host both zircon types have u pb ages of ca 2464 ma broadly contemporaneous with granulite facies events in the adjacent gneisses zircon megacrysts locally form 10 of the assemblage and may be associated to zones of localized nucleation or physically concentrated during movement of the siliceous melts their unusual size is linked to the suppression of zircon nucleation and increased zr solubility in the si undersaturated melts the metasomatism between crustal melts and peridotite may represent an analog for processes in the mantle wedge above subducting slabs as such the crystallization of abundant zircon in ultramafic host rocks has implications for geochemistry of melts generated in the mantle and the widely reported depletion of high field strength elements in arc magmas

role of partial melting in the evolution of the sulu eastern china ultrahigh pressure terrane strongly deformed potassium feldspar rich dikes are widely distributed in the northern part of the sulu ultrahigh pressure uhp metamorphic terrane eastern china the fact that the crystallization ages of these dikes overlap with the age of peak uhp metamorphic conditions implies the presence of melt during metamorphism sr isotopic ratios of the dikes are compatible with their origin as partial melts of the dominant felsic sulu gneiss partial melting may be the key to solving several unusual features of the sulu and other uhp terranes such as the almost complete lack of mineralogical evidence for uhp conditions and the limited growth of zircon during uhp conditions in the dominant felsic gneiss in addition because partial melting will cause a drastic reduction in the strength of the uhp gneisses the most likely exhumation mechanism is diapiric rise of a low viscosity partially molten mass containing entrained blocks of eclogite and not a thin sheet as usually proposed

solute and isotopic geochemistry and ground water flow in the central wasatch range utah ground water flow systems in the rugged central wasatch range utah were investigated by solute and isotopic methods six types of ground water systems were identified on the basis of rock type and structure the systems are broadly grouped into four categories 1 granitic systems 2 non granitic systems a unconsolidated alluvial systems b consolidated sedimentary rock systems and c fault controlled systems 3 thermal systems 4 mine drainage systems these ground water systems have distinctive solute and isotopic chemistries based on an analysis of 3h and 14c data all of the ground waters have a component of post 1952 recharge water and most of the ground waters are composed almost entirely of post 1952 recharge water a few of the ground water systems contain some water which is hundreds to perhaps thousands of years old the 18o and 2h data plot on a local meteoric water line and none of the data exhibit a positive 18o shift the absence of a positive 18o shift suggests maximum aquifer temperatures are approximately 100 c granitic terrains are dominated by fracture controlled local ground water flow systems which respond rapidly to recharge events the granitic ground water systems have estimated maximum circulation depths of about 160 m below land surface all the alluvial most of the consolidated sedimentary bedrock and some of the fault controlled ground water systems are shallow circulating local flow systems with short travel times ground water circulation of most of the local flow systems is within 200 m of land surface and is bedding plane controlled some of the consolidated bedrock and many of the fault controlled ground water systems are of the intermediate type and have circulation depths to 500 m four thermal ground water systems have been identified the thermal systems are heated by the geothermal gradient and have not circulated deeper than 2 2 5 km the thermal systems are of the intermediate and regional type the solute chemistry of most of the non granitic ground waters may be attributed to the dissolution of carbonate minerals and minor amounts of gypsum contributions of external co2 g to some of the thermal systems from either metamorphic or silicate hydrolysis processes is suspected

dating of zircon from ti clinohumite bearing garnet peridotite implication for timing of mantle metasomatism garnet peridotites from the kokchetav ultrahigh pressure uhp massif contain abundant volatile and high field strength element hfse bearing minerals such as ti clinohumite and zircon these characteristics are interpreted to be related to fluid infiltrated mantle metasomatism from the oceanic lithosphere that had been subducted below the area the zircons from the peridotites were dated by using sensitive high resolution ion microprobe shrimp and yielded apparent u pb ages of 554 494 ma weighted mean age 528 ma that are mostly consistent with the timing of uhp metamorphism deduced from diamond bearing country rocks in this massif these zircons have an almost flat rare earth element ree pattern and very low ree concentrations these characteristics are similar to those observed in kimberlitic zircons inherited zircon cores although only rarely preserved yielded apparently proterozoic ages and have different trace element characteristics compared to the overgrowth rims these features indicate that the mantle metasomatic events and the recrystallization of ti clinohumite and zircon were due to hfse enriched fluid infiltration during the uhp metamorphism at great depths the metasomatized mantle may have been transported farther into the deep upper mantle and contributed to the source of intraplate magmas such as kimberlites and alkali basalts because these rocks have characteristically high volatile and hfse concentrations such as those of the ti clinohumite bearing garnet peridotites

records of crustal metasomatism in the garnet peridotites of the ulten zone upper austroalpine eastern alps peridotites we infer that the metasomatizing hydrous fluids could peridotites in the ulten zone upper austroalpine eastern alps represent the residual fluids left after the crystallization of leucosomes occur as small bodies within lower crustal rocks gneisses and starting from water undersaturated melts produced during migmigmatites subducted at eclogite facies conditions during the vamatization of the host gneisses riscan orogeny they record a complex metamorphic and deformation evolution as indicated by the transition from coarse grained spinelbearing peridotites to fine grained garnet amphibole bearing peridotites and are interpreted as portions of mantle wedge that

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

redox condition in the nanhua basin during the waning of the sturtian glaciation a chromium isotope perspective abstract the neoproterozoic oxygenation event noe is often considered to be synchronously and possibly causally associated with the cryogenian glaciations reconstructing early cryogenian redox conditions is therefore essential for understanding the onset of the noe we analyzed cr isotope compositions 53cr as well as major and rare earth element ree concentrations of the banded iron formations bifs in the basal fulu formation of the xiajiajiang section nanhua basin which are thought to have been deposited during the waning stage of the sturtian glaciation strongly positively fractionated authigenic 53cr values were obtained after detrital correction ranging from 0 64 to 1 22 and averaging at 0 91 0 40 2 n 10 or after detrital evaluation 0 71 this suggests extensive oxidative cr cycling occurred at the earth s surface and implies that the po2 was probably higher than 1 pal during this period the lack of ce anomalies 0 90

geochemical and statistical evidence of deep carbonate groundwater within overlying volcanic rock aquifers aquitards of southern nevada usa abstract samples collected from springs and wells in southern nevada were analyzed for major solutes and trace elements as part of a larger study to characterize the geochemical signatures of these groundwaters in this study principal component analysis pca was used to reduce the large data sets including the four major cations ca mg na k and 27 trace elements analyzed in these groundwater samples principal components analysis of the major cation data indicates that groundwaters from cenozoic felsic volcanic rock aquifers aquitards of southern nevada exhibit strong chemical associations to each other but weak relationships to groundwaters from the regional carbonate aquifer which were instead chemically similar to each other however pca of the trace element data demonstrates that some groundwaters from the volcanic aquifers aquitards are chemically similar to those of the underlying regional carbonate aquifer the pca also reveals that these groundwaters from the volcanic aquifers aquitards have significantly different trace element compositions than perched groundwaters contained within similar felsic volcanic rocks moreover rare earth element ree data from groundwaters collected from wells finished in the volcanic aquifers aquitards of southern nevada have similar concentrations and similar shale normalized patterns to the carbonate aquifer groundwaters as well as local carbonate rocks these same southern nevada well waters do not exhibit ree concentrations or shale normalized signatures that resemble the perched volcanic groundwaters or the tuffs of southern nevada the ree data and trace element pca along with previous carbon isotope analyses water temperature data hydraulic head relations and results of a recent pump test of a well near yucca mountain suggest close contact of the regional carbonate groundwaters and groundwaters from the overlying volcanic rocks of southern nevada and possible upwelling of the carbonate groundwaters into the overlying volcanic rock units in the vicinity of yucca mountain

ultrahigh pressure metamorphism in the forbidden zone the xugou garnet peridotite sulu terrane eastern china the xugou garnet peridotite body of the southern sulu ultrahigh pressure uhp terrane is enclosed in felsic gneiss bounded by faults and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite the peridotite is composed of variable amounts of olivine fo91 enstatite en92 93 garnet alm20 23prp53 58knr6 9grs12 18 diopside and rare chromite the ultramafic protolith has a depleted residual mantle composition indicated by a high mg number very low cao al2o3 and total ree contents compared to primary mantle and other sulu peridotites most garnet prp44 58 clinopyroxenites are foliated except for rare kyanite bearing eclogitic bands most eclogites contain a simple assemblage of garnet alm29 34prp32 50grs15 39 omphacite jd24 36 minor rutile clinopyroxenite and eclogite exhibit lree depleted and lree enriched patterns respectively but both have flat hree patterns normalized la sm and yb contents indicate that both eclogite and garnet clinopyroxenite formed by high pressure crystal accumulation variable trapped melt from melts resulting from two stage partial melting of a mantle source recrystallized textures and p t estimates of 780 870 c 5 7 gpa and a metamorphic age of 231 11 ma indicate that both mafic and ultramafic protoliths experienced triassic uhp metamorphism in the p t forbidden zone with an extremely low thermal gradient 5 c km 1 and multistage retrograde recrystallization during exhumation develop of prehnite veins in clinopyroxenite eclogite felsic blocks and country rock gneiss and replacements of eclogitic minerals by prehnite albite white mica and k feldspar indicate low temperature metasomatism

submarine thermal springs associated with young volcanoes the teahitia vents society islands pacific ocean abstract hydrothermal solutions were sampled close to two summits of teahitia a submarine volcano from the southern extension of the society volcanic chain the temperatures measured within two vents were 30 c the vent solutions have a ph value around 5 3 and are enriched in fe mn si ba li and rees relative to ambiant seawater but show only a small decrease in mg and so4 concentrations the lowest measured 87sr 86sr is 0 70867 while the end is identical to those of the surrounding alkali basalt 2 5 the vent solutions from teahitia have chemical characteristics similar to pele s vents loihi seamount another mid plate volcano the exact temperature of seawater basalt interaction is difficult to constrain however the absence of positive eu anomaly in the fluids the particles and in the hydrothermal deposits from teahitia seems to exclude the participation of a high temperature component comparable as those involved in mid ocean ridge axis vents

melting induced fluid flow during exhumation of gneisses of the sulu ultrahigh pressure terrane abstract hydrothermally altered rocks are products of fluid rock interactions and typically preserve numerous quartz veins that formed as chemical precipitates from fluids that fill up cracks thus quartz veins are the record of the fluid system that involved fracture flow in the direction of changing temperature or pressure in order to decipher the fluid activity in the sulu ultrahigh pressure uhp terrane in eastern china quartz veins together with an adjacent eclogite lens and the host gneiss were studied in one location a deformed quartz vein is located at the boundary between the host gneiss and the eclogite lens the amphibolite facies overprinting of the eclogite lens decreases from the rim to the core of the lens with fresh eclogite preserved in the core the foliated biotite gneiss contains felsic veins and residual phengites zircon rims from the gneiss are characterized by melt related signatures with steep hree patterns high hf contents and negative eu anomalies and a pool of weighted average 206 pb 238 u analyses reveal an age of 219 3 ma 2 which is younger than the uhp metamorphic age 236 2 ma 2 recorded by zircons from the eclogite lens this suggests that the gneiss in the sulu uhp terrane could have suffered from partial melting due to phengite dehydration during the hot exhumation stage the formation age of the quartz vein 219 2 ma 2 defined by zircon rims agrees well with the partial melting time 219 3 ma 2 of the host gneiss the initial 176 hf 177 hf ratios of zircon rims from the quartz vein are obviously lower than zircons from the eclogite lens but overlap with the coeval zircon domains from the nearby granite dikes produced by partial melting of orthogneiss these observations suggest that the quartz vein and corresponding fluid flow could be associated with partial melting of the host gneiss on the other hand amphibole bearing and hree rich zircon rims from the amphibolite pool an amphibolite facies metamorphic age of 217 5 ma 2 overlap with the formation age of the quartz vein this implies that retrogression of the eclogite lens could have been caused by melting induced fluid flow based on the above observations we speculate that partial melting of the gneiss in the continental subduction related uhp belt could have induced a significant fluid flow during the exhumation stage and thus contributed significantly to the extensive retrogression of eclogites in the sulu uhp terrane

dehydration melting of ultrahigh pressure eclogite in the dabie orogen evidence from multiphase solid inclusions in garnet several types of multiphase solid ms inclusions are identified in garnet from ultrahigh pressure uhp eclogite in the dabie orogen the mineralogy of ms inclusions ranges from pure k feldspar to pure quartz with predominance of intermediate types consisting of k feldspar quartz silicate plagioclase or epidote barite the typical ms inclusions are usually surrounded with radial cracks in the host garnet similar to where garnet contains relict coesite barite aggregates display significant heterogeneity in major element composition with total contents of only 57 73 and highly variable sio2 contents of 0 32 25 85 that are positively correlated with bao and so3 contents the occurrence of ms inclusions provides petrographic evidence for partial melting in the uhp metamorphic rock the occurrence of barite aggregates with variably high sio2 contents suggests the coexistence of aqueous fluid with hydrous melt under hp eclogite facies conditions thus local dehydration melting is inferred to take place inside the uhp metamorphic slice during continental collision this is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust as a consequence the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial melting during the continental subduction zone metamorphism

metamorphic growth and recrystallization of zircon distinction by simultaneous in situ analyses of trace elements u th pb and lu hf isotopes in zircons from eclogite facies rocks in the sulu orogen simultaneous in situ analyses of trace elements u th pb and lu hf isotopes were carried out on distinct domains of zircons in ultrahigh pressure uhp eclogite facies metamorphic rocks from the main hole of the chinese continental scientific drilling ccsd in the sulu orogen for the first time trace elements are directly linked to lu hf isotopes in metamorphic zircons with reference to their u pb dates this enables methodological integration to distinguish four types of metamorphic zircon solid state replacement and dissolution recrystallizations of protolith zircons and new growth from the aqueous fluid metamorphically grown zircons are characterized by concordant u pb ages for the metamorphism flat hree patterns typical of the garnet effect low contents of ree especially hree y nb ta and th u high contents of hf low lu gd n lu hf and th u 0 1 ratios and elevated 176hf 177hf ratios relative to solid state recrystallized zircons this suggests the effects of both garnet and fluid on the growth of metamorphic zircons in contrast metamorphic recrystallization has reset the u th pb isotope system of protolith zircons to different extents depending on the extents of fluid action during metamorphism solid state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons which are characterized by discordant u pb ages close to or below the protolith age steep mree hree patterns typical of magmatic origin high contents of trace elements and their ratios and low 176hf 177hf ratios on the other hand dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant u pb ages for the metamorphism steep mree hree patterns lowered contents of trace elements such as ree th u y nb ta and ti relative to the protolith zircons and almost unchanged hf isotope ratios replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes in between while the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of hree with flat pattern and the low contents of trace elements the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of mree to hree without the flat hree pattern therefore the simultaneous in situ analyses of metamorphic zircons have the advantage over single term analyses in making distinction between the new growth and the different types of recrystallization

unusually low δ18o ultra high pressure metamorphic rocks from the sulu terrain eastern china eclogite and quartzite interlayers containing ultra high pressure uhp metamorphic mineral assemblages occur at qinglongshan in the sulu terrain eastern china these rocks exhibit some of the most negative oxygen isotope compositions that have ever been reported for metamorphic rocks the 18 value of minerals in eclogite ranges from 10 4 to 9 0 o and in quartzite ranges from 10 2 to 7 3 near isotopic equilibrium between minerals and rocks suggests that these rocks must have acquired such low oxygen isotope compositions before the uhp metamorphism through near surface meteoric water rock interactions the rocks were then subducted to a great depth during continent continent collision forming uhp minerals with concomitant local oxygen isotope redistribution it is significant that the rocks have retained such negative oxygen isotope compositions despite subduction into the upper mantle

global variability of chromium isotopes in seawater demonstrated by pacific atlantic and arctic ocean samples abstract seawater chromium cr isotope and concentration data are presented from multiple sites in the arctic ocean and three locations in the atlantic and pacific oceans a 2400 m profile illustrates the heterogeneity of 53cr in the arctic ocean with depth and water mass source pacific vs atlantic the highest 53cr values occur in pacific sourced waters which also have the lowest cr concentration chromium concentration and 53cr data from these locations in conjunction with published data for the south atlantic ocean yield a simple logarithmic function as predicted by rayleigh fractionation in a closed system the observed cr isotope signature is hypothesized to arise from fractionation during the reduction of cr vi in surface waters and oxygen minimum zones scavenging of isotopically light cr iii to deeper water and sediment and subsequent release of this seawater derived cr iii back into seawater either as organic complexes with cr iii or after oxidation to cr vi the isotopic fractionation factor e associated with cr cycling in seawater is estimated to be 0 80 0 03 2 samples from the sea ice affected surface mixed layer of the arctic ocean 10 m depth deviate from the general trend and samples proximal to rivers illustrate geographic variation in 53cr values for continental runoff but prompt loss of this signature away from the source

fluids in deeply subducted continental crust petrology mineral chemistry and fluid inclusion of uhp metamorphic veins from the sulu orogen eastern china abstract the complex vein associations hosted in southern sulu ultrahigh pressure uhp eclogites contain quartz omphacite or jadeite kyanite allanite zoisite rutile garnet these minerals have chemical compositions similar to those of host eclogites inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet and coesite inclusions were found in vein zircon these facts suggest that the veins together with host eclogites have been subjected to synchronous uhp metamorphism the vein minerals contain relatively high concentrations of rare earth elements ree high field strength elements hfse and transition metal elements tme a kyanite quartz vein has a whole rock composition similar to adjacent uhp metamorphic granitic gneisses abundant primary multi solid fluid inclusions trapped within uhp vein minerals contain complex daughter minerals of muscovite calcite anhydrite magnetite pyrite apatite celestite and liquid and gas phase of h 2 o with solids up to 30 70 of the inclusion volume the presence of daughter minerals anhydrite and magnetite indicates the subduction fluids were oxidizing and provides a possible interpretation for the high oxygen fugacity of subduction zone magmas these characteristics imply that the uhp vein minerals were crystallized from supercritical silicate rich aqueous fluids that were in equilibrium with peak uhp minerals and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as hree hfse and tme such fluids might have resulted in major fractionation between nb and ta i e the uhp fluids have subchondritic nb ta values whereas the host eclogites after extraction of the fluids have suprachondritic nb ta values therefore voluminous residual eclogites with high nb ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs

u pb hf and o isotope evidence for two episodes of fluid assisted zircon growth in marble hosted eclogites from the dabie orogen abstract a combined study of internal structure u pb age and hf and o isotopes was carried out for metamorphic zircons from ultrahigh pressure eclogite boudins enclosed in marbles from the dabie orogen in china cl imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain respectively the metamorphic zircons have low th and u contents with low th u ratios yielding two groups of 206 pb 238 u age at 245 3 to 240 2 ma and 226 4 to 223 2 ma respectively anomalously high 18 o values were obtained for refractory minerals with 9 9 to 21 4 for garnet and 16 9 for zircon this indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction zone metamorphism most of the zircons are characterized by very low 176 lu 177 hf ratios of 0 000001 0 000028 indicating their growth in association with garnet recrystallization a few of them falling within the older age group have comparatively high 176 lu 177 hf ratios of 0 000192 0 000383 suggesting their growth prior to the formation of garnet in the late stage of subduction the variations in the lu hf ratios for zircons can thus be used to correlate with garnet growth during eclogite facies metamorphism in either case the zircons have variable e hf t values for individual samples suggesting that their protolith is heterogeneous in hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle nevertheless a positive correlation exists between 206 pb 238 u ages and lu hf isotope ratios for the metamorphically recrystallized zircons suggesting that eclogite facies metamorphism in the presence of fluid has the identical effect on zircon lu hf and u th pb isotopic systems we conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh pressure metamorphism whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high pressure eclogite facies regime

oxidative release of chromium from archean ultramafic rocks its transport and environmental impact a cr isotope perspective on the sukinda valley ore district orissa india abstract this study investigates cr isotope fractionation during soil formation from archean 3 1 3 3 ga ultramafic rocks in a chromite mining area in the southern singhbhum craton orissa india the cr isotope signatures of two studied weathering profiles range from non fractionated mantle values to negatively fractionated values as low as 53 cr 1 29 0 04 local surface waters are isotopically heavy relative to the soils this supports the hypothesis that during oxidative weathering isotopically heavy cr vi is leached from the soils to runoff the impact of mining pollution is observed downstream from the mine where surface water cr concentrations are significantly increased accompanied by a shift to less positive 53 cr values relative to upstream unpolluted surface water a microbial mat sample indicates that microbes have the potential to reduce and immobilize cr vi which could be a factor in controlling the hazardous impact of cr vi on health and environment the positive cr isotope signatures of the brahmani estuary and coastal seawater collected from the bay of bengal further indicate that the positively fractionated cr isotope signal from the catchment area is preserved during its transport to the sea isotopically lighter cr vi downstream from the mine is probably back reduced to cr iii during riverine transport leading to similar cr isotope values in the estuary as observed upstream from the mine

iodine to calcium ratios in marine carbonate as a paleo redox proxy during oceanic anoxic events periods of globally distributed extreme oxygen depletion so called oceanic anoxic events oaes have been recognized in the mesozoic geological record and appear to be characteristic of episodes of extreme warmth here we explore the application of iodine calcium ratios i ca in marine carbonate as a new geochemical proxy to constrain seawater redox change and provide additional insights into the response of ocean chemistry to ancient climatic warming iodine has long been known as a redox sensitive and biophilic element mainly present as iodate and iodide in seawater iodate converting to iodide under anoxic conditions laboratory experiments growing calcite crystals from solutions spiked with iodate show that this is the ionic species incorporated into the carbonate structure likely substituting for the co 3 2 ion a fall in the i ca ratio measured in carbonates formed in shallow water by marine calcifiers during the early toarcian and cenomanian turonian oaes is interpreted both as a response to a decrease in the iodate iodide ratio in ocean waters and the drawdown of the global iodine inventory under conditions of accelerated organic matter burial the results suggest that i ca ratios in carbonates may be used to monitor seawater oxidation levels throughout earth history

zircon u pb age and o isotope evidence for neoproterozoic low 18o magmatism during supercontinental rifting in south china implications for the snowball earth event identification of neoproterozoic low 18 o igneous rocks is a key to demonstrate that high t water rock interaction and low 18 o magmatism in rift tectonic zones can serve as an efficient interface to transport heat and material from the earth s interior to exterior at the onset of the snowball earth event low 18 o values of 10 9 to 4 4 permil characteristic of meteoric origin were observed on an areal extent over 30 000 km2 for mid neoproterozoic metaigneous zircons from the ultrahigh pressure eclogite facies metamorphic belt of triassic ages along the dabie sulu orogenic belt china although the zircon o isotope system could be reset to acquire its low 18 o values by exchange with matrix minerals during wet eclogite facies metamorphism the abnormal 18 o depletion is confirmed by occurrence of neoproterozoic low 18 o zircons in low grade metaigneous rocks in the same belt cathodoluminscence imaging of zircons from low grade metagranitoid and metavolcanic rocks shows preservation of the internal structures of oscillatory zoning typical of magmatic origin hornblende from two granites of low 18 o zircons gave ar ar plateau ages of 747 6 and 776 12 ma ruling out that phanerozoic metamorphism effected mineral o isotope values two groups of u pb ages were measured at 756 5 ma and 782 5 ma respectively for the low 18 o zircons this indicates that low 18 o zircons crystallized during two episodes of low 18 o magmatism in response to tectonic evolution from supercontinental rifting at 780 ma to breakup at 750 ma the second episode of low 18 o magmatism is contemporaneous with the continental kaigas iceage a plate rift model is advanced to account for the tectonic connection between the low 18 o magmatism the continental glaciation and the supercontinental rifting during the cryogenian period high t meteoric water rock interaction occurred prior to low 18 o magmatism in rift tectonic zones and caldera collapse was responsible for melting of hydrothermally altered low 18 o rocks during rifting of the south china block from the rodinia supercontinent

oxygen isotope geochemistry of ultrahigh pressure metamorphic rocks from 200 4000 m core samples of the chinese continental scientific drilling abstract oxygen isotope studies were carried out for ultrahigh pressure metamorphic rocks from the main hole of the chinese continental scientific drilling in the sulu orogen the samples of interest include various types of lithology mainly eclogite and gneiss in a depth of 200 to 4000 m containing five continuous core segments between contrasting lithologies the results show a large variation in 18o value from 10 41 to 9 63 for constituent minerals distinct 18o depletions are observed in the frequently alternated layers of eclogite and gneiss in which the variations of 18o values are gradual regardless of lithologies both equilibrium and disequilibrium o isotope fractionations are observed between quartz and the other minerals special attention was paid to the relationship between distance petrography and 18o value of adjacent samples the results show o isotope heterogeneities between the different and same lithologies on scales of about 20 to 50 cm corresponding to the maximum scales of fluid mobility during the continental collision amphibolite facies retrograde metamorphism during exhumation caused mineral reactions and o isotope disequilibria between some of the minerals considerable changes occur in 18o and petrography at the contact between eclogite and gneiss suggesting that the contact between different lithologies is the most favorable place for fluid activity despite the widespread retrogression retrograde fluid was internally buffered in the stable isotope compositions the retrograde fluid is of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl although locally external fluids became available along fault zones and lithological layers it still acted within the exhumed slabs with internal origins fluid flow also took place after the amphibolite facies retrogression but it only affects the o isotope compositions of feldspar and mica premetamorphic protoliths are deduced to have heterogeneous 18o values due to varying degrees of meteoric water rock interaction before continental collision minimum depth of the 18o depletion is up to 3300 m together with areal 18o depletion in surface outcropped rocks along the dabie sulu orogenic belt at least 66 000 km3 of supracrustal rocks were interacted with meteoric water along the northern margin of the south china block during the neoproterozoic

biologically recycled continental iron is a major component in banded iron formations banded iron formations bifs record a time of extensive fe deposition in the precambrian oceans but the sources and pathways for metals in bifs remain controversial here we present fe and nd isotope data that indicate two sources of fe for the large bif units deposited 2 5 billion y ago high end and 56fe signatures in some bif samples record a hydrothermal component but correlated decreases in end and 56fe values reflect contributions from a continental component the continental fe source is best explained by fe mobilization on the continental margin by microbial dissimilatory iron reduction dir and confirms for the first time to our knowledge a microbially driven fe shuttle for the largest bifs on earth detailed sampling at various scales shows that the proportions of hydrothermal and continental fe sources were invariant over periods of 100 103 y indicating that there was no seasonal control although fe sources varied on longer timescales of 105 106 y suggesting a control by marine basin circulation these results show that fe sources and pathways for bifs reflect the interplay between abiologic hydrothermal and biologic processes where the latter reflects dir that operated on a basin wide scale in the archean

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

chromium geochemistry in serpentinized ultramafic rocks and serpentine soils from the franciscan complex of california weathering of ultramafic rocks and serpentinites in the franciscan complex of california produces serpentine soils containing high concentrations of cr as well as other potentially toxic elements including ni co and mn chromium concentrations in serpentine soils from jasper ridge biological preserve in the central coast range are as high as 4 760 mg kg 1 nearly three times greater than the serpentinite protolith 1800 mg kg 1 chromium containing minerals within the bedrock include chlorite 0 3 cr wt enstatite 0 4 cr wt augite 0 7 cr wt chromite 10 8 cr wt magnetite 8 2 10 3 cr wt and an ultra fine grained mixture of spinel and a silicate phase containing 13 3 cr weight percent chromium concentrations in serpentine soil profiles fluctuate between 1 725 to 4 760 mg kg 1 and do not correspond to variations in soil ph organic carbon or electrical conductivity the enrichment and variability of soil cr is directly related to the modal abundance and weathering of chromite cr magnetite and the spinel silicate mixture by comparison cr silicates account for

multiple partial melting events in the sulu uhp terrane zircon u pb dating of granitic leucosomes within amphibolite and gneiss thin layers and lenses of granitic leucosome are widely distributed within amphibolites paragneisses and orthogneisses of the sulu uhp terrane they are parallel to or cross cut foliations in the host rocks at different scales and show evidence of coalescence and migration to form centimetre to decimetre scale segregations variously migmatized rocks extend at least 350 km from sw sulu maobei to ne sulu weihai in a band at least 50 km wide a combined study of mineral inclusions cathoduluminescence cl images u pb la icp ms dates and in situ trace element compositions of zircon provide clear evidence on the nature and timing of partial melting in these uhp rocks most zircon from the granitic leucosomes occurs as distinct overgrowths around inherited igneous or metamorphic cores or as new euhedral crystals the overgrowths and new crystals commonly show perfectly euhedral shapes have pronounced oscillatory zoning and contain felsic mineral inclusions such as kfs pl qtz ilm monazite mon in contrast the inherited igneous or metamorphic cores are rounded or irregular contain low p or uhp mineral inclusions and show clear dissolution textures these data suggest that the new zircon is anatectic in origin and that it grew during partial melting of the uhp rocks the ree patterns of the anatectic zircon show steep slopes from the hree to lree with strongly to moderately negative eu anomalies eu eu 0 31 0 72 and pronounced positive ce anomalies ce ce 6 8 26 5 abundant u pb spot analyses of the anatectic zircon reveal two discrete and meaningful ages of partial melting within the sulu uhp terrane anatectic zircon from 12 granitic leucosomes within amphibolites paragneisses and orthogneisses from sulu uhp slices ii and iii yields consistent mean u pb ages of 219 0 1 2 to 218 3 1 6 ma 218 8 2 0 to 217 3 1 7 ma and 218 2 1 4 to 215 0 1 5 ma respectively in contrast anatectic zircon from six granitic leucosomes within paragneisses and orthogneisses from sulu uhp slice iii records younger mean u pb ages of 151 9 1 3 to 151 1 1 8 ma and 155 9 1 8 to 153 7 1 7 ma respectively these data imply that the sulu uhp terrane experienced two mesozoic partial melting events the first partial melting event 219 215 ma was probably associated with a late triassic granulite facies stage of hot exhumation whereas the second 156 151 ma is interpreted as the result of middle late jurassic extension and thinning of the previously thickened crust of the sulu uhp terrane both partial melting events induced extensive retrograde metamorphism of the eclogites and their country rocks

yttrium and rare earth elements in fluids from various deep sea hydrothermal systems rare earth element ree and yttrium y concentrations were measured in fluids collected from deep sea hydrothermal systems including the mid atlantic ridge mar i e menez gwen lucky strike tag and snakepit the east pacific rise epr i e 13 n and 17 19 s and the lau vai lili and manus vienna woods pacmanus desmos back arc basins bab in the south west pacific in most fluids y is trivalent and behaves like ho chondrite normalized y ree y reen concentrations of fluids from mar epr and two bab sites i e vai lili and vienna woods showed common patterns with lree enrichment and positive eu anomalies ree analysis of plagioclase collected at lucky strike strengthens the idea that fluid ree contents are controlled by plagioclase phenocrysts other processes however such as ree complexation by ligands cl f so42 secondary phase precipitation and phase separation modify ree distributions in deep sea hydrothermal fluids ree speciation calculations suggest that aqueous ree are mainly complexed by cl ions in hot acidic fluids from deep sea hydrothermal systems ree concentrations in the fluid phases are therefore influenced by temperature ph and duration of rock fluid interaction unusual y reen patterns found in the pacmanus fluids are characterized by depleted lree and a positive eu anomaly the demos fluid sample shows a flat y reen pattern which increases regularly from lree to hree with no eu anomaly these manus basin fluids also have an unusual major element chemistry with relatively high mg so4 h2s and f contents which may be due to the incorporation of magmatic fluids into heated seawater during hydrothermal circulation ree distribution in pacmanus fluids may stem from a subseafloor barite precipitation and the ree in demos fluids are likely influenced by the presence of sulfate ions

the enigma of crustal zircons in upper mantle rocks clues from the tumut ophiolite southeast australia we suggest a new explanation for the presence of crustally derived zircons in the upper mantle rocks of ophiolitic complexes as an alternative to subduction related models integrated isotopic u pb hf and o isotopes and trace element data for zircons from the tumut ophiolitic complex southeast australia indicate that these grains are related to granitic magmatism and were introduced into the mantle rocks after their emplacement into the crust these observations emphasize that a clear understanding of the origin of individual zircon populations and their relationship to the host rock is essential to interpretations of the tectonic history of upper mantle rocks and the dynamics of crust mantle interactions

cerium anomaly variations in ediacaran earliest cambrian carbonates from the yangtze gorges area south china implications for oxygenation of coeval shallow seawater abstract the late neoproterozoic ocean witnessed the naissance of animals which is believed to have been stimulated by an increase in oxygen levels in the ocean however apart from the emergence of animal fossils little supporting evidence has been found for the rise of oxygen in shallow seawater here we present ce anomaly data a redox proxy with a higher reduction potential than many other redox proxies for carbonates from well preserved marine successions the doushantuo formation dengying formation and lower yanjiahe formation of the ediacaran period and the upper yanjiahe formation of early cambrian on the neoproterozoic yangtze platform in the yangtze gorges area in order to constrain the redox evolution of the shallow marine environment calculated ce anomalies were screened using the following criteria al 36 in acetic acid leached carbonate to target samples that preserve primary seawater ree features and ce anomalies the samples satisfying this screening show seawater like ree distribution patterns in leached carbonates and may have recorded ce anomalies of the seawater from which the carbonates precipitated these data show that the doushantuo formation has ce ce values between 0 92 and 0 71 the dengying formation between 0 90 and 0 40 and the yanjiahe formation between 0 52 and 0 72 the ce ce values of the doushantuo formation suggest that shallow waters during the doushantuo stage 635 551 ma were anoxic to suboxic from the bottom to the top of the dengying formation ce ce values decrease systematically which cannot be explained by a change in depositional depth of the carbonate but suggests that the shallow waters became more oxygenated during the dengying stages from 551 ma to ediacaran cambrian boundary these results may provide direct evidence for an increase in oxygen levels in the shallow marine environment during the ediacaran period

hydrochemical and 14c constraints on groundwater recharge and interbasin flow in an arid watershed tule desert nevada summary geochemical data deduced from groundwater and vein calcite were used to quantify groundwater recharge and interbasin flow rates in the tule desert southeastern nevada 14 c age gradients below the water table suggest recharge rates of 1 2 mm yr which correspond to a sustainable yield of 5 10 4 km 3 yr to 1 10 3 km 3 yr uncertainties in the applied effective porosity value and increasing horizontal interbasin flow components at greater depths may bias these estimates low compared to those previously reported using the water budget method the deviation of the groundwater 18 o time series pattern for the pleistocene holocene transition from that of the devils hole vein calcite which is considered a proxy for local climate change allows interbasin flow rates of northerly derived groundwater to be estimated the constrained rates 75 0 120 m yr are slightly higher than those previously calculated using darcy s law but translate into hydraulic conductivity values strikingly similar to those obtained from pump tests data further indicate that production wells located closer to the western mountainous margin will be producing mainly from locally derived mountain system recharge whereas wells located closer to the eastern margin are more influenced by older regionally derived carbonate groundwater

sources and fractionation processes influencing the isotopic distribution of h o and c in the long valley hydrothermal system california u s a abstract the isotopic ratios of h o and c in water within the long valley caldera california reflect input from sources external to the hydrothermal reservoir a decrease in d in precipitation of 0 5 km 1 from west to east across long valley is caused by the introduction of less fractionated marine moisture through a low elevation embayment in the sierra nevada mountain range relative to seasonal fluctuations in precipitation 158 to 35 d ranges in hot and cold surface and groundwaters are much less variable 135 to 105 only winter and spring moisture reflecting higher precipitation rates with lighter isotopic signatures recharge the hydrological system the hydrothermal fluids are mixtures of isotopically heavy recharge d 115 18 o 15 derived from the mammoth embayment and isotopically lighter cold water d 135 18 o 18 this cold water is not representative of current local recharge the 13 c values for dissolved carbon in hot water are significantly heavier 7 to 3 than in cold water 18 to 10 denoting a separate hydrothermal origin these 13 c values overlie the range generally attributed to magmatic degassing of co 2 however 13 c values of metamorphosed paleozoic basement carbonates surrounding long valley fall in a similar range indicating that hydrothermal decarbonization reactions are a probable source of co 2 the 13 c and 18 o values of secondary travertime and vein calcite indicate respective fractionation with co 2 and h 2 o at temperatures approximating current hydrothermal conditions

rare earth element fractionation and concentration variations along a groundwater flow path within a shallow basin fill aquifer southern nevada usa abstract rare earth element ree concentrations were measured in 5 well water samples and 3 springs located along a groundwater flow path in a shallow tuffaceous alluvial aquifer from southern nevada usa the ree concentrations in these groundwaters decrease in the direction of groundwater flow a previous investigation demonstrated that ree solid liquid phase partitioning coefficients i e kd s for groundwaters from tuffaceous alluvial aquifers in southern nevada are relatively high mean kd 102 6 our groundwater ree data in conjunction with these kd s support strong sorption of aqueous rees to aquifer surface sites as the primary removal mechanism of rees from these groundwaters in addition relatively high aqueous ree concentrations occur at distinct locations along the groundwater flow path the elevated ree concentrations are explained by addition of deeper groundwaters influx of geothermal waters from a hot spring system differences in solution complexation and or mixtures of regional and local recharge sources solution complexation modelling of rees in the groundwaters indicate that carbonate complexes account for more than 99 of each rees in solution moreover groundwater yb nd ratios a measure of ree fractionation are associated with alkalinity hco3 co32 r 0 71 the data and speciation model results indicate that ree fractionation i e the observed heavy ree hree enrichments compared to rock sources is controlled by formation of progressively stronger carbonate complexes in solution with increasing atomic number which inhibits hree sorption compared to light rees lree and a greater affinity for the lrees to sorb to surface sites in the local tuffaceous alluvial aquifers compared to the hrees

zirconological tracing of transition between aqueous fluid and hydrous melt in the crust constraints from pegmatite vein and host gneiss in the sulu orogen abstract zircon can grow from aqueous fluid and hydrous melt in crustal rocks when they undergo metamorphic dehydration and partial melting under high pressure hp to ultrahigh pressure uhp conditions a distinction between fluid and melt has important bearing on the development of crustal anatexis in collisional orogens a genetic transition between fluid and melt is recorded by zircons from pegmatite vein and host uhp gneiss in the sulu orogen this transition is elucidated by an integrated study of sims u pb and o isotope analyses with la icpms u pb isotope and trace element analyses in the zircons pegmatite veins yield zircon u pb ages of 147 153 ma for new growths and 700 800 ma for relict cores a one meter width pegmatite vein exhibits two episodes of zircon growth at 153 3 ma for inner domain and at 147 2 ma for outer domain the two types of domains have a series of differences in cl image inclusion type ree content and pattern trace element contents and ratios and ti in zircon temperature the inner domain grew from the hydrous melt at 730 840 c whereas the outer domain grew from the aqueous fluid at 520 650 c nevertheless they have similarly low 18o values of 1 0 2 3 suggesting their growth from o isotope homogeneous media despite the transition from melt to fluid the host gneiss of this pegmatite vein exhibits three generations of zircon growth with 180 205 ma at 700 770 c during amphibolite facies metamorphism 157 3 ma at 610 670 c for fluid assisted growth and 147 2 ma at 780 860 c for melt assisted growth the last two types of growth record the development of crustal anatexis from hydration melting to dehydration melting the other decimeter width pegmatite vein exhibits only one episode of zircon growth from melt at 149 2 ma and 660 860 c its host gneiss exhibits a residual melt texture but no significant growth of zircon indicating low degree of dehydration melting therefore the zircon domains in the host gneisses record their growth during a transition from aqueous fluid to hydrous melt along a temperature increasing path whereas the zircon domains in the pegmatite veins record their growth during a transition from hydrous melt to aqueous fluid along a temperature decreasing path despite the difference in the direction of fluid melt evolution the all zircon domains in these gneisses and pegmatite veins record the same event of crustal anatexis in the uhp gneisses the present study also provides insights into a genetic definition of anatectic melt and thus anatectic zircon

multivariate statistical analysis of arsenic and selenium concentrations in groundwaters from south central nevada and death valley california abstract arsenic and selenium concentrations along with the major solutes were measured in ground waters sampled from springs in pahranagat valley and ash meadows nevada death valley california and from wells from the nevada test site and yucca mountain nevada the multivariate statistical technique correspondence analysis was applied to the data to determine relationships between the groundwaters from these areas the aquifer materials and the as and se concentrations and to examine the relationships between as and se and the other chemical parameters included in the statistical analysis the correspondence analysis indicates that a strong relationship exists between chloride and se in the groundwaters and that as is not associated with chloride the strong association between chloride and se suggests that se behaves more conservatively than as in these oxygenated circumneutral ph groundwaters no strong association was observed between the as and or se concentrations of the groundwaters and the aquifer material with which these waters interact i e regional paleozoic carbonate aquifer tertiary tuffaceous volcanic rocks and or basin fill deposits however it is likely that sampling of groundwaters from the various aquifers was insufficient to determine relationships between the aquifer materials and groundwater chemistry associations were observed between the groundwaters of the nevada test site and yucca mountain regions and the groundwaters of the regional carbonate aquifer that discharge at ash meadows and death valley suggesting mixing of these waters ground water from the regional carbonate aquifer in pahranagat valley which is upgradient from the nevada test site yucca mountain ash meadows and death valley exhibited no association with groundwaters from these regions

using multivariate statistical analysis of groundwater major cation and trace element concentrations to evaluate groundwater flow in a regional aquifer groundwater samples were collected from 11 springs in ash meadows national wildlife refuge in southern nevada and seven springs from death valley national park in eastern california concentrations of the major cations ca mg na and k and 45 trace elements were determined in these groundwater samples the resultant data were subjected to evaluation via the multivariate statistical technique principal components analysis pca to investigate the chemical relationships between the ash meadows and death valley spring waters to evaluate whether the results of the pca support those of previous hydrogeological and isotopic studies and to determine if pca can be used to help delineate potential groundwater flow patterns based on the chemical compositions of groundwaters the results of the pca indicated that groundwaters from the regional paleozoic carbonate aquifers all of the ash meadows springs and four springs from the furnace creek region of death valley exhibited strong statistical associations whereas other death valley groundwaters were chemically different the results of the pca support earlier studies where potentiometric head levels 18o and d geological relationships and rare earth element data were used to evaluate groundwater flow which suggest groundwater flows from ash meadows to the furnace creek springs in death valley the pca suggests that furnace creek groundwaters are moderately concentrated ash meadows groundwater reflecting longer aquifer residence times for the furnace creek groundwaters moreover pca indicates that groundwater may flow from springs in the region surrounding scotty s castle in death valley national park to a spring discharging on the valley floor the study indicates that pca may provide rapid and relatively cost effective methods to assess possible groundwater flow regimes in systems that have not been previously investigated copyright 1999 john wiley sons ltd

search for traces of the late heavy bombardment on earth results from high precision chromium isotopes abstract high precision mass spectrometric analyses of the chromium isotopic composition of metamorphosed turbiditic and pelagic sedimentary rocks and banded iron stones from the 3 7 gyr isua supracrustal belt isb in west greenland cannot be distinguished from the standard terrestrial 53 cr 52 cr ratio at our present level of resolution as a consequence our search for chemical traces of possible impact derived meteoritic components asteroidal and or cometary material or accreted cosmic dust in the earth s oldest chemical and detrital sediments was negative our results based on the 53 mn 53 cr short lived radionuclide system half life of 3 7 myr cannot confirm the recent findings by 1 r schoenberg b s kamber k d collerson s moorbath tungsten isotope evidence from approximately 3 8 gyr metamorphosed sediments for early meteorite bombardment of the earth nature 418 2002 403 405 of tungsten isotope anomalies based on the 182 hf 182 w short lived radionuclide system half life of 9 myr in these sediments which were interpreted as indicating a component derived from meteorites possible reasons for the failure to trace cosmic material in the isb metasediments are various 1 the samples studied are not representative 2 the sedimentation period did not overlap with the period of late heavy bombardment of the moon and 3 the potential chromium anomalies if present are too small to be traceable by our present levels of detection unequivocal evidence of a late heavy bombardment on the early earth therefore remains elusive and uncertain

synexhumation anatexis of ultrahigh pressure metamorphic rocks petrological evidence from granitic gneiss in the sulu orogen abstract petrological evidence is provided for partial melting of ultrahigh pressure uhp metamorphic granitic gneiss in the sulu orogen petrographic observations show the occurrence of elongated highly cuspate feldspars in grain boundaries interstitial cuspate feldspars in triple junctions felsic veinlets mainly consisting of k feldspar quartz and feldspar crystal faces against quartz these features indicate that the feldspar and quartz would have grown from anatectic melts in the granitic gneiss zircon domains grown from these melts were identified based on cl images mineral inclusions and ree patterns some large zircon grains 100 m contain small relict domains of magmatic origin suggesting nearly complete dissolution of the protolith zircon during the anatexis all newly grown zircon domains are categorized into two groups based on the presence or absence of coesite inclusions one group of domains contain no coesite inclusion and exhibit high u contents but low th u ratios pb ages of 217 2 to 224 2 ma the other group of domains contain coesite inclusions and exhibit low u contents and very low th u ratios pb ages of 221 5 to 226 3 ma the two groups of zircon domains are thus interpreted as growing from the anatectic melts at different pressures because they exhibit marked negative eu anomalies that are absent for metamorphic zircons grown from aqueous fluids the zircon u pb ages of 217 2 to 226 3 ma are close to but slightly younger than known ages for major uhp metamorphism in the sulu orogen therefore the uhp gneiss experienced incipient melting during the initial exhumation subsequent to the peak uhp metamorphism and extensive anatexis later at lower pressures muscovite relicts coexist with cuspate feldspars suggesting that the anatectic melts originate from dehydration melting due to decompression breakdown of phengitic muscovite in the uhp granitic gneiss as the degree of partial melting increases with temperature significant fractions of the anatectic melts would be produced in the regional gneiss provided that the uhp rocks experienced hot exhumation such melts can be gathered together being eventually emplaced as synexhumation granitic intrusions

weathering on land and transport of chromium to the ocean in a subtropical region misiones nw argentina a chromium stable isotope perspective abstract we have investigated the pathway of chromium from its mobilization on land and along its riverine transport in a subtropical region of south america misiones province argentina in an attempt to link cr stable isotope compositions recently measured in seawater with signals prevailing in rivers and ultimately with cr isotope effects observed during oxidative surface weathering in subtropical red soils cr concentrations and stable cr isotopic compositions expressed as 53cr values in two typical and representative surface profiles of weathered basalt show significant depletion of cr in the soils of up to 50 together with pronounced negatively fractionated 53cr values which are indicative of oxidative mobilization of heavy cr vi into the run off the behavior of cr in the studied weathering profiles is not correlated with that of other redox sensitive elements such as ce and u this is essentially due to the affinity of ree and u but not cr with secondary phosphates which form during weathering processes smaller tributaries in nw argentina to the parana river second largest river in south america carry dissolved cr in the order of 0 7 1 4 ppb 13 27 nm with 53cr values of 0 2 to 0 4 balancing the negatively fractionated weathering products the isotope composition and concentration of dissolved cr in the ca 1200 km long parana river from misiones to its estuary and discharge area into the south atlantic ocean remains relatively constant with an average cr concentration around 2 4 ppb 46 nm and an average 53cr value of 0 32 the cr concentration in the estuary itself drops by ca 50 but with only minor change in its cr isotope composition results from the parana estuary are identical with recently analyzed surface seawater from the argentine basin with cr contents of 0 3 ppb 6 nm and 53cr values 0 4 bonnand et al 2013 and indicate that there is only a minimal cr isotopic variability during riverine transport even during long transport distances as shown in our example of the parana river simple cr input flux calculations reveal that the parana river accounts for 5 of the total yearly cr flux to the world s oceans today and that its isotopic signature seems to be at least locally imparted to the surface seawater of the argentine basin whether or not this cr isotope signature is generally exhibited by the world s oceans needs further investigations particularly the characterization of seawater around the globe

regional groundwater flow in structurally complex extended terranes an evaluation of the sources of discharge at ash meadows nevada summary ash meadows nevada usa is a site of major groundwater discharge 38 000 l min in the arid mojave desert and hosts a number of endemic and threatened wetland species in addition to these resources ash meadows may also represent the future discharge location of radionuclide laden waters from nuclear weapons testing at the nevada test site more importantly however ash meadows provides the opportunity to understand the controls on water transfer between basins through fractured bedrock 4000 solute analyses were assembled from the literature into a single database the data were screened for spatial distribution completeness charge balance and elevated temperatures 20 c and within regional flow systems with 246 candidate up gradient water remaining distributed among six potential source areas in addition to and ash meadows itself these potential sources include both carbonate volcanic and perhaps valley fill aquifer systems these waters were characterized by cluster analysis in order to sort similar waters in an objective fashion into potential flow paths and to establish representative endmember waters for inverse geochemical models and other modes of analysis isotopic tracers both conservative and those reflecting water rock interaction all suggest that waters at ash meadows are derived by southward flow from volcanic terranes parallel to the preferred permeability structure induced by active regional east west extension solute balances support this conclusion however this runs counter to the prevailing model that waters at ash meadows are derived from easterly and northeasterly flows from the spring mountains and pahranagat valley areas by interbasin flow through a continuous fractured carbonate aquifer this work suggests that carbonate aquifer systems in extended terranes are more compartmentalized than previously appreciated and that anisotropy in fracture permeability is key to compartmentalization and the control of flow directions

a rapid and synchronous initiation of the wide spread cryogenian glaciations abstract the neoproterozoic glaciations represent a remarkable milestone in earth evolution due to their major influence on atmosphere biosphere and hydrosphere the early cryogenian sturtian glaciations evidence for which has been recorded worldwide may have lasted for about 50 million years but the precise timing and synchroneity of these glaciations have been disputed jiangkou glacigenic strata in south china can be attributed to the sturtian glacial episode as they share sedimentological stratigraphic and lithological similarities with sturtian strata in south australia as such dating the onset of the jiangkou glaciation would contribute to testing the synchroneity of the worldwide sturtian glaciation in this paper we demonstrate by means of sims u pb zircon dating that the jiangkou glaciation in south china was initiated at 715 9 2 8 ma the new age is within uncertainty synchronous with dates obtained from the lower parts of glacigenic successions in laurentia and oman thereby indicating thereby indicating a synchroneity and global extent to the sturtian glaciation the synchronous occurrence of the sturtian glaciation in mid to low latitude regions implies rapid ice sheet advancement in response to ice albedo feedback as supported by numerical climatic modeling

provenance and fate of arsenic and other solutes in the chaco pampean plain of the andean foreland argentina from perspectives of hydrogeochemical modeling and regional tectonic setting extensive arsenic as enriched groundwater is known to occur in the aquifers of the chaco pampean plain of argentina previous studies speculated that the as mobilization in these groundwaters was a direct result of their elevated ph and oxidative conditions the volcanic glass layers present in the aquifer matrix were hypothesized as one of the possible sources of as to the groundwaters here we examine the groundwater chemistry of the santiago del estero province of chaco pampean plains of argentina and test these hypotheses by using hydrogeochemical modeling within the framework of the regional geologic tectonic setting the study area is located in the active foreland of the andean orogenic belt which forms a continental arc setting and is dotted with several hot springs rhyolitic volcanic glass fragments derived from arc volcanism are abundant within the aeolian fluvial aquifer sediments and are related to the paleo igneous extrusion in the vicinity hydrogeochemical analyses show that the groundwater is in predominantly oxidative condition in addition some of the groundwaters exhibit very high na cl and so42 concentrations it is hypothesized in this study that the groundwater chemistry has largely evolved by dissolution of rhyolitic volcanic glass fragments contained within the aquifer sediments along with mixing with saline surface waters from adjoining salinas which are thought to be partially evaporated remnants of a paleo inland sea flow path modeling stability diagrams and thermodynamic analyses undertaken in this study indicate that the dominant evolutionary processes include ion exchange reactions chemical weathering of silicate and evaporites in monosialitization dominated weathering geochemical modeling predicts that plagioclase feldspar and volcanic glass are the major solids phases that contribute metal cations and dissolved silica to the local groundwaters co influxed oxyanions with similar ionic radii and structure e g mo si v po43 compete with as for mineral surface site leading to as mobilization to the groundwaters without considering the influence of microbial activities further the transition of the ca rich groundwater to na rich groundwater by mixing with water from the salinas and or evaporative concentration might also have led to counter ion effects a type of ion exchange reactions and hence further enrichment of groundwater by as some of the as may also have been contributed from mixing of meteoric water with magmatic sourced water in the geothermal springs

zr complexation in high pressure fluids and silicate melts and implications for the mobilization of hfse in subduction zones abstract field observations and solubility experiments show evidence for the efficient mobilization of nominally insoluble hfse i e ti zr nb and hf by high pressure fluids probably via complexation with polymerized alkali silica dissolved species and halogens f and cl here we investigate the complexation of zr in subduction related fluids aqueous fluids and hydrous haplogranite melts up to 800 c and 2 4 gpa using x ray absorption spectroscopy xanes and exafs in a hydrothermal diamond anvil cell and provide evidence for the formation of zr o si na polymeric species in alkali alumino silicate fluids at high pressure zr 4 speciation in dilute fluids 2 5 wt hcl is dominated by 8 fold coordinated zr h 2 o 8 4 hydrated complexes at room conditions and no evidence for extensive zr cl complexation in the fluid was found up to 420 c as confirmed by ab initio xanes calculations of various zro 8 x cl x clusters the addition of na and si dissolved species from 35 to 60 wt dissolved na 2 si 2 o 5 ns2 into the fluid favors the formation of alkali zirconosilicate clusters zr o si na similar to those found in vlasovite na 2 zrsi 4 o 11 with zr 4 in octahedral coordination with oxygen zr o distance 2 09 0 04 a and 6 si na second neighbors zr si na distance 3 66 0 06 a this coordination environment also dominates zr speciation in f free and f bearing ns2 and haplogranite glasses and high pressure hydrous haplogranite melts 15 5 33 wt dissolved h 2 o in the investigated pressure temperature range the xas analyses assisted by ab initio xanes calculations are not conclusive concerning the extent of zr f complexation in hydrous granitic melts alkali zirconosilicate zr o si na clusters such as those identified in this study may explain the enhanced solubility of zircon zrsio 4 and other hfse bearing minerals in alkali aluminosilicate bearing aqueous fluids produced by dehydration and melting of the slab and provide a favorable mechanism for the mobilization of hfse in subduction zones fluid rock interactions and or p t variations as fluids migrate through the mantle wedge could affect the stability of these complexes triggering the precipitation of hfse bearing accessory phases that are eventually recycled into the mantle contributing to the dispersion of hfse these processes provide a possible explanation for the characteristic hfse depletion recorded in arc magmas

co evolution of eukaryotes and ocean oxygenation in the neoproterozoic era the oxygenation of the earth s deep oceans is often thought to have triggered the evolution of simple animals a review article proposes that instead the evolution of animal life set off a series of biogeochemical feedbacks that promoted the oxygenation of the deep sea

re os evidence for the age and origin of peridotites from the dabie sulu ultrahigh pressure metamorphic belt china serpentinized garnet peridotites from the xugou peridotite body of the sulu ultrahigh pressure uhp metamorphic terrane central eastern china are refractory olivines have fo91 7 93 1 indicating their origin as residual mantle negative correlations between whole rock mgo and tio2 a l2o3 total fe2o3 and cao r 0 90 to 0 95 and positive correlations between wholerock al2o3 and cao and incompatible elements li v cu ga sr y zr heavy rare earth elements hrees hf pb and u r 0 69 to 0 98 likely reflect melt depletion trends four highly refractory samples were selected for re os isotopic analysis although they show evidence of variable enrichment of incompatible elements during serpentinization metasomatism no correlations exist between 187 re 188 os or 187 os 188 os with either la or re r 0 00 to 0 17 these results indicate that any re addition was fairly recent and did not affect the os isotopic composition significantly the correlation between 187 os 188 os and 187 re 188 os ratios thus most likely reflects an ancient melt extraction event the trd tma and errorchron ages of the xugou peridotites are all similar suggesting that these peridotites formed around 2 0 ga ago this age is similar to os model ages of mantle peridotites from the dabie terrane but contrasts markedly with the archean ages of the continental lithospheric mantle clm beneath the eastern block of the north china craton ncc if we assume that the dabie sulu belt formed by the triassic collision of the yangtze craton with the eastern block of ncc and that the archean aged clm of the latter persisted until the triassic the paleoproterozoic ages suggest derivation of these dabie sulu mantle peridotites from the yangtze craton a yangtze craton origin is consistent with the existing tectonic model of the dabie sulu uhp belt our results support the hypothesis that the crust and underlying lithospheric mantle of the yangtze craton were subducted to depths of n180 200 km to form the world s largest uhp belt

record of multiple stage channelized fluid and melt activities in deeply subducted slab from zircon u pb age and hf o isotope compositions abstract geochemical features of island arc rocks require two slab derived components of a melt and a fluid added to their sources these two liquids may unite to an intermediate aqueous melt at the base of volcanic arc direct evidence of this process has been scarce yet here we present an integrated study of in situ u pb age trace element and o hf isotope for zircons from a suite of a quartz vein a felsic vein their host eclogite and surrounding gneiss at the xitieshan uhp terrane of the north qaidam metamorphic belt nqmb oscillatory zoned zircons from the quartz vein are enriched in u li nb and ree with a formation age of 442 6 ma consistent with the timing of the peak uhp metamorphism this demonstrates there is no temporal gap between the uhp metamorphism and fluid flow in the xitieshan terrane in particular the oscillatory zoned zircons of the quartz vein have similar hf isotope compositions to zircons from the surrounding gneiss but variable o isotope compositions between the host eclogite and surrounding gneiss the fluid from surrounded gneiss was thus an aqueous melt with high transport ability for lu and hf and o while the fluid from the host eclogite was an aqueous fluid only with the ability to transport o according to the similar behaviors of granitic rock and sediment it is inferred that a channelized hydrous melt can be formed in the downgoing sediment layer where an aqueous fluid can be formed from underlying basalt and or peridotite layers at the sub arc depth the two types of fluid are fed to the overlying mantle wedge from subducting slab to form island arc rock by contrast the zircons in the felsic vein yield a weighted mean 206pb 238u age of 420 4 ma which can be regard as the timing of partial melting during the exhumed stage for the xitieshan uhp rocks the zircon o and hf isotopic features of the felsic vein are between the country gneiss and host eclogite compatible with their emanating from hydrous melt of both the eclogite and gneiss during the exhumation stage of the uhp rocks this indicates that hydrous melt would be released from both the sediment and basalt or peridotite layers of the subducting slabs at relatively shallow depth

hydrogen and oxygen isotope evidence for fluid rock interactions in the stages of pre and post uhp metamorphism in the dabie mountains abstract hydrogen and oxygen isotope studies were carried out on high and ultrahigh pressure metamorphic rocks in the eastern dabie mountains china the 18 o values of eclogites cover a wide range of 4 2 to 8 8 but the d values of micas from the eclogites fall within a narrow range of 87 to 71 both equilibrium and disequilibrium oxygen isotope fractionations were observed between quartz and the other minerals with reversed fractionations between omphacite and garnet in some eclogite samples the 18 o values of 4 to 1 for some of the eclogites represent the oxygen isotope compositions of their protoliths which underwent meteoric water rock interaction before the high to ultrahigh pressure metamorphism heterogeneous 18 o values for the eclogite protoliths implies not only the varying degrees of the water rock interaction before the metamorphism at different localities but also the channelized flow of fluids during progressive metamorphism due to rapid plate subduction retrograde metamorphism caused oxygen and hydrogen isotope disequilibria between some of the minerals but the fluid for retrograde reactions was internally buffered in the stable isotope compositions and could be derived from structural hydroxyls dissolved in nominally anhydrous minerals

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

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

hydrochemistry and isotope composition of springs in the tecopa basin southeastern california usa abstract springs and seeps in the tecopa basin were sampled and analyzed for their major ion chemistry and oxygen hydrogen and boron isotope compositions in order to assess the possible origins of the waters the tecopa basin is a groundwater discharge area along the amargosa river drainage basin south of the nevada test site and proposed high level nuclear waste repository at yucca mountain nv the spring waters are categorized into three end member hydrochemical facies 1 ph neutral na ca mg so4 hco3 water with moderate to high total dissolved solids 2 mildly alkaline na cl so4 hco3 water with high total dissolved solids and 3 alkaline na hco3 co3 water with high total dissolved solids the temperatures of most of the springs range from 17 c to 32 c however waters in the tecopa hot spring area range from 9 c to 47 c the oxygen and hydrogen isotope compositions normalized to smow of the waters are as follows the facies 1 springs have a mean 18o of 12 77 0 14 1 and d of 94 9 0 5 1 the facies 2 springs have a mean 18o of 12 73 0 19 1 and d of 97 7 0 9 1 the facies 3 spring has a mean 18o of 13 09 0 17 1 and d of 103 5 0 7 1 the mean boron concentrations and isotope compositions range from 2 4 mg l and 11b of 2 5 for facies 1 springs to 72 8 mg l and 11b of 3 8 for the facies 3 spring the facies 1 waters have chemical and isotope compositions consistent with meteoric recharge in the spring mountains nv groundwater flow through the regional carbonate aquifer and minor mixing with more concentrated water the thermal facies 2 waters are isotopically and chemically distinct from waters in the regional basin fill and carbonate aquifer systems these waters are interpreted to originate from fractured water bearing strata beneath the regional carbonate aquifer that discharge only in the southernmost part of the great basin the facies 3 spring is isotopically and chemically similar to basin fill aquifer waters in the amargosa desert but has higher sodium alkalinity and boron contents due to interaction with saline alkaline lake deposits of the plio pleistocene lake tecopa beds geologic geochemical and isotope data suggest that these three distinct water types flow through hydrogeologic compartments in the tecopa basin the compartments are interpreted to have been created by the impermeable lake tecopa beds and geologic structures within the basin

petrology of ultrahigh pressure rocks from the southern su lu region eastern china abstract in the su lu ultrahigh p terrane eastern china many coesite bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet quartz jadeite rock and kyanite quartzite in addition to garnet omphacite rutile coesite other peak stage minerals in some eclogites include kyanite phengite epidote zoisite talc nyboite and high al titanite the garnet quartz jadeite rock and kyanite quartzite contain jadeite quartz garnet rutile zoisite apatite and quartz kyanite garnet epidote phengite rutile omphacite assemblages respectively coesite and quartz pseudomorphs after coesite occur as inclusions in garnet omphacite jadeite kyanite and epidote from both eclogites and metasediments study of major elements indicates that the protolith of the garnet quartz jadeite rock and the kyanite quartzite was supracrustal sediments most eclogites have basaltic composition some have experienced variable crustal contamination or metasomatism and others may have had a basaltic tuff or pyroclastic rock protolith the su lu ultrahigh p rocks have been subjected to multi stage recrystallization and exhibit a clockwise p t path inclusion assemblages within garnet record a pre eclogite epidote amphibolite facies metamorphic event ultrahigh p peak metamorphism took place at 700 890 c and p 28 kbar at c 210 230 ma the symplectitic assemblage plagioclase hornblende epidote biotite titanite implies amphibolite facies retrogressive metamorphism during exhumation at c 180 200 ma metasedimentary and metamafic lithologies have similar p t paths several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in situ ultrahigh p metamorphism during the triassic collision between the sino korean and yangtze cratons

metamorphic zirconology of continental subduction zones abstract zircon is widely used to date geological events and trace geochemical sources in high pressure hp to ultrahigh pressure uhp metamorphic rocks of continental subduction zones however protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid state transformation metasomatic alteration and dissolution reprecipitation new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks metamorphic zircon or peritectic reactions above the wet solidus peritectic zircon as a consequence there are different origins of zircon domains in high grade metamorphic rocks from collisional orogens thus determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing we advocate an integrated study of zircon mineragraphy internal structure and external morphology u pb ages mineral inclusions trace elements and lu hf and o isotope compositions only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction zone conditions the metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in hp to uhp metamorphic rocks of collisional orogens these different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions while the protolith nature of metamorphosed rocks dictates water availability the p t conditions of subduction zones dictate the property of subduction zone fluids the fluids of different properties may be produced at different positions of subducting and exhuming crustal slices and they may physically and chemically mix with each other in continental subduction channels such fluids can act as an important agent not only for the physical transport of protolith zircons but also for the chemical transport of element zr and other fluid mobile incompatible trace elements from the subducted crust to the mantle wedge therefore the discrimination between the different types of zircons provides a powerful means to decipher the role of fluids in subduction zone processes