primary platinum mineralization in the nizhny tagil and kachkanar ultramafic complexes urals russia a genetic model for pge concentration in chromite rich zones the nizhny tagil uralian alaskan type ultramafic complex is composed of a dunite core with a rim of wehrlite and pyroxenite the dunite unit which has a uniform mineral composition olivine fo 89 8 92 1 avg 0 25 wt cao represents an accumulation of olivine and minor chromite from a mafic magma in an open system magma chamber after solidification of the dunite differentiation began with the formation of the wehrlite olivine fo 77 9 88 8 and pyroxenite corresponding to the closure of the magmatic system two main types of chromitite occurrence are observed in the dunite body small 100 5 cm scattered schlieren of chromite crystals within the ultramafic unit and much larger up to 100 5 m concentrations the chromite composition in both types is similar and uniform cr cr al fe 3 66 02 mg mg fe 2 49 05 and cr cr al fe 3 69 14 mg mg fe 2 49 81 respectively the larger concentrations of chromite include brecciated and net textured ore or massive veins that reflect the dynamic accumulation of chromite crystals in cavities along magma conduits inside the dunite body these cavities are analogous to small magma chambers within the consolidated part of the dunite body and similar to podiform chromitites in ophiolites were created by continuous feeding of magma to the system the platinum group element pge content of the chromite free facies of the dunite is relatively low pt being the only significant pge typically between 0 3 35 ppb but as much as 98 ppb in one sample in contrast both types of chromite occurrence are enriched in pge two pge mineralizing episodes are distinguished an ir ru rh os rich pt pd poor episode in which the platinum group minerals pgm are mainly ir alloys and a pt pd rich episode in which the pgm are mainly pt alloys i e pt 2 fe ni cu ptfeni 2 and tetraferroplatinum and tulameenite like compositions along with rare hollingworthite and an unidentified rhnias mineral the primary pgm have been affected locally by a serpentinization related low temperature alteration that is characterized by addition of copper with alteration to complex pt fe ni cu alloys of varied stoichiometry including tulameenite and platinian copper placer concentrations derived from the nizhny tagil complex have a slightly different assemblage of pgm with abundant isoferroplatinum the presence of gold commonly aggregating the pgm and unusual au sn and au pb alloys the kachkanar complex situated 120 km north of the nizhny tagil complex has essentially the same characteristics apart from a slightly different composition of the ultramafic cumulates suggesting that they were derived from a magma of different composition the pgm distribution also differs in that it contains more abundant isoferroplatinum than other pgm the high pt concentrations are the result of the affinity of platinum minerals for chromite and the mode of chromite deposit formation in the cavities in which chromite crystallized pgm tended to be incorporated in chromite or remained attached to chromite crystals to explain the very high concentrations of chromite associated pgm we define a cr factor which is the ratio of the mass of silicate magma to the mass of chromite in contact with the ascending magma where the cr is high i e a relatively low proportion of chromite in contact with a large amount of magma crystallizing the pt minerals the efficiency of the mechanical collection of pt minerals is at a maximum and the pt concentration i e amount of pt minerals will be high possible variations due to the collection of pge by a sulfide liquid have not been established a model is proposed whereby the observed chromite and associated pt mineralization can result from the normal magmatic evolution of the complex provided that a structural hydrodynamic trap is available to concentrate the chromite

characterization of dolomitic rocks from the coastal mixing zone of the floridan aquifer florida u s a abstract petrographic xrd sem cathodoluminescence and microprobe analyses were conducted to characterize dolomitic strata from a core penetrating the coastal mixing zone of the floridan aquifer lithologic geochemical and water chemistry data were used to identify diagenetic trends including modern water rock interaction the depositional environment of these middle eocene carbonate sediments of the avon park formation was intertidal to supratidal sabkha conditions caused penecontemporaneous deposition of evaporites and accessory minerals present water chemistry indicates undersaturation for gypsum and its survival is attributed to sluggish groundwater circulation dolomite crystals were commonly found to be compositionally zoned with crystal centers generally calcium rich and less stable than the more stoichiometric crystal rims three stages of dissolution were recognized from sem studies of these crystals 1 initial development of dissolution pits 2 partial dissolution of the unstable crystal center and 3 complete dissolution of the unstable crystal center leaving hollow dolomite present day mixing zone water is saturated with respect to stoichiometric dolomite near stoichiometric dolomite cement may be filling pores in this zone and non stoichiometric dolomite may be dissolving the instability of calcian dolomite in the mixing zone may be related to compositionally modulated crystal lattice defects pervasive dissolved crystal centers in the saline zone are believed to have been highly disordered penecontemporaneous dolomite stoichiometric dolomite does not appear to be precipitating in this zone because of the presence of competing ions stable carbon and oxygen isotopic values for dolomite are comparable to those reported for plio pleistocene dolomites 14 c activity implies some relatively young dolomite exists probably as rims on older crystals

analytical solutions of seawater intrusion in sloping confined and unconfined coastal aquifers sloping coastal aquifers in reality are ubiquitous and well documented steady state sharp interface analytical solutions for describing seawater intrusion in sloping confined and unconfined coastal aquifers are developed based on the dupuit forchheimer approximation specifically analytical solutions based on the constant flux inland boundary condition are derived by solving the discharge equation for the interface zone with the continuity conditions of the head and flux applied at the interface between the freshwater zone and the interface zone analytical solutions for the constant head inland boundary are then obtained by developing the relationship between the inland freshwater flux and hydraulic head and combining this relationship with the solutions of the constant flux inland boundary it is found that for the constant flux inland boundary the shape of the saltwater interface is independent of the geometry of the bottom confining layer for both aquifer types despite that the geometry of the bottom confining layer determines the location of the interface tip this is attributed to that the hydraulic head at the interface is identical to that of the coastal boundary so the shape of the bed below the interface is irrelevant to the interface position moreover developed analytical solutions with an empirical factor on the density factor are in good agreement with the results of variable density flow numerical modelling analytical solutions developed in this study provide a powerful tool for assessment of seawater intrusion in sloping coastal aquifers as well as in coastal aquifers with a known freshwater flux but an arbitrary geometry of the bottom confining layer this article is protected by copyright all rights reserved

saltwater circulation patterns within the freshwater saltwater interface in coastal aquifers laboratory experiments and numerical modeling summary groundwater flow patterns within the freshwater saltwater interface in coastal aquifers include rotation in the flow direction of saltwater that originates from the sea and circulates in the aquifer using two types of tracer experiments we analyze the configuration of the rotating flow lines the experimental results are numerically reconstructed and quantitatively compared to the salinity distribution along the interface the results show that the rotation in the direction of the saltwater flow lines begins at the lowermost part of the interface i e contour c cmax 99 and completes within its lower tenth contour c cmax 94 at the upper part of the interface after the rotation is completed the flow is dictated by the freshwater flow seaward based on these results the well known chemical freshwater saltwater interface is divided into two different parts defined by their physical properties 1 the lower part is the flow rotation region defined by convective circulating flow lines and 2 the upper part is the dispersive region defined by dispersive dilution sensitivity analysis shows that the physical configuration of the interface depends on the transversal dispersivities at higher dispersivities the rotation width increases but completes within the lower third of the interface at most the sensitivity analysis also shows that the rotation begins at the lowermost part of the interface for dispersivities therefore since no flow occurs below a line of 99 the saline water that flows seaward is always diluted with respect to its original salinity these flow patterns might affect coastal processes such as submarine groundwater discharge sgd and chemicals transport through the aquifer

anomalous sulfur poor platinum group element mineralization in komatiitic cumulates mount clifford western australia an unusual occurrence of sulfur poor platinum group element pge mineralization has been discovered at mount clifford in the agnew wiluna belt western australia within the lower part of a 1 000 m thick komatiitic dunite body the host body consists primarily of strongly serpentinized olivine adcumulates with accessory chromite high precision whole rock icp ms data show that the pge mineralization extends over tens of meters whereas the lateral extent is unknown palladium and pt abundances are closely associated with base metal sulfides but show offset distribution patterns with maximum concentrations associated with the relatively sulfide poor base of a disseminated sulfide bearing layer whole rock concentrations range up to 4 100 ppb pd and 920 ppb pt iridium rh and ru follow the trends in pd and pt but are strongly depleted ir 0 4 ppb ru 0 5 ppb rh 0 1 ppb platinum group minerals pgm were located in essentially sulfide free serpentine 2 m below the peak in sulfide abundance and can be categorized into three groups pd antimonides pt dominated pgm and pt bearing ni antimonides the pge mineralization zone is underlain by an interval of strong ni depletion within olivine mesocumulates that is characterized by an unusual negative correlation between ni and the mg number of the rock this relationship is interpreted to be the result of an equilibration of olivine of variable forsterite content with a relatively large mass of magmatic sulfide however there is no direct evidence of this sulfide mass anywhere in the entire mount clifford dunite body the ni depletion is accompanied by strong depletion in ipge ir ru and rh the association of the pge zone with a reversal in the mg fe ratio of the host cumulates and a distinctive offset relationship of multiple pt pd and cu s peaks is suggestive of a primary magmatic origin however this is inconsistent with current models for offset reef pge concentrations which involve the onset of sulfide liquid saturation from a previously sulfur undersaturated pge undepleted magma the association of enriched pt and pd with highly depleted ni and ipge in immediately underlying cumulates conflicts with this model we suggest a combination of processes early sulfide segregation at low r values producing ni and ipge depleted olivine partial redissolution and removal of most of this sulfide component during a major replenishment event and subsequent overprint by a migrating pt pd cu rich fluid

a global climatology of ionospheric irregularities derived from gps radio occultation 1 gps radio occultation measurements from champ grace a and formosat 3 cosmic are used to derive global information on small scale ionospheric irregularities such as sporadic e layers between january 2002 and december 2007 the investigations are based on the analysis of amplitude variations of the gps radio occultation signals the global distribution of ionospheric irregularities shows strong seasonal variations with highest occurrence rates during summer in the middle latitudes the long term data set of champ allows for first climatological studies while the data coverage increases significantly with the combination of champ grace and formosat 3 cosmic measurements this allows for global maps of sporadic e occurrence rates of very high spatial resolution where the influence of the earth s magnetic field becomes visible in global sporadic e maps for the first time

chondritic late accretion to mars and the nature of shergottite reservoirs abstract mars is considered to have formed as a planetary embryo that experienced extensive differentiation early in its history shergottite meteorites preserve evidence for this history and for late accretion events that affected their mantle sources within mars here we report the first coupled 187re 187os 87sr 86sr highly siderophile element hse os ir ru pt pd re and major element abundance dataset for martian shergottites that span a range of mgo contents from 6 4 to 30 3 wt the shergottites range from picro basalt to basaltic andesite compositions have enriched to depleted incompatible trace element compositions and define fractional crystallization trends enabling the determination of hse compatibility for martian magmatism in the order os ir ru pt pd re this order of compatibility is like that defined previously for earth and the moon but the fractionation of strongly compatible os ir and ru appears to take place at higher mgo contents in martian magmas due to early onset of sulfide fractionation in general enriched shergottites have lower mgo contents than intermediate or depleted shergottites and have fractionated hse patterns re pd pt ru ir os and more radiogenic measured 87sr 86sr 0 7127 0 7235 and 187os 188os 0 140 0 247 than intermediate or depleted shergottite meteorites 87sr 86sr 0 7010 0 7132 187os 188os 0 127 0 141 osmium isotope compositions corrected for crystallization age define compositions that are implausibly unradiogenic in some enriched shergottites implying recent mobilization of re in some samples filtering for the effects of alteration and high re os through crystal liquid fractionation leads to a positive correlation between age corrected sr and os isotope compositions mixing between hypothetical martian crustal and mantle reservoirs are unable to generate the observed sr os isotope compositions of shergottites which require either distinct and discrete long term incompatible element depleted and enriched mantle sources or originate from hybridized melting of deep melts with metasomatized martian lithosphere using mgo regression methods we obtain a modified estimate of the bulk silicate mars hse composition of in ng g 1 0 4 re 7 4 pd 9 6 pt 6 2 ru 3 7 ir 4 os and a long term chondritic 187os 188os ratio 0 1312 this result does not permit existing models invoking high pressure and temperature partitioning of the hse instead our estimate implies 0 6 0 7 by mass of late accretion of broadly chondritic material to mars our results indicate that mars could have accreted earlier than earth but that disproportional accretion of large bodies and a relative constant flux of accretion of available materials in the first 50 100 ma of solar system led to the broad similarity in hse abundances between earth and mars

variable density flow and solute transport simulation of regional aquifers containing a narrow freshwater saltwater transition zone variable density flow and solute transport simulation of aquifer systems containing narrow transition zones between freshwater and saltwater requires particular attention to certain aspects of the numerical method and its application to be successful typically only cases involving wide transition zones have been simulated with variable density transport models possibly because of inaccuracies in the modeling approaches used the major components of a successful approach are threefold first functionally consistent approximation of terms involved in fluid velocity calculations is necessary in the case of galerkin finite element methodology a significant modification is required to the standard approach in order to achieve consistency second the simulator must be verified in a particular series of tests the usual tests using henry s problem for verification of density dependent transport simulators are inadequate to check for consistency of the velocity approximations and for the accuracy of simulating flow driven by bouyancy forces third adequately fine spatial discretization is required when applying the simulator for spatial stability of the numerical transport solution and to allow accurate representation of narrow transition zones and the effects of low transverse dispersivity the effectiveness of this approach is demonstrated through simulation of the flow of fresh and saline groundwater in the layered basalt aquifer of southern oahu hawaii the transition zone in this regional flow system is narrow except near the discharge area where it is broadly dispersed simulation of this common situation with an inconsistent approximation gives grossly incorrect results while simulation with a consistent model provides a robust tool for analysis of system hydrology

crystallization of sulfide liquids and the interpretation of ore composition we have been exploring ways to quantitatively assess the extent to which fractionation of sulfide melt has effected variations in composition within magmatic sulfide ore bodies our approach has been to determine by experiment the crystallization paths of sulfide liquids in temperature and composition dimensions in this paper the results of new major element partitioning experiments below 1050 c in the nickel free system are presented and summarized along with new and previous work in the fe ni cu s quaternary the partition coefficients d for cu between monosulfid solid solution mss and sulfide liquid in the ni free system dcu wt cu in mss wt cu in liquid cluster near 0 3 but decrease to nearly 0 1 for cu rich s poor liquids near 1000 c dni also declines with decreasing sulfur content of the liquid but increases with decreasing temperature preliminary data indicate that dni exceeds 1 0 in low ni liquids with greater than 16 wt cu at 1050 c the quality of available data on the

partition coefficients for ni cu pd pt rh and ir between monosulfide solid solution and sulfide liquid and the formation of compositionally zoned ni cu sulfide bodies by fractional crystallization of sulfide liquid many nickel copper sulfide orebodies contain cu and fe rich portions the fe rich ore is generally richer in os ir ru and rh and poorer in pt pd and au than the cu rich ore in komatiite hosted ores ni tends to be concentrated in the cu rich ore whereas in tholeiitic ores it tends to be concentrated in the fe rich ore the origin of this zonation could be due to crystal fractionation of fe rich monosulfide solid solution from a sulfide liquid the crystal fractionation would produce an fe rich cumulate enriched in os ir ru and rh and a fractionated liquid enriched in cu pt pd and au this model can be tested for zoned orebodies by applying experimentally determined partition coefficients for the metals into monosulfide solid solution we have compared our experimental results with those of other workers to show that the partition coefficients are strongly influenced by the sulfur content of the system there is a positive correlation between the partition coefficients and sulfur content of th

an overview of the platinum group element nanoparticles in mantle hosted chromite deposits abstract this paper reviews the occurrence of platinum group elements nanoparticles pge nps in mantle hosted chromite deposits showing that pge nps are more frequently found in these deposits than previously thought a comparison of published experimental data with observations in natural samples reveal that pge nps might form at the pressure 1 0 5 gpa temperature 1200 c and f o 2 fmq 1 conditions in which chromite deposits form in the upper mantle the crystallization of pge nps may take place earlier or simultaneously to the segregation of chromite and or sulfide liquids from silicate melts if the pges are dissolved in the silicate melt the segregation of pge nps could be linked to local changes in f o 2 and f s 2 during the early crystallization of chromite and or olivine mobile crystals of chromite and droplets of sulfide melts entrained in the basaltic parent melt may also play an additional role as physical collectors of pge nps providing a complementary or even an alternative mechanism for the fractionation of pges in high temperature silicate melts furthermore hydrothermal alteration of the chromite deposits during seafloor metamorphism or exhumation i e retrograde metamorphism of upper mantle rocks has also a significant effect on the internal structure of the oxides and sulfides that host the pge nps frequently pge nps are found along the replacement contacts between primary and secondary minerals suggesting that pge nps can also form during low temperature alteration events 200 600 c finally heating events overimposed on chromite deposits previously affected by hydrous metamorphism may enhance fluid infiltration subsequently triggering chemical mineralogical or textural responses in the oxide or sulfide matrices hosting the pge nps promoting coarsening of metal nanoparticles the interplay between these processes can explain the wide spectrum of particle sizes of pge inclusions that are observed in many altered chromite deposits which can vary from a few nanometers to larger than a micron these studies provide evidence that pge nps can eventually form under a wide spectrum of thermal and pressure conditions and show that aqueous fluids may play a relevant role in producing pge nps during metamorphism and metasomatism of mantle hosted chromite deposits

chalcophile and platinum group element pge concentrations in the sulfide minerals from the mccreedy east deposit sudbury canada and the origin of pge in pyrite magmatic sulfide deposits consist of pyrrhotite pentlandite chalcopyrite pyrite and platinum group minerals pgm understanding the distribution of the chalcophile and platinum group element pge concentrations among the base metal sulfide phases and pgm is important both for the petrogenetic models of the ores and for the efficient extraction of the pge typically pyrrhotite and pentlandite host much of the pge except pt which forms pt minerals chalcopyrite does not host pge and the role of pyrite has not been closely investigated the ni cu pge ores from the south range of sudbury are unusual in that sulfarsenide pgm rather than pyrrhotite and pentlandite are the main carrier of pge probably as the result of arsenic contribution to the sulfide liquid by the as bearing metasedimentary footwall rocks in comparison the north range deposits of sudbury such as the mccreedy east deposit have as poor granites in the footwall and the ores commonly contain pyrite our results show that in the pyrrhotite rich ores of the mccreedy east deposit os ir ru rh ipge and re are concentrated in pyrrhotite pentlandite and surprisingly in pyrite this indicates that sulfarsenides which are not present in the ores were not important in concentrating pge in the north range of sudbury palladium is present in pentlandite and together with pt form pgm such as ptpd tebi 2 platinum is also found in pyrite two generations of pyrite are present one pyrite is primary and locally exsolved from monosulfide solid solution mss in small amounts 2 wt together with pyrrhotite and pentlandite this pyrite is unexpectedly enriched in ipge as pt and the concentrations of these elements are oscillatory zoned the other pyrite is secondary and formed by alteration of the mss cumulates by late magmatic hydrothermal fluids this pyrite is unzoned and has inherited the low concentrations of ipge and re from the pyrrhotite and pentlandite that it has replaced

long term persistence of oil from the exxon valdez spill in two layer beaches oil spilt from the tanker exxon valdez more than 20 years ago still persists in the gravel beaches of prince william sound alaska field data and numerical modelling indicate that some of the oil was trapped in the anoxic environment of the lower layers of the beaches when the water table was low oil spilled from the tanker exxon valdez in 1989 refs 1 2 persists in the subsurface of gravel beaches in prince william sound alaska the contamination includes considerable amounts of chemicals that are harmful to the local fauna3 however remediation of the beaches was stopped in 1992 because it was assumed that the disappearance rate of oil was large enough to ensure a complete removal of oil within a few years here we present field data and numerical simulations of a two layered beach with a small freshwater recharge in the contaminated area where a high permeability upper layer is underlain by a low permeability lower layer we find that the upper layer temporarily stored the oil while it slowly and continuously filled the lower layer wherever the water table dropped below the interface of the two layers as a result of low freshwater recharge from the land once the oil entered the lower layer it became entrapped by capillary forces and persisted there in nearly anoxic conditions that are a result of the tidal hydraulics in the two layered beaches we suggest that similar dynamics could operate on tidal gravel beaches around the world which are particularly common in mid and high latitude regions4 5 with implications for locating spilled oil and for its biological remediation

tidal influence on seawater intrusion in unconfined coastal aquifers studies of seawater intrusion in unconfined coastal aquifers typically neglect oceanic oscillations such as tides and assume a static seaward boundary condition defined by the mean sea level laboratory experiments and numerical simulations were conducted to investigate the influence of tidal oscillations on the behavior of the saltwater wedge for the conditions examined the experiments showed that an upper saline plume formed in the intertidal zone due to tide induced seawater circulation the presence of the upper saline plume shifted the fresh groundwater discharge zone seaward to the low tide mark and restricted the intrusion of the saltwater wedge the overall seawater intrusion extent as indicated by the wedge toe location was reduced significantly compared with the nontidal static case results from the numerical model matched these experimental observations and further demonstrated the similar type of tidal influence on the saltwater wedge in a field scale aquifer system the glover 1959 solution for predicting the saltwater wedge was modified to account for the tidal effect by including the tide induced circulation as a recharge to the aquifer the findings highlight the significant impact of the tide in modulating the groundwater behavior and salt freshwater dynamics not only within but also landward of the intertidal zone

experiments and models bearing on the role of chromite as a collector of platinum group minerals by local reduction chromite is widely recognized to act as a collector for platinum group elements pge which tend to be observed as discrete grains of platinum group minerals included within magmatic chromite grains in the course of experiments involving the re equilibration or growth of chromite and cr spinel in molten silicate we observe that platinum group minerals pgm including metal alloys and laurite form at the mineral melt interface the formation of pgm to the extent observed requires a mechanism involving sustained transport of pge from a source within the experiment to the site of deposition we propose that the driving force for this process is a redox gradient developed in response to mineral growth or re equilibration with the surrounding melt the mechanism is local reduction within the mineral melt interfacial region as a consequence of the selective uptake of trivalent cr and fe from the melt by spinel relative to the divalent species we have modeled the transient perturbation of fo2 in a compositional boundary layer melt around spinel for both crystal growth and diffusive re equilibration of mineral and melt we find that metal solubilities decrease by several per cent in the silicate melt at the melt crystal interface during crystal growth providing the driving force for pgm formation in magmas that are saturated with pgm as a result of falling temperature and oxygen fugacity during spinel crystallization nucleation of pgm will be impeded by interfacial tension everywhere except in the reduced boundary layer around spinel crystals the resulting concentration and trapping of alloy particles in the growing chromite crystals can produce apparent bulk chromite melt partition coefficients exceeding 20 even if there is no solid solution of pge in the chromite the introduction of spinel grains initially equilibrated with a mafic magma into a more primitive magma with higher cr al would lead to disequilibrium between chromite and melt the perturbation of fo2 in the compositional boundary layer surrounding a chromite xenocryst would exceed 0 1 log unit leading to local reduction of alloy solubility of the order of 13 18 a small number of spinel xenocrysts could serve as collection sites for all of the excess pge in the magma leading to the eventual observation that a few chromite crystals contain many pgm inclusions whereas the rest of the chromite population may be relatively free of pgm

aragonite to calcite transformation during fresh water diagenesis of carbonates insights from pore water chemistry dissolved strontium and calcium concentrations in fresh water lenses fwl and associated mixing zones mz on two small holocene ooid sand islands in the schooner cays bahamas were monitored during a 1 yr period to quantitatively analyze the transformation of aragonite to calcite the observed characteristics of this mass transfer are functions of climate and hydrology aragonite to calcite transformation in all hydrologic zones is primarily associated with meteoric recharge the transformation occurs throughout the fwl and in the mz to relative salinities of 19 and 36 sea water on the two islands rates of transformation are rapid in all zones and are greatest in the fwl a limestone composed of 100 calcite should form from an aragonite precursor within 4 700 to 15 600 yr in the fwl and within 8 700 to 60 000 yr in the upper mz efficiencies of transformation can vary between hydrologic zones due to pco2 effects yet the efficiency of the entire system fwl mz is high 87 this indicates that most caco3 derived from aragonite dissolution is reprecipitated as calcite somewhere in the fresh water system or upper mixing zone co2 effects fresh water sea water mixing and the differing solubilities between aragonite and calcite all drive the mass transfer the latter is the most significant accounting for up to nine times more mass transfer than co2 effects and at least ten times more mass transfer than fresh water sea water mixing differing solubilities should also cause mass transfer to occur throughout the hydrologic cycle but it apparently becomes ineffective after the rainy season possibly due to the inhibition of calcite precipitation

multiple events in the neo tethyan oceanic upper mantle evidence from ru os ir alloys in the luobusa and dongqiao ophiolitic podiform chromitites tibet in situ analysis of major elements platinum group elements and os isotope composition data for 170 grains of ru os ir alloys from podiform chromitites in the luobusa and dongqiao ophiolites tibet provide new constraints on the sources of the alloy grains the os isotope composition of the neo tethyan oceanic upper mantle and the timing of melt rock reactions in the ultramafic peridotites of the host ophiolites most alloy grains are osmiridium and iridosmine based on elemental and isotopic compositions two populations can be identified group i found in both ophiolite bodies has fractionated pge patterns typical of alloys and homogeneous 187 os 188 os 0 12645 4 2s n 145 group ii alloys from the dongqiao ophiolites show smooth pge patterns and 187 os 188 os ranges from 0 12003 to 0 12194 yielding re depletion ages 1 1 ga both the pge patterns and the os isotopic compositions of group ii resemble those of sulfides residual after melting group i alloys represent an early cumulate phase during the crystallization of magmas and are interpreted as derived from an asthenospheric melt in contrast group ii alloys are inferred to derive from the breakdown of residual sulfides from the mantle section of the ophiolite which may represent relics of a sub continental lithospheric mantle the coexistence of the two groups of alloys in dongqiao samples reflects interaction between asthenosphere derived melts and old depleted lithospheric mantle and suggests that the chromitites originated through melt rock reactions the data on the group i alloys support the hypothesis that the re os evolution of the convective oceanic mantle resembles that of enstatite chondrites 2007 elsevier b v all rights reserved

non chondritic platinum group element ratios in the earth s mantle the earth s upper mantle has an overabundance of highly siderophile elements platinum group elements gold and rhenium relative to what would be expected from equilibrium with the metallic core1 3 this excess is now widely believed to have been introduced as a late veneer by meteorite bombardment during early earth history but after separation of the core4 here we report high precision analyses of platinum group elements pges from fertile upper mantle iherzolites which show variations in relative abundances that exceed those in the chondritic meteorites that are thought to have furnished the late veneer5 in particular the pd ir ratio at about 1 76 is significantly greater than that of known chondritic meteorites6 our results combined with previous indications of both non chondritic and nearly chondritic pge ratios in the few other fertile mantle iherzolites that have been analysed2 3 7 11 strongly suggest that the mantle is heterogeneous in its pge content on scales of 100 kilometres if the observed pge ratios can be reconciled with a plausible meteoritic source the observed heterogeneity may reflect an original spatial variation in the late veneer otherwise an additional fractionation mechanism seems to be required

thermospheric vertical wind activity maps derived from dynamics explorer 2 wats observations large scale distribution of thermospheric vertical wind activity was studied using observations from the wind and temperature spectrometer on the dynamics explorer 2 satellite we calculated the vertical velocity standard deviation vz within a sliding window of width 120 seconds corresponding to an along track distance of 900 km maps of vz in local magnetic time and invariant latitude reveal a high latitude region of enhanced activity that largely fills the polar cap activity appears to be a maximum in the midnight dawn sector of the cap possibly at the nominal auroral oval latitude we provisionally interpret our results as evidence of polar cap gravity waves with sources in or near the midnight dawn auroral oval waves propagating poleward effectively fill the cap with vertical wind oscillations equatorward propagating waves move approximately parallel to the local thermospheric wind resulting in the possibility of critical layer dissipation

differential dissolution of a pleistocene reef in the ground water mixing zone of coastal yucatan mexico a geochemical explanation is provided for the extensive dissolution observed along the carbonate coast of the yucatan peninsula mexico mixing of fresh ground water with subterranean caribbean seawater generates a highly reactive geochemical zone that enhances aragonite and calcite dissolution and permits neomorphism of aragonite mixing zone dissolution caused by ground water discharge is a major geomorphic process in developing caves coves and crescent shaped beaches along the yucatan coast such dissolution has probably been a significant control on permeability and porosity distribution in carbonate rocks in the geologic record

the influence of connected heterogeneity on groundwater flow and salinity distributions in coastal volcanic aquifers abstract geological heterogeneity is a key factor affecting rates and patterns of groundwater flow and the evolution of salinity distributions in coastal aquifers the hydrogeologic systems of volcanic aquifers are characterized by lava flows that can form connected geologic structures in the subsurface surface based geostatistical techniques were adopted to generate geologically realistic statistically equivalent model realizations of a hydrogeologic system based on that of the big island of hawaii conduit models the density dependent groundwater flow and solute transport code seawat was used to perform 3d simulations to investigate subsurface flow and salt transport through these random realizations statistically equivalent geological systems generated with sequential indicator simulation sis and equivalent homogeneous systems were also simulated for comparison simulation results show that conduit realizations tend to have more complex salinity distributions with larger mixing zones for which the center of mass is farther offshore compared to homogeneous and sis realizations the zone of groundwater discharge is also farther offshore than for homogeneous and sis models with higher point fluxes of both fresh and saline water and greater spatial variability this highlights the importance of the geometry of geologic features in coastal groundwater flow and solute transport processes in highly heterogeneous aquifers it also points to the importance of preferential flow with implications for both coastal groundwater resource management and solute fluxes to the ocean

vertical leakage in sharp interface seawater intrusion models of layered coastal aquifers summary previous sharp interface studies of seawater intrusion swi adopt various approaches to the treatment of vertical fluxes in regions where alternating saltwater and freshwater are found in overlying aquifers in this study we compare dispersive modeling and sand tank experiments to the results of sharp interface models to evaluate assumptions regarding vertical fluxes in coastal multi aquifer systems the sand tank experiments one transient and two steady state cases consist of two coarse sand layers separated by a lower permeability layer of fine sand vertical freshwater leakage in sharp interface models is treated in one of three ways case 1 upward freshwater leakage flows only into freshwater in the aquifer above bypassing any overlying saltwater case 2 no upward freshwater leakage occurs if there is overlying saltwater case 3 freshwater leaks into any overlying saltwater without modifying the saltwater salinity sharp interface models over predicted the toe position of the saltwater wedge in both the experiments and numerical models regardless of the vertical leakage assumption in agreement with previous studies nonetheless case 1 produced improved prediction of the sand tank results relative to cases 2 and 3 freshwater leakage fluxes in areas where the interface was absent were reasonably well represented by all three sharp interface leakage assumptions compared to those of the dispersive model in regions where saltwater overlies freshwater the upward freshwater fluxes predicted by dispersive modeling were found to be consistently closest to the upward freshwater flux of case 1 compared to cases 2 and 3 saltwater to saltwater leakages from the dispersive models were poorly represented by the sharp interface models vertical flux assumptions were then evaluated for idealized field scale situations and case 1 again best matched the dispersive modeling results streamlines from dispersive modeling show that upward freshwater leakage tends to flow around and bypass overlying saltwater this matches optimally the assumption of case 1 we conclude that case 1 is the ideal approach to the treatment of upward freshwater fluxes in sharp interface models of multiple aquifer systems whereas cases 2 and 3 may create unrealistic swi predictions especially for situations where overlying aquifers are separated by a layer of low conductance

platinum group element distribution in base metal sulfides of the merensky reef from the eastern and western bushveld complex south africa base metal sulfides in magmatic ni cu pge deposits are important carriers of platinum group elements pge the distribution and concentrations of pge in pentlandite pyrrhotite chalcopyrite and pyrite were determined in samples from the mineralized portion of four merensky reef intersections from the eastern and western bushveld complex electron microprobe analysis was used for major elements and in situ laser ablation inductively coupled plasma mass spectrometry la icp ms for trace elements pge ag and au whole rock trace element analyses were performed on representative samples to obtain mineralogical balances in merensky reef samples from the western bushveld both pt and pd are mainly concentrated in the upper chromitite stringer and its immediate vicinity samples from the eastern bushveld reveal more complex distribution patterns in situ la icp ms analyses of pge in sulfides reveal that pentlandite carries distinctly elevated pge contents whereas pyrrhotite and chalcopyrite only contain very low pge concentrations pentlandite is the principal host of pd and rh in the ores palladium and rh concentrations in pentlandite reach up to 700 and 130 ppm respectively in the samples from the eastern bushveld and up to 1 750 ppm pd and up to 1 000 ppm rh in samples from the western bushveld only traces of pt are present in the base metal sulfides bms pyrrhotite contains significant though generally low amounts of ru os and ir but hardly any pd or rh chalcopyrite contains most of the ag but carries only extremely low pge concentrations mass balance calculations performed on the merensky reef samples reveal that in general pentlandite in the feldspathic pyroxenite and the pegmatoidal feldspathic pyroxenite hosts up to 100 of the pd and rh and smaller amounts 10 40 of the os ir and ru chalcopyrite and pyrrhotite usually contain less than 10 of the whole rock pge the remaining pge concentrations and especially most of the pt up to 100 are present in the form of discrete platinum group minerals such as cooperite braggite sperrylite moncheite and isoferroplatinum distribution patterns of whole rock cu ni and s versus whole rock pd and pt show commonly distinct offsets the general sequence of offset patterns of pge and bms maxima in the order from bottom to top is pd in pentlandite pd in whole rock cu ni and s the relationship is not that straightforward in general some of the reef sequences studied only partially show similar trends or are more complex in general however the highest pd concentrations in pentlandite appear to be related to the earliest volumetrically rather small sulfide liquids at the base of the merensky reef sequence a possible explanation for the offset patterns may be rayleigh fractionation

how does layered heterogeneity affect the ability of subsurface dams to clean up coastal aquifers contaminated with seawater intrusion abstract the main purpose of this work was to examine how aquifer layering impacts the ability of subsurface dams to retain seawater intrusion swi and to clean up contaminated coastal aquifers using both experimental and numerical techniques four different layering configurations were investigated including a homogeneous case case h and three different layered cases where a low permeability layer was set at the top of the aquifer case lh at the middle part of the aquifer as interlayer case hlh and at the lower part of the aquifer case hl the subsurface dam was able to retain the saltwater wedge associated with a drop of the hydraulic gradient from 0 0158 down to 0 0095 in all the cases thereby achieving up to 78 reduction in the saltwater toe length in cases lh and hlh the start of the saltwater spillage was delayed compared to the homogeneous case and the time taken for the freshwater zone to be fully contaminated post spillage was twice and three times longer respectively by contrast the existence of a low k layer at the bottom of the aquifer case hl considerably weakened the ability of dams to retain the intrusion allowing for quicker saltwater spillage past the wall the natural cleanup of swi contaminated coastal aquifers was for the first time evidenced in heterogeneous settings depending on the stratification pattern the presence of stratified layers however prolonged the cleanup time to various degrees compared to the homogeneous scenario particularly in case hl where the cleanup time was nearly 50 longer

tide induced recirculation across the aquifer ocean interface a parametric analysis is conducted to examine the influence of tides inland hydraulic conditions and aquifer properties on the rate of tide induced seawater recirculation through the nearshore aquifer understanding such influence is crucial for accurate prediction of subsurface chemical fluxes to coastal waters via groundwater discharge the analysis is based on numerical simulations of density dependent groundwater flow in a coastal aquifer subject to tidal oscillations across a sloping beach face the results reveal that the amplitude of tidal oscillations and the inland hydraulic gradient are the primary parameters controlling the tide induced recirculation rates significant tidal exchange is expected when the ratio of tidal to inland forcing is large the horizontal tidal shoreline excursion and aquifer depth both display asymptotic behavior influencing recirculation rates for only small values where the exchange process is limited by the potential for infiltration and shallowness of the aquifer respectively the analysis also indicates that tidal effects increase density driven recirculation rates due to enhanced convective flow within the saltwater wedge

freshwater seawater mixing experiments in sand columns summary a study was performed on vertical mixing between freshwater and seawater in controlled column experiments the aim was to quantify the variation in the thickness of the mixing zone caused by water table fluctuations in aeolian dune sandy sediments a series of column experiments were setup first a study on in well mixing was completed to understand the suitability of down hole logging in existing piezometers versus multi level sampling then an unsaturated saturated column experiment with sandy aeolian dune sediments of the target aquifer was performed to characterize the mixing zone thickness finally the third column experiment was conducted by imposing a groundwater fluctuation of 0 1 m to quantify the increase of the freshwater saltwater mixing zone due to vertical water table oscillations the freshwater saltwater mixing zone increase was very limited and did not exceed 0 1 m all experiments were modeled by means of the seawat numerical code and calibrated against saltwater concentrations the model fit was good for all experiments providing insights into the vertical dispersivity of sandy aeolian dune sediments and also into the reliability and precision of the freshwater saltwater interface monitoring methods the down hole logging technique in fact was demonstrated to be inefficient when horizontal flow velocity is negligible in this case only multi level sampling techniques provided reliable results

the roles of magmatic and hydrothermal processes in pge mineralization ferguson lake deposit nunavut canada the ferguson lake ni cu co platinum group element pge deposit in nunavut canada occurs near the structural hanging wall of a metamorphosed gabbroic sill that is concordant with the enclosing country rock gneisses and amphibolites massive to semi massive sulfide occurs toward the structural hanging wall of the metagabbro and a low sulfide high pge style of mineralization sulfide veins and disseminations locally occurs 30 50 m below the main massive sulfide water rock interaction in the ferguson lake ni cu co pge deposit is manifested mostly as widespread post metamorphic epidote chlorite calcite veins and replacement assemblages that contain variable amounts of sulfides and platinum group minerals pgm pgm occur as inclusions in magmatic pyrrhotite and chalcopyrite in both the massive sulfide and high pge zones at the contact between sulfides and hornblende or magnetite inclusions in the massive sulfide in undeformed sulfide veins and adjacent chlorite and or epidote halos in hornblende adjacent to hydrothermal veins and in plagioclase chlorite aggregates replacing garnet cemented by sulfide the pgm are mostly represented by the kotulskite pdte sobolevskite pdbi solid solution but also include michenerite pdbite froodite pdbi2 merenskyite pdte2 mertieite ii pd8 sb as 3 and sperrylite ptas2 and occur in variety of textural settings those that occur in massive and interstitial sulfides interpreted to be of magmatic origin and formed through exsolution from base metal sulfides at temperatures 600 c are dominantly bi rich i e te bearing sobolevskite whereas those that occur in late stage hydrothermal sulfide silicate veins and their epidote chlorite alteration halos tend to be more te rich i e bi bearing kotulskite the chemistry and textural setting of the various pgm supports a genetic model that links the magmatic and hydrothermal end members of the sulfide pgm mineralization the association of pgm with magmatic sulfides in the massive sulfide and high pge zones has been interpreted to indicate that pge mineralization was initially formed through exsolution from base metal sulfides which formed by magmatic sulfide liquid segregation and crystallization however the occurrence of pgm in undeformed sulfide bearing veins and in their chlorite epidote halos and differences in pgm chemistry indicate that hydrothermal fluids were responsible for post metamorphic redistribution and dispersion of pge

platinum group element gold silver and base metal distribution in compositionally zoned sulfide droplets from the medvezky creek mine noril sk russia concentrations of ag au cd co re zn and platinum group elements pge have been determined in sulfide minerals from zoned sulfide droplets of the noril sk 1 medvezky creek mine the aims of the study were to establish whether these elements are located in the major sulfide minerals pentlandite pyrrhotite chalcopyrite and cubanite to establish whether the elements show a preference for a particular sulfide mineral and to investigate the model which suggests that the zonation in the droplets is caused by the crystal fractionation of monosulfide solid solution mss nickel cu ag re os ir ru rh and pd were found to be largely located in the major sulfide minerals in contrast less than 25 of the au cd pt and zn in the rock was found to be present in these sulfides osmium ir ru rh and re were found to be concentrated in pyrrhotite and pentlandite palladium and co was found to be concentrated in pentlandite silver cd and zn concentrations are highest in chalcopyrite and cubanite gold and platinum showed no preference for any of the major sulfide minerals the enrichment of os ir ru rh and re in pyrrhotite and pentlandite exsolution products of mss and the low levels of these elements in the cubanite and chalcopyrite exsolution products of intermediate solid solution iss support the mss crystal fractionation model because os ir ru rh and re are compatible with mss the enrichment of ag cd and zn in chalcopyrite and cubanite also supports the mss fractionation model these minerals are derived from the fractionated liquid and these elements are incompatible with mss and thus should be enriched in the fractionated liquid gold and pt do not partition into either iss or mss and become sufficiently enriched in the final fractionated liquid to crystallize among the iss and mss grains as tellurides bismithides and alloys during pentlandite exsolution pd appears to have diffused from the cu rich portion of the droplet into pentlandite

laboratory scale investigation of saltwater intrusion dynamics 1 experiments were completed in a laboratory scale porous media tank to study the transport patterns of a saltwater wedge in a freshwater aquifer three types of experiments were performed to develop 1 steady state salt wedge data observed under different hydraulic gradient conditions 2 transient salt wedge data observed under intruding wedge conditions and 3 transient salt wedge data observed under receding wedge conditions furthermore flux measurements were made to quantify the flow characteristics of three distinct steady state experiments the saltwater intrusion model seawat was used to simulate these data sets the model results along with the experimental data are presented as benchmark problems for testing density coupled groundwater flow models a worthiness analysis was completed to test the sensitivity of these experimental problems to density coupling effects the results of our analysis show that the proposed benchmark is a more robust alternative to the traditional henry problem these new experimental data sets can be used to assess the performance of saltwater intrusion models under both steady state and transient conditions

use of tritium and helium to define groundwater flow conditions in everglades national park the concentrations of tritium 3 h and helium isotopes 3 he and 4 he were used as tracers of groundwater flow in the surficial aquifer system sas beneath everglades national park enp south florida from ages determined by 3 h 3 he dating techniques groundwater within the upper 28 m originated within the last 30 years below 28 m waters originated prior to 30 years before present with evidence of mixing at the interface interannual variation of the 3 h 3 he ages within the upper 28 m was significant throughout the 3 year investigation corresponding with varying hydrologic conditions in the region of taylor slough bridge younger groundwater was consistently detected below older groundwater in the biscayne aquifer suggesting preferential flow to the lower part of the aquifer an increase in he with depth in the sas indicated that radiogenic 4 he produced in the underlying hawthorn group migrates into the sas by diffusion higher 4 he values in brackish groundwaters compared to fresh waters from similar depths suggested a possible enhanced vertical transport of 4 he in the seawater mixing zone groundwater salinity measurements indicated the presence of a wide 6 28 km seawater mixing zone comparison of groundwater levels with surface water levels in this zone indicated the potential for brackish groundwater discharge to the overlying everglades surface water

effects of kinetic mass transfer and transient flow conditions on widening mixing zones in coastal aquifers 1 the width of a mixing zone between freshwater and seawater is important primarily because it directly reflects the extent of mixing and the growth and decay of the mixing zone indicates changes of the flow regime and water exchange between freshwater and coastal seawater wide mixing zones have been found in many coastal aquifers all over the world however the mechanisms responsible for wide mixing zones are not well understood this work examines the hypothesis that kinetic mass transfer coupled with transient conditions which create the movement of the mixing zone may widen mixing zones in coastal aquifers the hypothesis is tested by conducting two dimensional numerical simulations based on a variable density groundwater model for a scaled tank model and a field scale model periodic water levels representing periodic tidal motion and freshwater table fluctuations are imposed at the seaward and landward boundary respectively which cause the movement of the mixing zone both the scaled tank model and the field model show that the combination of the moving mixing zone and kinetic mass transfer may significantly enhance the extent of mixing and create a wider mixing zone than the models without kinetic mass transfer in addition sensitivity analyses indicate that a larger capacity ratio immobile porosity mobile porosity of mass transfer leads to a wider mixing zone and the maximum width of the mixing zone may be reached for a given capacity ratio when the mean retention time scale in the immobile domain the reciprocal of mass transfer rate and the period of water level fluctuations are comparable

metal saturated sulfide assemblages in nwa 2737 evidence for impact related sulfur devolatilization in martian meteorites nwa 2737 a martian meteorite from the chassignite subclass contains minute amounts 0 010 0 005 vol of metal saturated fe ni sulfides these latter bear evidence of the strong shock effects documented by abundant fe nanoparticles and planar defects in northwest africa nwa 2737 olivine a ni poor troilite fe s 1 0 0 01 sometimes cr bearing up to 1 wt coexists with micrometer sized taenite tetrataenite type native ni fe alloys ni fe 1 and fe os ir ru alloys a few hundreds of nanometers across the troilite has exsolved flame like pentlandite fe fe ni 0 5 0 6 chalcopyrite is almost lacking and no pyrite has been found as a hot desert find nwa 2737 shows astonishingly fresh sulfides the composition of troilite coexisting with ni fe alloys is completely at odds with chassigny and nahkla sulfides pyrite metal deficient monoclinic type pyrrhotite it indicates strongly reducing crystallization conditions close to iw several log units below the fo2 conditions inferred from chromites compositions and accepted for chassignites fmq 1 log unit it is proposed that reduction in sulfides into base and precious metal alloys is operated via sulfur degassing which is supported by the highly resorbed and denticulated shape of sulfide blebs and their spongy textures shock related s degassing may be responsible for considerable damages in magmatic sulfide structures and sulfide assemblages with concomitant loss of magnetic properties as documented in some other martian meteorites

the formation of pt ir alloys and cu pd rich sulfide melts by partial desulfurization of fe ni cu sulfides results of experiments and implications for natural systems abstract we propose a model of high temperature formation of platinum group element pge alloys from base metal sulfides subjected to decreasing sulfur fugacity f s 2 for this purpose we experimentally investigated the effect of partial desulfurization of monosulfide solid solution mss with variable ni and cu contents on the distribution behaviour of pt ir and pd at 1000 c we partially removed sulfur from the pge bearing mss using the tube in tube technique and pyrrhotite as an f s 2 buffer we found that mss undergoing s loss can produce 1 pt and ir exsolution from the mss matrix in the form of pge bearing alloys 2 partial melting of the cu ni bearing mss to form fe rich mss cu ni pd rich sulfide liquid and fe ir pt alloy both of these processes are geologically important the partial melting of the s depleted sulfides could explain the presence of two types of sulfides found in the mantle mss dominated sulfides and s poor sulfides consisting mainly of pentlandite and chalcopyrite we also suggest that the cu ni pd rich liquid formed by partial melting of the sulfides could migrate away from the mss and fe ir pt alloys thus spatially decoupling ir pt and cu pd as observed in reefs and mantle nodules in addition the formation of pge alloys in response to the desulfurization processes potentially could occur 1 after pressure falls during transport of the basalt magma with entrained sulfide droplets 2 after sulfur removal by s undersaturated melts or fluids from pge enriched sulfide proto ore 3 after degassing of the sulfide droplets occurring in a sub volcanic chamber and 4 as a result of sulfide interaction with chromite

experimental saltwater intrusion in coastal aquifers using automated image analysis applications to homogeneous aquifers summary this paper presents the applications of a novel methodology to quantify saltwater intrusion parameters in laboratory scale experiments the methodology uses an automated image analysis procedure minimising manual inputs and the subsequent systematic errors that can be introduced this allowed the quantification of the width of the mixing zone which is difficult to measure in experimental methods that are based on visual observations glass beads of different grain sizes were tested for both steady state and transient conditions the transient results showed good correlation between experimental and numerical intrusion rates the experimental intrusion rates revealed that the saltwater wedge reached a steady state condition sooner while receding than advancing the hydrodynamics of the experimental mixing zone exhibited similar traits a greater increase in the width of the mixing zone was observed in the receding saltwater wedge which indicates faster fluid velocities and higher dispersion the angle of intrusion analysis revealed the formation of a volume of diluted saltwater at the toe position when the saltwater wedge is prompted to recede in addition results of different physical repeats of the experiment produced an average coefficient of variation less than 0 18 of the measured toe length and width of the mixing zone

impact of layered heterogeneity on transient saltwater upconing in coastal aquifers abstract this research investigated the effect of layered heterogeneity on transient saltwater upconing in a laboratory scale coastal aquifer the experiments were conducted in a 2d laboratory flow tank and the response of the saltwater wedge to pumping was analysed in a heterogeneous aquifer system where a low permeability layer was constructed in the middle of the aquifer the seawat code was used for validation and to perform additional simulations to explore the sensitivity of the critical pumping rate and the critical time to the main parameters characterising the low permeability layer which included its permeability thickness and position the experimental results showed that the presence of layered heterogeneity noticeably altered the shape and the intrusion length of the upconing wedge without inducing a change in the abstraction rate triggering saltwater upconing mechanism compared to the homogeneous case the numerical results of the layered aquifer provided excellent agreement with the experimental data for both the transient toe length and the shape of the steady state saltwater wedges the sensitivity analysis revealed that the critical pumping rate and the critical time was found to decrease considerably with decreasing hydraulic conductivity and thickness of the middle layer which evidences the higher vulnerability of such layered aquifer systems to the saltwater upconing in comparison to idealised homogeneous systems the results nonetheless showed that varying the position of the interlayer induced very little change on the critical pumping rate but the critical time would tend to decrease as the low permeability layer was moved deeper away from the pumping well particularly for smaller middle layer thickness

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

experimental and numerical investigation of saltwater intrusion dynamics in flux controlled groundwater systems 1 sea level rise and reduction of groundwater fluxes due to changes in rainfall patterns are the two major climate change induced hydrological variables that can severely affect saltwater intrusion in coastal aquifers in this study we use a combination of laboratory experiments and numerical simulations to study the impacts of changes in one of these climate change induced hydrological variables groundwater flux on saltwater intrusion process we have completed experiments in a laboratory scale model to study the changes in two types of groundwater fluxes areal recharge flux and regional flux the experimental results were modeled using the numerical code seawat the transient data sets reported in this study are useful benchmarks for testing numerical models that employ flux type boundary conditions also based on the experimental observations we hypothesize that when the fluxes are perturbed it would require relatively less time for a salt wedge to recede from an aquifer when compared to the time required to advance into the aquifer this rather counterintuitive hypothesis implies that saltwater intrusion and receding processes are asymmetric and the timescales associated with these processes will be different we use a combination of laboratory and numerical experiments to test this hypothesis and use the resulting data set to explain the reason for the difference in intrusion rates

mineralogical changes along the freshwater saltwater interface of a modern aquifer abstract occurrence of dolomite is related to the distribution of geological and hydrochemical parameters in cores and wells penetrating the floridan aquifer along portions of the west coast of peninsular florida trace element analyses of the cores indicate correlations between strontium and sodium concentrations and the particular carbonate phase chloride sulfate and conductivity values indicate the position and extent of the freshwater saltwater interface thick sequences of carbonate rocks in western peninsular florida have been dolomitized in the freshwater saltwater mixing zone of the coastal aquifer correlations between the occurrence of dolomite and specific stratigraphic formations suggest that dolomitization was rock selective evidence of continuous formation of dolomite indicates that the process is actively occurring

ni cu au and platinum group element contents of sulphides associated with intraplate magmatism a synthesis the tectonic setting of intraplate magmas typically a plume intersecting a rift is ideal for the development of ni cu platinum group element bearing sulphides the plume transports metal rich magmas close to the mantle crust boundary the interaction of the rift and plume permits rapid transport of the magma into the crust thus ensuring that no sulphides are lost from the magma en route to the crust the rift may contain sediments which could provide the sulphur necessary to bring about sulphide saturation in the magmas the plume provides large volumes of mafic magma thus any sulphides that form can collect metals from a large volume of magma and consequently the sulphides will be metal rich the large volume of magma provides sufficient heat to release large quantities of s from the crust thus providing sufficient s to form a large sulphide deposit the composition of the sulphides varies on a number of scales i there is a variation between geographic areas in which sulphides from the noril

tidal effects on sea water intrusion in unconfined aquifers abstract a variable density groundwater model is used to analyse the effects of tidal fluctuations on sea water intrusion in an unconfined aquifer it is shown that the tidal activity forces the sea water to intrude further inland and it also creates a thicker interface than would occur without tidal effects moreover the configuration of the interface is radically changed when the tidal fluctuations are included this is because of the dramatic changes in the flow pattern and velocity of the groundwater near the shoreline for aquifer depths much larger than tidal amplitudes the tidal fluctuation does not have much effect on how far the sea water intrudes into the aquifer nevertheless a significant change in the configuration of concentration contours because of the effect of tidal fluctuations is observed this change is more noticeable at the top of the aquifer near the water table than at the bottom of the aquifer and is caused by the infiltration of salt water into the top of the aquifer at higher tidal levels a flatter beach slope therefore intensifies this phenomenon the interface configurations do not change noticeably over the course of a tidal cycle neglecting tidal fluctuation effects results in an inaccurate evaluation of the water table elevation at the land end of the aquifer although no distinguishable difference is seen between the water tables near the shoreline where the landward boundary condition is a constant head the effects of tidal fluctuations on sea water intrusion are more pronounced than for cases where the landward boundary condition is a specified flux also it is shown that the effects of tidal fluctuations are more significant for a sloping beach than for a vertical shoreline and the salt water intrudes further inland for the sloping case

a new physical barrier system for seawater intrusion control abstract the construction of subsurface physical barriers is one of various methods used to control seawater intrusion swi in coastal aquifers this study proposes the mixed physical barrier mpb as a new barrier system for seawater intrusion control which combines an impermeable cutoff wall and a semi permeable subsurface dam the effect of the traditionally used physical barriers on transient saltwater wedge dynamics was first explored for various hydraulic gradients and the workability of the mpb was thereafter thoroughly analysed a newly developed automated image analysis based on light concentration conversion was used in the experiments which were completed in a porous media tank the numerical code seawat was used to assess the consistency of the experimental data and examine the sensitivity of the performance of the barriers to various key parameters the results show that the mpb induced a visible lifting of the dense saline flux upward towards the outlet by the light freshwater this saltwater lifting mechanism observed for the first time induced significant reduction to the saline water intrusion length the use of the mpb yielded up to 62 and 42 more reduction of the saltwater intrusion length than the semi permeable dam and the cutoff wall respectively the performance achieved by the mpb with a wall depth of 40 of the aquifer thickness was greater than that of a single cutoff wall with a penetration depth of 90 of the aquifer thickness about 13 extra reduction this means that the mpb could produce better seawater intrusion reduction than the traditionally used barriers at even lower cost

platinum group element chromitite associations in the bushveld complex new and recently published platinum group element data of chromite rich layers in the bushveld complex are evaluated chondrite normalized platinum group element plots of these data show that trends for chromitite layers differ from that of the merensky reef the data furthermore indicate remarkable consistencies in compositional trends for many kilometers along strike of individual layers suggesting that the conditions of formation of chromitite layers and their associated platinum group element mineralization were very uniform over large areas of the magma chamber significant differences in the proportions of platinum group elements within a particular layer are only evident on a regional scale the platinum group minerals of the middle group of chromitite layers in the marikana area of the western bushveld are mainly sulfides and arsenides laurite is the most common platinum group mineral and is mostly enclosed by chromite a variety of other platinum group minerals occur in close association with base metal sulfides extremely low bulk sulfur content in the chromitite layers is ascribed to a postcumulus loss of sulfur postcumulus loss of sulfur and the existence of laurite enclosed within chromite casts doubt on the validity of any petrogenetic models for platinum mineralization in chromitite rich layers based on partition coefficients and the r factor

a novel approach for characterizing the mixing zone of a saltwater wedge 1 the mixing zone between freshwater and seawater in a coastal aquifer is an important transition region that controls regional groundwater flow dynamics and reactive transport processes while experimental procedures have been developed to study the movement of salt wedges in aquifers so far no one has attempted to exclusively map the mixing zone associated with the wedge in a laboratory scale aquifer model we propose a colorimetric experimental method which employs the reaction between alkaline freshwater and acidic saltwater to map the mixing zone the experiment was completed in a two dimensional aquifer model to simulate a steady saltwater wedge we used the density dependent flow model sutra to simulate these experimental results and to estimate the mixing parameters i e dispersivity coefficients the results show that the proposed colorimetric technique is a useful experimental approach to visualize and quantify the level of mixing in a saltwater intrusion experiment

the geologic setting of the platiniferous ug 2 chromitite layer on the farm maandagshoek eastern bushveld complex the platiniferous ug 2 chromitite layer forms part of the upper chromitite group of the critical zone on the farm maandagshoek 254kt in the eastern bushveld complex the upper chromitite group is made up of four cyclic units namely ug 1 ug 2 ug 3 and ug 3a each of which consists of a basal chromitite layer with or without olivine overlain successively by melanorite and norite and terminated with anorthosite the ug 2 chromitite layer and its associated basal pegmatoid is found close to the base of the ug 2 cyclic unit the cyclic units are separated by sharp contacts and have a remarkable persistence along strike and downdip whole rock cu total ni pt and pd show an upward enrichment from the base of the ug 2 unit reaching a maximum in the main chromitite layer they are depleted in the hanging wall sequence of melanorite bulk analyses of the ug 2 chromitite and its enclosing lithologies for pt pd ru rh ir au s sulfide ni cu and co show that the platinum group elements and base metals are concentrated 1 close to the top and base of the main chromitite layer 2 in the leader layer and 3 in minor chromitite lenses and layers both in the footwall pegmatoid and the hanging wall melanorite the pt pt pd and cu ni ratios are low in the footwall pegmatoid and base of the main chromitite layer values of these ratios however increase upward with pt pt pd decreasing at the upper contact of the ug 2 layer the calculated concentration of platinum group metals in the sulfide phase in the ug 2 chromitite layer ranges from 3 000 to just under 12 000 ppm the above determination is primarily controlled by the concentration of s in the layer with the average precious metal distribution remaining relatively constant the calculated d super sulfide liquid sub silicate liquid with the parental liquid values in parentheses are pt 52 575 40 ppb pd 132 940 16 ppb rh 22 080 8 ppb ir 730 000 0 4 ppb and au 2 190 21 ppb evidence of an influx of magma into the eastern lobe of the bushveld complex at a level between the upper and middle chromitite groups is indicated by discontinuous and disturbed layers in borehole drilling on maandagshoek it is suggested that this magma batch was slightly more enriched in the platinum group elements in order to produce the increased value of these elements in the ug 2 chromitite layer it is proposed that preexisting sulfides in the ug 2 chromitite layer collected platinum group elements from upward migrating highly fractionated intercumulus fluids under highly reducing conditions during the early stages of formation of the layer the relatively large volume of sulfide initially present in the layer was later reduced by volatiles enriched in o 2 passing through the layer resorbing the sulfides and blowing off so 2 the partial resorption of the sulfides accounts for the presence of minor pt fe alloys and the abnormally high partition coefficients of the platinum group elements in the sulfide liquid of the ug 2 chromitite layer

petrogenesis of pelitic xenoliths at the babbitt cuni deposit duluth complex minnesota u s a abstract the babbitt deposit consists of disseminated cu fe ni sulfides found within mafic rocks of the duluth complex generally near contacts with underlying metasedimentary rock types host rocks for the deposit include troctolites olivine gabbros gabbronorites norites and occasionally country rock hornfels xenoliths of country rocks are abundant in the deposit and suggest a relationship between sulfide mineralization and country rock contamination country rocks in the babbitt area include those of the middle precambrian biwabik iron formation and both calcareous and non calcareous pelites of the virginia formation xenoliths contain the assemblage cordierite plagioclase biotite orthopyroxene and are thought to have been derived from virginia formation protoliths comparison of protoliths and xenoliths using composition volume element ratio and mass balance techniques suggests that xenoliths have been strongly depleted in volatiles alkalis and si footwall rocks show only a depletion in volatiles neither fluid phase transport nor diffusion through an intergranular fluid can account for the mass of material transferred extensive partial melting of xenoliths with residual enrichment of feo mgo and al2o3 is the most viable transfer process the lack of sio2 concentration gradients around xenoliths and anomalous igneous rock compositions suggest that desilicification occurred at a time when physical mixing of extracted partial melt and host magma was possible and prior to final emplacement sulfide saturation may have been initiated due to si assimilation with an auxiliary magma chamber however the composition of ores in the babbitt deposit is consistent with saturation being achieved by addition of sediment derived volatile sulfur independent of major element assimilation

tidal effects on variations of fresh saltwater interface and groundwater flow in a multilayered coastal aquifer on a volcanic island jeju island korea summary we conducted various field studies at the seawater intrusion monitoring wells located in the eastern part of jeju island korea to observe the tidal effect on groundwater seawater flow in the coastal aquifer studies included monitoring the fluctuations of groundwater and tide levels electrical and temperature logging and 2 d heat pulse flowmeter tests according to time series analysis tidal effects on groundwater level reached up to 3 km inland from the coastline water level variation was more sensitive to tidal fluctuations near the coast and more related to rainfall toward inland areas temporal and spatial variations in the shape and location of the freshwater saltwater interface were analyzed using data from nine monitoring wells the results indicated that the interface toe is located at a distance of 6 8 km from the coastline and its location was related to geological layers present long term seasonal variations revealed no major changes in the interface minor variations were due to moving boundary conditions induced by tidal fluctuations using the two dimensional heat pulse flowmeter groundwater flow directions and velocities at four tidal stages were measured on three monitoring wells drilled into the multilayered aquifers this direct measurement enabled us to relate the differences of flow velocities and directions with geology and tidal fluctuations combining the results of ec logging and flowmeter tests we found a zone where freshwater and saltwater moved alternately in opposite directions as influenced by the tidal fluctuations integrating various physical logging and flowmeter data with water level fluctuations improved our understanding of the behavior of fresh and seawater flow in the coastal aquifers

image analysis and composition of platinum group minerals in the j m reef stillwater complex detailed imaging bulk geochemical analyses and laser ablation icp ms analyses were conducted on mineralized samples from the platinum group element pge bearing j m reef stillwater complex montana the aims of the study were to determine which phases are the main hosts of the pge to examine the textural relationships of the pge bearing minerals and the associated base metal sulfides silicates and secondary magnetites the platinum group minerals pgm observed in the studied samples consist of pd or pt sulfides pt fe alloy isoferroplatinum pd or pt tellurides and pd cu alloy skaergaardite with minor native pd ru ir os s sub 2 laurite and au pd ag alloy palladian electrum these minerals account for all the pt half the pd ru and ir and a smaller proportion of the os in whole rocks the balance being found in the base metal sulfide most of these pgm are closely associated with base metal sulfides either included in the base metal sulfide or located at the contact between the base metal sulfide and the silicate or oxide minerals the textures of the pgm within and at the margins of the base metal sulfide suggest that they exsolved from the sulfides we suggest that the pge ni cu and te partitioned into a magmatic sulfide liquid that crystallized into base metal sulfide the pt fe alloys and some of the pd sulfides exsolved from the base metal sulfide during desulfurization at fairly high temperatures in contrast the skaergaardite and some pd sulfides are found in association with secondary magnetite and probably formed at relatively low temperatures approximately 250 degrees 465 degrees c during a second fluid migration event the pd pt ratio of the reef approximately 3 3 is higher than most mafic magmas and the pd pt ratio of the footwall is lower 0 3 0 7 it is possible that some of the pd in the reef was leached from the underlying rocks and deposited in the reef as pgm associated with secondary magnetite

precious metal mobility during serpentinization and breakdown of base metal sulphide abstract serpentinization of ultramafic rocks drives geochemical exchange between the hydrosphere biosphere and lithosphere in surficial environments and at depth precious metals are implicated in these hydration reactions because of the well known association between peridotite hosted au and placer platinum group mineral pgm deposits sourced from serpentinized ophiolite complexes at surface however the distribution of precious metals in mantle rocks and their mode of occurrence at the onset of serpentinization are not well understood because early stage features are typically obliterated with progressive hydration herein we report electron probe microanalysis epma and laser ablation inductively coupled mass spectrometry la icpms spot and mapping results for a suite of base metal sulphide pentlandite pyrrhotite chalcopyrite native metal cu and fe and ni fe alloy awaruite from variably serpentinized peridotite and pyroxenite late permian to early triassic nahlin ophiolite cache creek terrane atlin british columbia canada pentlandite and pyrrhotite occur with magmatic clinopyroxene and cr spinel as ultrafine inclusions few m and as coarser interstitial base metal sulphides 200 m that are enveloped by native metal cu and fe and ni fe alloy because native fe and awaruite bearing mineral assemblages require reduced and or low fs2 conditions we suggest that these replacement textures document destabilization of base metal sulphide phases during the conversion of olivine to serpentine and magnetite desulphurization reactions decoupled precious metals from relict pentlandite at the microscale with ag pd pt and au partitioning from sulphides into both awaruite and native metal cu and fe at concentrations up to 100s of ppm new high resolution la icpms maps also point to clusters of ultrafine pgm tellurides bismuthides and metal alloys that were either remobilized within the serpentinized mesh and or represent the completely desulphidized product of ultrafine intergranular base metal sulphide other pge os and ir are mostly hosted within relict pentlandite and or pyrrhotite that were preserved during incomplete desulphurization along with lesser microscale remobilization of these elements into ultrafine veins new whole rock pge nickel sulphide fire assay nis fa results demonstrate that precious metal remobilization was limited to the microscale during the earliest stages of serpentinization which in the case of au is consistent with its low solubility within such reduced low fs2 fluids precious metal mobility during the early stages of serpentinization may therefore depend on the stability of ni fe alloy native metal cu and fe and pgm progressive serpentinization complete replacement of olivine and destabilization of the reduced low fs2 mineral assemblage likely represents an important process to liberate transport and concentrate precious metals within more oxidized and or sulphur bearing fluids in surficial and deep subduction environments

platinum ore in three dimensions insights from high resolution x ray computed tomography platinum group elements referred to as pge comprising pt pd rh ru ir and os are strategic metals with a wide variety of industrial applications most of the world s pge production is mined from large mafic ultramafic intrusions such as the bushveld complex in south africa which currently provides 75 of the world s pt production the pge mineralization is found within distinctive layers tens to hundred of centimeters thick but extending laterally for many tens of kilometers where the pge occur at low parts per million levels as platinum group minerals pgm and in solid solution within disseminated base metal sulfides there is still heated debate at the most fundamental level about the mode of formation of this class of deposit genetic models range from primary magmatic sulfide collection to concentration by migrating halogen rich fluids a crucial line of evidence is the spatial relationship between the pgm which are the most important pge bearing phases and the base metal sulfide aggregates or blebs so far all observations have been carried out using two dimensional mineralogical studies where textural relationships with other minerals are ambiguous and with statistical limitations owing to sampling of trace phases intersecting random surfaces we present the first detailed three dimensional in situ analysis of the pgm at the sample scale using high resolution x ray computed tomography coupled with conventional microscopic and mineralogical study we find a striking and highly consistent relationship of pgm grains with the edges of complex shaped magmatic sulfide blebs and the intersection of these blebs with chromite silicate grain boundaries these new three dimensional observations strongly support an orthomagmatic model coupled with nucleation and growth of pgm at the margins of sulfide liquid droplets

hydrodynamics in a gravel beach and its impact on the exxon valdez oil 1 this paper investigated the interaction of groundwater and seawater in a tidally influenced gravel beach field observations of water table pore water salinity were performed the two dimensional finite element model marun was used to simulate observed water table and salinity based on field observations and model calibrations a two layered beach structure was identified which is characterized by a high permeability surface layer underlain by a low permeability lower layer the salt wedge seaward of the low tide line was almost invariant in comparison with the strong fluctuations of the salinity plume in the surface layer of the intertidal zone the presence of the two layers prevented the presence of a freshwater discharge tube between the upper saline plume and salt wedge this is in contrast with the previous works where freshwater discharge tube was observed the tide induced submarine groundwater discharge sgd was estimated at 9 m3 d 1 m 1 a large value that is probably due to the large tidal range of 4 8 m and the very permeable surface layer the freshwater seawater dynamics revealed here may provide new insights into the complexity intensity and time scales of mixing between fresh groundwater and seawater in tidal beaches the simulated water table of the beach was higher than the interface between the surface layer and the lower layer which prevented exxon valdez oil from penetrating into the lower layer in 1989

mineralogy of the hse in the subcontinental lithospheric mantle an interpretive review abstract the highly siderophile elements hse os ir ru rh pt pd re au exist in solid solution in accessory base metal sulfides bms as well as nano to micron scale minerals in rocks of the subcontinental lithospheric mantle sclm the latter include platinum group minerals pgm and gold minerals which may vary widely in morphology composition and distribution the pgm form isolated grains often associated with larger bms hosted in residual olivine located at interstices in between peridotite forming minerals or more commonly in association with metasomatic minerals pyroxenes carbonates phosphates and silicate glasses in some peridotite xenoliths the pgm found inside residual olivine are mainly os ir and ru rich sulfides and alloys in contrast those associated with metasomatic minerals or silicate glasses of peridotite xenoliths consist of pt pd and rh bonded with semimetals like as te bi and sn nanoscale observations on natural samples along with the results of recent experiments indicate that nucleation of pgm is mainly related with the uptake of hse by nanoparticles nanominerals or nanomelts at high temperature 900 c in both silicate and or sulfide melts regardless of the residual or metasomatic origin of their host minerals a similar interpretation can be assumed for gold minerals our observations highlight that nanoscale processes play an important role on the ore forming potential of primitive mantle derived magmas parental to magmatic hydrothermal deposits enriched in noble metals the metal inventory in these magmas could be related with the physical incorporation of hse bearing nanoparticles or nanomelts during processes of partial melting of mantle peridotite and melt migration from the mantle to overlying continental crust

platinum group minerals in the middle group of chromitite layers at marikana western bushveld complex indications for collection mechanisms and postmagmatic modification the platinum group minerals in a drill core taken through the middle group of chromitite layers in the critical zone at marikana in the western bushveld complex were found to consist mainly of laurite as inclusions in chromite grains the platinum group minerals containing pt pd and rh are concentrated in the intercumulus silicates and frequently associated with base metal sulphides up to about 20 of all platinum group minerals in the investigated chromitite layers contain sub stantial amounts of as the base metal sulphides are strongly modified in the postmagmatic stage which led to a significant loss of fe and s in this way concentrating cu ni and the platinum group elements by factors of up to 10 interaction between chromite and base metal sulphides cannot account for all the fe lost in chromite poor samples and the importance of additional processes is indicated inclusions in chromite and orthopyroxene indicate the formation of discrete platinum group minerals and as rich phases before the

the distribution of platinum group elements pge and other chalcophile elements among sulfides from the creighton ni cu pge sulfide deposit sudbury canada and the origin of palladium in pentlandite concentrations of platinum group elements pge ag as au bi cd co mo pb re sb se sn te and zn have been determined in base metal sulfide bms minerals from the western branch 402 trough orebodies of the creighton ni cu pge sulfide deposit sudbury canada the sulfide assemblage is dominated by pyrrhotite with minor pentlandite chalcopyrite and pyrite and they represent monosulfide solid solution mss cumulates the aim of this study was to establish the distribution of the pge among the bms and platinum group minerals pgm in order to understand better the petrogenesis of the deposit mass balance calculations show that the bms host all of the co and se a significant proportion 40 90 of os pd ru cd sn and zn but very little 35 of the ag au bi ir mo pb pt rh re sb and te osmium and ru are concentrated in equal proportions in pyrrhotite pentlandite and pyrite cobalt and pd 1 ppm are concentrated in pentlandite silver cd sn zn and in rare cases au and te are concentrated in chalcopyrite selenium is present in equal proportions in all three bms iridium rh and pt are present in euhedrally zoned pge sulfarsenides which comprise irarsite irass hollingworthite rhass pge ni rich cobaltite coass and subordinate sperrylite ptas2 all of which are hosted predominantly in pyrrhotite and pentlandite silver au bi mo pb re sb and te are found predominantly in discrete accessory minerals such as electrum au ag alloy hessite ag2te michenerite pdbite and rhenium sulfides the enrichment of os ru ni and co in pyrrhotite pentlandite and pyrite and ag au cd sn te and zn in chalcopyrite can be explained by fractional crystallization of mss from a sulfide liquid followed by exsolution of the sulfides the early crystallization of the pge sulfarsenides from the sulfide melt depleted the mss in ir and rh the bulk of pd in pentlandite cannot be explained by sulfide fractionation alone because pd should have partitioned into the residual cu rich liquid and be in chalcopyrite or in pgm around chalcopyrite the variation of pd among different pentlandite textures provides evidence that pd diffuses into pentlandite during its exsolution from mss the source of pd was from the small quantity of pd that partitioned originally into the mss and a larger quantity of pd in the nearby cu rich portion intermediate solid solution and or pd bearing pgm the source of pd became depleted during the diffusion process thus later forming pentlandite rims of coarse granular veinlets and exsolution flames contains less pd than early forming pentlandite cores of coarse granular

the lower banded series of the stillwater complex montana whole rock lithophile chalcophile and platinum group element distributions the principal rock types of the lower banded series of the stillwater complex are gabbronorite and norite with minor troctolite and anorthosite the whole rock composition is largely controlled by the cumulate minerals plagioclase and pyroxene olivine the rocks are very poor in incompatible elements at the 0 1 to 2 times mantle levels indicating 10 liquid fraction the pd and pt are enriched in a layer known as the j m reef which occurs in a zone containing olivine bearing rocks ob i this zone contains the same rock types as the rest of the lower banded series but troctolite olivine gabbronorite and anorthosite are more abundant in ob i the concentrations of lithophile elements in ob i are similar to the rocks above and below ob i in particular the reef rocks show no additional signs of continental crust contamination such as additional th or lree enrichment and the reef rocks are not enriched in incompatible elements the strongly chalcophile elements cu ni se bi and au all correlate with s indicating that these elements are controlled by sulfides the pge show two trends the rocks below the j m reef and the reef rocks have high pge au to s ratios and fall on a single trend whereas those above the reef have low metal to s ratios these low ratios for the rocks above the reef could arise because the magma was depleted in pge au as it had already segregated some pge au rich sulfides the relatively high pge au cu and se concentrations of the rocks below the reef imply that the rocks contain 0 1 wt cumulate sulfides however they have very low s contents 300 ppm the low s se ratios of these rocks 2000 and most of the rocks outside of the reef suggest that they have lost more than half of their s the pd pt and pd ir ratios of the reef 3 4 and 800 respectively are much higher than those of the rocks below and above the reef the high pd pt and pd ir ratios of the reef imply that the sulfides formed from a fractionated magma however there is no correlation between the pd ir and pd pt ratios and the host rock type these ratios are dissimilar to the bushveld reefs and the main sulfide zone of the great dyke they more closely resemble the reefs in more fractionated rocks such as the ap reef of the penikat intrusion the roby zone of the lac des iles complex or the disseminated sulfides of the noril sk i intrusion the very high pd s and low cu pd ratios of the j m reef require that the sulfides collected pd from a very large volume of magma none of the current models is entirely satisfactory however we favor a model where at depth the ob1 magma partially melted a komatiitic massive sulfide and then transported the sulfide droplets into the magma chamber at the level of the j m reef

a multilayered sharp interface model of coupled freshwater and saltwater flow in coastal systems model development and application a quasi three dimensional finite difference model that simulates freshwater and saltwater flow separated by a sharp interface has been developed to study layered coastal aquifer systems the model allows for regional simulation of coastal groundwater conditions including the effects of saltwater dynamics on the freshwater system vertically integrated freshwater and saltwater flow equations incorporating the interface boundary condition are solved within each aquifer leakage through confining layers is calculated by darcy s law accounting for density differences across the layer the locations of the interface tip and toe within grid blocks are tracked by linearly extrapolating the position of the interface the model has been verified using available analytical solutions and experimental results application of the model to the soquel aptos basin santa cruz county california illustrates the use of the quasi three dimensional sharp interface approach for the examination of freshwater saltwater dynamics in regional systems simulation suggests that the interface today is still responding to long term pleistocene sea level fluctuations and has not achieved equilibrium with present day sea level conditions

fractional crystallization induced variations in sulfides from the noril sk talnakh mining district polar siberia russia abstract the distribution of platinum group elements pge within zoned magmatic ore bodies has been extensively studied and appears to be controlled by the partitioning behavior of the pge during fractional crystallization of magmatic sulfide liquids however other chalcophile elements especially tabs te as bi sb and sn have been neglected despite their critical role in forming platinum group minerals pgm tabs are volatile trace elements that are considered to be mobile so investigating their primary distribution may be challenging in magmatic ore bodies that have been somewhat altered magmatic sulfide ore bodies from the noril sk talnakh mining district polar siberia russia offer an exceptional opportunity to investigate the behavior of tabs during fractional crystallization of sulfide liquids and pgm formation as the primary features of the ore bodies have been relatively well preserved in this study new petrographic 2d and 3d and whole rock geochemical data from cu poor to cu rich sulfide ores of the noril sk talnakh mining district are integrated with published data to consider the role of fractional crystallization in generating mineralogical and geochemical variations across the different ore types disseminated to massive despite textural variations in cu rich massive sulfides lenses veins and breccias these sulfides have similar chemical compositions which suggests that cu rich veins and breccias formed from fractionated sulfide liquids that were injected into the surrounding rocks numerical modeling using the median disseminated sulfide composition as the initial sulfide liquid composition and recent dmss liq and diss liq predicts the compositional variations observed in the massive sulfides especially in terms of pt pd and tabs therefore distribution of these elements in the massive sulfides was likely controlled by their partitioning behavior during sulfide liquid fractional crystallization prior to pgm formation our observations indicate that in the cu poor massive sulfides the pgm formed as the result of exsolution from sulfide minerals whereas in the cu rich massive sulfides the pgm formed by crystallization from late stage fractionated sulfide liquids we suggest that the significant amount of sn bearing pgm may be related to crustal contamination from granodiorite whereas as bi te and sb were likely added to the magma along with s from sedimentary rocks large pgm that are scarce and randomly distributed may account for most of the whole rock pt budget based on our results we propose a holistic genetic model for the formation of the magmatic sulfide ore bodies of the noril sk talnakh mining district

reduction of saltwater intrusion by modifying hydraulic conductivity we present an approach for reducing saltwater intrusion in coastal aquifers by artificially reducing the hydraulic conductivity in the upper part of selected areas by using a precipitate we apply a previously presented analytical approach to develop formulas useful for the design of artificial barriers equations for the location of the tip of the saltwater wedge are presented and verified through a sand tank experiment the analysis is capable of computing discharges exactly but requires the dupuit forchheimer approximation to compute points of the interface between flowing fresh and stationary saltwater we consider a vertical coastline and boundaries in the fresh water zone of either given discharge or given head we demonstrate in the paper that reduction of the hydraulic conductivity in the upper part of a coastal aquifer will result in a decrease of saltwater intrusion and present analytic expressions that can be used for design purposes the previously presented analytical approach can be applied to design systems to reduce salt water intrusion caused by pumping inland from the zone that contains salt groundwater this article is protected by copyright all rights reserved

platinum group elements in sulphide minerals platinum group minerals and whole rocks of the merensky reef bushveld complex south africa implications for the formation of the reef the concentrations of platinum group elements pge co re au and ag have been determined in the base metal sulphide bms of a section of the merensky reef in addition we performed detailed image analysis of the platinum group minerals pgm the aims of the study were to establish 1 whether the bms are the principal host of these elements 2 whether individual elements preferentially partition into a specific bms 3 whether the concentration of the elements varies with stratigraphy or lithology 4 what is the proportion of pge hosted by pgm 5 whether the pgm and the pge found in bms could account for the complete pge budget of the whole rocks in all lithologies most of the pge 65 up to 85 are hosted by pgm essentially pt fe alloy pt pd sulphide pt pd bismuthotelluride lesser amounts of pge occur in solid solution within the bms in most cases the pgm occur at the contact between the bms and silicates or oxides or are included within the bms pentlandite is the principal bms host of all of the pge except pt and contains up to 600 ppm combined pge it is preferentially enriched in pd rh and co pyrrhotite contains rh os ir and ru but excludes both pt and pd chalcopyrite contains very little of the pge but does concentrate ag and cd platinum and au do not partition into any of the bms instead they occur in the form of pgm and electrum in the chromitite layers the whole rock concentrations of all the pge except pd are enriched by a factor of five relative to s ni cu and au this enrichment could be attributed to bms in these layers being richer in pge than the bms in the silicate layers however the pge content in the bms varies only slightly as a function of the stratigraphy the bms in the chromitites contain twice as much pge as the bms in the silicate rocks but this is not sufficient to explain the strong enrichment of pge in the chromitites in the light of our results we propose that the collection of the pge occurred in two steps in the chromitites some pgm formed before sulphide saturation during chromitite layer formation the remaining pge were collected by an immiscible sulphide liquid that percolated downward until it encountered the chromitite layers in the silicate rocks pge were collected by only the sulphide liquid

metamorphic evolution of sulphide rich chromitites from the chernichevo ultramafic massif se bulgaria abstract the upper mantle rocks of the metamorphosed ophiolite of chernichevo rhodope metamorphic complex in southern bulgaria host small chromite ores with unusual mineralization of base metal sulphides rich in platinum group elements mineralogical and chemical data indicate that after their formation in the mantle the chernichevo s chromite ores were modified by the intrusion of an alkaline mafic melt which resulted in the precipitation of a suite of metasomatic minerals sulphides calcite apatite and ilmenite accompanied by an increase in feo tio2 ga zn vn mn and especially ti and fe2o3 contents in the chromite the degree of chemical modification and abundance of metasomatic minerals are positively correlated and mark the extent of reaction of the chromitite with the intruding melt sulphide segregation promoted the concentration of high amounts of pges up to 3661 ppb particularly pt and pd yielding chromite ores with a typical flat to positive sloped chondrite normalized pattern subsequently the chromite ores were deformed and metamorphosed together with their host rocks at ultra high pressure uhp 2 5 gpa 1200 c to be later retrograded under eclogite and finally hydrous amphibolite facies conditions giving rise to three microstructural types metamorphism of the most metasomatized i e sulphide rich chromitites at temperatures 700 c within the conditions of uhp and eclogite facies resulted in the formation of 1 non porous recrystallized chromite consisting of a granoblastic microstructure made up of coarse grained blasts and finer grained chromite neoblasts in contrast hydrous metamorphism on the less metasomatized i e sulphide poor chromitite under the conditions of amphibolite facies ca 482 483 c resulted in the formation of 2 partly altered chromite characterized by unaltered cores surrounded by fe2 rich and al depleted porous chromite containing abundant clinochlore and 3 porous chromite corresponding to a chromite that was entirely transformed by the metamorphic alteration to fe2 rich and al depleted porous chromite during metamorphism magmatic ni fe cu sulphides originally formed during the metasomatic event in the mantle were altered resulting in a major leaching of cu rich sulphides leading to significant remobilization of pt and pd

noble metal enrichment processes in the merensky reef bushveld complex we have analysed sulphides silicates and chromites of the merensky introduction reef for platinum group elements pges re and au using laser the enrichment of noble metals in layered gabbroic ablation inductively coupled plasma mass spectrometry and synthetic intrusions to form stratiform merensky type ore horizons pyrrhotite standards annealed with known quantities of noble metals is an extremely efficient yet poorly understood process os ir and ru reside in solid solution in pyrrhotite and pentlandite metal enrichment must be related to the formation of rh and part of the reef s pd in pentlandite whereas pt au re magmatic layering as the ore horizons form an integral and some pd form discrete phases olivine and chromite often part of the magmatic stratigraphies of their host insuspected to carry os ir and ru are pge free all phases analysed trusions metal concentration is also related to the prescontain noble metals as discrete micro inclusions with diameters ence of magmatic sulphides because the noble metals are either dissolved in base metal sulphides or occur as typically 100 nm inclusions in sulphides commonly have the discrete noble mineral phases intergrown with sulphides element combinations os ir pt and pt pd au inclusions in vermaak hendriks 1976 ballhaus ryan 1995 olivine and chromite are dominated by pt au pd few inclusion ballhaus ulmer 1995 spectra can be related to discrete noble metal phases and few there is considerable controversy as to where the inclusions have formed by sub solidus exsolution rather some metals came from and how metal enrichment worked pge inclusions notably those in olivine and chromite are earlyfor a stratiform platinum group element pge deposit magmatic nuggets trapped when their host phases crystallized we inside the magmatic stratigraphy of a layered intrusion suggest that the silicate melt layer that preceded the merensky reef there are two possibilities either the metals were derived was pge oversaturated at early cumulus times experiments from the melt column above the ore horizon or they came combined with available sulphide silicate partition coefficients sugfrom the magma equivalent to the cumulate package gest that a silicate melt in equilibrium with a sulphide melt underlying the ore owing to the large density contrast containing the pge spectrum of the merensky ore would indeed be between silicate and sulphide melt a popular view is that oversaturated with respect to the least soluble noble metals sulphide the metals were enriched by gravitational settling of melt apparently played little role in enriching the noble metals in sulphide melt i e out of the magma column above the the merensky reef rather its role was to immobilize a pre existing ore campbell et al 1983 barnes naldrett 1985 in situ stratiform pge anomaly in the liquid stratified magma naldrett et al 1987 on the other hand it is well chamber established that gravitational crystal settling plays little role in producing magmatic layering and magmatic stratigraphies mcbirney noyes 1979 morse 1986 campbell 1987 and this cannot remain without consequence for stratiform noble metal deposits a third alternative is noble metal enrichment by late

carbonate dissolution in a modern mixing zone the potential for dissolution of calcite and aragonite and precipitation of dolomite by the mixing of fresh and saline ground waters has been demonstrated theoretically1 3 although geologists have used these processes in the explanation of the diagenesis of recent4 7 and ancient8 12 carbonates there has been only limited investigation of the geochemical processes of modern mixing zones13 14 here we report the results of chemical and rock profiling through a modern mixing zone on andros island in the bahamas our results demonstrate undersaturation of the waters in the mixing zone with respect to aragonite and calcite and corresponding pervasive dissolution of the carbonate rocks at this level undersaturation is significantly enhanced by bacterial processes which are also responsible for precipitation of an iron rich crust despite chemical supersaturation precipitation of dolomite was not observed thereby casting some doubt on the mixing zone model of dolomitization

modelling of colloidal particle and heavy metal transfer behaviours during seawater intrusion and refreshing processes the proper management of coastal aquifers commonly requires an understanding of regional mass flow and complete seawater freshwater circulation in this study time series observations of seawater intrusion and refreshing were conducted using a column experiment based on natural flow conditions in coastal groundwater and a sampled medium from a coastal sandy aquifer without chemical treatment ranges of hydrodynamic and hydrochemical variables were tested and analysed the results showed that the zeta potential of suspended colloids in aqueous solution in an aquifer polluted with 0 5 g kg of heavy metals exhibited an isoelectric point for ph values ranging from 5 70 to 6 07 when freshwater or seawater completely occupied the aquifer pores which is representative of natural hydrochemical conditions in this scenario a high background concentration of heavy metals induced colloidal immobilization otherwise seawater freshwater circulation enabled colloid mobilization due to ionic strength and ph fluctuations the migration of multiple heavy metals occurred at a characteristic time of approximately 1 pore volume after each intrusion stage began and when the peak rate of colloid release was reached at these times the colloid behaviour determined the quantity and pathway of heavy metal transport on the basis of the influences of seawater and freshwater interactions the quantity of mobilized particles generally decreased and was uniformly distributed in each fraction due to particle loss and decreased porous connectivity we speculate that the decrease in the total surface area of the migratory colloids may cause colloid associated heavy metal transport to decrease the experimental results provide a useful basis for testing coastal groundwater flow and mass transport models because these phenomena require full characterization to precisely evaluate the associated fluxes from the field scale to the microscopic dimension

paragenesis of multiple platinum group mineral populations in shetland ophiolite chromitite 3d x ray tomography and in situ os isotopes abstract chromitite from the harold s grave locality in the mantle section of the shetland ophiolite complex is extremely enriched in ru os and ir at g g concentrations high resolution x ray computed tomography on micro cores from these chromitites was used to determine the location size distribution and morphology of the platinum group minerals pgm there are five generations of pgm in these chromitites small average 5 m in equivalent sphere diameter esd euhedral laurites often with os ir alloys are totally enclosed in the chromite and are likely to have formed first by direct crystallisation from the magma as the chromite crystallised also within the chromitite there are clusters of larger 50 m esd aligned elongate crystals of pt rh ir os and ru bearing pgm that have different orientations in different chromite crystals these may have formed either by exsolution or by preferential nucleation of pgms in boundary layers around particular growing chromite grains thirdly there is a generation of large 100 m esd composite os ir ru rich pgm that are all interstitial to the chromite grains and sometimes form in clusters it is proposed that os ir and ru in this generation were concentrated in base metal sulfide droplets that were then re dissolved into a later sulfide undersaturated magma leaving pgm interstitial to the chromite grains fourthly there is a group of almost spherical large 80 m esd laurites hosting minor os ir ru rich pgm that form on the edge or enclosed in chromite grains occurring in a sheet crosscutting a chromitite layer these may be hosted in an annealed late syn or post magmatic fracture finally a few of the pgm have been deformed in localised shear zones through the chromitites the vast majority of the pgm including small pgm enclosed within chromite larger interstitial pgm and elongate aligned pgm have os isotope compositions that give re depletion model ages approximately equal to the age of the ophiolite at 492 ma a number of other pgm not confined to a single textural group fall to more or less radiogenic values with four pgm giving anomalously unradiogenic os corresponding to an older age of 1050 ma the 187 os 188 os isotopic ratios for pgm from cliff and quoys from the same ophiolite section are somewhat more radiogenic than those at harold s grave this may be due to a distinct mantle source history or possibly the assimilation of radiogenic crustal os

medium and small scale ionospheric irregularities detected by gps radio occultation method 1 the radio occultation technique is used to study plasma irregularities in the f2 layer ionosphere and below the data acquired by the gps receiver onboard the geophysical research satellite champ for more than three years allow to study ionospheric irregularities under different geophysical conditions on a global scale the dual frequency gps signals provide a possibility to trace irregularities with a spatial size ranging from about 20 to 250 km whereas the irregularities characterized by longer wavelengths dominate at low latitudes the shorter scale perturbations are very intense at high latitudes it is found that the high latitude irregularities occur mostly at night time when the ionization level is low

influence of a coarse interlayer on seawater intrusion and contaminant migration in coastal aquifers vertical 2d slice laboratory experiments were carried out in homogenous and layered sand tanks to elucidate the effects of a highly permeable coarse grained sand interlayer on seawater intrusion and transport of contaminants to a coastal sea tidal fluctuations produced oscillations in the seawater freshwater transition zone fluctuations of the contaminant infiltration rate and a zigzag contaminant plume outline the seawater wedge became discontinuous at the vertical edges of the interlayer due to increased lateral movement of the seawater freshwater interface within the interlayer the contaminant plume formed a tail within the interlayer depending on the tidal stage and similarly to the wedge its movement was accentuated a simple analytical model that neglected vertical flow reliably predicted steady state seawater intrusion into the coastal aquifer numerical modeling was used to gain insight into the groundwater hydrodynamics and contaminant migration the numerical results confirmed the experimental findings i e that a highly permeable interlayer can provide a rapid transit path for contaminants to reach the seaward boundary and that the interlayer amplifies the effects of tidal fluctuations resulting in wider transition zones for the seawater wedge and contaminant plume numerical simulations further showed that with increasing interlayer hydraulic conductivity the maximum seawater intrusion distance inside the interlayer increases approximately linearly for the fixed head contaminant injection condition used the model showed that contaminant infiltration increases approximately logarithmically with increasing interlayer hydraulic conductivity as other factors held fixed

homogenization and effective parameters for the henry problem in heterogeneous formations 1 general upscaling of density dependent flow is investigated for the classical two dimensional henry problem of saltwater intrusion in coastal aquifers combined theoretical and numerical results are here presented effective flow and transport parameters for saline intrusions could be derived for statistically isotropic and anisotropic heterogeneous permeability fields by use of homogenization theory applying also to the preasymptotic regime our numerical results indicate that heterogeneities in permeability affect foremost the transient evolution of saltwater intrusion whereas the steady state saltwater distribution is less sensitive to spatially varying permeabilities and longitudinal dispersion for the isotropic media the effective permeability is found to correspond to the geometric mean as under conditions without fluid density contrast the extension of results to anisotropic media requires nontrivial corrections for the effective permeability which include the variance of log k and the directional correlation lengths the appropriate dispersion coefficients for the problem correspond to the local dispersion coefficients rather than macrodispersion coefficients these results are discussed in light of the specific flow configuration posed in the henry problem

trace elements in sulfide inclusions from yakutian diamonds sulfide inclusions in diamonds may provide the only pristine samples of mantle sulfides and they carry important information on the distribution and abundances of chalcophile elements in the deep lithosphere trace element abundances were measured by proton microprobe in 50 sulfide inclusions sdi from yakutian diamonds about half of these were measured in situ in polished plates of diamonds providing information on the spatial distribution of compositional variations many of the diamonds were identified as peridotitic or eclogitic from the nature of coexisting silicate or oxide inclusions known peridotitic diamonds contain sdis with ni contents of 22 36 consistent with equilibration between olivine monosulfide solid solution mss and sulfide melt whereas sdis in eclogitic diamonds contain 0 12 ni a group of diamonds without silicate or oxide inclusions has sdis with 11 18 ni and may be derived from pyroxenitic parageneses eclogitic sdis have lower ni cu and te than peridotitic sdis the ranges of the two parageneses overlap for se as and mo the mo and se contents range up to 700 and 300 ppm respectively the highest levels are found in peridotitic diamonds among the in situ sdis significant zn and pb levels are found in those connected by cracks to diamond surfaces and these elements reflect interaction with kimberlitic melt significant levels of ru 30 1300 ppm and rh 10 170 ppm are found in many peridotitic sdis sdis in one diamond with wustite and olivine inclusions and complex internal structures have high levels of other platinum group elements pges as well and high chondrite normalized ir pd comparison with experimental data on element partitioning between crystals of monosulfide solid solution mss and sulfide melts suggests that most of the inclusions in both parageneses were trapped as mss while some high cu sdis with high pd rh may represent fractionated sulfide melts spatial variations of sdi composition within single diamonds are consistent with growth histories shown by cathodoluminescence images in which several stages of growth and resorption have occurred within magmatic environments that evolved during diamond formation

steady state freshwater seawater mixing zone in stratified coastal aquifers summary the mixing zone between freshwater and intruding seawater which controls regional groundwater flow dynamics and reactive transport processes is the most important component in coastal aquifers laboratory experiments and numerical simulations were conducted to investigate the effects of aquifer stratification on the thickness of a steady state freshwater seawater mixing zone a series of experiments were implemented in a laboratory tank packed with three layers of coastal sands and manipulated by two flow cells for maintaining constant head boundary conditions packed aquifers included a homogenous aquifer a high hydraulic conductivity k low k high k stratified aquifer and a low k high k low k stratified aquifer both tank scale and field scale density dependent models were developed to simulate the laboratory experiments and to systematically investigate aquifer stratification effects on the thickness of a steady mixing zone with varying layer configurations results showed that when an aquifer layer with a low k overlies a layer with a relatively high k slanting upward flow of diluted saltwater and re circulated seawater flow are refracted at the interface between the two layers resulting in streamline separation and a broad mixing zone in the low k layer by contrast the mixing zone in a high k layer is slightly narrowed when it overlies a layer with a relatively low k because flow refraction squeezes the streamlines within the mixing zone the mixing zone profile in head controlled systems is a function of k contrast but independent of the magnitude of k our results demonstrated the significant impact of aquifer stratification on controlling groundwater flow and solute transport in coastal subsurface environments and would have significant implications for studying physical chemical and biological processes in coastal subsurface environments as aquifer stratification is naturally prevalent

effects of wave forcing on a subterranean estuary wave and tide are important forcing factors that typically co exist in coastal environments a numerical study was conducted to investigate individual and combined effects of these forces on flow and mixing processes in a near shore subterranean estuary a hydrodynamic model based on the shallow water equations was used to simulate dynamic sea level oscillations driven by wave and tide the oscillating sea levels determined the seaward boundary condition of the coastal aquifer where variably saturated variable density flow was modeled the simulation results showed that waves induced an onshore upward tilt in the phase averaged sea level wave set up the resulting hydraulic gradient generated pore water circulations in the near shore zone of the coastal aquifer which led to formation of an upper saline plume usp similar to that due to tides however mixing of recirculating seawater in the usp with underlying fresh groundwater was less intensive under the high frequency wave oscillations in the case of combined forcing wave induced circulations coupled with the intra tidal flows strengthened the averaged circulating pore water flows in the near shore zone over the tidal period the circulating flows increased exchange between the subterranean estuary and ocean contributing 61 of the total submarine groundwater discharge for the simulated condition in comparison with the 40 and 49 proportions caused by the same but separate tidal and wave forcing respectively the combined forces also created a more extensive usp with the freshwater discharge zone shifted further seaward the freshwater flow paths in the intertidal subterranean estuary were altered with a significant increase of associated transit times the interplay of wave and tide led to increased mixing between discharging fresh groundwater and recirculating seawater these results demonstrated the complexity of near shore groundwater systems and have implications for future investigations on the fate of land sourced chemicals in the subterranean estuary prior to discharge to the ocean

understanding re os systematics and model ages in metamorphosed archean ultramafic rocks a single mineral to whole rock investigation abstract 187rhenium 187os model ages are used to infer the timing of crust mantle differentiation events and evolution of lithospheric mantle however ancient samples often have long and complex histories of metamorphism metasomatism and deformation that may be problematic if these processes caused disturbance the re os system such processes have been shown to disturb the re os systematics of ophiolitic chromitites but the effect on stratiform chromitites has not previously been investigated here we present a detailed petrographic and re os isotopic study of chromitites chromite bearing meta peridotites and single chromite grains from the ujaragssuit nun t layered ultramafic intrusion southwest greenland which has experienced intense deformation and at least two episodes of metamorphism up to amphibolite facies we report the first ever re os isotope and platinum group element concentration data for single chromite grains achieved by single grain dissolution and isotope dilution micrometric os ir ru sulphide and sulpharsenide inclusions in chromite occur in the massive chromitites these accessory phases host a significant portion of the os ir and ru in these samples the remaining os ir ru budget along with pt and pd appears to be homogeneously distributed within the chromite occurring either in the lattice itself as evenly distributed sub micrometric alloy inclusions in chromite unresolvable using the techniques applied in this study or a combination of both rhenium is hosted in silicates as predicted by previous studies both single grain and whole rock re os isotope systematics yield unfeasibly young re os model ages due to their radiogenic 187os 188os compositions this could result from country rock contamination of the original melt from which the ujaragssuit nun t intrusion crystallised and or from disturbance of the re os isotope systematics of these rocks during regional metamorphic events at 2 8 and or 3 75 ga we conclude that it is vital to establish a complete sample history before interpreting re os isotope data and the validity of model ages as the re os system is sensitive to disturbance it is feasible that in some cases single chromite grains or chromite mineral separates may offer a more robust and meaningful isotopic record than silicates because re is preferentially hosted in silicates relative to chromite alternatively the 190pt 186os decay system applied to whole rock samples may provide a more reliable chronometer than 187re 187os model ages in samples that have experienced crustal contamination and or metamorphic disturbance