inclusions of phlogopite and phlogopite hydrates in chromite from the hongguleleng ophiolite in xinjiang northwest china

stratigraphic and textural variations in the chromite composition of the ophiolitic sakhakot qila complex pakistan the deposits and their host rocks compare well with podiform ores hosted by alpine type peridotites of ophiolites unlike most alpine type complexes predominant stratigraphic variations in the chemical composition of chromite exist the chromite deposits fall in two major zones the first consisting of three subzones regular compositional correlations between the accessory chromites of the host rocks and the associated segregated chromites are drawn indicating relatively later crystallization of the former relict cumulate chromitite textures cover the podiform range and are examined in the light of compositional data the inequigranular textural varieties probably developed by crystallization of successive crops from magmatic fractionation with their coarser grained massive chromite components always being slightly higher in mg mg fe super 2 than the associated finer grained chromite components the mg fe super 2 exchange between chromite and silicates seems to start during the cumulus stage and continues during the postcumulus stage but is reduced to a very small intensity during the subsolidus stage until it ends at about 700 degrees c modified journal abstract

genesis and geodynamic significance of chromitites from the orhaneli and harmancik ophiolites bursa nw turkey as evidenced by mineralogical and compositional data abstract chromitites from orhaneli and harmancik ophiolites bursa nw turkey are mainly associated with dunites they occur as banded and lenticular bodies showing massive and disseminated textures and are composed of magnesiochromite grains characterized by the following composition cr 2 o 3 58 30 61 19 wt al 2 o 3 8 32 10 03 wt and tio 2 0 14 0 21 wt the cr cr cr al values of magnesiochromite from both ophiolites range between 0 80 and 0 83 and their mg mg mg fe 2 values vary from 0 52 to 0 69 however one chromitite sample bo13b from the orhaneli ophiolite contains magnesiochromite with relatively low cr 2 o 3 48 38 wt high al 2 o 3 15 16 wt and tio 2 0 30 wt contents and average cr and mg values of 0 68 and 0 56 respectively although the total content of platinum group elements pge in most chromitite samples varies between 83 and 169 ppb two samples from the orhaneli ophiolite show enrichments in total pge up to 500 ppb several platinum group mineral pgm grains occur as minute inclusions in magnesiochromite grains 1 15 m across laurite is dominant pgm phase followed by erlichmanite unidentified ru ni fe sulfide irarsite hollingworthite native osmium and iridium millerite and pentlandite represent the most abundant primary base metal mineral inclusions in magnesiochromite whereas bornite awaruite heazlewoodite violarite and orcelite are mostly observed in the secondary silicate groundmass olivine clinopyroxene and amphibole are also identified as silicate inclusions in the magnesiochromite the composition of the magnesiochromite and the parental melts in equilibrium indicate that these magnesiochromite grains were most likely crystallized from boninitic melts in an arc setting however bo13b chromitite sample from the orhaneli chromitites may have crystallized from arc related melt of probably an island arc tholeiite affinity

controls on platinum group elemental distributions of podiform chromitites a case study of high cr and high al chromitites from chinese orogenic belts abstract a study of podiform chromite deposits from the asiatic orogenic belt and the qilian qiangling kunlun himalaya tectonic domain provides new insights into the geochemistry of the pges in podiform chromite deposits and the genesis of the deposits themselves the bulk of deposits which occur in mantle peridotites of ophiolites have typical ophiolitic pge patterns that are depleted in pt and pd relative to the average upper mantle and have negatively sloping distributions on mantle normalized diagrams type i high cr chromitites have higher os ir ru and rh contents than type ii high al chromitites although both have similar pd and pt most of the type i and ii chromite deposits have lower pd and pt contents than the upper mantle peridotites in which they occur podiform chromitites are essentially products of melt rock interaction in the upper mantle their cr and pges were contributed by not only the invading magmas but also by the upper mantle host the chromite deposits are in part metasomatic replacement bodies the type i high cr chromitite pge patterns were produced by interaction between s undersaturated boninitic magmas and depleted harzburgites whereas the type ii high al chromitite pge patterns were formed by interaction between initially s saturated tholeiitic magmas and depleted harzburgites the low to very low pd and pt contents of both type i and type ii chromitites require that the mantle assemblage in which the chromite deposits were formed had lost their sulfides and hence pd and pt prior to formation of the chromite deposits in addition no or little pd and pt were deposited by the invading magma which either remained s undersaturated boninite or became morb s undersaturated due to interaction with the s depleted harzburgitic mantle it is suggested that the very low ir os and ru contents of boninites in general might be due to loss of ir during the formation of podiform chromitites it is suggested that podiform chromitites with type i pge patterns were formed in an island arc environment whereas those with type ii pge patterns were formed in a back arc setting

compositional variations in the mesoarchean chromites of the nuggihalli schist belt western dharwar craton india potential parental melts and implications for tectonic setting the chromite deposits in the archean nuggihalli schist belt are part of a layered ultramafic mafic sequence within the western dharwar craton of the indian shield the 3 1 ga ultramafic mafic units occur as sill like intrusions within the volcano sedimentary sequences of the nuggihalli greenstone belt that are surrounded by the tonalite trondhjemite granodiorite ttg suite of rocks the entire succession is exposed in the tagdur mining district the succession has been divided into the lower and the upper ultramafic units separated by a middle gabbro unit the ultramafic units comprise of deformed massive chromitite bodies that are hosted within chromite bearing serpentinites the chromitite bodies occur in the form of pods and elongated lenses 60 500 m by 15 m detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate rich chromitite 50 modal chromite and serpentinite 2 modal chromite throughout the entire ultramafic sequence however the primary composition of chromite is preserved in the massive chromitites 60 75 modal chromite from the byrapur and the bhaktarhalli mining district of the nuggihalli schist belt these are characterized by high cr ratios cr cr al 0 78 0 86 and moderate mg ratios mg mg fe2 0 38 0 58 the compositional variability occurs due to sub solidus re equilibration in the accessory chromite in the serpentinite mg ratio 0 01 0 38 cr ratio 0 02 0 99 and in silicate rich chromitite mg ratio 0 06 0 48 cr ratio 0 60 0 99 in the massive chromitites the sub solidus re equilibration for chromite is less or absent however the re equilibration is prominent in the co existing interstitial and included olivine fo96 98 and pyroxene grains mg numbers 97 99 compositional variability on the scale of a single chromite grain occurs in the form of zoning and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite in the zoned grains the composition of the core is modified and the rim is ferritchromit in general ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains in this case ferritchromit formation is not very extensive this indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization in some occurrences the ferritchromit rim is very well developed and only a small relict core appears to remain in the chromite grain however complete alteration of the chromite grains to ferritchromit without any remnant core is also present the regular well developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries the complete alteration of the chromite grains and the modification of the core composition indicate the alteration in the nuggihalli schist belt to be intense pervasive and affected by later low grade metamorphism the primary composition of chromite has been used to compute the nature of the parental melt the parental melt calculations indicate derivation from a high mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the western dharwar craton tectonic discrimination diagrams using the primary composition of chromites indicate a supra subduction zone setting ssz for the archean chromitites of nuggihalli and derivation from a boninitic magma the composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back arc rift settings formation of the massive chromitites in nuggihalli may be due to magma mixing process involving hydrous high mg magmas or may be related to intrusions of chromite crystal laden magma however there is little scope to test these models because the host rocks are highly altered serpentinized and deformed the present configurations of the chromitite bodies are related to the multistage deformation processes that are common in archean greenstone belts

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

the recycling of chromitites in ophiolites from southwestern north america abstract podiform chromitites occur in mantle peridotites of the late triassic puerto nuevo ophiolite baja california sur state mexico these are high cr chromitites cr cr cr al atomic ratio 0 61 0 69 that contain a range of minor and trace elements and show whole rock enrichment in ipge os ir ru that are similar to those of high cr ophiolitic chromitites crystallised from melts similar to high mg island arc tholeiites iat and boninites in supra subduction zone mantle wedges crystallisation of these chromitites from s undersaturated melts is consistent with the presence of abundant inclusions of platinum group minerals pgm such as laurite rus2 erlichmanite oss2 osmium and irarsite irass in chromite that yield tma trd model ages peaking at 325 ma thirty three xenocrystic zircons recovered from mineral concentrates of these chromitites yield ages 2263 44 ma to 278 4 ma and hf o compositions hf t 18 7 to 9 1 and 18o values

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

chromitite and ultramafic rock compositional zoning through a paleotransform fault poum new caledonia the major mylonitic shear zone of the northern part of the ophiolitic peridotite sheet of new caledonia has been described as a paleotransform fault secher 1981 located along this structure the poum ultramafic massif shows a wide spectrum of peridotite compositions ranging from plagioclase lherzolite to dunite across a 2 km thick transition zone the mylonitized plagioclase lherzolite contains abundant pyroxenite and mafic veins the poum ultramafic massif includes more than 30 chromitite orebodies their composition vanes according to the nature of the host peridotite al rich in the plagioclase lherzolite zone 31 43 al 2 o 3 and 25 36 cr 2 o 3 cr rich 39 60 cr 2 o 3 in the spinel lherzolite and clinopyroxene harzburgite zones and still richer in the harzburgite dunite zone 49 64 cr 2 o 3 in peridotites the accessory cr spinel also displays a cr enrichment from lherzolite to dunite segregations of cr rich chromite 50 60 cr 2 o 3 are present in orthopyroxenite veins displaying boninitic affinities the poum chromitites display low platinum group element pge contents and pd ir ratios as do the chromitite segregations of the orthopyroxenite veins suggesting a common parent magma or and common fractionation processes the compositional zoning of the poum massif can be explained by the contrasting behavior of ultramafic rocks and upwelling mafic melts away from the axis of the transform fault using the current ideas about the genesis of boninitic melts and the formation of dunite and harzburgite by magma rock interactions during melt percolation nicolas and prinzhofer 1983 kelemen 1990 kelemen et al 1992 takazawa et al 1992 in the lithospheric lherzolite zone that was cooled and mylonitized along the older wall of the transform fault melts were channeled along fractures leaving al rich chromitite pods by contrast pervasive percolation along the lithosphere asthenosphere boundary allowed important melt rock interactions resulting in the progressive formation of harzburgite and dunite and generating mg cr rich melts from which precipitated cr rich chromitites

irarsite hollingworthite solid solution series and other associated ru os ir and rh bearing pgm s from the shetland ophiolite complex pure end and intermediate members of the irarsite hollingworthite solid solution series occur in the shetland ophiolite complex hollingworthite frequently rims irarsite their compositions are unusually pt poor compared with analyses of these minerals from elswhere suggesting the existence of a pt poor environment during their formation ir sb s and rh sb s have been identified as inclusions within irarsite ir sb s and rh sb s together with rh ni sb are thought to be new platinum group minerals pgm s in ophiolite complexes two types of laurite are present an os rich up to 22 os variety is entirely enclosed by chromite whereas an os free variety is located in the silicate matrix interstitial to the chromite laurites in the rims of chromite grains are os free but contain tiny inclusions of native osmium it is suggested that either the availability of os decreased during crystallisation of the laurites or that os has been removed from laurites not totally enclosed by chromite in a few cases laurite is surrounded by a ruthenian pentlandite containing up to 12 ru

seasonal circulation over the catalan inner shelf northwest mediterranean sea 1 this study characterizes the seasonal cycle of the catalan inner shelf circulation using observations and complementary numerical results the relation between seasonal circulation and forcing mechanisms is explored through the depth averaged momentum balance for the period between may 2010 and april 2011 when velocity observations were partially available the monthly mean along shelf flow is mainly controlled by the along shelf pressure gradient and by surface and bottom stresses during summer fall and winter the along shelf momentum balance is dominated by the barotropic pressure gradient and local winds during spring both wind stress and pressure gradient act in the same direction and are compensated by bottom stress in the cross shelf direction the dominant forces are in geostrophic balance consistent with dynamic altimetry data

along shelf current variability on the catalan inner shelf nw mediterranean 1 we examine the circulation over the inner shelf of the catalan sea using observations of currents obtained from three acoustic doppler current profilers two at 24 m and one at 50 m during march april 2011 the along shelf current fluctuations during that period are mainly controlled by local wind stress on short time scales and by remote pressure gradients on synoptic time scales different forcing mechanisms are involved in the along shelf momentum balance during storm conditions wind stress sea level gradients and the nonlinear terms dominate the balance during weak wind conditions the momentum balance is controlled by the pressure gradient while during periods of moderate wind in the presence of considerable stratification the balance is established between the coriolis and wind stress terms vertical variations of velocity are affected by the strong observed density gradient the increased vertical shear is accompanied by the development of stratified conditions due to local heating when the wind is not able to counteract and break stratification the occasional influence of the besos river plume is observed in time scales of hours to days in a limited area in near the city of barcelona the area affected by the plume depends on the vertical extent of the fresher layer the fast river discharge peak and the relaxation of cross shore velocities after northeast storm events this contribution provides a first interpretation of the inner shelf dynamics in the catalan sea

precious metals in magmatic fe ni cu sulfides from the potosi chromitite deposit eastern cuba abstract the moa baracoa ophiolite in eastern cuba is one of the few known ophiolites that display sulfide mineralization attributable to a magmatic origin in association with podiform chromite ores hosted in the mantle crust transition these sulfide ores chiefly consist of fe ni cu sulfides namely pyrrhotite pentlandite chalcopyrite and cubanite partly altered to valleriite the sulfide mineralization is located along the contact between the podiform like chromite ores and intruding pegmatitic gabroic dykes the detailed mineralogical study of the sulfide mineralization coupled with the first ever laser ablation icp ms analysis reveals that this sulfide mineralization show contents of the precious metals os ir ru pt re au ag and other semi metals co ni cu se te bi pb as sb comparable to those sulfides from the magmatic sulfide deposits associated with mafic complexes hosted in the continental crust the results obtained from this study confirm that fe ni cu sulfides at potosi are magmatic in origin and very likely derived from the solidification of droplets of sulfide melt segregated by immiscibility from the intruding mafic melts once they interacted with the pre existing chromitite at the mantle crust transition zone of the ophiolite the immiscibility of sulfide melt was achieved as a result of a progressive increase of fs2 very likely triggered by a set of circumstances including the progressive fractionation of the intruding mafic melt leading to increase of asio2 and accumulation of volatiles as well as the crystallization of oxides two main generations of pentlandite were observed one generation is primary in origin and it was locally exsolved along with pyrrhotite from monosulfide solid solution mss during low temperature cooling the second type of pentlandite resulted from the reaction of mss with coexisting droplets of cu and ni rich sulfide melt la icp ms analysis reveals that most precious metals ru os ir re au ag were concentrated along with the base metal sulfides bms although their distribution among the different bms pyrrhotite pentlandite chalcopyrite and cubanite does not strictly follow the expected distribution according to the known melt solid and solid solid partition coefficients unlike the other analyzed pges pt was not preferentially concentrated in bms but as discrete micrometer sized sperrylite grains the crystallization of sperrylite took place before and contemporaneous to sulfide segregation and pt as nanoparticles probably played an important role in the pt uptake as nucleation seeds for the formation of micron sized sperrylite grains these observations highlight the open system nature of the ore forming system as well as the important role of arsenic in concentrating pge in high temperature silicate and sulfide melts

zircon ages and geochemical compositions of the manlay ophiolite and coeval island arc implications for the tectonic evolution of south mongolia abstract numerous small dismembered ophiolite fragments occur in south mongolia but they are very poorly studied the lack of age data and geochemical analysis hampers our understanding of the paleozoic tectonic evolution of the region we conducted detailed studies on the manlay ophiolitic complex and huree volcanic rocks south of the main mongolian lineament mml to provide some constraints on these rocks the manlay ophiolite consists of dunite harzburgite pyroxenite gabbro plagiogranite basalt and chert locally with chromite mineralization in dunite the gabbro and plagiogranite yielded shrimp zircon weighted mean 206 pb 238 u ages of 509 5 ma and 482 4 ma respectively the basalt and dolerite samples of this complex show enrichment in lree and lile and negative nb ta and ti anomalies and the chrome spinel from the chromitite lens in the dunite is characterized by high cr and low tio 2 contents these features suggest a supra subduction zone ssz origin for the ophiolitic complex the huree volcanic rocks ranging from basalt to dacite display enrichment in lree and lile weak eu anomalies and distinctly negative nb ta and ti anomalies consistent with those of typical magmas in a subduction environment an andesite sample from this arc yielded a shrimp 206 pb 238 u zircon age of 487 5 ma which is the oldest reliable age for an island arc in south mongolia recognition of an early paleozoic ophiolitic complex and a coeval island arc indicates that south mongolia underwent a period of active volcanism during late cambrian to ordovician additionally the tuff overlying the ophiolitic complex and a granite intruding the ophiolite have shrimp zircon u pb ages of 391 5 ma and 304 4 ma respectively combining the available data we propose that the early paleozoic subduction accretionary complexes likely constitute the basement of the late paleozoic arc formations and correlate with the lake zone in western mongolia

the genesis of archaean chromitites from the nuasahi and sukinda massifs in the singhbhum craton india abstract the chromitite deposits of the nuasahi and sukinda massifs are part of layered ultramafic bodies which occur within archaean low grade metamorphic rocks of the iron ore group iog in the singhbhum craton of the indian shield the chromitite seams are interlayered with dunite and associated with orthopyroxenite detailed electron microprobe study reveals very high mg numbers 0 66 0 82 and cr numbers 0 75 0 87 for chromite in massive chromitite from the seams high fo content fo 92 95 for olivine in dunite and high en content en 89 94 for orthopyroxene in orthopyroxenite our study suggests that the original igneous compositions for these minerals are preserved in the respective monomineralic rocks and can be used to evaluate the primary magmatic petrogenesis of the rocks the primitive compositions of the minerals in monomineralic rocks suggest high degree of partial melting of the source mantle during the mesoarchaean the parental magma from which the massive chromitites crystallized was of siliceous high mg basaltic or boninitic compositions similar to the compositions of spatially associated chromite bearing siliceous high mg basalts of the iron ore group the parental magma may have generated due to the interaction of a depleted mantle with a fluid enriched melt possibly derived in response to the dehydration of a subducting slab monomineralic chromitite layers formed either in response to mixing of magmas or due to the suppression of silicate mineral crystallization in response to elevated h 2 o concentrations in the parent magmas the parental magmas that produced chromitites may have intruded into the volcano sedimentary greenstone belts of a supra subduction zone setting

chromite typology and composition characterized through multispectral specular reflectance abstract determination of multispectral specular reflectance is an important tool for ore identification in reflected light microscopy and may be used for automated characterization of ores however reflectance values can be affected by compositional variations in a way that is seldom understood the aim of the present work is to investigate this problem in chromite an ore whose composition may show typically large natural variations as member of the spinel family and whose relatively high reflectance variations are apparently unpredictable for this research eighteen samples of chromian spinel covering a large range of compositions in the base of the hagerthy prism were selected for microprobe analysis and reflectance measure on polished sections the samples belong to a variety of deposits and types ultramafic massif ophiolitic and metamorphosed types ojen and ronda massifs spain mayari moa baracoa camaguey and sagua de tanamo cuba golyamo kemenyane avren and yakovitsa bulgaria the specular reflectances are characterized as multispectral values measured at thirteen intervals 50 nm each in the vnir region visible and near infrared 400 1000 nm using the automated cameva system the relationship between compositional and reflectance values is studied by multivariate analysis and subsequently tested on independent samples for this purpose the samples have been grouped in two sets a larger population of fifteen samples constitutes the initial data set for mathematical processing while a selection of three samples of widely differing compositions is used to test the resulting functions so as to gain a critical appraisal of their validity the results obtained show that the specular reflectance of chromite depends on composition and can be used to estimate compositional parameters as cr cr cr al or mg mg mg fe2 but this relationship is complex and does not allow simple direct determinations due to the multiplicity of possible changes and coupled substitutions e g al cr fe3 or mg fe2 ni zn ti in the chromian spinel family on the other hand the relationship of chromite composition with deposit type is also detectable through the reflectance values but with a higher uncertainty reflectance increases with increasing cr and fe3 contents in all cases but the relationship of reflectance with mg seems to be specific of the deposit type while reflectance increases with increasing mg in the ojen chromites it shows the opposite behaviour in podiform chromites although these results should be regarded as preliminary until further studies on larger sample populations can be achieved they are suggestive of possible practical applications for exploration e g to approach cr spinel compositions or chromite deposit typology early in an exploration campaign by inexpensive reflected light determination of specular reflectance in a few samples

formation of fine sediment deposit from a flash flood river in the mediterranean sea we identify the mechanisms controlling fine deposits on the inner shelf in front of the besos river in the northwestern mediterranean sea this river is characterized by a flash flood regime discharging large amounts of water more than 20 times the mean water discharge and sediment in very short periods lasting from hours to few days numerical model output was compared with bottom sediment observations and used to characterize the multiple spatial and temporal scales involved in offshore sediment deposit formation a high resolution 50 m grid size coupled hydrodynamic wave sediment transport model was applied to the initial stages of the sediment dispersal after a storm related flood event after the flood sediment accumulation was predominantly confined to an area near the coastline as a result of preferential deposition during the final stage of the storm subsequent reworking occurred due to wave induced bottom shear stress that resuspended fine materials with seaward flow exporting them toward the midshelf wave characteristics sediment availability and shelf circulation determined the transport after the reworking and the final sediment deposition location one year simulations of the regional area revealed a prevalent southwestward average flow with increased intensity downstream the circulation pattern was consistent with the observed fine deposit depocenter being shifted southward from the river mouth at the southern edge bathymetry controlled the fine deposition by inducing near bottom flow convergence enhancing bottom shear stress according to the short term and long term analyses a seasonal pattern in the fine deposit formation is expected

distribution of aluminum rich and chromium rich chromite pods in ophiolite peridotites detailed study of the acoje and the coto districts zambales philippines and comparison with new caledonia the troodos massif and other ophiolites cr rich pods are located near the lherzolite harzburgite transition zone whereas the al rich pods are distributed along the harzburgite dunite transition zone the chromite pods have probably been generated from different sources during the upwelling of mantle peridotites modified journal abstract

podiform chromitites of the tari misaka ultramafic complex southwestern japan as mantle melt interaction products petrological and geochemical data for chromian spinel from the tari misaka ultramafic complex southwest japan are discussed in this complex the spinels form a chromitite pod coated by a dunite envelope and are collectively enclosed by clinopyroxene bearing harzburgite chromitite and its dunite envelope are distinctly different from the surrounding harzburgite in terms of the chromian spinel chemistry the ti content is five to ten times higher in spinels of the enveloped chromitite than in those of the harzburgite although the cr ratio cr cr al the atomic ratio for both is almost constant or varies only slightly 0 4 0 5 this eliminates a comagmatic origin for the harzburgite dunite chromitite association spinel chemistry for both major and trace elements is almost the same in the chromitite and its dunite envelope except for the mg ratio and the co and mn contents which have a changed subsolidus as a function of the spinel mode thus the dunite and chromitite could be comagmatic but fractional crystallization of the dunite after or before chromitite deposition is not possible the chemistry of the chromian spinel and the mode of occurrence of the dunite chromitite association are consistent with an origin by interaction of an exotic melt with wall rock harzburgite the dunite may be of replacive origin the exotic melt would react with the harzburgite at the new contact to form dunite and a secondary si rich melt the melt thus formed could mix with a successive inflow of a more primitive melt the mixed melt would be expected to have chromite chromian spinel as the only liquidus phase precipitating chromitite next to the replacive dunite the dunite envelope podiform chromitite is most commonly formed in the mantle beneath island arcs the spread in cr ratios of spinel in podiform chromitites is almost comparable with that of spinel in primitive arc related magmas the presence of chromitite in the upper mantle beneath the japan arcs is consistent with this interpretation

complex human vegetation climate interactions in the late holocene and their significance for paleotemperature reconstructions using 3 980 14c ages and 43 669 pollen data from 632 globally distributed lakes 88 of them in north america and europe fig 1 a jenny et al 1 demonstrate that increased soil erosion rates fig 1 b and decreased tree coverage fig 1 c have occurred in lake watersheds across a substantial portion of earth s surface since 4 000 y bp besides using climate model results fig 1 d they propose that the late holocene tree coverage decreases were caused mainly by anthropogenic deforestation 1 notably the model deduced late holocene slight cooling trends in north america and europe 1 fig 1 d are the opposite of the slight global warming trends indicated by other models 2 fig 1 e arguably it is problematic to use model results to determine a climate vegetation relationship moreover the model deduced slight cooling trends since 4 000 y bp 1 fig 1 d are supported by a synthesized global mean temperature record 3 fig 1 f however only 4 out of the 73 selected records fig 1 a comprising the synthesized record are terrestrial records 3 fig 1 a locations of the study sites cited herein the blue squares represent the 1 1to whom correspondence may be addressed email raozhg at hunnu edu cn 1 xref corresp 1 1

formation of podiform chromitites by melt rock interaction in the upper mantle high cr and high al podiform chromitites are derived from melts formed by high and low degrees of partial melting in the upper mantle respectively the close association of the chromitites regardless of compositions with dunites and harzburgites can be explained by interaction between melts and harzburgites or depleted lherzolites in the upper mantle continuous reaction between melt and wallrock modifies the compositions of the magmas towards the precipitation of chromite alone under upper mantle conditions the modified magmas would have higher silica and lower magnesia and would be extracted into the crust or erupted leaving chromite pods in situ with dunite rinds that grade to harzburgites or lherzolites

os isotope heterogeneity of the upper mantle evidence from the mayari baracoa ophiolite belt in eastern cuba in an attempt to quantify the extent of geochemical heterogeneity within a restricted and well dated portion of the upper mantle 27 chromite separates from the 90 my old chromite deposits in the mayari baracoa ophiolite belt in eastern cuba have been investigated for platinum group element pge concentrations and re os isotopic systematics the samples are characterized by systematically subchondritic initial 187 os 188 os ratios and substantial heterogeneity the initial 187 os 188 os ratios vary with chromite chemistry and with geographical distribution reflecting differences in the os isotopic evolution for the different upper mantle sections represented by the ophiolite accordingly the os isotope data might be divided into three groups in the moa baracoa district where the chromite bodies are located in the mantle crust transition zone the calculated initial gos values average 0 97f0 69 n 13 in the sagua de tanamo district where chromite chemistry is highly variable and their location in relation the mantle sequence is less clear the initial gos values are intermediate with an average of 1 77f0 80 n 7 in the mayari district where the chromite bodies are located in the lower part of the mantle sequence initial gos values average 2 66f0 29 n 7 these subchondritic i e negative initial gos values are most simply explained by re depletion during ancient partial melting and or melt percolation events the os isotope heterogeneity documented here indicates a high degree of geochemical complexity on small to intermediate length scales in the upper mantle our results in combination with data on chromites from the literature show that an baverage present day os isotopic compositionq for the hypothetical depleted morb mantle dmm reservoir cannot be precisely established beyond the statement that it is bbroadly chondriticq indeed the upper mantle cannot be considered a sufficiently homogeneous geochemical breservoirq to serve meaningfully as a baseline against which geochemical banomaliesq are evaluated on the other hand our findings are consistent with the bstatistical upper mantle assemblageq or bsumaq concept according to which a high level of geochemical heterogeneity is maintained in the upper mantle at all relevant length scales as a result of the plate tectonic cycle and intra mantle processes such as melt migration and metasomatism d 2005 elsevier b v all rights reserved

high pge contents and extremely abundant pge minerals hosted in chromitites from the veria ophiolite complex northern greece abstract more than 100 small isolated chromite occurrences form elongated lenses that follow the nw se trend of the host serpentinized dunite within a 60 km 2 area 15 km se of veria northern greece they occur only within mylonite shear zones characterized by a strong overprint of late brittle deformation detailed investigation of chromitites by scanning electron microscope with energy dispersive x ray analysis shows that chromite is dominantly of high cr composition with the atomic cr cr al ratio ranging from 0 55 to 0 84 and an average value of 0 7 interstitial primary silicates in chromitites have been replaced by secondary phases consisting of mostly cr andradite cr chlorite and serpentine flakes in lesser amounts the most salient feature is the high total platinum group element pge content up to 25 ppm coupled with the presence of extremely abundant solitary platinum group element minerals pgm within massive chromite samples located along a shear zone these pgm fragments are angular rarely corroded grains or fine grained aggregates and occur in a matrix of cr garnet they are mostly os ru ir fe alloys containing small amounts of cr co mn followed by irarsite laurite rus 2 with variable os and ir contents occurs as remnants surrounding by ru os ir oxides of varying composition available geological mineralogical and geochemical data indicate a prolonged multistage evolution at least four stages for the pge mineralization the relatively high ipge enrichment in chromitites seems to be related to post magmatic processes encompassing a long period of deformation episodes beginning with plastic deformation in the asthenospheric mantle and culminating with brittle deformation in the crust metasomatic fluids have during a process of rodingitization and serpentinization at a low state of sulfidation substantially modified the composition of primary laurite towards fe cr co and mn bearing fe os ir ru alloys and oxides the most prospective locations for chromitite pgm targets seem to exclusively be small chromite occurrences that are found along shear zones within ophiolite complexes these shear zones post date the initial magmatic deposition of the host chromitites

composition and textures of chromite and platinum group minerals in chromitites of the western ophiolitic belt from pampean ranges of cordoba argentina abstract chromitite bodies hosted in the neoproterozoic western ophiolitic belt of pampean ranges of cordoba argentina were studied at los congos and los guanacos ultramafic bodies with regard to the composition and textures of the chromite and platinum group minerals primary chromite composition is only preserved in some massive chromitites from the los guanacos ultramafic body and is similar to al rich ophiolitic chromitites suggesting that they crystallized from melts with back arc basin basalts babb affinity in the suprasubduction mantle subsequently these chromitites underwent a prograde metamorphism chromites from chromitites and associated metamorphosed ultramafic rocks show complex replacement and exsolution textures mineral chemistry and texture indicate that the chromite composition records two main metamorphic trends a first trend defined by chromite from massive chromitite in which there is an enrichment in fe3 and fe2 cr remain relatively constant and slightly depleted in al mg a second trend is defined by chromite from disseminated chromitite and metamorphosed dunite and harzburgite in which a fe rich phase is replacing the al rich chromite this alteration trend is characterized by enrichment in the total iron content fe3 fe2 and a strong depletion in al and mg the chemical composition of all analyzed spinels from los guanacos and los congos as plotted on the ternary fe3 cr al diagram correlates well with the cr spinels from the upper amphibolite to granulite facies metamorphism platinum group minerals pgm identified include native osmium laurite erlichmanite irarsite platinum and a number of inadequately identified phases such as an oxide or hydroxide of ru pt and ir ru pt telluride ir ru as se and ir ru ti compounds native osmium was the only pgm which remained unaltered other pgm underwent mineralogical reworking during metamorphism although it is difficult to establish the extent of platinum group element mobilization based on mineralogical observation our results suggest that the ru os ir pgm in the los guanacos and los congos chromitites were modified in situ producing re distribution of these pge on a small scale the presence of rare pt and pge as se minerals was possibly related to remobilization of pt as and se by fluids during the alteration processes

reply to li et al human societies began to play a significant role in global sediment transfer 4 000 years ago in a recent study 1 we conducted a global synthesis of long term records of reconstructed sediment accumulation rates sars in lakes and diagnosed that a dominant anthropogenic imprint on soil erosion rates started 4 000 y ago our approach was based on the assumption that lake sars are watershed integrators of terrestrial biogeochemical responses to atmospheric climate and land surface dynamics spanning several millennia 2 4 our statistical analysis was based on sar reconstructions for the last 12 000 y 1 global pollen records spanning the same period to infer past vegetation changes 5 and model based climate reconstructions from transient holocene simulation with 1 1to whom correspondence may be addressed email jean philippe jenny at inrae fr 1 xref corresp 1 1

petrological characteristics of podiform chromitites and associated peridotites of the pan african proterozoic ophiolite complexes of egypt mafic ultramafic fragments of a dismembered ophiolite complex are abundant in the late precambrian pan african belt of the eastern desert of egypt and north east sudan the ultramafic bodies in the eastern desert of egypt are mostly characterised by the harzburgite dunite chromitite association because of their severe metamorphism almost all primary silicates were converted to secondary minerals and we use the chrome spinel as a reliable petrogenetic indicator the podiform chromitite deposits are common as small and irregularly shaped masses in the central and southern parts of the eastern desert they strongly vary in texture degree of alteration and chemical composition of chrome spinel the podiform chromitites exhibit a wide range of composition from high cr to high al varieties the cr of chrome spinel ranges from 0 65 to 0 85 in dunite quite similar in the high cr chromitite whereas it is around 0 5 in harzburgite primary hydrous mineral inclusions amphibole and phlogopite in chrome spinel are reported for the first time from the pan african proterozoic podiform chromitites the petrological characteristics of pan african podiform chromitites and associated peridotites of egypt are similar to those of phanerozoic ophiolites the proterozoic podiform chromitites may have formed in the same way as the phanerozoic ones namely by melt harzburgite reaction and subsequent melt mixing the similarity of the mantle section of the late proterozoic and the phanerozoic ophiolites suggests that the thermal conditions controlling genesis of the crust mantle system basically have not changed since the late proterozoic era the pan african harzburgite is very similar to abyssal peridotite at fast spreading ridges and the high cr low ti character of spinel in chromitite and dunite indicates a genetic link with a supra subduction zone setting the late proterozoic ophiolites of egypt are possibly a fragment of oceanic lithosphere modified by arc related magmatic rocks or a fragment of back arc basin lithosphere

significance of melt wall rock reaction a comparative anatomy of three ophiolites the ascent of basaltic melts through the upper mantle results in chemical disequilibrium between the melts and the pyroxene and plagioclase of the wall rock peridotite phase and cryptic variations in ophiolitic peridotites demonstrate that the resulting reactions deplete the mantle in magmatophile components and enrich ascending melts in ca na al and incompatible trace elements while buffering their mg mg fe ratios at primitive values 0 6 a comparative anatomy of the trinity oman and darb zubaydah ophiolites illustrates both the significance of this process in shaping the composition of the shallow lithospheric mantle and how the integrated effects may reflect tectonic setting a first order correlation between crustal thickness and degree of mantle depletion exists but multiple rifting events may remove part of the crustal record so that melt rock ratios are difficult to quantify most impressive in ophiolitic peridotites is the abundance of melt crystallization and reaction products in zone

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

systematics of chromitite occurrences in central palawan philippines the chromitite occurrences in the central palawan ophiolite can be classified into four groups based on their chemistry and geological relationships group i chromites from a deep tectonite level of the ophiolite complex show cr cr al values of between 0 78 and 0 90 whereas group ii from a shallower level gives values of 0 64 0 78 and group iii from the immediate gabbro lower contact dunite cumulates and diapirs show values of 0 5 0 64 group iv chromites from the gabbro zone have ratios around 0 38 0 5 larger orebodies type a representing a first stage of partial melting of a primordial mantle show cr fe ratios between 2 5 and 4 5 whilst those of small chromitite bodies with a thin dunite envelope type b lie below 2 5 these small bodies belong to a second stage of magma generation for both types the cr fe ratio increases with the chromite silicate ratio within individual occurrences an additional method which may help to subdivide these chromitites involves their silicate and ore inclusions the platinum group minerals for example occur mainly in chromites of group ii type a the intensive tertiary tectonic activity to which the ophiolite was subjected during and after emplacement disturbs the regular pattern of the ophiolite stratigraphy and its chromitite occurrences

the giant chromite deposits at kempirsai urals constraints from trace element pge ree and isotope data the investigation of stable and radiogenic isotopes and of platinum group pge and rare earth elements ree in chromitites and associated ultramafic rocks of the kempirsai massif southern urals gives strong evidence for a multistage formation of giant ophiolitic podiform chromite deposits present in the southeastern part of the massif the kempirsai ophiolite massif is divided by a shear zone into two parts in the northwestern area small bodies of al rich chromite formed from basaltic melts between 420 to 400 ma according to sm nd mineral isochrons of harzburgite pyroxenite websterite and gabbro harzburgites and pyroxenites in this area are enriched in light ree and have nd 400 6 and sr 400 5 chromitites have scattered pge distributions pd ir 0 4 7 0 being partly enriched in pd and pt os 400 of one chromitite is 4 4 the southeastern part of the kempirsai massif well known for its world class deposits of podiform low al magnesiochromite is characterized by harzburgite and dunite enriched in light ree with very low nd 400 4 3 to 17 1 and positive sr 400 10 values chromitites are strongly enriched in ir os and ru and depleted in pd and pt os 400 of three chromitites is uniform and approaches c1 and dmm compositions in veins and pods postdating crystallization of massive chromite pargasitic amphibole formed in equilibrium with fluid inclusion bearing chromite at temperatures close to 1000 c these amphiboles give 40ar 39ar stepwise heating ages of 365 to 385 ma and are characterized by low nd 400 0 6 to 4 6 and general enrichment in ree the cooling ages correspond to a 379 3 1 6 ma rb sr mineral isochron produced from amphibole and phlogopite of a pyroxenite vein in the western part of the massif from these data it is concluded that parts of the kempirsai massif have been pervasively metasomatized by large amounts of fluids and melts derived from a subducted slab composed of oceanic crust and sediments subduction occurred at least 15 35 ma after a melting event that produced a typical ophiolitic sequence in the paleozoic sakmara zone we conclude that large chromite orebodies formed from second stage high mg melts that interacted with depleted mantle and fluids on their way upward in a suprasubduction zone regime and in a fore arc position to the magnitogorsk island arc

compositions of chromite associated minerals and parental magmas of podiform chromite deposits the role of slab contamination of asthenospheric melts in suprasubduction zone environments abstract podiform chromitites in the mantle sections of ophiolites belong to either high cr metallurgical or high al refractory varieties their highly variable compositions are reflected by different cr s 100cr cr al and cr2o3 and al2o3 contents of the chromite falling in the boninitic and morb fields respectively parental magmas of high cr chromitites have higher sc mn co and ni and lower ti v zn and ga concentrations than morb melts their trace element patterns are similar to those of boninites except for ni and zn in contrast high al chromitites have parental magmas characterized by generally flat morb normalized patterns showing slight enrichments in v mn and co and depletion in ni and zn regardless of their compositions both types of chromitites have chondrite normalized platinum group element pge patterns showing enrichment in ipge and depletion in ppge a variety of platinum group minerals are typically present in both types occurring either as euhedral inclusions or along fractures in chromite grains these minerals have a wide span of re os isotopic compositions reflecting a variety of origins there is a diversity of unusual minerals and mineral inclusions associated with podiform chromitites the presence of these minerals suggest that grains of amphibolite plagioclase amphibole and zircon and eclogite coesite kyanite and garnet were present in the magmas from which chromite crystallized multiphase mineral inclusions demonstrate that podiform chromitites form from hydrous mafic magmas in suprasubduction zone environments ssz we propose a new model in which chromitite formation was involved in intra oceanic subduction zones initiated in closing oceanic basins continued subduction carries oceanic and possibly continental crustal materials to deep levels where they are metamorphosed under greenschist amphibolite and eclogite facies conditions the tearing and breakoff of the subducted slab possibly along the transitional contact between amphibolites and eclogites create a slab window through which the underlying asthenosphere rises and melts to generate cr rich mafic magmas these upward migrating magmas pass through the subduction zone and assimilate the subducted slab as a result of slab contamination these magmas become more siliceous more oxidized and more hydrous rapidly triggering chromite crystallization minute grains of chromite are suspended in the upward moving magmas as they migrate through the overlying metasomatized mantle wedge such chromite bearing magmas eventually deposit chromite in magma conduits in the uppermost mantle close to the moho where the upward flow changes from vertical to subhorizontal and velocity is greatly reduced highly reduced and ultrahigh pressure minerals including diamonds are reported in literature both in podiform chromitites and host peridotites of ophiolites some of these minerals in association with host peridotites may have been brought by the uprising asthenosphere at mid oceanic ridges due to the mantle convection it is also possible that some diamonds may have formed in the subducted slab below about 150 km some minerals of subducted slabs are preserved because they are encapsulated in chromite grains where they are protected from the ssz melts some of these ssz mantle wedges are emplaced on land to become podiform chromitite bearing ophiolites

unexpectedly high pge chromitite from the deeper mantle section of the northern oman ophiolite and its tectonic implications unusually high platinum group element pge enrichments are reported for the first time in a podiform chromitite of the northern oman ophiolite the chromitite contains 1 5 ppm of total pge being highly enriched in the ipge subgroup ir os and ru and strongly depleted in the ppge subgroup rh pt and pd its platinum group minerals pgms are classified into three types arranged in order of abundance 1 sulphides os rich laurite laurite erlishmanite solid solution and an unnamed ir sulphide 2 alloys os ir alloy and ir rh alloy and 3 sulpharsenides irarsite and hollingworthite the high pge concentrations are observed only in a discordant chromitite deep in the mantle section which has high cr 0 7 spinel with an olivine matrix all the other types of chromitite in the moho transition zone mtz and concordant pods in the deeper mantle section are poor in pges and tend to have spinels with lower cr up to 0 6 this diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis in response to a switch of tectonic setting the first is residual from lower degree partial melting of peridotite which produced low cr pge poor chromitites at the moho transition zone and to a lesser extent within the mantle possibly beneath a fast spreading mid ocean ridge the second chromitite forming event involves higher degree partial melting which produced high cr pge rich discordant chromitite in the upper mantle possibly in a supra subduction zone setting

compositional variations of chromite and solid inclusions in ophiolitic chromitites from the southeastern turkey implications for chromitite genesis abstract chromitites associated with upper mantle peridotites of ophiolites from southeastern turkey are lenticular bodies or veinlets made up of massive banded and disseminated chromite chromite shows a wide range of cr cr cr al varying between 0 42 and 0 81 and mg mg mg fe2 of 0 45 to 0 75 geochemically they are divided into two compositional groups such as high cr cr 0 7 and high al cr

deposition and flux of sediment from the po river italy an idealized and wintertime numerical modeling study abstract recent studies of sediment dynamics and clinoform development in the northern adriatic sea focused on winter 2002 2003 and provided the data and motivation for development of a detailed sediment transport model for the area near the po river delta we used both idealized test cases and more realistic simulations to improve our understanding of seasonal sediment dynamics there we also investigated the relationship between physical processes and the observed depositional products e g the accumulation of sediment very near the po river distributary mouths sediment transport near the po river was evaluated using a three dimensional ocean model coupled to sediment transport calculations that included wave and current induced resuspension suspended sediment transport multiple grain classes and fluvial input from the po river high resolution estimates from available meteorological and wave models were used to specify wind wave and meteorological forcing model results indicated that more than half of the discharged sediment remained within 15 km of the po river distributary mouths even after two months of intensive reworking by winter storms during floods of the po river transport in the middle to upper water column dominated sediment fluxes otherwise sediment fluxes from the subaqueous portion of the delta were confined to the bottom few meters of the water column and correlated with increases in current speed and wave energy spatial and temporal variation in wind velocities determined depositional patterns and the directions of sediment transport northeasterly bora winds produced relatively more eastward transport while southwesterly sirocco winds generated fluxes towards both the north and the south eastward transport accounted for the majority of the sediment exported from the subaqueous delta most likely due to the frequent occurrence of bora conditions progradation of the po river delta into the adriatic sea may restrict the formation of the western adriatic coastal current increasing sediment retention at the po delta and reducing the supply of sediment to the apennine margin a positive morphodynamic feedback may therefore be present whereby the extension of the delta into the adriatic increases sediment accumulation at the delta and facilitates further progradation

distribution of platinum group elements and gold within the vourinos chromitite ores greece a geochemical study of chromite ores and host rocks of the vourinos ophiolite complex leads to the distinction of three geotectonic units corresponding to the north vourinos south vourinos and kissavos geographic areas this is based on the range and variability of cr cr al pt pd os ir ru and pd ir ratios and the relative abundance of compatible platinum group elements pge os ir and ru within the three blocks the platinum group element and gold contents are low average concentrations in ppb are os 23 ir 19 ru 58 rh 12 pt 5 9 pd 3 3 and au 2 7 however a few pge enriched chromite samples up to 3 030 ppb were also found the south vourinos area differs from north vourinos as far as its remarkably constant chemical composition major and trace elements its lower pd ir and pt pd os ir ru ratios its greater abundance of the os ir sum relative to ru and its absence of pge enriched chromite ores there is no significant correlation between chromite composition and pge data throughout the complex the only exception is the kissavos area where the pge content is higher and the pd ir ratio lower in cr rich ores than in al rich ones showing a strong negative correlation between the cr cr al and pd ir ratios r 0 8 the low pge content in the majority of the vourinos chromite ores may reflect a lack of sulfur saturation during an early stage of their crystallization whereas pge enrichment may suggest a mixing of primitive and partially fractionated magmas assuming that the vourinos complex formed in a suprasubduction zone environment it seems likely that south vourinos chromitites formed directly above the spreading center whereas the north vourinos chromitites formed at a distance from the spreading center in addition the strong depletion of pge in the kissavos cr rich ores particularly os ir and ru may suggest that these chromitites were crystallized from magmas derived by partial melting of an already depleted mantle source the geochemical trend throughout the vourinos complex seems to provide valuable confirmation of a stratigraphic orientation in the mantle sequence of the complex

lake sediments erosion and landscape change during the holocene in britain and ireland abstract lakes are excellent repositories of air borne and especially stream borne materials it has long been recognised that lake sediments contain a strong record of catchment soils via the inputs of minerogenic and chemical erosional products to these may be added a variety of palaeoecological indicators including pollen fungi sphagnum leaves and faunal remains pollen analysis in particular enables land use change to be assessed over many millennia and demonstrates the landscape impacts of woodland clearance grazing and crop cultivation radiocarbon dating provides a chronology for environmental and agricultural change and acts as an indicator of erosion in its own right the use of such indicators to reconstruct past instances and patterns of erosion is demonstrated with reference mainly to 50 british and irish lake sites and especially those which have produced signals of erosion in the form of accelerated sediment accumulation and inversed or reversed sequences of radiocarbon dates during the mid and late holocene sites displaying even or decreased sedimentation through time are concentrated in northern and northwestern scotland and have either no clear signs of human impact or only so towards the latter part of the holocene this pattern probably reflects a combination of factors including low population densities thin soils and the sealing effect of blanket peat in contrast all sites with accelerated erosion have indications of anthropogenic impact this is often prior to levels of increased sedimentation showing a delay in system response while the spread of dates for the start of the rise in sediment accumulation spans 6200 940 bp three clusters are evident at 5295 4970 4530 4235 and 2980 2810 bp radiocarbon inversions mostly reversals are evident for 18 sites no obvious causal patterning is indicated and they probably demonstrate no more than the fact that catchment soils including peats around a particular site were pushed beyond an erosional threshold sufficient to age the sediments deposited within the lakes it is hoped that improvements in data quality will allow the refinement of such exercises in the future and that lakes will be valued as repositories of long term landscape sensitivity

intermediate chromitite in kizildag ophiolite se turkey formed during subduction initiation in neo tethys abstract chromitites in the kizildag ophiolite mostly occur in the mantle harzburgites and some of them in crustal cumulate dunites olivine in both the dunites and chromitites is highly magnesian fo 90 5 92 8 and 90 2 96 1 respectively while the coexisting chromite varies widely in composition cr 52 9 68 6 and 57 1 76 2 respectively placing it between high al and high cr varieties concentrations of ni v and ga in the chromite ni 741 1310 ppm v 521 1227 ppm ga 20 51 ppm are also highly variable straddling the range of high al and high cr chromite olivine in the dunites and chromitites has a large range of li concentrations 0 27 2 47 ppm and 7li values 7 20 29 90 based on the calculated al2o3 12 1 17 2 wt contents feo mgo values 0 92 1 57 and trace element concentrations of the parental magmas of the dunites and chromitites it is suggested that they are transitional between morb and boninitic melts and should have been derived by mixing of geochemically and spatially distinct magmas generated by partial melting of different sources the li isotopic compositions of olivine from the dunites and chromitites except kz14 38 and kz15 38 are neither controlled by diffusion nor fractional crystallization and thus may be intrinsic features of their parental melts the range of 7li values of olivine in the dunites and chromitites is analogous to that of arc lavas and marine sediments indicating that the chromitites record various degrees of melt penetration the wide range of li isotopic variations in olivine and chemistry in chromite from the kizildag chromitites suggest that the parental melts were originated in a proto forearc mantle during subduction initiation

high cr and high al chromitites from the sagua de tanamo district mayari cristal ophiolitic massif eastern cuba constraints on their origin from mineralogy and geochemistry of chromian spinel and platinum group elements article i nfo several chromitite bodies of variable sizes are hosted in dunite harzburgite of the small b3 km mining district of sagua de tanamo in the mayari cristal ophiolitic massif eastern of cuba the chromian spinel in these bodies displays a large range in cr cr cr al atomic ratio from 0 74 to 0 45 in high cr chromitites cr and ti contents increase from harzburgite to dunite to chromitite a roughly opposite trend of variation in cr is observed in high al chromitites although in this case ti distributes randomly the differences in the type of chromian spinel i e high cr or high al in the chromitite coincide with a different behavior of the platinum group elements pge whereas high cr chromitites are rich in pge and contain abundant grains of platinum group minerals pgm high al chromitites are systematically poor in pge and in pgm the calculated melts in equilibrium with chromian spinel of high cr chromitite are island arc thoeliites iat with boninitic affinity whereas those in equilibrium with chromian spinel of high al chromitites are back arc basin basalts babb the formation of high cr chromitites is interpreted as a result of the extensive reaction of harzburgite with migrating island arc tholeiite melts of boninitic affinity melt rock reaction produces boninitic melts with variable composition and porous dunitic channels in which the mixing mingling of melts promotes crystallization of mononomineralic high cr chromian spinel in contrast high al chromitites formed by the mixing mingling of babb melts within conduits not in equilibrium with dunite percolation of primitive babb melts through pre existing dunite dissolved olivine producing melt conduits in which babb melts mixed and formed high al chromian spinel the higher pge and pgm in high cr chromitites is not only a function of the degree of partial melting but is linked to interaction between the migrating melt and mantle harzburgite during the formation of the chromitite the coexistence of both types of chromitites in one small mining district reflects the temporal and or spatial variations of separate melt intrusions emplaced in different subarc mantle domains during the opening of a back arc basin in a supra subduction zone environment

chromite and pge geochemistry of the elekdag ophiolite kastamonu northern turkey implications for deep magmatic processes in a supra subduction zone setting abstract the geochemistry of chromium spinel and platinum group elements pge provides significant information about conditions in the mantle during the emplacement and genesis of ophiolitic complexes the elekdag ophiolite is a greenschist to amphibolite facies ophiolite that was emplaced onto the sakarya zone in the central part of the pontides of northern turkey nw se trending elekdag ophiolite hosts small scale chromitite occurrences in serpentinized harzburgite and dunite and have maximum resource of several thousands t cr2o3 the chromitites the elekdag ophiolite can be divided in to high cr and high al types the calculated melt composition of the high cr chromitites is 9 4 13 2 wt al2o3 0 2 0 4 wt tio2 with feo mgo ratios of 0 4 1 9 which are indicative of boninitic compositions however the high al chromitites have 14 7 17 3 wt al2o3 and 0 3 0 8 wt tio2 with feo mgo ratios of between 0 3 and 0 8 which are consistent with morb like tholeiitic affinities the total pge content of the chromitites range between 82 and 1466 ppb and the average pge content of the high cr and high al chromitites are 267 ppb and 195 ppb respectively the chromitites are characterized by low pd ir ratios of 0 02 0 20 that is characteristic of non fractionated chromitites the chondrite normalized pge pattern of the chromitites resemble those of mantle derived cr rich chromitites in ophiolites of the supra subduction zone ssz elsewhere mineral and bulk rock pge data for the elekdag chromitites indicate that high cr chromitites formed from cr rich and al and ti poor boninitic melts most probably in the fore arc setting in the deeper part of the mantle this was succeeded by the mineralization of high al chromitites from a differentiated boninitic melt at shallow depths close to the moho discontinuity

stabilities of laurite rus2 and monosulfide liquid solution at magmatic temperature we have reversed the equilibria ru s2 rus2 and 2feo s2 2fes o2 under controlled temperature oxygen fugacity fo2 and sulfur fugacity fs2 at 1 atm to test if the platinum group mineral laurite rus2 has a stability field at magmatic temperature outside that of magmatic monosulfide liquid the fs2 in equilibrium with ru and rus2 is given by log fs2 6 9 12190 t 1173 to 1673 k whereas the fs2 at sulfide saturation of a basaltic melt may be approximated from log fs2 6 7 12800 t 2 log xfeo log fo2 1473 to 1673 k our experiments support the notion that the sulfide mineral laurite can crystallize directly out of a sulfur bearing sulfide undersaturated silicate melt as implied by laurite chromite intergrowth textures in natural chromite ore

platinum group element distribution in chromite ores from ophiolite complexes implications for their exploration abstract compilation of some new data on ophiolites for greece and yugoslavia and published data from previous studies indicate that platinum group element pge and gold concentrations in chromite ores are generally low ranging from less than 100 ppb to a few hundred ppb however samples from several ophiolite complexes exhibit an enrichment of a few ppm a only in os ir and ru b only in pt and or pd or c in all pge this enrichment up to 10s ppm is mainly related with chromitites hosted in supra moho dunites and dunites of the uppermost stratigraphic levels of the mantle sequence and it seems to be local independent of the chromitite major element composition and the chromite potential of the ophiolite complexes the contents of pge combined with less chalcophile elements ni co cu the ratios of incompatible compatible elements and pge patterns provide evidence for discrimination between chromitites derived from primitive magmas and those derived from partially fractionated magmas although they have a similar major element composition thus they can be used for a stratigraphic orientation in the mantle sequence and therefore for exploration targets moreover pge data offer valuable information for the evaluation of the chromite potential in ophiolite complexes the most promising ophiolites seem to be those which apart from the petrological and geochemical characteristics indicating extensive degree of partial melting in the mantle source contain only one chromite ore type the other type being only in small proportion of limited compositional variation in both major elements and pge low ratios of pd ir while pge enriched chromitites in the mantle sequence are only occasionally present in contrast ophiolites which contain both high cr and al chromitites and where their chalcophile element data implies relatively extensive fractionation trend are not good exploration targets for chromite ores although they are related with a ssz environment

nanoscale constraints on the in situ transformation of ru os ir sulfides to alloys at low temperature abstract we report new results of a combined focused ion beam high resolution transmission electron microscopy fib hrtem investigation of two grains of platinum group minerals pgm corresponding to the laurite rus2 erlichmanite oss2 solid solution series the grains are included within unaltered chromite and the serpentine dominant matrix of the mantle hosted chromitite body of monte bueno in eastern cuba at the micron scale the grain hosted in unaltered chromite preserves magmatic features including a complex oscillatory zoning defined by a core of erlichmanite surrounded by alternating bands of os rich os poor laurite revealed by conventional field emission scanning electron microscopy fe sem and x ray mapping by electron microprobe analyzer epma combining high magnification hmem high angle annular dark field haadf images and the corresponding energy dispersive spectra eds the elemental mappings acquired using hrtem showed that the oscillatory zoning occurs at both the micron and nano scales a specific feature highlighted by hrtem observations is the rather complex structure of the erlichmanite core consisting of an aggregation of much smaller cores

coexistence of compositionally heterogeneous chromitites in the antalya isparta ophiolitic suite sw turkey a record of sequential magmatic processes in the sub arc lithospheric mantle abstract the antalya isparta region in southwestern turkey is well known for its large ophiolitic peridotite exposures which host various chromite orebodies these are small sized massive to disseminated in texture chromitites occurring in the form of lenses or veinlets and commonly surrounded by dunite envelopes of variable thickness chromitite seams from the antalya mantle suite are both high and intermediate cr varieties cr 0 56 0 83 whereas chromitites in the isparta mantle sequence are exclusively cr rich cr 0 75 0 85 in situ minor and trace element abundances obtained by la icp ms analyses of unaltered cr spinel from the cr rich chromitites are comparable to those reported in cr spinel of chromitites from typical fore arc peridotite complexes however minor and trace element concentrations in cr spinel from intermediate chromitites are dissimilar to those acquired from cr spinels of chromitites from well known back arc basin derived ultramafic massifs calculation of parental magma compositions indicates that both types of chromitites share a common parentage with progressively fractionating arc related melts the studied chromitites are characterized by a systematic enrichment in ipge os ir and ru 41 317 ppb with respect to ppge rh pt and pd 3 49 ppb resulting in negatively sloping chondrite normalized pge patterns that are less fractionated in intermediate chromitites their noble mineral assemblage is vastly dominated by tiny 10 m euhedral laurite crystals followed by subsidiary irarsite and trivial amounts of os ir alloy grains pgm grains are not encountered in the intermediate chromitites potentially due to crystallization resulting from pge poor melt laurite is os poor and exhibits a narrow range of os for ru substitution ru ru os 0 75 0 99 however the concomitance of laurite and millerite in the cr rich chromitites of the mutual antalya isparta mantle suite is in favour of their precipitation from an os depleted melt characterized by local and rapid variations of fs2 prior to or coevally with cr spinel crystallization moreover the presence of amphibole inclusions in cr spinel indicates that the melt triggered chromitite genesis potentially involved a hydrous component overall data suggest that investigated orebodies were produced by a successively fractionating arc derived melt that generated compositionally distinct chromitites at two different pseudo stratigraphic levels within the antalya isparta arc type mantle suite

significance of chromian spinels from the mantle sequence of the andaman ophiolite india paleogeodynamic implications abstract the mantle section of the andaman ophiolite between rutland island in the south and north andaman in the north has been studied the restitic peridotite of rutland island is comprised of depleted harzburgite to clinopyroxene bearing harzburgite whereas that in middle and north andaman is mostly less depleted lherzolite dominated mantle chromian spinels from the mantle section of the andaman ophiolite belong to four major groups group 1 spinels from rutland chromitite pods characterized by high cr cr cr a1 atomic ratio 0 71 group 2 spinels from north andaman chromitite pods with two subgroups 2a characterized by high cr 0 70 0 79 and 2b characterized by medium cr 0 48 0 51 and high mg mg mg fe 2 atomic ratio 0 74 0 77 group 3 residual accessory grains from rutland peridotite characterized by medium cr 0 37 0 55 and low mg 0 41 0 54 and group 4 residual accessory grains from middle and north andaman peridotites characterized by low cr 0 09 0 23 and high mg 0 71 0 81 group 1 chromian spinels of rutland island and group 2a of north andaman are likely to have crystallized from boninitic melts formed by relatively high degrees of partial melting whereas group 2b varieties from north andaman are from tholeiitic melts formed by lower degrees of melting the chemistry of the residual accessory chromian spinels groups 3 and 4 suggests that the mantle peridotites of rutland island towards the south are similar to fore arc peridotites of suprasubduction zone environments whereas those of north andaman are less depleted this spatial and or temporal directional change in spinel compositions may reflect variations linked to the melting history where the same sliver of oceanic mantle underwent different styles of melting in different tectonic settings at different points in time the coexistence of both chromitite types with high and low cr spinels in the same area from north andaman possibly reflects temporal variations of separate melt intrusions produced through specific melting stages it is inferred that a similar geodynamic setting to the present day also existed during the cretaceous period with a fore arc at rutland island and a back arc towards the north at north andaman

solid inclusions in chrome spinels and platinum group element concentrations from the hochgrossen and kraubath ultramafic massifs austria a great variety of platinum group mineral sulfide and silicate inclusions in chrome spinel from hochgrossen and kraubath ultramafic massifs and platinum group element contents of three different rock types have been investigated both ultramafic massifs are tectonically isolated bodies variably serpentinized and metamorphosed greenschist to lower amphibolite facies and show ophiolitic geochemical affinities the chromite from massive chromitites and disseminated in serpentinized dunites and serpentinites exhibits compositional zonation as the result of alteration during serpentinization and metamorphism three distinctive alteration stages are indicated in the chrome spinels from the hochgrossen whereas alteration is less significant in chromites from kraubath the core of chrome spinel represents the least altered part surrounded by an inner rim characterized by slight compositional differences in cr mn fe2 and al with respect to the core the outer rim is formed by ferritchromite with a sharp boundary to the inner rim and shows a significant decrease of al mg cr and increase of fe2 fe3 and ni compared to the core two different groups of inclusions in chrome spinel are present the first group occurs within the chromite core and comprises olivine orthopyroxene amphibole sulfides and platinum group minerals i e dominated by ru os ir sulfides the second group is formed by chlorite serpentine galena pyrite arsenopyrite pt pd rh dominated sulfarsenides and sperrylite in particular the abundance of pt pd rh sulfarsenides and arsenides is typical of both ultramafic massifs and is very unusual for chromitites from ophiolites morphology paragenesis and chemical composition indicate a different origin for these two groups of inclusions the first group is intimately related to the crystallisation of the chromite host the second group of inclusions clearly displays a secondary formation during serpentinization and metamorphism closely related to the alteration of chrome spinel and the development of ferritchromite the distribution patterns of the platinum group elements from massive chromitites disseminated chrome spinel bearing serpentinites and serpentinites exhibit variable enrichment of rh pt and pd rh pt for the hochgrossen and kraubath massifs respectively these results are in accordance with the occurrence and distribution of platinum group mineral phases a remobilisation of pt pd and rh together with ni cu and possibly fe as bisulfide and or hydroxide complexes and deposition of metals by the reaction of the metal bearing hydrothermal fluid with chromite is proposed

metallogeny of the chrome ores of the xerolivado skoumtsa mine vourinos ophiolite greece implications on the genesis of ipge bearing high cr chromitites within a heterogeneously depleted mantle section abstract chrome ore deposits comprise less than 1 of the volume of a pervasively serpentinized dunite body 3 5 km 3 that constitutes the xerolivado skoumtsa mine of the vourinos ophiolite in northwestern greece ores have been examined to aid determination of their geological mode of occurrence their mineralogy and mineral chemistry cr spinel and olivine and platinum group element pge geochemistry the ore bodies are highly deformed and of the schlieren type where cr spinel rich and serpentine after olivine rich layers 1 20 cm thick alternate the cr spinel displays a limited range in cr 100 cr cr al atomic ratio 77 85 and tio 2 content 0 05 0 22 wt the mg 100 mg mg fe 2 of cr spinel shows a wider variation 52 74 indicating that subsolidus mg sp fe 2 ol re equilibration occurred remnant fresh olivine in the serpentine rich layers is typical forsterite fo 100 mg mg fe 2 93 95 the cr rich chromitite ores have total pge and au abundances that range between 74 3 and 205 ppb they show a general enrichment in ir and ru over ppge rh pt and pd resulting mostly in a negatively sloping c1 chondrite normalized pge profile with a quite strong ru peak field observations indicate that the schlieren banding of the deformed ore bodies is a tectonometamorphic feature acquired within ductile deformation of the ophiolitic slab geochemical calculations demonstrate that the parental melts of the xerolivado skoumtsa high cr chromitites were boninitic their high ru content and variation in cr sp values indicate a polygenetic origin from geochemically similar but spatially distinct melt inputs these melts originated within a hydrated mantle wedge beneath a forearc basin within an evolving lithospheric slab we emphasize that the pge budget of the xerolivado skoumtsa chromitites was not inherited from interactions of the migrating melts with their parental harzburgites but it is an intrinsic feature of the deep seated and heterogeneously depleted mantle source of these magmas

origin of podiform chromite deposits by magma mingling abstract i have oversaturated the immiscible system sio2 al2o3 feo k2o with fecr2o4 to study the fractionation of chromite between conjugate siliceous and fayalitic melts although both melts are oversaturated with chromite chromite only nucleates in the fayalite rich melt where the crystal melt interfacial energy sl is lowest in less than three days at 1150 c all excess chromium in the bulk composition is concentrated as cumulus chromite in the fayalitic melt pools producing nodular cumulate textures as observed in podiform chromite ore when these results are applied to the lithospheric mantle podiform chromite deposits may form when droplets of olivine normative high pressure low viscosity melt mingle with siliceous low pressure more viscous melt in shallow magma conduits it is inferred from the experimental analogue that chromite only nucleates in the melt where sl is lowest which in the mantle would be the high pressure melt the low pressure melt only serves as the diffusive chromium reservoir the richest chromite ores form when the volume ratio of the melt nucleating chromite is small relative to the ambient melt

the temperature of europe during the holocene reconstructed from pollen data we present the first area average time series reconstructions of warmest month coldest month and mean annual surface air temperatures across europe during the last 12 000 years these series are based on quantitative pollen climate reconstructions from over 500 pollen sites assimilated using an innovative four dimensional gridding procedure this approach combines three dimensional spatial gridding with a fourth dimension represented by time allowing data from irregular time series to be focussed onto a regular time step we provide six regional reconstructed temperature time series as well as summary time series for the whole of europe the results suggest major spatial and seasonal differences in holocene temperature trends within a remarkably balanced regional and annual energy budget the traditional mid holocene thermal maximum is observed only over northern europe and principally during the summer this warming was balanced by a mid holocene cooling over southern europe whilst central europe occupied an intermediary position changes in annual mean temperatures for europe as a whole suggest an almost linear increase in thermal budget up to 7800 bp followed by stable conditions for the remainder of the holocene this early holocene warming and later equilibrium has been mainly modulated by increasing winter temperatures in the west which have continued to rise at a progressively decreasing rate up to the present day

tracing ancient events in the lithospheric mantle a case study from ophiolitic chromitites of sw turkey abstract new major minor and trace element data on high cr chromites from several ophiolitic podiform chromitites from lycian and antalya peridotites in southwestern turkey reveal a polygenetic origin from a range of arc type melts within forearc and back arc settings these forearc and the back arc related high cr chromitites are interpreted to reflect the tectonic juxtaposition of different lithospheric mantle segments during the obduction the diversity of the os t 0 values 8 28 to 13 92 in the antalya and lycian chromitite pgms and their good correlations with the sub to supra chondritic 187os 188os ratios 0 1175 0 1459 suggests a heterogeneous mantle source that incorporated up to 40 recycled crust probably due to subduction processes of the orogenic events the few model ages calculated define two significant peaks in trd model ages at 1 5 and 0 25 ga suggesting that the chromitites are younger than 0 25 ga and include relics of an at least mesoproterozoic or older 1 0 ga mantle protolith eight of the nine zircon grains separated from the chromitites are interpreted as detrital and or resorbed xenocrystic relics whilst a significantly less reworked resorbed one is considered to be of metasomatic origin in situ u pb dating of the xenocrystic zircon grains yielded a spread of ages within ca 0 6 2 1 ga suggesting recycling of crustal rocks younger than 0 6 ga late neoproterozoic the notable coincidence between the lower age limit of the older zircons ca 1 6 ga and the oldest os model age peak ca 1 5 ga from the pgm may suggest a mesoproterozoic rifting stage these findings imply a paleoproterozoic sub continental lithospheric mantle sclm protolith for the sw anatolian mantle which was later converted into an oceanic lithospheric mantle domain possibly following a rifting and continental break up initiated during mesoproterozoic ca 1 5 1 0 ga the single metasomatic zircon of ca 0 09 ga age coinciding with the initiation of the contraction of the neotethys may be interpreted to date the timing of a metasomatic event beneath the intra oceanic subduction zone

inter element fractionation of highly siderophile elements in the tonga arc due to flux melting of a depleted source highly siderophile element concentrations hses os ir ru pt pd and re have been determined for a suite of fresh submarine mafic lavas from the northern tonga arc front and the nascent backarc fonualei spreading centre fsc prior melt depletion of the tongan mantle wedge combined with a high degree of fluid fluxed melting is thought to have produced boninitic magmas at several arc and fsc locations as such this arc system provides an opportunity to assess the fluid mobility of hses and to investigate the effects of fluid induced melting and prior melt depletion on hse behaviour during both mantle melting and magma evolution tongan lavas display extreme enrichment of pt 2 5 32 ng g and pd over os 0 002 0 6 ng g ir and ru significantly greater than basalts from mid ocean ridges magma evolution increases the degree of fractionation resulting in the highest recorded pt ru ratios 300 in arc front samples with mgo 8 wt this increasing fractionation is due to the mild incompatibility of pt and pd and concurrent compatibility of ru during sulphide undersaturated magma evolution however the fractionation of pt and pd from os ir and ru is observed in the highest mgo samples indicating source inheritance prior melt depletion of the mantle and elevated oxygen fugacity both increase the likelihood of complete consumption of sulphide in the source during melting which typically leads to melts with high concentrations of all the hse indeed modelling indicates that 25 aggregate partial melting of a depleted morb mantle source proposed for the tonga arc will lead to complete base metal sulphide consumption unless there is considerable addition of s by the slab flux at least 200 g g although source enrichment of pt pd and re by slab fluids may take place the fractionation of pt and pd from os ir and ru can largely be explained by relatively low temperature yet high degree melting of fluid fluxed melt depleted mantle the high pt and pd contents can be produced by the exhaustion of sulphide in the source while the presence of ru os ir alloys or sulphides e g laurite associated with cr spinel can explain os ir and ru retention in the source residue such phases have been documented in fluid fluxed sub arc mantle from ophiolites osmium isotopes co vary negatively with os abundance and thus appear to be dominated by shallow level contamination the most os rich samples however have 187os 188os ratios 0 126 0 132 which are typical of dmm and morb suggesting an indistinguishable flux of radiogenic os from the slab the significant fractionation of pt and re from os in arc settings will lead over time to elevated 186os and 187os which may be relevant to the observed enrichments of these isotopes in some mantle regions in addition the differing behaviour of ru and ir and the implication of a mantle source containing ru rich microphases may have consequences for the estimation of the hse composition of primitive upper mantle

pge rich podiform chromitites in the al ays ophiolite complex saudi arabia an example of critical mantle melting to extract and concentrate pge the al ays ophiolite complex in saudi arabia is an example of an ophiolite that contains anomalous concentrations of all six platinum group elements pge in podiform chromitite with maximum values of 2 570 ppb pt 6 870 ppb pd 840 ppb rh 5 800 ppb ru 6 200 ppb ir and 3 300 ppb os smooth chondrite normalized pge profiles indicate igneous pge ratios these suggest that in situ alteration of the pgm caused only minor mobility of pge during secondary modification of the mineralogy thus the geochemistry of the igneous concentration processes can be examined despite the mineralogical changes caused by subsequent alteration three main types of pge mineralization are observed that are defined by their relative abundances of individual pge type 1 has ru both pt and pd with negative slope chondrite normalized profiles type 2 has ru either pt or pd pt pd ir ratios of 1 to 5 and convex upward chondrite normalized profiles type 3 has ru either pt or pd pt pd ir ratios of 5 to 60 positive slope chondrite normalized profiles and is associated with elevated cu and ni concentrations the unaltered centers of chromite grains in the chromitite within this complex have an unusually large range of composition for example cr2o3 varies from 39 to 69 wt percent pge mineralization types 1 2 and 3 are related to the composition of the chromite type 1 occurs across the range of chromitite compositions from 39 to 69 wt percent cr2o3 type 2 occurs in chromitite having a range of 53 to 61 wt percent cr2o3 and type 3 occurs in chromitite having a range of 39 to 51 wt percent cr2o3 the pge form a great variety of platinum group minerals pgm and they differ among the three types of pge mineralization type 1 is characterized by euhedral os ir and ru ipge alloys and laurite both commonly enclosed in chromite as well as members of the irarsite hollingworthite solid solution series and pt ipge bearing pgm both commonly interstitial to the chromite grains type 2 pge enrichment is characterized by ipge pt and rh bearing pgm type 3 pge enrichment hosts predominantly pd and pt bearing pgm associated with ni and cu bearing minerals where exposed to the serpentinization process the pgm are altered to alloys arsenides antimonides and oxides that form irregular shapes or may form pseudomorphs of former pgm they are commonly associated with ni and cu bearing minerals including ruthanian pentlandite millerite arsenides and pge bearing awaruite mantle melting and subsequent crystallization were at an optimum to concentrate pge in the al ays ophiolite complex crystallization of ipge commonly prior to chromite crystallization and latterly with some pt and rh occurred across the range of chromitite composition crystallization of pd with some remaining pt occurred during sulfur saturation in chromitite formed from a more evolved magma we propose that this crystallization was from a magma that was enriched in pge because the degree of mantle melting was just sufficient to extract the pge but not dilute them in a melt that includes further mantle melting this feature is likely to be common to other pge rich ophiolite complexes such as in shetland in the united kingdom leka in norway thetford in canada pindos in greece tropoja in albania and in new caledonia if equilibrium partial melting had continued in al ays then the magma would have been diluted by subsequent pge poor melt this would have prevented sulfur saturation until much higher in the sequence producing pt and pd bearing base metal sulfides in the crustal wehrlite and gabbro as has occurred in the cyprus and oman ophiolites

chemical composition and origin of platinum group minerals from placers of the aunik river buryatia russia abstract platinum group minerals pgms are occasionally encountered in gold placers around the world most research related to the gold placer deposits focuses on the origin migration and concentration of gold the primary commercial mineral in the placers both origin and fate of pgm are rarely studied the present study of pgm grains reveals complex relationships between the mineral phases within the grains conforms to the published results on immiscibility within certain pgm solid solutions presents evidence of alteration of pgms and shows the presence of unusual secondary minerals within the grains the studied pgm grains are os ir ru alloys and can be divided into two types based on their chemical composition the first type is solid solutions enriched in os and ru bearing inclusions of silicates such as olivine pyroxene and amphibole the second type is enriched in ir and pt with inclusions of sulfides tellurides and selenides such as cuproiridsite iridisite malanite cooperite laurite erlichmanite and tolovkite these two types of pgm have been described in other regions and the first type was attributed to ophiolitic origin of the mineralization the authors confirm this theory and find that the pgm of the aunik river placers likely originated from ophiolites of the shamanskaya spreading zone with a possible contribution from ultrabasic rocks of ural alaskan type the secondary mineralization of the pgm especially prominent in the peripheral parts of many studied samples is most likely the result of post magmatic metasomatic alteration interestingly an unnamed new mineral phase ir os se2 was detected in a hydrothermally altered grain

distribution of platinum group elements and os isotopes in chromite ores from mayari baracoa ophiolitic belt eastern cuba the mayari baracoa ophiolitic belt in eastern cuba hosts abundant chromite deposits of historical economic importance among these deposits the chemistry of chromite ore is very variable ranging from high al cr 0 43 0 55 to high cr cr 0 60 0 83 compositions platinum group element pge contents are also variable from 33 ppb to 1 88 ppm and corre late positively with the cr of the ore bulk pge abundances correlate negatively with the pd ir ratio showing that chromite concentrates mainly os ir and ru which gives rise to the characteristic negatively sloped chrondrite normalized pge patterns in many chromitites this is consistent with the mineralogy of pges which is dominated by members of the laurite erlichmanite solid solution series rus2 oss2 with minor amounts of irarsite irass os ir alloys ru os ir fe ni alloys ni rh as and sulfides of ir os rh cu ni and or pd measured 187 os 188 os ratios from 0 1304 to 0 1230 are among the lower values reported for podiform chromitites the 187 os 188 os ratios de crease with increasing whole rock pge contents and cr of chromite furthermore cos values of all but one of the chromitite samples are negative indicating a subchondiritc mantle source cos decrease with increasing bulk os content and decreasing 187 re 188 os ratios these mineralogical and geochemical features are interpreted in terms of chromite crystallization from melts varying in composition from back arc basalts al rich chromite to boninites cr rich chromite in a su prasubduction zone setting chromite crystallization occurs as a consequence of magma mixing and assimi lation of preexisting gabbro sills at the mantle crust transition zone cr pge abundances and bulk os isotopic composition of chromitites are determined by the combined effects of mantle source heterogeneity the degree of partial melting the extent of melt rock inter actions and the local sulfur fugacity small scale l mt o cm chemical and isotopic heterogeneities in the plati num group minerals are controlled by the mechanism s of chromite crystallization in a heterogeneous environ ment created by the turbulent regime generated by suc cessive inputs of different batches of melt

deposits associated with ultramafic mafic complexes in mexico the loma baya case abstract several ultramafic mafic complexes of assumed ophiolitic affinity that contain ore deposits occur mostly along the pacific margin of mexico and the boundaries of the guerrero composite terrane which have been described in association with various tectonomagmatic settings these are 1 roots of volcanic arcs 2 ophiolites 3 oceanic basins 4 continental fragments underlying a magmatic arc and 5 subduction complexes or rift transform magmatic suites however the petrogenesis of such ultramafic mafic complexes and the associated ore deposits remains ill defined to date as their comprehensive study virtually lacks an appropriate geochemical approach ultramafic mafic complexes with known or estimated ages in mexico are mostly of paleozoic and mesozoic ages and very few of them formed once the crustal dynamics of the mexican pacific margin switched from generally extensional to compressional tectonic settings during the late cretaceous a few deposits aged late cretaceous to paleocene were interpreted to have formed in association with tectonomagmatic settings different from most deposits in the guerrero composite terrane to move forward in the understanding on how these mafic ultramafic complexes and their associated ore deposits formed the genesis of the chromitite deposits within the loma baya ultramafic mafic complex southernmost guerrero composite terrane is hereby endeavored the combination of the geochemical study of both major and trace elements in chromite including platinum group elements pge the mineralogy pge geochemistry and radiogenic isotopes i e re os in platinum group minerals pgm provides new assessment on the genesis of this ultramafic mafic complex from the obtained results it stems that the loma baya complex and the chromitite deposits associated with it are closely related with the formation of a back arc basin in an active supra subduction zone during the lower cretaceous and the parental melts resulted from re melting of an ultramafic protolith of paleozoic or older age the melts that originated in such back arc basin were of tholeiitic affinity and produced al rich chromitites with a moderate enrichment in pge associated with rates of partial melting of a depleted mantle source of at least 20 such geological framework is in frank disagreement with previous work thus highlighting the necessity of further research in other mexican ultramafic mafic complexes to better constrain their geodynamic setting of formation the obtained re os model ages in pgm of the loma baya chromitite deposits ca 300 and 130 ma suggest a possible correlation between these deposits and those in the californias in the sense that both areas record similar geological processes mantle rock melting re melting and chromitite emplacement within similar tectonomagmatic environments and time frame for their occurrence such possibility further highlights the necessity to carry on similar studies in chromitite deposits elsewhere along the pacific margin in mexico and southwestern usa in order to better reconstruct its tectonomagmatic evolution during the mesozoic

platinum group minerals in alpine chromitites from southwestern oregon electron microprobe and scanning electron microscopy the platinum group minerals found in the chromitites are laurite porous ru rich alloys os ir and pt rich alloys with highly variable compositions an ir cu rich sulfide a pt arsenide and three relatively rare platinum group sulfides the platinum group minerals are 1 to 30 mu m in diameter and occur as inclusions in chromite grains some of these inclusions also contain silicates and millerite base metal alloys largely ni fe occur in cracks in the chromitites along with magnetite ferritchromite chromian chlorite and serpentine the primary platinum group mineral assemblage is interpreted to have formed before or during crystallization of the chromite grains and not as a result of exsolution from the chromite modified journal abstract

the chromite deposits of the troodos complex cyprus evidence for the role of a fluid phase accompanying chromite formation the troodos ophiolite of cyprus hosts chromite deposits both as podiform bodies within the basal harzburgite and as lesser layers and schlieren within the overlying dunites and clinopyroxene dunites of the metacumulate sequence all chromitite types have been studied from both field and mineralogical aspects with particular emphasis on the chemistry of the ubiquitous silicate inclusions within chromite grains two distinct inclusion assemblages have been documented chromites from podiform bodies contain abundant hornblende serpentine ortho and clinopyroxene and lesser olivine while those from the metacumulates host pargasite albite serpentine and rare pyroxenes this implies that no single silicate inclusion assemblage in chromite can be considered typically ophiolitic the presence of an alkalirich fluid phase during chromite crystallization as evidenced by the unusual mineralogy of these inclusions has already been widely discussed in the literature however the marked stratigraphic control of inclusion types in chromite as reported here enables more complete modelling of the evolution of this phase to be made

high pressure phase transitions of natural chromitite from tibetan ophiolites abstract the podiform chromitites in the tibetan ophiolites have been thought to have experienced metamorphism corresponding to depths at the top of the mantle transition zone mtz however little is known about the phase relations of natural podiform chromitite in the mtz as well as the implications for processes related to formation of the ultra high pressure uhp chromitite an experimental study is presented to constrain the phase relations of natural chromitite at pressures and temperatures of 11 18 gpa and 1400 1600 c the experimental results show that the chromitite transforms into phase assemblages composed of chromite eskolaite garnet for pressures at the base of the upper mantle and chromite eskolaite garnet mc phase at the top of the mtz chromitite is replaced by a cati2o4 structured phase ct phase eskolaite garnet in the middle of the mtz the mc phase has an average composition of mg fe 3 cr al 2 2x 4 3y mg fe xsix y o6 x 0 2 y 0 3 with a cubic space group of fd3m it is suggested that the mc phase is a new silicon bearingmg and cr rich phase with idealized stoichiometry of mg3cr2o6 the observation of the mc phase in a natural rock system is helpful for understanding the incorporation of mg and cr minerals in the mtz and helps to constrain the related geological processes that controlled the formation of the tibetan chromitites

spinel compositions of mantle hosted chromitite from the eastern anatolian ophiolite body turkey implications for deep and shallow magmatic processes abstract the eastern anatolian accretionary complex includes several ophiolitic megablocks and or tectonic slivers within a 150 180 km long east west trending complex formed during the late cretaceous tertiary in eastern turkey the alabayir mollatopuz yukaribalcikli and mehmetalan ophiolites are megablocks or tectonic slivers containing locally massive nodular or schlieren banded chromitite layers these podiform chromitites formed in mantle sequences and are classified as high cr chromitites cr 0 63 0 88 mg 0 50 0 64 0 01 0 5 wt tio 2 5 7 18 8 wt al 2 o 3 calculated parental melt compositions of these chromitites indicate boninite magma characteristics 8 2 13 4 wt al 2 o 3 0 64 1 50 feo mgo the pge patterns also support the view that they crystallized from a boninitic melt the total pge contents of chromitites vary between 79 and 390 ppb pd ir ratios of chromitites vary from 0 07 to 0 28 and are consistent with an ipge fractionated nature the examined chromitites were divided into two groups in terms of their mineral chemistry group i alabayir mollatopuz yukaribalcikli and mehmetalan i chromitites group ii mehmetalan ii chromitites mineral chemistry of these chromitites and their parental melt composition indicate that group i chromitites were probably formed at shallow mantle depths and that group ii chromitites formed in deeper parts of the mantle mineral chemistry of these eastern anatolia high cr chromitites indicate that they formed not only in the deeper mantle but also in shallower parts of the moho transition zone