fenicu sulfides in tectonite peridotites from the maqsad district sumail ophiolite southern oman implications for the origin of the sulfide component in the oceanic upper mantle abstract forty samples of harzburgites and dunites from the maqsad oceanic mantle diapir southern oman have been investigated for the cu fe ni sulfide component and analyzed for sulfur the residual harzburgites contain less than 180 ppm of s the sulfide component is considered to be residual and to have survived the partial melting event giving rise to s saturated morb type magma the primary sulfide assemblage is closely associated with orthopyroxene dominated by pentlandite and chalcopyrite and very poor in pyrrhotite it has been partly altered into secondary sulfides and native metals awaruite and native copper by serpentinization related reducing fluids residual peridotites infiltrated by silicate melts subsequent to plastic deformation are strongly enriched in sulfur this applying especially to the dunites which contain up to 570 ppm s in this case the sulfide component has a metasomatic origin as demonstrated by its close association with the impregnation silicates clinopyroxene and to a lesser degree plagioclase and with cr spinel re equilibrated with the percolating silicate melts moreover the sulfide assemblage of the impregnated rocks differs from that of the harzburgites by virtue of a high cu ni bulk ratio the primary sulfide assemblage included in cr spinel has resisted low temperature alteration and is composed of pentlandite chalcopyrite bornite and very small amounts of pyrrhotite the metasomatic sulfide component of impregnated rocks is ascribed to s saturated silicate melts coming from deeper parts of the mantle and trapped at the top of the mantle diapir nevertheless sulfide precipitation differs from a simple gravity settling model and seems to be dependent on the precipitation of fe mg non sulfide minerals probably because the latter were capable of modifying the fe activity of the silicate melt which controls the s solubility

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

geochemistry of the platinum group elements in various types of spinels from the vourinos ophiolitic complex greece abstract samples of spinel from three different environments in the vourinos ophiolitic complex were analysed for the platinum group elements pge os ir ru pd pt and au chondrite normalized pge patterns of both spinels from dunites and massive chromites are characterized by high os ir and ru normalized values as compared to the low pd and pt values whereas spinels from harzburgite have nearly chondritic patterns the bulk chemistry of os ir ru of spinels from dunites and chromitites reflects the composition of the platinum group minerals pgm trapped in the spinels the analysed pge mostly pd and pt in spinels from harzburgites are considered to be trapped at grain boundaries as sulfides the geochemical behaviour of the pge during partial melting and crystallization is used in order to define the conditions of formation of chromitites and or pge concentrations

evidence for an ancient osmium isotopic reservoir in earth iridosmine grains from placer deposits associated with peridotite bearing ophiolites in the klamath mountains have extremely radiogenic 186 os 188 os ratios and old re os minimum ages from 256 to 2644 million years this indicates the existence of an ancient platinum group element reservoir with a supra chondritic pt os ratio such a ratio may be produced in the outer core as a result of inner core crystallization that fractionates os from pt however if the iridosmine os isotopic compositions are a signature of the outer core then the inner core must have formed very early within several hundred million years after the accretion of earth

geothermometry using minor and trace elements in igneous and hydrothermal magnetite abstract the temperature t and oxygen fugacity fo2 dependence of minor and trace elements mg al mn cr ni in magnetite mag in equilibrium with water bearing silicate liquids varying in composition from basalt to rhyolite 49 77 wt sio2 is examined in a compilation of experimental data n 299 from 700 to 1050 c 50 900 mpa and five order of magnitude range in fo2 the xmg mg mg fetot of mag is strongly dependent on t and can be formulated into an empirical geothermometer tmg mag c 8344 320 lnxmg 4 1 0 28 273 r2 0 83 that reproduces 72 of the experimentas to 50 c and conventional fe ti oxide thermometry in tuffs and vitrophyres to better than 60 c for 85 of the samples the tmg mag thermometer may have potential application in igneous or hydrothermally altered rocks where conventional thermometry or oxybarometry is not possible application to natural mag n 476 compiled from several settings shows igneous rocks mostly record 700 c but with some lower temperatures in plutonic rocks due to slow cooling hydrothermal samples skarns porphyry deposits show consistently subsolidus conditions tmg mag of 700 350 c consistent with other mineral and o isotopic equilibria the element correlations in natural mag from various settings are explored using multivariate statistical analysis to identify which minor or trace elements most critically depend on t fo2 or fluid composition

exhumation of lower mantle inclusions in diamond atem investigation of retrograde phase transitions reactions and exsolution a multiphase inclusion kk 83 in a diamond from the kankan deposits in guinea resulting from complex reactions between primary lower mantle phases was examined in detail using analytical transmission electron microscopy the inclusion consists of a large 300 m diopside crystal with a small symplectitic intergrowth in one corner comprising olivine and tetragonal almandine pyrope phase tapp the olivine part of the symplectite contains small exsolutions of mg al chromite and rare ca carbonate a second inclusion of ferropericlase observed in the same diamond indicates a primary origin within the lower mantle the clinopyroxene formed through a series of reactions at the expense of touching inclusions of casi and mgsi perovskite during convective ascent of the diamond through the transition zone the proposed reaction sequence is mgsipvk casipvk mgsiilm casipvk ringwoodite stishovite casipvk clinopyroxene relict ringwoodite the high al content of the bulk symplectite indicates ringwoodite as precursor phase coexisting with clinopyroxene a slight deficiency of sio2 during the diopside forming reaction leads to a small amount of relict ringwoodite 0 1 vol of the total inclusion is occupied by ringwoodite coexisting with diopside the ringwoodite captures the observed high amount of al subsequent breakdown of ringwoodite to wadsleyite tapp produces the observed symplectitic intergrowth during the phase transformation of wadsleyite to olivine exsolution of mg al chromite occurs further decreasing the original solubility of al cr and ti in accordance with experimental data on the element partitioning between wadsleyite and olivine based on these observations tapp can form as a retrograde phase within the transition zone of the earth s mantle and is not restricted to the upper part of the lower mantle high fe3 contents may favour its formation

comment on the importance of auroral features in the search for substorm onset process by syun ichi akasofu a t y lui and c i meng

platinum group element fractionation in a komatiitic basalt lava lake precise pge os ir ru pt pd abundance data for bulk rock samples and mineral separates from a deep differentiated komatiitic basalt lava lake in the vetreny belt baltic shield were obtained using an icp ms id technique the composition of the erupted liquid mgo 5 15 is characterized by a highly fractionated pge pattern pd os n 5 48 with subchondritic os ir n 5 0 43 analysis of separated olivine and chromite revealed that ru os and ir were slightly compatible to moderately incompatible in olivine d 5 1 7 0 8 and were compatible with chromite d 5 100 150 platinum and pd were highly incompatible with olivine d 5 0 08 0 03 and moderately compatible with chromite d 5 1 6 3 3 bulk solid liquid partition coefficients for pges ni cr and cu estimated from the whole rock regressions were os 7 9 ir 6 6 ru 4 5 pt 0 53 pd 0 09 ni 6 2 cr 4 4 and cu 0 01 the highly incompatible behavior of cu and pd indicates that there was no separation of fenicu immiscible sulfide liquid during the differentiation of the lava lake comparison of the bulk partition coefficients with those calculated from the olivine and chromite compositions implies that the fractionating mineral assemblage included 10 2561 mass fraction of pyrrhotite the data obtained provide new constraints on the effects of fractional crystallization on the pge compositions of mafic ultramafic magmas copyright 2001 elsevier science ltd

mineralogical variation in platinum group element within altered chromitite of the kondapalli layered igneous complex southern india implication on magmatic evolution and its petrogenetic significance abstract the kondapalli layered igneous complex klic is deformed and dismembered stratiform type of mafic ultramafic complex emplaced into high grade metamorphic rocks of eastern ghat belt egb which contains chromite with relatively oxidized compositions that occur as bands pockets pods stringer and dissemination this chromite hosts traces of pgm eg laurite irarsite ir ru rh pt ass os ir ru alloys and fe ni as s minerals the present study helps to understand the changes and or reworking of chromite and pgm due to the effect of alteration petrographic observations and epma study indicate that the chromite core is still consistent with magmatic fractionation processes whereas sequential alteration of the chromite rim suggests upper greenschist to lower amphibolite facies of metamorphism 500 to 550 c the platinum group minerals pgm however display evidence of partial hydrothermal metamorphic reworking especially laurite and irarsite which occur near the chromite grain boundary in contrast the primary ones which probably formed at high t and low fs2 and occur in the chromite cores are relatively less affected by alteration the calculated parental magma composition was similar to that of modern primitive boninitic tholeiitic basalt formed by a high degree of mantle melting present study ascribes the formation of the subduction generated arc mafic ultramafic magmatism in the kondapalli to the closure of the ocean that led to the formation of the suture between eastern dharwar craton edc and egb the parent magma of the klic is interpreted as mantle derived arc related boninites with s undersaturated nature like other layered intrusions this is indicating that an initial percolation of sulphur poor melts for long period without significant contamination for the genesis of klic the mineral chemistry supports the very limited crustal contamination and non availability of the sufficient meta sediments during assimilation precludes the sulphur saturation in the klic which is mainly constrained for the development of potential ni cu pge mineralization as a result the klic fever the deposition of only primary orthomagmatic chromite pge mineralization these findings determined that the investigated klic are low priority ni cu pge targets the compositions of the chromites can be used to determine the petrologic history of the intrusions and may prove to be a useful exploration tool in such mineralised belt

importance of auroral features in the search for substorm onset processes 1 auroral features at about the onset time of substorms are revisited to emphasize their importance in considering substorm onset processes this study is based on all sky camera photographs and meridian scanning photometer records from a single station first in considering substorm onset processes it is crucial to pay attention to the implication of the fact that an auroral arc located just poleward side of the initially brightening arc becomes active only after not before onset second prior to substorm onset there occurs a rapid equatorward shift or more like narrowing the width of the equatorward half of the oval not the whole oval without any distinct and immediate changes of the interplanetary magnetic field imf bz component except that the shift tends to occur after the southward turning of the imf these phenomena suggest that some processes take place spontaneously at a distance of less than 10 re perhaps near the boundary between the central plasma sheet and the boundary plasma sheet after the magnetosphere is primed by the imf southward turning thus it is concluded that substorm models in the near earth initiation category satisfy the observational constraints provided by the present paper because of our limited data set in terms of observations at single stations new ground based observations are suggested to confirm our results

supergene neoformation of pt ir fe ni alloys multistage grains explain nugget formation in ni laterites ni laterites from the dominican republic host rare but extremely platinum group element pge rich chromitites up to 17 5 ppm without economic significance these chromitites occur either included in saprolite beneath the mg discontinuity or as floating chromitites within limonite above the mg discontinuity both chromitite types have similar iridium group pge ipge enriched chondrite normalized patterns however chromitites included in limonite show a pronounced positive pt anomaly investigation of heavy mineral concentrates obtained via hydroseparation techniques led to the discovery of multistage pge grains i os ru fe ir grains of porous appearance are overgrown by ii ni fe ir and ir fe ni pt phases which are overgrown by iii pt ir fe ni mineral phases whereas ir dominated overgrowths prevail in chromitites from the saprolite pt dominated overgrowths are observed within floating chromitites the following formation model for multistage pge grains is discussed i hypogene platinum group minerals pgm e g laurite are transformed to secondary pgm by desulphurization during serpentinization ii at the stages of serpentinization and or at the early stages of lateritization ir is mobilized and recrystallizes on porous surfaces of secondary pgm serving as a natural catalyst and iii at the late stages of lateritization biogenic mediated neoformation and accumulation of pt ir fe ni nanoparticles occurs the evidence presented in this work demonstrates that in situ growth of pt ir fe ni alloy nuggets of isometric symmetry is possible within ni laterites from the dominican republic

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

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

activity variations attending tungsten skarn formation pine creek california an integrated geochemical analysis of the well exposed pine creek california tungsten skarn deposit has been undertaken to evaluate changes in chemical gradients across various lithologies thermodynamic calculations using available experimental and thermodynamic data allow limits to be assigned to the activities of important chemical components in the metasomatic environment quantifiable changes in non volatile component activites cao mgo al2o3 fe2o3 wo3 and in fugacities o2 f2 have been traced across the system the activities of al2o3 fe2o3 and wo3 generally increase from the marble 102 10 6 10 5 respectively through the outer skarn zone and into the massive garnet skarn 10 1 7 0 3 10 3 4 0 4 10 4 8 0 1 while cao and mgo activities decrease for the same traverse from 10 5 and 10 2 1 1 respectively to 10 5 7 and 10 3 calculated oxygen fugacities are 10 23 5 1 0 at t 800 k 527 c about one log unit below qfm and more reducing than that required by mt py po the high variance of the garnet pyroxene quartz assemblages adds sufficient uncertainty to the calculated activities for individual specimens that only the large scale trends survive the small scale scatter none of the chemical variables emerge as major independent or controlling factors for the mineralogy or phase compositions changes in the activity of one component may be offset by compensatory changes in another resulting in an environment that while different from pine creek could still host scheelite mineralization mass balance calculations indicate that the exposed endoskarn cannot have supplied the necessary chemical components to convert the country rock to skarn

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

diversity of platinum group minerals in podiform chromitites of the late proterozoic ophiolite eastern desert egypt genetic implications abstract podiform chromitites are frequently distributed as lensoidal pods in the central and southern parts of the eastern desert egypt they are in most cases hosted by fully serpentinized peridotite which is a part of dismembered ophiolite complexes of the pan african belt of late precambrian age serpentinites are the predominant components in the ophiolitic melange either as a matrix or as variably sized blocks and are derived from harzburgite and subordinate dunite the central eastern desert ced chromitites have a wide compositional range from high cr to high al varieties whilst those of the southern eastern desert sed have a very restricted compositional range the cr ratio of spinel ranges from 0 5 up to 0 8 in the former while it is around 0 8 in the latter platinum group element pge mineralization has been recently reported in podiform chromitites from the late proterozoic pan african ophiolite of the eastern desert of egypt the populations of platinum group minerals pgm in the studied ced and sed chromitites are quite distinguishable they are mainly sulfides os rich laurite in the former and os ir alloy in the latter sulfarsenides and arsenides are also found in subordinate amounts from both chromitites the most abundant base metal sulfides bms in the eastern desert chromitites of egypt are millerite heazlewoodite pentlandite chalcopyrite and pyrite the sulfur fugacity and temperature conditions are the main controlling factors of pge mineralogy in the host chromitite at the initial stage within the upper mantle os rich laurite is stable at high sulfur fugacity and low temperature conditions whereas os ir alloy is stable at lower sulfur fugacity and higher temperature conditions the diversity of pge mineralogy combined with the differences in petrological characteristics of chromian spinels from ced chromitites to sed ones suggests different degrees of partial melting of the mantle rocks of this ophiolite which in turn may be attributed to different tectonic settings during the post magmatic processes i e serpentinization the primary pgm e g os rich laurite and os ir alloy can be modified at low temperatures to secondary pgm

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

platinum group elements in mantle xenoliths from eastern china the platinum group element pge composition of a suite of anhydrous spinel peridotite xenoliths from five different localities in eastern china were determined in order to evaluate the geochemical behaviour of these elements during mantle processes the samples are composed of harzburgites of granoblastic textures and lherzolites mainly of protogranular and porphyroclastic textures the total pge contents vary from 16 20 to 50 7 ppb ruthenium ir and pd are correlated with cao and are interpreted to define a partial melting trend the positive correlation between cu and pd supports this interpretation however with the exception of au all peridotitic xenoliths display a downward concave pge pattern indicating pt enrichment this pattern is very different from the flat or negatively sloped pge pattern observed commonly for mantle xenoliths and peridotitic massifs the pt enrichment is ascribed to percolating basaltic magma which is precipitating only small amounts of sulphides as pt is less strongly chalcophilic than pd it may fractionate from the sulphides which host the pd and form pt alloys this process also explains the au enrichment and the pt au correlation as au is also less strongly chalcophilic than pd the strong ir depletion in the protogranular samples is ascribed to ir retention in the deep mantle due to the high metal silicate distribution coefficient of ir at fo2 iw 1

correlation between chromite composition and pge mineralization in the critical zone of the western bushveld complex detailed mineralogical investigations of chromite in the lower and critical zones in the northwestern sector of the bushveld complex have revealed significant compositional variations with regard to modal proportions host rock lithology and stratigraphic height superimposed on these variations are long range systematic trends in the composition of chromite in the massive layers these long range trends are closely linked with the evolution of the silicate cumulates the massive chromitite layers are divided into two types type 1 comprises the chromitites hosted entirely within ultramafic cumulates while type 2 chromitites are within cyclic units in which plagioclase cumulates occur the types are also distinguishable by their respective contents of platinum group elements pges and distribution patterns thereof viz the ratios between ru os ir and pt pd rh or relative element proportions both of which display a systematic change with height in accordance with chromite composition the relation between silicate geochemistry chromite composition and pge tenor leads to the development of a model explaining the formation of pge mineralized sulphide poor chromitite layers in the critical zone of the bushveld complex

crystal chemistry of a mg vesuvianite and implications of phase equilibria in the system cao mgo al2o3 sio2 h2o co2 chemical analysis including h f feo fe o of a mg vesuvianite from george town calif usa yields a formula r 9 mgl bbfe 0 a 10 7si1 81 06 oh 8 4 0 4 in good agreement on a cation basis with the analysis reported by pabst 1936 x ray and electron diffraction reveal sharp reflections violat ing the space group p4 nnc as consistent with domains having space groups p4 n and p4nc refinement of the average crystal structure in space group p4 nnc is consistent with occupancy of the a site with ai of the half occupied b site by 0 8 mg and 0 2 fe of the half occupied c site by ca of the ca i 2 3 sites by ca and the oh and o 10 sites by oh and 0 we infer an idealized formula for mg vesuvianite to be cai9mg mga1 ai4si 8069 oh 9 which is related to fe3 vesuvianite by the substi tutions mg oh fe3 0 in the b and o 10 sites and fe a1 in the alfe site thermodynamic calculations using this formula for mg vesuvianite are consistent with the phase equilibria of hochella liou keskinen kim 1982 but inconsistent with those of olesch 1978 further work is needed in determining the composition and entropy of synthetic us natural vesuvianite before quantitative phase equilibria can be dependably generated a qualitative analysis of reactions in the system cao mgo

the first occurrence of platinum group minerals pgm in a chromite deposit in the eastern desert egypt the platinum group mineralogy pgm of the chromitite from gebel lawi in the southeastern desert has been investigated the most abundant base metal sulfides bms associated with the lawi chromite are pentlandite millerite and heazlewoodite the major platinum group minerals identified were as follows laurite irosru s2 osmian iridium osir hollingworthite rhass tellurian arsenopalladinite pdtesbas polarite pdhg besides cuprian palladian gold cupdau a pd sb hg and hgte phases laurite and osmian iridium occur preferentially in chromite os ir commonly forms composite pgm with laurite hollingworthite and tellurian arsenopalladinite are included within serpentine and close to the base metal sulfides the cuprian palladian gold shares boundaries with chromite polarite together with the pd sb hg and hgte phases are embedded in serpentine palladium is the most abundant pge in the gebel lawi chromite

silicate and base metal sulfide inclusions in chromites from the maqsad area oman ophiolite gulf of oman a model for entrapment abstract al rich chromites from the maqsad area oman ophiolite have been studied to propose a genetic model for silicate and base metal sulfide bms inclusion entrapment samples were collected in both concordant and discordant nodular pods and in a peculiar stratiform ore chromite ore contains olivine clinopyroxene plagioclase as interstitial silicates regardless of the ore type the silicate inclusions are in decreasing order of abundance composed of ti and na rich phlogopite ti pargasite enstatite mg diopside olivine albite and secondary hydrothermal products chromite hosted bms are dominated by pentlandite heazlewoodite millerite and are nearly devoid of fe and cu sulfides the samples which come from concordant ores are very poor in mineral inclusions a consequence of the strain induced grain boundary migration and recrystallization which they suffered during plastic deformation in contrast the nodular and stratiform samples which escaped post magmatic deformation and densification are inclusion rich the inclusions display negative crystal shape following the chromite symmetry if present in nodular samples they may be arranged as coronas which mimic the shape of the host chromite grains all these textural features are considered to be evidence for mineral inclusion entrapment during magmatic precipitation of chromite by reference to a previous dynamic model of chromite ore genesis we consider that the nodules of chromite crystallized from basaltic magma were flowing inside a narrow cavity in which a non turbulent convective system was established we deduce from the presence of several inclusion coronas or patches in the largest nodules and their aabsence in the smallest that the inclusions were trapped preferentially at the point in which the primitive magma flow meets the convective current the peculiar mineralogy of mineral inclusions dominated by hydroxyl sodic phases indicates a complex interaction at the magmatic stage between the chromite precipitating magma and an unrelated aqueous fluid phase this fluid which was sodium and sulfur bearing travelled through the top of the mantle sequence and the basis of cumulate rocks of the ophiolite it could have been injected at certain stages of its evolution in the same conduits as the chromite precipitating magma locally giving rise to volatile enriched differentiated melts the latter have been sealed together with the liquidus phases of magma olivine clinopyroxene orthopyroxene and plagioclase as inclusions during chromite growth and or dissolution olivine and pyroxenes have partly reacted with the trapped volatile rich melt producing phlogopite and pargasite several hundred degrees below trapping temperatures bms have formed through sulfidization reactions with enclosed silicates and or host chromite

comparative reos smnd and rbsr isotope and trace element systematics for archean komatiite flows from munro township abitibi belt ontario abstract a series of komatiite flows from munro township ontario have re and os concentrations that range from 0 545 to 1 27 ppb and 1 00 to 2 20 ppb respectively the187re 186os in the suite range from 14 05 to 52 37 and the187os 186os range from 1 49 to 3 22 giving a crystallization age of2726 93ma and an of initial187os 186os of0 873 0 035 this age is in good agreement with the crystallization ages that have been reported for these rocks using other isotopic systems the re os isochron indicates that the system was resistant to the low grade metamorphism that altered the rocks the initial os isotopic composition is consistent with the derivation of the melts from a mantle source s that originated with a chrondritic 187os 186os i and evolved with an approximately chondriticre os sm nd data for the same suite cluster about anend value at 2 72 ga of 2 to 3 rb sr isotope data indicate significant mobilization of these elements by the greenschist grade metamorphism that has altered the rocks comparisons of re and os concentrations with those of mgo feo ni cr and s suggests that re was incompatible d 1 os concentrations were likely controlled by the presence or precipitation of trace metallic or sulfide phases both during melting and crystallization and not by the modally predominant silicate minerals

platinum group minerals pgm in the falcondo ni laterite deposit loma caribe peridotite dominican republic two ni laterite profiles from the loma caribe peridotite dominican republic have been investigated for their platinum group element pge geochemistry and mineralogy one profile loma peguera i

paragenesis and composition of laurite from chromitites of othrys greece implications for os ru fractionation in ophiolitic upper mantle of the balkan peninsula os rich laurite and erlichmanite are the dominant pgm inclusions in chromitites of the othrys ophiolite complex close association of the pgm sulfides with enstatite na rich pargasite clinopyroxene phlogopite and cu ni sulfides indicates crystallization at high temperature in the presence of an alkali rich fluid phase under relatively high sulfur fugacity because of the predominance of ru os ir phases the pgm assemblage of othrys is similar to that of chromitites located in the mantle unit of other ophiolite complexes of the balkan peninsula vourinos skyros island rhodope although it is distinguished because of the os rich composition of laurite and the presence of erlichmanite comparison among literature data for these complexes indicates that the os ru ratios vary consistently in laurite and bulk chromitite with respect to the chondritic os ru value this suggests that fractionation between the two elements occurred being apparently registered in the composition of laurite inclusions among other factors fluctuation of sulfur fugacity during fractionation as well as variation of the os ru ratio in the parent melts of the chromitites might be invoked to explain the ru os decoupling

the age of the inner core abstract the energy conservation law when applied to the earth s core and integrated between the onset of the crystallization of the inner core and the present time gives an equation for the age of the inner core in this equation all the terms can be expressed theoretically and given values and uncertainties of all relevant physical parameters the age of the inner core can be obtained as a function of the heat flux at the core mantle boundary and the concentrations in radioactive elements it is found that in absence of radioactive elements in the core the age of the inner core cannot exceed 2 5 ga and is most likely around 1 ga in addition to have an inner core as old as the earth concentrations in radioactive elements needed in the core are too high to be acceptable on geochemical grounds

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

on the formation of peridotite derived os rich pge alloys osmium rich pt group element pge alloys occur worldwide in association with chromite in ultramafic peridotite complexes it has been suggested that these os rich alloys formed under extreme p t conditions in the lowermost mantle before the metallic core of the earth formed or later in the outer core and have been transported to the upper mantle as xenoliths in deep rooted mantle plumes

origin of extremely pge rich mafic magma system an example from the jinbaoshan ultramafic sill emeishan large igneous province sw china abstract the 260 ma jinbaoshan sill is part of the permian emeishan large igneous province in sw china and is mainly composed of wehrlite and hosts a pge deposit containing 45 tonnes of pt and pd with ore grades ranging from 1 to 5 ppm pt pd the sill is composed of several pge rich horizons at different stratigraphic heights a 5 m thick pge rich horizon at the base of the sill which is underlain by a chromite rich pge poor layer accounts for 44 of the total ore reserve the jinbaoshan sill is rich in pges with ir ranging from 1 7 to 138 ppb ru from 0 9 to 39 ppb rh from 3 1 to 133 ppb and a total pt and pd from 76 to 5371 ppb all the rocks from the sill display primitive mantle normalized chacophile element patterns enriched in pges relative to ni and cu the low cu pd ratios 20 to 1938 of the rocks relative to the primitive mantle cu pd 7000 imply that there was no sulfide fractionation of the magmas the pge enrichment of the sill indicates that it formed from extremely pge rich magmas modeling results show that such pge rich magmas likely resulted from a multistage dissolution upgrading process in an open magma conduit system large amounts of chromite crystallization triggered sulfide saturation when pge rich magmas emplaced into the upper magma chamber of the system settling and sorting of chromite and most sulfide liquids along with olivine resulted in the formation of the largest pge rich horizon at the base of the sill the rest minor sulfide liquid accumulated to form pge rich horizons in the middle part of the sill aided by turbulence and convection within the magma chamber

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

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

native gold and native copper grains enclosed by olivine phenocrysts in a picrite lava of the emeishan large igneous province sw china a native gold bleb found in an olivine phenocryst in a picrite lava from the emeishan large igneous province elip may be the first documented case of the transport of gold as a distinct precious metal phase in a mantle derived magma four picrite layers have been recognized in the lower part of the volcanic succession in the lijiang area in the western part of the elip the native gold bleb was found enclosed in an olivine phenocryst in the second picritic layer of a basalt picrite succession in the elip the gold bleb is spheroidal about 30 m in diameter consists of pure gold and does not contain any other elements in addition native copper grains were also discovered in the serpentinized olivine phenocrysts and native zinc and moissanite sic were separated from an 20 kg sample of picrite the paragenesis of these minerals suggests that the primary magmas were s unsaturated the native gold and native copper grains are considered to be xenocrysts from the mantle transported to shallow depths by a rising plume and then captured by the picritic melts the discovery of native gold and native copper grains provides direct evidence that the gold in the hydrothermal gold deposits and the native copper deposits in the elip lavas ultimately may be derived from a mantle plume

platinum group element resources in podiform chromitites from california and oregon rock samples from about 280 podiform chromite deposits in california and oregon were analyzed for pt pd rh and ir by fire assay atomic absorption and spectrographic techniques to estimate by product platinum group element potential of chromite mining these deposits include ophiolites from the sierra nevada klamath mountains coast ranges and the canyon mountain complex the deposits vary from paleozoic to mesozoic age and vary in size shape texture and degree of metamorphism the maximum platinum group element contents in ppm measured for chromitites are pt 2 53 pd 0 2 rh 0 14 ru 4 93 and ir 2 93 correlations between pt and rh 0 60 rh and ir 0 64 rh and ru 0 71 and ir and ru 0 85 are significant at the 1 percent level relatively small but significant differences in platinum group element content exist between geologic terranes chromite production from california and oregon amounts to about 692 000 metric tons 1914 1985 the mean contained platinum group element amounts in troy ounces are pt 850 pd 100 rh 260 ru 4200 and ir 1 700 using estimated grades previous investigators have suggested that relatively few samples can represent the platinum group element content of a podiform chromite deposit therefore these analyses are assumed to approximate grades total world resources of platinum group elements in podiform chromite deposits are estimated in troy ounces to be pt 210 000 pd 25 000 rh 64 000 ru 1 000 000 and ir 420 000

the tectonic significance of pgm bearing chromitites at the ranomena mine toamasina chromite district madagascar this paper presents new data on platinum group mineral pgm bearing chromitites from the ranomena mine the only past producing chromite mine in the north toamasina chromite district of eastern madagascar such data can provide an insight on tectonic setting that led to emplacement of their host ultramafic body and can help to highlight the differences with other chromite deposits of madagascar chromitite host rocks are harzburgite and pyroxenite and gangue minerals are orthopyroxene and minor olivine chromite mineral chemistry shows a strong affinity with chromitites from layered intrusions and pge patterns are similar to those from the stratigraphically lowest chromitites in the bushveld complex pgms are dominated by laurite and are free of alloys pge mineralogy is consistent with early crystallization of laurite erlichmanite at low fs2 previous studies have suggested that the north toamasina ultramafic bodies are fragments of dismembered ophiolites and that their presence within the so called suture zone supports the suggestion that they mark the closure of an ocean between east and west gondwana even though these ultramafic bodies have been interpreted as ophiolitic our data better support an origin as small layered intrusions rather than as fragments of oceanic crust

the activation energy values estimated by the arrhenius equation as a controlling factor of platinum group mineral formation abstract in ophiolite complexes and ural alaskan type intrusions the platinum group element minerals pgm occur as laurite rus2 erlichmanite oss2 irarsite irass and alloys os ir ru and pt fe they are commonly found as small inclusions normally less than 10 m occasionally up to 100 m in chromite the origin of coarse grained pgm in the form of 0 5 10 mm nuggets in placer deposits related with mafic ultramafic complexes remains still unclear literature data on grain size r of platinum group minerals pgm and their formation temperature range of temperatures between 700 and 1100 c revealed an arrhenius temperature dependence correlation of the rate of crystal formation that depends on temperature t with the size r of the grain results in a linear relationship between ln r and 1 t from the slope of the line n ln r const eact rt the activation energy for the formation of ipgm ir platinum group minerals was estimated for the first time in the present study to be approximately 450 45 kj mol 1 applying the arrhenius equation the corresponding formation temperature for extremely large ipgm grains up to 1 3 mm in chromite ores related to ophiolites was found to be approximately 740 c it seems to be consistent with a lower formation temperature than with the typical formation temperature of small pgm grains associated with ophiolitic chromitites this suggests that coarse grained pgm in mafic ultramafic complexes along the permeable shear zones may have been re crystallized during plastic deformation at relatively lower temperatures 700 800 c under appropriate pressure temperature redox conditions and an increased h2o content thus applying the plot of ln r versus 1 t on large os ir ru minerals sulfides or alloys characterized by an r value falling into the linear part of the graph and having evidence supporting their formation at relatively high temperatures then the corresponding formation temperature of those ipgm can be found

an overview of the pge concentrations in the shetland ophiolite complex extremely anomalous platinum group element pge concentrations are known at one site cliff in the shetland ophiolite complex systematic pge analysis of all the mafic and ultramafic igneous and alteration lithologies in the ophiolite has revealed many lower grade anomalous pge occurrences throughout the cumulate ultramafic sequence and their distribution is described here more sites with pge enrichments as anomalous as those at cliff have not been found at cliff the anomalous values of over 60 ppm pt pd are restricted to a chromite rich sulphide bearing dunite forming a small part of a 200 m dunite lens in mantle harzburgite close to the basal thrust of the ophiolite lower values of up to 6 4 ppm pt pd occur in chromite poor sulphide rich dunites also within this dunite lens the only other chromite rich dunite lens in the cliff area also has detected pt and pd but with low pt pd values of 200 ppb in contrast harzburgites metasediments underlying the basal thrust serpentinites from the basal thrust and internal faults are all barren of pt and pd detection limit 20 ppb in the cliff area therefore detected pge values are restricted to magmatic chromite rich dunite lenses pge analyses of drill core from beneath the disused chromite quarries at cliff are low 227 ppb pt pd indicating that the extremely anomalous pge values are very restricted this supports the idea that pge concentrated magmatically within the dunite lens at cliff may have been remobilized locally to produce extremely anomalous pge values in an alteration zone only a few metres in diameter elsewhere in the ophiolite anomalous pt pd values of up to 4 ppm occur in chromite rich sulphide bearing dunite within the dunite unit which stratigraphically overlies the harzburgite pt pd concentrations of up to 1 ppm are present in the pyroxenites and wehrlites of the upper part of the ultramafic sequences high level wehrlites and pyroxenites within the gabbro unit representing late ultramafic differentiates contain lower levels of pge of up to 310 ppb pt pd the gabbros all have less than 20 ppb pt and pd this distribution indicates that the pge concentrations occur in specific primary magmatic lithologies and pathfinder analysis shows that they are associated with ni cu au and cr pge have not been found above detection limits in fault zones away from primary pge mineralization sites and it is concluded that pge are not significantly hydrothermally reconcentrated at cliff remobilization may have occurred over short distances of only a few metres at most

review of platinum group element distribution and mineralogy in chromitite ores from southern iran abstract the platinum group element pge potential of four chromite mining districts in southern and south eastern iran was investigated using geochemical and mineralogical methods a total of 20 mines and prospects were sampled in the neyriz ophiolite the abdasht and sikhuran complexes in the esfandagheh district and the faryab district chromitite and dunite were analysed for major trace element and pge geochemistry and the platinum group element mineralogy pgm in all mining districts chromites analysed from mantle tectonite probable transition zone and cumulate zone chromitites are refractory low in tio2 pge concentrations range from 5000 ppb with median values around 200 ppb in all complexes investigated the 187os 188os isotope ratios of chromitite indicate chondritic to slightly suprachondritic initial os mantle normalised pt ir ratios are generally low in the sikhuran complex pods of sulphide bearing chromitite 0 2 1 2 sulphides occur in a dunite zone interpreted as an ultramafic cumulate zone chromites in chromitite are characterised by high cr 75 81 and intermediate mg numbers 45 65 in associated dunite chromites cr 74 80 are more fe rich mg 29 50 chromitite and dunite carry up to 5 2 ppm total pge and are characterised by high normalised pt ir up to 5 pt pd large degrees of partial melting in a supra subduction zone geotectonic setting probably in a two stage melting process produced boninitic melts that crystallised refractory cr mg rich and ti poor chromite in some cases sulphide saturation was achieved during and after chromite formation saturation during chromite crystallisation is evidenced by ni cu rich sulphide droplets preserved in some chromitites sulphur saturation after chromite crystallisation governed the formation of ni rich sulphide liquids that exsolved monosulphide solid solution which later crystallised to form pge bearing pentlandite platinum and pd did not partition into pentlandite but crystallised as pt and pd rich pgm associated with the pentlandite

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

effects of mother lode type gold mineralization on 187os 188os and platinum group element concentrations in peridotite alleghany district california osmium isotope compositions and concentrations of re platinum group elements pge and au were determined for host peridotites serpentinites and harzburgites and hydrothermally altered ultramafic wall rocks associated with mother lode type hydrothermal gold quartz vein mineralization in the alleghany district california the host peridotites have os isotope compositions and re pge and au abundances typical of the upper mantle at their presumed formation age during the late proterozoic or early paleozoic the hydrothermally altered rocks have highly variable initial os isotope compositions with os values deviation of 187os 188os from the chondritic average calculated for the approx 120 ma time of mineralization ranging from 1 4 to 8 3 the lowest os isotope compositions are consistent with re depletion of a chondritic source e g the upper mantle at ca 1 6 ga most of the altered samples are enriched in au and have depleted and fractionated abundances of re and pge relative to their precursor peridotites geochemical characteristics of the altered samples suggest that re and some pge were variably removed from the ultramafic rocks during the mineralization event in addition to re the pt and pd abundances of the most intensely altered rocks appear to have been most affected by mineralization the 187os depleted isotopic compositions of some altered rocks are interpreted to be a result of preferential 187os loss via destruction of re rich phases during the event for these rocks os evidently is not a useful tracer of the mineralizing fluids the results do however provide evidence for differential mobility of these elements and mobility of 187os relative to the initial bulk os isotope composition during hydrothermal metasomatic alteration of ultramafic rocks

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

contact skarn formation at elkhorn montana i p t component activity conditions of early skarn formation black butte stock the initial stage of skarn development i is characterized by a variety of three phase and less often four phase assemblages which usually include garnet and pyroxene and occasionally epidote vesuvianite or clintonite the second stage of skarn development ii was marked by local replacement of garnet and pyroxene upon cooling by one or more of epidote amphibole clintonite chlorite calcite and quartz electron microprobe analyses indicate wide variations in pyroxene and garnet compositions in stage i application of experimental and calculated phase equilibria to observed skarn assemblages has allowed estimations of general physical chemical conditions of formation for stage i skarns p 1 0 or 0 3 kb t 525 or 35 degrees c 0 04

implications of 187os isotopic heterogeneities in a mantle plume evidence from gorgona island and curacao recent work has suggested that the mafic ultramafic volcanism in evidence throughout portions of the caribbean central america and northern south america including the islands of gorgona and curacao was generated as part of a middle cretaceous large igneous province new re os isochron results for tholeiitic basalts from gorgona and curacao indicate crystallization ages of 89 2 5 2 and 85 6 8 1 ma respectively consistent with reported ar ages the gorgona ultramafic suite shows a large range in initial os isotopic composition with os values ranging from 0 5 to 12 4 this large range reflects isotopic heterogeneities in the mantle source similar to those observed for modern ocean island basalts in contrast to ocean island basalts however os isotopic compositions do not correlate with variations in nd sr or pb isotopic compositions which are within the range of depleted mid ocean ridge basalts the processes that produced these rocks evidently resulted in the decoupling of os isotopes from the nd sr and pb isotopic systems picrites from curacao have very uniform chondritic initial os isotopic compositions with initial os values ranging only from 0 4 to 1 4 basalts from curacao however define an isochron with a 187os enriched initial isotopic composition os 9 5 in contrast to the 187os enriched ultramafic rocks from gorgona the enrichment in these basalts could have resulted from lithospheric contamination if the gorgona and curacao rocks were derived from the same plume os results combined with sr nd and pb data indicate a heterogeneous plume with multiple compositionally and isotopically distinct domains the os isotopic results require derivation of os from a minimum of two distinct reservoirs one with a composition very similar to the chondritic average and one with long term enriched re os oceanic crustal recycling has been invoked to explain most of the 187os enrichments that have been observed in ocean island basalt sources and could potentially apply to the gorgona suite crustal recycling however requires large proportions of very ancient recycled basaltic crust in the sources of the 187os enriched ultramafic rocks to explain the magnitude of 187os enrichments observed for example addition of 20 oceanic crust to fertile mantle and nearly 3 billion years are necessary to generate a reservoir with the os isotopic composition of the most radiogenic komatiites if the recycled oceanic crust was added to basalt depleted mantle as may be indicated by nd values for the komatiites averaging about 10 even larger proportions of older crust are required large proportions of oceanic mafic crust in the sources of the 187os enriched komatiites although petrologically conceivable under certain melting conditions is unlikely here given the limited trace element and lithophile isotope system variations these results raise questions about the efficacy of using os isotopes to constrain the proportion of recycled oceanic crust in other plumes other possible mechanisms for generating 187os enriched mantle include invoking the existence of a 187os enriched lower mantle and minor outer core lower mantle interactions

au ir ni and co in some chromitites of the eastern bushveld complex south africa abstract instrumental neutron activation analyses of the lower group middle group and upper group chromitite layers of the eastern bushveld complex are reported the elements analysed were ir 300 ppb au 240 ppb ni 540 to 2240 ppm co 101 to 370 ppm ir and au are higher in the upper critical zone compared to the lower critical zone the relationship of the element abundance to the amount and composition of the chromite in the sample and the possible controls on the distribution of the elements are discussed at low levels of concentration ir is most likely controlled by discrete primary ir bearing platinum group minerals and au is related to small amounts of base metal sulphide both being sympathetic with the volume percent chromite in the sample higher ir and au levels are possibly related to sulphides present in even greater amounts

platinum group element pge mineralisation and chromite geochemistry in the berit ophiolite elbistan kahramanmaras se turkey abstract the high al chromitite bearing stratigraphic levels cr ranges from 0 38 to 0 47 are located at the mantle crust transition zone mtz and in the cumulate dunites in the berit ophiolite in se turkey iridium group elements ipge ir os and ru and related mineralisation were previously reported from ophiolitic high cr chromitites in the berit elbistan area palladium group elements ppge pt pd and rh and associated mineralisation are predominantly found in the high al chromitites and they are presented in this study for the first time our results indicate that ipge enriched phases of the pgms such as laurite are present in both the ppge enriched and the ppge poor chromitites in berit the high al chromitite chondrite normalised patterns in the berit ophiolite show an enrichment of ppge with values of up to 1700 ppb pt and up to 4469 ppb pd during their occurrence sulphur saturation is consistent with the production of minor quantities of base metal sulphides bms which took place in the form of droplets enclosed in chromite commonly with ppge enriched patterns these ppgm assemblages include pd pt tellurides sperrylite platarsite pd antimonide pd sb as and pt pd cu au alloys the ppge enrichment in the berit ophiolite is unusual and the presence of such occurrences is known only in a few other ophiolites worldwide the geochemical evolution of the high cr ratios of the high cr chromitites indicates an origin due to high degrees of partial melting whereas the high al chromitites indicate lower degrees of partial melting both high degree and low degree melts may have been fluxed through the upper mantle either synchronously or possibly with a hiatus between the generation of different melts

the influence of meteoric water on skarn formation and late stage hydrothermal alteration at the evciler skarn occurrences kazdag nw turkey abstract skarn ore deposits are generally characterized by two distinctly different alteration styles an early prograde stage with anhydrous minerals such as garnet grossular andradite and pyroxene diopside hedenbergite which forms in the presence of relatively high temperature fluid and a later retrograde stage with hydrous minerals such as epidote amphibole actinolite and chlorite from most to least abundant plus sulfides which forms in the presence of the lower temperature fluid these two alteration stages commonly have been thought to reflect a dominance of magmatic and meteoric water respectively with relevance to the source of ore metals we report data from the evciler au cu skarn deposit kazdag nw anatolia the aim of the analyses was to determine temperatures of formation the extent of equilibration among the mineral phases and or of possible alteration through fluid mineral isotopic exchange stable isotope compositions of anhydrous and hydrous alteration minerals indicate that garnet grossular andradite pyroxene diopside hedenbergite skarn prograde stage was produced by predominantly magmatic fluids during initial skarn forming metasomatism in the study area and actinolite epidote rich skarn retrograde stage was formed by magmatic water mixed with meteoric water the 18 o values of garnet from the evciler district range from 4 6 to 8 2 mean 6 7 18 o values of pyroxene range from 6 2 to 10 3 mean 8 2 18 o values of amphibole range from 6 3 to 8 2 mean 7 4 and the 18 o values of epidote range from 3 3 to 5 1 mean 0 9 however the d values of retrograde alteration minerals such as amphibole and epidote indicate both magmatic and lighter values for evciler skarn deposit that could be explained by mixing with meteoric water we conclude that the isotopic evolution of the hydrothermal fluid can be accounted for by circulation of meteoric water through a convection system heated by the evciler granitoid causing exchange of oxygen isotopes with the granitoid and country rock and possibly involving some admixture of magmatic water

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

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

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

the josephine ophiolite northwestern california the 157 m y old josephine ophiolite is a complete ophiolite consisting of the following units harzburgite tectonite cumulate ultramafics cumulate gabbro interlayered with cumulate ultramafics cumulate gabbro high level gabbro and diorite a 1 5 km thick sheeted dike complex and 0 4 km of pillow lava and breccia the ophiolite is conformably overlain by the galice formation consisting of a thin siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental margin sources the ophiolite and overlying galice formation were intruded by numerous 150 to 151 m y old calc alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite pumpellyite to lower greenschist facies the josephine peridotite makes up the base of the ophiolite and covers 800 km2 it is an alpine type peridotite that represents upper mantle material and the residue of partial melting ultramafic rocks overlying the josephine peridotite are olivine spinel clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene olivine plagioclase cumulates and finally to clinopyroxene plagioclase cumulates high level gabbros and diorites are characterized by highly variable textures complex intrusive relationships strongly zoned plagioclase and abundant mafic dikes plagiogranites occur at this level as dikes small intrusions and screens between mafic dikes but they are rare the sheeted dike complex consists of mafic dikes averaging 0 6 m in thickness chilled margins are locally well preserved and many of the dikes have been split and intruded by younger dikes resulting in half and marginless dikes the dike complex is composed of 100 sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts the dikes are inclined at 40 to bedding after correcting for folding and are inferred to have been rotated shortly after formation the extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas some of the breccias formed as talus deposits whereas others are graded and matrix supported indicating transport and redeposition possibly in an area of rugged submarine topography the dikes and lavas have geochemical affinities to island arc tholeiites but are in some aspects transitional to mid ocean ridge basalts the dikes and lavas are cogenetic with the underlying plutonic sequence as indicated by similarity in crystallization sequence contradictory intrusive relationships between sheeted dikes and high level plutonic rocks and lack of diabase dikes in the lowermost cumulates and harzburgite the upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center resulting in downward changes in mineralogy texture and 18o in addition metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs a geothermal gradient of 250 350 c km is inferred from mineral assemblages on the basis primarily of its regional setting the ophiolite is inferred to have formed in a back arc basin in particular it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex remnant arc this interpretation is consistent with abundant volcanic arc detritus in graywackes overlying the ophiolite the island arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back arc spreading

genesis of the olden wollastonite skarn sharbot lake domain central metasedimentary belt grenville province southeastern ontario canada with a resource of 2 8 mt at 30 35 wollastonite occurring at a depth of about 75 m the olden formerly hawley prospect is the largest of a swarm of skarns hosted by amphibolite facies dominantly calcitic marble that occurs adjacent to and as inliers within the mountain grove pluton the post kinematic intrusion comprises units with a wide compositional range from anorthositic gabbro to alkali feldspar granite and syenite and widely exhibits megascopic fabrics recording magma comingling and mixing isotope dilution thermal ionization mass spectrometry id tims dating of zircon separates from a hornblende diorite unit yields a 207pb 206pb age of 1153 2 ma significantly older than the contiguous 1070 3 ma mclean granite al in hornblende geobarometry on several mountain grove units indicates that the intrusion crystallized at pressures of 250 470 mpa equivalent to mesozonal depths of 10 15 km the main exoskarn body 200 m long and up to 50 m wide is dominated by wollastonite clinopyroxene

kankan diamonds guinea i from the lithosphere down to the transition zone peridotitic inclusions in alluvial diamonds from the kankan region of guinea in west africa are mainly of lherzolitic paragenesis nevertheless extreme cr2o3 contents max 17 wt in some of the exclusively lherzolitic garnets document that the diamond source experienced a previous stage of melt extraction in the spinel stability field this initial depletion was followed by at least two metasomatic stages 1 enrichment of lree and sr and 2 introduction mainly of mree hree and other hfse ti y zr hf the ti and hfse poor character of stage 1 points towards a cho rich fluid or carbonatitic melt the high hfse in stage 2 favour silicate melts as enriching agent eclogitic inclusions are derived from a large depth interval ranging from the lithosphere through the asthenosphere into the transition zone the occurrence of negative eu anomalies in garnet and clinopyroxene from both lithosphere and transition zone suggests a possible relationship to subducted oceanic crust lithospheric eclogitic inclusions are derived from heterogeneous sources that may broadly be divided into a low ca group with lree depleted trace element patterns and a high ca group representing a source with negative lree hree slope that is moderately enriched in incompatible elements relative to primitive mantle high ca inclusions of majoritic paragenesis are significantly more enriched in incompatible elements such as in sr and lree calculated whole rock compositions require metasomatic enrichment even if a derivation from morb is assumed

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

reply to comment by y i feldstein v g vorobjev and v l zverev on the importance of auroral features in the search for substorm onset process

pge distribution in 2 7 ga layered komatiite flows from the belingwe greenstone belt zimbabwe the well preserved anderson s and onias s komatiite flows from the late archaean belingwe greenstone belt 2 7 ga zimbabwe were sampled in the saskmar drill hole anderson s flow possesses a 1 1 m thick b1 subzone and a clinopyroxene spinifex subzone whereas onias s flow has spinifex and cumulate layers typical of komatiite flows platinum group element pge abundances throughout these two flows provide constraints on the formation of layered komatiite flows all samples from both flows have similar chondrite normalized pge patterns that are enriched in rh pt and pd relative to os and ir which are attributed to large degrees 30 of partial melting of the upper mantle their pd ir ratios are highly variable 2 0 31 in onias s flow and 2 3 61 in anderson s flow pge variations within both flows except the spinifex a zone and b1 subzone of anderson s flow together with those of major and other trace elements are consistent with a model involving olivine fractionation the spinifex a zone of anderson s flow has low pd ir ratios 6 8 which occur in an interval having negative eu anomalies caused by hydrothermal alteration the pt fractionation from pd and rh is documented from their relationship to mgo contents behaviours of pd and rh are similar and more incompatible than pt at the early stage of relatively slow cooling but less incompatible at the late stage during the formation of komatiite flows pge in the b1 subzone of anderson s flow do not follow an olivine fractionation trend pd ir ratios in this subzone increase from 28 at the top to 61 at the base these unusually high values can be accounted for by late stage development of the subzone and strong fractionation

chemical and physical transfers in an ultramafic rock weathering profile part 2 dissolution vs accumulation of platinum group minerals the chemical weathering of ultramafic rocks has resulted in eluvial concentration of pt group minerals pgm in lateritic weathering profiles of southern new caledonia the pt mineralization interpreted as being primary consists of pt group minerals included within chromite crystals the oc currence of pgm as free particles in the weathering profile results from the supergene dissolution of pt bearing chromite traore et al 2008 following their release in the profile supergene dissolution processes variably affect the pgm particles the behavior of pt group elements in the weathering profile is characterized by significant loss of pd and relative enrichment of pt indicating that pd is more mobile than pt in the exogenous cycle unstable pt fe cu pd alloys and pge oxides undergo chemical and mineralogical changes to acquire the chemical configuration of isoferroplatinum pt3fe which is the most stable pt phase in a lateritic environment the isoferroplatinum phase may also be dispersed throughout the weathering mantle and or accumulated in the lower parts of profiles ac cording to a translocation mechanism of residual pt rich fine particles driven by percolation of water through the connected pore spaces