salinity driven thermocline transients in a wind and thermohaline forced isopycnic coordinate model of the north atlantic abstract an isopycnic coordinate oceanic circulation model formulated with the aim of simulating thermodynamically and mechanically driven flow in realistic basins is presented special emphasis is placed on the handling of diabatic surface processes and on thermocline ventilation the model performance is illustrated by a 30 year spinup run with coarse horizontal resolution 2 mesh in a domain with north atlantic topography extending from 10 to 60 n latitude the vertical structure encompasses 10 isopycnic layers in steps of 0 2 units capped by a thermodynamically active mixed layer from an initially isohaline state with isopycnals prescribed by zonally averaged climatology the model is forced by seasonally varying wind stress radiative and freshwater fluxes and by a thermal relaxation process at the surface after a mechanical spinup time of about 15 years a quasi stationary pattern of mean circulation and annual variability ensues characterized by pronounced subtropical mode water formation

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

a wind driven isopycnic coordinate model of the north and equatorial atlantic ocean 1 model development and supporting experiments numerical approximations to the dynamic equations are given which allow basin size ocean circulation models formulated in isopycnic coordinates to accommodate variable bottom topography and irregular coastlines emphasis is placed on computational economy through the use of a split explicit time integration scheme on the proper formulation of the advection and coriolis terms in the momentum equation in case of strongly varying layer thickness and on the correct estimation of the horizontal pressure gradient force in grid boxes truncated by steep bottom slopes the algorithms are tested in a series of two and three layer double gyre experiments in cases of steady forcing leading to a steady circulation we are able to reproduce the expected motionless final state in coordinate layers that are below the direct influence of the wind forcing this includes layers intersecting topographic obstacles a long term 25 30 year vacillation tentatively associated with the outcropping of isopycnals along the edge of the cyclonic gyre in a steadily forced model is documented the paper forms the basis for a subsequent study in which circulation features obtained in a realistic north atlantic setting will be discussed

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

melting and melt segregation in the mantle wedge above a subduction zone evidence from the chromite bearing peridotites of the miyamori ophiolite complex northeastern japan

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

petrogenesis and tectonic significance of jurassic iat magma types in the hellenide ophiolites as deduced from the rhodiani ophiolites pelagonian zone greece abstract the rhodiani ophiolites are represented by two tectonically superimposed ophiolitic units the lower ultramafic unit and the upper volcanic unit both bearing calcareous sedimentary covers the ultramafic unit consists of mantle harzburgites with dunite pods and chromitite ores and represents the typical mantle section of supra subduction zone ssz settings the volcanic unit is represented by a sheeted dyke complex overlain by a pillow and massive lava sequence both including basalts basaltic andesites andesites and dacites chemically the volcanic unit displays low ti affinity typical of island arc tholeiite iat ophiolitic series from ssz settings having as most distinctive chemical features low ti v ratios the rare earth element and incompatible element composition of the more primitive basaltic andesites from the rhodiani ophiolites can be successfully reproduced with about 15 non modal fractional melting of depleted lherzolites which are very common in the hellenide ophiolites the calculated residua correspond to the depleted harzburgites found in the rhodiani and othrys ophiolites both field and chemical evidence suggest that the whole sequence of the rhodiani volcanic unit from basalt to dacite originated by low pressure fractional crystallization under partially open system conditions the modelling of mantle source melt generation and mantle residua carried out in this paper provides new constraints for the tectono magmatic evolution of the mirdita pindos oceanic basin

simulating the dynamics and intensification of cyclonic loop current frontal eddies in the gulf of mexico 1 the dynamics associated with the loop current lc variability in the gulf of mexico gom are studied using a 5 year free running numerical simulation with the hybrid coordinate ocean model hycom the dynamics of major gom circulation features are represented the extension of the lc and the associated anticyclonic warm core loop current eddies lces and cyclonic loop current frontal eddies lcfes the study focuses on the dynamics of the lcfes and their role during the lces shedding which dramatically affects the gom circulation we analyze several characteristics of the lc frontal dynamics modeled lcfes have a coherent vertical structure which extends to the deep layers of the gom they may split in two separate upper and lower layer eddies deep and surface remnants from different frontal eddies are able to align to form new coherent structures lcfes intensify along the extended lc northern edge when flowing over the deep northern gom shelf slope that forms the mississippi fan through a promontory effect in which the incoming cyclone aggregates positive potential vorticity anomalies in lower layers leading to the intensification of the whole vortex structure lcfes may also expand further along the lc path by horizontal vortex merging when they are blocked between the lc and the northeast corner of the continental shelf in the gom the intensification and merging due to topographic effects explain the enlarged frontal eddies observed on the eastern side of the loop current these larger eddies further migrate along the lc front and may play a role in the shedding sequence

the role of komatiitic and picritic magmatism and s saturation in the formation of ore deposits abstract the introduction of high temperature high magnesium komatiitic and picritic magmas into the earth s upper crust has given rise either directly or indirectly to many of the world s major ore deposits there is a direct association between komatiitic magmatism and kambalda style ni cu pge deposits the link between magma type and mineralization is not as clear cut in the case of the noril sk cu ni pge deposits but nevertheless there is an association between picritic magmatism and ore deposit development komatiitic and or picritic magmatism may also have contributed to the formation of many types of hydrothermal ore deposits recent nd isotope data generated by kent et al 1994 provide strong evidence for the hypothesis proposed by keays in 1983 that the au in archaean au deposits was derived either directly or indirectly from komatiites through the action of metamorphic fluids the importance of komatiitic and picritic magmas in ore forming processes arises from the fact that they are among the few mafic ultramafic magma types that are s undersaturated at the time of magma formation s saturation of magmas leads to marked depletion in the chalcophile metals for example czamanske and moore 1977 estimated that a third of the original cu and ni contents of morb were lost from the magmas due to extraction into magmatic sulphides the depletion in au and the pge is much greater due to their much larger partition coefficients keays and scott 1976 komatiitic and picritic magmas are s undersaturated because they are high temperature magmas produced by large degrees of partial melting of upper mantle sourc regions that were already depleted in s through earlier partial melting events most mafic magmas produced by less than 25 partial melting are s saturated and hence depleted in the ore forming chalcophile elements in contrast komatiitic and picritic magmas have their full complement of chalcophile metals when they do become s saturated they form sulphides that are strongly enriched in ni cu au pge and other strongly chalcophile metals these may accumulate to form ore deposits directly e g magmatic ni cu pge sulphides or be dispersed in komatiitic picritic rocks or their fractionation products making these source rocks for hydrothermal au or vms deposits

role of late magmatic fluids in merensky type platinum deposits a discussion a number of features of platinum group element pge rich layers in layered intrusions provide evidence for the involvement of late magmatic fluids the presence of pegmatitic textures intergrowth of sulfides with intercumulus volatile rich phases high cl content in volatile bearing phases and association with graphite and fluid inclusions have been taken by some authors as evidence for postcumulus transport and deposition of pge by a fluid phase of late magmatic derivation we argue that these apparently hydrothermal features are the result of postcumulus processes superimposed on layers whose extensive uniform pge concentrations were formed by magmatic cumulus processes the association with pegmatites and volatiles arises from the relatively low solidus temperatures of orthocumulate layers which cause these layers to act as traps for fluids migrating through the crystal pile these fluids are mostly derived by vapor exsolution from fractionated intercumulus melt and are consequently enriched in ci redissolution of these fluids into vapor undersaturated trapped liquid in orthocumulate layers causes recrystallization and formation of gabbro pegmatites immiscible sulfide oxide liquid solidifies at near solidus temperatures in the interstitial pore space between early crystallizing silicate minerals and it is forced into areas occupied by late crystallizing ci bearing silicates this accounts for intergrowths of cu and pge rich sulfides with amphiboles and micas a ubiquitous feature of sulfide bearing gabbros further exsolution of a vapor phase during late stages of crystallization causes deuteric alteration formation of fluid inclusions and possible local redistribution of sulfides and pge cooling of this vapor phase may lead to precipitation of graphite at or below the orthocumulate solidus graphite deposition may be catalyzed by sulfides and pge

geochemistry of meteoric diagenesis in carbonate islands of the northern bahamas 2 geochemical modelling and budgeting of diagenesis geochemical characterization and numerical modelling of surface water and ground water combined with hydrological observations provide quantitative estimates of meteoric diagenesis in pleistocene carbonates of the northern bahamas meteoric waters equilibrate with aragonite but water rather than mineral controlled reactions dominate dissolutional lowering of the undifferentiated bedrock surface is an order of magnitude slower than that within soil filled topographic hollows generating small scale relief at a rate of 65 140 mm ka 1 and a distinctive pocketed topography oxidation of organic matter within the subsoil and vadose zones generates an average pco2 of 4 0 10 3 atm which drives dissolution during vadose percolation and or at the water table however these dissolution processes together account for 60 of the average rock derived calcium in groundwaters pumped from the freshwater lens the additional calcium may derive from oxidation of organic carbon within the lens accounting for the high pco2 of the lens waters mixing between meteoric waters of differing chemistry is diagenetically insignificant but evapotranspiration from the shallow water table is an important drive for subsurface cementation porosity generation in the shallow vadose zone averages 1 6 3 2 ka 1 phreatic meteoric diagenesis is focused near the water table where dissolution generates porosity at 1 4 2 8 ka 1 maximum dissolution rates however are similar to those of evaporation driven precipitation which occludes porosity of 4 0 s 0 6 ka 1 this drives porosity inversion from primary interparticle to secondary mouldic vug and channel porosity in the deeper freshwater lens oxidation of residual organic carbon and reoxidation of reduced sulphur species from deeper anaerobic oxidation of organic carbon may generate porosity up to 0 06 ka 1 meteoric diagenesis relies critically on hydrological routing and vadose thickness controlled by sea level as well as the geochemical processes active a thin vadose zone permits direct evaporation from the water table and drives precipitation of meteoric phreatic cements even where mineral stabilization is complete copyright 2007 john wiley sons ltd

assimilation and partial melting of continental crust evidence from the mineralogy and geochemistry of autoliths and xenoliths this paper presents a model for the partial melting of quartz diorite and greywacke in the upper crust based on the mineralogy and geochemistry of enclaves within the loch doon granitic intrusion of southern scotland the melting of quartz diorite was modelled using autoliths which represent fragments of cogenetic igneous rocks that became incorporated in the fractionating magma compared to their quartz diorite parents the autoliths are enriched to varying degrees in some elements notably rb nb ta sm y yb and depleted in others sr and ba eu and p are also depleted in the more assimitated autoliths the compositions of melts that could be derived from assimilation of the autoliths have also been calculated their ree patterns reveal a light ree enrichment low concentrations of heavy rees 1 3 x chondrite and a positive eu anomaly the calculated degrees of melting vary from 35 in the least assimilated to 84 in the most assimilated autolith assuming a bulk distribution coefficient of 10 for the most compatible element results from modelling of xenolith compositions derived from metasediments are also reported but because of uncertainties in the composition of the parental sediment these data are subject to larger errors they do however indicate that resultant partial melts are distinctly different from those derived by partial melting of autoliths in particular the ree pattern of a greywacke derived melt shows a slight enrichment in light rees greater concentrations of heavy rees 10 x chondrite and a small negative eu anomaly the calculated degrees of melting of the xenoliths fall in the range of 66 88 assuming a bulk distribution coefficient of 10 for the most compatible element the results have direct implications for assimilation and melting of the upper crust by taking into account how the nature of residual phases is likely to change with depth it can be demonstrated that some archaean tonalite gneisses could represent liquids derived by partial melting of igneous material

mesoscale eddies in the fram strait marginal ice zone during the 1983 and 1984 marginal ice zone experiments during the summer marginal ice zone experiment in fram strait in 1983 and 1984 fourteen mesoscale eddies in both deep and shallow water were studied between 78 o and 81on sampling combined satellite and aircraft remote sensing observations conductivity temperature depth observations drift of surface and subsurface floats and current meter measurements typical scales of these eddies were 20 40 km rotation was mainly cyclonic with a maximum speed in several cases subsurface of up to 40 cm s observations further suggest that the eddy lifetime was at least 20 to 30 days five generation sources are suggested for these eddies several of the eddies were topographically trapped while others primarily formed by combined baroclinic and barotropic instability moved as much as 10 15 km d with the mean current the vorticity balance in the nontrapped eddies is dominated by the stretching of isopycnals accompanied by a change in the radial shear in the most completely observed eddy south of 79on the available potential energy exceeded the kinetic energy by a factor of 2 quantitative estimates suggest that the abundance of these eddies enhances the ice edge melt up to 1 2 km d

biogenic β fabric features in the hard laminated crusts of the mersin and adana regions southern turkey and the role of soil organisms in the formation of the calcrete profiles abstract this paper describes fabric features and their distribution in the calcretes and emphasizes the role of soil organisms in the formation of the calcrete profiles quaternary calcretes are common in the mersin and adana provinces occurring as powdery nodular tubular fracture infill and various forms of crusts the stable isotope values indicate calcrete development under semi arid climatic conditions and a pedogenic origin biogenic or fabric constituents are abundant in the hard laminated crust these are the rhizoliths alveolar septal structures needle fiber calcites calcified filaments spherical calcite bodies microcodium and vadose calcrete pisoliths rhizoliths have mainly formed by the petrification of roots and often contain cellular cortex structures the alveolar septal structure comprises root voids separated by a micritic wall or a bundle of tangentially oriented calcite needles needle fiber calcite is common in the hard laminated crust as monocrystalline rods and polycrystalline needles monocrystalline rods are smooth and small

new application of gis and statistical analysis in mapping the distribution of quaternary calcrete tensift al haouz area central morocco abstract quaternary calcretes are widespread in soils of many regions of the world especially in semi arid and arid areas their development has been studied worldwide for more than a century involving various disciplines and resulting in large amounts of data concerning their evolution in this article the geographical distribution of calcrete has been studied by combining geographical information systems gis and statistical analysis in this paper we present a qualitative approach that incorporates topographic geological hydrogeological and climatic data as well as field observations after getting all four parameters they were divided into classes and an index value has been assigned to each of them according to influence degree on calcrete development finally all the parameters are superimposed to prepare the final map of calcrete distribution the multivariate statistical analysis techniques used were principal component analysis and cluster analysis both methods were performed to discuss the potential relations among the investigated parametres caco3 palygorskite slope and calcrete evolution the qualitative approach adopted provided a distribution map predicting the possible areas where different calcrete forms might occur and allowed to elucidate the degree of influence of environmental factors on the evolution of calcrete the accuracy of this method has been estimated by comparing it with 52 cross section data from the field this approach proved to be successful in this study the results of statistical analysis confirm the results obtained using gis method and highlight the influence of the studied environmental factors on the spatial distribution of different types of calcrete

chromite alteration processes within vourinos ophiolite the renewed interest in chromite ore deposits is directly related to the increase in cr price ruled by international market trends chromite an accessory mineral in peridotites is considered to be a petrogenetic indicator because its composition reflects the degree of partial melting that the mantle experienced while producing the chromium spinel bearing rock burkhard in geochim cosmochim acta 57 1297 1306 1993 however the understanding of chromite alteration and metamorphic modification is still controversial e g evans and frost in geochim cosmochim acta 39 959 972 1975 burkhard in geochim cosmochim acta 57 1297 1306 1993 oze et al in am j sci 304 67 101 2004 metamorphic alteration leads to major changes in chromite chemistry and to the growth of secondary phases such as ferritchromite and chlorite in this study we investigate the vourinos complex chromitites from the mines of rizo aetoraches xerolivado and potamia with respect to textural and chemical analyses in order to highlight the most important trend of alteration related to chromite transformation the present study has been partially funded by the aliakmon project in collaboration between the public power corporation of greece and institute of geology and mineral exploration of kozani

chromitites platiniferes des complexes ophiolitiques de l estrie beauce appalaches du sud du quebec des chromitites contenant du platine du palladium et du rhodium viennent d etre mises a jour dans les appalaches du sud du quebec les complexes ophiolitiques hotes de ces mineralisations sont d ge ordovicien inferieur et d affinite boninitique de plus ils sont etroitement lies a un arc insulaire les chromitites a pt pd rh se situent dans les dunites de la sequence a cumulat a leur passage aux pyroxenites le platine le palladium et le rhodium sont exprimes soit sous forme d alliages et de sulfures complexes automorphes ou soit sous forme d alliages a pt pd et d arseniures rhodiferes xenomorphes cette derniere paragenese accompagne la serpentinisation des cumulats ultramafiques

a classification of rhizogenic root formed calcretes with examples from the upper jurassic lower cretaceous of spain and upper cretaceous of southern france reply abstract studies of fluvial sequences from the upper jurassic and lower cretaceous of the cameros basin spain and the upper cretaceous of the aquitaine basin france have revealed large numbers of calcrete profiles which have formed wholly or largely through the action of plant root calcification those from spain have formed by the calcification of root mats whereas those from france resulted from the accumulation of calcified root cells microcodium these root related rhizogenic calcretes developed in stages allowing developmental time series chronosequences to be identified contrasting with existing models for calcrete profile growth they provide unique examples where vegetation has been the dominant control on profile development and probably indicate specific vegetation types and palaeoecological settings the role of root activity in forming some quaternary calcretes is also discussed

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

generation and separation of mesoscale eddies from surface ocean fronts abstractthis study is motivated by arctic ocean observations of sub mixed layer eddies found at large distances from their assumed formation region of a surface ocean front eddy formation is explored through high resolution numerical simulations of surface fronts separating two mixed layers with a range of configurations similar to those observed in the arctic ocean this study finds that frontal instabilities lead to the development of self propagating dipoles which have the potential to propagate far from the front if interactions with other eddies are avoided however most dipoles are unbalanced consisting of a dominating surface cyclone and a weaker anticyclone below and thus propagate on curved trajectories with eventual recirculation back to the front their maximum separation distance from the front depends on the ratio of self advecting velocities balanced dipoles that have 1 and the ability to propagate far from the front for dipoles generated numerically this study estimates u

palladium platinum rhodium ruthenium and iridium in chromitites from the massif du sud and tiebaghi massif new caledonia massive and disseminated podiform chromitite from 43 mines and other occurrences in the massif du sud and tiebaghi massif both ophiolites in new caledonia contain up to 9 ppb pd up to 45 ppb pt up to 31 ppb rh up to 410 ppb ir and up to 1 130 ppb ru the chromitites are strongly depleted in pt and pd and mildly depleted in ir and ru with respect to an average chondrite composition as are podiform chromitites from oman turkey greece and northern california stratiform chromitites from the stillwater complex montana and the bushveld complex south africa show inverse patterns of enrichment and depletion these patterns suggest that either chromitite from ophiolites represents mantle rocks depleted in pt and pd or that the magmas that formed the ophiolites were depleted in pt and pd or that the process of forming the chromitite involved the concentration of os and ir from a magma the relatively high contents of ir and ru in the new caledonian chromitite suggest the potential for the occurrence of placers that contain concentrations of os ir alloys

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

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

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

sulfur saturation and second stage melts application to the bushveld platinum metal deposits a model is presented for the formation of platinum group element ores by precipitation from sulfur deficient platinum group element rich second stage magmas numerous investigations indicate that basaltic magmas originating from undepleted or mildly depleted source regions are sulfur saturated at the time of eruption mantle derived peridotites residual from midocean ridge basalt magma genesis have pd abundances that are uniformly too high for it to have originated solely from silicate melt retained in the source region at the time of segregation they imply the presence of an accessory immiscible sulfide melt to host the platinum group elements the very high sulfide melt silicate melt partition coefficients of the platinum group elements would result in strong fractionation of these metals into the accessory sulfide component during the melting event the extreme platinum group element enrichment of this accessory sulfide component is evidenced by the discovery ofplatinoid minerals in residual mantle nodules containing pd and pt abundances at the low ppb level keays et al 1981 magmas generated by low to moderate degrees of partial melting of undepleted mantle are therefore sulfur saturated at the time of segregation these first stage magmas become impoverished in platinum group elements during the early stages of fractional crystallization because of coprecipitation of an immiscible sulfide phase e g midocean ridge basalt glasses average

on the diversity of the pge content in chromitites hosted in ophiolites and in porphyry cu systems controlling factors abstract the present study on the diversity of the platinum group elements pge content in chromitites and porphyry cu deposits from greece and everywhere literature data aims to define factors controlling precious metal enrichment in ophiolite complexes and the unknown pd and pt potential in porphyry cu systems through an integrated set of mineralogical and geochemical approaches the pge content in chromitites associated with ophiolite complexes is commonly low a few 100 s of ppb however a significant pge enrichment up to 10 s of ppm is a feature of relatively small chromite occurrences regardless their major element composition plots of the pd ir versus normalized pt pt ratios defined separate fields corresponding to various chromitite types large chromite deposits exhibit low pd ir values reflecting low degree of fractionation primitive magmas negative pt pt partial melting of already depleted mantle and increasing partial melting trend from high al towards to high cr deposits the association with hydrous silicates cr bearing hydrogrossular chlorite and serpentine and texture features of os ir ru rich compatible or ipge minerals coupled with very low pd ir and negative pt pt values were attributed to an origin by partial melting of already depleted mantle and increased partial melting degree during a subsequent crustal mantle recycling stage via fluid driven processes assuming that the increasing pd ir reflects a fractionation trend of parent magma a significant enrichment in pt and pd incompatible or ppge occurring in small occurrences either high cr or high al type may be a sensitive fingerprint for the orientation in the mantle sequence of the ophiolites and exploration for chromite commonly the pge content in porphyry cu mo au deposits is less than 10 ppb however elevated pd pt contents over 6 ppm in sulfide rich samples are a characteristic feature of certain porphyry cu au alkaline intrusions evidence of a favorable tectonic setting post collisional for the origin of porphyry cu au pd pt elevated porphyry deposits include elevated up to hundreds of ppm contents in cr co ni and re and elevated 187 os 188 os ratios reflecting mixing and contribution to the magma composition the oxidized nature of parent magmas abundant magnetite and the evolved mineralized fluid system in porphyry deposits are considered to be critical requirements for the transportation and deposition of sufficient au and pge elevated pd pt and au contents in porphyry deposits combined with the association of the major pd mineral merenskyite with bornite and chalcopyrite major cu minerals are encouraging economic factors for the contribution to the global pd pt production

reflux dolomitization a holocene example beneath a coastal salina west caicos island turks and caicos islands abstract holocene reflux dolomitization occurred beneath the company point evaporitic coastal salina on west caicos island less than 1 km in from the edge of caicos platform in the southeastern bahamas poorly ordered calcian rich protodolomites replaced coralgal grainstones shed into the salina as storm washover deposits and also cemented them finer grained carbonate sands were replaced by fine crystalline dolomites but coarser grained skeletal fragments including halimeda sp dissolved out to create late holocene precompactional secondary moldic porosity precipitation of overlying evaporitic deposits principally gypsum generated magnesium rich brines below these evaporites the downward increase in the degree of replacement dolomitization toward the impermeable calcrete that caps pleistocene grainstones at the base of the holocene sequence strongly implies reflux dolomitization company point dolomitization less than 1 km in from the western edge of caicos platform demonstrated that this process is not limited to platform or ramp interior settings well removed from the open ocean a semi arid climate combined with depositional ridge topography and periodic storm surge and groundwater recharge allowed this coastal salina to initiate and evolve as these beach ridge and evaporitic deposits continue to prograde as they are doing on the eastern side of west caicos island then one can expect associated evaporitic deposits to have the potential not only to serve as an updip sealing facies but to generate mg rich fluids to create precompactional porous dolostones with sheet like reservoir quality

calcareous crusts on exposed pleistocene limestones a case study from grand cayman british west indies abstract calcareous crusts which include laminar up to 6 cm thick and non laminar calcretes have developed on ooid grainstones and skeletal wacke pack grain floatstone in the pleistocene ironshore formation exposed at morgan s harbour tarpon springs and rogers wreck point on grand cayman british west indies non laminar calcretes which consist largely of micrite with variably altered skeletal fragments and lithoclasts derived from the host limestone contain peloids in their basal parts in contrast the laminar calcretes are formed of porous laminae and low porosity laminae calcified roots are common in the porous laminae but rare in the low porosity laminae the 18o values of calcretes from 5 22 to 0 16 have a positive correlation with the 13c values that vary from 10 03 to 6 41 at tarpon springs from 11 57 to 5 43 at morgan s harbour and from 9 89 to 8 68 at rogers wreck point the non laminar calcretes reflect progressive micritization of the bedrock with a basal peloidal zone that passes upward into the micrite zone in contrast laminar calcretes are attributed to accretionary build up dictated by microscale processes that were controlled largely by the interplay of plant roots and their associated microorganisms the positive correlation between the 18o and 13c values from the calcareous crusts indicates that evaporation influenced calcite precipitation the 13c values of the laminar calcretes record a change from vegetation dominated by c3 plants that grew in a cool wet climate to a c3 c4 vegetation that grew in a hotter drier climate the cayman calcretes resemble those found on the pleistocene miami limestone in florida and highlight the possibility that these calcareous crusts may reflect regional climates and may therefore enable regional stratigraphic correlation

possible sources driving the potential vorticity structure and long wave instability of coastal upwelling and downwelling currents it is well known that upwelling and downwelling currents are unstable to perturbations less is however known about the physical mechanism responsible for the observed and modeled instabilities it is shown that the origin of the long wave barotropic baroclinic instability observed on upwelling currents has to be sought among diabatic or thermobaric mechanisms in particular the role of mixing associated with kelvin helmholtz instability and of wind forcing is investigated low richardson numbers occur when the pycnocline outcrops at the sea surface the criterion for instability ri 1 4 can be reached in a narrow region close to the upwelling front permitting kelvin helmholtz instability and mixing this can precondition the current for long wave instability by transforming the current s potential vorticity a constant wind can likewise modify the potential vorticity the resulting potential vorticity anomaly is always negative for both upwelling and downwelling currents and this anomaly interacts with the outcropped front destabilizing it examples are provided via numerical calculations using an idealized front a wind stress is an effective means of inducing the negative pv necessary for instability with wind kelvin helmholz instability when present merely modifies the instability characteristics in addition upwelling fronts are always less stable than comparable downwelling fronts

initial subduction of neo tethyan ocean geochemical records in chromite and mineral inclusions in the pozanti karsanti ophiolite southern turkey abstract chromitites in the pozanti karsanti ophiolite in turkey mainly occur as podiform chromitites within mantle harzburgite and stratiform like chromitites in mantle crust transition zone chromites in chromitites have varing cr from 62 8 to 80 3 and can be divided into two types namely intermediate cr 62 8 69 2 and high cr cr 73 9 80 3 types major elements of the high cr chromitite have an affinity with boninite whereas the intermediate chromitite shows transitional features between morb and boninite the compositional differences in clinopyroxene inclusions between intermediate and high cr chromitite coupled with the relatively high trace element contents e g v ga in the high cr chromitite indicate distinctive parental magmas trace elemental profile analysis of a nodular chromite grain in one nodular chromitite sample pk14 41 demonstrates significant but non systematic variations from the core to the rim which also confirmed the compositional heterogeneity of the parental magmas the presence of primary hydrous mineral inclusions such as amphibole in chromite together with ca rich minerals e g calcite reflect the water rich and ca rich characteristics of the parental magma the higher fo2 of high cr chromitite evidenced by the lower v mn values may be due to more oxidized fluids released from downgoing crustal materials thus we conclude that the parental magmas of the pozanti karsanti chromitite were derived from a proto forearc mantle and evolved to higher fo2 with the subduction initiation at that time but were water and ca rich in general

oceanographic conditions in the marginal ice zone north of svalbard in early fall 1979 with an emphasis on mesoscale processes during september october 1979 the norwegian remote sensing experiment was carried out in the marginal ice zone north of svalbard convergence of the ice cover is correlated with along ice edge winds with the ice to the right while divergence occurs during office winds or calm conditions a wind driven ice edge jet is observed wind driven upwelling of the pycnocline of up to 7 m was present along the ice edge during a 10 to 15 m s easterly wind event the upwelling is due to ekman divergence at the ice edge caused by higher wind stress over ice than over open water the ice edge meanders with a scale of 20 40 km and sheds eddies with a scale of 5 to 15 km into the open water this scale is of the same order as the rossby radius of deformation eddies with the same scale are also seen in the conductivity temperature and depth observations conditions during the experiment were such that barotropic instabilities could have generated these eddies

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

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

morphology and significance of super mature calcretes from the upper old red sandstone of scotland thick calcrete profiles occur in the late devonian early carboniferous kinnesswood formation of the midland valley of scotland these profiles consist of highly brecciated peloidal and coated grain fabrics with laminar calcrete horizons and linings to fractures the profiles resemble supermature calcretes associated with late cenozoic surfaces in the southwestern united states and reflect pedogenesis over periods of several hundred thousand years or more the possible origins of the highly complex fabrics seen are reviewed and the polygenetic nature of such profiles is stressed it is suggested that brecciation takes place co incident with subaerial exposure of the calcrete mature profiles may be predisposed to brecciation and surficial weathering because of their susceptibility to soil erosion as a result of changes in vegetation cover the calcretes underwent phreatic cementation burial dolomitization and silicification

regional trends in the platinum group mineralogy of the critical zone of the bushveld complex south africa a wealth of quantitative data on the types volume distribution and mode of occurrence of the platinum group minerals residing in the merensky reef the ug 2 chromitite layer and platreef has been obtained over a twelve year period with the result that general platinum group mineral types for unexplored areas can perhaps now be predicted the platinum group minerals of the ug 2 and overlying merensky layers although locally different in composition mode of occurrence and size show close ties when viewed on a regional scale especially with regard to their pt pd sulfide and pt fe alloy contents thus if in one particular area the merensky reef contains pt pd sulfides as the predominant platinum group minerals then it can be safely inferred that pt pd sulfides will be a significant component of the ug 2 layer in that area the occurrence of pt fe alloy also follows this principle the distribution of pt fe alloy is in fact a major factor in differentiating the ores in the vicinity north and south of the pilansberg alkaline complex from those of the same layers farther along the arcuate strike northeastern and southeastern horns of the western bushveld and has led to an overall concept relating platinum group mineral types to postulated centers of increased volatile activity or perhaps feeder sites of the complex the mineralogy of reef disturbances such as potholes is discussed the mode of occurrence of the platinum group minerals varies between base metal sulfide silicate and chromite associations and the size of platinum group minerals in the various reefs is given solid solution or lattice substitutions of platinum group elements in the base metal sulfides account for the very important latent fraction of the total platinum group elements as distinct from platinum group minerals of the bushveld complex rhodium in the merensky reef occurs almost wholly in solid solution within pentlandite whereas rh minerals occur in the ug 2 layer chromite tends to host microscopic to submicroscopic inclusions of laurite and does not appear to take ru into the crystal lattice the probability of solid solution of platinum group elements in silicate and other oxide phases is discussed

topographic and vegetative controls on calcrete formation turks and caicos islands british west indies abstract vegetation and topography of the limestone islands on caicos platform british west indies are decisive factors in controlling the morphologic petrographic and geochemical characteristics of calcretes forming on pleistocene exposure surfaces valley calcretes form in low areas as a result of vegetative and soil forming processes which penetrate downward and reconstitute host limestone valley calcretes vary in degree of maturity which is expressed petrographically and geochemically enrichment in fe and al in valley calcretes is caused by gradual concentration of aerosol derived material in topographically low and therefore stable settings ridge caleretes form on ridge crests and slopes and are associated with downward penetrating roots but they undergo a cycle of formation and destruction ridge calcrete cycle which prevents them from achieving the progressive maturity seen in valley calcrete profiles the vegetative role in the calcrete cycle is twofold first root mats spreading out laterally within the host limestone enhance dissolution of limestone and subsequent reprecipitation of carbonate cements these processes result in well indurated calcrete layers second once formed these indurated layers are uprooted by downward and laterally displacive rooting and subjected to physical and chemical erosion recognition of the two different modes of calcrete formation in the rock record can yield valuable information about paleotopography and erosional history

cobalt arsenide orebodies related to an upper proterozoic ophiolite bou azzer morocco the bou azzer cobalt deposits which have produced to date 50 000 tons of cobalt metal are apparently unique they are associated with serpentinites of an upper proterozoic ophiolite and they consist of cobalt ni fe arsenides with accessory sulfoarsenides copper sulfides molybdenite and gold in a quartz carbonate gangue the ore has undergone several phases of brecciation and recrystallization related to late pan african and hercynian deformations which produced the varying shapes of the orebodies lodes veins stocks complex shells and flat lenses the cobalt ore is mainly controlled by tectonic structures along the borders and top of the precambrian serpentinite massifs the orebodies which are buried under the infracambrian volcanic cover may be localized by geochemical zoning the serpentinites are the most obvious source rock for cobalt the quartz carbonate gangue grades laterally into a weathering crust of late proterozoic age however the origin of the arsenic is not yet resolved it may be consanguineous with the serpentinites or derived from an unknown hydrothermal source

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

erosion of a surface vortex by a seamount abstract numerical experiments are carried out on the f plane using a shallow water isopycnal model to analyze the behavior of a surface intensified anticyclonic vortex when it encounters an isolated seamount the advection by the vortex of deep fluid parcels across the isobaths is known to generate deep anticyclonic and cyclonic circulations above and near the bathymetry respectively these circulations are shown to exert a strong shear on the upper layers which causes an erosion of the initial vortex by filamentation the erosion often results in a subdivision of the eddy while the eroded original structure forms a dipole with the deep cyclone and is advected away the filaments torn off from the original core aggregate into a new eddy above the seamount splitting in more than two structures is sometimes observed the erosion process is quantified by the bulk volume integral of the eddy potential vorticity anomaly a sensitivity study to different parameters of the configuration distance between

geochemistry of chromitites and host rocks from the pindos ophiolite complex northwestern greece abstract representative chromite samples of schlieren and massive ore types from various localities of the pindos ophiolite complex were analyzed for platinum group elements pge s ni co and cu and mineral chemistry host dunites and harzburgite were also analyzed for the chalcophile elements the chromite ores show a wide compositional variation the cr cr al ratio ranges from 0 47 to 0 83 while mg mg fe 2 ranges between 0 42 and 0 73 but two populations can be distinguished the high cr and high al types which show a more restricted variation ranges of pge contents in ppb are os 8 150 ir 4 320 ru 15 550 rh 2 82 pt 3 150 and pd 1 5 20 gold ranges between 1 and 13 ppb the pge and less chalcophile elements ni co and cu the incompatible compatible element ratios the pge s ratio and pge patterns provide valuable information for the discrimination of chromite ores with a similar major element composition a fractionation trend in both chromitites and associated dunites can be used for stratigraphic orientation in the mantle sequences of ophiolite complexes and evaluation of their chromite potential

geochemistry of meteoric diagenesis in carbonate islands of the northern bahamas 1 evidence from field studies processes driving carbonate diagenesis in islands of the northern bahamas are investigated using major ion dissolved oxygen and dissolved organic carbon analyses of water samples from surface and ground waters and measurements of soil gas pco2 meteoric waters equilibrate with aragonite but reactions are water controlled rather than mineral controlled and drive dissolution rather than concurrent precipitation of calcite surface runoff waters equilibrate with atmospheric pco2 and rapidly recharge the vadose zone limiting subaerial bedrock dissolution to only 6 6 15 mg l 1 ca pco2 of soil gas measured in the summer wet season 7 4 s 3 7 10 3 atm is elevated compared with that of the atmosphere despite the thin skeletal organic nature of the soil and the discontinuous soil cover soil waters retained in surface pockets are equilibrated with respect to aragonite and have dissolved 51 s 19 mg l 1 ca this is substantially less than the 93 s 18 mg l 1 ca in samples from pumping boreholes that sample meteoric waters from the freshwater lens the high pco2 of the freshwater lens 16 s 8 3 10 3 atm for pumping boreholes suggests that significant additional co2 may be derived by oxidation of soil and surface derived organic carbon within the lens the suboxic nature of the majority of the freshwater lens and the observed depletion in sulphate support this suggestion and indicate that both aerobic and anaerobic oxidation may take place shallow lens samples from observation boreholes are calcite supersaturated and have a lower pco2 than deeper lens waters indicating that co2 degasses from the water table driving precipitation of calcite cements we suggest that the geochemical evolution of waters in the vadose zone and upper part of the freshwater lens may be determined by the presence of a body of ground air with pco2 controlled by production in the freshwater lens and soil and by degassing to the atmosphere copyright 2007 john wiley sons ltd

chromite in komatiites ii modification during greenschist to mid amphibolite facies metamorphism chromite compositions in komatiites are influenced by metamorphic chromite is highly susceptible to modification during processes particularly above 500 c metamorphosed chromite is early hydrous alteration and subsequent prograde metasubstantially more iron rich than igneous precursors as a result of morphism of host rocks this modification is the subject mg fe exchange with silicates and carbonates chromite metaof this paper morphosed to amphibolite facies is enriched in zn and fe and metamorphic modification of chromite has been disdepleted in ni relative to lower metamorphic grades relative cussed extensively in the literature in the context mainly of proportions of the trivalent ions cr al and fe are not ophiolitic or alpine ultramafic complexes onyeagocha greatly modified by metamorphism up to lower amphibolite facies 1974 ulmer 1974 evans frost 1975 hoffman although minor fe depletion occurs during talc carbonate alwalker 1978 loferski lipin 1983 kimball 1990 teration at low temperature significant al is lost from chromite burkhard 1993 and in a few studies of komatiitic rocks cores above 550 c as a result of equilibration with fluids in bliss maclean 1975 donaldson 1983 gole hill equilibrium with chlorite elevated zn content in chromite is 1990 and in a detailed study of the pechenga intrusions restricted to rocks with low metamorphic mg fe ratios and is abzalov 1998 these studies have highlighted two the result of introduction of zn during low temperature alteration important effects first chromites become rimmed and with further concentration and homogenization during prograde progressively replaced by chromian magnetite or fermetamorphism cobalt and mn also behave similarly except where ritchromit second chromite core compositions become carbonate minerals are predominant in the metamorphic assemblage progressively modified during prograde metamorphism chromite at amphibolite facies is typically extensively replaced by as a result of exchange of components with surrounding magnetite this is the result of incomplete metamorphic reaction silicate minerals evans frost 1975 abzalov 1998 between chromite and chlorite bearing silicate assemblages magthis paper examines the nature and magnitude of these netite compositions at the inner chromite magnetite boundary are effects in komatiites from a variety of localities in the indicators of metamorphic grade eastern goldfields province of the archaean yilgarn block in western australia fig 1 brief locality descriptions are given in appendix b

microcodium revisited root calcification products of terrestrial plants on carbonate rich substrates abstract microcodium is a problematic calcitic microfeature of many calcretes and calcareous paleosols in the cretaceous and tertiary continental and marine successions of the peri tethyan realm the main controversy about the microcodium structures is their origin and possible relation with calcified plant roots microcodium and rhizogenic root formed microfabrics were studied in calcrete profiles within the paleocene shallow marine carbonate succession in southwestern slovenia the prominent laminar calcrete horizons contain abundant calcite aggregates 150 m to 1 cm in size with perfectly preserved structural details of plant root tissues morphology and structure of these aggregates indicate that they formed through biologically controlled precipitation of calcium carbonate within the root cortical cells the morphologies intermediate between the typical microcodium aggregates composed of a single layer of individual elongate pyramidal or prismatic crystals of calcite measuring 100 500 m in length and 20 70 m in width and calcified roots with multilayer arrangement of isodiametric cells were observed and this supports previous rhizogenic interpretations of microcodium structures intermediate forms show that the typical microcodium aggregates formed through morphological transformation of the root tissue by growth of the calcite within the cortical cells which distorted the cell shape calcification of roots and the creation of microcodium structures can be explained as an effective nutrient acquiring mechanism used by certain types of terrestrial plants inhabiting nutrient poor calcareous substrates the widespread occurrence of microcodium in almost unaltered shallow marine limestones indicates that its formation took place during early stages of paleosol development probably reflecting specific types of vascular plants of a pioneer community that were able to colonize carbonate substrates during the early phases of subaerial exposure

rotational deformation in the jurassic mesohellenic ophiolites greece and its tectonic significance abstract the jurassic pindos and vourinos ophiolites in the western hellenides of greece are part of the mesohellenic mafic ultramafic slab underlying the eocene miocene sedimentary basin mesohellenic trough the tectonic incorporation of this oceanic slab into the western edge of the pelagonian subcontinent occurred via trench passive margin collision in the late jurassic much of the tectonic architecture of these ophiolites particularly vourinos was acquired during progressive inhomogeneous deformation associated with the initial displacement of the jurassic oceanic crust from its original igneous environment of formation and its subsequent tectonic emplacement eastward onto the pelagonian margin the heterogeneous deformation in the mantle and crustal sequences of the pindos vourinos ophiolites occurred in ductile ductile brittle and brittle fields synchronously as the jurassic oceanic lithosphere was translated eastward it also resulted in differential horizontal rotations within the displaced oceanic slab areas retaining high temperature diapiric mantle fabric appear to have been passively translated by lower temperature ductile shearing commonly along mylonite zones individual dunite bodies in the harzburgite tectonites indicate movement distances of at least kilometer scale pervasively mylonitic domains within the upper mantle peridotites suggest elongation on the order of five to ten times in the direction of ophiolite emplacement seafloor spreading related high temperature mantle structures within the pindos and vourinos ophiolites are sub parallel imprinted ductile kinematic indicators and ductile shear zones pervasive to the mantle and lower crustal sections in both ophiolites are also sub parallel consistent with the direction of tectonic vergence and appear traceable across the sedimentary overburden of the mesohellenic trough these geometric relations imply that the pindos vourinos ophiolites retain their relative orientations from the time of cessation of ductile deformation within the ophiolitic slab more than 90 anticlockwise horizontal rotation of the vourinos ophiolite seems to have taken place during progressive simple shear deformation associated with the eastward emplacement of the jurassic oceanic lithosphere this kind of rotational deformation postdating the igneous accretion of ancient oceanic crust may be common in many ophiolites and should be carefully distinguished from tectonic extension related vertical rotational deformation developed during their seafloor spreading evolution

an index to distinguish surface and subsurface intensified vortices from surface observations in this study the authors first show that it is difficult to reconstruct the vertical structure of vortices using only surface observations in particular they show that the recent surface quasigeostrophy sqg and interior and surface quasigeostrophy isqg methods systematically lead to surface intensified vortices and those subsurface intensified vortices are thus not correctly modeled the authors then investigate the possibility of distinguishing between surface and subsurface intensified eddies from surface data only using the sea surface height and the sea surface temperature available from satellite observations a simple index based on the ratio of the sea surface temperature anomaly and the sea level anomaly is proposed while the index is expected to give perfect results for isolated vortices the authors show that in a complex environment errors can be expected in particular when strong currents exist in the vicinity of the vortex the validity of the index is then analyzed using results from a realistic regional circulation model of the peru chile upwelling system where both surface and subsurface eddies coexist the authors find that errors are mostly associated with double core eddies aligned surface and subsurface cores and that the index can be useful to determine the nature of mesoscale eddies surface or subsurface intensified from surface satellite observations however the errors reach 24 and some possible improvements of the index calculations are discussed

entry flux and exit of potential vorticity in ocean circulation the flux form of the potential vorticity pv equation is employed to derive simple expressions for the boundary and interior flux of pv in ocean circulation using bernoulli functions the formulas are discussed and physically interpreted and used to map the flux of pv through a model of ocean circulation

two layer model of shear driven coastal upwelling in the presence of bottom topography a stratified two layer fluid is brought to solid body counterclockwise rotation inside a cylindrical tank having a conical bottom with a radial topographic ridge a stress is then applied to the top surface by means of a clockwise differentially rotating disk the resulting ekman layer flux causes the top layer to spin down and the interface to rise near the wall and to descend a t the centre of the tank as this process continues the interface front between the two layers intersects the disk surface and after that migrates away from the wall and allows the bottom fluid to contact the disk directly the migration of the front continues until a steady state is reached the front becomes stationary and the system is in geostrophic balance the first sign of upwelled flow at the surface always occurs as a high speed jet like plume at the bottom topography and only at a later time does a uniform upwelled flow appear a t the surface upstream of the topography this plume which always forms near the downstream edge of the topography migrates in advance of the upstream front to produce an upwelling maximum at the bottom topography the final width of the upstream flow in steady state conditions is estimated by a simple theoretical model which is in good agreement with the experimental results we have an experimental criterion to predict the occurrence of travelling baroclinic waves on the upstream front on the downstream side of the ridge large standing waves are observed with significant upwelling within them under some circumstances cyclones pinch off from the upstream front the plume on the ridge and the crests of the large downstream standing waves we present criteria to predict the occurrence of these pinch off processes

the response of quasigeostrophic oceanic vortices to tropical cyclone forcing abstractthe response of quasigeostrophic qg oceanic vortices to tropical cyclone tc forcing is investigated using an isopycnic ocean model idealized oceanic currents and wind fields derived from observational data acquired during hurricane katrina are used to initialize this model it is found that the upwelling response is a function of the curl of wind driven acceleration of oceanic mixed layer oml currents rather than a function of the wind stress curl upwelling downwelling regimes prevail under the tc s eye as it translates over cyclonic anticyclonic qg vortices oml cooling of 1 c occurs over anticyclones because of the combined effects of downwelling instantaneous turbulent entrainment over the deep warm water column weak stratification and vertical dispersion of near inertial energy by contrast oml cooling of 4 c occurs over cyclones due to the combined effects of upwelling instantaneous turbulent entrainment over regions of tight vertical thermal gradients strong stratificati

tectono stratigraphy and evolution of the mesozoic pindos ophiolite and related units northwestern greece the northwest pindos mountains expose mesozoic and tertiary thrust sheets which include the jurassic pindos ophiolite composed of ultramafic and mafic oceanic crustal rocks the tectono stratigraphy of these units is i mainly ultramafic ophiolitic thrust sheet and basal metamorphic sole dramala complex and loumnitsa unit ii late cretaceous platformal limestones orliakas group iii dismembered intrusive and extrusive ophiolitic rocks aspropotamos complex iv tectonic melange and olistostromes dominated by triassic jurassic volcanic and sedimtntary rocks avdella melange v coherent thrust sheets of late jurassic late cretaceous deep water sediments dio dendra group vi early tertiary flysch pindos flysch immobile trace element studies indicate that triassic and jurassic extrusives of the volcanic sedimentary melange formed mainly at within plate and or mid ocean ridge settings by contrast structurally overlying ophiolitic extrusives include boninitic volcanics and island arc tholeiites of supra subduction zone origin initial ophiolite displacement c 165 ma is recorded in metamorphic sole formation including basal peridotite mylonite amphibolites and greenschists the sole rocks have morb and wpb chemical affinities in our favoured model the pindos ophiolite formed above a mid jurassic westerly dipping intra oceanic subduction zone accompanied by accretion to form melange followed by collision and emplacement with the pelagonian margin final suturing of the pindos ocean however did not take place until early tertiary times

comments on anatomy of a late quaternary carbonate island constraints on timing and magnitude of sea level fluctuations west caicos turks and caicos islands bwi by kerans et al quat sci rev 205 2019 193 223 abstract we comment on interpretations of the evolution of late pleistocene geology on west caicos island caicos platform b w i by kerans et al 2019 which omits our documentation of the interrelationship of carbonate facies in outcrop and in boreholes dravis and wanless 2017 we further disagree with their geochronological dating assumptions results that are the basis for interpreting late quaternary sea level fluctuations

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

on the dynamics of mid ocean ridges most extant flow models for ocean ridges suffer either from unrealistic geometry vertical conduit models or a stress singularity at the ridge axis corner flow models i present a model which avoids these difficulties by incorporating a weak plate boundary zone pbz between diverging surface plates an analytical solution for the flow of a constant viscosity asthenosphere and a coexisting melt phase is obtained in elliptic coordinates and used to predict the dynamically supported topography axial valley on the pbz the force required to pull the plates apart and the rate of melt extraction at the ridge stress in the asthenosphere increases linearly with spreading rate but decreases as the plates become thinner and the dynamics of ridge crests is controlled by the balance between these opposing effects the depth of the axial valley is controlled by the plate thinning effect and is therefore a decreasing function of spreading rate the depth of the axial valley on the mid atlantic ridge can be explained if the viscosity of the asthenosphere is 6 1019 1021 pa s and the lower surface of the plate is the 600 c isotherm the plate separation force is controlled by the spreading rate effect and therefore increases with spreading rate about five times as much force is required to maintain a fast spreading ridge 9 cm yr 1 half rate as a slow spreading ridge 1 cm yr 1 melt extraction can produce 6 km of basaltic crust if the volume fraction of melt present in the asthenosphere total degree of melting ranges from about 2 5 4 0 for a slow spreading ridge to 4 0 6 5 for a fast spreading ridge the dynamics of melt extraction does not depend significantly on the finite width of the pbz

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

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

a coupled ice ocean model of upwelling in the marginal ice zone a dynamical coupled ice ocean numerical model for the marginal ice zone miz is suggested and used to study upwelling dynamics in the miz the nonlinear sea ice model has a variable ice concentration and includes internal ice stress the model is forced by stresses on the air ocean and air ice surfaces the main coupling between the ice and the ocean is in the form of an interfacial stress on the ice ocean interface the ocean model is a linear reduced gravity model the wind stress exerted by the atmosphere on the ocean is proportional to the fraction of open water while the interfacial stress ice ocean is proportional to the concentration of ice a new mechanism for ice edge upwelling is suggested based on a geostrophic equilibrium solution for the sea ice medium the upwelling reported in previous models invoking a stationary ice cover is shown to be replaced by a weak downwelling due to the ice motion most of the upwelling dynamics can be understood by analysis of the divergence of the across ice edge upper ocean transport on the basis of numerical model an analytical model is suggested that reproduces most of the upwelling dynamics of the more complex numerical model

heterogeneous mantle complex crustal processes and obduction kinematics in a unified pindos vourinos ophiolitic slab northern greece

paleosols in shallow marine carbonate sequences abstract shallow water carbonate sequences in the geological record commonly exhibit numerous exposure surfaces reflecting emersion of sediment due to progradation processes and sealevel oscillations some ancient platform sequences contain hundreds of such surfaces associated with paleosols fossil soils typically calcretes a wide range of macro and microstructures are associated with these calcretes which are reviewed profile types and stages in profile development can be defined the nature of the exposure surface and associated paleosols can be used to identify the nature of the prevailing climate although hydromorphic effects related to marine flooding of the platforms can modify the paleosols a potentially important use of paleosols is to define the nature of sequence boundaries several studies have identified sets of paleosols with varying maturities reflecting changes in rates of accommodation space creation during sealevel rises and falls

evidence for the emplacement of ca 3 0 ga mantle derived mafic ultramafic bodies in the ukrainian shield abstract the archean bedrock of the ukrainian shield is divided into western and central blocks by a n w trending tectonic zone known as the golovanevsk suture zone within this zone there are numerous mafic ultramafic bodies including the chromitite bearing lipovenki and kapitanov massifs the chromitites of these massifs possess mineralogical and textural features and platinum group element systematics similar to those typically observed in ophiolitic podiform chromitites although there is no field evidence for an ophiolitic structure the geochemical evidence strongly suggests an origin in the oceanic lithosphere the re os ratios of the chromites and whole rock chromitites are extremely low and consequently their 187 os 188 os ratios have increased little since crystallization osmium isotope model ages indicate formation at approximately 3 0 ga suggesting that the massifs represent portions of the archean oceanic lithosphere

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

mesoscale variability in denmark strait the pv outflow hypothesis abstract the outflow through denmark strait shows remarkable mesoscale variability characterized by the continuous formation of intense mesoscale cyclones just south of the sill these cyclones have a diameter of about 30 km and clear signatures at the sea surface and in currents measured near the bottom they have a remnant of arctic intermediate water aiw in their core the authors hypothesis is that these cyclones are formed by stretching of the high potential vorticity pv water column that outflows through denmark strait the light upper layer of the outflow the east greenland current remains on the surface in the irminger sea while the dense overflow water descends the east greenland continental slope the midlevel waters mostly aiw could thus be stretched by more than 100 which would induce very strong cyclonic relative vorticity the main test of this new hypothesis is by way of numerical experiments carried out with an isopycnal coordinate ocean model configured to have a marginal sea

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

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

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

structural classification of chromite pods in southern new caledonia a structural classification of podiform chromite orebodies from southern new caledonia results in a division of deposits into three major types discordant subconcordant and concordant with penetrative structures foliation and lineation in the enclosing peridotite discordant deposits are very irregular in shape and clearly crosscut banding and foliation subconcordant deposits are generally tabular in shape and lie within 10 degrees to 25 degrees in strike and or dip to the foliation concordant deposits are also tabular and lie parallel to foliation in peridotite pyroxene lineation in host peridotite always indicates the elongation direction of these deposits within subconcordant and discordant deposits chromite lineations are often oblique to those in the surrounding rocks they follow local variations in the orebody shape and indicate possible deposit extensions at depth the classification of chromite bodies corresponds to three stages of increasing deformation as evidenced by chromite ore textures discordant deposits which are the least deformed are characterized by primary textures such as the nodular one the foam texture the chromite net and the occluded silicate texture within subconcordant and mainly concordant deposits massive disseminated and antinodular ores show evidence of strong deformation the lack of geochemical distinctions between the different deposit types supports such an hypothesis these chromite deposits in ophiolitic harzburgites are thought to have been formed beneath an oceanic spreading ridge if the discordant pods are considered as representative of the original situation it is proposed that the chromite has crystallized and has been dynamically concentrated along steep conduits traversing the enclosing harzburgite and feeding a magma chamber next the chromite enriched pipes are caught up by plastic deformation in the mantle and tectonically reoriented toward the foliation the chromite deposits in the massif du sud are located within a domain about 1 5 km thick in the harzburgites and dunite zones this domain is limited upward by the first cumulates and gabbros and downward by the transition to a different plastic flow regime in the harzburgites finally some guides to chromite prospecting and exploration are cited applicable at different geologic scales and based on lithologic and structural criteria

noble metals in thetford mines ophiolites quebec canada part i distribution of gold iridium platinum and palladium in the ultramafic and gabbroic rocks the gold iridium palladium and platinum contents of ophiolites from the thetford mines area quebec were determined by radiochemical neutron activation analysis the main lithologies studied are the tectonite peridotites harzburgites of the lower structural unit and the ultramafic mafic plutonic members of the overlying cumulates average noble metal contents of the main rock types are au ppb ir ppb pt ppb pd ppb tectonite peridotitesharzburgite 1 5 3 2 10 0 3 8dunite lenses in harzburgite 0 27 3 6 2 1 0 29cumulate rocksolivine chromitite 0 28 30 0 1 5 0 33dunite 1 3 2 4 28 0 21 0pyroxenite 1 4 0 24 17 0 29 0gabbro 1 7 0 017 4 5 3 0the harzburgites have chondritelike noble metal abundance patterns and relatively uniform noble metal contents both features are compatible with an origin for the harzburgite as a residue from extensive partial melting of mantle peridotite the cumulates are highly variable in noble metal content iridium is enriched in early olivine chromitite and progressively depleted in later rocks palladium increases from early to late ultramafic cumulates and decreases in gabbros whereas pt shows no systematic trend in the cumulates gold increases very slightly from early to late cumulates the noble metal trends in cumulate rocks result mainly from fractional crystallization but other processes including sulfur saturation of magma and adcumulus growth may enrich rocks in noble metals sulfur rich late dunites and early pyroxenites probably have the best mineralization potential of the cumulate rocks there is little preferential partition of noble metals into any ferromagnesian silicate but the platinum group elements particularly ir are strongly associated with chromite acid leaching experiments suggest that in harzburgites ir is concentrated along chromite silicate grain boundaries whereas in cumulus chromitites ir occurs partly within chromite grains it is suggested that ir was originally incorporated into the chromite structure during a high temperature magmatic stage and in the case of the harzburgites has diffused to grain boundaries in response to cooling and to recrystallization induced in part by tectonic stress serpentinization probably occurred in both oceanic and continental environments cumulates suffered intense serpentinization but little noble metal mobility resulted harzburgites were serpentinized in both oceanic and continental environments and the latter process probably caused some mobilization of au in conjunction with formation of asbestos veins

experimental study of the solubility of platinum and iridium in basic silicate melts implications for the differentiation of platinum group elements during magmatic processes this study presents evidence for the dissolution of two platinum group elements pge s pt and ir in silicate melts of basic composition in the absence of any associated fluid phase the dissolution of these pge s is attributed to the formation of a complex ion in which the noble metal is uncharged an increase in oxygen fugacity in equilibrium with the melt brings about a considerable decrease in ir solubility in this way we can explain the early enrichment in ir group pge s ipge s ir ru os observed during the crystallization of ultrabasic cumulates by contrast however pt remains soluble in the melt an increase in the fs2 results in a pronounced increase in the solubility of pt in the silicate melt the sulfur saturation which leads to the precipitation of the base metal sulfides bms s also brings about the fixation of pt group pge s ppge s pt pd rh which are dissolved in bms s this accounts for the presence of ppge s in the nickel sulfide horizons of stratiform deposits the occurrence of platiniferous fe crystallizing with chromite in alaskan type complexes at oxygen fugacities of 10 6 bar can be explained by the low activity of fe in fe pt alloys such alloys have been formed in our laboratory at such fo2

mid ocean ridge and supra subduction affinities in the pindos ophiolites greece implications for magma genesis in a forearc setting abstract the pindos ophiolitic massif is considered an important key area within the albanide hellenide ophiolitic belt and is represented by two tectonically distinct ophiolitic units 1 a lower unit including an intrusive and a volcanic section and 2 an upper ophiolitic unit mainly including mantle harzburgites both units share similar metamorphic soles and tectono sedimentary melanges at their bases the intrusive section of the lower unit is composed by an alternation of troctolites with various ultramafic rock types including dunites lherzolites olivine websterites olivine gabbros anorthositic gabbros gabbros and rare gabbronorites the volcanic and subvolcanic sequence of the lower unit can geochemically be subdivided into three groups of rocks 1 basalts and basaltic andesites of the lower pillow section showing a clear high ti affinity 2 basaltic andesites of the upper pillow section with high ti affinity but showing many geochemical differences with respect to the first group 3 very low ti boninitic basaltic and basaltic andesitic lava flows separating the lower and upper pillow sections and dykes widespread throughout the pindos ophiolites these different magmatic groups originated from fractional crystallization from different primary magmas which were generated in turn from partial melting of mantle sources progressively depleted by previous melt extractions group 1 volcanics may have derived from partial melting ca 20 of an undepleted lherzolitic source while group 2 basaltic rocks may have derived from partial melting ca 10 of a mantle that had previously experienced mid ocean ridge basalt morb extraction finally the group 3 boninites may have derived from partial melting ca 12 17 of a mantle peridotite previously depleted by primary melt extraction of groups 1 and 2 primary melts in order to explain the coexistence of these geochemically different magma groups two petrogenetic models formerly proposed for the albanian ophiolites are discussed

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

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