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

formation of iron carbon alloys in basaltic magma at uivfaq disko island the role of carbon in mafic magmas two types of metallic iron were identified in a collection of samples from the tertiary iron bearing basalts of uivfaq disko island greenland high carbon iron has 2 9 4 0 wt carbon and eutectic textures of iron and cohenite fe 3 c similar to those of commercial white cast iron it occurs in approximately spherical bodies up to 15 mm in diameter that appear to have been immiscible metallic liquids within basaltic magma low carbon samples have 1 8 2 0 wt carbon and textures similar to those of commercial hypereutectoid steel they occur principally as large up to 22 tons cumulate masses both types of metal could have crystallized at low pressure from iron carbon liquids of their present bulk carbon contents fe ni co cu and p oxides in basaltic magma were reduced by carbon derived principally from assimilated cretaceous tertiary sediments niand ni cu ratios of the metal are similar to those of terrestrial basalts the magma contained excess carbon graphite so the metal formed as

spatial and temporal distribution of gold deposits in the urals during the past 10 years the paleozoic urals mountain belt has been the subject of internationally based deep seismic and ancillary geologic studies aimed at improving our knowledge of the lithospheric evolution of the orogenic belt in general and its prominent crustal root in particular in view of its wide ranging mineral endowment especially for gold this work also provides important keys to help clarify relationships between collisional tectonic processes and gold mineralization following an outline of current ideas on the tectonic evolution of the urals we present an overview of the distribution of the majority of known gold deposits this information together with the available isotopic and geologic age data of associated alteration assemblages is compared to available geochronological data for various magmatic and deformation episodes spatial and temporal relationships suggest that hydrothermal mineralizing processes and the geodynamic evolution of the orogen are linked in an attempt to analyze the deep structural framework of the orogen down to the mantle we have also examined a number of subcontinental and regional scale continuous geophysical datasets the urals have been subdivided traditionally into a series of north south striking tectonic zones but more recently into three principal tectono magmatic sectors which comprise a suture sector along the main uralian fault zone and two sectors with tectonically imbricated island arc transitional viewed as active continental margin and continental zones gold deposits are found in all tectono magmatic sectors of the orogen minor gold occurs in siluro devonian volcanic hosted massive sulfide deposits magnetite copper skarns and porphyry copper deposits in a number of cases younger structures shear zones faults and their intersections have controlled the deposition of higher gold grades in the urals this has led to the concept of progressive concentration of gold during later tectonic overprinting and the understanding that regional deformation and hydrothermal fluid rock interaction in the late carboniferous to early triassic upgraded gold contents of earlier deposits during the late carboniferous to early triassic gold bearing quartz vein lodes which effectively encompass the majority of the larger gold deposits in the urals were also formed in structural chemical traps some deposits such as mindyak that is hosted by tectonic melanges along the main uralian fault zone are clearly typical late orogenic lode gold deposits others occur within and along the margins of early to middle carboniferous older granite and although local orthomagmatic relationships have been described most recent observations now favor a strong structural control on the lodes and no direct genetic association with granite intrusion in the two largest gold deposits kochkar and berezovskoe gold mineralization was controlled by structural and combined structural chemical traps in dilational jogs in both shear and contact zones locally gold mineralization has been shown to occur during a change from orthogonal to transpressional compression the inferred ages of the lode gold deposits suggest they are mostly coeval with the generally undeformed permian early triassic younger granites however a spatial relationship with gold mineralization has not been observed the younger granites were formed in an extensional regime following ural wide magma generation caused by a thermal flux attributed to underplating of the crust by mantle derived mafic magmas the widespread distribution of this late thermal event during changes in stress regime and the involvement of magma generating mantle processes suggest regional delamination in the lithosphere and concomitant upwelling of the asthenosphere these processes may have largely occurred prior to the formation of the prominent crustal root of the urals which formed during subsequent transpressional convergence another triassic ural wide phase of hydrothermal alteration has been recognized but associated gold deposits have been documented in only two cases we tentatively attribute this phase to destabilization of the lithosphere of the wider altaids reflected by the formation of the west siberian and central asian basins east of the urals during the mesozoic the integrated analysis of magnetic bouguer gravity heat flow and crustal thickness data suggests that a major indenter of the east european craton is responsible for a singular transcrustal sidewall ramp in the subsurface of the middle urals this ramp may extend into the mantle and formed the principal conduit zone for mineralizing fluids the world class kochkar and berezovskoe gold deposits are adjacent to this proposed conduit

ferro andesites in the grande ronde basalt their composition and significance in studies of the origin of the columbia river basalt group flows from the grande ronde basalt of the columbia river basalt group include several with the chemical composition of ferro andesites these flows have sio2 55 mgo 4 and also have higher fe mg than the average value for grande ronde basalt they are also distinctly richer in cs rb k ba la ce th and u than the remainder of the grande ronde basalt flows and possess small negative eu anomalies their pb isotopic compositions define a mixing line with a negative slope on a 206pb 204pb versus 207pb 204pb plot their nd isotopic compositions lie between 143nd 144nd 0 51252 and 0 51264 and their sr isotopic compositions lie between 87sr 86sr 0 7055 and 0 7060 these values define the enriched end of the columbia river basalt group spectrum on a sr nd epsilon diagram excluding the saddle mountains basalt these ferro andesite flows also form a compositional end member of the columbia river basalt group and are sufficiently distinctive to warrant special consideration we compare them with

a case of simple magma mixing in the columbia river basalt group the wilbur creek lapwai and asotin flows saddle mountains formation three small sequential saddle mountains flows occupying similar areas on the columbia plateau were erupted over a short interval of time in the lewiston basin area the middle flow of the trio lapwai has intermediate mineralogy and lies on a straight mixing line between the other two flows for virtually all twenty five elements analyzed systematic changes in the ratios of incompatible elements demonstrate that these relationships are a result of magma mixing rather than either crystal fractionation or variable degrees of partial melting the lapwai flow has a composition approximately midway between the two homogeneous end members and is itself relatively homogeneous this implies efficient mixing between equal amounts of asotin and wilbur creek magmas and suggests that mixing was completed in a magma reservoir prior to eruption the wilbur creek and asotin end members have isotopic features which are believed to result from different degrees of assimilation of crustal material by magma derived from an enriched mantle source carlson 1984 the mixing processes described here cannot be related to that earlier mantle crust mixing process

kibaran a type granitoids and mafic rocks generated by two mantle sources in a late orogenic setting burundi in the mesoproterozoic northeastern kibaran belt of burundi central africa two distinct late kibaran magmatic suites coexist both including a type granitoids they are located along the boundary zone between the kibaran mobile belt western internal domain and the archaean tanzanian craton overlain by mesoproterozoic foreland deposits eastern external domain intense deformation high temperature metamorphism and intrusion of abundant peraluminous anatectic crustal granites occur only in the former domain whereas the mesoproterozoic sedimentary cover of the latter is much less or even nearly undeformed nor metamorphosed the first late kibaran magmatism 350 km long kabanga musongati with an emplacement age of 1275 ma u pb on zircon is mainly composed of mafic and ultramafic layered rocks with subordinate a type acidic differentiates moderately enriched in incompatible elements initial isotopic ratios srlr 0 708 nd 8 indicate an old continental lithospheric mantle origin the emplacement of these late kibaran magmatic rocks was controlled by late lateral shear possibly contemporaneous with the latest intrusions of the kibaran peraluminous synkinematic granites of the western internal domain 1330 1260 ma the second late kibaran magmatism 40 km long gitega makebuko and bukirasazi alignment with an emplacement age of 1249 s ma u pb on zircon is limited in volume it is mainly granitic in composition a type can be strongly enriched in incompatible elements and comprises both syenites and mafic rocks initial isotopic ratios srlr 0 702 na 4 5 to 1 4 point to an oib type asthenospheric lower continental lithospheric mantle origin with only slight contamination by the lower crust during differentiation this group was also intruded during the late lateral shear in both groups liquid lines of descent can be reconstructed although some of the rocks have been strongly albitized this indicates that the granites are produced by differentiation of less evolved magmas and not by crustal anatexis upwelling of the asthenosphere along the tanzanian craton can generate by adiabatic pressure release the oibtype basic melts and provide the heat necessary to melt the continental lithospheric mantle sources this mechanism assigns a major role to a lithosphere scale late kibaran shear event occurring at the end of the regional compressive deformation between two rheologically contrasted domains ascent of the asthenosphere continental lithospheric mantle delamination and late orogenic extensional collapse of the western internal domain are suggested as a possible geodynamic model for the entire northeastern kibaran belt additional work is however necessary to test this model finally our results indicate that in the northeastern kibaran belt the kibaran orogeny ended at 1250 ma despite various reactivation events occurring later e g at 1137 ma

sources of metals in the porgera gold deposit papua new guinea evidence from alteration isotope and noble metal geochemistry the porgera gold deposit is spatially and temporally associated with the late miocene mafic alkalic epizonal porgera intrusive complex pic located in the highlands of papua new guinea png the highlands region marks the site of a tertiary age continent island arc collision zone located on the northeastern edge of the australasian craton the pic was emplaced within continental crust near the lagaip fault zone which represents an oligocene suture between the craton and volcano sedimentary rocks of the sepik terrane magmatism at porgera probably occurred in response to the late miocene elimination of an oceanic microplate and subsequent early pliocene collision between the craton margin and an arc system located on the bismarck sea plate gold mineralization occurred within 1 ma of the time of magmatism metasomatism accompanying early disseminated au mineralization in igneous host rocks resulted in additions of k rb mn s and co2 and depletions of fe mg ca na ba and sr rare earth and high field strength elements remained largely immobile pervasive development of illite k feldspar quartz carbonate alteration assemblages suggests alteration by mildly acidic 200 to 350 c fluids at high water rock ratios strontium and lead isotopic compositions of minerals from early base metal sulphide veins associated with k metasomatism and later quartz roscoelite veins carrying abundant free gold and tellurides are remarkably uniform e g 87sr86sr 0 70745 0 00044 n 10 207pb204pb 15 603 0 004 n 15 these compositions fall between those of unaltered igneous and sedimentary host rocks and specifically sedimentary rocks from the jurassic om formation which underlies the deposit igneous rocks 87sr86sr 0 7035 207pb204pb 15 560 om formation 87sr86sr t 0 7153 207pb204pb 15 636 it is therefore suggested that the hydrothermal fluids acquired their sr and pb isotopic signatures by interaction with or direct derivation from a plutonic root of the pic and host sedimentary rocks of the om formation it is likely that au was also derived from one or both of these two sources concentrations of au in unaltered igneous and sedimentary rocks from porgera 10 ppb au do not indicate that either lithology represents a significantly enriched protore although au and platinum group element pge abundances in the igneous rocks suggest a mild primary magmatic enrichment of au relative to the pge average au pt pd mantlenormalized 14 0 6 5 n 8 evidence that the porgera magmas were rich in volatiles permits speculation that au may have been concentrated in a magmatic fluid phase but alternative possibilities such as derivation of au by hydrothermal leaching of solidified igneous materials or sedimentary rocks cannot be excluded at this time

arctic underwater noise transients from sea ice deformation characteristics annual time series and forcing in beaufort sea a 13 month time series of arctic ocean noise from the marginal ice zone of the eastern beaufort sea is analyzed to detect under ice acoustic transients isolated from ambient noise with a dedicated algorithm noise transients due to ice cracking fracturing shearing and ridging are sorted out into three categories broadband impulses frequency modulated fm tones and high frequency broadband noise their temporal and acoustic characteristics over the 8 month ice covered period from november 2005 to mid june 2006 are presented and their generation mechanisms are discussed correlations analyses showed that the occurrence of these ice transients responded to large scale ice motion and deformation rates forced by meteorological events often leading to opening of large scale leads at main discontinuities in the ice cover such a sequence resulting in the opening of a large lead hundreds by tens of kilometers in size along the margin of landfast ice and multiyear ice plume in the beaufort chukchi seas is detailed these ice transients largely contribute to the soundscape properties of the arctic ocean for both its ambient and total noise components some fm tonal transients can be confounded with marine mammal songs especially when they are repeated with periods similar to wind generated waves

under ice ambient noise in eastern beaufort sea canadian arctic and its relation to environmental forcing this paper analyzes an 8 month time series november 2005 to june 2006 of underwater noise recorded at the mouth of the amundsen gulf in the marginal ice zone of the western canadian arctic when the area was 90 ice covered the time series of the ambient noise component was computed using an algorithm that filtered out transient acoustic events from 7 min hourly recordings of total ocean noise over a 0 4 1 khz frequency band under ice ambient noise did not respond to thermal changes but showed consistent correlations with large scale regional ice drift wind speed and measured currents in upper water column the correlation of ambient noise with ice drift peaked for locations at ranges of 300 km off the mouth of the amundsen gulf these locations are within the multi year ice plume that extends westerly along the coast in the eastern beaufort sea due to the large beaufort gyre circulation these results reveal that ambient noise in eastern beaufort sea in winter is mainly controlled by the same meteorological and oceanographic forcing processes that drive the ice drift and the large scale circulation in this part of the arctic ocean

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

siderophile element abundances in the upper mantle evidence for a sulfide signature and equilibrium with the core abstract measurements of the intrinsic oxygen fugacities of samples from the earth s upper mantle have shown that the characteristic oxidation state ranges from values close to fe ni metal saturation to more oxidized values tectonic processes can account for the progressive and variable oxidation of a mantle that was originally in equilibrium with metal so that the oxygen fugacity data give strong support to the possibility of protocore protomantle equilibrium the data for apparently overabundant siderophile elements in the mantle are reviewed and 2 groups group a ge au pd pt ru rh ir re os group b sn sb ag ga cu as co ni w mo are recognized group a abundances result from an overprinting by a late accreting chondritic component of a mantle that had suffered major extraction of the indigenous complement by core forming metal chou 1978 group b however are 10 more abundant than group a and may be distinguished as a group by their greater chalcophile tendencies this is highlighted for example by the large cl normalized sn ge ratio of the mantle which is unexpected as a result of metal silicate fractionation alone but can be accounted for if some sn together with sulfur was retained in the mantle during core separation industrial experience in the equilibrium of molten fe and silicate slags shows that sulfur is not overwhelmingly partitioned into metal so that the concept of partial 20 of original accreting component retention of sulfur and group b elements in the mantle has experimental and theoretical support following core separation progressive loss of sulfide from the mantle is indicated by the anomalously high ni at a given mg number in archaean basalts and siliceous mg poor sediments it is also a feature of oceanic and continental basalts that some u pb fractionation must have occurred in the mantle source regions following core separation and this is most satisfactorily achieved if pb is partially removed from u in a sulfide phase

platiniferous chromitite and the tectonic setting of ultramafic rocks in cabo ortegal nw spain high cr number cr chromitites with exceptional concentrations of platinum up to 7000 ppb and palladium up to 4400 ppb occur in sulphide poor dunites exposed along the western side of the ultramafic herbeira massif at cabo ortegal nw spain the pt pd enriched chromitites are restricted to the exposed top of the c 800 m thick herbeira layered complex which comprises two dunite dominated units lower and upper dunites separated by a garnet bearing pyroxenite dominated unit the herbeira layered complex is interpreted as lower crustal and is underlain by high al spinel harzburgite interpreted as mantle tectonite dunite lenses within this underlying tectonized harzburgite also contain high cr chromitites but these are pt pd depleted pt and pd have been strongly fractionated into the overlying herbeira layered complex highest pt and pd concentrations are restricted to the top of the upper dunite where a negative ru anomaly also becomes apparent the cabo ortegal ultramafics form part of a high grade terrane with hp granulites garnet pyroxenites and eclogites and are similar to pge rich chromite concentrations in arc root complexes in pakistan jijal and alaska talkeetna and tonsina the cabo ortegal chromitites are interpreted as having crystallized from a sulphide poor mantle derived suprasubduction melt deep within an arc massif subsequently thrust over the gondwanan continent during variscan collision

osmium isotopes in ophiolites re and os concentrations and os isotopic composition are reported for ultramafic samples from ophiolites these data may be divided into two groups on the one hand isotopic values higher than mean upper mantle 187os 186os 1 05 correspond to back arc type ophiolites and could reflect injection of crustal material sediments on the other hand ophiolites referred to as being typical n morb plot very close to previous estimates for mean upper mantle 1 05 together with refined osmiridium results from well dated ultramafic suites the data on ophiolites lead to the following constraints 1 present day 187os 186os for the upper mantle seems for the time being constrained around 1 05 1 06 2 the mantle evolution curve for osmium isotopes through time when associated to recent os isotopic determinations from the literature for archaean samples shows a chondritic 187re 186os ratio of 3 33 and a linear closed system evolution

re os isotopic constraints on the source of platinum group minerals pgms from the vestřev pyrope rich garnet placer deposit bohemian massif abstract mineralogical studies of the heavy fraction from a holocene pyrope rich garnet placer deposit at vest ev krkonose piedmont basin bohemian massif have identified the presence of very rare grains of platinum group minerals pgm pt fe alloy grains are accompanied by os ir ru minerals native osmium iridium and ruthenium with inclusions of pt fe alloy and hongshiite ptcu this mineral assemblage is typical for several mantle settings including ophiolites the chemistry of the os ir ru minerals shows an enrichment of the pgm in ru which is typical of ophiolites the grain morphology of pgm and pyrope rich garnet mostly rounded with numerous euhedral subhedral grains does not exclude a common source in situ laser ablation mc icp ms was used to measure the re os isotopic compositions of single os rich grains which show heterogeneous subchondritic os isotopic compositions 187 os 188 os 0 12082 0 12505 0 00003 this precludes their low temperature origin and indicates derivation of platinum group elements pges essentially from mantle derived rocks without a significant contribution of crustal os the mantle model age t ma and re depletion model age t rd model ages range from 0 4 to 1 0 ga and most likely reflect a long history of melt depletion that affected the mantle sources of pgm

pge and pgm in the luanga mafic ultramafic intrusion in serra dos carajas para state brazil abstract the late archean luanga mafic ultramafic complex intrudes an archean greenstone belt that is mainly composed of ultramafic and mafic metavolcanics the luanga intrusion consists of dunite peridotite gabbro and norite chromitite seams and layers are present in the ultramafic rocks a metamorphic overprint transformed the primary paragenesis into a serpentine talc chlorite tremolite and magnetite association the magnetite is commonly altered to fe hydroxides unaltered chromite commonly displays atoll like textures and a chemical composition typical of stratiform chromites cr 2 o 3 below 45 wt base metal sulfides base metal alloys platimum group minerals and platinum group element bearing phases are present in the form of inclusions in the silicate assemblages and in or on the edges of chromite grains the main minerals detected are pentlandite pyrrhotite millerite chalcopyrite and mackinawite fe ni alloy braggite sperrylite and platinum group elements pge bearing sulfo arsenides braggite is associated with the chromite whereas sperrylite lies on the edges of or is included in silicates the pge content of the massive and disseminated chromities is dominated by pt up to 8900 ppb and the chondrite normalized pge profile shows a cuspidal shape with a pt peak the main hypothesis for the source of the pge rich magma which fractionated the chromitite bearing ultramafic magma consists of a relatively primitive mantle that partially melted in the late archean

the fracture of ice on scales large and small arctic leads and wing cracks from observations and calculations of crack patterns in ice it is suggested that a similar mechanism may account for cracking over a wide range of scales

variations of olivine fo ni contents and highly chalcophile element abundances in arc ultramafic cumulates southern alaska abstract recent drilling in the duke island complex shows that some olivine clinopyroxenites from hall cove contain low s but high pt pd in contrast dunites from the duke island complex and the yellow hill intrusion on nearby annette island are low in pt pd as well as s new data from this study show that the low s ultramafic rocks from duke and annette islands are extreme cumulates of olivine and or clinopyroxene and all contain trace to minor amounts of cumulus sulfide as a result the ratios of the least mobile highly chalcophile elements in these rocks mainly reflect those of original sulfide liquids which in turn reflect those of their parental magmas our data also show that all dunite samples from annette island contain ni depleted primitive olivine with fo contents 90 mol and ni contents

early proterozoic cu au and fe ore deposits associated with regional nacl metasomatism in northern fennoscandia scapolite is widely distributed in 1 9 2 5 ga volcano sedimentary rocks and 1 77 2 2 ga igneous rocks over several hundred square kilometres in northern fennoscandia comprising northern sweden no

core growth and siderophile element depletion of the mantle during homogeneous earth accretion abstract a simple mechanism is put forward to explain the abundances of siderophile elements in the earth s mantle within the frame of a homogeneous accretion hypothesis the model involves repeated equilibrium fractionation in a portion of the mantle between solid and liquid silicate and metal phases fractionation events are followed by segregation of metal phases into the core and extensive mixing of different materials newly accreted matter terrestrial unfractionated matter and silicate mantle material which has undergone fractionation within the mantle the time scale of a fractionation and mantle mixing event is very short compared to that of accretion the process leads to a decrease of the metal fraction in the mantle with time therefore the modelling was done in two stages in the first envisaged during accretion metal fractions around 0 1 were considered a second stage is set after the completion of accretion and metal fractions modelled are around 0 001 the depletion in the mantle by such a two stage process was calculated for seven involatile siderophile elements characterized by different partitioning between liquid metal solid metal liquid silicate and solid silicate in the first stage the mantle is depleted equally in all siderophile elements irrespective of differences in partition coefficients abundances are controlled solely by the mass balance of metal and silicate phases within the fractionation events in the second stage abundances of the moderately siderophile elements w co ni are not changed while strongly siderophile elements re ir au are further depleted calculated depletions for five of the seven elements agree well with the observed concentrations cl and al normalized as follows w 0 053 co 0 074 re 0 0047 ir 0 0062 and au 0 0056 results for ni 0 028 and mo 0 0046 are by factor 2 5 and 5 below the observed concentrations respectively an implication of the model is that the mainly convective remixing flux within the silicate mantle during accretion was about ten times larger than the metal flux into the core the silicate mantle is depleted in pb by a factor 2 5 during core formation the already large increase in value caused by volatilization of pb is thus augmented with the probable time scale of accretion taken as around 100 ma this means that the discrepancy between meteoritic and terrestrial lead isotope systematics need not be a serious paradox

sea ice thickness measurement based on the dispersion of ice swell the dispersion of flexural waves propagating in the arctic sea ice cover is exploited in order to locally measure the ice thickness the observed dispersion for waves filtered in the 4 20 s period interval at up to 4 broad band seismometers deployed in spring 2007 near the north pole is compared to a parameterized model that accounts for a complex wavefield made of a superposition of independent plane waves with different amplitudes and back azimuth angles the parameterization that includes finding the best modeled ice thickness is performed by using the cross correlation functions between the seismometers the ice thickness is estimated to 2 5 0 2 m for the 1 km large floe the seismic stations were deployed on which is coherent with other independent measurements at this site this study thus demonstrates the feasibility of using broad band seismometers deployed on the sea ice in order to passively measure the ice thickness without requiring active sources nor human intervention

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

lead isotopes and the sources of the columbia river basalt group a detailed study of 208pb 204pb 207pb 204pb and 206pb 204pb ratios suggests that the number of pb isotopic reservoirs required for the columbia river basalt group crbg as currently defined must be increased from the presently accepted four to at least six the identities of the six reservoirs are two of mid ocean ridge basalt morb slightly contaminated with sediment rl and r2 rl being cascades pb one a probable plume component r3 two of unspecified mantle material but probably of rl type contaminated with local crustal pb r4 and r5 and the sixth a complex enriched reservoir of 2150 ma age r6 rl is an end member for all crbg except the saddle mountains basalts r2 is a second end member for the picture gorge basalt and appears to be unique to it both rl and r2 are related to the juan de fuca subduction zone and the back arc spreading associated with it r3 is the principal source for most of the wanapum basalt as well as for most of the chemically evolved portions of the grande ronde basalt volumetric and tectonic considerations require that one source of the crbg be a plume or at least a nonlocal crustal or uppermost mantle source and r3 is the ideal candidate for that role r4 is the second source for the imnaha basalt and r5 is the source for the 206pb rich varieties of the grande ronde basalt r4 and r5 are identified with contamination by local phanerozoic crust because of their similarity to local crustal pb and because the rocks which contain them also have cu contents correlated with their pb isotopic compositions r6 is the parent for pb in all the saddle mountain basalts this last source appears to have been homogeneous 2150 m y ago but has since split into a number of separate discrete pockets each with its own characteristic pb isotopic signature some of these appear in individual saddle mountain basalt flows while other sources have mixed with r4 to produce individual saddle mountain basalt flows of the ice harbor set r6 is similar in all its isotopic properties to the precambrian basement that is exposed to the immediate north of the columbia river basalt group the overall model for columbia river basalt group genesis is termed plume reinforced back arc magmatism

basalt geochemistry and tectonic discrimination within continental flood basalt provinces abstract continental flood basalts are usually regarded as a single tectonomagmatic entity but frequently quoted examples exhibit a variety of tectonic settings in one well studied classic flood basalt province the mesozoic karoo province of southern africa magmatism occurred in the following tectonic settings a continental rifting leading to ocean floor spreading in the south atlantic ocean etendeka suite of namibia b stretched continental lithosphere and rifting not leading directly to ocean floor formation lebombo suite of southeastern africa and c an a tectonic within plate continental setting characterized by an absence of faulting or warping lesotho highlands and karoo dolerites of south africa by means of spidergrams of the elements rb ba th nb k la ce sr nd p hf zr sm ti tb y v ni and cr uncontaminated tholeiites from c above i e the lesotho type continental flood basalts ltcfb are compared with mid ocean ridge basalts morb ocean island tholeiites oit and tholeiites and calc alkali basalts from subduction environments the comparison reveals the ltcfbs are geochemically distinct the differences are reflected in relative enrichments or depletions of the more incompatible elements rb ce to less incompatible elements ce y i e the overall slope of the spidergrams and in anomalous enrichments or depletions of one or more of the elements th k nb sr ti hf and zr the distinctive geochemical character of the lesotho ltcfbs is interpreted in terms of a lithospheric mantle source for the basalts this is supported by isotopic data there are no major geochemical differences between lesotho cfbs and basalts of the rift related etendeka and lebombo suites although the latter are somewhat enriched in rb ba and k however unlike the lesotho basalts the lebombo and etendeka basalts are associated with voluminous silicic volcanics or intrusive centres and late stage dolerites having morb oit i e asthenospheric geochemical characteristics the flood basalt silicic magmatism late stage dyke swarm association is characteristic of several rift or thinned lithosphere environments e g ethiopia skye eastern greenland but in many of these the flood basalts have ocean island basalt oit geochemical characteristics the lesotho type cfb geochemistry is exhibited by the grande ronde basalt of the columbia river group a possible subduction related flood basalt province and the basic rocks associated with mesozoic rifting in the north and south atlantic basalt geochemistry alone is unhelpful in determining the tectonic setting of cfbs although the rift related environments may be identified by the petrology and geochemistry of the whole igneous suite a two source model is proposed for the mantle derived basic rocks in rift related cfb provinces early enriched basalts are derived from the lithosphere and following pronounced lithospheric attenuation or rifting later morb like melts are emplaced from the rising asthenosphere the presence of both lesotho and oit type geochemical patterns in rift related cfbs suggests that the lithosphere exhibits different styles of enrichment

highly siderophile elements and 187re 187os isotopic systematics of the allende meteorite evidence for primary nebular processes and late stage alteration abstract the abundances of highly siderophile elements hse re os ir ru pt and pd as well as 187 re 187 os isotopic systematics were determined for calcium aluminum rich inclusions cais chondrules and matrix separated from the cv3 carbonaceous chondrite allende consistent with prior studies cais are characterized by significant depletions in pd relative to the other hse while the other hse are in generally bulk chondritic relative abundances the depletions in pd can be linked with initial formation of cais via condensation or subsequent processing by evaporative processes chondrules generally have relative hse patterns similar to cais although they have lower absolute abundances palladium depletions in chondrules may reflect solid metal liquid metal fractionation at the time of formation or alternatively be the result of processes that acted on precursor materials matrix samples have nearly chondritic absolute abundances of all hse measured consequently matrix is the only major chondritic component examined here that shows no relative depletion in pd mass balance suggests the existence of an unidentified pd rich carrier although it is possible that the dataset presented here is too limited to represent typical hse abundances of some chondritic components e g chondrules the 187 re 187 os isotopic systematics of only six out of twenty four allende chondritic components analyzed plot within uncertainties of a 4568 ma primordial reference isochron the deviations from the expected isochron most likely reflect late stage open system behavior within the last 2 billion years and in some cases could even have resulted from terrestrial alteration the open system behavior is most readily observed in small millimeter size sub samples of allende consistent with re and or os mobility on that scale

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

the partitioning of fe ni and co between olivine metal and basaltic liquid an experimental and thermodynamic investigation with application to the composition of the lunar core abstract the partitioning of fe ni and co between mg rich olivine lunar basaltic liquid and metal has been measured over a range of temperatures and pressures the results of the olivine metal partitioning may be compared to those predicted from thermodynamic calculation in general the agreement is good although the calculated distribution coefficients are slightly greater then those experimentally determined this discrepancy increases with increasing temperature and pressure but it is not possible to ascertain unambiguously which thermodynamic data might be responsible at lower temperatures inversion of the equilibria to yield cosmothermometers for measuring the temperature of equilibration between olivine and metal gives results for the pallasite meteorites in excellent agreement with independent estimates the results have been applied to apollo 15 green glass presumed to approximate a primitive melt from the lunar mantle to deduce the composition of fe ni co metal in equilibrium with the lunar mantle this composition is approximately by weight 54 60 fe 38 45 ni and 1 co consequently if metal has separated from the lunar mantle under equilibrium conditions to form a small lunar core this core will be nickel rich with a ni fe ratio of 0 7 0 15

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

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

nickeliferous pyrite tracks pervasive hydrothermal alteration in martian regolith breccia a study in nwa 7533 martian regolith breccia nwa 7533 and the seven paired samples is unique among martian meteorites in showing accessory pyrite up to 1 by weight pyrite is a late mineral crystallized after the final assembly of the breccia it is present in all of the lithologies i e the fine grained matrix icm clast laden impact melt rocks climr melt spherules microbasalts lithic clasts and mineral clasts all lacking magmatic sulfides due to degassing pyrite crystals show combinations of cubes truncated cubes and octahedra polycrystalline clusters can reach 200 lm in maximum dimensions regardless of their shape pyrite crystals display evidence of very weak shock metamorphism such as planar features fracture networks and disruption into subgrains the late fracture systems acted as preferential pathways for partial replacement of pyrite by iron oxyhydroxides interpreted as resulting from hot desert terrestrial alteration the distribution and shape of pyrite crystals argue for growth at moderate to low growth rate from just saturated near neutral 6 fmq 2 log units it is inferred from the maximum ni contents 4 5 wt that pyrite started crystallizing at 400 500 c during or shortly after a short duration relatively low temperature thermal event that lithified and sintered the regolith breccias 1 4 ga ago as deduced from disturbance in several isotope systematics

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

chalcophile siderophile element systematics of hydrothermal pyrite from martian regolith breccia nwa 7533 unlike other martian meteorites studied so far martian regolith breccia nwa 7533 and 56 paired meteorites that have sampled 4 4 ga old impact lithologies show only sulfides of 57 hydrothermal origin mostly pyrite 1 the reasonably good positive correlation r 2 0 72 between se and 65 ni reflects a temperature control on the solubility of both elements apart from the chalcogens s se 66 and te pyrite appears to be a minor contributor 400 c temperature crystallization for nwa 7533 pyrite as deduced from 69 its se and ni contents ii magmatic sulfide depletion of brecciated early martian crust iii 70 precipitation from near neutral h 2 s hs h 2 o rich hydrothermal fluids that did not provide halogen 71 ligands for extensive transport of chalcophile siderophile metals it is suggested that the 1 4 ga 72 lithification event that precipitated hydrothermal pyrite left the chalcophile siderophile element 73 budget of the early martian crust nearly unmodified except for s se and te 74 75 76

the sound of ice break up and floe interaction when landfast ice breaks up and starts to move under the influence of wind and current it produces a distinctive acoustical signature that can include nearby pure tones observations of this signal obtained in amundsen gulf are interpreted in terms of the initial fracturing mechanism followed by the rubbing of adjacent ice floes the ice rubbing sound excites sh waves in the ice that evidently dominate the response resulting in the pure tone that is observed the frequency of this signal is determined by ice thickness and the shear wave speed

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

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

contrasting platinum group element concentration patterns in cumulates of the bushveld complex chromite rich lithologies in both the lower and the critical zones of the bushveld complex in the potgietersrus area display flat chondrite normalized platinum group element pge concentration patterns whereas those of associated sulphide bearing but chromite poor rocks are considerably steeper the low pt pd os ir ru ratio in the chromite bearing rocks is maintained irrespective of the amount of sulphide or chromite in the rock this feature suggests that the partitioning of the individual pge into pge bearing phases during conditions in the magma under which crystallization of chromite in excess of the normal cotectic amounts was favoured differed from conditions under which an immiscible sulphide liquid separated from the same magma in the absence of enhanced chromite crystallization these changes in the partitioning coefficients of the individual pge are considered to reflect changes in the solubility of these elements in response to variations in the intensive parameters in the magma necessary to bring about the enhanced crystallization of chromite

preliminary fractionaltion patterns of ore metals through earth history abstract a systematic method of comparing fractionation patterns of ore metals to those of common rock forming elements is developed using the coherent electronic framework provided by the periodic table through its progressive filling of transition metal d and f orbitals four generalized fractionation processes in earth history are addressed 1 nebular condensation and accretion 2 core mantle interaction 3 continental crustal growth and 4 ore formation in supracrustal environments through each of these simplified processes preliminary fractionation patterns of ore metals within periods appear to vary smoothly with atomic number fractionation trends for each of these four processes can be ascribed respectively to 1 differential condensation volatility 2 multiple stages of core formation with partial separation of highly siderophile metals into the core 3 enrichment of the continental crust in ore metals and incompatible elements by ascent of mantle derived magmas and 4 efficient multistage magmatic and hydrochemical ore forming processes within the crust fractionation trends of ore metals are contiguous and collinear with those of rock forming metals in the same period implying common geochemical controls the systematic variation of enrichment and depletion factors across groups of elements within the same period are termed chemical fractionation spectra

chemical equilibration of the earth s core and upper mantle the oxygen fugacity fo2 of the earth s upper mantle appears to lie somewhat above that of the iron wustite buffer its fo2 is assumed to have been similar to the present value at the time of core formation in the upper mantle the fe rich liquid protocore that would form under such conditions of fo2 at elevated temperatures would lie predominantly in the system fe s o distribution coefficients for co cu ni ir au ir w re mo ag and ga between such liquids and basalt are known and minimum values are known for ge from these coefficients upper mantle abundances for the above elements can be calculated by assuming cosmic abundances for the whole earth and equilibrium between the fe s o protocore and upper mantle these calculated abundances are surprisingly close to presently known upper mantle abundances agreements are within a factor of 5 except for cu w and mo therefore siderophile element abundances in the upper mantle based on known distribution coefficients do not demand a late stage meteoritic bombardment and a protocore formed from the upper mantle containing s and o seems likely as upper mantle abundances fit a local equilibrium model then either the upper mantle has not been mixed with the rest of the mantle since core formation or else partition coefficients between protocore and mantle were similar for the whole mantle regardless of p t and fo2 the latter possibility seems unlikely over such a p t range

platinum group elements as petrological indicators in mafic ultramafic complexes of the central and southern urals preliminary results abstract platinum group elements pge and gold were analyzed by icp ms after the ni sulphide fire assay pre concentration step in 44 rocks from 5 orogenic mafic ultramafic massifs of the central and southern urals they comprise a ring zoned ural alaskan type bodies of the pt bearing belt kachkanar tagil uktus and b lherzolite harzburgite complexes overlain by layered wehrlite gabbro and diorite nurali mindyak supposed to be ophiolite type suites total pge and au contents are generally higher in ural alaskan type complexes compared with lherzolite harzburgite complexes mantle normalized pge patterns range from nearly unfractionated in the dunites pd ir 0 57 3 68 to mildly and strongly fractionated in wehrlite clinopyroxenite and gabbro pd ir 9 1 187 exceptionally high values of pd ir 761 2622 are observed in amphibole rich lithologies at kachkanar possibly indicating a relationship between fractionation of the less refractory pd and increased fluid activity in the parental magma pronounced positive pt anomalies up to 20 times the mantle estimate distinguishes the ural alaskan type dunites and progressively reduces in pyroxenites and gabbros in the lherzolite harzburgite complexes of mindyak and nurali layered wehrlites and pyroxenites as well as rocks from the gabbro diorite units have fractionated pge patterns pd ir 2 94 92 39 and positive pt anomaly as in the ural alaskan complexes conversely the pt anomaly is negative in the peridotites and increases from lherzolites and harzburgites to dunites similarly to residual mantle from alpine type orogenic massifs the pd ir 0 71 3 7 is higher than in ophiolites and resembles that of lherzolite massifs of the western mediterranean representing sub continental mantle variation of the pt anomaly versus pd ir in ural alaskan type complexes and in the layered sequences overlaying lherzolite harzburgite mantle suites is consistent with magmatic fractionation in which pt behaves as a compatible element it reflects a progressive depletion by partial melting in mantle peridotites the pt becoming incompatible in the last stage of melting in which pt poor residual dunites are formed the positive pt anomaly in ural alaskan type complexes is striking and would appear as a distinctive feature of the parent magma probably inherited from its mantle source

back arc convection model for columbia river basalt genesis abstract genesis of the columbia river basalt group crbg may be accounted for by the progressive thermal erosion of an attenuated heterogenous subcontinental lithosphere scl by back arc convection behind the cascade arc assembly of the source components of the crbg began during the early mesozoic when slices of former oceanic lithosphere were isolated below terranes accreted to the continental margin subsequent lithospheric shearing accompanying northward translation of the terranes allowed the injection of small degree partial melts from the asthenosphere which solidified in the scl as pyroxenite veins the scl source of the crbg underwent a further episode of shearing accompanied by attenuation during the eocene and oligocene when it was caught between contrasting right lateral terrane translation and e w extension regimes in the northern cordillera and basin and range province respectively the attenuation resulted in the raising of the geotherm close to that of the pyroxenite and iherzolite solidi melting was triggered by back arc convection behind the cascade arc the pyroxenite was the first component mobilised constituting the principal source of the imnaha formation continued back arc convection resulted in the failure of the scl and the generation of the voluminous grande ronde formation from pyroxenite and the surrounding scl the change to wanapum volcanism marks the stage of exhaustion of in situ sources and their replacement with a combination of proterozoic scl from the east and asthenospheric material by back arc convection the saddle mountains formation was derived entirely from proterozoic scl small magma volumes resulted in a susceptibility to local contamination such that the distinct isotopic arrays of the saddle mountains formation comprise three mixing lines and do not have any age significance

rheological conditions for emplacement of ural alaskan type ultramafic complexes article i nfo ural alaskan or alaskan type complexes correspond to a particular class of ultramafic intrusions that attract particular attention due to their deep mantle origin and their platinum group element pge mineralization when defined as massifs of dunite clinopyroxenite only forty six complexes are reported in the literature these large scale dunite pipe like structures are rarely isolated and they even can appear in clusters to better understand genesis of these relatively young b460 ma complexes a worldwide compilation has been built and three categories have been defined single circular or elliptical bodies twin bodies with similar shapes and dismembered dunite bodies pge enrichment in alaskan type complexes is highest for the second category where twin bodies are interpreted as horizontal sections of y shaped dunite pipes to constrain mechanical properties of the lithosphere allowing emplacement of the alaskan type complexes the forceful diapiric ascent hypothesis is investigated through numerical thermo mechanical models one hundred high resolution experi ments accounting for realistic phase changes and softening mechanisms have been performed the experiments show that with no rheological softening of the host rock and in case of a relatively weak ductile lower crust the uprising magma tends to spread laterally without reaching the surface to account for the forceful ascent of deep magmas it is hence necessary to assume a strong lower crust rheology and strong local softening mechanisms besidesreproducingtheclustereddistributionoftheweaknesszonesrepresentingmagmapathways theselatter experiments reproduce large scale pipe like cylindrical structures y shaped and funnel shaped bodies and laterally shifted structures interestingly zones of highest strain rates are located at the bottom parts of the inclined edges of y shaped and funnel shaped bodies the restricted age range of alaskan type complexes b460 ma would mean that prior to this time the lower crust was less resistant due to the hotter geotherm prohibiting the possibility of alaskan type magmatism

implications of oxygen isotope data and trace element modeling for a large scale mixing model for the columbia river basalt current debate regarding the origin of the columbia river basalt group centers on whether the considerable geochemical diversity in these basalts is the result of derivation from a heterogeneous mantle or of extensive interaction of the magmas with crustal rocks compositionally diverse rocks such as the pomona and elephant mountain members of the saddle mountains formation have identical 87 sr 86 sr ratios 0 7078 0 7080 these radiogenic rocks have nearly identical 18 o values 6 24 6 36 to those of picture gorge 6 24 the least radiogenic of the columbia river basalts 87 sr 86 sr 0 7038 this suggests that the mantle beneath the columbia plateau is heterogeneous at least with respect to sr and probably other components basalts with low 18 o values 6 19 6 36 can be found in all formations of the columbia river basalt an observation not compatible with models requiring extensive assimilation of upper crustal rocks to explain isotopic differences between formations evidence for crustal contamination for some flow rocks in the grande ronde wanapum and saddle mountains basalts does exist contamination however seems to have affected isolated members of formations rather than formations as a whole this conclusion is consistent with the results of trace element modeling which suggests that the grande ronde and saddle mountains basalts were produced as numerous independent magmas from a heterogeneous mantle the magmas ascended following independent paths some undergoing crustal contamination others not

nearfield noise measurements from an arctic pressure ridge this paper presents initial analysis results of ambient noise measurements from an active ice pressure ridge over a 3 day period in april 1979 the site was a research ice camp 180 miles from the pole in the eurasian basin of the arctic ocean the ridge was instrumented with calibrated hydrophones paired in both the horizontal and vertical directions their distance from the ridge was about 100 m and their outputs were tape recorded dimensional surveys of the active portions of the ridge were made at the surface source level linear density is deduced from ambient noise measurements near the pressure ridge by assuming the ridge to be a line source of spatially incoherent point sources measured arctic transmission loss data are used to deduce an active pressure ridge average separation compatable with previously measured omnidirectional ambient noise data statistics for the month of april the effects of surface image interference are present in the data and allow a preliminary estimate of acoustic sourc

arctic ambient noise in the beaufort sea seasonal relationships to sea ice kinematics various modes of sea ice motion in the beaufort sea are correlated with under ice noise at 10 32 and 1000 hz seasonal variations are considered and a parameterization of ice microfracturing caused by sensible heat flux is included in the correlations during the summer the correlations indicate that all frequencies are primarily a response to the ice rushing through the water during the fall 10 and 32 hz noise correlate best with a linear combination of the speed of the ice parcel plus the total rate of change in the shape of the ice parcel this latter factor indicates noise generation due to the individual ice floes moving past one another as they rearrange into new shapes the correlations indicate differential motions of other forms primarily ice convergence become important in generating lower frequency noise only during winter as for 1000 hz during fall and winter the correlations are low and the results are ambiguous several factors are discussed that emphasize our lack of knowledge ab

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

scaling properties of sea ice deformation from buoy dispersion analysis 1 a temporal and spatial scaling analysis of arctic sea ice deformation is performed over timescales from 3 h to 3 months and over spatial scales from 300 m to 300 km the deformation is derived from the dispersion of pairs of drifting buoys using the iabp international arctic buoy program buoy data sets this study characterizes the deformation of a very large solid plate the arctic sea ice cover stressed by heterogeneous forcing terms like winds and ocean currents it shows that the sea ice deformation rate depends on the scales of observation following specific space and time scaling laws these scaling properties share similarities with those observed for turbulent fluids especially for the ocean and the atmosphere however in our case the time scaling exponent depends on the spatial scale and the spatial exponent on the temporal scale which implies a time space coupling an analysis of the exponent values shows that arctic sea ice deformation is very heterogeneous and intermittent whatever the scales i e it cannot be considered as viscous like even at very large time and or spatial scales instead it suggests a deformation accommodated by a multiscale fracturing faulting processes

platinum group element geochemistry of sulfide rich horizons in the ural alaskan type ultramafic complex of duke island southeastern alaska the duke island ultramafic complex in southeastern alaska is a ural alaskan type complex that hosts sulfide rich horizons primarily in the olivine clinopyroxenite unit although placer deposits hosting platinum group elements pges have been found in association with ural alaskan type complexes in several locations around the world these ultramafic complexes have traditionally been viewed as sulfide poor systems and therefore unlikely hosts of economic ni cu sulfide deposits sulfide mineralization in the duke island complex consists of massive net textured and disseminated assemblages that formed as a result of the interaction between magma and sulfidic graphitic country rocks the sulfide mineralization at duke island is characterized by strong depletion in ir os and ru ipges and relative enrichment in pt pd and cu the sulfide bearing olivine clinopyroxenites formed from a derivative magma produced as a result of the fractional crystallization of olivine and chromite although ipges may be compatible within olivine and chromite fractional crystallization of up to 20 cannot explain the observed ipge depletion we propose that early crystallization of laurite and ir alloys accompanied the accumulation of olivine and chromite r factor modeling indicates that much of the inter element variation in the sulfide mineralization can be attributed to variations in the degree of equilibration between sulfide and silicate liquids mixing between monosulfide solid solution and fractionated sulfide liquid may also have contributed to the variability the modeling suggests that the pre sulfide saturation concentrations of os pt and pd in the magmas were approximately 0 03 2 and 3 ppb respectively the concentration of pd is near that of primitive mantle and the low pt concentration can be explained by the crystallization of pt alloy with early formed dunites and chromitites this is consistent with models for pt rich placers in the urals that have been derived from chromitites and dunites the relative enrichments in pd and cu are the results of fractional crystallization before sulfide saturation was achieved taken together the ni cu pge elemental characteristics at the duke island complex are similar to those shown by flood basalt related sulfide deposits and cu rich sulfide veins studied in different locations around the world

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

eastern arctic ambient noise on a drifting vertical array ambient noise in the eastern arctic was studied from april to september 2013 using a 22 element vertical hydrophone array as it drifted from near the north pole 89 23 n 62 35 w to north of fram strait 83 45 n 4 28 w the hydrophones recorded for 108 min day on six days per week with a sampling rate of 1953 125 hz after removal of data corrupted by non acoustic transients 19 days throughout the transit period were analyzed noise contributors identified include broadband and tonal ice noises bowhead whale calling seismic airgun surveys and earthquake t phases the bowhead whale or whales detected are believed to belong to the endangered spitsbergen population and were recorded when the array was as far north as 86 24 n median power spectral estimates and empirical probability density functions along the array transit show a change in the ambient noise levels corresponding to seismic survey airgun occurrence and received level at low frequencies and transient ice noises at high frequencies

platinum group element distribution in subcontinental mantle evidence from the ivrea zone italy and the betic rifean cordillera spain and morocco the six platinum group elements pge s os ir ru rh pt and pd and au were analyzed by instrumental neutron activation analysis after nickel sulfide fire assay in peridotites and dyke rocks from the orogenic ultramafic massifs of the ivrea zone in the italian western alps baldissero balmuccia finero and the betico rifean cordillera in southern spain and northern morocco ronda beni bousera the peridotites are considered as variably depleted and reenriched low lithosphere whereas the dyke rocks represent polybaric derivatives of basaltic melts pyroxenites and gabbros most coming from the underlying asthenosphere the peridotites have total pge content in the range 8 6 54 7 ppb while mantle normalized patterns generally grade from nearly flat and pge rich in less depleted lherzolites to negative and pge poor in residual harzburgites and dunites dyke rocks have total pge s in the range 5 4 250 ppb and positive mantle normalized patterns negative anomalies of ir pt are frequently ob

a miocene subcontinental plume in the pacific northwest geochemical evidence most of the recent discussions on the number and kinds of components which can be distinguished in the columbia river basalt group crbg magmas have used arguments developed from isotopic evidence in this paper we consider relative contents of excluded trace elements interpreted by means of spidergrams and abundance ratios and compare data from the crbg and related lavas with those of selected oceanic basalts although many recent models for crbg petrogeneses do not include a plume component spidergrams for the american bar flows of the imnaha basalt of the crbg have broad humps at ba th and nb ta this feature suggests that an enriched mantle plume component is present in the american bar magmas an inference which is weakly confirmed by 87sr 86sr data this plume may have been identical to that presently beneath yellowstone the inferred hot spot track lies almost directly east west and requires a vector of motion of the pacific northwest part of the north american plate significantly different from that inferred by minster and jordan 1978 31 our proposed vector for the north american plate agrees however with that suggested by the anahim belt a volcanic lineament in southern british columbia that has been proposed as a hot spot track this proposed vector requires alterations in current models for pacific and north american plate movement during the cenozoic

platinum group element behaviour and thermochemical constraints in the ultrabasic basic complex of the vizcaino peninsula baja california sur mexico the vizcaino complex is characterized by tectonised harzburgites overlain by serpentinized dunite bodies and wherlites layers containing several cumulate chromitite deposits and by fractional crystallization sequences cyclic repetititons of dunites pyroxenites with pegmatoidal textures and gabbros overlain by a succession of gabbronorites to diorites in which olivine is followed by pyroxene and by plagioclase enriched sulfide horizons generally associated with pegmatoidal rocks are lacking the fractionation of platinum group element pge displays evidence of erratic dispersion throughout the intrusion pge patterns of chromitites show high ir and ru contents relatively high rh content and an unusual pt pd depletion lower than in ophiolitic chromitites these chromitites do not contain laurite type sulfides which are commonly observed in association with os ir ru alloys dunites show relatively flat patterns and gabbros present typical trends with depletions in the iridium group ipge relative to ppge ru rh and especially ir are very low in these silicate rocks indicating the precipitation of these elements during the earliest stages of fractional crystallization and their concentration in chromitites spilitized dolerite dykes from the metamorphic basement show significant pge enrichments similar to those of gabbros based on our previous thermochemical data an extremely low fs2 10 2 atm was in equilibrium with the silicate melt as indicated by the absence of laurite included in chromitites in the absence of complexation of pt pd by sulfur in the early stage of magmatic differentiation a complexation of these elements by cl as chloride complexes in a cl rich fluid phase may be expected this hypothesis is reinforced by the notable absence of any sulfide horizon throughout the intrusion part of precious metals pt pd au may have been remobilized at low temperature and transported as chloride or hydroxide complexes by fluid phases introduced during low grade metamorphic processes or later during deformation events in this interpretation the erratic behaviour of pge at vizcaino was partially controlled by solubilities of pt and pd as chloride complexes at magmatic temperatures as early as chromite crystallization and the control of local saturation of the magma by variation of fs2 was overshadowed

principles of mobilization dissolution of metals in mafic and ultramafic rocks the role of immiscible magmatic sulphides in the generation of hydrothermal gold and volcanogenic massive sulphide deposits abstract immiscible magmatic sulphides exert a major control over both the absolute precious and base metal contents of magmas and on the availability of these metals from rocks during ore formation it is concluded that only those mafic magmas that attained s saturation at a late stage are not depleted in the chalcophile elements and are hence suitable source rocks for metals in hydrothermal ore deposits immiscible magmatic sulphides are not only strongly enriched in the ore forming metals but they are also much more reactive than silicates the ore forming metals may be liberated from the sulphides at the sea floor alteration stage and deposited in interflow chemical sediments au deposits or volcanogenic massive sulphide deposits subsequent mobilization remobilization of au from the interflow sediments may concentrate au within the interflow sediments to form economic au deposits the mobilized remobilized au may also be transported to form epigenetic style gold deposits in archaean greenstone belts and in palaeozoic slate belts

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

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

solubilities of mantle oxides in molten iron at high pressures and temperatures implications for the composition and formation of earth s core the solubilities of several oxide components of the mantle in molten iron have been measured at 16 gpa and 1700 2000 c their relative solubilities are as follows feo cr2o3 mno v2o3 tio2 sio2 mgo al2o3 oxide contents reach significant levels varying from 10 wt feo to 1 wt sio2 at the respective metal oxide eutectics but are very small for mgo and al2o3 the solubilities of these oxides are expected to increase substantially at temperatures above 2000 c and at pressures above 16 gpa and are relevant to the core formation process in the earth individual oxide species dissolve quasi congruently under conditions which maintain oxygen fugacities near the iron wustite buffer however the dissolution of mantle mineral phases is highly incongruent if a mixture of metallic iron and mantle silicates were subjected to increasing pressures and temperatures above 16 gpa and 2000 c the most soluble species feo would first be extracted into the metallic liquid when the feo activity had been lowered sufficiently significant amounts of sio2 would then enter this melt at extremely high temperatures and pressures appreciable amounts of mgo could even dissolve these results provide a background for interpreting recent diamond anvil experiments in which molten iron was observed to react with mantle silicates at temperatures of 2700 3700 c and pressures of 20 70 gpa they also elucidate the nature of chemical reactions between the core and mantle which may occur in the d layer of the lower mantle the differing solubilities of mantle oxides in molten iron place constraints on models of accretion of the earth and of accompanying core formation processes the observation that tio2 is undepleted in the earth s mantle relative to ci chondrites makes it improbable that the core contains a substantial amount of dissolved si or sio2 it therefore seems likely that oxygen as dissolved feo is the principal light element component of the core this conclusion is reconciled more readily with a modified version of homogeneous accretion of the earth than with some current models of heterogeneous accretion accretion of the earth from a population of giant planetesimals would have led to complete melting of the mantle and core at very high temperatures under these conditions nearly all feo and other transition metal oxides would have been extracted from the mantle into the core together with a significant amount of sio2 it is difficult to reconcile the geochemical consequences of this accretion scenario with the present composition of the mantle

platinum group mineral inclusions in chromite from the bird river sill manitoba platinum group minerals pgm have been identified as inclusions in chromite from the bird river sill manitoba the inclusions are small 20 microns and are commonly euhedral the pgm inclusions are ru os ir s2 laurite and os ir ru alloy rutheniridosmine laurites contain up to 2 99 wt palladium arsenic content is negligible and no platinum or rhodium has been detected one platinum group element alloy contains 0 96 wt rhodium but neither platinum nor palladium has been detected laurite inclusions in chromite from the ultramafic zone record two compositional trends first increasing and then decreasing ru ru os ir up section pgm inclusions and other solid inclusions occur as discrete phases in chromite and are part of the chromite precipitation event increasing oxygen fugacity by wall rock assimilation or new magma injection initiates chromite precipitation locally increasing the sulphur content of the magma to convert pge alloys to sulphides

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

tourmalinites from the golden dyke dome northern australia tourmalinites occur at five stratigraphic levels within the low metamorphic grade multiply deformed lower proterozoic metasediments and metavolcanics of the golden dyke dome the geological setting exhalite associations sedimentary structures and tourmaline chemistry all preclude a granitic origin tourmalinite derived from the isochemical metamorphism of an exhalite comprising silica and tourmaline tourmalinite and other exhalites are hosted by carbonaceous pelitic metasediments and were deposited in deeper basinal areas the ore fluid was probably dolerite heated seawater which leached a thick pile of argillaceous sediments and acquired metals and boron by leaching the first and second increase in geothermal gradient produced tourmalinite the third produced auriferous tourmalinite and iron formations with tourmalinite and exhalative pb zn deposits forming during the period of maximum geothermal gradient more tourmalinite formed during the subsequent decline in geothermal gradient tourmalinites are regarded as b rich iron formations which can be the host for or associated with exhalative deposits of au and base metals

reaction between magnesiowusite of lower mantle composition and core forming fe ni alloy at 1 40 gpa one class of models for the early history of the earth requires the present day inventory of siderophile elements in the mantle to have been established by equilibrium partitioning between core forming metal and mantle minerals at high pressures and temperatures deep inside the earth we have accordingly carried out reconnaissance experiments on the partitioning of nickel between model lower mantle magnesiowustite mg9 85 and 1 3 wt nio and a model core forming alloy fe 94 ni 6 approximately 7 wt ni at pressures between 1 40 gpa and temperatures ranging from 1200 degrees c to 2000 degrees c reversal experiments were also attempted our results highlight the difficulty of attaining equilibrium partitioning in this system and imply that partition coefficients derived from unreversed experiments should accordingly be viewed with reservation our data nevertheless imply that the concentration of nio in lower mantle magnesiowustite in equilibrium with core forming metal with approximately 7 wt ni would be extremely low e g about 0 2 wt nio moreover equilibrium seems to be fairly insensitive to the effects of either pressure or temperature and so it is unlikely that magnesiowustite could acquire 1 3 wt nio simply by equilibrating with core forming metal under special high p t conditions early in earth history alternative hypotheses for the present day siderophile element inventory of the mantle are accordingly preferred

sulfide and platinum mineralization in the merensky reef evidence from hydrous silicates and fluid inclusions the base metal sulfides of the merensky reef are associated with hydrous silicates and intense deuteric hydrous alteration of cumulus and postcumulus silicates biotite and phlogopite crystallized in the vicinity of sulfides from a volatile enriched highly fractionated intercumulus melt amphibole chlorite and talc are later alteration phases of cumulus pyroxene and intercumulus plagioclase biotite is often accompanied by zircon rutile and quartz accessory quartz hosts a complex suite of h2o nacl cacl2 co2 ch4 fluid inclusions which have thus far not been described from the merensky reef the earliest fluid inclusion compositions are nacl h2o with less than 10 vol water co2 coexisting with a halite daughter crystal and brine and polyphase inclusions with up to six daughter and accidental phases and high contents of divalent cations the maximum trapping temperature is around 730 c at 4 to 5 kb pressure later inclusion generations are h2o nacl co2 h2o and pure co2 and ch4 the presence of cl rich fluids during the intercumulus stage of the crystallizing merensky reef is directly related to the mode of sulfide precipitation prior to sulfide unmixing in a hydrous magma sulfur is likely to be present as h2s sulfur saturation causes reaction of h2s with oxides of the silicate melt to form a sulfide melt plus water during reaction the magma is enriched in water until a separate fluid unmixes it carries all compounds with high fluid melt partition coefficients as well as metals capable of forming oh and cl complexes precious metals are assumed to have fractionated into the cl rich fluid as cl complexes rather than being dissolved in the sulfide melt during the cooling evolution of the fluid the precious elements precipitate around the periphery of sulfide melt droplets the model proposed explains the distribution pattern of platinum group minerals in the merensky reef better than any orthomagmatic mineralization concept offered so far

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

acoustical radiation from thermally stressed sea ice observations of the sound generated by cracking ice in the canadian arctic during a cooling period provide an opportunity for studying the response of different types of ice to thermal stress synthetic aperture radar sar images and air photographs of ice around the experimental site allow mapping of cracking events back to their points of origin thus providing a correlation between ice type and acoustical activity it is shown that there are only a few cracking events in the first year ice during periods of air temperature variation whereas multiyear ice produces a large number and is responsible for most of the ambient sound recorded analysis of several individual events including both natural and artificial sources reveals that failure processes cause acoustic emissions with an angularly dispersive radiation pattern in which higher frequencies tend to be radiated downward and lower frequencies radiated toward the horizontal this pattern can be explained in terms of two types of eigenmodes used in

geochemistry of platinum group elements of the zoned ultramafic alto condoto complex northwest colombia the alto condoto complex dated at 20 ma is a zoned ultramafic intrusion with a magmatic rock sequence ranging from dunite to hornblendite the different petrographic units are cumulates formed by fractional crystallization processes parts of the central dunite are anomalously enriched in ptfe alloys and to a minor degree in iros alloys all other lithologic units of the complex display normal fractionation trends of platinum group elements pge with a general increase of total pge from wehrlite to horn blende clinopyroxenite and hornblendite this normal trend is best described by increasing pd and decreasing ir abundances the controlling factor for pge mineralization in zoned ultramafic complexes is probably an effective ptfe alloy precipitation during the crystallization of olivine this may be initiated by rapid tectonic movements causing pressure decrease and overheating of the magma with pge alloy formation as a result of the f sub o 2 shift during olivine crystallization

alteration of platinum group minerals and dispersion of platinum group elements during progressive weathering of the aguablanca ni cu deposit sw spain the distribution mineralogy and mobility of the platinum group elements pge in the surface environment are poorly understood this study of the lower less altered and upper more altered gossan overlying the aguablanca ni cu pge magmatic deposit spain has shown that the platinum group minerals pgm are progressively oxidised and dispersed into iron oxides that form the gossan a combination of the characterization of pge in host pgm using a scanning electron microscope and measurement of pge at lower concentrations in host iron oxides using laser ablation inductively coupled plasma mass spectrometry la icp ms has for the first time allowed the total distribution of the pge within a gossan to be documented this study has revealed a complete in situ alteration and dispersion sequence of the pgm including 1 breakdown of both the more stable pt arsenides pt pd tellurides and the less stable bismuthotellurides 2 formation of partially oxidised pgm 3 development of a wide range of oxidised pt and pd bearing phases 4 subsequent formation of fe pge oxides and pge hydroxides 5 incorporation of pge into ferruginous supergene products and lastly 6 concentration of pge at the edges of veins and iron oxides dispersion of pd is greater than for the other pge with pd being widely distributed throughout the iron oxides this oxidising environment produced pge oxides rather than pge alloys also commonly found in the surface environment especially in placers these results provide critical evidence for the stages of mineralogical change from pge host mineralogy in magmatic ores to surface weathering producing pge oxides

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