protists of the upper mesoproterozoic ruyang group in shanxi province china abstract organic walled microfossils described in this paper have been obtained from the beidajian formation of the mesoproterozoic ruyang group ca 1300 ma in southern shanxi province china they are characterized by many eukaryotic organisms such as acanthomorphic acritarchs sphaeromorphic acritarchs with a distinct circular excystment acritarchs with multilayered walls and ornamentation and multicellular filaments and algal thallose recently more eukaryotic microfossils have been found and the triaromatic dinosteroid biomarker was extracted from the whole rock sample containing abundant acritarch specimens of dictyosphaera delicata and shuiyousphaeridium one acritarch form spiromorpha segmentata shows morphological similarities to the extant green alga spirotaenia it means that sexual reproduction by conjugation probably arose in the late mesoproterozoic a new genus spiromorpha is proposed here in combination with known mesoproterozoic microfossil assemblages from the roper group of australia and the bahraich group of ganga basin india we infer that marine protests had a rapid development during the mesoproterozoic era

microfossils in 2000 ma old cherty stromatolites of the franceville group gabon abstract a well preserved microbial assemblage has been found in paleoproterozoic cherty stromatolites from the 2000 ma old franceville group gabon this assemblage contains coccoids and star like forms filaments and budding tubular forms these various forms are mostly of cyanobacterial affinity and among them nostocalean akinetes aerchaeoellipsoides are reported for the first time from rocks older than the mesoproterozoic the most noteworthy discovery in the franceville biota is the gunflint type association huroniospora gunflintia eoastrion archaeorestis known also without archaeorestis in other paleoproterozoic assemblaes throughout the world the present assemblage comprises 13 taxa consisting of 8 species belonging to 10 genera and 2 unnamed forms it is dominated by gunflintia filaments regarded as a mat bulding community and by huroniospora coccoids probably mat dwellers and or plankters the rare and large forms such as akinetes of archaeoellipsoides were allochthonous and presumably planktonic

the effect of a low viscosity layer on convection in the mantle summary in this paper we attempt to find a solution of the following problem it appears reasonable to expect that if thermal convection occurs in the earth s mantle it may also occur within the moon and mars the dimensions of these latter two bodies are comparable to the thickness of the earth s mantle presumably the amount of radioactive heat generated per unit mass is similar in all three bodies yet the surface morphology of the earth which many scientists believe arises ultimately from mantle convection differs markedly from that of the moon or mars the explanation advanced here for this difference is based on the effect produced on convection in the mantle by the presence of a low viscosity or low creep strength layer it is assumed that the low velocity layer of the mantle is such a low creep strength layer a low viscosity layer changes the amount of coupling between the outer crust and mantle convection the crust is coupled if mantle convection produces stresses in the crust which are large enough to deform it plastically the crust is decoupled from the mantle if these stresses are insufficient to produce plastic deformation the theory assumes that the viscosity or creep strength is essentially zero in the low viscosity layer the analysis is similar to that developed earlier for the calculation of stresses within the mantle we find that the deeper within the mantle lies the low viscosity layer the greater is the coupling of the outer crust to the mantle convection currents if the low creep strength layer lies close to the surface the outer crust is decoupled from the interior according to the literature the depth of the low velocity layer is determined by the temperature and pressure profiles within the mantle if the temperature profile were kept constant but the pressure reduced by changing g the gravitational acceleration the low velocity zone would be moved to a level closer to the surface it is proposed that because of this interdependence between temperature pressure and depth of the low velocity layer the low velocity layers in mars and the moon if such layers exist in these bodies lie much closer to the surface than does the corresponding layer in the earth hence the outer crusts of mars and the moon are more nearly decoupled from the interior than is the earth s crust

thermal convection and the origin of ice streams ice streams are a fact of ice sheet dynamics draining up to 90 of the ice thermal convection in ice below the density inversion is a speculation an attempt is made to meld the two in such a way that the speculation becomes an explanation for the fact

on the pulling power of ice streams gravity wants to pull an ice sheet to the center of the earth but cannot because the earth s crust is in the way so ice is pushed out sideways instead or is it the ice sheet sees nothing preventing i t from spreading out except air which is much less massive than ice therefore does not ice rush forward to fill this relative vacuum does not the relative vacuum suck ice into i t because nature abhors a vacuum if so the ice sheet is not only pulled downward by gravity i t is also pulled outward by the relative vacuum this pulling outward will be most rapid where the ice sheet encounters least resistance the least resistance exists along the bed of ice streams where ice bed coupling is reduced by a basal water layer especially if the ice stream becomes afloat and the floating p art is relatively unconfined around its perimeter and unpinned to the sea floor ice streams are t herefore fast currents of ice that develop near the margins of an ice sheet where t hese conditions exist because of these condit ions ice streams pull ice out of ice sheets and have pulling power equal to the longitudinal gravitational pulling force multiplied by the ice stream velocity t hese boundary conditions beneath and beyond ice streams can be quantified by a basal buoyancy factor that provides a life cycle classification of ice streams into inception growth mature declining and terminal stages during which ice streams disintegrate the ice sheet surface profiles of ice streams are diagnostic of the stage in a life cycle and hence of the vitality of the ice sheet

organic walled microfossils from the tonian gouhou formation huaibei region north china craton and their biostratigraphic implications abstract the meso and neoproterozoic huaibei group in the huaibei region north china craton ncc is emerging as a target for geobiological and tectonic studies due to its thick unmetamorphosed sediments with well preserved organic walled fossils however the lack of accurate age constraints for this sedimentary sequence dramatically hampers our ability to take full advantage of the rich geological and paleontological history recorded in the huaibei group particularly the depositional age of the uppermost unit of the huaibei group the gouhou formation is a controversial issue although it is widely accepted that there is an unconformity between the gouhou formation and the overlying early cambrian houjiashan formation the magnitude of this unconformity and hence the scale of the underlying tectonic event are unconstrained due to the uncertainty of the sedimentary age for the gouhou formation which has been variously interpreted as cambrian cryogenian ediacaran 720 ma to 541 ma and tonian 1000 ma to 720 ma to improve our knowledge about this sedimentary gap we carried out a biostratigraphic study of the gouhou formation our investigation using a low manipulation maceration technique revealed a diverse organic walled microfossil assemblage a total of 22 taxa have been revealed including a new species dictyosphaera tacita n sp importantly the co occurrence of trachyhystrichosphaera valeria and dictyosphaera in the gouhou formation suggests a likely tonian age the biostratigraphic data indicate an infra cambrian depositional gap of 200 myr between the tonian gouhou formation and cambrian houjiashan formation perhaps driven by a major tectonostratigraphic event

magmatic unmixing in spinel from late precambrian concentrically zoned mafic ultramafic intrusions eastern desert egypt abstract spinel is widespread in the ultramafic core rocks of zoned late precambrian mafic ultramafic complexes from the eastern desert of egypt these complexes gabbro akarem genina gharbia and abu hamamid are precambrian analogues of alaskan type complexes they are not metamorphosed although weakly altered each intrusion is composed of a predotite core enveloped by pyroxenites and gabbros at the margin silicate mineralogy and chemistry suggest formation by crystal fractionation from a hydrous magma relatively high cr 2 o 3 contents are recorded in pyroxenes up to 1 1 wt and amphiboles up to 1 4 wt from the three plutons the chrome spinel crystallized at different stages of melt evolution as early cumulus inclusions in olivine inclusions in pyroxenes and amphiboles and late magmatic intercumulus phase the intercumulus chrome spinel is homogenous with narrow range of chemical composition mainly fe 3 rich spinel spinel inclusions in clinopyroxene and amphibole reveal a wide range of al 27 44 wt al 2 o 3 and mg 6 13 wt mgo contents and are commonly zoned the different chemistries of those spinels reflect various stages of melt evolution and re equilibration with the host minerals the early cumulus chrome spinel reveals a complex unmixing structures and compositions three types of unmixed spinels are recognized crystallographically oriented irregular and complete separation unmixing products are al rich type i and fe 3 rich type ii spinels with an extensive solid solution between the two end members the compositions of the unmixed spinels define a miscibility gap with respect to cr al fe 3 extending from the fe 3 al join towards the cr corner spinel unmixing occurs in response to cooling and the increase in oxidation state the chemistry and grain size of the initial spinel and the cooling rate control the type of unmixing and the chemistry of the final products causes of spinel unmixing during late magmatic stage are analogous to those in metamorphosed complexes the chemistry of the unmixed spinels is completely different from the initial spinel composition and is not useful in petrogenetic interpretations spinels from oxidized magmas are likely to re equilibrate during cooling and are not good tools for genetic considerations

thermal convection in ice sheets we look but do not see un critere de rayleigh generalise de la convection thermique dans les solides polycristallins chauffes par en dessous a ete developpe et applique a la convection dans les calottes glaciaires une analyse du profil de temperature a l aval du forage de byrd station suggere qu environ mille metres de glace en dessous du point d inversion de la densite entretiendrait la convection

tappania bearing association of the siberian platform biodiversity stratigraphic position and geochronological constraints abstract a new fossil microbiota yielding eukaryotic tappania is described from the mesoproterozoic kamo group of the central angara basin siberian craton the assemblage also includes valeria satka lophosphaeridium pulvinosphaeridium miroedichia osculosphaera tasmanites obruchevella glomovertella as well as branching and anastomosing filamentous microfossils of putative fungal affinities and peculiar short trichomes that together substantially increase the biodiversity of the tappania association stratigraphic position and geochronological constraints indicate a relatively young late mesoproterozoic age of the tappania bearing sediments there appears to be a continuity between the tappania association and the neoproterozoic microbiota the latter being firmly rooted in the late mesoproterozoic

organic walled microfossils in 3 2 billion year old shallow marine siliciclastic deposits the discovery of relatively large cell like structures in early archaean shale and siltstone deposits adds further evidence that diversified living organisms thrived on earth at a very early date claims for fossils of this age are often controversial as non biological processes can produce life like microstructures and chemical signatures resembling those of the remains of living organisms extensive efforts were therefore made to rule out a non biological origin for these microstructures they survive this examination and appear to be organic walled microfossils derived from large microorganisms that cohabited with previously reported microbial mats living in the sunlit shallows of earth s early oceans around 3 2 billion years ago claims that life existed on earth in the early archaean eon 3 2 billion years ago are often controversial as non biological processes can produce life like microstructures and chemical signatures that mimic those of the remains of living organisms now however the discovery of relatively large carbonaceous spheroidal microstructures interpreted as organic walled microfossils in early archaean deposits adds further evidence that life existed thrived and survived on earth at a very early date although the notion of an early origin and diversification of life on earth during the archaean eon has received increasing support in geochemical sedimentological and palaeontological evidence ambiguities and controversies persist regarding the biogenicity and syngeneity of the record older than late archaean1 2 3 non biological processes are known to produce morphologies similar to some microfossils4 5 and hydrothermal fluids have the potential to produce abiotic organic compounds with depleted carbon isotope values6 making it difficult to establish unambiguous traces of life here we report the discovery of a population of large up to about 300 m in diameter carbonaceous spheroidal microstructures in mesoarchaean shales and siltstones of the moodies group south africa the earth s oldest siliciclastic alluvial to tidal estuarine deposits7 these microstructures are interpreted as organic walled microfossils on the basis of petrographic and geochemical evidence for their endogenicity and syngeneity their carbonaceous composition cellular morphology and ultrastructure occurrence in populations taphonomic features of soft wall deformation and the geological context plausible for life as well as a lack of abiotic explanation falsifying a biological origin these are the oldest and largest archaean organic walled spheroidal microfossils reported so far our observations suggest that relatively large microorganisms cohabited with earlier reported benthic microbial mats8 in the photic zone of marginal marine siliciclastic environments 3 2 billion years ago

the chromite deposit of the ipueira medrado sill sao francisco craton bahia state brazil the ipueira medrado mafic ultramafic sill hosts the largest chromite deposit in brazil the sill is a 7 km long and 300 m thick elongated body consisting mainly of interlayered dunite and harzburgite but also containing minor mafic rocks the most distinctive feature of the ipueira medrado sill is a continuous 5 to 8 m thick massive chromitite layer that is currently being mined the anomalous thickness of this chromitite layer provides important constraints on current genetic models for the origin of massive chromitites the sill is divided into three zones marginal ultramafic and mafic the marginal zone 5 20 m thick consists of sheared gabbro and orthopyroxene rich harzburgite the ultramafic zone up to 250 m thick is subdivided into the lower ultramafic unit the main chromitite layer and the upper ultramafic unit the lower ultramafic unit consists of dunite with minor harzburgite and chain textured chromitite the main chromitite layer is composed of chain textured and massive chromitites and the upper ultramafic unit consists mainly of harzburgite but also minor chain textured chromitite and dunite pyroxene abundance increases toward the top in this zone and grades to an orthopyroxenite at the top the presence of magmatic intercumulus amphibole is significant up to 20 vol in this unit the mafic zone up to 40 m thick consists of leuconorites to melanorites that are partially metamorphosed under amphibolite facies conditions the variation of olivine and orthopyroxene compositions throughout the sill layers reveals two intervals of distinct magmatic evolution the interval located below the main chromitite layer is characterized by gradual upward evolution of mineral compositions toward more mg rich compositions fo93 and en94 the interval located above the main chromitite layer is characterized by rapid evolution toward more fe rich compositions fo85 and en84 with increasing stratigraphic height the interval below the main chromitite layer suggests crystallization in a dynamic magma chamber undergoing frequent replenishment with primitive magma the interval above the main chromitite layer suggests crystallization in a mainly closed fractionating magma chamber mass balance considerations suggest that an enormous amount of magma 10 000 m thick was associated with the formation of the main chromitite layer at the ipueira medrado sill such a volume of magma is obviously not represented in the stratigraphy of the ipueira medrado sill mineral chemistry data indicate that the composition of the resident magma was most primitive at the stratigraphic interval immediately below the main chromitite layer we suggest that the sill acted as a conduit through which a large volume of magma flowed under these conditions contamination of the parental magma with crustal material and fluids is a likely mechanism for explaining the origin of the chromitite abundant intercumulus amphibole in the upper ultramafic unit and high fe3 in chromites intercumulus orthopyroxene in main chromitite layer chromitites and sm nd and re os isotope data for silicate rocks and chromitites respectively indicate that the ipueira medrado sill formed from magmas contaminated by crustal material the host rocks of the ipueira medrado sill consist mainly of high grade gneiss and marbles that would have been appropriate source of fluids and sio2 and could have caused the increase in f o2

reaction relationships of chrome spinels in igneous rocks further evidence from the layered intrusions of rhum and mull inner hebrides scotland compositions of chrome spinels in peridotites from the layered igneous rocks of rhum and of the ben buie intrusion mull show evidence of post depositional reaction relationships with the cumulus olivine and or the intercumulus liquid some seam forming spinels in the ben buie intrusion are highly aluminous more so than those in the rhum intrusion occasional zoned spinels in both intrusions show enrichment in al towards their rims the evidence suggests that the spinel reaction trend involving cr al exchange proceeds in the direction of al enrichment equilibrium between chrome spinel and cumulus olivine appears to have been attained rarely thus limiting the application of the olivine spinel geothermometer

the kemi stratiform chromitite deposit northern finland the kemi chromite deposit is hosted by an early proterozoic 2 44 ga layered intrusion the lower part of the intrusion is composed of peridotitic and pyroxenitic cumulates and chromitite layers olivine and chromite have not been observed in the upper part where plagioclase is the predominant cumulus phase occurring either alone or together with pyroxenes the intrusion was metamorphosed during the svecokarelidic orogeny mafic minerals of the lower and upper parts of the complex are completely altered to chlorite serpentine talc amphiboles and carbonates whereas the middle part is well preserved the chromite grains too suffered from alteration but the cores of the grains still exhibit primary compositions a continuous chromitite layer can be traced for the entire length of 15 km of the intrusion it varies in thickness from a few millimeters to as much as 90 m in the central part of the intrusion the mineable thicker part of the main chromitite layer is about 4 5 km long the main chromitite layer is overlain by a sequence of thin chromite rich layers which continues upward to the stratigraphic level of 500 m above the basal contact of the intrusion the upper part of the main chromitite is layered but the lower part is more massive and brecciated the ore reserves for open pit mining are ca 40 million metric tons with an average grade of 26 6 percent cr 2 o 3 and a cr fe ratio of 1 53 the whole area has additional reserves of about 110 million metric tons of ore the ore zone is intensely fractured the host rocks have undergone alteration which has degraded the quality of the ore the present surface section of the kemi intrusion is interpreted as a cross section typical of a funnel shaped intrusion and plunging about 70 degrees to the northwest the cumulate sequence especially the main chromitite layer is at its thickest in the middle of the intrusion and the magmatic conduit which fed the intrusion is also thought to have been located just below this thickening it is suggested that the chromitite was deposited when the input of magma into the kemi intrusion was contaminated by salic material from the underlying basement complex this magma mixed with the fresh input of primitive magma resulting in chromite saturation the chromite crystals formed during mixing in a plume according to the model of huppert et al 1986 and accumulated preferentially around the magmatic conduit evidence of contamination is found in chromite grains in the form of small inclusions rich in alkalies which are thought to represent trapped droplets of the contaminant salic melt

kotuikan formation assemblage a diverse organic walled microbiota in the mesoproterozoic anabar succession northern siberia abstract abundant and diverse microfossils from shales of the lower member of the 1500 ma old kotuikan formation in northern siberia document early mesoproterozoic life along the siberian platform similar to many meso and neoproterozoic microbiota the kotuikan assemblage is dominated by prokaryotic cyanobacteria both filamentous oscillatorialeans and nostocaleans which are represented primarily by cellular trichomes and by empty sheaths and coccoidal chroococcaleans including solitary and colonial specimens however unlike mesoproterozoic microbiota of shallow water settings the kotuikan compressed organic walled microfossil assemblage in the open shelf facies includes diverse microscopic eukaryotes large up to 1 mm diameter megasphaeromorph acritarchs branched filaments that are most likely of eukaryotic algae and coccoidal monostromatic colonies of chlorococcaleans as well as other morphologically complex microorganisms the kotuikan assemblage contains numerous taxa previously recorded from neoproterozoic successions e g elatera eosolena palaeastrum pterospermopsimorpha rugosoopsis and navifusa but these are long ranging genera and no neoproterozoic index fossils have been recorded the microbiota is considered to be mesoproterozoic in age because it contains a mixed assemblage of prokaryotic and eukaryotic microorganism remains including new taxa unknown in younger deposits the kotuikan assemblage documents the status of the marine biosphere at an early mesoproterozoic time preceding the primary radiation of eukaryotes and representing an evolutionary stage transitional between the predominantly prokaryote dominated paleoproterozoic and the eukaryote dominated neoproterozoic microbiota as reported in this study 35 taxa were identified in the kotuikan assemblage of which five forms are described informally that are assigned to 18 genera of microscopic prokaryotes and eukaryotes two new genera and five new species are proposed hirudiforma lancetica gen and sp nov h simmetrica gen and sp nov lineaforma elongata gen and sp nov elatera minor sp nov eosolena minuta sp nov

textural and compositional features of chromite in the lower and critical zones of the bushveld complex south of potgietersrus chromite is developed as an accessory mineral in a 1 700 m thick sequence of lower and lower critical zone cumulates in the area south of potgietersrus and as a major constituent in a number of chromitite layers within this sequence the lower zone consists of a 1 500 m thick sequence of ultramafic rocks characterized by varying proportions of olivine orthopyroxene and chromite which allows the pile of cumulates to be subdivided into 37 cyclic units the critical zone on the other hand is only 350 m thick of which only the lower 150 m contains chromite bearing lithologies electron microprobe analyses of chromite and coexisting silicates indicate a strong positive correlation between the mg mg fe super 2 and cr fe super 2 fe super 3 ratios of chromite and the mg mg fe super 2 ratio of the mafic silicates the composition of the chromite is also strongly influenced by the mineralogical composition of the host rock and probably reflects conditions within the parental magma during crystallization of these rocks the analytical data have also shown the tio 2 content of the disseminated chromite to be the most informative indicator of the degree of differentiation of the magma a detailed investigation of the chromite across the contact of a thin chromitite layer has shown that the chromitite layers crystallized in response to an influx of magma and that chromite crystallization could have been enhanced by an increase in the f sub o 2 and a rise in the cr 2 o 3 content of the liquid thereby causing an increase in the cr 2 o 3 content of the chromite within the chromitite layers postcumulus modification of the chromitite layers is extensive and primary textural features have been obliterated essentially by sintering sintering took place in the presence of a reactive mg rich liquid which resulted in an increase in the mg mg fe super 2 ratio of the chromite this process resulted in a densification of the chromitite layers from an estimated initial porosity of 30 to 45 percent to less than 10 percent

reaction of the accumulation zone portions of glaciers to climatic change the response of the accumulation regions of glaciers to changes in accumulation rate and in surface temperature is calculated by considering perturbations to the ice flow the analysis is limited to glaciers in which flow is dominated by internal shear and not by bottom sliding and for which flowlines are geographically parallel in general glaciers begin to alter thickness immediately after a change in accumulation rate but the effect of a change in surface temperature is delayed by the time for this temperature change to penetrate to depth in the glacier a warming leads to glacial thinning the amount and timing of the response is very different for different glaciers characteristic times are on the order of tens of years for mountain glaciers and tens of thousands of years for the east antarctic ice sheet as an example the theory is applied to the ice sheet near byrd station antarctica for hypothetical changes in surface accumulation rate and temperature rates and amounts of thickness change are calculated and it is found that the measured thinning is probably too fast to have been caused by climatic variation alone the effects of hypothetical climatic variations on the depth age and depth temperature relationships through the ice sheet are also calculated it is hoped that the theory will prove valuable in the interpretation of future drill hole results

flow of ice sheets in the vicinity of subglacial peaks although changes of surface slope across a subglacial peak can be explained at least semi quantitatively on a two dimensional basis in terms of gradients of longitudinal and shear stresses along a flow line we lack appreciation of flow in the third dimension which may tend to cause lower layers to flow around rather than over a sharp peak this paper collects and surveys evidence relevant to flow in the third dimension and discusses the processes involved particularly the evidence from radio echo layering and also discusses possible reasons for the lack of radio echo layers near the base of the antarctic ice sheet

microbiotas from the late proterozoic chuar group northern arizona and uinta mountain group utah and their chronostratigraphic implications abstract samples of shales from the dominantly clastic upper proterozoic chuar and uinta mountain groups in northern arizona and utah yielded relatively abundant assemblages of acritarchs these are taxonomically varied and are interpreted as consisting of openwater cosmopolitan eukaryotic plankton the assemblages are compared with late proterozoic late riphean and early vendian c 800 700 ma sequences in the southern urals of the ussr the russian platform northern and southern scandinavia svalbard and east and north west greenland in all instances both the assemblages and discrete form taxa predate the varangerian vendian glacial event of the north atlantic region and this is taken to indicate a comparable age for glaciogenic rock units in western north america one new acritarch species vandalosphaeridium walcottii is erected and the partly megascopic species chuaria circularis is emended

bending shear the rate controlling mechanism for calving ice walls bending shear was observed to produce nearly vertical shear bands in a calving ice wall standin on dry land on deception island iat 63 0 os long 60 6 w and slabs calved straight downward when shear rupture occurred along these shear bands hughes 1989 a formula for the calving rate was developed from the deception island data and we have attempted to justify generalizing this formula to include ice walls standing along beaches or in water these are environments in which a wave washed groove develops along the base of the ice wall or along a water line above the base the rate of wave erosion provides an alternative mechanism for controlling the calving rate in these environments we have determined that the rate at which bending creep produces nearly vertical shear bands along which shear rupture occurs controls the calving rate in all environments shear rupture occurs at a calving shear stress of about i bar our results justify using the calving formula to compute the calving rate of ice walls in computer models of ice sheet dynamics this is especially important in simulating retreat of northern hemisphere ice sheets during the last deglaciation when marine and lacustrine environments were common along retreating ice margins these margins would have been ice walls standing along beaches or in water because floating ice shelves are not expected in the ablation zone of retreating ice sheets

reaction rims of orthopyroxene and plagioclase around chrome spinels in olivine from skye and rhum nw scotland reaction between chrome spinel and its supernatant magma due to changes in magmatic composition or temperature is well known its effects can be observed in these rocks however evidence of solid solid reaction between chrome spinels and enclosing olivine is also present in the form of relict diffusion profiles in some of the olivines and an orthopyroxene reaction product rimming the spinel in addition the rhum specimens contain rims of plagioclase resulting from solid liquid reaction around chrome spinels within the margins of olivine detailed microprobe traverses and analysis of reaction products have been made in an attempt to determine the equations for these reactions

geochemistry and mineralogy of platinum group elements in the ransko gabbro peridotite massif bohemian massif czech republic the ransko gabbro peridotite massif in eastern bohemia is a strongly differentiated intrusive complex of lower cambrian age the complex hosts low grade ni cu ores mainly developed close to the contact of olivine rich rocks with gabbros in troctolites and to a much lesser extent in both pyroxene and olivine gabbros and plagioclase rich peridotites the ore zone is characterized by strong serpentinization and uralitization the total ni cu locally reaches up to 4 wt anomalous concentrations of platinum group elements pge s maximum 532 ppb pd 182 ppb pt 53 ppb rh 15 ppb ru 41 ppb ir were detected in samples of cu ni and ni cu ores maximum 2 63 wt ni and 2 31 wt cu from the jezirka orebody the main ore paragenesis includes pyrrhotite pentlandite chalcopyrite cubanite pyrite magnetite mackinawite valleriite ilmenite and sphalerite during this work michenerite froodite sperrylite gold native bismuth altaite tsumoite hessite an unnamed bi ni telluride cobaltite gersdorffite and galena were newly identified the host rocks originated through partial melting of a slightly depleted mantle source with noble metals scavenged from this primitive magma prior to the development of these rocks

compositional effects on the solubility of minor and trace elements in oxide spinel minerals insights from crystal crystal partition coefficients in chromite exsolution chromite from los congos and los guanacos in the eastern pampean ranges of cordoba argentinian central andes shows homogenous and exsolution textures the composition of the exsolved phases in chromite approaches the end members of spinel mgal 2 o 4 spl and magnetite fe 2 fe 2 3 o 4 mag that define the corners of the spinel prism at relatively constant cr 3 r 3 ratio where r 3 is cr al fe 3 the exsolution of these phases from the original chromite is estimated to have accounted at 600 c on the basis of the major element compositions of chromite with homogenous and exsolution textures that are in equilibrium with forsterite rich olivine fo 95 the relatively large size of the exsolved phases in chromite up to ca 200 m provided for the first time the ability to conduct in situ analysis with laser ablation inductively coupled plasma mass spectrometry for a suite of minor and trace elements to constrain their crystal crystal partition coefficient between the spinel rich and magnetite rich phases d i spl mag minor and trace elements listed in increasing order of compatibility with the spinel rich phase are ti sc ni v ge mn cu sn co ga and zn d i spl mag values span more than an order of magnitude from d ti spl mag 0 30 0 06 to d zn spl mag 5 48 0 63 our results are in remarkable agreement with data available for exsolutions of spinel rich and magnetite rich phases in other chromite from nature despite their different cr 3 r 3 ratio the estimated crystal crystal partitioning coefficients reflect the effect that crystal chemistry of the exsolved phases from chromite imposes on all investigated elements excepting cu and sc and only slightly for mn the observed preferential partitioning of ti and sc into the magnetite rich phase is consistent with high temperature chromite melt experiments and suggests a significant dependence on fe 3 substitution in the spinel structure a compositional effect of major elements on ga co and zn is observed in the exsolved phases from chromite but not in the experiments this might be due to crystal chemistry differences along the mgfe 1 al 2 fe 2 3 exchange vector which is poorly covered experimentally this inference is supported by the strong covariance of ga co and zn observed only in chromite from layered intrusions where this exchange vector is important a systematic increase of zn and co coupled with a net decrease in ga during hydrous metamorphism of chromitite bodies cannot be explained exclusively by compositional changes of major elements in the chromite which are enriched in the magnetite component the most likely explanation is that the contents of minor and trace elements in chromite from metamorphosed chromitites are controlled by interactions with metamorphic fluids involved in the formation of chlorite

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

the biostratigraphic conundrum of siberia do true tonian cryogenian microfossils occur in mesoproterozoic rocks abstract abundant and diverse microfossils from shales of the 1500 ma old ust il ya formation in northern siberia document early mesoproterozoic life along the siberian platform similar to many meso and neoproterozoic microbiota the ust il ya assemblage is dominated by cyanobacteria both filamentous oscillatorialeans and nostocaleans represented primarily by cellular trichomes and empty sheaths and coccoidal chroococcaleans including solitary and colonial specimens microbiotas from shallow water settings that contain morphologically simple and small microfossils were originally taken to be representative of mesoproterozoic life however the ust il ya and other compressed organic walled microfossil assemblages in open shelf facies include diverse microscopic eukaryotes large up to 1 mm diameter megasphaeromorph acritarchs branched filaments that are most likely of eukaryotic algae and other morphologically complex microorganisms the ust il ya assemblage contains numerous taxa previously reported from tonian and cryogenian successions e g elatera pterospermopsimorpha rugosoopsis and chuaria but these are long ranging genera and no neoproterozoic index fossils have been recorded the microbiota is considered to be mesoproterozoic in age because it contains a few taxa reported from the calymmian microfossil assemblages e g lineaforma aff l elongata satka favosa and elatera aff e minor the ust il ya mixed assemblage of prokaryotic and eukaryotic microorganisms records an evolutionary stage transitional between the prokaryote dominated paleoproterozoic and the eukaryote dominated neoproterozoic world as reported in this study 38 taxa were identified in the ust il ya assemblage of which six forms are described informly that are assigned to 17 genera of microscopic eukaryotes presumably 17 20 entities and prokaryotes a new species is proposed heliconema ustilense

the distribution of microfossil assemblages in proterozoic rocks abstract a biostratigraphic model of the temporal distribution of distinctive proterozoic microfossil assemblages is suggested based on studies of upper precambrian chert embedded and compression preserved organic walled microfossils from the reference sections of eurasia north america and australia microfossils from 2 0 to 0 542 ga can be divided into seven successive informal global units which can be compared to standard units of the international and russian time scales each unit is characterized by a particular association of taxa typified by the fossil assemblage that gives it its name these form broad biostratigraphic units comparable to assemblage zones of phanerozoic successions in general but with minor differences they correspond to chronostratigraphic units accepted by the internal commission on stratigraphy the units are 1 labradorian the upper part of the paleoproterozoic orosirian and statherian 2 0 1 65 ga 2 anabarian lower mesoproterozoic calymmian ectasian lower riphean lower middle riphean 1 65 1 2 ga 3 turukhanian upper mesoproterozoic stenian upper middle riphean 1 2 1 03 ga 4 uchuromayan lower neoproterozoic late stenian tonian lower upper riphean 1 03 0 85 ga 5 yuzhnouralian upper neoproterozoic cryogenian upper upper riphean 0 85 0 63 ga 6 amadeusian lower ediacaran lower vendian 0 63 0 55 ga 7 belomorian upper ediacaran upper vendian 0 55 0 542 ga

heterogeneous redox conditions and a shallow chemocline in the mesoproterozoic ocean evidence from carbon sulfur iron relationships abstract oxygenation of earth s oceans in the proterozoic was a protracted process that began prior to the great oxidation event goe and culminated in deep ocean ventilation following neoproterozoic glacial episodes with both of these major thresholds accompanied by profound changes in the evolution of earth s biosphere our understanding of the redox structure of mesoproterozoic oceans however is considerably less certain limited proxy investigations of oceanic redox in the mesoproterozoic have focused largely on early mesoproterozoic shelf to basinal settings with comparatively little attention paid to epeiric sea environments that dominate the late mesoproterozoic sedimentary record here we report multi proxy shale redox data from epicratonic and pericratonic environments of the 1 1 ga atar and el mreiti groups taoudeni basin mauritania these strata were deposited across the west african craton during global sea level highstand and provide insight into the redox structure of mesoproterozoic epeiric seas iron speciation pyrite sulfur isotope and trace metal concentration data suggest a shallow chemocline and highly heterogeneous redox conditions with fundamental heterogeneities in sulfate concentration and the extent and persistence of euxinia between onshore and offshore environments we suggest that a protracted increase in biospheric oxygenation in the late mesoproterozoic was accompanied by unstable and heterogeneous redox conditions in shallow nearshore areas contributing to the fragmentary pattern of early eukaryotic evolution and diversification

longitudinal variations in glacial flow theory and test using data from the byrd station strain network antarctica application de la theorie de budd 1970 des effets du lit sur les variations d ecoulement du glacier aux observations de la calotte de glace a la station byrd on utilise une rheologie visqueuse lineaire et on resout une equation biharmonique des variations des efforts et des vitesses de deformation les conditions aux limites comportent une topographie du fond avec ondulations glissements et cisaillements

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

chromian spinel mineralogy of the stare ransko gabbro peridotite czech republic and its implications for sulfide mineralization chromian spinels from the stare ransko gabbro peridotite czech republic display a distinct development trend from primary cr rich spinels towards ferro ferri ti enriched magnetite and magnesio alumina spinel sensu stricto spinels zoning in the spinels is thought to be the result of a combination of exsolution postdepositional diffusion and replacement processes multiphase globular silicate inclusions mainly biotite and amphibole also orthopyroxene serpentine and chlorite are present in euhedral largely homogeneous chromian spinels which occur in a matrix of poikilitic amphibole the enclosed minerals have similar compositions to the minerals in the host rock these silicate inclusions are interpreted as being due to a late magmatic introduction of a hydrous k ca rich melt phase contaminated by wall rock interaction into a semi solidified olivine pyroxenes cumulate