zoning of platinum group mineral assemblages in the ug2 chromitite determined through in situ sem eds based image analysis in situ scanning electron microscopy energy dispersive x ray spectrometry analysis of platinum group minerals pgm and base metal sulfides in the ug2 chromitite shows that this ore body is zoned along at least 6 km of strike the uppermost part of the ug2 chromitite referred to as the leader seam is 16 cm thick and has a pgm assemblage that is dominated by pge arsenides sulpho arsenides and alloys 70 vol of all pgm which are typical secondary pgm assemblages in other segments of ug2 this is the first time such laterally persistent secondary assemblages have been identified in the ug2 chromitite as previously they were only known to occur adjacent to transgressive fluid bearing structures e g pipes faults the underlying main seam is thicker one to nine seams totaling 130 cm and has a pgm assemblage that consists mostly of pt sulfide pt pd sulfide pt rh cu sulfide laurite and fe pt alloys 85 vol of all pgm typically regarded as primary magmatic constituents of ug2 chromitite there are however some subtle vertical changes in the pgm assemblages of the main seam that include the occasional presence of secondary assemblages in the top and bottom parts the origin of these secondary pgm assemblages is related to alteration by hydrothermal fluids and or fluid rich melts that infiltrated during crystallization of the ug2 and may possibly have been derived from the ug2 chromitite itself

greenland meltwater as a significant and potentially bioavailable source of iron to the ocean the micronutrient iron is thought to limit primary production in large regions of the global ocean meltwater measurements suggest that the greenland ice sheet serves as a significant source of potentially bioavailable iron to the surrounding coastal ocean

analysis of twenty microseim storms during the winter of 1987 1988 and comparison with wave hindcasts abstract in this paper an attempt is made to establish a quantitative relationship between the spectra of secondary microseisms recorded at menai bridge and the parent sea wave spectra in the sea areas in the western approaches secondary microseisms have double the frequency of the parent waves the sea wave spectra were obtained from a hindcasting model the problem was simplified by restricting the consideration of the microseismic energy to that associated with periods of 7 s or over and relating this to the energy spectra of sea waves of period 14 s or over in this way only the strongest storms usually only found in the north atlantic were taken into account the hindcasting model gives a complete frequency directional distribution of the sea wave energy and so in coastal areas the energy moving normal to the coast can be ascertained in the event a workable relationship was found between the microseism spectra and those for waves in the bristol channel area and also the area bounded by the north coast of ireland and the south west coast of scotland including the north channel from the theory due to longuet higgins it was possible to calculate the coastal reflection coefficients from the relationship by putting in the areas for these two regions these were found to be between 0 15 and 0 25 which are too high for these coastal regions it is probable therefore that these two regions act as monitors for larger sea areas an investigation was also made into the existence of primary microseisms these have the same frequency as the parent waves the results confirmed a previous finding that with the vertical component the energy is about 100 times less than that of the secondary

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

estimates of atmospheric processed soluble iron from observations and a global mineral aerosol model biogeochemical implications 1 desert dust deposition to the ocean may be a significant source of biogeochemically important elements specifically iron the bioavailability of iron in the oceans requires it to be in a soluble form and because atmospheric iron in desert dust is typically insoluble understanding the atmospheric processes that convert insoluble iron to more soluble forms is essential understanding these relationships is especially important in remote ocean regions where iron may be the limiting nutrient observations of soluble iron from 2001 cruise based aerosol measurements over the atlantic and pacific oceans ranged from 0 to 45 mean of 4 9 in the fine mode 2 5 m in diameter and 0 to 87 mean of 2 10 in the coarse mode we test two simple hypotheses of soluble iron enhancement in the atmosphere using a global model of mineral aerosols the first method assumes that iron solubility increases as iron is exposed to solar radiation approximating photoreduction reactions that are important pathways for enhancement of soluble iron in the presence of acidic solutions the second process imitates cloud processing of iron by increasing the amount of soluble iron when the mineral aerosol comes into contact with a cloud both methods resulted in similar average magnitudes of percent soluble iron compared to observations but did not capture specific events or have sufficient variability perhaps because the model does not include aerosol interactions between species other than mineral dust or other processes that may be important

generation of microseisms in the coastal area abstract this paper considers the regularities and properties of microseisms and swell in the coastal area of east kamchatka based on extensive experimental data dependences were obtained of the value of coastal swell and the amplitude of microseisms on the period a relationship was revealed between the vertical and horizontal components of microseisms the dependence of the amplitude of microseisms on the value of swell is non linear for both the horizontal and vertical components for the horizontal component the non linearity is much more significant the mechanism of the generation of horizontal components by bottom friction is considered analysis of the hydrodynamic conditions of swell in the coastal area shows the possibility of the generation of vertical and horizontal components by respectively bottom pressure and bottom friction estimations were made of the dimensions of the microseismic generation areas in the coastal zone

consistent relationship between global climate and surface nitrate utilization in the western subarctic pacific throughout the last 500 ka 1 the open subarctic pacific is at present a high nitrate low chlorophyll hnlc region where nitrate is perennially abundant at the surface theoretically the hnlc status of this region is subject to modification by ocean circulation and or micronutrient supply with implications for the effectiveness of the biological pump and hence carbon sequestration in the ocean interior records of biogenic detritus in sediments from throughout the subarctic pacific indicate that export production was generally lower during glacial maxima while nitrogen isotope measurements from the bering sea have shown that nitrate consumption there was more complete during the last glacial period than it is today here nitrogen isotopic analyses of bulk sediments 15nbulk from three deep water sites in the open subarctic pacific are evaluated in terms of regional nitrate isotopic composition and local relative nitrate utilization the eastern subarctic pacific 15nbulk record bears great similarity to 15nbulk records from the western margin of north america over the last glacial cycle suggesting that variability in the isotopic composition of subeuphotic zone nitrate the growth substrate is reasonably coherent throughout the northeast pacific and dominates at these sites however the two western subarctic pacific records which lie at the heart of the hnlc region display a different pattern implying that significant changes in local relative nitrate utilization overlie the regional background variability after a novel correction intended to remove the background signal associated with denitrification in the eastern tropical north pacific these nitrate utilization records are correlated with a benthic oxygen isotope stack reflecting global deep ocean temperature and ice volume r2 0 65 the correlation implies a strong link between global climate and subarctic pacific nitrate utilization with nearly complete nitrate consumption during glacial periods when export production was low

iron solubility driven by speciation in dust sources to the ocean particulate aerosols are thought to be the primary source of iron to the oceans but the factors determining their solubility and thus bioavailability are unclear x ray absorption analysis reveals that speciation controls the solubility of iron in three large sources of aerosol iron

origin of the ug2 chromitite layer bushveld complex chromitite layers are common in large mafic layered intrusions a widely accepted hypothesis holds that the chromitites formed as a consequence of injection and mixing of a chemically relatively primitive magma into a chamber occupied by more evolved magma this forces supersaturation of the mixture in chromite which upon crystallization accumulates on the magma chamber floor to form a nearly monomineralic layer to evaluate this and other genetic hypotheses to explain the chromitite layers of the bushveld complex we have conducted a detailed study of the silicate rich layers immediately above and below the ug2 chromitite and another chromitite layer lower in the stratigraphic section at the top of the lower critical zone the ug2 chromitite is well known because it is enriched in the platinum group elements and extends for nearly the entire 400 km strike length of the eastern and western limbs of the bushveld complex where we have studied the sequence in the central sector of the eastern bushveld the ug2 chromitite is embedded in a massive 25 m thick plagioclase pyroxenite consisting of 60 70 vol granular cumulus orthopyroxene with interstitial plagioclase clinopyroxene and accessory phases throughout the entire pyroxenite layer orthopyroxene exhibits no stratigraphic variations in major or minor elements mg number1 4 79 3 81 1 however the 6 m of pyroxenite below the chromitite footwall pyroxenite is petrographically distinct from the 17 m of hanging wall pyroxenite among the differences are 1 phlogopite k feldspar and quartz are ubiquitous and locally abundant in the footwall pyroxenite but generally absent in the hanging wall pyroxenite and 2 plagioclase in the footwall pyroxenite is distinctly more sodic and potassic than that in the hanging wall pyroxenite an45 60 vs an70 75 the lower critical zone chromitite is also hosted by orthopyroxenite but in this case the rocks above and below the chromitite are texturally and compositionally identical for the ug2 we interpret the interstitial assemblage of the footwall pyroxenite to represent either interstitial melt that formed in situ by fractional crystallization or chemically evolved melt that infiltrated from below in either case the melt was trapped in the footwall pyroxenite because the overlying ug2 chromitite was less permeable if this interpretation is correct the footwall and hanging wall pyroxenites were essentially identical when they initially formed however all the models of chromitite formation that call on mixing of magmas of different compositions or on other processes that result in changes in the chemical or physical conditions attendant on the magma predict that the rocks immediately above and below the chromitite layers should be different this leads us to propose that the bushveld chromitites formed by injection of new batches of magma with a composition similar to the resident magma but carrying a suspended load of chromite crystals the model is supported by the common observation of phenocrysts including those of chromite in lavas and hypabyssal rocks and by chromite abundances in lavas and peridotite sills associated with the bushveld complex indicating that geologically reasonable amounts of magma can account for even the massive 70 cm thick ug2 chromitite the model requires some crystallization to have occurred in a deeper chamber for which there is ample geochemical evidence

u pb geochronology and isotope geochemistry of the archean and proterozoic rocks of north central madagascar abstract new u pb zircon ages and sm nd and rb sr isotopic data are presented for orthogneisses from north central madagascar including ile sainte marie alaotra beforona maevatanana and ambatolampy ambatomarina a migmatite tonalite gneiss from ile sainte marie is dated precisely at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 3187 pm 2 end document ma and has a sm nd model age tdm of 3204 ma thereby establishing a middle archean age for the oldest juvenile gneisses in northeast madagascar dated orthogneisses int

madagascar a continental fragment of the paleo super dharwar craton of india the morphological kinship of madagascar to its immediate neighbors on the west african continent and east indian subcontinent during the early and middle cretaceous has been debated for the past two decades on the basis of available geologic tectonic and paleomagnetic information most of the paleoreconstructions of madagascar have shown its attachment to the east african continent we present magnetic satelite and gravity data and morphological geophysical and geotectonic similarities to hypothesize that in the period before the breakup of gondwana madagascar was a continental fragment of the paleo super dharwar craton of india

magnetization carriers and remagnetization of bedded chert abstract article history received 22 october 2010received in revised form 11 january 2011accepted 25 february 2011editor p demenocalkeywords bedded chertremanencebiogenic magnetitehematite bedded chert is the only sediment type representative of the paleozoic to early mesozoic pelagic marineenvironment becauseoftheirassociationwithophiolitesandislandarcrocks presenceofdatablemicrofossilsandpaleohorizontalreferenceprovidedbywell developedbeddingsurfaces beddedchertsequencesareoftentargeted for paleomagnetic tectonic studies however processes of magnetization acquisition in biosiliceoussediments and consequently the signi cance of their magnetic record are not well understood our rockmagnetic study of a triassic jurassic radiolarian chert sequence the mino terrane central japan shows thatthe ferrimagnetic assemblage of the gray chert units is of detrital origin while the red chert s assemblage isdominated by authigenic phases pigmentary hematite and biogenic magnetite which contribute to thenatural remanent magnetization the presence of magnetofossils places red oxic chert in the category ofprospective environmental archives magnetite producing magnetotactic bacteria were apparently able totolerate elevated concentrations of dissolved silica as well as a steep redox gradient in sedimentary pore waters during the deposition of red chert layers a strong uniaxial anisotropy due to chain alignment of thebiogenic magnetite grains likely contributes to the acquisition of anomalously stable partial thermoviscousmagnetization by chert even at low metamorphic temperatures 2011 elsevier b v all rights reserved

influence of anthropogenic combustion emissions on the deposition of soluble aerosol iron to the ocean empirical estimates for island sites in the north atlantic the results of several recent studies challenge the reigning paradigm that continental soil dust provides the only significant atmospheric source of dissolved iron to the surface ocean this evidence includes correlations between the operational solubility of aerosol iron and atmospheric loadings of black carbon and aluminum normalized vanadium and nickel each of which are associated with emissions from the combustion of fossil fuel oil these observations suggest that the relative solubility of aerosol iron hence the eolian flux of soluble iron to the surface ocean may be significantly impacted by anthropogenic oil combustion products using recent field data from the bermuda region we have developed an empirical method to estimate the solubility of aerosol iron using bulk aerosol concentrations of fe v and al we apply this method to a large body of published data from the aeroce program for north atlantic island sites on tenerife barbados bermuda and ireland where the relative proportions of anthropogenic aerosols range from minor to major respectively our aerosol iron solubility estimates suggest that anthropogenic emissions contribute approximately 70 and 85 of the annual dry deposition of soluble iron to the surface ocean near bermuda and ireland respectively implying that human activities have profoundly affected the iron budget of the north atlantic region the annual mean dry deposition of soluble iron at barbados and izana is dominated by soil dust the anthropogenic contribution at these two sites ranges from 12 to 30 and is highly dependent on the soil dust solubility of fe employed in the model the low end 12 estimate appears to be more representative of these high dust sites

iron mineralogy and speciation in clay sized fractions of chinese desert sediments iron released from asian desert dust may be an important source of bioavailable iron for the north pacific ocean and thereby may stimulate primary productivity however the fe species of the fine dusts from this source region are poorly characterized here we investigate iron species and mineralogy in the clay sized fractions 2 m the size fraction most prone to long distance transport as dust samples were analyzed by sequential chemical extraction x ray diffraction and diffuse reflectance spectrometry our results show that fe dissolved from easily reducible iron phases ferrihydrite and lepidocrocite and reducible iron oxides dominated by goethite are 0 81 wt and 2 39 wt respectively and fe dissolved from phyllosilicates extracted by boiling hcl dominated by chlorite is 3 15 wt dusts originating from deserts in northwestern china particularly the taklimakan desert are relatively enriched in easily reducible fe phases probably due to abundant fe contained in fresh weathering products resulting from the rapid erosion associated with active uplift of mountains to the west data about fe speciation and mineralogy in asian dust sources will be useful for improving the quantification of soluble fe supplied to the oceans especially in dust models

chromite in the platreef bushveld complex south africa occurrence and evolution of its chemical composition the regional distribution and chemical composition of massive and disseminated chromitites through a platreef sequence and along a strike distance of over 20 km were investigated to correlate them both within the framework of the northern limb and to the eastern and western limbs of the bushveld complex the chromitite layers and seams of the platreef form two main chromite bearing zones the upper chromitite that occurs as two to three discontinuous seams in feldspathic pyroxenite at approximately 20 m below the platreef top contact and the lower chromitite that is composed of multiple seams in feldspathic harzburgite pyroxenite and norite close to the bottom contact of the platreef with footwall electron micro probe analyses reveal that the chemical composition of chromite depends on the host rock type norite and pyroxenite host chromite with the highest cr2o3 content while harzburgite hosted chromites are cr and mg poor the wide range in chromite compositions is explained by the influence of late magmatic processes including post cumulus growth and re equilibration interaction with fluid and sulphide saturated magmatic liquid and contact metamorphism each of these processes is characterised by its own distinct geochemical signature but generally they lead to a decrease in mg and al and an increase in both di and tri valent fe in the chromite the occurrence of chromitite locally on the different distance from the contact between the upper platreef sills and the overlying main zone magma suggests erosion of the upper platreef by the main zone as it was emplaced the localisation of chromitites supports an independent development of the northern limb during the critical zone emplacement although the chemical composition of chromite and co existing silicates from ultramafic rocks suggest a critical zone affinity with the eastern and western limbs of the bushveld complex

transport and reaction of iron and iron stable isotopes in glacial meltwaters on svalbard near kongsfjorden from rivers to estuary to ocean abstract glacial meltwater has been suggested as a significant source of potentially bioavailable iron to the oceans however the supply of dissolved iron dfe in glacial meltwaters is poorly constrained as few sites have been studied and because the chemical processing of fe during transport from glaciers to the adjacent coastal ocean is not well understood in order to better constrain glacial fluxes of dfe to the ocean iron concentrations iron stable isotopes 56fe and other supporting chemical and physical measurements were made along a 4 km long glacial meltwater river on svalbard and in estuarine waters that it flows into dissolved iron concentrations in the bayelva river decreased from a maximum of 734 nm near the glacier to an average value of 116 nm near the mouth of the river measurements in the kongsfjorden estuary suggest that 3 to 10 nm of dfe from the bayelva river is stabilized in glacial waters by the time it mixes into the ocean incubation of bayelva river waters over two weeks in both the light and dark show similar results with the majority of dfe being quickly precipitated and 4 to 7 nm fe stabilized in the dissolved phase evidence suggests that fe is most likely lost from the dissolved phase by aggregation and adsorption of nanoparticulate and colloidal fe to particles dissolved 56fe was between 0 11 and 0 09 for all river samples and did not vary systematically with dfe concentrations we infer that the fe is lost from the dissolved phase by a process that fractionates fe isotopes by less than 0 05 indicating that the fe bonding environment does not change during precipitation this is consistent with doc loss that is much faster than predicted photo oxidation rates suggesting that doc is also lost through adsorption and precipitation dissolved fe concentrations in the bayelva river 15 734 nm and fe concentrations which are stabilized in the dissolved phase 4 7 nm are much lower than some previous estimates of fe in glacial meltwaters with roughly 80 of dfe lost during transit in the bayelva river and roughly 90 of the remaining dfe lost in the estuary this may mean that glaciers are a less significant source of dissolved fe to the global oceans than has been previously hypothesized that cold base glaciers of the type studied here do not contribute significantly to the dissolved fe flux or that the flux of reactive particulate fe to the oceans is more important than the dissolved flux in arctic regions with similar proglacial environments bedrock composition weathering intensity and as precipitation of colloidal and nanoparticulate fe may all play an important role in regulating the glacial meltwater iron flux to the ocean

osmium isotope systematics of the proterozoic and phanerozoic ophiolitic chromitites in situ ion probe analysis of primary os rich pgm in situ 187 os 188 os ratios are determined on os rich platinum group minerals in podiform chromitites both in the proterozoic ophiolite eastern desert egypt and in the phanerozoic oman ophiolite because they have very low re os these primary minerals reflect the initial 187 os 188 os ratios of their parental magmas the platinum group minerals pgm in the central eastern desert chromitites exhibit sub chondritic to chondritic 187 os 188 os ratios 0 1226 on average which is lower than the primitive upper mantle evolution trend of a comparable age those of the southern eastern desert chromitites have more radiogenic os with supra chondritic 187 os 188 os ratio of about 0 1293 on average which could be due to crustal contamination the three chromitite types in the northern part of the oman ophiolite are almost indistinguishable in terms of their 187 os 188 os ratios they have overlapping values ranging from sub chondritic to supra chondritic ratios the pge rich mantle chromitite samples have a wide range of 187 os 188 os ratio from 0 1230 up to 0 1376 with an average of 0 1299 the values of the pge poor mantle chromitites overlap in their 187 os 188 os ratios with pge rich chromites but are less variable and have a significantly higher average ratio the moho transition zone mtz chromitites are highly variable in the 187 os 188 os ratio ranging from 0 1208 up to 0 1459 the wide range of 187 os 188 os ratios from 0 1192 to 0 1459 in platinum group minerals in egyptian and oman ophiolites can be attributed to the diversity of origin of their podiform chromitites the os isotope data combined with spinel chemistry indicate that the way involved in podiform chromitite formation was not substantially different between the proterozoic ophiolite of egypt and the phanerozoic ophiolite in northern oman the os isotope compositions of the mantle chromitites in the proterozoic ophiolite of egypt clearly suggest crustal contamination the heterogeneity of 187 os 188 os ratios combined with the spinel chemistry and high pge contents of the pge rich chromitite in the oman ophiolite may give reliable evidence for high degree partial melting at a supra subduction zone setting crustal contamination from the subducted slab and assimilation of previously altered lower crustal gabbro may have contributed to the high cr spinel and radiogenic os characteristics in chromitite formed in the mantle section and along the moho transition zone respectively

toward understanding the dust deposition in antarctica during the last glacial maximum sensitivity studies on plausible causes 1 understanding the plausible causes for the observed high dust concentrations in antarctic ice cores during the last glacial maximum lgm is crucial for interpreting the antarctic dust records in the past climates and could provide insights into dust variability in future climates using the geophysical fluid dynamics laboratory gfdl general circulation models we conduct an investigation into the various factors modulating dust emission transport and deposition with a view toward an improved quantification of the lgm dust enhancements in the antarctic ice cores the model simulations show that the expansion of source areas and changes in the antarctic ice accumulation rates together can account for most of the observed increase of dust concentrations in the vostok dome c and taylor dome cores but there is an overestimate of the lgm present ratio in the case of the byrd core the source expansion due to the lowering of sea level yields a factor of 2 3 higher contribution than that due to the reduction of continental vegetation the changes in other climate parameters e g sh precipitation change are estimated to be relatively less important within the context of this sensitivity study while the model simulated lgm surface winds yield a 20 30 reduction rather than an increase in dust deposition in antarctica this research yields insights toward a fundamental understanding of the causes for the significant enhancement of the dust deposition in the antarctic ice cores during the lgm

atmospheric iron deposition in the northwestern pacific ocean and its adjacent marginal seas the importance of coal burning this study applied a regional air quality model incorporated with an emission module to quantitatively differentiate the atmospheric iron sources originating from lithogenic dusts or coal burning fly ashes deposited in the northwestern pacific ocean and its marginal seas particular attention was paid to the high iron content of fly ashes emitted from steel and iron plants burning coals using the year 2007 as an example the modeling results exhibit large seasonal variations in iron deposition with highest deposition fluxes occurred during spring and autumn which are comparable to the seasonal fluctuation of chlorophyll a concentrations estimated by satellite images in the oceanic regions fly ash from coal burning accounted for 7 2 of the total iron deposited over the northwestern pacific ocean and 15 of that over the northern south china sea after considering the difference of iron solubility in the aerosols anthropogenic aerosol associated with coal burning would be the major bioavailable iron source in the surface water of the oceanic regions

constraints on the formation of platinum group element deposits in igneous rocks mechanisms of concentration of the platinmn group and other chalcophile elements iu several platinum group element deposits in layered intrusions are evaluated in light of recently published data on element partitioning between immiscible sulfide and silicate melts the similarity of the partition coefficieut d super sulfide melt silicate melt of ir to that of pd indicates that the observed fractionation of these elements in nature resulted from a process other than or in addition to sulfide liquid immiscibility in experiments designed to test the possibility that this fractionation was due to the presence of os ir alloy sulfide and silicate melts were equilibrated with ir metal at 1 450 degrees c 8 kbars total pressure and under sulfur and oxygen fugacity conditions appropriate for nature the ir content of the sulfide liquid was found to be or 3 7 wt percent and that of the coexisting silicate melt inferred to be or 1 000 ppb these concentrations are far in excess of the ir contents of natural sulfides or rocks and indicate that os ir alloys are not stable in the presence of sulfide or silicate melts in nature using our preferred values of d super sulfide melt silicate melt for the chalcophile elements and their estimated concentrations in the mantle limits are determined for the possible ranges of concentrations and relative abundances of these elements in immiscible sulfide liquid formed in magma derived by partial melting of the mantle the ratios of highly chalcophile to moderately chalcophile elements and of metal to sulfur of several deposits including the j m reef stillwater complex the ug 2 bushveld complex and the lac des isles complex are inconsistent with concentration solely by segregation of sulfide liquid in contrast the merensky reef fits the geochemical criteria for formation by accumulation of magmatic sulfide stratigraphic variations in chalcophile element concentrations through the ug 2 and the main sulfide zone of the great dyke also indicate that metals must have been redistributed on at least the meter scale our analysis implies either different relative mobilities of the chalcophile elements relative to one another in a process postdating initial sulfide liquid deposition or concentration by a different mechanism

mineralogy as a critical factor of dust iron solubility 1 to understand the impact of dust deposition on ocean primary productivity the evaluation of dust input is not sufficient and the soluble iron supply is also needed measurements of iron in aerosols suggest a much higher solubility than in soil implying substantial atmospheric processing of chemical or physical nature here we have investigated the effect of the mineralogy on iron solubility we have measured iron solubility from some minerals commonly found in dust clay feldspar and iron hydr oxides firstly we observe a greater solubility of iron resulting from clays 4 in comparison to iron hydr oxides 1 independently of specific surface of minerals secondly by considering amount of dissolved iron and the mineralogical composition of dust our results indicate that dissolved iron fraction mainly comes from clay dissolution in contrast to that assumed in some biogeochemical models in consequence it appears that iron solubility is closely linked to the mineralogical composition of aerosol

biochemical vs detrital mechanism of remanence acquisition in marine carbonates a lesson from the k t boundary interval abstract an apparently complete carbonate rich cretaceous tertiary boundary interval in odp section 119 738c 20r 5 from the southern kerguelen plateau provides a unique insight into processes of magnetization acquisition in marine carbonates the boundary interval is characterized by a 1 m thick clay rich zone distinct depositional lamina are preserved within the basal 15 cm of this zone the upper part is bioturbated previous studies have demonstrated that the bulk of the detrital fraction in the laminated and bioturbated carbonates has the same local source and hence the two intervals likely had similar initial detrital assemblages magnetic properties of these rocks however differ significantly the laminated sediments have a higher content of non silicate bound iron yet approximately an order of magnitude lower intensity of the natural remanent magnetization compared to the bioturbated rocks our detailed rock magnetic study indicates that in the bioturbated interval the dominant iron bearing phase is single domain magnetite likely of biogenic origin in the laminated interval apart from a small ferromagnetic fraction with multi domain like behavior non silicate bound iron is mainly sequestered in paramagnetic phases likely poorly crystalline oxyhydroxides it appears that a shut down of biological productivity after the k t event allowed preservation of the initial detrital early authigenic iron phases that are dominated by reactive iron oxyhydroxides with recovery of the normal biological activity as evidenced by resumption of bioturbation the oxyhydroxides had been replaced with biogenic magnetite thus produced biochemical magnetization led to a several fold increase in the remanence our results suggest that in areas where bioavailable iron constitutes a significant part of the detrital input such as in pelagic marine environments distant from clastic sources the biochemical remanent magnetization may be the dominant process of magnetization acquisition

spatial and particle size distributions of atmospheric dissolvable iron in aerosols and its input to the southern ocean and coastal east antarctica 1 to characterize atmospheric dissolvable iron over the southern ocean so and coastal east antarctica cea bulk and size segregated 0 056 18 m in diameter aerosols were collected from 34 s 109 e to 69 s 76 e and between 69 s 76 e and 66 s 110 e during a cruise during november 2010 to march 2011 aerosols were analyzed for total dissolvable fe and fe ii by uv visible spectroscopy and total fe by inductively coupled plasma mass spectrometry the average concentrations of total fe were 19 ng m 3 range 10 38 ng m 3 over the so and 26 ng m 3 range 14 56 ng m 3 in cea the average fe ii concentrations were 0 22 ng m 3 range 0 13 0 33 ng m 3 over the so and 0 53 ng m 3 range 0 18 1 3 ng m 3 over cea the total dissolvable fe followed the same trend over the so a single peak size distribution of fe ii existed over cea a bimodal size distribution of fe ii appeared with the first peak at 0 32 0 56 m and the second peak at 5 6 10 m higher fe concentrations over cea than over the so and the existence of coarse mode fe ii over cea suggest potential dust sources in antarctica the fractional fe ii solubility ranged from 0 58 to 6 5 and decreased with total fe concentration increase the estimated atmospheric fluxes of fe ii were 0 007 0 092 mg m 2 yr 1 over the so and 0 022 0 21 mg m 2 yr 1 in cea total dissolvable fe fluxes were 0 007 0 52 mg m 2 yr 1 over the so the atmospheric dissolvable fe input contributes to the dissolved fe pool in the so surface waters

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

mesoproterozoic ophiolitic melange from the se periphery of the indian plate u pb zircon ages and tectonic implications abstract the kanigiri melange within the proterozoic nellore khammam schist belt in southern peninsular india includes ophiolitic fragments that represent the remnants of an oceanic plate the ophiolitic units were accreted along a ne trending suture that juxtaposes the proterozoic eastern ghats granulite belt eggb against the archean nellore schist belt of the dharwar craton the ophiolite components in the kanigiri melange include plagiogranites and gabbros which show mutually intrusive relations indicating their coeval nature we report laser ablation icp ms age data and ree geochemistry of zircons from the gabbro and granite the zircons from both gabbro and granite show high ree contents prominent hree enrichment and a conspicuous negative eu anomaly suggesting a common melt source zircon ree abundances and normalized patterns show little intersample and intrasample variations u pb dating of the zircons reveals prominent mesoproterozoic ages for the plagiogranite with the ca 1 33 ga age of the kanigiri ophiolitic melange offering important clues for arc continent collision during the final stages of amalgamation of the columbia derived fragments within the neoproterozoic supercontinent assembly

characteristics of seismic noise fundamental and higher mode energy observed in the northeast of the netherlands we study seismic noise recorded in the northeast of the netherlands by beamforming and by using empirical green s functions obtained by seismic interfero metry from beamforming we found differences in noise directions in different fre quency bands the main source region for primary microseisms 0 05 0 08 hz is in the west northwest direction while the secondary microseisms 0 1 0 14 hz have a west southwest back azimuth furthermore we observed a fast 4 km s arrival corresponding to the rayleigh wave overtone this arrival is also in the secondary microseism band between 0 15 and 0 2 hz but has a west northwest back azimuth both arrivals in the secondary microseism band gain in strength during winter as does the average wave height in the north atlantic we measured phase velocity dispersion curves from both beamforming and noise cross correlations as well as group velocity from the latter these are then jointly inverted for an average 1d s wave model the results show how the combination of different methods leads to a more complete char acterization of the propagation modes and an improved knowledge of the subsurface especially as the group velocity measurements increase the upper frequency limit of analysis providing valuable information of the shallowest subsurface

microseisms formed off the coast of norway abstract microseisms recorded at many stations in eastern europe during october 1983 could be associated with storms off the coast of norway it is the object of the present investigation to ascertain how these microseisms could be formed from the wind data the three dimensional wave spectra were hindcasted these showed no evidence of waves travelling in opposite directions over this area and so any such effect must be due to reflection off the coast the energies of the wave components within 22 5 to the normal to the mean coastline were evaluated from the hindcast spectra their variation showed a good correspondence with both the double period and the amplitude of the observed microseisms the bottom topography of the norwegian coast was investigated and although the actual coastline is very indented the neighbouring shelf contours are much more regular and show a very steep gradient it is possible to calculate the reflection coefficients due to the depth variation offshore and these proved to be quite appreciable being of the order of 20 with values of this order there is no difficulty in explaining how microseisms of the observed magnitude could be caused

biogeochemistry of iron in australian dust from eolian uplift to marine uptake 1 dust is an important vector for iron supply to the ocean which subsequently impacts ocean productivity atmospheric co2 concentrations and hence global climate here we synthesize the processes influencing the biogeochemistry of australian dust and compare them with those from other southern hemisphere dust sources our observations range from soil and dust physical properties to abrasion and cloud chamber chemistry experiments to dust storms and their dispersion and deposition we then present satellite observations of the impact of episodic dust deposition events on the productivity of low iron oceanic waters north i e low nitrate low chlorophyll lnlc and south i e high nitrate low chlorophyll hnlc of australia dust deposition from the largest dust storm in over 40 years did not result in iron mediated algal blooms in either oceanic region a comparison of australia with other southern hemisphere source regions reveals that the relatively well sampled australian system is a poor generic model furthermore there are marked distinctions between southern and northern hemisphere iron dust biogeochemistry that must be recognized by modelers and included in future simulations better information is required on the relative role of the atmosphere and ocean on influencing iron biogeochemistry and how their relative influences might change in the future due to climate change

influence of the seismic noise characteristics on noise correlations in the baltic shield it has recently been shown that correlations of seismic noise can contain significant information about the green s function along the station profile using an array of 38 temporary broad band stations located in finland between 1998 september and 1999 march we study the resulting 703 noise correlations to understand how they are influenced by the directivity of the noise field the latter information is obtained through f k analysis of data from two permanent seismic arrays in germany and norway and from a subset of stations of the array in finland both types of analysis confirm that the characteristics of the seismic noise is strongly frequency dependent at low frequencies 0 02 0 04 hz we observe diffuse noise and or randomly distributed sources in contrast the noise is strongly direction dependent and not fully diffuse in the intermediate period ranges 0 04 0 25 hz which correspond to the first and second microseismic peak created at the irish and scottish coast and the western coast of norway in this frequency interval the noise is sufficiently close to a plane wave to introduce systematic errors in group velocities for station pairs which are not parallel to the direction of the dominant incident noise phase velocities calculated by slant stack over many traces are however correct independently of profile direction in the high frequency band 0 25 1 0 hz the situation is a mix between the low frequency and the intermediate frequency cases average phase velocities and individual group velocities from well oriented profiles are in excellent agreement with results from rayleigh wave studies of the area

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

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

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

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

a further investigation of microseisms recorded in north wales abstract investigation of microseisms and their origin has been continued in the previous investigation a vertical seismograph only was used and so it was not possible to study phase relationships between components a north south seismograph has now been used and it has been possible to relate the phase difference between the north south and vertical seismograph to the position of the microseism generating areas described in the previous paper in december 1989 there was a long period of intense coastal activity in the southwest sector bristol channel during this time the north south components were consistently 90 behind the vertical in january however the position changed as the maximum activity came from the northwest sector north channel and then the north south components were 90 ahead of the vertical these phase relationships became apparent only after prolonged activity in one sector only the situation is usually very confused when there is much activity in both sectors but occasionally one can observe the phase difference swinging round as a storm goes from south to north these phase relationships indicate that the microseisms have a strong rayleigh wave component it is usually very difficult to identify microseism caused by storms in mid atlantic even when there are areas of appreciable wave interference because they are often overshadowed by those caused by coastal waves however during 11 13 november 1989 there were two storms about 1000 and 2000 km to the southwest of the british isles where wind components in a sector of one storm were directly opposed to those in a sector of the other giving rise to waves moving in opposite directions in areas between them the storms were of only medium intensity giving rise to waves of a 10 12 s period a corresponding increase in microseism activity of a 5 6 s period was apparent at this time and as there was very little coastal activity there could be no ambiguity about the origin of these microseisms

impact of anthropogenic combustion emissions on the fractional solubility of aerosol iron evidence from the sargasso sea we report empirical estimates of the fractional solubility of aerosol iron over the sargasso sea during periods characterized by high concentrations of saharan dust summer 2003 and by low concentrations of aerosols in north american maritime north atlantic air masses spring 2004 and early summer 2004 we observed a strong inverse relationship between the operational solubility of aerosol iron defined using a flow through deionized water leaching protocol and the total concentration of aerosol iron whereby the operational solubility of aerosol iron was elevated when total aerosol iron loadings were low this relationship is consistent with source dependent differences in the solubility characteristics of our aerosol samples and can be described by a simple mixing model wherein bulk aerosols represent a conservative mixture of two air mass end members that carry different aerosol types saharan air which contains a relatively high loading of aerosol iron 27 8 nmol fe m 3 that has a low fractional solubility 0 44 and north american air which contains a relatively low concentration of aerosol iron 0 5 nmol fe m 3 that has a high fractional solubility 19 historical data for aerosols collected on bermuda indicate that the low iron loadings associated with north american air masses are typically accompanied by elevated v al fe al and v mn mass ratios in the bulk aerosol relative to saharan dust which are indicative of anthropogenic fuel combustion products the identification of similar compositional trends in our sargasso sea aerosol samples leads us to suggest that the elevated solubility of iron in the aerosols associated with north american air masses reflects the presence of anthropogenic combustion products which contain iron that is readily soluble relative to iron in saharan soil dust we thus propose that the source dependent composition of aerosol particles specifically the relative proportion of anthropogenic combustion products is a primary determinant for the fractional solubility of aerosol iron over the sargasso sea this hypothesis implies that anthropogenic combustion emissions could play a significant role in determining the atmospheric input of soluble iron to the surface ocean

distribution transport and deposition of mineral dust in the southern ocean and antarctica contribution of major sources 1 a model based investigation of the transport distribution and deposition of mineral dust in the southern hemisphere sh is performed by using the gfdl atmospheric model am2 the study represents an attempt to quantify the contribution of the major sources by tagging dust based on its origin we evaluate the contribution of each source to the emission distribution mass burden and deposition of dust in the southern ocean and antarctica and show that each source produces distinctive meridional transport vertical distribution and deposition patterns the dust in sh originates primarily from australia 120 tg a 1 patagonia 38 tg a 1 and the inter hemispheric transport from northern hemisphere 31 tg a 1 a small fraction of it 7 tg a 1 is transported and deposited in the southern ocean and antarctica where dust from south america australia and northern hemisphere are essentially located in the boundary layer mid troposphere and upper troposphere respectively these three sources contribute to nearly all the dust burden in the southern ocean and antarctica south america and australia are the main sources of the dust deposition but they differ zonally with each one dominating half of a hemisphere along 120 e 60 w the half comprising the atlantic and indian oceans in the case of the south american dust and the pacific half in the case of the australian dust our study also indicates a potentially important role of northern hemisphere dust as it appears to be a significant part of the dust burden but contributing little to the dust deposition in antarctica

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 element mineralization in the critical zone of the western bushveld complex i sulfide poor chromitites below the ug 2 primary stratiform platinum group element pge mineralizations associated with chromitites in the critical zone of the bushveld complex south africa are confined to cumulate layers that are regionally persistent over tens of kilometers four categories ofchromitite are recognized layers at the bases of cycles in the lower critical zone type i layers at the bases of cycles in the upper critical zone type ii thin layers in the intermediate parts of cycles type iii and stringers associated with orthopyroxene pegmatoids type iv these latter may not be of primary magmatic origin nevertheless probably each type of chromitite contains anomalous concentrations of pge chromitites at the bases of cycles comprising the lower group lg middle group mg and upper group ug are further categorized into types ia lg 1 lg 4 ib lg 5 mg 1 iia mg 2 ug 1 and iib ug 2 and above on the basis of lithostratigraphy chromite and pge chemistry and sulfide content type iib chromitites contain significant quantities of sulfides and are not investigated here new whole rock and electron microprobe chemical analyses of the sulfide poor type ia ib and iia chromitites cu contents 1 8 low pge contents 2 type ib chromitites occur in pyroxenite cycles and form an intermediate group these regular variations with increased height are interpreted as a differentiation trend pge are concentrated in sulfide poor chromitites as a result of chromitite control a process not fully understood but thought to involve a combination of direct nucleation of platinum group minerals and localized s saturation it is the crystallization of copious quantities of chromite from hybridized melts close to the crystal liquid interface that triggers this process but magma mixing essential to drive the melt into the chromite field is the overall cause replenishment of the magma chamber was from a single magma batch because the chromitites below the ug 2 are not associated with major isotopic inflections huge volumes of dense hot magma were fed as basal flows along the crystal liquid interface the magma chamber was compartmentalized at an early stage possibly in response to basement tectonics and lateral mixing over distances of 150 km is an alternative mechanism for achieving sufficiently high r values repeated replenishment of the magma chamber resulted in mixing with resident liquid that became increasingly differentiated with height type ia chromitites are ascribed to replenishment and mixing with the relatively primitive lower zone residue type ib crystallized as a result of mixing with lower critical zone resident liquid and type iia due to mixing with more evolved upper critical zone resident liquid differentiation of pge with height is a critical part of the process which culminates in the occurrence of higher grade pge mineralization in sulfide bearing chromitites ug 2 merensky reef in the uppermost part of the upper critical zone

platinum group element chromitite associations in the bushveld complex new and recently published platinum group element data of chromite rich layers in the bushveld complex are evaluated chondrite normalized platinum group element plots of these data show that trends for chromitite layers differ from that of the merensky reef the data furthermore indicate remarkable consistencies in compositional trends for many kilometers along strike of individual layers suggesting that the conditions of formation of chromitite layers and their associated platinum group element mineralization were very uniform over large areas of the magma chamber significant differences in the proportions of platinum group elements within a particular layer are only evident on a regional scale the platinum group minerals of the middle group of chromitite layers in the marikana area of the western bushveld are mainly sulfides and arsenides laurite is the most common platinum group mineral and is mostly enclosed by chromite a variety of other platinum group minerals occur in close association with base metal sulfides extremely low bulk sulfur content in the chromitite layers is ascribed to a postcumulus loss of sulfur postcumulus loss of sulfur and the existence of laurite enclosed within chromite casts doubt on the validity of any petrogenetic models for platinum mineralization in chromitite rich layers based on partition coefficients and the r factor

mid ludfordian coeval carbon isotope natural gamma ray and magnetic susceptibility excursions in the mielnik ig 1 borehole eastern poland dustiness as a possible link between global climate and the silurian carbon isotope record abstract the graptolite bearing upper silurian succession of the mielnik ig 1 borehole eastern poland represents a periplatform setting on a neritic carbonate platform located on the palaeocontinent of baltica and records widely recorded positive mid ludfordian carbon isotope excursion cie the initial increase of 13 c values occurs in the uppermost part of the neocucullograptus kozlowskii zone whereas pronounced increase and maximal values 6 74 are noted in the n kozlowskii pseudomonoclimacis latilobus interzone pristiograptus dubius bloom interval the thickness time relation in the section indicates that the main initial shift in carbon isotope ratio of 2 7 occurred over a very short time estimated to be around 4000 years and is coeval with a rapid initiation of deposition of rock forming silt sized detrital dolomite and quartz the cie decline is interpreted as steeper than in inner shelf sections and is coincident with a flooding event in the ps latilobus slovinograptus balticus zone whereas the end of the excursion is coeval with the next flooding surface marked by the ps latilobus uncinatograptus acer graptolite assemblage combined sgr spectral gamma ray facies and faunal records during the cie suggest prolonged sea level lowstand conditions the abundant detrital material dolomite siltstone facies occurring exclusively in the cie interval is noted in microfacies as well as in the geophysics record as pronounced negative excursions in total natural gamma ray and magnetic susceptibility both resulting from dilution by diamagnetic and low radioactivity minerals the perfectly sorted vertically equally sized silt material contains unique constant and low diversified grain assemblage euhedral dolomite quartz minute ooids suggesting its provenance exclusively from the shallower and or emerged parts of the carbonate platform interior the spot findings of graptolites and nautiloids along with delicate parallel lamination of sediments indicates relatively deep environment with absence of traction processes these features along with the lack of dolomite detritus in accompanying redeposited carbonates suggest its derivation by wind with regard to the earlier noted presence of aeolian detritus in the middle ludfordian we suggest dustiness as a plausible driving force of the cie in the hypothesis proposed bioavailable fe bearing dust inflow in concert with antiestuarine circulation on shelfs caused nutrient n p ratio disturbances resulting in methanogenesis in surface layer of the seas the surface layer methanogenesis should resulting in bathymetric gradient of isotope fractionation what is consistent with the cie record coincidence of sea level lowstand low latitude aridity and gustiness of the global climate are in accordance with the hypothetical short lived glaciation in mid ludfordian time

the iron isotopic composition of subglacial streams draining the greenland ice sheet abstract in this study we present the first measurements of iron fe stable isotopic composition 56 fe of subglacial streams draining the greenland ice sheet gis we measure the 56 fe values 56 fe 56 fe 54 fe sample 56 fe 54 fe standard 1 10 3 of both dissolved and suspended sediment fe in subglacial outflows from five distinct land terminating glaciers suspended sediments have 56 fe values that lie within the crustal array 56 fe 0 in contrast the 56 fe values of dissolved fe in subglacial outflows are consistently less than 0 reaching a minimum of 2 1 in the outflow from the russell glacier the 56 fe values of dissolved fe vary geographically and on daily time scales major element chemistry and mineral saturation state modeling suggest that incongruent silicate weathering and sulfide oxidation are the likely drivers of subglacial stream fe chemistry and that the extent of chemical weathering influences the 56 fe of dissolved fe the largest difference in 56 fe between dissolved and suspended load is 2 1 and occurs in the subglacial system from the russell glacier southwest gis major element chemistry indicates this outflow to be the least chemically weathered while more mature subglacial systems i e that exhibit greater extents of subglacial weathering have dissolved loads with 56 fe that are indistinguishable from suspended sediments 56 fe suspended dissolved 0 ultimately the dissolved fe generated in some subglacial systems from the gis is a previously unrecognized source of isotopically light fe into the hydrosphere the data illustrate that the dissolved fe supplied by subglacial weathering can have variable 56 fe values depending on the degree of chemical weathering thus fe isotopes have potential as a proxy for subglacial chemical weathering intensity or mode finally based on our regional fe concentration measurements from each glacial outflow we estimate a flux weighted continental scale dissolved iron export of 2 1 gg fe yr 1 to the coastal ocean which is within the range of previous estimates

dust deposition in the eastern indian ocean the ocean perspective from antarctica to the bay of bengal atmospheric deposition is an important but still poorly constrained source of trace micronutrients to the open ocean because of the dearth of in situ measurements of total deposition i e wet dry deposition in remote regions in this work we discuss the upper ocean distribution of dissolved fe and al in the eastern indian ocean along a 95 e meridional transect spanning the antarctic margin to the bay of bengal we use the mixed layer concentration of dissolved al in conjunction with empirical data in a simple steady state model to produce 75 estimates of total dust deposition that we compare with historical observations and atmospheric model estimates except in the northern bay of bengal where the ganges brahmaputra river plume contributes to the inventory of dissolved al the surface distribution of dissolved al along 95 e is remarkably consistent with the large scale gradients in mineral dust deposition and multiple source regions impacting the eastern indian ocean the lowest total dust deposition fluxes are calculated for the southern ocean 66 60 mg m 2 yr 1 and the highest for the northern end of the south indian subtropical gyre up to 940 mg m 2 yr 1 at 18 s and in the southern bay of bengal 2500 570 mg m 2 yr 1 our total deposition fluxes which have an uncertainty on the order of a factor of 3 5 are comparable with the composite atmospheric model data of mahowald et al 2005 except in the south indian subtropical gyre where models may underestimate total deposition using available measurements of the solubility of fe in aerosols we confirm that dust deposition is a minor source of dissolved fe to the southern ocean and show that aeolian deposition of dissolved fe in the southern bay of bengal may be comparable to that observed underneath the saharan dust plume in the atlantic ocean

the age of iron and iron source attribution in the ocean we use tracers to partition dissolved iron dfe into the contributions from each source within a numerical model of the iron cycle without perturbing the system these contributions are further partitioned according to the time since injection into the ocean which defines their iron age spectrum and mean iron age the utility of these diagnostics is illustrated for a family of inverse model estimates of the iron cycle constrained by a data assimilated circulation and available dfe measurements the source contributions are compared with source anomalies defined as the differences between solutions with and without the source in question we find that in the southern ocean euphotic zone the hydrothermal and sediment contributions range from 15 to 30 of the total each which the anomalies underestimate by a factor of 2 because of the nonlinearity of scavenging the iron age is only reset by scavenging and attains a mean of several hundred years in the southern ocean euphotic zone revealing that aeolian iron there is supplied primarily from depth as regenerated dfe tagging iron according to source region and pathways shows that 70 80 of the aeolian dfe in the euphotic zone near antarctica is supplied from north of 46 s via paths that reach below 1 km depth hydrothermal iron has the oldest surface mean ages on the order of mid depth ventilation times a measure of uncertainty is provided by the systematic variations of our diagnostics across the family of iron cycle estimates each member of which has a different aeolian source strength

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

the contribution of aeolian sand and dust to iron fertilization of phytoplankton blooms in southwestern ross sea antarctica iron is a limiting micronutrient for primary production in the ross sea antarctica recent observations reveal low dissolved fe dfe concentrations in the ross sea polynya following high initial rates of primary production in summer after the dfe winter reserve has been consumed significant new sources of dfe are therefore required to further sustain phytoplankton blooms iron from aeolian sand and dust asd released from melting sea ice is one potential source to constrain aeolian fe inputs we determined asd mass accumulation rates and the total and soluble fe content of asd on sea ice in mcmurdo sound southwestern sw ross sea the mean mass accumulation rate was 1 5 g m 2 yr 1 total fe content of this asd was 4 1 wt and the percentage of soluble fe was 11 1 our mean estimate of the bulk aeolian dfe flux of 122 1 mol m 2 yr 1 for the mcmurdo sound region suggests that aeolian fe can support between 9 0 109 and 4 1 1011 mol c yr 1 0 1 4 9 tg c yr 1 of new primary production this equates to only 15 of new primary production in the sw ross sea suggesting that aeolian dfe is a minor component of seasonal fe supply the very high asd accumulation on sea ice in mcmurdo sound compared to other regions of antarctica suggests that our results represent the upper limit of dfe supply to the ocean from this source in the ross sea

aeolian input of bioavailable iron to the ocean 1 atmospheric deposition of mineral dust supplies much of the essential nutrient iron to the ocean presumably only the readily soluble fraction is available for biological uptake previous ocean models assumed this fraction was constant here the variable solubility of fe in aerosols and precipitation is parameterized with a two step mechanism the development of a sulfate coating followed by the dissolution of iron hydr oxide on the dust aerosols the predicted soluble fe fraction increases with transport time from the source region and with the corresponding decrease in dust concentration the soluble fraction is 1 percent near sources but often 10 40 percent farther away producing a significant increase in soluble fe deposition in remote ocean regions our results may require more rapid biological and physicochemical scavenging of fe than used in current ocean models we further suggest that increasing so2 emission alone could have caused significant fe fertilization in the modern northern hemisphere oceans

iron in east antarctic snow implications for atmospheric iron deposition and algal production in antarctic waters to evaluate the deposition and solubility of aerosol iron in the antarctic seasonal sea ice zone ssiz iron was measured in snow samples collected from three areas in the ssiz prydz bay dumont d urville sea and ross sea and one continental area princess elizabeth land of east antarctica concentrations of total dissolvable iron that soluble at ph 2 ranged from 20 2950 pg g 1 with the lowest concentrations measured in snow from the dumont d urville sea using estimates of snow accumulation rates we calculate atmospheric iron deposition fluxes of 0 017 0 11 mg m 2 yr 1 0 30 2 0 mol m 2 yr 1 which are generally lower than previously published estimates measurements of iron in filtered meltwaters of snow samples from prydz bay and princess elizabeth land suggest that 10 90 of the total atmospheric iron is readily soluble assuming our results to be broadly representative of atmospheric deposition over seasonally ice covered high nutrient antarctic waters we use our mean estimates of atmospheric iron deposition 1 1 mol m 2 yr 1 and solubility 32 to calculate that atmospheric iron potentially supports annual phytoplankton production of 1 1 1012 mole c in the antarctic ssiz which is less than 5 of the estimated total annual primary production in this ocean region

diversity of platinum group mineral assemblages in banded and podiform chromitite from the kraubath ultramafic massif austria evidence for an ophiolitic transition zone we report highly unusual platinum group mineral pgm assemblages from geologically distinct chromitites banded and podiform of the kraubath massif the largest dismembered mantle relict in the eastern alps the banded chromitite has a pronounced enrichment of pt and pd relative to the more refractory platinum group elements pges of the ipge group os ir ru similar to crustal sections of ophiolites on the contrary the podiform chromitite displays a negatively sloping chondrite normalised pge pattern typical of ophiolitic podiform chromitite the chemical composition of chromite varies from cr 73 77 in the banded type to 81 86 in the podiform chromitite thirteen different pgms and one gold rich mineral are first observed in the banded chromitite the dominant pgm is sperrylite 53 of all pgms which occurs in polyphase assemblages with an unnamed pt base metal bm alloy and pd rich minerals such as stibiopalladinite mayakite mertieite ii unnamed pd rh as and pd pt as sb minerals this banded type also contains pge sulphides about 7 represented by a wide compositional range of the laurite erlichmanite series and irarsite 8 os ir alloy geversite an unnamed pt pd bi cu phase and tetrauricupride are present in minor amounts by contrast the podiform chromitite which yielded 21 different pgms is dominated by laurite 43 of all pgms which occurs in complex polyphase assemblages with pge alloys ir os os ir pt fe pge sulphides kashinite bowieite cuproiridsite cuprorhodsite unnamed fe cu ir rh 2s4 braggite unnamed bm ir and bm rh sulphides and pd telluride keithconnite a variety of pge sulpharsenides 33 including irarsite hollingworthite platarsite ruarsite and a number of intermediate species have been identified whereas sperrylite and stibiopalladinite are subordinate 2 the occurrence of such a wide variety of pgms from only two 2 5 kg chromitite samples is highly unusual for an ophiolitic environment our novel sample treatment allowed to identify primary pgm assemblages containing all six pges in both laurite dominated podiform chromitite as well as in uncommon sperrylite dominated banded chromitite we suggest that the geologically geochemically and mineralogically distinct banded chromitite from kraubath characterises the transition zone of an ophiolite closely above the mantle section hosting podiform chromitite rather than being representative of the crustal cumulate pile

gabbro akarem eastern desert egypt cu ni pge mineralization in a concentrically zoned mafic ultramafic complex the gabbro akarem late precambrian intrusion is concentrically zoned with a dunite core surrounded by lherzolite clinopyroxenite enveloped by olivine plagioclase hornblendite and plagioclase hornblendite cu ni pge mineralization is closely associated with peridotite especially in the inner olivine rich core dunite pipes where net textured and massive sulfides pyrrhotite pentlandite chalcopyrite are found in association with al mg rich spinel and cr magnetite primary magmatic textures are well preserved however deformation and mobilization due to shearing are locally observed platinum group minerals pgm documented from the deposit are merenskyite pdte2 and michenerite pdtebi as well as palladian bismuthian melonite ni pd te bi 2 these minerals occur in intimate association with hessite ag2te and electrum au0 65ag0 31bi0 04 in two distinct textural positions 1 as inclusions in pyrrhotite pentlandite and rarely chalcopyrite and 2 at sulfide silicate grain boundaries and on microfractures in base metal sulfides bms and olivine associated with serpentine and secondary magnetite textural features suggest that pgm were exsolved from monosulfide solid solution over a wide range of temperatures late stage low temperature hydrothermal solutions led to redistribution of pge mineralized samples show ni cu ratios ranging from 0 2 to 2 with an average of 1 0 the pt pd rh os ir ru ratio is generally 6 in most samples and os ru and ir are below the detection limit 2 ppb the pge contents show positive correlation with s only at low sulfur contents the pge patterns of gabbro akarem are similar to those of alaskan type deposits compared with stratiform deposits gabbro akarem is depleted in pge the consistently low pge contents of the mineralization and their uniform distribution in the ultramafic rocks despite the high sulfur content of the rock is attributed to rapid crystallization of sulfides in a highly dynamic environment

on observations of storm microseismic vibrations by seismic stations of the u s s r abstract in the present work systematic observations of storm microseismic vibrations smv are discussed vibrations of the intensities of smv for oceans of the northern and southern hemispheres in the period 1979 1983 are estimated due to an example of a deep cyclone which had occurred in the northern part of the atlantic and the barents sea records of smv performed by seismic stations of europe and asia are considered locations of sources of excitation of smv their power and energy are determined the mean power of smv in their maximum phase from 18 h 23 october 1983 to 18 h 24 october 1983 had a value of 10 11 j s 1 and the energy transferred to the earth s crust in the time span mentioned was 10 16 j all the parameters determined for the smv may characterize excitations in the atmosphere and the hydrosphere areas of standing sea waves and areas of origin of acoustic radiation

simulation of high concentration of iron in dense shelf water in the okhotsk sea abstract an ocean general circulation model coupled with a simple biogeochemical model was developed to simulate iron circulation in and around the sea of okhotsk the model has two external sources of iron dust iron at the sea surface and sedimentary iron at the seabed shallower than 300 m the model represented characteristic features reasonably well such as high iron concentration in the dense shelf water dsw and its mixing which extends southward in the intermediate layer from the northwestern shelf along sakhalin island and finally flows into the pacific sensitivity experiments for the solubility of dust iron in seawater suggest that a solubility of 1 is appropriate in our simulation higher solubilities 5 and 10 result in too low phosphate in the northwestern north pacific in summer as well as too high iron concentrations at the sea surface compared with observations besides these experiments show that dust iron hardly contributes to the high iron concentration in the intermediate layer to investigate locations from which the iron in the intermediate layer originates the fate of sedimentary iron input from four regions in the okhotsk sea was examined results suggest that the western and central parts of the northern shelf are important

asteroid impact vs deccan eruptions the origin of low magnetic susceptibility beds below the cretaceous paleogene boundary revisited abstract the respective roles of an asteroid impact and deccan traps eruptions in biotic changes at the cretaceous paleogene k pg boundary are still debated in many shallow marine sediments from around the world the k pg boundary is marked by a distinct clay layer that is often underlain by a several decimeter thick low susceptibility zone a previous study of the gubbio section italy lowrie et al 1990 attributed low magnetization intensity in this interval to post depositional dissolution of ferrimagnetic minerals dissolution was thought to be a consequence of downward infiltration of reducing waters that resulted from rapid accumulation of organic matter produced by mass extinctions after the k pg event we compare the magnetic properties of sediments from the gubbio section with those of the bidart section in southern france the two sections are similar in their carbonate lithology and the presence of a boundary clay and low susceptibility zone when compared to background cretaceous sediments the low susceptibility zone in both sections is marked by an absence of biogenic magnetite a decrease in total ferrimagnetic mineral content and a preferential loss of magnetite with respect to hematite features that are consistent with reductive dissolution however unlike the gubbio section where the low susceptibility zone starts immediately below the boundary clay the low susceptibility zone and the clay layer at bidart are separated by a 4 cm carbonate interval that contains abundant biogenic magnetite such separation casts doubt on a causal link between the impact and sediment bleaching more likely the low susceptibility layer marks a different environmental event that preceded the impact an episode of increased atmospheric and oceanic acidity associated with deccan traps volcanism that occurred well before the k pg impact is argued here to account for the distinct magnetic properties of the low susceptibility intervals

cryptic variations within chromitites of the upper critical zone northwestern bushveld complex the close association of platinum group elements with chromite in the upper critical zone calls for the appraisal of structures textures and composition of chromitites in the development of genetic models cryptic variations in successive chromitite layers of the ug 1 unit are consistent with fractionation of limited melt volumes individual chromitite bands within the composite ug 2 chromitite layer exhibit distinctive compositions but all display long range compositional trends with height superimposed on short range homogeneity accessory chromites are substantially different from massive chromitites in composition reaction with residual liquids and subsolidus reactions were significant recrystallization of chromitites was ubiquitous but the importance of in situ growth is shown by relict textures long range settling adcumulus growth or deposition from density currents are processes that find little support in the data presented

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

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

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

large flux of iron from the amery ice shelf marine ice to prydz bay east antarctica the antarctic continental shelf supports a high level of marine primary productivity and is a globally important carbon dioxide co2 sink through the photosynthetic fixation of co2 via the biological pump sustaining such high productivity requires a large supply of the essential micronutrient iron fe however the pathways for fe delivery to these zones vary spatially and temporally our study is the first to report a previously unquantified source of concentrated bioavailable fe to antarctic surface waters we hypothesize that fe derived from subglacial processes is delivered to euphotic waters through the accretion fe storage and subsequent melting fe release of a marine accreted layer of ice at the base of the amery ice shelf ais using satellite derived chlorophyll a data we show that the soluble fe supplied by the melting of the marine ice layer is an order of magnitude larger than the required fe necessary to sustain the large annual phytoplankton bloom in prydz bay our finding of high concentrations of fe in ais marine ice and recent data on increasing rates of ice shelf basal melt in many of antarctica s ice shelves should encourage further research into glacial and marine sediment transport beneath ice shelves and their sensitivity to current changes in basal melt currently the distribution volume and fe concentration of antarctic marine ice is poorly constrained this uncertainty combined with variable forecasts of increased rates of ice shelf basal melt limits our ability to predict future fe supply to antarctic coastal waters

iron mobilization in mineral dust can anthropogenic so2 emissions affect ocean productivity 1 for fe contained in long range transported aeolian dust to act as a micronutrient for oceanic phytoplankton it must be first dissolved or mobilized we propose that fe mobilization can occur in mineral dust from east asia by the incorporation of so2 into the advecting dust plumes and subsequent acidification of the dust through heterogeneous so2 oxidation to test this hypothesis we consider a dust plume that originated from the gobi deserts and advected over the pacific ocean data collected over the yellow sea confirm that this plume contained high concentrations of dust and so2 significant gaseous hno3 concentrations indicate that the dust particles were acidified i e ph 2 at these ph s 1 2 of the fe contained in a deliquescent mineral dust particle would be mobilized within 3 5 days these results suggest a possible link between the rate of c fixation in so called high nitrate low chlorophyll regions of the pacific ocean and so2 emissions from east asia

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

rock magnetic and geochemical evidence for authigenic magnetite formation via iron reduction in coal bearing sediments offshore shimokita peninsula japan iodp site c0020 sediments recovered at integrated ocean drilling program iodp site c0020 in a forearc basin offshore shimokita peninsula japan include numerous coal beds 0 3 7 m thick that are associated with a transition from a terrestrial to marine depositional environment within the primary coal bearing unit 2 km depth below seafloor there are sharp increases in magnetic susceptibility in close proximity to the coal beds superimposed on a background of consistently low magnetic susceptibility throughout the remainder of the recovered stratigraphic sequence we investigate the source of the magnetic susceptibility variability and characterize the dominant magnetic assemblage throughout the entire cored record using isothermal remanent magnetization irm thermal demagnetization anhysteretic remanent magnetization arm iron speciation and iron isotopes magnetic mineral assemblages in all samples are dominated by very low coercivity minerals with unblocking temperatures between 350 580 c that are interpreted to be magnetite samples with lower unblocking temperatures 300 400 c higher arm higher frequency dependence and isotopically heavy 56fe across a range of lithologies in the coal bearing unit between 1925 1995 mbsf indicate the presence of fine grained authigenic magnetite we suggest that iron reducing bacteria facilitated the production of fine grained magnetite within the coal bearing unit during burial and interaction with pore waters the coal peat acted as a source of electron donors during burial mediated by humic acids to supply iron reducing bacteria in the surrounding siliciclastic sediments these results indicate that coal bearing sediments may play an important role in iron cycling in subsiding peat environments and if buried deeply through time within the subsequent deep biosphere

submarine groundwater discharge revealed by 228 ra distribution in the upper atlantic ocean submarine groundwater discharge estimated from a 228ra inventory across the upper atlantic ocean provides a flux of 2 4 1013 m3 yr 1 equivalent to 80 160 of the influx from rivers into the atlantic ocean