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1.
Pyroxenitic layers are a minor constituent of ultramafic mantle massifs, but are considered important for basalt generation and mantle refertilization. Mafic spinel websterite and garnet-spinel clinopyroxenite layers within Jurassic ocean floor peridotites from the Totalp ultramafic massif (eastern Swiss Alps) were analyzed for their highly siderophile element (HSE) and Os isotope composition.Aluminum-poor pyroxenites (websterites) display chondritic to suprachondritic initial γOs (160 Ma) of −2 to +27. Osmium, Ir and Ru abundances are depleted in websterites relative to the associated peridotites and to mantle lherzolites worldwide, but relative abundances (Os/Ir, Ru/Ir) are similar. Conversely, Pt/Ir, Pd/Ir and Re/Ir are elevated.Aluminum-rich pyroxenites (clinopyroxenites) are characterized by highly radiogenic 187Os/188Os with initial γOs (160 Ma) between +20 and +1700. Their HSE composition is similar to that of basalts, as they are more depleted in Os, Ir and Ru compared to Totalp websterites, along with even higher Pt/Ir, Pd/Ir and Re/Ir. The data are most consistent with multiple episodes of reaction of mafic pyroxenite precursor melts with surrounding peridotites, with the highest degree of interaction recorded in the websterites, which typically occur in direct contact to peridotites. Clinopyroxenites, in contrast, represent melt-dominated systems, which retained the precursor melt characteristics to a large extent. The melts may have been derived from a sublithospheric mantle source with high Pd/Ir, Pt/Ir and Re/Os, coupled with highly radiogenic 187Os/188Os compositions. Modeling indicates that partial melting of subducted, old oceanic crust in the asthenosphere could be a possible source for such melts.Pentlandite and godlevskite are identified in both types of pyroxenites as the predominant sulfide minerals and HSE carriers. Heterogeneous HSE abundances within these sulfide grains likely reflect subsolidus processes. In contrast, large grain-to-grain variations, and correlated variations of HSE ratios, indicate chemical disequilibrium under high-temperature conditions. This likely reflects multiple events of melt-rock interaction and sulfide precipitation. Notably, sulfides from the same thick section for the pyroxenites may display both residual-peridotite and melt-like HSE signatures. Because Totalp pyroxenites are enriched in Pt and Re, and depleted in Os, they will develop excess radiogenic 187Os and 186Os, compared to ambient mantle. These enrichments, however, do not possess the requisite Pt-Re-Os composition to account for the coupled suprachondritic 186Os-187Os signatures observed in some Hawaiian picrites, Gorgona komatiites, or the Siberian plume.  相似文献   

2.
《Applied Geochemistry》2001,16(5):559-570
Fe(II)–Fe(III) layered double hydroxysalt green rusts, GRs, are very reactive compounds with the general formula, [FeII(1−x) FeIIIx (OH)2]x+·[(x/n) An−·(m/n) H2O]x−, where x is the ratio FeIII/Fetot, and reflects the structure in which brucite-like layers alternate with interlayers of anions An− and water molecules. Two types of crystal structure for GRs, GR1 and GR2, represented by the hydroxychloride GR1(Cl−) and the hydroxysulphate GR2(SO42−) are distinguished by X-ray diffraction due to different stacking. By analogy with GR1(Cl−) the structure of the fougerite GR mineral, [FeII(1−x) FeIIIx (OH)2]x+·[x OH−·(1−x) H2O]x-  Fe(OH)(2+x)·(1−x) H2O, is proposed displaying interlayers made of OH− ions and water molecules (in situ deprotonation of water molecules is necessary for explaining the flexibility of its composition). The space group of mineral GR1(OH−) would be R3̄m, with lattice parameters a≅0.32 and c≅2.25 nm. Stability conditions and the Eh-pH diagram of Fe(OH)(2+x) (the water molecules are omitted) are determined from hydromorphic soil solution equilibria with GR mineral in Brittany (France). Computed Gibbs free energies of formation from soil solution/mineral equilibrium fit well with a regular solid solution model: μ°[Fe(OH)(2+x)]=(1−x) μ°[Fe(OH)2]+x μ°[Fe(OH)3]+RT [(1−x) ln (1−x)+x ln x]+A0 x (1−x), where μ°[Fe(OH)2]=−492.5 kJ mol−1, μ°[Fe(OH)3]=−641 kJ mol−1 and A0=−243.9 kJ mol−1 at the average temperature of 9±1°C. The upper limit of occurrence of GR mineral at x=2/3, i.e. Fe3(OH)8, is explained by its unstability vs. α-FeOOH and/or magnetite; Fe(OH)3 is thus a hypothetical compound with a GR structure which cannot be observed. These thermodynamic data and Eh-pH diagrams of Fe(OH)(2+x) can be used most importantly to predict the possibility that GR minerals reduce some anions in contaminated soils. The cases of NO3−, Se(VI) or Cr(VI) are fully illustrated.  相似文献   

3.
Thirty-three whole-rock drill core samples and thirteen olivine, chromite, and sulfide separates from three differentiated komatiite lava flows at Alexo and Pyke Hill, Canada, were analyzed for PGEs using the Carius tube digestion ID-ICP-MS technique. The emplaced lavas are Al-undepleted komatiites with ∼27% MgO derived by ∼50% partial melting of LILE-depleted Archean mantle. Major and minor element variations during and after emplacement were controlled by 30 to 50% fractionation of olivine Fo93-94. The emplaced lavas are characterized by (Pd/Ir)N = 4.0 to 4.6, (Os/Ir)N = 1.07, and Os abundances of ∼2.3 ppb. Variations in PGE abundances within individual flows indicate that Os and Ir were compatible (bulk DOs,Ir = 2.4-7.1) and that Pt and Pd were incompatible (bulk DPt,Pd < 0.2) during lava differentiation, whereas bulk DRu was close to unity. Analyses of cumulus olivine separates indicate that PGEs were incompatible in olivine (DPGEsOl-Liq = 0.04-0.7). The bulk fractionation trends cannot be accounted for by fractionation of olivine alone, and require an unidentified Os-Ir-rich phase. The composition of the mantle source (Os = 3.9 ppb, Ir = 3.6 ppb, Ru = 5.4 ppb, Pt and Pd = 5.7 ppb) was constrained empirically for Ru, Pt, and Pd; the Os/Ir ratio was taken to be identical to that in the emplaced melt, and the Ru/Ir ratio was taken to be chondritic, so that the absolute IPGE abundances of the source were determined by Ru. This is the first estimate of the PGE composition of a mantle source derived from analyses of erupted lavas. The suprachondritic Pd/Ir and Os/Ir of the inferred Abitibi komatiite mantle source are similar to those in off-craton spinel lherzolites, orogenic massif lherzolites, and enstatite chondrites, and are considered to be an intrinsic mantle feature. Bulk partition coefficients for use in komatiite melting models derived from the source and emplaced melt compositions are: DOs,Ir = 2.3, DRu = 1.0, DPt,Pd = 0.07. Ruthenium abundances are good indicators of absolute IPGE abundances in the mantle sources of komatiite melts with 26 to 29% MgO, as Ru fractionates very little during both high degrees of partial melting and lava differentiation.  相似文献   

4.
《Geochimica et cosmochimica acta》1999,63(13-14):2001-2007
Stable oxygen isotope ratios of foraminiferal calcite are widely used in paleoceanography to provide a chronology of temperature changes during ocean history. It was recently demonstrated that the stable oxygen isotope ratios in planktonic foraminifera are affected by changes of the seawater chemistry carbonate system: the δ18O of the foraminiferal calcite decreases with increasing CO32− concentration or pH. This paper provides a simple explanation for seawater chemistry dependent stable oxygen isotope variations in the planktonic foraminifera Orbulina universa which is derived from oxygen isotope partitioning during inorganic precipitation. The oxygen isotope fractionation between water and the dissolved carbonate species S = [H2CO3] + [HCO3−] + [CO32−] decreases with increasing pH. Provided that calcium carbonate is formed from a mixture of the carbonate species in proportion to their relative contribution to S, the oxygen isotopic composition of CaCO3 also decreases with increasing pH. The slope of shell δ18O vs. [CO32−] of Orbulina universa observed in culture experiments is −0.0022‰ (μmol kg−1)−1 (Spero et al., 1997), whereas the slope derived from inorganic precipitation is −0.0024‰ (μmol kg−1)−. The theory also provides an explanation of the nonequilibrium fractionation effects in synthetic carbonates described by Kim and O’Neil (1997) which can be understood in terms of equilibrium fractionation at different pH. The results presented here emphasize that the oxygen isotope fractionation between calcium carbonate and water does not only depend on the temperature but also on the pH of the solution from which it is formed.  相似文献   

5.
《Geochimica et cosmochimica acta》1999,63(19-20):3417-3427
In order to verify Fe control by solution - mineral equilibria, soil solutions were sampled in hydromorphic soils on granites and shales, where the occurrence of Green Rusts had been demonstrated by Mössbauer and Raman spectroscopies. Eh and pH were measured in situ, and Fe(II) analyzed by colorimetry. Ionic Activity Products were computed from aqueous Fe(II) rather than total Fe in an attempt to avoid overestimation by including colloidal particles. Solid phases considered are Fe(II) and Fe(III) hydroxides and oxides, and the Green Rusts whose general formula is [FeII1−xFeIIIx(OH)2]+x· [x/z A−z]−x, where compensating interlayer anions, A−, can be Cl−, SO42−, CO32− or OH−, and where x ranges a priori from 0 to 1. In large ranges of variation of pH, pe and Fe(II) concentration, soil solutions are (i) oversaturated with respect to Fe(III) oxides; (ii) undersaturated with respect to Fe(II) oxides, chloride-, sulphate- and carbonate-Green Rusts; (iii) in equilibrium with hydroxy-Green Rusts, i.e., Fe(II)-Fe(III) mixed hydroxides. The ratios, x = Fe(III)/Fet, derived from the best fits for equilibrium between minerals and soil solutions are 1/3, 1/2 and 2/3, depending on the sampling site, and are in every case identical to the same ratios directly measured by Mössbauer spectroscopy. This implies reversible equilibrium between Green Rust and solution. Solubility products are proposed for the various hydroxy-Green Rusts as follows: log Ksp = 28.2 ± 0.8 for the reaction Fe3(OH)7 + e− + 7 H+ = 3 Fe2+ + 7 H2O; log Ksp = 25.4 ± 0.7 for the reaction Fe2(OH)5 + e− + 5 H+ = 2 Fe2+ + 5 H2O; log Ksp = 45.8 ± 0.9 for the reaction Fe3(OH)8 + 2e− + 8 H+ = 3 Fe2+ + 8 H2O at an average temperature of 9 ± 1°C, and 1 atm. pressure. Tentative values for the Gibbs free energies of formation of hydroxy-Green Rusts obtained are: ΔfG° (Fe3(OH)7, cr, 282.15 K) = −1799.7 ± 6 kJ mol−1, ΔfG° (Fe2(OH)5, cr, 282.15 K) = −1244.1 ± 6 kJ mol−1 and ΔfG° (Fe3(OH)8, cr, 282.15 K) = −1944.3 ± 6 kJ mol−1.  相似文献   

6.
A comprehensive method for the precise determination of Re, Os, Ir, Ru, Pt and Pd concentrations as well as Os isotopic compositions in geological samples is presented. Samples were digested by the Carius tube method, and the Os was extracted by conventional CCl4 method. The Re, Ir, Ru, Pt and Pd were first subgroup separated from the matrix elements into Re‐Ru, Ir‐Pt and Pd by a 2‐ml anion exchange column. Subsequently, the Re‐Ru was further purified by a secondary 0.25 ml anion exchange column or by microdistillation of Ru using CrO3‐H2SO4 as an oxidant followed by a secondary 0.25 ml anion exchange separation of Re. The Pd and Ir‐Pt were further successively purified by an Eichrom‐LN column to completely remove Zr and Hf, respectively. Rhenium, Ir, Ru, Pt and Pd were individually measured by multi‐collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS), except for Ru after microdistillation purification was analysed by negative‐thermal ionisation mass spectrometry (N‐TIMS). The analytical results for peridotite reference material WPR‐1 agree well with the previously published data. Finally, several mafic rock reference materials including TDB‐1, WGB‐1, BHVO‐2, BCR‐2, BIR‐1a and DNC‐1a were analysed for Re‐Os isotopes and platinum‐group element concentrations to test their suitability for certification.  相似文献   

7.
The nature of PGE-Re (PGE = Pt, Pd, Os, Ir, Ru) behavior in subcontinental lithospheric mantle was investigated using new, high precision PGE-Re abundance measurements and previously published Re-Os isotopic analyses of peridotite xenoliths from the Sierra Nevada and Mojave Province, California. Ru/Ir ratios and Ir concentrations are constant over a wide range in S content and major-element fertility indices (e.g., Mg/(Mg+Fe)), indicating that Ru and Ir are not only compatible during partial melting, but also that their partitioning behaviors may not be controlled entirely by sulfide. Pt/Ir, Pd/Ir, Os/Ir, and Re/Ir ratios range from slightly superchondritic to distinctly subchondritic for all xenoliths except for one anomalous sample (1026V), which is characterized by radiogenic 187Os/188Os, low Re/Os ratio, and large enrichments in Cu, Os, Pt, Pd, and S relative to Ir (COPPS metasomatism). Assuming chondritic initial relative abundances, the magnitudes of some of the depletions in Pt, Pd, Os, and Re relative to Ir and Ru require incompatible behavior or substantial secondary loss. In detail, some samples, which are otherwise characterized by fertile major-element indices, exhibit low S contents and subchondritic Os/Ir and Pd/Ir ratios, indicating that depletions in Pd and Os relative to Ir are not simple functions of the degree of melting as inferred from major elements. Possible mechanisms for depleting Pt, Pd, Os, and Re relative to Ir and Ru include partitioning into chromian spinels and alloys, partitioning between sulfide and sulfide liquids, mobilization by aqueous fluids, or secondary loss associated with late-stage sulfide breakdown. However, it is not possible to explain all of the depletions in Pt, Pd, Os, and Re by any single mechanism.The preferential enrichment in Os over Re and Ir in sample 1026V is somewhat paradoxical because this sample’s radiogenic 187Os/188Os requires a metasomatic agent, originating from a source with a high time-integrated Re/Os ratio. The abundant garnet websterite xenoliths may be a suitable source because they have high Re/Os ratios, radiogenic Os, and abundant garnet, which may sequester Re over Os during partial melting. However, their extremely low Os contents require the processing of large amounts of garnet websterite to concentrate enough Os into the metasomatic sulfides needed to enrich sample 1026V in Os. The homogeneity in 187Os/188Os ratio in the remaining xenoliths suggest that their Os isotopic compositions were not significantly affected by PGE metasomatism. The singular nature of 1026V’s composition emphasizes the rarity of COPPS metasomatism.  相似文献   

8.
Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to measure distributions of the siderophile elements V, Fe, Co, Ni, Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in Fremdlinge with a spatial resolution of 15 to 25 μm. A sulfide vein in a refractory inclusion in Allende (CV3-oxidized) is enriched in Rh, Ru, and Os with no detectable Pd, Re, Ir, or Pt, indicating that Rh, Ru, and Os were redistributed by sulfidation of the inclusion, causing fractionation of Re/Os and other siderophile element ratios in Allende CAIs. Fremdlinge in compact Type-A inclusions from Efremovka (CV3-reduced) exhibit subsolidus exsolution into kamacite and taenite and minimal secondary formation of V-magnetite and schreibersite. Siderophile element partitioning between taenite and kamacite is similar to that observed previously in iron meteorites, while preferential incorporation of the light PGEs (Ru, Rh, Pd) relative to Re, Os, Ir, and Pt by schreibersite was observed. Fremdling EM2 (CAI Ef2) has an outer rim of P-free metal that preserves the PGE signature of schreibersite, indicating that EM2 originally had a phosphide rim and lost P to the surrounding inclusion during secondary processing. Most Fremdlinge have chondrite-normalized refractory PGE patterns that are unfractionated, with PGE abundances derived from a small range of condensation temperatures, ∼1480 to 1468 K at Ptot = 10−3 bar. Some Fremdlinge from the same CAI exhibit sloping PGE abundance patterns and Re/Os ratios up to 2 × CI that likely represent mixing of grains that condensed at various temperatures.  相似文献   

9.
Ab initio, molecular orbital calculations at the 6-31G1 level including second-order Møller-Plesset electron correlation predict that the species [Si(OH)5]1− is dynamically stable in a distorted trigonal bipyramid configuration. Reaction pathways for Si(OH)4 + (OH)− → [Si(OH)5]1− → [(OH)3SiO]1−—H2O are also calculated. The first reaction represents the formation of pentacoordinate Si from orthosilicic acid and hydroxide. The activation energy for adding a fifth Si-(OH)− bond to the Si(OH)4 molecule is ≈0.1 eV /molec (≈10kJ/mol). The second reaction is the deprotonation of the Si(OH)4 which forms as a hydroxyl group leaves the [Si(OH)5]1− molecule. Removal of a bond from this complex requires 0.9 eV/molecule (≈85 kJ/mol). Lengthening the Si—OH2 distance results in the isolated molecules [(OH)3SiO]1− + H2O. This represents dehydration of the deprotonated orthosilicic acid.[Si(OH)5]1− and [(OH)3SiO]1−- H2O have the same energetic stability within the accuracy of these calculations. The potential energies of the isolated molecular systems [(OH)3SiO]1−+ H2O and Si(OH)4 + (OH)− are considerably higher. These results suggest that [Si(OH)5]1− may be a stable species or reaction intermediate in dissolution of silicate minerals in basic aqueous solutions.  相似文献   

10.
Osmium, Ru, Ir, Pt, Pd and Re abundances and 187Os/188Os data on peridotites were determined using improved analytical techniques in order to precisely constrain the highly siderophile element (HSE) composition of fertile lherzolites and to provide an updated estimate of HSE composition of the primitive upper mantle (PUM). The new data are used to better constrain the origin of the HSE excess in Earth’s mantle. Samples include lherzolite and harzburgite xenoliths from Archean and post-Archean continental lithosphere, peridotites from ultramafic massifs, ophiolites and other samples of oceanic mantle such as abyssal peridotites. Osmium, Ru and Ir abundances in the peridotite data set do not correlate with moderately incompatible melt extraction indicators such as Al2O3. Os/Ir is chondritic in most samples, while Ru/Ir, with few exceptions, is ca. 30% higher than in chondrites. Both ratios are constant over a wide range of Al2O3 contents, but show stronger scatter in depleted harzburgites. Platinum, Pd and Re abundances, their ratios with Ir, Os and Ru, and the 187Os/188Os ratio (a proxy for Re/Os) show positive correlations with Al2O3, indicating incompatible behavior of Pt, Pd and Re during mantle melting. The empirical sequence of peridotite-melt partition coefficients of Re, Pd and Pt as derived from peridotites () is consistent with previous data on natural samples. Some harzburgites and depleted lherzolites have been affected by secondary igneous processes such as silicate melt percolation, as indicated by U-shaped patterns of incompatible HSE, high 187Os/188Os, and scatter off the correlations defined by incompatible HSE and Al2O3. The bulk rock HSE content, chondritic Os/Ir, and chondritic to subchondritic Pt/Ir, Re/Os, Pt/Re and Re/Pd of many lherzolites of the present study are consistent with depletion by melting, and possibly solid state mixing processes in the convecting mantle, involving recycled oceanic lithosphere. Based on fertile lherzolite compositions, we infer that PUM is characterized by a mean Ir abundance of 3.5 ± 0.4 ng/g (or 0.0080 ± 0.0009*CI chondrites), chondritic ratios involving Os, Ir, Pt and Re (Os/IrPUM of 1.12 ± 0.09, Pt/IrPUM = 2.21 ± 0.21, Re/OsPUM = 0.090 ± 0.002) and suprachondritic ratios involving Ru and Pd (Ru/IrPUM = 2.03 ± 0.12, Pd/IrPUM = 2.06 ± 0.31, uncertainties 1σ). The combination of chondritic and modestly suprachondritic HSE ratios of PUM cannot be explained by any single planetary fractionation process. Comparison with HSE patterns of chondrites shows that no known chondrite group perfectly matches the PUM composition. Similar HSE patterns, however, were found in Apollo 17 impact melt rocks from the Serenitatis impact basin [Norman M.D., Bennett V.C., Ryder G., 2002. Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis. Earth Planet. Sci. Lett, 217-228.], which represent mixtures of chondritic material, and a component that may be either of meteoritic or indigenous origin. The similarities between the HSE composition of PUM and the bulk composition of lunar breccias establish a connection between the late accretion history of the lunar surface and the HSE composition of the Earth’s mantle. Although late accretion following core formation is still the most viable explanation for the HSE abundances in the Earth’s mantle, the “late veneer” hypothesis may require some modification in light of the unique PUM composition.  相似文献   

11.
Voluminous platinum-group mineral(PGM) inclusions including erlichmanite(Os,Ru)S_2, laurite(Ru,Os)S_2, and irarsite(Ir,Os,Ru,Rh)As S, as well as native osmium Os(Ir) and inclusions of base metal sulphides(BMS), including millerite(NiS), heazlewoodite(Ni_3S_2), covellite(CuS) and digenite(Cu_3S_2), accompanied by native iron, have been identified in chromitites of the Zedang ophiolite, Tibet. The PGMs occur as both inclusions in magnesiochromite grains and as small interstitial granules between them; most are less than 10 μm in size and vary in shape from euhedral to anhedral. They occur either as single or composite(biphase or polyphase) grains composed solely of PGM, or PGM associated with silicate grains. Os-, Ir-, and Ru-rich PGMs are the common species and Pt-, Pd-, and Rh-rich varieties have not been identified. Sulfur fugacity and temperature appear to be the main factors that controlled the PGE mineralogy during crystallization of the host chromitite in the upper mantle. If the activity of chalcogenides(such as S, and As) is low, PGE clusters will remain suspended in the silicate melt until they can coalesce to form alloys. Under appropriate conditions of ?S_2 and ?O_2, PGE alloys might react with the melt to form sulfides-sulfarsenides. Thus, we suggest that the Os, Ir and Ru metallic clusters and alloys in the Zedang chromitites crystallized first under high temperature and low ?S_2, followed by crystallization of sulphides of the laurite-erlichmanite, solid-solution series as the magma cooled and ?S_2 increased. The abundance of primary BMS in the chromitites suggests that ?S_2 reached relatively high values during the final stages of magnesiochromite crystallization. The diversity of the PGE minerals, in combination with differences in the petrological characteristics of the magnesiochromites, suggest different degrees of partial melting, perhaps at different depths in the mantle. The estimated parental magma composition suggests formation in a suprasubduction zone environment, perhaps in a forearc.  相似文献   

12.
Experimental studies, performed under oxidized conditions (fO2 > QFM + 2, where QFM is quartz–fayalite–magnetite oxygen buffer), have shown that Rh, Ru, Ir and Os are strongly compatible with Cr spinel, whereas empirical studies of Cr spinels from ultramafic–mafic rocks suggest that the experimental results may overestimate the partition coefficients. We report laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of platinum-group elements (PGE), Au and Re abundances in Cr spinels from the Ambae volcano, Vanuatu (fO2 = QFM + 2.5), the Jimberlana layered intrusion, western Australia, and the Bushveld complex, South Africa (fO2 ～ QFM). The results show that Rh and IPGEs (Iridium-group PGE; Ru, Ir, Os) partition strongly into the Cr spinels that crystallized from the oxidized Ambae lavas whereas most of the Cr spinels from the more reduced Jimberlana layered intrusion and the Bushveld complex contain no detectable PGE, Au or Re, with exception of ～10 ppb of Ir in some Jimberlana Cr spinels. In the Ambae Cr spinels, Rh, Ru and, to lesser extent Os, are positively correlated with Fe3+, Ni and V. The homogeneous distribution of Rh and IPGEs in LA-ICP-MS time-resolved spectra indicates that these elements are in solid solution in Cr spinels. Pt–Fe alloys occur as inclusions within the Ambae Cr spinels, which indicate that the Ambae melt was saturated with Pt.Our results show that partitioning of Rh, Ru and Ir into Cr spinels increases with increasing oxygen fugacity, which suggests that the high concentrations of these elements in the Ambae Cr spinels are due to the high oxygen fugacity of the host magma. Therefore, Cr spinels may play an important role in controlling the concentrations of Rh and IPGEs during fractional crystallization of oxidized ultramafic–mafic magmas and during partial melting of oxidized arc mantle.  相似文献   

13.
Platinum-group minerals (PGM) have been identified as inclusions in chromite from the Bird River Sill, Manitoba. The inclusions are small (<20 microns) and are commonly euhedral. The PGM inclusions are (Ru, Os, Ir) S2, laurite, and (Os, Ir, Ru alloy), rutheniridosmine: Laurites contain up to 2.99 wt. % palladium. Arsenic content is negligible and no platinum or rhodium has been detected. One platinum-group element alloy contains 0.96 wt. % rhodium but neither platinum nor palladium has been detected. Laurite inclusions in chromite from the ultramafic zone record two compositional trends; first increasing and then decreasing Ru/(Ru+Os+Ir) up section. PGM inclusions and other solid inclusions occur as discrete phases in chromite and are part of the chromite precipitation event. Increasing oxygen fugacity by wall rock assimilation or new magma injection initiates chromite precipitation, locally increasing the sulphur content of the magma to convert PGE alloys to sulphides.  相似文献   

14.
The Tagish Lake meteorite is a primitive C2 chondrite that has undergone aqueous alteration shortly after formation of its parent body. Previous work indicates that if this type of material was part of a late veneer during terrestrial planetary accretion, it could provide a link between atmophile elements such as H, C, N and noble gases, and highly siderophile element replenishment in the bulk silicate portions of terrestrial planets following core formation. The systematic Re-Os isotope and highly siderophile element measurements performed here on five separate fractions indicate that while Tagish Lake has amongst the highest Ru/Ir (1.63 ± 0.08), Pd/Ir (1.19 ± 0.06) and 187Os/188Os (0.12564-0.12802) of all carbonaceous chondrites, these characteristics still fall short of those necessary to explain the observed siderophile element systematics of the primitive upper mantles of Earth and Mars. Hence, a direct link between atmophile and highly siderophile elements remains elusive, and other sources for replenishment are required, unless an as yet poorly constrained process fractionated Re/Os, Ru/Ir, and Pd/Ir following late accretion on both the Earth and Mars mantles.The unique elevated Ru/Ir combined with elevated 187Os/188Os of Tagish Lake may be attributed to Ru and Re mobility during aqueous alteration very early in its parent body history. The Os, Ir, Pt, and Pd abundances of Tagish Lake are similar to CI chondrites. The elevated Ru/Ir and the higher Re/Os and consequent 187Os/188Os in Tagish Lake, are balanced by a lower Ru/Ir and lower Re/Os and 187Os/188Os in CM-chondrites, relative to CI chondrites. A model that links Tagish Lake with CI and CM chondrites in the same parent body may explain the observed systematics. In this scenario, CM chondrite material comprises the exterior, grading downward to Tagish Lake material, which grades to CI material in the interior of the parent body. Aqueous alteration intensifies towards the interior with increasing temperature. Ruthenium and Re are mobilized from the CM layer into the Tagish Lake layer. This model may thus provide a potential direct parent body relationship between three separate groups of carbonaceous chondrites.  相似文献   

15.
The concentrations of Rh, Au and other highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd and Au), and 187Os/188Os isotope ratios have been determined for samples from peridotite massifs and xenoliths in order to further constrain HSE abundances in the Earth's mantle and to place constraints on the distributions processes accounting for observed HSE variations between fertile and depleted mantle lithologies. Concentrations of Re, Os, Ir, Ru, Pt and Pd were determined by isotope dilution ICP-MS and N-TIMS. The monoisotopic elements Rh and Au were quantified by standardization relative to the concentrations of Ru and Ir, respectively, and were determined from the same digestion aliquot as other HSE. The measurement precision of the concentration data under intermediate precision conditions, as inferred from repeated analyses of 2 g test portions of powdered samples, is estimated to be better than 10% for Rh and better than 15% for Au (1 s).Fertile lherzolites display non-systematic variation of Rh concentrations and constant Rh/Ir of 0.34 ± 0.03 (1 s, n = 57), indicating a Rh abundance for the primitive mantle of 1.2 ± 0.2 ng/g. The data also suggest that Rh behaves as a compatible element during low to moderate degrees of partial melting in the mantle or melt–mantle interaction, but may be depleted at higher degrees of melting. In contrast, Au concentrations and Au/Ir correlate with peridotite fertility, indicating incompatible behaviour of Au during magmatic processes in the mantle. Fertile lherzolites display Au/Ir ranging from 0.20 to 0.65, whereas residual harzburgites have Au/Ir < 0.20. Concentrations of Au and Re are correlated with each other and suggest similar compatibility of both elements. The primitive mantle abundance of Au calculated from correlations displayed by Au/Ir with Al2O3 and Au with Re is 1.7 ± 0.5 ng/g (1 s).The depletion of Pt, Pd, Re and Au relative to Os, Ir, Ru and Rh displayed by residual harzburgites, suggests HSE fractionation during partial melting. However, the HSE abundance variations of fertile and depleted peridotites cannot be explained by a simple fractionation process. Correlations displayed by Pd/Ir, Re/Ir and Au/Ir with Al2O3 may reflect refertilization of previously melt depleted mantle rocks due to reactive infiltration of silicate melts.Relative concentrations of Rh and Au inferred for the primitive mantle model composition are similar to values of ordinary and enstatite chondrites, but distinct from carbonaceous chondrites. The HSE pattern of the primitive mantle is inconsistent with compositions of known chondrite groups. The primitive mantle composition may be explained by late accretion of a mixture of chondritic with slightly suprachondritic materials, or alternatively, by meteoritic materials mixed into mantle with a HSE signature inherited from core formation.  相似文献   

16.
《Applied Geochemistry》1998,13(4):509-520
A gravity-fed, battery-powered, portable continuously-stirred tank reactor has been developed to directly measure aqueous reaction rates in the field. Dye and tracer experiments indicate the reactor is well-mixed. Rates of Fe2+ oxidation at untreated and passively treated coal mine drainage sites in Pennsylvania were measured under ambient conditions and with the addition of either O2 gas or NaOH solutions. Rates at 5 sites ranged from below the detection limit for this technique (approximately 10−9 mol L−1 s−1) to 3.27±0.01×10−6 mol L−1 s−1. Uncertainties in rates ranged from 70% near the lower limit of measurement to as little as 1% at higher rates of reaction. Multiple linear regressions showed no universal correlations of rates to Fe2+, dissolved O2, and pH (Thiobacillus populations were not measured), although data for two more acidic sites were found to fit well for the model log rate=log K+a log [Fe2+]+b log [OH−]+c log [O2]. Field rates of Fe oxidation from this and other studies vary by 4 orders of magnitude. A model using the ambient field rate of Fe oxidation from this study successfully reproduced independently-measured Fe2+ concentrations observed in a passive wetland treatment facility.  相似文献   

17.
《Applied Geochemistry》2000,15(8):1203-1218
Ca6[Al(OH)6]2(CrO4)3·26H2O, the chromate analog of the sulfate mineral ettringite, was synthesized and characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, thermogravimetric analyses, energy dispersive X-ray spectrometry, and bulk chemical analyses. The solubility of the synthesized solid was measured in a series of dissolution and precipitation experiments conducted at 5–75°C and at initial pH values between 10.5 and 12.5. The ion activity product (IAP) for the reaction Ca6[Al(OH)6]2(CrO4)3·26H2O⇌6Ca2++2Al(OH)−4+3CrO2−4+4OH−+26H2O varies with pH unless a CaCrO4(aq) complex is included in the speciation model. The log K for the formation of this complex by the reaction Ca2++CrO2−4=CaCrO4(aq) was obtained by minimizing the variance in the IAP for Ca6[Al(OH)6]2(CrO4)3·26H2O. There is no significant trend in the formation constant with temperature and the average log K is 2.77±0.16 over the temperature range 5–75°C. The log solubility product (log KSP) of Ca6[Al(OH)6]2(CrO4)3·26H2O at 25°C is −41.46±0.30. The temperature dependence of the log KSP is log KSP=A−B/T+D log(T) where A=498.94±48.99, B=27,499±2257, and D=−181.11±16.74. The values of ΔG0r,298 and ΔH0r,298 for the dissolution reaction are 236.6±3.9 and 77.5±2.4 kJ mol−1. the values of ΔC0P,r,298 and ΔS0r,298 are −1506±140 and −534±83 J mol−1 K−1. Using these values and published standard state partial molal quantities for constituent ions, ΔG0f,298=−15,131±19 kJ mol−1, ΔH0f,298=−17,330±8.6 kJ mol−1, ΔS0298=2.19±0.10 kJ mol−1 K−1, and ΔC0Pf,298=2.12±0.53 kJ mol−1 K−1, were calculated.  相似文献   

18.
《Geochimica et cosmochimica acta》1999,63(13-14):1969-1980
The solubility of ettringite (Ca6[Al(OH)6]2(SO4)3 · 26H2O) was measured in a series of dissolution and precipitation experiments at 5–75°C and at pH between 10.5 and 13.0 using synthesized material. Equilibrium was established within 4 to 6 days, with samples collected between 10 and 36 days. The log KSP for the reaction Ca6[Al(OH)6]2(SO4)3 · 26H2O ⇌ 6Ca2+ + 2Al(OH)4− + 3SO42− + 4OH− + 26H2O at 25°C calculated for dissolution experiments (−45.0 ± 0.2) is not significantly different from the log KSP calculated for precipitation experiments (−44.8 ± 0.4) at the 95% confidence level. There is no apparent trend in log KSP with pH and the mean log KSP,298 is −44.9 ± 0.3. The solubility product decreased linearly with the inverse of temperature indicating a constant enthalpy of reaction from 5 to 75°C. The enthalpy and entropy of reaction ΔH°r and ΔS°r, were determined from the linear regression to be 204.6 ± 0.6 kJ mol−1 and 170 ± 38 J mol−1 K−1. Using our values for log KSP, ΔH°r, and ΔS°r and published partial molal quantities for the constituent ions, we calculated the free energy of formation ΔG°f,298, the enthalpy of formation ΔH°f,298, and the entropy of formation ΔS°f,298 to be −15211 ± 20, −17550 ± 16 kJ mol−1, and 1867 ± 59 J mol−1 K−1. Assuming ΔCP,r is zero, the heat capacity of ettringite is 590 ± 140 J mol−1 K−1.  相似文献   

19.
The nature of Re-platinum-group element (PGE; Pt, Pd, Ir, Os, Ru) transport in the marine environment was investigated by means of marine sediments at and across the Cretaceous-Tertiary boundary (KTB) at two hemipelagic sites in Europe and two pelagic sites in the North and South Pacific. A traverse across the KTB in the South Pacific pelagic clay core found elevated levels of Re, Pt, Ir, Os, and Ru, each of which is approximately symmetrically distributed over a distance of ∼1.8 m across the KTB. The Re-PGE abundance patterns are fractionated from chondritic relative abundances: Ru, Pt, Pd, and Re contents are slightly subchondritic relative to Ir, and Os is depleted by ∼95% relative to chondritic Ir proportions. A similar depletion in Os (∼90%) was found in a sample of the pelagic KTB in the North Pacific, but it is enriched in Ru, Pt, Pd, and Re relative to Ir. The two hemipelagic KTB clays have near-chondritic abundance patterns. The ∼1.8-m-wide Re-PGE peak in the pelagic South Pacific section cannot be reconciled with the fallout of a single impactor, indicating that postdepositional redistribution has occurred. The elemental profiles appear to fit diffusion profiles, although bioturbation could have also played a role. If diffusion had occurred over ∼65 Ma, the effective diffusivities are ∼10−13 cm2/s, much smaller than that of soluble cations in pore waters (∼10−6 cm2/s). The coupling of Re and the PGEs during redistribution indicates that postdepositional processes did not significantly fractionate their relative abundances. If redistribution was caused by diffusion, then the effective diffusivities are the same. Fractionation of Os from Ir during the KTB interval must therefore have occurred during aqueous transport in the marine environment. Distinctly subchondritic Os/Ir ratios throughout the Cenozoic in the South Pacific core further suggest that fractionation of Os from Ir in the marine environment is a general process throughout geologic time because most of the inputs of Os and Ir into the ocean have Os/Ir ratios ≥1. Mass balance calculations show that Os and Re burial fluxes in pelagic sediments account for only a small fraction of the riverine Os (<10%) and Re (<0.1%) inputs into the oceans. In contrast, burial of Ir in pelagic sediments is similar to the riverine Ir input, indicating that pelagic sediments are a much larger repository for Ir than for Os and Re. If all of the missing Os and Re is assumed to reside in anoxic sediments in oceanic margins, the calculated burial fluxes in anoxic sediments are similar to observed burial fluxes. However, putting all of the missing Os and Re into estuarine sediments would require high concentrations to balance the riverine input and would also fail to explain the depletion of Os at pelagic KTB sites, where at most ∼25% of the K-T impactor’s Os could have passed through estuaries. If Os is preferentially sequestered in anoxic marine environments, it follows that the Os/Ir ratio of pelagic sediments should be sensitive to changes in the rates of anoxic sediment deposition. There is thus a clear fractionation of Os and Re from Ir in precipitation out of sea water in pelagic sections. Accordingly, it is inferred here that Re and Os are removed from sea water in anoxic marine depositional regimes.  相似文献   

20.
Shield-stage high-MgO alkalic lavas from La Palma and El Hierro (Canary Islands) have been characterized for their O-Sr-Nd-Os-Pb isotope compositions and major-, trace-, and highly siderophile-element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances. New data are also reported for associated evolved rocks, and entrained xenoliths. Clear differences in Pd/Ir and isotopic ratios for high Os (>50 ppt) lavas from El Hierro (δ18Oolivine = 5.17 ± 0.08‰; 87Sr/86Sr = 0.7029 to 0.7031; εNd = +5.7 to +7.1; 187Os/188Os = 0.1481 to 0.1750; 206Pb/204Pb = 19.1 to 19.7; Pd/Ir = 6 ± 3) versus those from La Palma (δ18Oolivine = 4.87 ± 0.18‰; 87Sr/86Sr = 0.7031 to 0.7032; εNd = +5.0 to +6.4; 187Os/188Os = 0.1421 to 0.1460; 206Pb/204Pb = 19.5 to 20.2; Pd/Ir = 11 ± 4) are revealed from the dataset.Crustal or lithospheric assimilation during magma transport cannot explain variations in isotopic ratios or element abundances of the lavas. Shallow-level crystal-liquid fractionation of olivine, clinopyroxene and associated early-crystallizing minerals (e.g., spinel and HSE-rich phases) controlled compatible element and HSE abundances; there is also evidence for sub-aerial degassing of rhenium. High-MgO lavas are enriched in light rare earth elements, Nb, Ta, U, Th, and depleted in K and Pb, relative to primitive mantle abundance estimates, typical of HIMU-type oceanic island basalts. Trace element abundances and ratios are consistent with low degrees (2-6%) of partial melting of an enriched mantle source, commencing in the garnet stability field (?110 km). Western Canary Island lavas were sulphur undersaturated with estimated parental melt HSE abundances (in ppb) of 0.07 ± 0.05 Os, 0.17 ± 0.16 Ir, 0.34 ± 0.32 Ru, 2.6 ± 2.5 Pt, 1.4 ± 1.2 Pd, 0.39 ± 0.30 Re. These estimates indicate that Canary Island alkali basalts have lower Os, Ir and Ru, but similar Pt, Pd and Re contents to Hawai’ian tholeiites.The HIMU affinities of the lavas, in conjunction with the low δ18Oolivine and high 206Pb/204Pb for La Palma, and elevated 187Os/188Os for El Hierro implies melting of different proportions of recycled oceanic crust and lithosphere. Our preferred model to explain isotopic differences between the islands is generation from peridotitic mantle metasomatised by <10% pyroxenite/eclogite made from variable portions of similar aged recycled oceanic crust and lithosphere. The correspondence of radiogenic 206Pb/204Pb, 187Os/188Os, elevated Re/Os and Pt/Os, and low-δ18O in western Canary Island lavas provides powerful support for recycled oceanic crust and lithosphere to generate the spectrum of HIMU-type ocean island basalt signatures. Persistence of geochemical heterogeneities throughout the stratigraphies of El Hierro and La Palma demonstrate long-term preservation of these recycled components in their mantle sources over relatively short-length scales (∼50 km).  相似文献   

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