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1.
Osmium isotope composition (187Os/188Os) and concentrations of Os, Ir and Pt are reported for an early Pleistocene section from the ODP Site 849 in the eastern equatorial Pacific. Using the data obtained in this study, the contributions from detrital and extraterrestrial particulate matter to Os concentration and 187Os/188Os of sediment are estimated. Our calculations show that detrital contributions to sedimentary Os are too small (<2%) to significantly shift measured bulk sediment 187Os/188Os away from seawater values. A moderate but significant negative correlation between 187Os/188Os and 3He/188Os indicate that the average particulate extraterrestrial Os flux to this site is 1.21 ± 0.47 pg cm−2 kyr−1, which constitutes ?3% of total Os burial flux. The estimates of detrital and extraterrestrial Os are used to calculate the seawater 187Os/188Os in the early Pleistocene. The most notable features of this early Pleistocene 187Os/188Os record are: (1) glacial-interglacial 187Os/188Os differences are insignificant within errors of estimates, (2) glacial 187Os/188Os values are higher compared to those reported for the late Pleistocene glacials. Comparison of 187Os/188Os values at Site 849 to the late Pleistocene records suggests that average seawater 187Os/188Os change has been modest (∼5%) since the early Pleistocene. Assuming that 187Os/188Os difference between the glacial periods of the late and the early Pleistocene results solely from temperature dependence of weathering rates, it has been calculated that average surface temperature during the late Pleistocene glacials was 0.8 ± 0.2 °C lower than glacials in the early Pleistocene. This inference is consistent with temperature estimates based on a recent study of pCO2 reconstruction in the Pleistocene. This observation based on limited studies of marine 187Os/188Os records seems to suggest that temperature played an important role in influencing chemical weathering during the Pleistocene glacials. However, more studies are needed to confirm if this temperature-weathering feedback was operational throughout the Pleistocene. A significant down core Ir-3He co-variation coupled with similar burial fluxes of Ir at Site 849 and at LL44 GPC-3 in the north Pacific point to the utility of Ir concentration as a point paleoflux tracer. However, a twofold difference in Ir burial fluxes between the eastern and the western equatorial Pacific suggests that calibration in space and time is required to use Ir concentration as a robust indicator of paleoflux through time. Significant co-variation of concentrations of Os and total alkenone during the glacials coupled with lighter δ13C of benthic foraminifera indicates that productivity and carbon burial played a dominant control on scavenging of Os at Site 849. In a broader context, this data set encourages future investigation of response of PGE behavior to paleoceanographic processes.  相似文献   

2.
Paired analyses of Os isotope composition and concentration of bulk sediment and leachable Os in a metalliferous pelagic clay sequence from the North Pacific, ODP Site 886C, are used to reconstruct the marine Os isotope record and the particulate meteoritic Os flux between 65.5 and 78 Ma. Measured 187Os/188Os of bulk sediments ranges from approximately 0.64 to 0.32 and those of leach analyses are very similar to bulk analyses. Hydrogenous Os dominates the sedimentary Os inventory throughout most of the studied interval. As a result the measured 187Os/188Os of leachable Os approximates that of contemporaneous seawater. The ODP 886C record shows rising 187Os/188Os in the deepest portion of the core, with a local maximum of 0.66 close to 74 Ma. The 67-72 Ma portion of the record is characterized by nearly constant 187Os/188Os ratios close to 0.6. The structure of the marine Os isotope record from ODP 886C differs markedly from the seawater 87Sr/86Sr curve, which rises monotonically throughout the time interval studied here. Calculated particulate meteoritic Os fluxes are between 0.5 and 2 pg cm−2 kyr−1 throughout most of the studied interval. Two discrete intervals of the core (one of which is within Cretaceous Tertiary, boundary KTB interval) are characterized by higher fluxes of meteoritic Os. Excluding these two intervals, the average background flux of particulate meteoritic Os is roughly half of that estimated from analyses of Cenozoic marine sediments. These are the first Os isotope data to provide evidence of resolvable temporal variations in the background flux of particulate meteoritic material to the Earth.  相似文献   

3.
The flux of particulate extraterrestrial (ET) matter to the deep-sea has been calculated using a four-component mixing model based on osmium concentrations and isotope ratios in slowly accumulating pelagic sediments from the Pacific Ocean. Nineteen published bulk-leach osmium isotope data pairs that cover the last 80 million years have been used for the calculation. The calculated annual particulate flux, averaged over several 100 ka by slow accumulation and bioturbation, ranges from ∼18,000 to ∼67,000 tons, with a mean value of ∼37,000 ± 13,000 tons. The data indicate no significant variability in the flux with time, except at the K-T boundary and are thus compatible with results based on Ir accumulation in deep-sea sediments. The inferred constancy of the ET matter flux is in contrast to recent results based on the ET 3He flux that integrates only a small size fraction of the entire ET matter flux. 3He data indicate variability by a factor of 6 over the past 70 Ma and a fivefold increase in the ET flux between 3 Ma and 1 Ma that is not seen in the bulk ET matter flux based on osmium isotope data.The apparent constancy of the cosmic matter flux over the past 80 Ma stands in marked contrast to the dynamic nature of the marine osmium isotope record and indicates that dissolution of cosmic matter does not drive changes in this record, except at the K-T boundary. A comparison between osmium isotope and Ir data from two pelagic sediment cores from the Pacific (DSDP Site 596 and LL44-GPC3) indicates that the seawater-soluble fraction of ET Os cannot exceed ∼36 kg/a and most likely is significantly smaller. The maximum value of 36 kg/a can account for approximately one half of the seawater-soluble ET matter flux necessary to balance the radiogenic continental runoff of Os without any additional unradiogenic source. Simple mass balance calculations indicate that an additional unradiogenic source of Os to the oceans, most likely alteration of oceanic crust, is required to balance the present-day seawater osmium-isotopic composition. This source is probably more important in balancing radiogenic continental runoff than is dissolution of cosmic matter in seawater.  相似文献   

4.
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   

5.
《Geochimica et cosmochimica acta》1999,63(23-24):4053-4058
A detailed study was made of the oceanic 187Os/186Os variation through the Cenozoic by using a hydrogen peroxide leaching procedure on a pelagic clay core from the North Pacific (Long Lines-44 GPC3). A 6% H2O2 solution was used. The range of 187Os/186Os obtained by this procedure was between 2 and 8.2. A milder leach (0.15% H2O2) in three out of four samples yielded a slightly higher 187Os/186Os value than the stronger leach implying that the stronger leach released some meteoritic Os with 187Os/186Os of 1. Using published 3He concentrations in GPC3 and cosmic dust 3He and Os fluxes to deep-sea sediments, the extraterrestrial Os concentration was estimated for each core segment and subtracted to yield “corrected” values of 187Os/186Os. The results based on the milder leaching protocol and the 3He correction protocol yield similar values. The “corrected” ocean water 187Os/186Os pattern for the past 25 million years is similar to that obtained by other workers and is compatible with other results for the entire Cenozoic. The variation with time in GPC3, uniquely, provides a statigraphic signature for the Paleogene.  相似文献   

6.
Ultramafic xenoliths entrained in the late Miocene alkali basalts and basanites from NW Turkey include refractory spinel-harzburgites and dunites accompanied by subordinate spinel-lherzolites. Whole-rock major and trace element characteristics indicate that the xenoliths are mostly the solid residues of varying degrees of partial melting (～4–～15%), but some have geochemical signatures reflecting the processes of melt/rock interaction. Mantle-normalized trace element patterns for the peridotites vary from LREE-depleted to strongly LREE-enriched, reflecting multistage mantle processes from simple melt extraction to metasomatic enrichment. Rhenium and platinum group element (PGE) abundances and 187Os/188Os systematics of peridotites were examined in order to identify the nature of the mantle source and the processes effective during variable stages of melt extraction within the sub-continental lithospheric mantle (SCLM). The peridotites are characterized by chondritic Os/Ir and Pt/Ir ratios and slightly supra-chondritic Pd/Ir and Rh/Ir ratios, representing a mantle region similar in composition to the primitive mantle (PM). Moderate enrichment in PPGE (Pd–Pt–Rh)/IPGE (Ir–Os–Ru) ratios with respect to the PM composition in the metasomatized samples, however, reflects compositional modification by sulphide addition during possible post-melting processes. The 187Os/188Os ratios of the peridotites range from 0.11801 to 0.12657. Highly unradiogenic Os isotope compositions (γOs at 10 Ma from –7.0 to –3.2) in the chemically undisturbed mantle residues are accompanied by depletion in Re/Os ratios, suggesting long-term differentiation of SCLM by continuous melt extraction. For the metasomatized peridotites, however, systematic enrichments in PPGE and Re abundances, and the observed positive covariance between 187Re/188Os and γOs can most likely be explained by interaction of solid residues with basaltic melts produced by melting of relatively more radiogenic components in the mantle. Significantly, the wide range of 187Os/188Os ratios characterizing the entire xenolith suite seems to be consistent with multistage evolution of SCLM and suggests that parts of the lithospheric mantle contain materials that have experienced ancient melt removal (～1.3 Ga) which created time-integrated depletion in Re/Os ratios; in contrast, some other parts display evidence indicative of recent perturbation in the Re–Os system by sulphide addition during interaction with metasomatizing melts.  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.
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   

9.
Eighteen picrites (MgO > 13 wt.%) and three related basalts from six Hawaiian volcanoes were analyzed for 187Os/188Os and 186Os/188Os. Variations in these ratios reflect long-term Re/Os and Pt/Os differences in the mantle source regions of these volcanoes. 187Os/188Os ratios vary from ∼0.129 to 0.136, consistent with the range defined by previous studies of Hawaiian picrites and basalts. Samples with lower 187Os/188Os are mainly from Kea trend volcanoes (Mauna Kea and Kilauea), and the more radiogenic samples are mainly from Loa trend volcanoes (Mauna Loa, Hualalai, Koolau and Loihi). As previously suggested, differences in 187Os/188Os between volcanic centers are most consistent with the presence of variable proportions of recycled materials and/or pyroxenitic components in the Hawaiian source.186Os/188Os ratios vary from 0.1198332 ± 26 to 0.1198480 ± 20, with some samples having ratios that are significantly higher than current estimates for the ambient upper mantle. Although the range of 186Os/188Os for the Hawaiian suite is consistent with that reported by previous studies, the new data reveal significant heterogeneities among picrites from individual volcanoes. The linear correlation between 187Os/188Os and 186Os/188Os reported by a previous study is no longer apparent with the larger dataset. The postulated recycled materials and pyroxenites responsible for the dominant variations in 187Os/188Os are likely not responsible for the variations in 186Os/188Os. Such materials are typically characterized by both insufficiently high Os concentrations and Pt/Os to account for the 186Os/188Os heterogeneities. The lack of correspondence between 186Os/188Os variations and the Kea and Loa trends supports this conclusion.The primary cause of 186Os/188Os variations are evaluated within the framework of two mixing scenarios: (1) metasomatic transport of Pt and/or 186Os-rich Os into some portions of the Hawaiian source, and (2) interaction between an isotopically complex plume source with a common, Os- and 186Os-enriched reservoir (COs). Both scenarios require large scale, selective transport of Pt, Re and/or Os. Current estimates of HSE concentrations in the mantle source of these rocks, however, provide little evidence for either process, so the dominant cause of the 186Os/188Os variations remains uncertain.  相似文献   

10.
High precision Os isotope analysis of young marine manganese nodules indicate that whereas the composition of modern seawater is radiogenic with respect to 187Os/188Os, it has 186Os/188Os that is within uncertainty of the chondritic value. Marine Mn nodule compositions thus indicate that the average continental source of Os to modern seawater had long-term high Re/Os compared to Pt/Os. Analyses of loess and freshwater Mn nodules support existing evidence that average upper continental crust (UCC) has resolvably suprachondritic 186Os/188Os, as well as radiogenic 187Os/188Os. Modeling the composition of seawater as a two-component mixture of oceanic/cosmic Os with chondritic Os compositions and continentally-derived Os demonstrates that, insofar as estimates for the composition of average UCC are accurate, congruently weathered average UCC cannot be the sole continental source of Os to seawater. Our analysis of four Cambrian black shales confirm that organic-rich sediments can have 187Os/188Os ratios that are much higher than average UCC, but 186Os/188Os compositions that are generally between those of chondrites and average-UCC. Preferential weathering of black shales can result in dissolved Os discharged to the ocean basins that has a much lower 186Os/188Os than does average upper crust. Modeling the available data demonstrates that augmentation of estimated average UCC compositions with less than 0.1% additional black shale and 1.4% additional ultramafic rock can produce a continental end-member Os isotopic composition that satisfies the requirements imposed by the marine Mn nodule data. The interplay of these two sources provides a mechanism by which the 187Os/188Os of seawater can change as sources and weathering conditions change, yet seawater 186Os/188Os varies only minimally.  相似文献   

11.
New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presented for basalts from the three volcanic flank zones in Iceland and from Jan Mayen Island. The 187Os/188Os ratios in lavas with <30 ppt Os (n = 4) are elevated compared to ratios in coexisting olivine and appear to be contaminated at a shallow level. The 187Os/188Os ratios in the remaining lavas with >30 ppt Os (n = 17) range between 0.12117 and 0.13324. These values are surprisingly low for oceanic island basalts and include some samples that are less than putative present-day primitive upper mantle (PUM with 187Os/188Os of 0.1296). These low 187Os/188Os preclude significant shallow-level contamination from oceanic crust. The 187Os/188Os ratios for Jan Mayen lavas are less than PUM, severely limiting the presence of any continental crust in their mantle source. A positive correlation between 143Nd/144Nd and 187Os/188Os ratios in Iceland and Jan Mayen lavas likely reflects the presence in their source of ancient subcontinental lithosphere that has undergone incompatible trace element enrichment that did not affect the Re-Os system. In addition, the Jan Mayen lava isotopic signature cannot be explained solely by the presence of subcontinental lithospheric mantle, and the influence of another geochemical component, such as a mantle plume appears required. Combined 87Sr/86Sr, 143Nd/144Nd, 3He/4He and 187Os/188Os data indicate a genetic relationship between Jan Mayen Island and the Iceland mantle plume. Material from the Iceland mantle plume likely migrates at depth until it reaches the tensional setting of the Jan Mayen Fracture Zone, where it undergoes low-degree partial melting. At a first-order, isotopic co-variations can be interpreted as broadly binary mixing curves between two primary end-members. One end-member, characterized in particular by its unradiogenic 187Os/188Os and 143Nd/144Nd, low 3He/4He and high 87Sr/86Sr, is represented by subcontinental lithospheric mantle stranded and disseminated in the upper mantle during the opening of the Atlantic Ocean. The second end-member corresponds to a hybrid mixture between the depleted-MORB mantle and the enriched Iceland mantle plume, itself resulting from mixing between recycled oceanic crust and depleted lower mantle. This hybrid accounts for the high 3He/4He (∼28 Ra), high 143Nd/144Nd (∼0.5132), high 187Os/188Os (∼0.14) and low 87Sr/86Sr (∼0.7026) composition observed in Iceland. Two different models may account for these observed mixing relationships between the end-members. In this first model, the Iceland mantle entrains pristine depleted material when rising in the upper mantle and allows refractory sub-lithospheric fragments to melt because of excess heat derived from the deep plume material. A second model that may better account for the Pb isotopic variations observed, uses the same components but where the depleted-MORB mantle is already polluted by subcontinental lithospheric mantle material before mixing with the Iceland mantle plume. Both cases likely occur. Though only three principal components are required to explain the isotopic variations of the Iceland-Jan Mayen system, the different possible mixing relationships may be accounted for by potentially a greater number of end-members.  相似文献   

12.
Picrites from the neovolcanic zones in Iceland display a range in 187Os/188Os from 0.1297 to 0.1381 (γOs = + 2.1 to +8.7) and uniform 186Os/188Os of 0.1198375 ± 32 (2σ). The value for 186Os/188Os is within uncertainty of the present-day value for the primitive upper mantle of 0.1198398 ± 16. These Os isotope systematics are best explained by ancient recycled crust or melt enrichment in the mantle source region. If so, then the coupled enrichments displayed in 186Os/188Os and 187Os/188Os from lavas of other plume systems must result from an independent process, the most viable candidate at present remains core-mantle interaction. While some plumes with high 3He/4He, such as Hawaii, appear to have been subjected to detectable addition of Os (and possibly He) from the outer core, others such as Iceland do not.A positive correlation between 187Os/188Os and 3He/4He from 9.6 to 19 Ra in Iceland picrites is best modeled as mixtures of 1 Ga or older ancient recycled crust mixed with primitive mantle or incompletely degassed depleted mantle isolated since 1-1.5 Ga, which preserves the high 3He/4He of the depleted mantle at the time. These mixtures create a hybrid source region that subsequently mixes with the present-day convecting MORB mantle during ascent and melting. This multistage mixing scenario requires convective isolation in the deep mantle for hundreds of million years or more to maintain these compositionally distinct hybrid sources. The 3He/4He of lavas derived from the Iceland plume changed over time, from a maximum of 50 Ra at 60 Ma, to approximately 25-27 Ra at present. The changes are coupled with distinct compositional gaps between the different aged lavas when 3He/4He is plotted versus various geochemical parameters such as 143Nd/144Nd and La/Sm. These relationships can be interpreted as an increase in the proportion of ancient recycled crust in the upwelling plume over this time period.The positive correlation between 187Os/188Os and 3He/4He demonstrates that the Iceland lava He isotopic compositions do not result from simple melt depletion histories and consequent removal of U and Th in their mantle sources. Instead their He isotopic compositions reflect mixtures of heterogeneous materials formed at different times with different U and Th concentrations. This hybridization is likely prevalent in all ocean island lavas derived from deep mantle sources.  相似文献   

13.
To characterize the compositions of materials accreted to the Earth-Moon system between about 4.5 and 3.8 Ga, we have determined Os isotopic compositions and some highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundances in 48 subsamples of six lunar breccias. These are: Apollo 17 poikilitic melt breccias 72395 and 76215; Apollo 17 aphanitic melt breccias 73215 and 73255; Apollo 14 polymict breccia 14321; and lunar meteorite NWA482, a crystallized impact melt. Plots of Ir versus other HSE define excellent linear correlations, indicating that all data sets likely represent dominantly two-component mixtures of a low-HSE target, presumably endogenous component, and a high-HSE, presumably exogenous component. Linear regressions of these trends yield intercepts that are statistically indistinguishable from zero for all HSE, except for Ru and Pd in two samples. The slopes of the linear regressions are insensitive to target rock contributions of Ru and Pd of the magnitude observed; thus, the trendline slopes approximate the elemental ratios present in the impactor components contributed to these rocks. The 187Os/188Os and regression-derived elemental ratios for the Apollo 17 aphanitic melt breccias and the lunar meteorite indicate that the impactor components in these samples have close affinities to chondritic meteorites. The HSE in the Apollo 17 aphanitic melt breccias, however, might partially or entirely reflect the HSE characteristics of HSE-rich granulitic breccia clasts that were incorporated in the impact melt at the time of its creation. In this case, the HSE characteristics of these rocks may reflect those of an impactor that predated the impact event that led to the creation of the melt breccias. The impactor components in the Apollo 17 poikilitic melt breccias and in the Apollo 14 breccia have higher 187Os/188Os, Pt/Ir, and Ru/Ir and lower Os/Ir than most chondrites. These compositions suggest that the impactors they represent were chemically distinct from known chondrite types, and possibly represent a type of primitive material not currently delivered to Earth as meteorites.  相似文献   

14.
Analyses of enriched mantle (EM)-basalts, using lithophile element-based isotope systems, have long provided evidence for discrete mantle reservoirs with variable composition. Upon partial melting, the mantle reservoir imparts its isotopic fingerprint upon the partial melt produced. However, it has increasingly been recognised that it may not be simple to delimit these previously well-defined mantle reservoirs; the “mantle zoo” may contain more reservoirs than previously envisaged.Here we demonstrate that a simple model with varying contributions from two populations of compositionally distinct mantle sulphides can readily account for the observed heterogeneities in Os isotope systematics of such basalts without additional mantle reservoirs. Osmium elemental and isotopic analyses of individual sulphide grains separated from spinel lherzolites from Kilbourne Hole, New Mexico, USA demonstrate that two discrete populations of mantle sulphide exist in terms of both Re-Os systematics and textural relationship with co-existing silicates. One population, with a rounded morphology, is preserved in silicate grains and typically possesses high [Os] and low [Re] with unradiogenic, typically sub-chondritic 187Os/188Os attributable to long term isolation in a low-Re environment. By contrast, irregular-shaped sulphides, preserved along silicate grain boundaries, possess low [Os], higher [Re] and a wider range of, but generally supra-chondritic 187Os/188Os ([Os] typically ? 1-2 ppm, 187Os/188Os ? 0.3729; this study). This population is thought to represent metasomatic sulphide.Uncontaminated silicate phases contain negligible Os (<100 ppt) therefore the Os elemental and isotope composition of basalts is dominated by volumetrically insignificant sulphide ([Os] ? 37 ppm; this study). During the early stages of partial melting, supra-chondritic interstitial sulphides are mobilised and incorporated into the melt, adding their radiogenic 187Os/188Os signature. Only when sulphides armoured within silicates are exposed to the melt through continued partial melting will enclosed sulphides add their high [Os] and unradiogenic 187Os/188Os to the aggregate melt. Platinum-group element data for whole rocks are also consistent with this scenario. The sequence of (i) addition of all of the metasomatic sulphide, followed by (ii) the incorporation of small amounts of armoured sulphide can thus account for the range of both [Os] and 187Os/188Os of EM-basalts worldwide without the need for contributions from additional silicate mantle reservoirs.  相似文献   

15.
Reported here are the first 187Os/188Os ratios and abundances of Os and Re for Taklimakan Desert sands and glacial moraines from the Kunlun Mountains. Osmium isotopic data are also reported for river sediments around the Taklimakan Desert, river sediments from the Kunlun and Tianshan Mountains, Tibetan soils and loesses from the Loess Plateau, as well as Sr and Nd isotopic data for these samples. The Taklimakan Desert sands from various regions show surprisingly homogeneous Os isotopic ratios (187Os/188Os = 1.29 ± 0.08) and abundances (Os = 11 ppt), with some variations in Re abundances (Re = 130 to 260 ppt) and 187Re/188Os ratios (60 to 140). The 187Os/188Os ratios for the Taklimakan Desert sands are close to the average for Kunlun moraines, river sediments around the Taklimakan Desert sands, and the Tibetan soils, supporting the idea that the Taklimakan Desert sands are derived from moraines and river sediments around the desert or from Tibetan soils and are homogenized by aeolian activity in the desert. Furthermore, the Os isotopic data for the sediments studied here are compared with those (187Os/188Os = 1.04, Os = 32 ppt, Re = 206 ppt, 187Re/188Os = 35) of loesses from the Loess Plateau reported elsewhere, and it is concluded that the Re-Os data for the loess can be used as proxy for the upper continental crust.  相似文献   

16.
The Mayarí-Baracoa ophiolitic belt in eastern Cuba hosts abundant chromite deposits of historical economic importance. Among these deposits, the chemistry of chromite ore is very variable, ranging from high Al (Cr#=0.43–0.55) to high Cr (Cr#=0.60–0.83) compositions. Platinum-group element (PGE) contents are also variable (from 33 ppb to 1.88 ppm) and correlate positively with the Cr# of the ore. Bulk PGE abundances correlate negatively with the Pd/Ir ratio showing that chromite concentrates mainly Os, Ir and Ru which gives rise to the characteristic negatively sloped, chrondrite-normalized PGE patterns in many chromitites. This is consistent with the mineralogy of PGEs, which is dominated by members of the laurite–erlichmanite solid solution series (RuS2–OsS2), with minor amounts of irarsite (IrAsS), Os–Ir alloys, Ru–Os–Ir–Fe–Ni alloys, Ni–Rh–As, and sulfides of Ir, Os, Rh, Cu, Ni, and/or Pd. Measured 187Os/188Os ratios (from 0.1304 to 0.1230) are among the lower values reported for podiform chromitites. The 187Os/188Os ratios decrease with increasing whole-rock PGE contents and Cr# of chromite. Furthermore, γOs values of all but one of the chromitite samples are negative indicating a subchondiritc mantle source. γOs decrease with increasing bulk Os content and decreasing 187Re/188Os ratios. These mineralogical and geochemical features are interpreted in terms of chromite crystallization from melts varying in composition from back-arc basalts (Al-rich chromite) to boninites (Cr-rich chromite) in a suprasubduction zone setting. Chromite crystallization occurs as a consequence of magma mixing and assimilation of preexisting gabbro sills at the mantle–crust transition zone. Cr#, PGE abundances, and bulk Os isotopic composition of chromitites are determined by the combined effects of mantle source heterogeneity, the degree of partial melting, the extent of melt-rock interactions, and the local sulfur fugacity. Small-scale (μm to cm) chemical and isotopic heterogeneities in the platinum-group minerals are controlled by the mechanism(s) of chromite crystallization in a heterogeneous environment created by the turbulent regime generated by successive inputs of different batches of melt.  相似文献   

17.
含有普通锇的辉钼矿Re-Os同位素定年研究   总被引：14，自引：3，他引：11  
通过大量数据统计,表明较高比例的辉钼矿中存在普通锇。普通锇可能以类质同像形式存在于辉钼矿样品中,理论上辉钼矿中可能含有较高含量普通锇。辉钼矿样品含有较高含量普通锇可能对Re-Os定年结果产生很大影响,从原理上并结合实例证实了普通锇含量对辉钼矿Re-Os年龄影响程度。对于一般辉钼矿样品来讲,如果187Os总量(放射成因187Os与非放射成因187Os之和)与普通锇比值小于20,需要考虑普通锇对Re-Os模式年龄的影响,并提出了对于含有普通锇辉钼矿模式年龄的计算方法。先做出187Os/188Os-187Re/188Os等时线,求得初始187Os/188Os值,再根据初始187Os/188Os值和单个样品的普Os含量求得非放射成因的普Os中187Os的量。最后根据Re含量以及放射成因187Os含量得到模式年龄。  相似文献   

18.
The marine osmium isotope record   总被引：3，自引：0，他引：3  
Over the past decade the marine osmium isotope record has been developed into a new tracer in palaeoceanographic research. Several analytical developments, particularly in the past few years, have significantly increased our ability to study the behaviour of osmium in the surficial environment. The 187Os/188Os and osmium concentration of seawater, river water, rain, and hydrothermal vent fluids have been measured directly. Recently, the behaviour of osmium in estuaries–critical for estimating the marine residence time of osmium–has been studied. Our knowledge of the surficial osmium cycle has thus significantly improved. In addition, reconstructions of past variations in the marine 187Os/188Os recently have been extended back into the Mesozoic. This review attempts to summarize our current understanding of the marine osmium system–present and past. The 187Os/188Os of seawater during the Cenozoic to first order mimics the marine 87Sr/86Sr record. It is therefore tempting to interpret both records as reflecting increased input of radiogenic osmium and strontium resulting from enhanced continental weathering regulated by climatic/tectonic processes. However, the marine osmium isotope system differs fundamentally from the marine strontium isotope system. This review emphasizes three important differences. First, large impacts are capable of resetting the 187Os/188Os to unradiogenic values without significantly affecting the marine strontium system. Second, organic‐rich sediments are characterized by high 187Re/188Os; resulting 187Os/188Os ingrowth‐trajectories are similar to the average slope of the Cenozoic 187Os/188Os seawater record. Trends towards more radiogenic 187Os/188Os seawater therefore can be caused by weathering of organic‐rich sediments at a constant rate. Third, the marine residence time of osmium is sufficiently short to capture short‐periodic (glacial‐interglacial) fluctuations that are inaccessible to the buffered marine strontium isotope system. This offers the opportunity to discriminate between high‐frequency (climatic) and low‐frequency (tectonic) forcing.  相似文献   

19.
Podiform chromite deposits occur in the mantle sequences of many ophiolites that were formed in supra-subduction zone (SSZ) settings. We have measured the Re-Os isotopic compositions of the major chromite deposits and associated mantle peridotites of the Dongqiao Ophiolite in the Bangong-Nujiang suture, Tibet, to investigate the petrogenesis of these rocks and their genetic relationships.The 187Os/188Os ratios of the chromite separates define a narrow range from 0.12318 to 0.12354, less variable than those of the associated peridotites. Previously-reported 187Os/188Os ratios of the Os-rich alloys enclosed in the chromitites define two clusters: 0.12645 ± 0.00004 (2 s; n = 145) and 0.12003 to 0.12194. The ultra-depleted dunites have much lower 187Os/188Os (0.11754, 0.11815), and the harzburgites show a wider range from 0.12107 to 0.12612. The average isotopic composition of the chromitites (187Os/188Os: 0.12337 ± 0.00001) is low compared with the carbonaceous chondrite value (187Os/188Os: 0.1260 ± 0.0013) and lower than the average value measured for podiform chromitites worldwide (0.12809 ± 0.00085). In contrast, the basalts have higher 187Os/188Os, ranging from 0.20414 to 0.38067, while the plagioclase-bearing harzburgite and cumulates show intermediate values of 187Os/188Os (0.12979 ~ 0.14206). Correspondingly, the basalts have the highest 187Re/188Os ratios, up to 45.4 ± 3.2, and the chromites have the lowest 187Re/188Os ratios, down to 0.00113 ± 0.00008. We suggest that melts/fluids, derived from the subducting slab, triggered partial melting in the overlying mantle wedge and added significant amounts of radiogenic Os to the peridotites. Mass-balance calculations indicate that a melt/mantle ratio of approximately 15:1 (melt: 187Re/188Os: 45.4, 187Os/188Os: 0.34484; mantle peridotite: 187Re/188Os: 0.0029, 187Os/188Os: 0.11754) is necessary to increase the Os isotopic composition of the chromitite deposits to its observed average value. This value implies a surprisingly low average melt/mantle ratio during the formation of the chromitite deposits. The percolating melts probably were of variable isotopic composition. However, in the chromitite pods the Os from many melts was pooled and homogenized, which is why the chromitite deposits show such a small variation in their Os isotopic composition. The results of this study suggest that the 187Os/188Os ratios of chromitites may not be representative of the DMM, but only reflect an upper limit. Importantly, the Os-isotope compositions of chromitites strongly suggest that such deposits can be formed by melt/mantle mixing processes.  相似文献   

20.
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   

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