Pt-Os同位素体系在地球科学研究中的应用

1 90 Pt通过α衰变生成1 86Os ,目前采用的衰变常数λ =1 .5 4 2× 1 0 -1 2 a-1 是由诺里尔斯克矿床的Pt Os等时线确定的。IIAB群铁陨石的1 90 Pt 1 86Os等时线年龄为 (4 2 90± 2 84 )Ma(2σ) ,说明Pt Os定年方法可能主要适合于古老样品。Pt Os体系的示踪研究以夏威夷现代地幔柱和诺里尔斯克矿床 (与西伯利亚地幔柱有关 )等为例 ,夏威夷苦橄岩的N(1 86Os) /N(1 88Os)值为 0 .1 1 98339～ 0 .1 1 985 2 6 ,γ(Os)为 1 .2 3～ 9.0 6 ,诺里尔斯克矿石的N(1 86Os) /N(1 88Os)为 0 .1 1 984 9,γ(Os)为 8,这种1 86Os/ 1 88Os 1 87Os/ 1 88Os双异常现象揭示了地幔柱物质中有来自地球外核的Os的贡献 ,这是迄今为止Pt Os体系对地球科学研究的最大贡献。Pt Os体系在壳源岩石方面的研究以肖德贝里Cu Ni硫化物矿床为例。研究支持肖德贝里的矿石主要来自地壳的结论 ,并可用二元混合模型 (太古宙苏必利尔省和元古宙Huronian省的沉积变质岩 )来解释其1 87Os/ 1 88Os同位素组成。但是该矿区的某些矿床的1 86Os/ 1 88Os比值偏高 ,需要有古老的Pt/Os比高的镁铁质岩的加入来解释     

Os mobilization during melt percolation: The evolution of Os isotope heterogeneities in the mantle sequence of the troodos ophiolite, Cyprus

This study focuses on the origin of the Os isotope heterogeneities and the behaviour of Os and Re during melt percolation and partial melting processes in the mantle sequence of the Troodos Ophiolite Complex. The sequence has been divided into an eastern (Unit 1) and a western part (Unit 2) (Batanova and Sobolev, 2000). Unit 1 consists mainly of spinel-lherzolites and a minor amount of dunites, which are surrounded by cpx-bearing harzburgites. Unit 2 consists of harzburgites, dunites, and contains chromitite deposits.Unit 1 (¹⁸⁷Os/¹⁸⁸Os: 0.1169 to 0.1366) and Unit 2 (¹⁸⁷Os/¹⁸⁸Os 0.1235 to 0.1546) peridotites both show large ranges in their Os isotopic composition. Most of the ¹⁸⁷Os/¹⁸⁸Os ratios of Unit 1 lherzolites and harzburgites are chondritic to subchondritic, and this can be explained by Re depletion during ancient partial melting and melt percolation events. The old Os isotope model ages (>800 Ma) of some peridotites in a young ophiolitic mantle show that ancient Os isotopic heterogeneities can survive in the Earth upper mantle. Most harzburgites and dunites of Unit 2 have suprachondritic ¹⁸⁷Os/¹⁸⁸Os ratios. This is the result of the addition of radiogenic Os during a younger major melt percolation event, which probably occurred during the formation of the Troodos crust 90 Ma ago.Osmium concentrations tend to decrease from spinel-lherzolites (4.35 ± 0.2 ng/g) to harzburgites (Unit 1: 4.06 ± 1.12 ng/g; Unit 2: 3.46 ± 1.38 ng/g) and dunites (Unit 1: 2.71 ± 0.84 ng/g; Unit 2: 1.85 ± 1.20 ng/g). Therefore, this element does not behave compatibly during melt percolation as it is observed during partial melting, but becomes dissolved and mobilized by the percolating melt. The Os contents and Re/Os ratios in the mantle peridotites can be explained if they represent mixing products of old depleted mantle with cpx- and opx-veins, which are crystallization products of the percolating melt. This mixing occurred during the melting of a continuously fluxed mantle in a supra-subduction zone environment.This study shows that Unit 1 and Unit 2 of the Troodos mantle section have a complex and different evolution. However, the Os isotopic characteristics are consistent with a model where the harzburgites and dunites of both units belong to the same melting regime producing the Troodos oceanic crust.     

Formation of pyroxenite layers in the Totalp ultramafic massif (Swiss Alps) - Insights from highly siderophile elements and Os isotopes

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 γ_Os (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 γ_Os (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.     

Melt/mantle mixing produces podiform chromite deposits in ophiolites: Implications of Re-Os systematics in the Dongqiao Neo-tethyan ophiolite, northern Tibet

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.     

Constraints on mantle evolution from Os/Os isotopic compositions of Archean ultramafic rocks from southern West Greenland (3.8 Ga) and Western Australia (3.46 Ga)

Initial ¹⁸⁷Os/¹⁸⁸Os isotopic compositions for geochronologically and geologically well -constrained 3.8-Ga spinel peridotites from the Itsaq Gneiss Complex of southern West Greenland and chromite separates from 3.46-Ga komatiites from the Pilbara region of Western Australia have been determined to investigate the osmium isotopic evolution of the early terrestrial mantle. The measured compositions of ¹⁸⁷Os/¹⁸⁸Os(0) = 0.10262 ± 2, from an olivine separate, and 0.10329 ± 3, for a spinel separate from ∼3.8-Ga peridotite G93/42, are the lowest yet reported from any terrestrial sample. The corrections for in situ decay over 3.8 Ga for these low Re/Os phases are minimal and change the isotopic compositions by only 0.5 and 2.2% for the spinel and the olivine, respectively, resulting in ¹⁸⁷Os/¹⁸⁸Os_{(3.8 Ga)} = 0.1021 ± 0.0002 and 0.1009 ± 0.0002, respectively. These data extend direct measurement of Os isotopic compositions to much earlier periods of Earth history than previously documented and provide the best constraints on the Os isotopic composition of the early Archean terrestrial mantle. Analyses of Pilbara chromites yield 3.46-Ga mantle compositions of 0.1042 ± 0.0002 and 0.1051 ± 0.0002.These new data, combined with published initial Os isotopic compositions from late Archean and early Proterozoic samples, are compatible with the mantle, or at least portions of it, evolving from a solar system initially defined by meteorites to a modern composition of ¹⁸⁷Os/¹⁸⁸Os(0) = 0.1296 ± 0.0008 as previously suggested from peridotite xenolith data ( Meisel et al., 2001); the associated ¹⁸⁷Re/¹⁸⁸Os(0) = 0.435 ± 0.005. Thus, chondritic ¹⁸⁷Os/¹⁸⁸Os compositions were a feature of the upper mantle for at least 3.8 billion years, requiring chondritic Re/Os ratios to have been a characteristic of the very early terrestrial mantle. In contrast, nonchondritic initial compositions of some Archean komatiites demonstrate that Os isotopic heterogeneity is an ancient feature of plume materials, reflecting the development of variable Re/Os mantle sources early in Earth history.The lower average ¹⁸⁷Os/¹⁸⁸Os = 0.1247 for abyssal peridotites (Snow and Reisberg, 1995) indicate that not all regions of the modern mantle have evolved with the same Re/Os ratio. The relative sizes of the various reservoirs are unknown, although mass balance considerations can provide some general constraints. For example, if the unradiogenic ¹⁸⁷Os/¹⁸⁸Os modern abyssal peridotite compositions reflect the prevalent upper mantle composition, then the complementary high Re/Os basaltic reservoir must represent 20 to 40% by mass of the upper mantle (taken here as 50% of the entire mantle), depending on the mean storage age. The difficulties associated with efficient long-term storage of such large volumes of subducted basalt suggest that the majority of the upper mantle is not significantly Re-depleted. Rather, abyssal peridotites sample anomalous mantle regions.The existence of 3.8-Ga mantle peridotites with chondritic ¹⁸⁷Os/¹⁸⁸Os compositions and with Os concentrations similar to the mean abundances measured in modern peridotites places an upper limit on the timing of a late accretionary veneer. These observations require that any highly siderophile element -rich component must have been added to the Earth and transported into and grossly homogenized within the mantle by 3.8 Ga. Either large-scale mixing of impact materials occurred on very short (0-100 myr) timescales or (the interpretation preferred here) the late veneer of highly siderophile elements is unrelated to the lunar terminal cataclysm estimated to have occurred at ∼3.8 to 3.9 Ga.     

Age and sources of the ore at Tharsis and Rio Tinto, Iberian Pyrite Belt, from Re-Os isotopes

Re-Os isotopes were used to constrain the source of the ore-forming elements of the Tharsis and Rio Tinto mines of the Iberian Pyrite Belt, and the timing of mineralization. The pyrite from both mines has simila]r Os and Re concentrations, ranging between 0.05–0.7 and 0.6–66 ppb, respectively. ¹⁸⁷Re/¹⁸⁸Os ratios range from about 14 to 5161. Pyrite-rich ore samples from the massive ore of Tharsis and two samples of stockwork ore from Rio Tinto yield an isochron with an age of 346 ± 26 Ma, and an initial ¹⁸⁷Os/¹⁸⁸Os ratio of about 0.69. Five samples from Tharsis yield an age of 353 ± 44 Ma with an initial ¹⁸⁷Os/¹⁸⁸Os ratio of about 0.37. A sample of massive sulfide ore from Tharsis and one from Rio Tinto lie well above both isochrons and could represent Re mobilization after mineralization. The pyrite Re-Os ages agree with the paleontological age of 350 Ma of the black shales in which the ores are disseminated. Our data do not permit us to determine whether the Re-Os isochron yields the original age of ore deposition or the age of the Hercynian metamorphism that affected the ores. However, the reasonable Re-Os age reported here indicates that the complex history of the ores that occurred after the severe metamorphic event that affected the Iberian Pyrite Belt massive sulfide deposits did not fundamentally disturb the Re-Os geochronologic system. The highly radiogenic initial Os isotopic ratio agrees with previous Pb isotopic studies. If the initial ratio is recording the initial and not the metamorphic conditions, then the data indicate that the source of the metals was largely crustal. The continental margin sediments that underlie the deposits (phyllite-quartzite group) or the volcanic rocks (volcanogenic-sedimentary complex) in which the ores occur are plausible sources for the ore-forming metals and should constrain the models for the genesis of these deposits. Received: 15 March 1999 / Accepted: 26 July 1999     

Platinum-osmium isotope evolution of the Earth’s mantle: Constraints from chondrites and Os-rich alloys

Separation of a metal-rich core strongly depleted the silicate portion of the Earth in highly siderophile elements (HSE), including Pt, Re, and Os. To address the issues of how early differentiation, partial melting, and enrichment processes may have affected the relative abundances of the HSE in the upper mantle, ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁶Os/¹⁸⁸Os data for chondrites are compared with data for Os-rich alloys from upper mantle peridotites. Given that ¹⁸⁷Os and ¹⁸⁶Os are decay products of ¹⁸⁷Re and ¹⁹⁰Pt, respectively, these ratios can be used to constrain the long-term Re/Os and Pt/Os of mantle reservoirs in comparison to chondrites. Because of isotopic homogeneity, H-group ordinary and other equilibrated chondrites may be most suitable for defining the initial ¹⁸⁶Os/¹⁸⁸Os of the solar system. The ¹⁸⁶Os/¹⁸⁸Os ratios for five H-group ordinary chondrites range only from 0.1198384 to 0.1198408, with an average of 0.1198398 ± 0.0000016 (2σ). Using the measured Pt/Os and ¹⁸⁶Os/¹⁸⁸Os for each chondrite, the calculated initial ¹⁸⁶Os/¹⁸⁸Os at 4.567 Ga is 0.1198269 ± 0.0000014 (2σ). This is the current best estimate for the initial ¹⁸⁶Os/¹⁸⁸Os of the bulk solar system. The mantle evolution of ¹⁸⁶Os/¹⁸⁸Os can be defined via examination of mantle-derived materials with well-constrained ages and low Pt/Os. Two types of mantle-derived materials that can be used for this task are komatiites and Os-rich alloys. The alloys are particularly valuable in that they have little or no Re or Pt, thus, when formed, evolution of both ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁶Os/¹⁸⁸Os ceases. Previously published results for an Archean komatiite and new results for Os-rich alloys indicate that the terrestrial mantle evolved with Pt-Os isotopic systematics that were indistinguishable from the H-group ordinary and some enstatite chondrites. This corresponds to a Pt/Os of 2.0 ± 0.2 for the primitive upper mantle evolution curve. This similarity is consistent with previous arguments, based on the ¹⁸⁷Os/¹⁸⁸Os systematics and HSE abundances in the mantle, for a late veneer of materials with chondritic bulk compositions controlling the HSE budget of the upper mantle. It is very unlikely that high pressure metal-silicate segregation leading to core formation can account for the elemental and isotopic compositions of HSE in the upper mantle.     

Os isotopic constraints on Archean mantle dynamics

The Re-Os isotopic systematics of two ca. 2.7-Ga komatiite flows from Belingwe, Zimbabwe are examined. Rhenium and Os concentrations in these rocks are similar to concentrations in other Archean, Proterozoic, and Phanerozoic komatiites. Despite the excellent preservation of primary magmatic minerals, the Re-Os systematics of whole-rock samples of the komatiites show open-system behavior. Consistent model ages for several whole-rock samples suggest a disturbance to the system during the Proterozoic. Despite the open-system behavior in the whole rocks, Re-Os systematics for concentrates of primary magmatic olivine and spinel indicate generally closed-system behavior since the magmatic event that produced the rocks. Regression of the data for the mineral concentrates yields an age of 2721 ± 21 Ga, which is consistent with Pb-Pb and Sm-Nd ages that have been previously reported for the komatiites (Chauvel et al., 1993), and an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.11140 ± 84 (γ_Os = +2.8 ± 0.8).The 2 to 3% enrichment in ¹⁸⁷Os/¹⁸⁸Os ratio of the mantle source of the komatiites, relative to the chondritic composition of the contemporaneous convecting upper mantle, most likely reflects either the incorporation of substantially older (≥ 4.2 Ga), Re-rich recycled mafic crust into the mantle source of the komatiites or the contribution of suprachondritic Os to the source from the putative ¹⁸⁷Os-enriched outer core. The former interpretation would indicate the Hadean formation and recycling of mafic crust. The latter interpretation would require early formation of a substantial inner core followed by upwelling of a mantle plume from the core-mantle boundary, at least as far back as the Late Archean. Either interpretation requires large-scale mantle convection during the first half of Earth history.     

Mineralogical and geochemical constraints on the shallow origin, ancient veining, and multi-stage modification of the Lherz peridotite

New major- and trace-element data of bulk-rocks and constituent minerals, and whole-rock Re-Os isotopic compositions of samples from the Lherz Massif, French Pyrenees, reveal complex petrological relationships between the dominant lithologies of lherzolite ± olivine-websterite and harzburgite. The Lherz peridotite body contains elongate, foliation parallel, lithological strips of harzburgite, lherzolite, and olivine-websterite cross-cut by later veins of hornblende-bearing pyroxenites. Peridotite lithologies are markedly bimodal, with a clear compositional gap between harzburgites and lherzolites ± olivine-websterite. Bulk-rock and mineral major-element oxide (Mg-Fe-Si-Cr) compositions show that harzburgites are highly-depleted and result from ∼20-25 wt.% melt extraction at pressures <2 GPa. Incompatible and moderately-compatible trace-element abundances of hornblendite-free harzburgites are analogous to some mantle-wedge peridotites. In contrast, lherzolites ± olivine-websterite overlap estimates of primitive mantle composition, yet these materials are composite samples that represent physical mixtures of residual lherzolites and clinopyroxene dominated cumulates equilibrated with a LREE-enriched tholeiitic melt. Trace-element compositions of harzburgite, and some lherzolite bulk-rocks and pyroxenes have been modified by; (1) wide-spread interaction with a low-volume LREE-enriched melt +/− fluid that has disturbed highly-incompatible elements (e.g., LREEs, Zr) without enrichment of alkali- and Ti-contents; and (2) intrusion of relatively recent, small-volume, hornblendite-forming, basanitic melts linked to modal and cryptic metasomatism resulting in whole-rock and pyroxene Ti, Na and MREE enrichment.Rhenium-Os isotope systematics of Lherz samples are also compositionally bimodal; lherzolites ± olivine-websterite have chondritc to suprachondritic ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁷Re/¹⁸⁸Os values that overlap the range reported for Earth’s primitive upper mantle, whereas harzburgites have sub-chondritic ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁷Re/¹⁸⁸Os values. Various Os-model age calculations indicate that harzburgites, lherzolites, and olivine-websterites have been isolated from convective homogenisation since the Meso-Proterozoic and this broadly coincides with the time of melt extraction controlled by harzburgite Os-isotope compositions. The association between harzburgites resulting from melting in mantle-wedge environments and Os-rich trace-phases (laurite-erlichmanite sulphides and Pt-Os-Ir-alloys) suggests that a significant portion of persistent refractory anomalies in the present-day convecting mantle of Earth may be linked to ancient large-scale melting events related to wide-spread subduction-zone processing.     

Further evaluation of the Re-Os geochronometer in organic-rich sedimentary rocks: a test of hydrocarbon maturation effects in the Exshaw Formation, Western Canada Sedimentary Basin

Re-Os isotopic analyses of a single organic-rich sedimentary rock unit (ORS) of known depositional age, and at three levels of regional hydrocarbon maturity, show that hydrocarbon maturation does not affect the ability the ¹⁸⁷Re-¹⁸⁷Os chronometer to yield a depositional age for such rocks. We present Re-Os isotope analyses from the Late Devonian Exshaw Formation in the subsurface of the Western Canada Sedimentary Basin, Alberta, and obtain a Re-Os isochron age of 358 ± 10 Ma (2σ, Model 3, λ = 1.666 × 10⁻¹¹.a⁻¹) for samples ranging from hydrocarbon immature to overmature. This age is within uncertainty of the established absolute age for the Exshaw Formation. Hydrocarbon immature, and mature plus overmature samples show no significant age differences if regressed individually, indicating that hydrocarbon maturation did not greatly disturb the Re-Os isotope system in the Exshaw Formation. As such, we propose that the Re-Os geochronometer may be used as a reliable tool for measuring the depositional ages of ORS regardless of their level of hydrocarbon maturity. We find that minimizing natural variation in the initial ¹⁸⁷Os/¹⁸⁸Os ratio is more important than avoiding hydrocarbon maturation in obtaining precise Re-Os ages. In particular, the Exshaw Formation appears to contain a nonhydrogenous component of unradiogenic Os, in addition to the hydrogenous Os load. A subset of Exshaw Formation samples with >5% total organic carbon (TOC), which should best reflect the hydrogenous Os load alone, yields a very well-fitted isochron having a depositional age of 358 ± 9 Ma (2σ, λ = 1.666 × 10⁻¹¹.a⁻¹) with an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.59 ± 0.05 (Model 3, Mean Square of Weighted Deviates (MSWD) = 1.8). The initial ¹⁸⁷Os/¹⁸⁸Os ratio of this regression may provide an estimate of the Os isotopic composition of local seawater at the time of deposition.     

Comparative Re-Os systematics of chondrites: Implications regarding early solar system processes

A suite of 47 carbonaceous, enstatite, and ordinary chondrites are examined for Re-Os isotopic systematics. There are significant differences in the ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios of carbonaceous chondrites compared with ordinary and enstatite chondrites. The average ¹⁸⁷Re/¹⁸⁸Os for carbonaceous chondrites is 0.392 ± 0.015 (excluding the CK chondrite, Karoonda), compared with 0.422 ± 0.025 and 0.421 ± 0.013 for ordinary and enstatite chondrites (1σ standard deviations). These ratios, recast into elemental Re/Os ratios, are as follows: 0.0814 ± 0.0031, 0.0876 ± 0.0052 and 0.0874 ± 0.0027, respectively. Correspondingly, the ¹⁸⁷Os/¹⁸⁸Os ratios of carbonaceous chondrites average 0.1262 ± 0.0006 (excluding Karoonda), and ordinary and enstatite chondrites average 0.1283 ± 0.0017 and 0.1281 ± 0.0004, respectively (1σ standard deviations). The new results indicate that the Re/Os ratios of meteorites within each group are, in general, quite uniform. The minimal overlap between the isotopic compositions of ordinary and enstatite chondrites vs. carbonaceous chondrites indicates long-term differences in Re/Os for these materials, most likely reflecting chemical fractionation early in solar system history.A majority of the chondrites do not plot within analytical uncertainties of a 4.56-Ga reference isochron. Most of the deviations from the isochron are consistent with minor, relatively recent redistribution of Re and/or Os on a scale of millimeters to centimeters. Some instances of the redistribution may be attributed to terrestrial weathering; others are most likely the result of aqueous alteration or shock events on the parent body within the past 2 Ga.The ¹⁸⁷Os/¹⁸⁸Os ratio of Earth’s primitive upper mantle has been estimated to be 0.1296 ± 8. If this composition was set via addition of a late veneer of planetesimals after core formation, the composition suggests the veneer was dominated by materials that had Re/Os ratios most similar to ordinary and enstatite chondrites.     

Re-Os and Pd-Ag systematics in Group IIIAB irons and in pallasites

Using improved analytical techniques, which reduce the Re blanks by factors of 8 to 10, we report new Re-Os data on low Re and low PGE pallasites (PAL-anom) and IIIAB irons. The new pallasite samples nearly double the observed range in Re/Os for pallasites and allow the determination of an isochron of slope 0.0775 ± 0.0008 (T = 4.50 ± 0.04 Ga, using the adjusted λ¹⁸⁷Re = 1.66 × 10⁻¹¹ a⁻¹) and initial (¹⁸⁷Os/¹⁸⁸Os)₀ = 0.09599 ± 0.00046. If the data on different groups of pallasites (including the “anomalous” pallasites) are considered to define a whole-rock isochron, their formation would appear to be distinctly younger than for the iron meteorites by ∼60 Ma. Five IIIAB irons (Acuna, Bella Roca, Chupaderos, Grant, and Bear Creek), with Re contents ranging from 0.9 to 2.8 ppb, show limited Re/Os fractionation and plot within errors on the IIAB iron meteorite isochron of slope 0.07848 ± 0.00018 (T = 4.56 ± 0.01 Ga) and initial (¹⁸⁷Os/¹⁸⁸Os)₀ = 0.09563 ± 0.00011. Many of the meteorites were analyzed also for Pd-Ag and show ¹⁰⁷Ag enrichments correlated with Pd/Ag, requiring early formation and fractionation of the FeNi metal, in a narrow time interval, after injection of live ¹⁰⁷Pd (t_1/2 = 6.5 Ma) into the solar nebula. Based on Pd-Ag, the typical range in relative ages of these meteorites is ≤10 Ma. The Pd-Ag results suggest early formation and preservation of the ¹⁰⁷Pd-¹⁰⁷Ag systematics, both for IIIAB irons and for pallasites, while the younger Re-Os apparent age for pallasites suggests that the Re-Os system in pallasites was subject to re-equilibration. The low Re and low PGE pallasites show significant Re/Os fractionation (higher Re/Os) as the Re and PGE contents decrease. By contrast, the IIIAB irons show a restricted range in Re/Os, even for samples with extremely low Re and PGE contents. There is a good correlation of Re and Ir contents. The correlation of Re and Os contents for IIIAB irons shows a similar complex pattern as observed for IIAB irons (Morgan et al., 1995), and neither can be ascribed to a continuous fractional crystallization process with uniform solid-metal/liquid-metal distribution coefficients.     

Pt-Re-Os and Sm-Nd isotope and HSE and REE systematics of the 2.7 Ga Belingwe and Abitibi komatiites

High-precision Pt-Re-Os and Sm-Nd isotope and highly siderophile element (HSE) and rare earth element (REE) abundance data are reported for two 2.7 b.y. old komatiite lava flows, Tony’s flow (TN) from the Belingwe greenstone belt, Zimbabwe, and the PH-II flow (PH) from Munro Township in the Abitibi greenstone belt, Canada. The emplaced lavas are calculated to have contained ∼25% (TN) and ∼28% (PH) MgO. These lavas were derived from mantle sources characterized by strong depletions in highly incompatible lithophile trace elements, such as light REE (Ce/Sm_N = 0.64 ± 0.02 (TN) and 0.52 ± 0.01 (PH), ε¹⁴³Nd(T) = +2.9 ± 0.2 in both sources). ¹⁹⁰Pt-¹⁸⁶Os and ¹⁸⁷Re-¹⁸⁷Os isochrons generated for each flow yield ages consistent with respective emplacement ages obtained using other chronometers. The calculated precise initial ¹⁸⁶Os/¹⁸⁸Os = 0.1198318 ± 3 (TN) and 0.1198316 ± 5 (PH) and ¹⁸⁷Os/¹⁸⁸Os = 0.10875 ± 17 (TN) and 0.10873 ± 15 (PH) require time-integrated ¹⁹⁰Pt/¹⁸⁸Os and ¹⁸⁷Re/¹⁸⁸Os of 0.00178 ± 11 and 0.407 ± 8 (TN) and 0.00174 ± 18 and 0.415 ± 5 (PH). These parameters, which by far represent the most precise and accurate estimates of time-integrated Pt/Os and Re/Os of the Archean mantle, are best matched by those of enstatite chondrites. The data also provide evidence for a remarkable similarity in the composition of the sources of these komatiites with respect to both REE and HSE. The calculated absolute HSE abundances in the TN and PH komatiite sources are within or slightly below the range of estimates for the terrestrial Primitive Upper Mantle (PUM). Assuming a chondritic composition of the bulk silicate Earth, the strong depletions in LREE, yet chondritic Re/Os in the komatiite sources are apparently problematic because early Earth processes capable of fractionating the LREE might also be expected to fractionate Re/Os. This apparent discrepancy could be reconciled via a two-stage model, whereby the moderate LREE depletion in the sources of the komatiites initially occurred within the first 100 Ma of Earth’s history as a result of either global magma ocean differentiation or extraction and subsequent long-term isolation of early crust, whereas HSE were largely added subsequently via late accretion. The komatiite formation, preceded by derivation of basaltic magmas, was a result of second-stage, large-degree dynamic melting in mantle plumes.     

Re–Os isotopic dating of molybdenite and pyrite in the Baishan Mo–Re deposit, eastern Tianshan, NW China, and its geological significance

The Baishan Mo–Re deposit is located in the eastern section of the eastern Tianshan orogenic belt, NW China. The deposit has a grade of 0.06% Mo and a high content of rhenium of 1.4 g/t. Rhenium and osmium isotopes in sulfide minerals from the Baishan deposit are used to determine the age of mineralization. Rhenium concentrations in molybdenite samples are between 74 and 250 g/g. Analysis of eight molybdenite samples yields an isochron age of 224.8±4.5 Ma (2). Pyrite samples have rhenium and osmium concentrations varying in the range 33.4–330.6 ng/g and 0.08–0.81 ng/g, respectively. Isotope data on seven pyrite samples yield an isochron age of 225±12 Ma (2) on the ¹⁸⁷Re/¹⁸⁸Os versus ¹⁸⁷Os/¹⁸⁸Os plot and an age of 233±14 Ma (2) on the ¹⁸⁷Os versus ¹⁸⁷Re correlation diagram. The ages of molybdenite and pyrite are consistent within the analytical errors. Combined with field observations, the data indicate that Mo–Re mineralization in the Baishan deposit is produced by a magmatic-hydrothermal event in an intracontinental extensional setting after late Paleozoic orogeny. The initial ¹⁸⁷Os/¹⁸⁸Os ratio of pyrite is 0.3±0.07. The ³⁴S values of molybdenite vary from +0.5 to +3.6. Both data indicate that mineralization is derived mainly from a mantle source.Editorial handling: J. Richards     

Osmium isotope systematics of ureilites

The ¹⁸⁷Os/¹⁸⁸Os for 22 ureilite whole rock samples, including monomict, augite-bearing, and polymict lithologies, were examined in order to constrain the provenance and subsequent magmatic processing of the ureilite parent body (or bodies). The Re/Os ratios of most ureilites show evidence for a recent disturbance, probably related to Re mobility during weathering, and no meaningful chronological information can be extracted from the present data set. The ureilite ¹⁸⁷Os/¹⁸⁸Os ratios span a range from 0.11739 to 0.13018, with an average of 0.1258 ± 0.0023 (1σ), similar to typical carbonaceous chondrites, and distinct from ordinary or enstatite chondrites. The similar mean of ¹⁸⁷Os/¹⁸⁸Os measured for the ureilites and carbonaceous chondrites suggests that the ureilite parent body probably formed within the same region of the solar nebula as carbonaceous chondrites. From the narrow range of the ¹⁸⁷Os/¹⁸⁸Os distribution in ureilite meteorites it is further concluded that Re was not significantly fractionated from Os during planetary differentiation and was not lost along with the missing ureilitic melt component. The lack of large Re/Os fractionations requires that Re/Os partitioning was controlled by a metal phase, and thus metal had to be stable throughout the interval of magmatic processing on the ureilite parent body.     

Investigation of an early Pleistocene marine osmium isotope record from the eastern equatorial Pacific

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.     

Reconstructing the marine Os/Os record and the particulate flux of meteoritic osmium during the late Cretaceous

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.     

Sources of osmium to the modern oceans: new evidence from the Pt-Os system

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.     

Os-Os systematics of Hawaiian picrites revisited: New insights into Os isotopic variations in ocean island basalts

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.     

Os and Os enrichments and high-He/He sources in the Earth’s mantle: Evidence from Icelandic picrites

Picrites from the neovolcanic zones in Iceland display a range in ¹⁸⁷Os/¹⁸⁸Os from 0.1297 to 0.1381 (γ_Os = + 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.