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1.
A simple, rapid method for the determination of Re and Os concentrations and isotope compositions using isotope dilution multi‐collector inductively coupled plasma‐mass spectrometry (ID‐MC‐ICP‐MS) combined with Carius tube digestion and sparging introduction of Os was developed. For Os measurement, four channeltron ion counters to detect different Os isotopes were used simultaneously, which led to a drastic reduction in the measurement time. Rhenium isotopes were measured by means of eight Faraday cups with solution nebulisation and an ultrasonic membrane desolvator. The representative 188Os count rate of an Os standard solution containing 50 pg of total Os was approximately 110000–120000 cps at the onset of measurement; the Re intensity of our in‐house 10 pg g?1 standard solution reached 1820 V/μg g?1 with a sample uptake rate of 95–99 μl min?1. These values indicate that the sensitivity of the method was sufficient even for samples with low Re and Os concentrations, such as chert. As the temporal variations of the amplification efficiency of the ion counters differed from one another, we adopted a sample‐calibrator bracketing method to correct the measured Re and Os isotope ratios. The Re and Os concentrations via the isotope dilution method and the 187Os/188Os ratios of two sedimentary rock reference materials (JMS‐2 and JCh‐1) on the basis of the isotope ratios determined by the MC‐ICP‐MS and by negative thermal ionisation mass spectrometry (N‐TIMS) were comparable within their ranges. Based on Os isotope measurement of the IAG reference material [Durham Romil Os (DROsS)], the average difference from the recommended value and precision of Os isotope measurements by the sparging method in combination with multi‐ion‐counters were 0.72% and 0.76% [1RSD (%), n = 29], respectively. The precisions in the 187Os/188Os ratios [1RSD (%)] of JMS‐2, JCh‐1 and DROsS were 0.35–0.71, 1.56–3.31 and 0.99–1.28%, respectively, which depended on their Os ion intensities. No systematic difference was observed between the Re and Os geochemical compositions of JCh‐1 and JMS‐2 obtained by means of digestion with inverse aqua regia and CrO3‐H2SO4 solutions, suggesting that either acid solution can be used for the sparging method of sedimentary rock samples. As CrO3‐H2SO4 solution is believed to liberate predominantly the hydrogenous Re and Os fraction from organic‐rich sediment, the sparging method combined with CrO3‐H2SO4 digestion and multi‐ion‐counters in the mass spectrometry is expected to be a powerful tool for reconstructing the secular change in marine Os isotope compositions with high sample throughput.  相似文献   

2.
This study presents two matrix‐matched reference materials developed for petroleum Re‐Os measurements. We present the Re and Os mass fractions and 187Re/188Os and 187Os/188Os values (ratio of the number of atoms of the isotopes) for repeatedly measured aliquots (ca. 120–150 mg test portions) of the NIST Research Material 8505 (RM 8505) crude oil, and its asphaltene and maltene fractions, and ~ 90 g of homogeneous asphaltene powder isolated from this oil. Measurements were performed using the Carius tube‐isotope dilution negative‐thermal ionisation mass spectrometry methodology. The RM 8505 crude oil contains 1.98 ± 0.07 ng g?1 Re and 25.0 ± 1.1 pg g?1 Os, with Re‐Os isotope amount ratios of 452 ± 6 for 187Re/188Os and 1.51 ± 0.01 for 187Os/188Os (n = 20, 95% conf.). The homogeneous asphaltene sample contains 16.52 ± 0.10 ng g?1 Re and 166.0 ± 0.9 pg g?1 total Os, and possesses isotope amount ratios of 574 ± 3 for 187Re/188Os and 1.64 ± 0.01 for 187Os/188Os (n = 24, 95% conf.). The intermediate precision of these data makes the RM 8505 whole oil and the (~ 90 g) homogenised asphaltene appropriate petroleum matrix‐matched reference materials for Re‐Os measurements. The asphaltene fraction of the oil is the main carrier of Re and Os of the RM 8505 whole oil, and caution is suggested in using asphaltene and maltene fractions of a single oil for Re‐Os geochronology.  相似文献   

3.
In recent years, the 187Re–187Os isotope system has been increasingly used to study samples containing very small quantities of Os. For such samples, optimisation of measurement procedures is essential to minimise the loss of Os before mass spectrometric measurements. Micro‐distillation is a necessary purification step that is applied after the main Os chemical separation procedure, prior to Os isotope ratio measurements by negative‐thermal ionisation mass spectrometry (N‐TIMS). However, unlike the other separation steps, this procedure has not yet been optimised for small samples. In this study, we present a refined micro‐distillation method that achieved higher yields and allowed high‐precision R(187Os/188Os) expressed as 187Os/188Os measurements for small‐sized geological samples that contain only a few pg Os. The Os recovery in the micro‐distillation step was tested by changing the operating conditions including heating time and temperature, and amounts of oxidant and reductant. Recoveries were measured by the isotope dilution ICP‐MS method after the addition of 190Os‐enriched spike solution. We found that the most critical factor controlling the chemical yield of Os during micro‐distillation is the extent of dilution of the reductant (HBr) by H2O evaporated from the oxidant. A refined micro‐distillation method, in which the amount of oxidant solution is reduced from the conventional method, achieved an improved chemical yield of Os (~ 90%). This refined method was applied to the measurement of 187Os/188Os by N‐TIMS of varying test portions of the geological reference material BIR‐1a. The resulting 187Os/188Os ratios of BIR‐1a matched the literature data, with propagated uncertainties of 0.2, 1.1 and 11% digested sample quantities containing 150, 10 and 1 pg of Os, respectively.  相似文献   

4.
N(187Os)/N(188Os) ratios of six geological reference materials were measured using static Faraday cups (FCs) with 1013 Ω amplifiers by N‐TIMS. Our results show that the repeatability precision was 2–3‰ (2 RSD, n = 3), when taking ~ 1 g of BHVO‐2 with 76 pg g?1 of Os mass fraction and ~ 2 g of BCR‐2 with 21 pg g?1 of Os mass fraction for each sample, whether measured by FCs or by secondary electron multiplier. The repeatability precision measured by FCs was 1–0.2‰ (2 RSD, n = 3) when taking ~ 1 g of BIR‐2 with 350 pg g?1 of Os mass fraction, ~ 1 g of WGB‐1 with 493 pg g?1 of Os mass fraction or ~ 0.5 g of WPR‐1 with 13.3 ng g?1 of Os mass fraction for each sample, which is much better than those measured by secondary electron multiplier. Instead, when taking ~ 2 g of AGV‐2 with 4 pg g?1 Os mass fraction, the repeatability precision measured by secondary electron multiplier is 3–4‰ (RSD, n = 3), which is better than those measured by FCs. Of the six reference materials analysed, WPR‐1 and BIR‐1a are the most homogeneous with regard to Os isotopic composition (2 RSD of 0.08% and 0.23%, respectively) when test portion masses are 0.5–1 g.  相似文献   

5.
pg-ng级Os同位素热表面电离质谱高精度分析测试技术   总被引：5，自引：4，他引：1  
随着Os同位素在地质学领域定年与示踪的广泛应用,对分析测试精度和检出限提出了更高的要求。本文利用热表面电离质谱负离子模式(NTIMS)对Os O-3进行测定,通过试剂纯化、器皿清洗等前处理步骤的优化保证了超低的化学流程空白和获得灵敏而稳定的仪器测量信号,建立了高精度pg-ng级Os同位素分析测试方法。采用逐级剥谱法扣除氧同位素干扰,采用内标迭代法按照指数规律对仪器的质量歧视效应进行校正,全流程Os空白为0.41 pg,1 pg的190Os计数约85000 cps,质量分馏系数在0.1%以内。pg级Os采用离子计数器动态跳峰扫描模式进行测量,187Os/188Os值测量精度小于在0.2%以内;ng级Os采用法拉第杯静态接收模式采集信号,187Os/188Os值测量精度在0.005%以内。经国际上Os同位素标准溶液N2、JMC以及国家一级Re-Os同位素标准物质JCBY的验证,分析精度达到了国际同类实验室先进水平。本方法具有低空白、高灵敏度、高精度的特点,可为地质学研究提供高精度Os同位素数据。  相似文献   

6.
Geological reference materials (RMs) with variable compositions and NIST SRM 612 were analysed by isotope dilution mass spectrometry for bulk rock concentrations of chalcogen elements (sulfur, selenium and tellurium), rhenium and platinum‐group elements (PGEs: Ru, Pd, Os, Ir and Pt), including the isotope amount ratios of 187Os/188Os. All concentrations were obtained from the same aliquot after HCl‐HNO3 digestion in a high pressure asher at 320 °C. Concentrations were determined after chemical separation by negative TIMS, ICP‐MS and hydride generation ICP‐MS (Se, Te). As in previous studies, concentrations of the PGEs in most RMs were found to be highly variable, which may be ascribed to sample heterogeneity at the < 1 g level. In contrast, S, Se and Te displayed good precision (RSD < 5%) in most RMs, suggesting that part of the PGE budget is controlled by different phases, compared with the chalcogen budget. The method may minimise losses of volatile chalcogens during the closed‐system digestion and indicates the different extent of heterogeneity of chalcogens, Re and PGEs in the same sample aliquot. OKUM, SCo‐1, MRG‐1, DR‐N and MAG‐1 are useful RMs for the chalcogens. NIST SRM 612 displays homogenous distribution of S, Se, Te, Pt and Pd in 30 mg aliquots, in contrast with micro‐scale heterogeneity of Se, Pd and Pt.  相似文献   

7.
A method was developed for the determination of platinum‐group elements (PGE) in geological samples by isotope dilution‐inductively coupled plasma‐mass spectrometry combined with sulfide fire assay preconcentration. Samples were fused and PGE analytes were concentrated in sulfide buttons. The buttons were dissolved using HCl leaving PGE analytes in insoluble residues, which were digested in HNO3 and simultaneously processed for the distillation of Os. The remaining solutions were further prepared for the purification of Ru, Rh, Pd, Ir and Pt using a tandem assembly of cation and Ln resin columns. The eluents were directly analysed by membrane desolvation‐ICP‐MS. Ruthenium, Pd, Os, Ir and Pt were determined by isotope dilution, whereas Rh was determined by conventional reference material calibration combined with 193Ir as the internal standard element. The method was validated using a series of PGE reference materials, and the measurement data were consistent with the recommended and the literature values. The measurement precision was better than 10% RSD. The procedural blanks were 0.121 ng for Ru, 0.204 for Rh, 0.960 ng for Pd, 0.111 ng for Os, 0.045 ng for Ir and 0.661 ng for Pt, and the limits of detection (3s) were 0.011 ng g?1 for Ru, 0.008 ng g?1 for Rh, 0.045 ng g?1 for Pd, 0.009 ng g?1 for Os, 0.006 ng g?1 for Ir and 0.016 ng g?1 for Pt when a test portion mass of 10 g was used. This indicates that the proposed method can be used for the determination of trace amounts of PGE in geological samples.  相似文献   

8.
A method for the determination of total sulfur in geological materials by inductively coupled plasma‐optical emission spectrometry (ICP‐OES) is described. We show that good results were obtained using this method even for sample types with very low (< 20 μg g?1) sulfur concentration (e.g., peridotite). Sulfur was determined in fifteen geological reference materials with different sulfur contents. For reference materials with certified sulfur contents, the ICP‐OES method gave results in excellent agreement with certified values, and uncertainties better than 4% RSD. ICP‐OES results for sulfur in other reference materials yielded RSDs better than 10%, where S concentrations were > 100 μg g?1 (except for diabase W‐2a, 16% RSD). Reference materials with lower sulfur contents (< 40 μg g?1) showed much higher RSDs (17–18%). Except for RMs with certified values for sulfur, most data obtained by the combustion infrared detection method generally showed higher concentrations than those measured by ICP‐OES and a better RSD (≤ 8% for all materials except DTS‐2b).  相似文献   

9.
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   

10.
To evaluate the homogeneity of geological reference material BIR‐1a (basalt; United States Geological Survey, USGS) for Re‐Os isotopic studies at the 0.2–1.0 g test portion size level, sixty‐three precise measurement results of Re and Os mass fractions and isotope amount ratios were obtained over an 18‐month period. These data reveal that the reference material has higher Re (0.691 ± 0.022 ng g?1, 2s, n = 63) and lower Os mass fractions (0.343 ± 0.089 ng g?1, 2s, n = 63) than UB‐N (serpentinite, CRPG) and is homogeneous in 187Os/188Os isotope amount ratio (0.13371 ± 0.00092, 2s, n = 63) at the 0.2–1.0 g test portion size level. The results are essentially consistent with previous views indicating that BIR‐1a gives precise measurement results for Re‐Os isotope amount ratio measurements at the 1 g test portion size level (Ishikawa et al., Chemical Geology, 2014, 384, 27–46; Meisel and Horan, Reviews in Mineralogy and Geochemistry, 2016, 81, 89–106). Based on these new Re‐Os data and previous studies, we propose BIR‐1a as a useful reference material that can be used in method validation and quality control and interlaboratory comparisons for studies dealing with mafic geological samples at test portion sizes of > 0.4 g.  相似文献   

11.
We report homogeneity tests on large natural apatite crystals to evaluate their potential as U reference materials for apatite fission‐track (AFT) thermochronology by laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS). The homogeneity tests include the measurements of major element concentrations by electron probe microanalysis (EPMA), whereas for U concentration, isotope dilution (ID) ICP‐MS and laser ablation (LA) ICP‐MS were employed. Two apatite crystals are potential reference materials for LA‐ICP‐MS analysis: a 1 cm3 fraction of a Durango crystal (7.5 μg g?1 U) and a 1 cm3 Mud Tank crystal (6.9 μg g?1 U). The relative standard deviation (1 RSD) of the U concentration determined by ID‐ICP‐MS of both apatite crystals was ≤ 1.5%, whereas 1 RSD for the LA‐ICP‐MS results was better than 4%, providing sufficient homogeneity for fission‐track dating. The results on the U homogeneity for two different apatite samples are an important step towards establishing in situ dating routines for AFT analysis by LA‐ICP‐MS.  相似文献   

12.
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   

13.
To precisely determine the abundances of fifty‐two elements found within natural water samples, with mass fractions down to fg g?1 level, we have developed a method which combines freeze‐drying pre‐concentration (FDC) and isotope dilution internal standardisation (ID‐IS). By sublimation of H2O, the sample solution was reduced to < 1/50 of the original volume. To determine element abundance with accuracy better than 10%, we found that for solutions being analysed by mass spectrometry the HNO3 concentration should be > 0.3 mol l?1 to avoid hydrolysis. Matrix‐affected signal suppression was not significant for the solutions with NaCl concentrations lower than 0.2 and 0.1 cg g?1 for quadrupole ICP‐MS and sector field ICP‐MS, respectively. The recovery yields of elements after FDC were 97–105%. The detection limits for the sample solutions prepared by FDC were ≤ 10 pg g?1, except for Na, K and Ca. Blanks prepared using FDC were at pg‐levels, except for eleven elements (Na, Mg, Al, P, Ca, Mn, Fe, Co, Ni, Cu and Zn). The abundances of fifty‐two elements in bottled drinking water were determined from five different geological sources with mass fractions ranging from the fg g?1 to μg g?1 level with high accuracy.  相似文献   

14.
湖南鲁塘石墨矿Re-Os同位素研究   总被引：5，自引：1，他引：4  
石墨具有较高的Re、Os含量,可望成为理想的Re-Os同位素测年对象,但迄今国内外研究较少,尤其在煤层经变质作用形成石墨过程中,其中Re-Os同位素体系的变化还有待研究。湖南鲁塘石墨矿是我国典型的隐晶质石墨矿床之一,矿体产于二叠系龙潭组煤系地层中。本文采用Carius管逆王水溶解样品,直接蒸馏、微蒸馏分离纯化Os,丙酮萃取法分离富集Re,热表面电离质谱法对鲁塘矿区石墨样品以及外围原煤进行了Re-Os同位素分析。结果表明:鲁塘石墨的Re含量为0.901~9.794 ng/g,Os含量为7.3~189.5 pg/g,Re-Os同位素等时线年龄为155.6±3.6 Ma,该年龄与鲁塘石墨矿东侧骑田岭岩体第二阶段中粒黑云母花岗岩锆石U-Pb年龄153~157 Ma一致,表明了龙潭组煤层受到骑田岭岩体"烘烤"作用,发生热接触变质作用,使得靠近骑田岭岩体原煤变质为石墨,形成石墨矿床。通过对比石墨、原煤和骑田岭岩体Re、Os含量及比值,发现石墨中的Re、Os主要来源于原煤,并根据石墨Re-Os等时线初始(187Os/188Os)i值(0.686±0.032),推测骑田岭岩体在侵入煤系地层过程中,有少量具有较低187Os/188Os值的Os被碳质吸附。  相似文献   

15.
A method has been developed for the in situ determination of Re-Os isotopes in single grains of sulfides in mantle-derived peridotites using a laser ablation microprobe attached to a multicollector-induced coupled plasma mass spectrometer (MC-ICPMS). High-precision Os isotope analysis by MC-ICPMS is demonstrated by the measurement of interlaboratory Os standards. Evaluation of mass bias correction procedures shows that the exponential law provides the best fit to the Os isotope data and that the ratio of the mass fractionation coefficients for Re and Os remains constant for the range of typical instrument operating conditions. This relationship enables the accurate and precise correction of the isobaric interference of 187Re on 187Os for 187Re/188Os values up to 1.6.Results are presented for single sulfide inclusions in olivine macrocryts from kimberlites in the Siberian and Slave Cratons, and sulfides enclosed in silicates and interstitial to silicates in peridotite xenoliths from the Slave Craton and Massif Central, France. Enclosed sulfides larger than 50 μm in diameter and with Os contents ≥40 ppm give 187Os/188Os ratios with a precision of 0.1% (2 SE), which is equivalent to N-TIMS whole-rock data. Interstitial sulfides typically have lower Os (10 to 30 ppm) and give analyses with lower precision (∼1 to 2%) but still provide valuable information on the movement of Os within the lithosphere. The sulfide inclusions in silicates preserve significantly less radiogenic Os isotopic compositions than interstitial sulfides and accordingly produce significantly older and more realistic Re-Os age information. Whole-rock Os isotope compositions reflect the proportions of different generations of enclosed and interstitial sulfides; this calls into question the significance of many published “depletion ages.”  相似文献   

16.
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   

17.
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.  相似文献   

18.
以经过负离子热表面电离质谱仪准确标定的1 86Os和1 88Os双同位素作为稀释剂 ,利用1 86Os和1 88Os计算1 87Os的含量 ,能有效地减少用等离子体质谱仪测定1 87Os时由于仪器本身的不稳定性及质量歧视效应所产生的误差 ,使测量准确度比用单同位素作为稀释剂有明显提高。方法应用于辉钼矿Re -Os年龄标准参考物及样品的测定 ,结果与参考值一致 ,5份试样测定的RSD <3%。  相似文献   

19.
Elevated 187Os/188Os ratios compared to ambient oceanic mantle, i.e.,187Os/188Os>0.13, have been reported for both arc lavas and mantle wedge xenoliths, which have been ascribed to the addition of crustal Os through slab dehydration or melting. By contrast, much lower 187Os/188Os ratios of spinels from Izu‐Bonin‐Mariana boninites indicate slight or no crustal Os was transferred from the slab to the forearc mantle. Here we report Os isotopic compositions of peridotites from New Caledonia ophiolites, which represent relics of a forearc mantle. Some New Caledonia peridotites are characterized by Os concentrations of <1 ppb, yet have187Os/188Os ratios comparable to the ambient oceanic mantle (i.e., 187Os/188Os<0.13). This confirms that little crustal Os was transported to the forearc mantle via slab dehydration. Contrasting Os isotopes between forearc peridotites and mantle wedge xenoliths may reflect the changing behavior of Os in diverse agents released from the descending slab as a function of depth, which is mainly controlled by the stability of sulfides in the slabs. During dehydration at shallow depths, sulfides keep stable and thus little Os is transported to the overlying mantle. In comparison, sulfides become unstable and tend to break down at deeper depths where slab melting or supercritical fluid generation occurs, and thus Os behaves like a mobile element.  相似文献   

20.
石灰岩铼-锇同位素分析方法研究及应用初探   总被引：2，自引：2，他引：0  
针对石灰岩样品Re-Os同位素分析,在选样和溶样方法上进行了改进,在Carius管封闭前加入HCl与石灰岩反应释放出大量CO2,然后加入氧化剂和稀释剂封闭Carius管溶解样品,大大增加了样品取样量。利用改进的方法对采自青海玉树地区二叠世九十道班组底部的灰黑色微细晶灰岩的Re-Os同位素体系进行了分析测定,得到了精确的沉积年龄(283.1±7.1)Ma(MSWD=0.61,Model1,n=7)。187Os/188Os同位素初始值为0.56±0.12,与二叠纪时海水的187Os/188Os值相一致,反映了石灰岩沉积时海水的187Os/188Os比值。所得石灰岩年龄与其中的生物化石年龄相吻合,并且与区域上岩浆岩锆石年龄相互印证,表明Re-Os同位素体系在该石灰岩中的封闭性较好。通过石灰岩中有机碳含量以及其中Re、Os含量关系研究,得出了Re、Os在灰岩中主要赋存于有机质中的结论。从原理上解释了Re-Os同位素体系在灰岩中的应用具有十分广泛的前景。  相似文献   

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