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1.
Osmium isotopic compositions, and Re and Os concentrations have been examined in one komatiite unit and two komatiitic basalt units at Dundonald Beach, part of the 2.7 Ga Kidd-Munro volcanic assemblage in the Abitibi greenstone belt, Ontario, Canada. The komatiitic rocks in this locality record at least three episodes of alteration of Re-Os elemental and isotope systematics. First, an average of 40% and as much as 75% Re may have been lost due to shallow degassing during eruption and/or hydrothermal leaching during or immediately after emplacement. Second, the Re-Os isotope systematics of whole rock samples with 187Re/188Os ratios >1 were reset at ∼2.5 Ga, possibly due to a regional metamorphic event. Third, there is evidence for relatively recent gain and loss of Re in some rocks.Despite the open-system behavior, some aspects of the Re-Os systematics of these rocks can be deciphered. The bulk distribution coefficient for Os (DOssolid/liquid) for the Dundonald rocks is ∼3 ± 1 and is well within the estimated D values obtained for komatiites from the nearby Alexo area and stratigraphically-equivalent komatiites from Munro Township. This suggests that Os was moderately compatible during crystal-liquid fractionation of the magmas parental to the Kidd-Munro komatiitic rocks. Whole-rock samples and chromite separates with low 187Re/188Os ratios (<1) yield a precise chondritic average initial 187Os/188Os ratio of 0.1083 ± 0.0006 (γOs = 0.0 ± 0.6) for their well-constrained ∼2715 Ma crystallization age. The chondritic initial Os isotopic composition of the mantle source for the Dundonald rocks is consistent with that determined for komatiites in the Alexo area and in Munro Township, suggesting that the mantle source region for the Kidd-Munro volcanic assemblage had evolved with a long-term chondritic Re/Os before eruption. The chondritic initial Os isotopic composition of the Kidd-Munro komatiites is indistinguishable from that of the projected contemporaneous convective upper mantle. The uniform chondritic Os isotopic composition of the Kidd-Munro komatiites contrasts with the typical large-scale Os isotopic heterogeneity in the mantle sources for ca. 89 Ma komatiites from the Gorgona Island, arc-related rocks and present-day ocean island basalts. This suggests that the Kidd-Munro komatiites sampled a late-Archean mantle source region that was significantly more homogeneous with respect to Re/Os relative to most modern mantle-derived rocks.  相似文献   

2.
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   

3.
The Suwałki anorthosite massif, located in extreme northeast Poland beneath more than a kilometer of Phanerozoic cover, hosts major Fe-Ti-V deposits. These deposits, discovered in 1962, are contained in Fe and Ti oxide minerals that coexist with subordinate quantities of Fe, Cu, Ni, and Co sulfides in massif-style anorthosites, norites, and gabbronorites. Accessibility and other considerations preclude development of this natural resource in the present economic climate. Detailed work by Polish geologists during the last 35 years provides a sound geologic framework for this Re-Os study of the age and origin of oxide and sulfide deposits associated with a major, but lesser known anorthosite massif. Rhenium and osmium abundances and Os isotopic compositions were measured for nine sulfides and four titanomagnetites from the Suwałki anorthosite massif. The titanomagnetites are over an order of magnitude lower in Re (0.4–1.5 ppb) and Os (0.036–0.144 ppb) concentrations than co-precipitated pyrrhotite, pyrite, and chalcopyrite that yield consistent concentrations for Re (30–55 ppb) and Os (1–6 ppb). Parallel lines connecting co-existing titanomagnetite and sulfides have slopes of ∼1 on Re versus common Os concentration plots, indicating that both Re and Os behave similarly during crystallization in their high preference for any sulfide phase over magnetite. Samples from three deposits within the anorthosite massif were analyzed. An age of 1559 ± 37 Ma (n=10) with an initial 187Os/188Os of 1.16 ± 0.06 for the Jezioro Okrągłe and Krzemianka deposits is essentially identical to an age of 1556 ± 94 Ma (n=3) for the Udryń deposit. Udryń, however, yielded a marginally lower initial 187Os/188Os of 0.87 ± 0.20. The high initial 187Os/188Os combined with the Proterozoic Re-Os age indicates that the source for Suwałki oxides-sulfides is older crust, and hypothetically, could involve Archean rocks. An average crustal value of 50 for 187Re/188Os yields a 2777 Ma age for Suwałki source rocks. Widespread Phanerozoic cover severely limits knowledge of basement rocks in Poland, however, and no Archean rocks are known in the immediate region. More likely, 187Re/188Os ratios may be higher than average continental crust, reflecting mafic crust in the source, and may move the source age for Suwałki anorthosite and mineral deposits toward younger values that easily include ∼2.0 Ga Proterozoic rocks. This more favorable case also accommodates Paleoproterozoic Nd model ages. Regardless of Archean or Proterozoic source age, the high initial 187Os/188Os ratios derived from the Re-Os isochron indicate that the source for the oxide-sulfide mineral deposits is more likely the crust and not the mantle. Given that these deposits are clearly magmatic, the Re-Os results add a new dimension to the long-standing “origin of anorthosite” problem, implying a crustal source for the anorthosite as well. The 1559 Ma Suwałki age is compatible with a well-exposed east-west band of 1530-1660 Ma rapakivi granite-anorthosite magmatism to the immediate north, transecting western Russia, southern Finland, Estonia and Latvia, and central Sweden. In particular, the age and isotopic character of Suwałki are not unlike those of the well-studied Salmi rapakivi granite-anorthosite batholith in western Russia (Karelia). Received: 4 December 1998 / Accepted: 11 November 1999  相似文献   

4.
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.  相似文献   

5.
We measured by negative thermal ionization mass spectrometry (NTIMS) Re, Os and 186Os/188Os and 187Os/188Os in 26 samples of 18 Ni-Cu sulfide ores from the Falconbridge, McCreedy West, and Strathcona mines at Sudbury, Ontario. At McCreedy West and Falconbridge, the isochron Re-Os ages are 1835 ± 70 Ma and 1827 ± 340 Ma, and the initial 187Os/188Os ratios 0.514 ± 0.019 and 0.550 ± 0.024, respectively. The ages agree with the canonical value of 1850 ± 1 Ma for the Sudbury Igneous Complex (SIC). For Hangingwall and Deep Zone ores at Strathcona, the age of 1780 ± 7 Ma may reflect resetting by dyke activity. The high initial 187Os/188Os of 0.934 ± 0.005 in these ores is distinct from those at McCreedy West and Falconbridge. Strathcona Deep Copper Zone ores have highly radiogenic Os giving a mean model age of 1883 ± 54 Ma that is similar to ages at McCreedy West and Falconbridge, but distinct from other Strathcona sulfides. Initial 186Os/188Os in two Strathcona ores with low 190Pt/188Os average 0.119 826 ± 0.000 009 (n = 3) and 0.119 827 ± 0.000 004 (n = 3), respectively, with a grand mean of 0.119 827 ± 0.000 003. This ratio may be slightly lower than the chondritic value at that time. Similar ores at Falconbridge and McCreedy West show more scatter, averaging 0.119 855 ± 0.000 008 (n = 6) and 0.119 867 ± 0.000 020 (n = 3), respectively. These values are substantially suprachondritic. The Re-Os isotope systematics of Sudbury ores are clearly of crustal origin and may be derived from a binary mixture of Superior Province and Huronian metasedimentary rocks, with Strathcona, Falconbridge, and McCreedy West ores containing, respectively, 55%, 16%, and 12% of Os from Superior sediments. The suprachondritic 186Os/188Os at McCreedy West and Falconbridge may be due to admixture of Archean or Paleozoic mafic rocks with 190Pt/188Os ≈ 0.1. No trace of the asteroid that produced the Sudbury Structure has been reported. At the Whistle mine S-poor olivine melanorite inclusions with high Ni and Os and low 187Os/188Os may contain the signature of a magmatically fractionated asteroidal core contributing 1 to 2.5 % metal. The S-poor melanorite Ni and Os data are equally well explained by admixture of ≈40% mantle peridotite, however.  相似文献   

6.
Major elements, highly siderophile elements (HSE) and Re-Os isotope ratios were analysed in situ on individual sulfide grains in spinel peridotite xenoliths hosted by Miocene intraplate basalts from the Penghu Islands, Taiwan. The xenoliths represent texturally and compositionally different mantle domains, and the geochemical characteristics of the sulfides show changes in HSE distribution and Re-Os isotope systematics, produced as their host rocks were metasomatised by percolating fluids/melts. In prophyroclastic and partly metasomatised peridotites from the Kueipi (KP) locality, the sulfides have subchondritic to superchondritic 187Re/188Os and 187Os/188Os ratios. Many of these sulfides reflect fluid/melt interaction with residual MSS and/or crystallization of fractionated sulfide melts, which produced high contents of Cu and PPGEs and high Re/Os; inferred melt/rock ratios are low. In contrast, sulfides in equigranular and extensively metasomatised peridotites from the Tungchiyu (TCY) locality are mainly more sulfur-rich Ni-(Co)-rich MSS, with subchondritic to chondritic 187Os/188Os and subchondritic 187Re/188Os. These sulfides are interpreted as products of interaction between pre-existing MSS and percolating silicate melts. Melt/rock ratios were high and the percolating melt was less differentiated than the melt that percolated the KP peridotites. Sulfides in a TCY pyroxenite are mainly MSS; they have the lowest HSE contents, subchondritic to superchondritic 187Os/188Os and subchondritic 187Re/188Os, and may have precipitated from sulfide melts that segregated from basaltic melts under S-saturated conditions. In most sulfides melt percolation appears to have induced fractionation among the HSEs and disturbed Re-Os isotope compositions. Despite the metasomatic effects, rare residual MSS, sulfides that from crystallised sulfide melts and sulfides modified by addition of Re (with no evidence for Os addition) can still provide useful chronological information. Such sulfides yield TRD age peaks of 1.9, 1.7-1.6, 1.4-1.3 and 0.9-0.8 Ga, which may record the timing of melt extraction and/or metasomatic events in the mantle. These periods are contemporaneous with the major crustal events recorded by U-Pb dates and Nd and Hf model ages in the overlying crust. This close correspondence indicates that the sulfide TRD ages reflect the timing of lithosphere-scale tectonothermal events (such as melting and metasomatism) that affected both the lithospheric mantle and the overlying crust. The sulfide TRD ages, taken together with the crustal data, suggest that most of the Cathaysia block had formed at least by Paleo-Proterozoic time, and that some domains are Archean in age.  相似文献   

7.
Chromites separated from the upper mantle or lower crustal portions of 18 ophiolites ranging in age from 900 Ma to 50 Ma are examined for Re-Os isotopic systematics. The ophiolites include both MORB and back arc types, although most are from supra-subduction zone (SSZ) settings. The chromites are robust indicators of the initial Os isotopic compositions of the systems sampled. There is very limited range in calculated initial γOs values, with the entire group averaging +1.31. Least squares linear regression of the age of chromite formation (in Ga) versus initial 187Os/188Os of a filtered suite yields a slope of −0.0058±0.0019 (2σ) and a present day intercept of 0.12809±0.00085 (2σ), equivalent to a γOs value of +0.9±0.6. Of the suite of 51 samples analyzed, 68% lie within ±1% of this evolution trajectory.Although most of the samples formed in SSZ environments, there is little evidence to suggest modification of the mantle Os isotopic composition via radiogenic melts or fluids derived from subducting slabs. The ophiolite data are interpreted as representative of the convecting upper mantle and suggest that the present isotopic composition of the convecting upper mantle averages approximately 1.2% less radiogenic than the estimated minimum composition of the primitive upper mantle of 0.1296±8 (Meisel et al., 2001). The most likely explanation for the difference is the formation, subduction and isolation of some portion of the mafic oceanic crust. Using models based on the assumption that the convecting upper mantle comprises 50% of the total mass of the mantle, and that the average isolation period for subducted oceanic crust is 1.5 to 2.0 Ga, it is estimated that approximately 2 to 3% of the total mass of the mantle is composed of subducted mafic oceanic crust that remains isolated from the convecting upper mantle. Because the isotopic compositions of the DMM and PUM overlap within uncertainties, however, the results do not require any isolated slab component.  相似文献   

8.
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.  相似文献   

9.
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.  相似文献   

10.
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 (t1/2 = 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.  相似文献   

11.
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   

12.
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   

13.
Re-Os同位素体系在蛇绿岩应用研究中的进展   总被引：2，自引：1，他引：2  
Re-Os不同于由亲石元素构成的同位素体系，在原始上地幔（PUN）部分熔融过程中，母体Re是中等不相容元素，优先进入熔体相，子体Os是强相容元素，富集在残留相中，是研究蛇绿岩的极好示踪剂。在蛇绿岩应用研究中已经取得了4个方面的进展：（1）明确了熔体相的Re／Os和^187Os／^188Os比值高，而残留相的低；（2）铬铁矿中铂族元素矿物（PGM）的Re亏损年龄（TRD）证实了蛇绿岩中复杂的超镁铁岩体是多阶段部分熔融的产物；（3）现代大洋橄榄岩和玄武岩的Re-Os同位素研究表明熔体相和残留相的^187Os／^188Os比值在高于亏损地幔值（DMM）的部分是一致的，而低于DMM的存在不一致性，为研究蛇绿岩中熔体相与残留相是否存在“耦合”关系提供了新的制约因素；（4）揭示了蛇绿岩地幔橄榄岩中含有古大陆岩石圈地幔，这是前所未知的。虽然取得了不少进展，但是由于Re-Os同位素体系用于蛇绿岩研究的时间较短，尚存在一些问题，如显生宙蛇绿岩地幔橄榄岩的定年问题，有待进一步深化研究。  相似文献   

14.
Exposure of the ca. 6 Ma Taitao ophiolite, Chile, located 50 km south of the Chile Triple Junction, allows detailed chemical and isotopic study of rocks that were recently extracted from the depleted mantle source of mid-ocean ridge basalts (DMM). Ultramafic and mafic rocks are examined for isotopic (Os, Sr, Nd, and O), and major and trace element compositions, including the highly siderophile elements (HSE). Taitao peridotites have compositions indicative of variable extents of partial melting and melt extraction. Low δ18O values for most whole rock samples suggest some open-system, high-temperature water–rock interaction, most likely during serpentinization, but relict olivine grains have δ18O values consistent with primary mantle values. Most of the peridotites analyzed for Nd–Sr isotopes have compositions consistent with estimates for the modern DMM, although several samples are characterized by 87Sr/86Sr and 143Nd/144Nd indicative of crustal contamination, most likely via interactions with seawater. The peridotites have initial 187Os/188Os ratios that range widely from 0.1168 to 0.1288 (γOs = −8.0 to +1.1), averaging 0.1239 (γOs = −2.4), which is comparable to the average for modern abyssal peridotites. A negative correlation between the Mg# of relict olivine grains and Os isotopic compositions of whole rock peridotites suggests that the Os isotopic compositions reflect primary mantle Re/Os fractionation produced by variable extents of partial melting at approximately 1.6 Ga. Recent re-melting at or near the spatially associated Chile Ridge further modified these rocks, and Re, and minor Pt and Pd were subsequently added back into some rocks by late-stage melt–rock or fluid–rock interactions.In contrast to the peridotites, approximately half of the mafic rocks examined have whole rock δ18O values within the range of mantle compositions, and their Nd and Sr isotopic compositions are all generally within the range of modern DMM. These rocks have initial 187Os/188Os ratios, calculated for 6 Ma, that range from 0.126 (γOs = −1) to as high as 0.561 (γOs = +342). The Os isotopic systematics of each of these rocks may reflect derivation from mixed lithologies that include the peridotites, but may also include pyroxenites with considerably more radiogenic Os than the peridotites. This observation supports the view that suprachondritic Os present in MORB derives from mixed mantle source lithologies, accounting for some of the worldwide dichotomy in 187Os/188Os between MORB and abyssal peridotites.The collective results of this study suggest that this >500 km3 block of the mantle underwent at least two stages of melting. The first stage occurred at 1.6 Ga, after which the block remained isolated and unmixed within the DMM. A final stage of melting recently occurred at or near the Chile Ridge, resulting in the production of at least some of the mafic rocks. Convective stirring of this mantle domain during a >1 Ga period was remarkably inefficient, at least with regard to Os isotopes.  相似文献   

15.
Sulphide-bearing diamonds recovered from the ∼20 Ma Ellendale 4 and 9 lamproite pipes in north-western Australia were investigated to determine the nitrogen aggregation state of the diamonds and Re-Os isotope geochemistry of the sulphide inclusions. The majority of diamond studies have been based on diamonds formed in the sub-continental lithospheric mantle (SCLM) below stable cratons, whereas the Ellendale lamproites intrude the King Leopold Orogen, south of the Kimberley craton. The sulphide inclusions consist of pyrrhotite-pentlandite-chalcopyrite assemblages, and can be divided into peridotitic and eclogitic parageneses on the basis of their Ni and Os contents. A lherzolitic paragenesis for the high-Ni sulphide inclusions is suggested from their Re and Os concentrations. Regression analysis of the Re-Os isotope data for the lherzolitic sulphides yields an age of 1426 ± 130 Ma, with an initial 187Os/188Os ratio of 0.1042 ± 0.0034. The upper limit of the uncertainty on the 187Os/188Os initial ratio gives a Re depletion age of 2.96 Ga, indicating the presence of SCLM beneath Ellendale since at least the Mesoarchaean, with the lherzolitic diamond-forming event much younger and unrelated to the craton keel stabilisation. The nitrogen aggregation state of the diamonds and calculated mantle residence temperatures suggest an origin and storage of the Ellendale diamonds in a stable cratonic SCLM, consistent with the King Leopold Orogen being cratonised by about 1.8 Ga. The diamonds do not show evidence for pervasive deformation or platelet degradation, which suggests that the diamonds had a relatively undisturbed 1.4 billion year mantle storage history.  相似文献   

16.
《China Geology》2021,4(4):593-599
The Cenozoic basalts with OIB-affinity in northern marginal region of the North China Craton are thought to experience minor even no crustal contamination during the magma evolution. The whole-rock Sr-Nd-Pb-Hf isotopes are attributed to a two-component mixing between depleted and enriched mantle sources, while the major element variations are controlled by the fractional crystallization of olivine and clinopyroxene. However, in this study, the new Os isotopic data proposes an opposite model for the Cenozoic basalts in northern marginal region of the North China Craton. In this model, the Jining basalts were contaminated by the Archean mafic rocks during the magma storage and ascent. The crustal contamination process is supported by (1) the highly radiogenic Os isotopic compositions, and (2) the positive correlation between 187Os/188Os and 1/Os of the Jining basalts. By modeling the Os isotopic composition of the basalts, an incorporation of < 10% mafic granulites/amphibolites to the parental magma can successfully explain the initial values of highly radiogenic Os. In contrast, the unradiogenic and uniform Os isotopic compositions of the Chifeng basalts suggest negligible crustal contamination. Os isotopic data acts as an indicator of crustal contamination during magma evolution, providing us a novel insight into the evolution of the intra-continental OIB-like basalts worldwide.©2021 China Geology Editorial Office.  相似文献   

17.
通过对日喀则蛇绿岩的镁铁质岩石进行岩石学和地球化学研究,探讨其岩石成因及源区性质,同时根据Re-Os同位素的地球化学约束来探索雅鲁藏布蛇绿岩的形成机制。研究表明,日喀则蛇绿岩镁铁质岩石微量元素的标准化配分型式与洋中脊玄武岩类似,又具有岛弧玄武岩的地球化学特征。结合微量元素和同位素特征,均表明岩石的形成与俯冲作用有关。日喀则蛇绿岩产出于远离大陆地壳的SSZ环境,其形成过程未受到陆壳的混染;同时,Re-Os同位素体系受蚀变作用的影响也不明显。日喀则蛇绿岩镁铁质岩石的Re、Os含量低,187Os/188Os同位素比值较高,主要是源区性质和俯冲作用影响的结果。特提斯洋早期发生的多次俯冲作用造成地幔源区不均一。新特提斯洋壳俯冲过程中,上述不均一地幔发生部分熔融产生的镁铁质岩浆上升,经过遭受了早期熔体/岩石作用的纯橄岩通道,发生强烈的Re-Os同位素分馏,使熔体与地幔残余Os同位素组成表现出明显的解耦现象,进而形成现今的日喀则蛇绿岩。  相似文献   

18.
Major elements and Re–Os isotope ratios were analysed in situ on individual sulfide grains in spinel peridotite xenoliths hosted by Quaternary intraplate basalts from the Tariat volcanic field, Central Mongolia. The sulfides are dominantly high-temperature (>900 °C) Fe-rich monosulfide solid solution (MSS). Some sulfides with low Ni contents may be residual MSS, whereas other sulfides defining a negative Ni–Cu correlation may be crystallization products of fractionated sulfide melts. The subchondritic 187Re/188Os and 187Os/188Os of some sulfides also indicate they are residual MSS. Os isotope compositions of sulfides reveal the presence of Archean to Proterozoic lithospheric mantle beneath the region. The sulfides have TRD model ages ranging from 3.0 to 0.2 Ga, with peaks at 1.5–1.3, 1 and 0.7–0.5 Ga. The peak ages are indicative of significant events in the lithospheric mantle at those times. The timing of these events is remarkably consistent with those of the major crust-building events within the Tarvagatay Terrane where the Tariat volcanic field is located. The similarity in the ranges of crustal U–Pb ages and Nd model ages, and our sulfide Os model ages, suggests that the sulfide ages may date metasomatic events in the underlying lithospheric mantle, which were related to tectonothermal events that affected the overlying crust. Radiometric ages from the Tarvagatay Terrane appear to correspond to the Archean model ages from its SCLM counterpart. The last two events (1.1 and 0.7–0.5 Ga) recorded in the Tarvagatay Terrane suggest involvement of the “CAOB mantle” and development of significant juvenile crustal growth in the orogeny.  相似文献   

19.
Reproducibility of Re-Os molybdenite ages depends on sample size and homogeneity, suggesting that Re and Os are decoupled within individual molybdenite crystals and do not remain spatially linked over time. In order to investigate the Re-Os systematics of molybdenite at the subgrain (micron) scale, we report LA-ICP-MS Re-Os ages for an Archean molybdenite crystal from Aittojärvi, Finland, analyzed in situ in a white aplite matrix. A related Aittojärvi molybdenite (A996D), in the form of a very fine-grained mineral separate, is used as one of our in-house NTIMS standards, and thus its age of 2760 ± 9 Ma is well established. Measurements of (187Re + 187Os)/185Re on micron scale spots along 200 μm traverses across the crystal yield a wide range of ages demonstrating that, in this case, microsampling of molybdenite does not produce geologically meaningful ages. Experimentation with mineral separations and sample size over a 7-yr period predicted that this would be the outcome. We suggest that 187Os is more likely to be the mobile species, based on its charge and ionic radius, and that 187Os becomes decoupled from parent 187Re with time on the micron and larger scale. Incompatible charge and ionic radius for Os ions formed during reduction of molybdenite-forming fluids may explain the widely observed absence of common (initial) Os in molybdenite. Geologically accurate ages for molybdenite can only be obtained for fully homogenized crystals (or crystal aggregates) so that any post-crystallization 187Re-187Os decoupling is overcome.A growing number of geologically accurate ID-NTIMS 187Re-187Os ages for homogenized molybdenite suggest that postcrystallization mobility of radiogenic 187Os must be limited to within the molybdenite mineral phase. We suggest that radiogenic 187Os may be stored in micron scale dislocations, kink bands, and delamination cracks produced by deformation, and that the unusual structure and deformation response of molybdenite results in an increased chemical stability in this mineral. Migration of 187Os into adjacent silicate phases is highly unlikely, but other contacting sulfides may take in Os. In an example from a Proterozoic skarn deposit at Pitkäranta (western Russia), we demonstrate minor loss of radiogenic 187Os from molybdenite and a corresponding gain in adjacent chalcopyrite such that the molybdenite age is not perceptibly disturbed, whereas the resulting chalcopyrite ages are impossibly old. Therefore, it is unadvisable to perform Re-Os analytical work on any sulfide in contact or intimate association with molybdenite. In addition to large errors in the age, if the isochron method is employed, initial 187Os/188Os ratios could be erroneously high, leading to seriously errant genetic interpretations.  相似文献   

20.
Re-Os同位素对峨眉山大火成岩省成因制约的探讨   总被引：6，自引：3，他引：3  
史仁灯  郝艳丽  黄启帅 《岩石学报》2008,24(11):2515-2523
峨眉山大火成岩省（ELIP）主要由玄武岩、玄武质火山碎屑岩及少量的苦橄岩（包括越南的科马提岩）、长英质岩石以及层状岩体和岩墙组成,其物质来源直接关系到其成因是否与地幔柱活动有关。Re-Os同位素体系是地核、地幔和地壳物质的最佳示踪剂。前人对ELIP内的Re-Os同位素研究表明,低Ti玄武岩的Os含量为0．006&#215;10^-9-0．40010^-9,^187Os／^188Os初始值为0．1371～1．403,并提出其与地幔柱活动有关;而高Ti玄武岩的Os含量为0．00410^-9～0．56010^-9,^187Os／^188Os初始值为0．1271～5．19,认为起源于大陆岩石圈地幔或地幔柱上升过程中受到大量岩石圈地幔“混染”（xu JF et al．,2007）;科马提岩的0s含量为1．2410^-9～7．0010^-9,^187Os／^188Os初始值为0．1251～0．1261,苦橄岩的Os含量为0．3210^-9～2．32910^-9,^187Os／^188Os初始值为0．1233～0．1266,指示苦橄岩和科马提岩均来自亏损地幔源区（Hanski et al．,2004;陈雷等,2007）。本文利用Os含量最低、^187Os／^188Os最高的高Ti玄武岩作为地壳端员,用铁质陨石、原始上地幔（PUM）和亏损地幔（DMM）作为地核和各种地幔端员,分别做二元混合计算,结果显示绝大多数玄武岩和所有苦橄岩及科马提岩均落在地壳和DMM混合曲线附近,并且邻区特提斯洋地幔岩与DMM具有相近的Os含量和^187Os／^188Os组成,据此推测峨眉山火成岩的形成与特提斯洋的活动有关,主要受控于地壳和亏损地幔的相互作用。  相似文献   

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