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1.
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   

2.
Rhenium (Re) is one of the least abundant elements in Earth, averaging 0.28 ppb in the primitive mantle. The unique occurrence of rheniite ReS2 (74.5 wt% of Re) in Kudryavy volcano precipitates raises questions about recycling of Re-rich reservoirs within the Kurile-Kamchatka volcanic Island arc setting. The sources of this unique Re enrichment have been inferred from studies of Re-Os isotope systematic and trace elements in volcanic gases, sulphide precipitates and host volcanic rocks. The fumarolic gas condensates are enriched in hydrophile trace elements relative to fluid-immobile elements and exhibit high Ba/Nb (133-204), Rb/Y (16-406) and Th/Zr (0.01-0.25) ratios. They are characterised by high Re (7-210 ppb) and Os abundances (0.4-0.9 ppb), with 187Os/188Os ratios in a range 0.122-0.152. This Os isotopic compositional range is similar to that of the peridotite xenoliths from the metasomatised mantle wedge above the subducted Pacific plate, the radiogenic isotopic signature of which is probably due to radiogenic addition from a slab-derived fluid.Re- and Os-rich sulphide and oxide minerals precipitate from volcanic gases within fumarolic fields. Molybdenite (MoS2), powellite (CaMoO4) and cannizzarite (Pb4Bi6S13) contain 1.5-1.7 wt%, 10 ppm, and 65-252 ppb of Re, respectively. Both molybdenite and rheniite contain normal Os concentrations, with total Os abundances in a range from 0.6 to 3.1 ppm for molybdenite, and 2.3-24.3 ppb for the rheniite samples. Repeated analyses of osmium isotope ratios for two rheniite samples form a best-fit line with an initial 187Os/188Os ratio of 0.32 ± 0.15 and an age of 79 ± 11 yr, which is the youngest age ever measured in natural samples. The high Re contents in molybdenite and rheniite led to high radiogenic 187Os values, even in the limited period of time, with 187Os/188Os ratios up to 3.3 for molybdenite and up to 4.4 for rheniite.The Os isotopic compositions of andesite-basaltic rocks from the Kudryavy volcano (187Os/188Os up to 0.326) are more radiogenic than those of residual peridotites and fumarolic gas condensates that are mainly constituted from magmatic vapor. Such radiogenic values can be attributed either to the addition of a radiogenic Os-rich subduction component to the depleted mantle, or to the assimilation of older dacitic caldera walls (187Os/188Os = 0.6) during arc magma ascent and emplacement. The latter hypothesis is supported by the correlation between 187Os/188Os ratio and indicators of fractionation such as MgO or Ni, and by low contents of potentially hydrophile trace elements such as Ba, Rb and Th relative to fluid-immobile elements such as Nb, Zr and Y. The high Re flux in the Kudryavy volcano (estimated at ∼46 kg/yr) can be explained by remobilisation of Re by Cl-rich water from an underplated mantle wedge and subducted organic-rich sediments of the Pacific plate.  相似文献   

3.
Highly siderophile element concentrations (HSE: Re and platinum-group elements (PGE)) are presented for gabbros, gabbroic eclogites and basaltic eclogites from the high-pressure Zermatt-Saas ophiolite terrain, Switzerland. Rhenium and PGE (Os, Ir, Ru, Rh, Pt, Pd) abundances in gabbro- and eclogite-hosted sulphides, and Re-Os isotopes and elemental concentrations in silicate phases are also reported. This work, therefore, provides whole rock and mineral-scale insights into the PGE budget of gabbroic oceanic crust and the effects of subduction metamorphism on gabbroic and basaltic crust.Chondrite-normalised PGE patterns for the gabbros are similar to published mid-ocean ridge basalts (MORB), but show less inter-element fractionation. Mean Pt and Pd contents of 360 and 530 pg/g, respectively, are broadly comparable to MORB, but gabbros have somewhat higher abundances of Os, Ir and Ru (mean: 64, 57 and 108 pg/g). Transformation to eclogite has not significantly changed the concentrations of the PGE, except Pd which is severely depleted in gabbroic eclogites relative to gabbros (∼75% loss). In contrast, basaltic eclogites display significant depletion of Pt (?60%), Pd (>85%) and Re (50-60%) compared with published MORB, while Os, Ir and Ru abundances are broadly comparable. Thus, these data suggest that only Pt, Pd and Re, and not Os, Ir and Ru, may be significantly fluxed into the mantle wedge from mafic oceanic crust. Re-Os model ages for gabbroic and gabbroic eclogite minerals are close to age estimates for igneous crystallisation and high-pressure metamorphism, respectively, hence the HSE budgets can be related to both igneous and metamorphic behaviour. The gabbroic budget of Os, Ir, Ru and Pd (but not Pt) is dominated by sulphide, which typically hosts >90% of the Os, whereas silicates account for most of the Re (with up to 75% in plagioclase alone). Sulphides in gabbroic eclogites tend to host a smaller proportion of the total Os (10-90%) while silicates are important hosts, probably reflecting Os inheritance from precursor phases. Garnet contains very high Re concentrations and may account for >50% of Re in some samples. The depletion of Pd in gabbroic eclogites appears linked, at least in part, to the loss of Ni-rich sulphide.Both basaltic and gabbroic oceanic crust have elevated Pt/Os ratios, but Pt/Re ratios are not sufficiently high to generate the coupled 186Os-187Os enrichments observed in some mantle melts, even without Pt loss from basaltic crust. However, the apparent mobility of Pt and Re in slab fluids provides an alternative mechanism for the generation of Pt- and Re-rich mantle material, recently proposed as a potential source of 187Os-186Os enrichment.  相似文献   

4.
The Yushui Cu-polymetallic deposit, which is associated with Ag, Pb, and Zn, is located in the middle part of the Yongan–Meixian Late Paleozoic Hercynian depression. It was discovered in eastern Guangdong Province in the late 1980s and is one of the richest copper deposits in China with high-grade copper averaging 3.25% and locally reaching 50–60%. The main ore body is located along the unconformity between the Upper Carboniferous Hutian Group limestone and the Lower Carboniferous Zhongxin Formation quartz sandstone with a bedded and lenticular morphology. The ores exhibit massive textures dominated by chalcopyrite, bornite, chalcocite, pyrite, sphalerite, galena, and a trace amount of argentite. Although researchers began studying the Yushui deposit in the early 1990s, the ore genesis remains controversial because of the lack of precise mineralisation age constraints. In this study, direct Re–Os dating of Cu sulphides aided in facilitating a better understanding of the timing of formation of the Yushui deposit. This study is the first attempt to use the Re–Os isotopic system for directly dating chalcopyrite and bornite ores for the Yushui deposit. The contents of Re, common Os, 187Re and 187Os in nine sulphides are 1.68–219.35 ppb, 0.003–0.427 ppb, 1.05–137.31 ppb, and 0.045–0.734 ppb, respectively. The isotope data yielded an isochron age of 308 ± 15 Ma (mean square weighted deviates = 2.4) using the 87Re/188Os–187Os/188Os plot, which is interpreted to represent the age of formation for these sulphides, suggesting that the mineralisation age of the Yushui deposit is close to the age of the host rocks. The 187Os/188Os initial value obtained from the Re–Os isochron is 1.81 ± 0.34, which corresponds to the γOs value of + 1349. This value indicates that the ore-forming materials were derived from the crust without mixing with materials from the mantle, and that the Yushui massive sulphide deposit may be of sedimentary exhalative origin.  相似文献   

5.
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   

6.
Lower crustal xenoliths recovered from Eocene to Cambrian kimberlites in the central and southern Slave craton are dominated by mafic granulites (garnet, clinopyroxene, plagioclase±orthopyroxene), with subordinate metatonalite and peraluminous felsic granulites. Geothermobarometry indicates metamorphic conditions of 650–800 °C at pressures of 0.9–1.1 GPa. The metamorphic conditions are consistent with temperatures expected for the lower crust of high-temperature low-pressure (HT-LP) metamorphic belts characteristic of Neoarchean metamorphism in the Slave craton. U–Pb geochronology of zircon, rutile and titanite demonstrate a complex history in the lower crust. Mesoarchean protoliths occur beneath the central Slave supporting models of an east-dipping boundary between Mesoarchean crust in the western and Neoarchean crust in the eastern Slave. At least, two episodes of igneous and metamorphic zircon growth occurred in the interval 2.64–2.58 Ga that correlate with the age of plutonism and metamorphism in the upper crust, indicating magmatic addition to the lower crust and metamorphic reworking during this period. In addition, discrete periods of younger zircon growth at ca. 2.56–2.55 and 2.51 Ga occurred 20–70 my after the cessation of ca. 2.60–2.58 Ga regional HT-LP metamorphism and granitic magmatism in the upper crust. This pattern of younger metamorphic events in the deep crust is characteristic of the Slave as well as other Archean cratons (e.g., Superior). The high temperature of the lower crust immediately following amalgamation of the craton, coupled with evidence for continued metamorphic zircon growth for >70 my after ‘stabilization’ of the upper crust, is difficult to reconcile with a thick (200 km), cool lithospheric mantle root beneath the craton prior to this event. We suggest that thick tectosphere developed synchronously or after these events, most likely by imbrication of mantle beneath the craton at or after ca. 2.6 Ga. The minimum age for establishing a cratonic like geotherm is given by lower crustal rutile ages of ca. 1.8 Ga in the southern Slave. Transient heating and possible magmatic additions to the lower crust continued through the Proterozoic, with possible additional growth of the tectosphere.  相似文献   

7.
Geochemistry and U-Pb ages of leucosomes and tonalites in a high pressure granulite unit of the Dulan area have been determined to constrain the tectonothermal evolution related to collision and thickening of lower crust in the North Qaidam Mountains (NQD). Leucosomes and tonalites show a marked chemical resemblance to adakites: (1) high La/Yb and Sr/Y, and low Y and HREE; (2) high Al2O3 and low Mg# values with obvious positive Eu anomalies; and (3) slightly positive εNd(t) values. Zircon U-Pb analysis of leucosomes and tonalites yielded 206Pb/238U ages of 428-437 Ma and 436-437 Ma respectively, which constrain the emplacement ages of the adakitic rocks. Petrological, geochronological and geochemical characters indicate that the adakitic rocks may have been derived from partial melting of a thickened mafic lower crust (> 50 km), suggesting a dominating source regime of synchronous high-pressure mafic granulites. Contemporary magmatism in other units of the NQD shows evidence of a widespread tectonothermal event during early Silurian (420-450 Ma) that includes metamorphism, magmatism and anatexis related to collision and thickening of lower crust.  相似文献   

8.
Shield-stage high-MgO alkalic lavas from La Palma and El Hierro (Canary Islands) have been characterized for their O-Sr-Nd-Os-Pb isotope compositions and major-, trace-, and highly siderophile-element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances. New data are also reported for associated evolved rocks, and entrained xenoliths. Clear differences in Pd/Ir and isotopic ratios for high Os (>50 ppt) lavas from El Hierro (δ18Oolivine = 5.17 ± 0.08‰; 87Sr/86Sr = 0.7029 to 0.7031; εNd = +5.7 to +7.1; 187Os/188Os = 0.1481 to 0.1750; 206Pb/204Pb = 19.1 to 19.7; Pd/Ir = 6 ± 3) versus those from La Palma (δ18Oolivine = 4.87 ± 0.18‰; 87Sr/86Sr = 0.7031 to 0.7032; εNd = +5.0 to +6.4; 187Os/188Os = 0.1421 to 0.1460; 206Pb/204Pb = 19.5 to 20.2; Pd/Ir = 11 ± 4) are revealed from the dataset.Crustal or lithospheric assimilation during magma transport cannot explain variations in isotopic ratios or element abundances of the lavas. Shallow-level crystal-liquid fractionation of olivine, clinopyroxene and associated early-crystallizing minerals (e.g., spinel and HSE-rich phases) controlled compatible element and HSE abundances; there is also evidence for sub-aerial degassing of rhenium. High-MgO lavas are enriched in light rare earth elements, Nb, Ta, U, Th, and depleted in K and Pb, relative to primitive mantle abundance estimates, typical of HIMU-type oceanic island basalts. Trace element abundances and ratios are consistent with low degrees (2-6%) of partial melting of an enriched mantle source, commencing in the garnet stability field (?110 km). Western Canary Island lavas were sulphur undersaturated with estimated parental melt HSE abundances (in ppb) of 0.07 ± 0.05 Os, 0.17 ± 0.16 Ir, 0.34 ± 0.32 Ru, 2.6 ± 2.5 Pt, 1.4 ± 1.2 Pd, 0.39 ± 0.30 Re. These estimates indicate that Canary Island alkali basalts have lower Os, Ir and Ru, but similar Pt, Pd and Re contents to Hawai’ian tholeiites.The HIMU affinities of the lavas, in conjunction with the low δ18Oolivine and high 206Pb/204Pb for La Palma, and elevated 187Os/188Os for El Hierro implies melting of different proportions of recycled oceanic crust and lithosphere. Our preferred model to explain isotopic differences between the islands is generation from peridotitic mantle metasomatised by <10% pyroxenite/eclogite made from variable portions of similar aged recycled oceanic crust and lithosphere. The correspondence of radiogenic 206Pb/204Pb, 187Os/188Os, elevated Re/Os and Pt/Os, and low-δ18O in western Canary Island lavas provides powerful support for recycled oceanic crust and lithosphere to generate the spectrum of HIMU-type ocean island basalt signatures. Persistence of geochemical heterogeneities throughout the stratigraphies of El Hierro and La Palma demonstrate long-term preservation of these recycled components in their mantle sources over relatively short-length scales (∼50 km).  相似文献   

9.
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   

10.
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   

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