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The Black Swan komatiite sequence, in the Eastern Goldfields province of the Archaean Yilgarn Craton in Western Australia, is a body of dominantly olivine-rich cumulates with lesser volumes of spinifex textured rocks, interpreted as a section through an extensive komatiite lava flow field. The sequence hosts a number of nickel sulfide orebodies, including the Silver Swan massive shoot and the Cygnet and Black Swan disseminated orebodies. The massive sulfide orebodies of the Black Swan Succession are pervasively depleted in all platinum group elements (PGEs), particularly Pt and Pd, despite very high Ni contents. This depletion cannot be explained by R-factor variations, which would also require relatively low Ni tenors. The PGE depletion could be explained in part if the ores are enriched in a monosulfide solid solution (MSS) cumulate component, but requires some additional fractional segregation of sulfide melt upstream from the site of deposition. The Silver Swan orebody shows a remarkably consistent vertical zonation in PGE contents, particularly in Ir, Ru, Rh, Os, which increase systematically from very low levels at the stratigraphic base of the sulfide body to maxima corresponding roughly with the top of a lower layer of the orebody rich in silicate inclusions. Platinum shows the opposite trend, but is somewhat modified by remobilisation during talc carbonate alteration. A similar pattern is also observed in the adjacent White Swan orebody. This zonation is interpreted and modelled as the result of fractional crystallisation of MSS from the molten sulfide pool. The strong IPGE depletion towards the base of the orebody may be a consequence of sulfide liquid crystallisation in an inverted thermal gradient, between a thin rapidly cooling upper rind of komatiite lava and a hot substrate.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial handling: Peter Lightfoot 相似文献
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The paper describes the variation pattern of magnetic susceptibility of Lameta sediments and isotopic variation of organic13C from Chui Hill, Bergi, Kholdoda, Pisdura and Girad. The susceptibility pattern and a negative carbon isotopic anomaly allows
fixation of the K/T boundary at these localities and they differ in these aspects from the inter-trappean sediments at Anjar.
Paleomagnetic measurements of the Anjar sediment and the overlying basalt flow demonstrate reversed polarity. The Lameta sediments
with dinosaur nests at Kheda and the overlying intertrappean sediments are of normal polarity.
The clay layers at Anjar, associated closely with Ir-enrichments, are strongly leached, rhyolitic bentonites containing low-quartz
paramorphs after high-quartz with glass inclusions.
It is concluded, that the inter-trappean lake deposits at Anjar were deposited in the early part of magnetochron 29R and are
unrelated to the K/T boundary. 相似文献
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针对低轨卫星快速空间几何变化和抗干扰能力强等特征,该文基于卫星工具包软件对全球导航定位系统和铱星系统星座进行了仿真,并假定铱星具有导航卫星的功能,分析铱星对GPS定位的增强作用。首先对GPS和铱星增强星座的可见卫星数量和几何精度因子值进行了分析,然后通过对不同的误差值建模,对GPS系统和铱星系统的观测值进行了仿真,分析了低轨卫星对双差定位浮点解和模糊度固定的增强作用,结果表明:低轨卫星的加入增加了可见卫星数量,几何精度因子也优于单GPS系统。单频双差模糊度浮点解的RMS值优于1周,双频双差模糊度浮点解的RMS值优于0.5周,与单GPS相比有了较明显的提高,同时,低轨卫星的加入更有利于单频短基线的模糊度固定。 相似文献
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We present new analyses that confirm Ir enrichment (up to 0.31 ng/g) in close proximity to the palynological Triassic–Jurassic boundary in strata near the top of the Blomidon Formation at Partridge Island, Nova Scotia. High Ir concentrations have been found in at least two samples within the uppermost 70 cm of the formation. Ratios of other PGEs and Au to Ir are generally higher by an order of magnitude than in ordinary chondrites. No impact-related materials have been identified at this horizon in the Blomidon Formation, therefore we cannot confirm an extraterrestrial source for the anomalous Ir levels. We consider, however, the possibility that regional basaltic volcanism is a potential source for the Ir in these sediments. The elevated Ir concentrations are found in reduced, grey colored mudstones, so redox concentration is a possible explanation for the distribution of Ir in these strata. 相似文献
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Dewey M. McLean 《Cretaceous Research》1985,6(3):235-259
Mantle degassing continually releases gases onto the earth's surface. Over geologically long time intervals, a general equilibrium probably exists between mantle CO2 release and uptake by surficial sinks. However, during periods of rapid plate movement, or continental flood basalt volcanism, the increased rate of mantle CO2 release may exceed that of uptake, leading to CO2 accumulation in the atmosphere and the marine mixed layer (top 50–100 m). This in turn triggers chemical changes in the mixed layer, climatic warming, and bioevolutionary turnover. The Cretaceous/Tertiary () transition at 65 Ma seems to have been a time of major mantle degassing which induced a perturbation of the carbon cycle. During the transition, Deccan Traps volcanism, perhaps the greatest episode of continental flood basalt volcanism in the Phanerozoic, flooded an estimated 2.6 × 106 km2 of India with basaltic lavas, releasing 5 × 1017 moles of CO2 into the earth's atmosphere over a duration 0.53–1.36 Ma at the rate of 3.9 × 1011 to 9.6 × 1011 moles CO2 per year. The modern mean annual rate of mantle CO2 release from all sources is 4.1 × 1012 moles CO2 per year; assuming a comparable rate of release prior to the Deccan Traps volcanism, the Deccan Traps addition would have elevated the rate of mantle CO2 release by 10–25%. Sluggish marine circulation and warm, deep, oceans (14–15°C) would have exacerbated CO2 buildup in the atmosphere, accounting for the Cretaceous to Tertiary drop in oxygen-18 via climatic warming, and, in the marine mixed layer (top 50–100 m), explaining the selective nature of the terminal Cretaceous marine extinctions via a pH change. The extinctions were most severe amongst the calcareous microplankton of the mixed layer; calcareous microplankton (planktonic foraminifera and coccolithophorids) begin to have pH problems at 7.8 and 7.5, respectively. Failure of the coccolithophorids would have disrupted the Williams-Riley pump (algal productivity-gravity pump of CO2 from the atmosphere and mixed layer into the deep oceans) producing dead ocean conditions (severely reduced photosynthesis and CaCO3 production). Failure of the Williams-Riley pump is reflected in the extinctions themselves, and in the loss of biogenic CaCO3 to the sea floor, causing the boundary hiatus and (or) the boundary clay. Failure of the pump today would elevate atmospheric pCO2 severalfold; the failure would have responded comparably. Dead ocean conditions would, in themselves, have produced a major CO2 buildup. Early Tertiary “Strangelove” conditions in the mixed layer, characterized by a dominance of the thoracosphaerids, braarudosphaerids and small planktonic foraminifera, were coeval with the main pulse of Deccan Traps volcanism. Overall, the record is one of gradual bioevolutionary turnover during a period of disequilibrium between the rate of mantle CO2 degassing and uptake by sinks. Mantle degassing during the Deccan Traps volcanism unifies the biological and physicochemical records. 相似文献
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Controls on variations of platinum-group element concentrations in the sulfide ores of the Jinchuan Ni-Cu deposit, western China 总被引：3，自引：0，他引：3
Ongoing underground exploration in the giant Jinchuan Ni-Cu sulfide deposit in western China is beginning to emphasize the
potential for Cu-, Pt-, and Pd-rich sulfide ores that may have formed by sulfide liquid fractionation. The success of such
an effort relies on whether or not fractional crystallization of sulfide occurred in the Jinchuan system. In this paper, we
used available PGE data to evaluate such a process. We found that about two thirds of the 126 samples analyzed to date exhibit
significant decoupling not only between Pt and Pd but also between Ru, Rh, and Ir. The best explanation for the decoupling
is postmagmatic hydrothermal alteration, which affected not only silicates but also sulfides. The effects of postmagmatic
alteration must be considered when using metal and isotopic ratios to evaluate primary mineralization. PGE variations in the
remaining one third of the samples with Ir/(Ir + Ru) = 0.3–0.7, Ir/(Ir + Rh) = 0.4–0.8, and Pt/(Pt + Pd) = 0.3–0.7 indicate
variable R-factors within individual ore bodies as well as the entire deposit, consistent with the interpretation that multiple
sulfide-bearing magmas from depth were involved in the formation of the Jinchuan deposit. The mantle-normalized PGE patterns
of the least-altered samples from the Jinchuan deposit are similar to the picrite-related Pechenga Ni-Cu sulfide deposit in
Russia. PGE variations that can be related to sulfide liquid fractionation are observed in orebody-1 and orebody-24 but not
in orebody-2 at Jinchuan. Exploration for Cu-, Pt-, and Pd-rich sulfide ores that may have been expelled into fractures in
the footwalls of orebody-1 and orebody-24 appears to be justified. 相似文献
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Electron paramagnetic resonance of Ir2+ in forsterite is studied at Q-band frequency and room temperature. There are four equivalent spectra superimposed along the three crystallographic axes. The individual spectrum consists of four hyperfine lines of approximately equal intensity separated from each other by 42 G; one axis of the g tensor is near the c axis. Ir2+ is certainly substituted for Mg2+. Because of the fourfold degeneracy of the EPR spectrum, it may be suggested that iridium occurs at M1. Taking into account that one eigenvector of the g tensor is nearly parallel to c, it seems also possible that the substitution takes place at the M2 position. In this case, the lattice relaxation of the crystal structure around Ir2+ at M2 must break the point symmetry m at M2. 相似文献