其他摘要 | In the long past the instrumental neutron activation analysis (INAA) has been mainly employed in the study of whole-rock metal phases in meteorites since this technique has the advantages of high sensitivity, no sample damage and simple sample preparation. Unfortunately, this method is not powerful enough to determine all the platinum-group elements (PGEs). With the development of analytical techniques, inductively coupled plasma mass spectrometry (ICP-MS) has found wider and wider applications in the geological field, but less in the study of extraterrestrial materials.
In this paper, on the basis of this analysis method, the methodological studies of ICP-AES and ICP-MS have been carried out for the first time in China to analyze the major and trace elements of metal phases in meteorites. We employed both the methods to analyze metal phases in four chondrites and four iron meteorites, preliminarily explore the variation characteristics of siderophile elements in the processes of nebular condensation and asteroid thermal metamorphism, and further disclose the process of nebular condensation and that of crystallization of iron meteorites. Research done on the newly found Ulasitai iron meteorite has revealed that this meteorite may pair to the Armanty meteorite. Moreover, our research has been expanded to the study of extraterrestrial impact event. On the basis of the results of analysis of the P/T boundary samples at Meishan, Zhejiang Province, reasonable explanations have been proposed for the mass extinction event at that time.
The Ulasitai iron meteorite is a piece of iron meteorite which has been newly found in Beitashan, Qinghe County, Xinjiang, whose find site is about 130 km to that of the Armanty (Xinjiang, IIIE) meteorite. We employed ICP-MS to analyze the bulk samples of Ulasitai, together with the Armanty(IIIE), Nandan (IIICD) and Mundrabilla (IIICD). The results are in agreement with literature data for the same meteorites, indicating that this method is highly feasible. Systematic petrological, mineralogical and trace element analyses were conducted of the Ulastai iron meteorite, indicating that this meteorite is a coarse octahedrite, with a kamacite bandwidth of 1.2±0.2 (0.9-1.8) mm. Plessite is abundant with various micro-structures. Schreibersite is Ni-rich (30.5-55.5 wt%) in plessite or coexisting with troilite and daubreelite. Through calculation it is known that this meteorite has a cooling rate of ~20 C/Ma. The Ulasitai is similar in petrography and mineral chemistry to the Armanty, both plotting within the IIIE field. Therefore, the Ulasitai is classified as a new IIIE iron meteorite, suggesting that it pairs with the Armanty. At the same time, studies of the metal phases in the Jilin chondrite (H5), the Anlong chondrite (H5), and Antarctic meteorites GRV 9919(L3)and GRV 021603(H3)have shown that the distribution patterns of siderophile elements in the metal phases of the chondrites are mainly related to the volatility of the elements, i.e., the refractory elements generally show a smooth distribution pattern, while the moderately and strongly volatile elements tend to be depleted with increasing volatility.
Research on the platinum-group elements (PGEs) is of great significance not only in dealing with the differentiation of siderophile elements, metal-silicate differentiation and core-mantle formation as well as the crystallization-fractionation of metal melt, but also has plaid a very important role in exploring the tracing of extraterrestrial materials. The platinum-group elements (PGEs) are enriched in most extraterrestrial materials, but highly depleted in the terrestrial crust materials. Therefore, Ir anomaly can be regarded as one important piece of evidence for the existence of extraterrestrial materials. The Permian-Triassic boundary sections (25.1 Ma B.P.) have recorded the most severe mass extinction event in the geological history. However, there is a long-standing controversy on whether bolide impact and/or basaltic flood volcanism triggered the mass extinction. We analyzed Ir, Ru, Rh, Pt and Pd in a set of samples from the global stratotype section and point (GSSP) of the Permian-Triassic boundary at Meishan, China, using isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) and nickel sulfide fire assay (NiS-FA) combined with Te coprecipitation. Results of fire assay showed that the content of Ir is 0.053 ng/g. The results of isotope dilution analysis for P/T boundary samples demonstrated that the contents of Ir are within the range of 0.005-0.028 ng/g. The analytical results obtained by the two methods both show no Ir anomaly and their PGE patterns normalized to chondrites are highly fractionated with Ir/Pd ratios of 0.02-0.03 CI, distinct from most meteorites. In contrast, the PGE distribution patterns of the P/T boundary samples are closely parallel to those of the Siberian and probably Emeishan flood basalts, suggestive of possible sources of PGEs from the basalts. The abundances of PGEs increase in order of the pyrite laminae at the top of bed 24, bed 25 and bed 26, and then decrease toward bed 28, probably indicative of a maximum eruption of flood basalts during deposition of bed 26. The results provide strong evidence in favor of massive volcanism, instead of extraterrestrial impact, as a major cause of the Permian-Triassic boundary mass extinction. |
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