Hf-W Isotopic Systematics : Constraints for Geochemical Evolution of the Planet Earth

点击翻译按钮获取中文摘要

Hf-W isotopic systematics has been given great attention in the field of Earth sciences because of its high time resolution to the cosmochemical events. However, the practical utility of the Hf-W chronometer has been retarded by the difficulty in sensitive and precise W isotopic measurements mainly caused by low ionisation efficiency in the mass spectrometer. We have developed a new analytical technique for W isotopes using a multiple collector-ICP mass spectrometer (MC-ICPMS) coupled with a high recovery ion chromatographic technique. In this study, W isotopic compositions for various geochemical samples, covering sediments, volcanic rocks or ore deposits, have been measured.The 182W/183W can change due to radiogenic contribution from beta-decay of radioactive nuclide 182Hf.In fact, there was significant variation in resulting 182W/183W ratio for metallic meteorites (iron meteorites). This can be attributed either to the difference in timing of metal/silicate segregation inside the mo … More ther planetesimals, or to the heterogeneous distribution of Hf/W in the primary solar nebula. Most interesting feature is that the present 182W/183W ratio for IVB iron meteorites did not agree with the some of the published values, indicating that Hf/W ratio was not totally homogenised at the early stage of the solar nebula. In strike contrast, resulting 182W/183W for geochemical materials did not vary measurably, suggestive of either slow accretion rate of the Earth, or total reset of the Hf-W isotopic system by a giant impact event. Thus the present 182W/183W ratio data could provide key information about the early sequence of the solar system.Despite the absence in variation in 182W/183W ratio for the geochemical samples, we found that W in some geochemical samples was isotopically fractionated through the sample formation sequence.Resulting 186W/183W (expressed as epsilon units) for JMn-1 and HSD show significantly higher 1.5ε and 4.5ε than the value for NIST-3163 (international isotopic standard), respectively. It is widely recognized that oxidation state of W can change through cosmo- and geo-chemical processes, and therefore, W in geochemical materials could provide us physico-chemical information for the production sequence of the sample. Although mechanism of the isotopic fractionation of W is not well understood at this moment, the small but significant variations in non-radiogenic W components indicated clearly that further detailed geochemical discussion is possible by the combination of Hf-W chronology and stable isotope geochemistry using W, which would open up the new research field in the Earth Sciences. Less