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

Hf-W同位素系统学因其对宇宙化学事件具有较高的时间分辨率而受到地球科学界的高度重视。然而，HF-W计时器的实际应用一直受到限制，主要是由于质谱计的电离效率低，导致难以进行灵敏和精确的W同位素测量。我们用多收集器-电感耦合等离子体质谱仪(MC-ICPMS)和高回收离子色谱技术建立了一种新的钨同位素分析技术。本研究测量了沉积物、火山岩或矿床等不同地球化学样品的W同位素组成。182W/183W可因放射性核素182Hf的β衰变而产生放射性贡献。实际上，金属陨石(铁陨石)的182W/183W比值发生了显著变化。这可以归因于mo…内部金属/硅酸盐分凝时间的不同。更多的行星体，或与原太阳星云中HF/W的不均匀分布有关。最有趣的特征是，目前IVB铁陨石的182W/183W比值与一些已发表的值不一致，表明在太阳星云早期，HF/W比值并不是完全均匀的。与之形成鲜明对比的是，地球化学物质的182W/183W没有可测量的变化，这要么表明地球的缓慢吸积速率，要么表明Hf-W同位素系统被一次巨大的撞击事件完全重置。目前的182W/183W比值数据可以提供关于太阳系早期序列的关键信息。尽管地球化学样品的182W/183W比值没有变化，但我们发现一些地球化学样品中的W通过样品形成序列进行了同位素分馏。结果表明，JMN-1和HSD的186W/183W(以epsilon单位表示)分别显著高于国际同位素标准NIST-3163的值1.5ε和4.5ε。人们普遍认为，W的氧化状态可以通过宇宙和地球化学过程来改变，因此，地球化学物质中的W可以为我们提供样品生产序列的物理化学信息。尽管目前还不清楚W的同位素分馏机制，但非放射性成因W组分的微小但显著的变化清楚地表明，结合HF-W年代学和稳定的W同位素地球化学，有可能进行更详细的地球化学讨论，这将开辟地球科学的新研究领域。较少

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