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

Hf-W同位素系统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领域的广泛关注。然而，由于质谱计电离效率低，导致难以进行灵敏、精确的W同位素测量，阻碍了Hf-W计时器的实际应用。我们开发了一种新的W同位素分析技术，使用多收集器- icp质谱仪（MC-ICPMS）结合高回收率离子色谱技术。本研究测量了沉积物、火山岩、矿床等不同地球化学样品的W同位素组成。182W/183W可能由于放射性核素182Hf的β衰变而改变。事实上，金属陨石（铁陨石）的182W/183W比值有显著变化。这可能是由于金属/硅酸盐在mo . More的星子中分离的时间不同，或者是由于主太阳星云中Hf/W的不均匀分布。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，这表明在太阳星云早期，Hf/W比值并没有完全均匀化。相反，地球化学物质的182W/183W没有可测量的变化，这表明地球的吸积速度缓慢，或者是一次巨大的撞击事件使Hf-W同位素系统完全重置。因此，目前的182W/183W比值数据可以提供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，但我们发现部分地球化学样品中的W通过样品地层序列发生了同位素分馏。JMn-1和HSD的186W/183W（以epsilon单位表示）分别比NIST-3163（国际同位素标准）高1.5ε和4.5ε。人们普遍认为，W的氧化态可以通过宇宙化学和地球化学过程发生变化，因此，地球化学材料中的W可以为样品的生产顺序提供物理化学信息。虽然目前对W的同位素分异机制尚不清楚，但非放射性成因W组分的微小但显著的变化表明，将Hf-W年代学与W稳定同位素地球化学相结合，进行更详细的地球化学讨论是可能的，这将开辟地球科学新的研究领域。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