The now-extinct short-lived ¹⁸²Hf-¹⁸²W radiometric isotope system with a half life of 8.9 Myrs has been widely used to provide significant geochronological constraints on planetary processes, such as core-mantle segregation and early crust-mantle differentiation [1]. Evolving from a chondriticreservoir (u¹⁸²W = ~-190 calculated as the deviation of ¹⁸²W/¹⁸⁴W from modern Earth’s mantle multiplied by 1 million), the modern Earth was long believed to consist of two components –a bulk silicate earth with a homogeneous ¹⁸²W isotopic composition(u¹⁸²W = 0) and by inference a metalliccore with u¹⁸²W = ~-220. Recently, high-precision (<±5 ppm 2σ SD on ¹⁸²W/¹⁸⁴W) measurements of W isotopic ratios [2-5] have been able to discriminate subtle ¹⁸²W isotopic anomalies in terrestrial rocks, triggeringconsiderable exploration of early Earth processes and their influence on modern Earth. In theory, the older rock samples and the deeper mantle-derived rocks are more likely to preserve the information of early-formed ¹⁸²W isotopic variation within the silicate Earth.

Previous studies have revealed that Hadean to Eoarchean supracrustal rocks are often characterized by positive u¹⁸²W ranging from +10 to +15 [2, 6-15], while negative values are also reported [9]. The positive ¹⁸²W anomalies could reflect the heritage of a pre-late accretionary mantle [2,8,13,14] or an early depleted,high Hf/W parental mantle reservoir (while ¹⁸²Hf was still extant) [6,7,10]. The negative values could record the influence of the mantle contaminated by more proportional meteoritic materials during localized late accretion or an early, enriched low Hf/W parentalmantle reservoir [9]. Given ~40% of the W budget of the silicate Earth resides in the continental crust, the accessible continental crust over time can provide the long-term evolution of W isotopic compositionfor the uppermost portion of the convecting upper mantle where the crust was derived. Archean upper continental crust sampled by diamictites is characterized by generally negative u¹⁸²W of -13 that progressively diminished to zero in diamictites through the Paleoproterozoic to Phanerozoic [16], most likely reflecting the efficient homogenization of W isotopic composition in the upper mantle by plate tectonics and crustal growth/rejuvenation. Provided by the W mobility in the crust-mantle reservoirs [7,17], another way to inspect the W isotopic composition is to analyze tungsten ore deposits (scheelite or wolframite) formed in ancient to modern continents. Recently, Mundl et al. [18] reported negative ¹⁸²W anomalies for the deep plume-derived basalts, the deep sources of which may not be well homogenized by subduction or it could reflect the primordial signature in the deep mantle or the interaction between the lowermost mantle and metalliccore. Surprisingly, Rizo et al. [19] found substantially largeru¹⁸²W values from +24 to +48 for Phanerozoic flood basalts from the Ontong Javal Plateau (120Ma) and the Baffin Bay (60Ma). However, Kruijer and Kleine [20] proposed that the ¹⁸²W excesses for the OJP sample reported by Rizo et al. [19] and perhaps also the Baffin Bay samples were caused by the effect of nuclear field shift leading tothe deficit of ¹⁸³W, which is used in the NTIMS analyses via a double normalization. Another critical issue is that significant mass dependent fractionation of W isotopes (e.g., δ¹⁸²W/¹⁸⁴W up to >0.2‰) can occur in the natural samples [21]. Whether such large amounts of mass dependent fractionation of W isotopes in the rocks can be well corrected deserves further investigation before concluding subtle variations of the W isotopic ratios on the terrestrial rocks. Collectively, the interpretation of the highly precise and accurate ¹⁸²W isotopic anomalies of the terrestrial rocks should be assisted by data of ¹⁴⁶Sm-¹⁴²Nd, ¹⁹⁰Pt-¹⁸⁷Re-^186,187Os, and highly siderophile element abundances. In addition, how the mobility of W in the mantle affects the magnitute of W anomaly demands detailed work coupled with elements with similar incompatibilities.

Acknowledgements:This work has been supported by the National Natural Science Foundationof China (No. 41822301) and China “1000 Youth Talents Program”.

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Dr. Jingao Liu is a professor at the China University of Geosciences (Beijing). He obtained a PhD at the University of Maryland-College Park in 2011 and did postdoc research in both University of Maryland and University of Alberta before joining the China University of Geosciences (Beijing) in 2016. Jingao is mainly focused on the mantle geochemistry and cosmochemistry using radiogenic isotope systems (¹⁹⁰Pt-¹⁸⁷Re-^186,187Os, ¹⁸²Hf-¹⁸²W and ¹⁴⁶Sm-¹⁴²Nd, as well as traditional Sr-Nd-Hf-Pb isotopes) coupled with highly siderophile element systematics, and has published over 30 articles, with a Google Scholar h-index 15 (https://scholar.google.com/citations?user=min5XQwAAAAJ&hl=en).