[S-GC33] Solid Earth Geochemistry, Cosmochemistry

[SGC33-P03] Tungsten elemental and isotopic composition of the Taishan komatiites: evidence for alloy saturation

*Mimi Chen^1,2, Katsuhiko Suzuki¹, Maria Luisa G. Tejada¹, Chang Qing¹, Wei Tian¹, Yuki Hibiya³, Junko Kikuchi¹ (1.Japan Agency for Marin-Earth Science and Technology, 2.Peking University, 3.The University of Tokyo)

Keywords:Siderophile element, 182Hf-182W isotope system, Komatiites, alloy saturation, early Earth mantle dynamics

This study reports the first high precision W concentrations and ¹⁸²W isotopic compositions for the ca. 2.7 Ga Taishan komatiites in the North China Craton. Geochemical characteristics of the analysed komatiites reveal olivine fractionation as a major control, though a minor crustal contamination on La is detected. The Taishan komatiites have unusually high W content ranging from 1.0 to 15.2 ppm, exceeding the W budgets of all the known W-rich reservoirs. The overabundance in W also lead to highly elevated W/Th and relatively restricted Hf/W ratios that are rarely seen in the ultramafic-mafic rocks closely affiliated with deep mantle. Another striking feature is that W is positively correlated with MgO and Ni, but inversely correlated with Na₂O, revealing a compatible behaviour of W during komatiitic magma differentiation. Therefore, the strong W overabundance, rather than secondary enrichment by post-magmatic processes, must be a pristine feature of the Taishan komatiites. The Taishan komatiites have µ¹⁸²W values averaging at -2.3 ± 1.6 ppm (2 SD, n = 18), falling within the present upper mantle range, but distinct from the ¹⁸²W excess found in the Archean TTG rocks from the Northeast North China Craton. The near-neutral ¹⁸²W compositions of the Taishan komatiites indicate that by ~2.7 Ga, the regional mantle homogenization beneath the North China Craton has been achieved, but the global mantle heterogeneity persisted. Our data provide strong evidence for W being a compatible trace element, probably in the form of metal alloy, during the early-stage fractionation of komatiitic magma. The significance of the crystallization of non-silicate phases from the deep mantle lies in their extraordinary capability of scavenging siderophile elements, especially W, from the parental melt. The unique geochemical composition of the Taishan komatiites provide hints on the presence of the rarely seen but potentially widespread metal alloy phase segregation and, more importantly, its role in the elemental and isotopic behavior of W and other siderophile elements during early melt extraction.