Tungsten Abundance and Isotope Composition of the Upper Mantle

Variations in the 182W/184W ratio of ancient and modern mantle-derived rocks could trace processes occurring early in Earths history, such as early silicate differentiation, late accretion, and even core-mantle interaction. However, the use of W as elemental and isotopic tracer requires knowing the W abundance of Earths reservoirs, including the upper mantle, and having a good understanding of its behavior during mantle processes. Based on melt products (MORBs and OIBs), W is currently considered highly incompatible and lithophile similar to Ba, Ta, Th, and U during mantle melting, and the W abundances of the mantle reservoirs have been constrained accordingly. Here, we present trace element and isotope-dilution W concentration data for 42 well-characterized mantle peridotites from Tariat (Mongolia), Yangyuan (China), Hannuoba (China), and Letlhakane (Botswana). Their W concentrations are higher than 12 ng g-1 (i.e, current W abundance estimate of the primitive mantle reservoir) even in the most Ba-Ta-Th-U depleted peridotites, and fractionation between W and these highly incompatible elements increases with degree of depletion. Incompatible element-enriched peridotites show no such fractionation. These geochemical patterns suggest that W is more compatible than Ba, Ta, Th and U in peridotite residues, and that current W abundances in the upper mantle reservoirs might be underestimated. The 182W/184W ratio of the upper mantle is also poorly constrained due to limited data. We analyzed MORBs from the Pacific-Antarctic-Ridge, a ridge devoid of any plume influence or recycled component. Tungsten concentrations in the MORBs are well-correlated with Ba, Ta, Th, and U, indicating a mantle origin without secondary W input. High-precision W isotope ratio measurements are currently underway. Preliminary results reveal that MORBs have small positive 182W anomalies relative to the Alfa Aesar W laboratory reference material. Based on our results, revised W concentration estimates and 182W/184W ratios for the depleted and primitive mantle reservoirs will be suggested.