[SIT26-P08] The tungsten isotopic compositions of kimberlites: constraints on material circulation in the deep Earth
Keywords:tungsten isotope, kimberlite
Mundl et al. (Mundl et al., 2017) reported negative μ182W (deviations in ppm from the 182W/184W isotopic composition of the terrestrial standard) values in oceanic island basalts (OIB) which negatively correlate with 3He/4He. This result suggests that there exist early-differentiated reservoirs in the present-day deep mantle, since OIB are considered to have their sources in the lower mantle. Several processes have been proposed to explain the deficits in 182W, for example, core-mantle interaction and the contribution from the EER. However, none of the hypothesis can satisfy geochemical constraints. This study provides new constraints on this issue by investigating plume-derived rocks, kimberlites.
In this study, we performed 182W isotope analysis of kimberlites from South Africa, China and Brazil with MC-ICP-MS (Thermo Fisher Scientific Neptune Plus). Kimberlites are ultrabasic rocks that are presumed to have their origin deep in the Earth like OIB. On the other hand, globally distributed kimberlites have relatively undifferentiated isotopic compositions close to those of bulk silicate Earth (BSE), whereas OIB have relatively differentiated values. From these results, kimberlites are considered to originate from an isotopically primordial reservoir like the Earth’s primitive mantle (Woodhead et al., 2019).
All measured 182W/184W values of kimberlites were within analytical error of a terrestrial standard, which is considered to have the value of the modern accessible mantle. The present result indicates that the depletion of the present-day upper mantle in incompatible elements has occurred after 182Hf was no longer extant, as there is no difference in 182W values between the primordial mantle and the already depleted mantle. In addition, given that OIB show μ182W negative anomalies (Mundl et al., 2017), whereas kimberlites didn't show any anomalies, the source mantle of OIB is considered to be more strongly affected by the core-mantle interaction or the EER than that of kimberlites.