The spatio-temporal evolution of 182W and 142Nd in the Deccan-La Réunion plume

Abstract

Variations of the short-lived decay products ¹⁸²W and ¹⁴²Nd that formed approximately during the first 60 and 500 million years after solar system formation are pivotal in our understanding of Hadean processes and homogenization of Earth's mantle. For example, a coupling of ¹⁴²Nd and ¹⁸²W anomalies for the Deccan-La Réunion plume has been previously suggested to mirror a combined involvement of core-mantle interaction and Hadean silicate material, possibly stored in Large Low Shear Wave Velocity Provinces (LLSVPs). However, the limited availability of such short-lived isotope data for basalts from the Deccan Volcanic Province (DVP) made it difficult to assess this model closely. In this study, we provide new combined µ¹⁴²Nd and µ¹⁸²W data for ten selected samples of the DVP and one TTG from the Dharwar Craton basement, now covering different mantle endmembers of the DVP. Additionally, we provide new µ¹⁴²Nd data for six volcanic rocks from La Réunion that were previously analyzed for ¹⁸²W. We do not find evidence for a correlation between µ¹⁴²Nd and µ¹⁸²W in contrast to previous suggestions. Our data show that the involvement of mantle lithosphere and crustal components influences the µ¹⁸²W compositions of the DVP samples. Values of µ¹⁸²W (-4.2 ± 3.0) of such contaminated DVP samples overlap with the compositional field of volcanic rocks from La Réunion (-4.9 ± 1.5). The asthenospheric endmember of the DVP displays resolvable larger µ¹⁸²W deficits as low as -12.0 ± 2.3. Additionally, the asthenospheric DVP endmember also displays more unradiogenic ²⁰⁶Pb/²⁰⁴Pb ratios and low Δ²⁰⁷Pb/²⁰⁶Pb compared to La Réunion lavas and DVP lavas that assimilated lithospheric material. With regards to the two endmember models previously proposed for W isotope anomalies in mantle plumes, neither core-derived W nor an ancient silicate component with anomalous ¹⁸²W can be completely ruled out at this stage. However, due to the covariation of Pb and W isotopes and lower W/Th ratios in asthenosphere-derived DVP lavas, we currently favour a recycled mafic restite component of Hadean age as a source for the ¹⁸²W deficits in the Deccan-La Réunion plume.