Where on Earth is the enriched Hadean reservoir?

Abstract

We report the results of an exhaustive search conducted to find ¹⁴²Nd anomalies in the rocks associated with the Deccan and Iceland plumes. High-precision Nd isotopic measurements for the samples were inter-leaved with that for laboratory standards; the latter demonstrating an external reproducibility of 6 ppm (2σ). No measurable ¹⁴²Nd anomalies were found in Deccan picrites and basalts despite earlier indications to the contrary. Primitive Icelandic ankaramites and basalts with primordial He and Ne isotopic signatures, and ultrabasic Deccan rocks (carbonatite/nephelinite) also do not show any anomalies. It is likely that deep mantle plume-derived lavas do not carry the negative ¹⁴²Nd anomalies that are expected for an enriched Hadean reservoir with subchondritic Sm/Nd. It is evident from meteorite data that an enriched Hadean reservoir has to exist, unless the Bulk Earth accreted with a Sm/Nd ratio that was at least 3.5% higher than the chondrite average. The enriched Hadean reservoir may be (i) subducted protocrust in the D" layer, in which case negative ¹⁴²Nd anomalies and primordial noble gases would be associated with each other, (ii) the first cumulates of a magma ocean in the lower mantle, in which case negative ¹⁴²Nd and primordial noble gases may be decoupled, or (iii) the core, in which case searches for negative ¹⁴²Nd anomalies in mantle derived rocks are futile. Experiments at core-forming temperatures, pressures, composition, and oxidation states are needed to constrain possible REE partitioning into the core.