Deep-rooted plumes sample Hadean refractory domains and the core

by

Reidar G. Trønnes

Centre for Earth Evolution and Dynamics

Petrological evidence indicates that the solidification of the magma ocean produced voluminous domains of early bridgmanite-dominated cumulates, as well as residues from remelting above the basal magma ocean (BMO). Planetary cooling with complementary core-BMO chemical exchange of SiO2 (from core to BMO) and FeO (from BMO to core) would have enhanced the production of refractory bridgmanitic material. Such viscous matter, neutrally buoyant in the mid-lower mantle, would tend to be convectively aggregated into bridgmanite-enriched ancient mantle structures (BEAMS), located in the convectively calm zones outside the margins of the rising mantle flow above the two antipodal large low S-wave velocity provinces. In the hot Hadean Earth, rapidly diffusing He and Ne would have recharged such U- and Th-depleted material, mostly before its aggregation into BEAMS. The Mm-sized BEAMS would preserve primordial-like He and Ne isotopic compositions during subsequent Earth evolution and cooling. Deep-rooted plumes seem to entrain such bridgmanitic material, which becomes pyroxenitic and slightly less refractory in the upper mantle melting zone. Oceanic island basalts with PREMA- or FOZO-like isotopic signatures (PREvalent Mantle or mantle FOcus ZOne) are derived from source materials with elevated 3He/4He, 143Nd/ 144Nd, and 142Nd/ 144Nd ratios. The associated low 182W/ 144W ratios, correlated with the high 3He/4He ratios in most OIBs, and with elevated 129Xe/136Xe ratios in some OIBs, are probably caused by minor entrainment of metallic alloy from the core via the ultra-low velocity zones (ULVZs), which thicken in the root-zones of the major plumes. The OIB geochemical data and high-pressure phase relations, therefore, indicate the ULVZs are partly open "windows" between the core and the mantle.

Published Mar. 29, 2021 11:36 AM - Last modified Apr. 7, 2021 9:14 AM