Pebble accretion and siderophile element partitioning between Earth's mantle and core

Abstract

Pebble accretion is an efficient mechanism for early terrestrial protoplanet growth and differentiation. Metal-silicate partitioning of moderately siderophile elements offers constraints on the role of pebble accretion in Earth's formation and the segregation of its core. Here, we determine pebble accretion properties of the proto-Earth that are consistent with metal-silicate partitioning measurements and siderophile abundances in the mantle and core. We combine a pebble accretion model that includes mass balances for siderophile abundances in the mantle and core of a growing terrestrial protoplanet with experimentally-determined partition functions for seven moderately siderophile elements: Ni, Co, V, Cr, Mo, Mn, and W. Mantle and core abundances of these elements during pebble accretion are calculated, as well as changes to their abundances following the addition of large and giant impactors built with pebbles. Model results are compared to the estimated abundances of these elements in Earth's primitive mantle and core. We find that metal-silicate partitioning of these elements is especially sensitive to the total mass of accreted pebbles. Best fits to primitive mantle and core siderophile abundances are found in cases where the proto-Earth accreted with pebbles to approximately 0.6 times its present mass under slightly reducing conditions, then added the remaining mass via one or more impactors with the same composition. We also find that pebbles consisting of chondritic components (chondrules, metal grains, AOAs, and CAIs) generally yield better partitioning results compared to pebbles made from chondrites.