Reconciling siderophile element data in the Earth and Moon, W isotopes and the upper lunar age limit in a simple model of homogeneous accretion

Abstract

A model of Earth accretion in which the composition of the accreting material does not change with time (broadly homogeneous accretion), and the core is formed through metal phase segregation in possibly relatively shallow-level (c. 60 kbar) fractionation zones in a mantle which may otherwise be largely solid, is investigated with regard to its consequences for siderophile element depletion and W isotope evolution. In a scenario following thermal models, core formation is triggered off as the Earth is c. 10% accreted, and its rate is subsequently limited by the accretion rate. The mantle is indirectly depleted in metallic phase content and siderophile elements as it is diluted by input and remixing of metal-free silicate material from the fractionating zones after completion of metal–silicate fractionation there, and removal of the metal fraction into the core. The metal fraction in the mantle decreases monotonously with time during accretion. After accretion has ceased, a residual metal content of <1% in the mantle enables core formation to continue. The behaviour of V, W, Mo, Co, Ni, Au, Re and Ir was modelled using partition coefficient data from the literature. As accretion and core formation proceed, the ever smaller metal phase fraction which segregates in the fractionation zones first fails to scavenge the weakly siderophile elements (V and W) efficiently, and later also leaves much of the medium siderophiles Co, Mo and Ni in the silicate fraction. The strongly siderophile elements Au, Re and Ir are still virtually quantitatively scavenged when the metal fraction is as low as 0.2%. The model reproduces the depletion data for the studied elements within a factor 2, and a close to chondritic abundance pattern results for Au, Re and Ir in spite of large differences in their partition coefficients. If whole-mantle melting occurred as a result of a giant impact, its effect on siderophile elements would be obliterated after a few Ma if the normal accretion-core formation process continued afterwards. Taking the Moon as a mantle sample at any time after the Earth is >70% accreted, and using its metal fraction to form a lunar core by equilibrium fractionation, produces a good fit to lunar siderophile element data, independent of the exact amount of metal in the mantle at the time of the Moon-forming event. W isotope constraints on the silicate Earth in relation to chondrites are satisfied in models where the last 20% of Earth accretion occurs between 60 and 100 Ma after its commencement, irrespective of the early history of the process. No oxidised late veneer needs to be invoked to explain siderophile element abundances in the Earth or the Moon, there is no conflict between the W isotope data and a minimum age of 4.49 Ma for the Moon, and no need for invoking a much delayed start to core formation in the Earth to explain W isotopes.