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 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.