Behavior of tungsten and hafnium in silicates: A crystal chemical basis for understanding the early evolution of the terrestrial planets

Abstract

The short‐lived isotopic system involving the decay of 182Hf to 182W (half life of 9 Ma) has been used to establish a time‐scale for planetary assembly and core formation. The W isotope signature of lunar and martian basalts appears to be correlated to characteristics of their source. On the Moon, the small amount of excess 182W is associated with basalts that may be derived from an ilmenite‐ and clinopyroxene‐bearing mantle that was produced during late‐stages of lunar magma ocean crystallization. On Mars, excess 182W is associated with a depleted mantle source. Our experimental measurements for the partitioning behavior of Hf and W between silicates and melt suggest that the early fractionation of Hf from W by phases such as ilmenite, high‐Ca clinopyroxene, and garnet may be significant enough to account for the generation of all or part of the radiogenic W signatures in the mantle sources for these basalts. The duration and nature of early mantle differentiation events on the Moon and Mars may be calculated by combining Hf‐W fractionation with the extent of radiogenic W enrichment.