PII S0016-7037(99)00315-4 Tungsten isotopes and the early development of the Earth and Moon

Abstract—The rates of accretion and core formation of the Earth and other inner solar system objects can be effectively studied using the newly developed 182Hf-182W chronometer. This approach offers far more leverage than any other short-lived nuclide system because: 1. the half-life of 9 Myrs is ideal for accretionary timescales; 2. both parent and daughter elements are highly refractory and in known, chondritic proportions in the Earth; 3. the initial 182Hf atomic abundance at the start of the solar system was relatively high; and 4. Hf and W are strongly fractionated by the early processes of melting and core formation. In this paper we review in detail the basic systematics of W isotope geochemistry and then discuss the presently available data for more than 80 meteorites and lunar samples in the context of various kinds of accretionary and core formation models. Tungsten isotope data are now available for metals from 30 iron meteorites and 8 ordinary chondrites, as measured by MC-ICPMS and N-TIMS and demonstrate a ubiquitous and well defined deficiency in the range 200 to 500 ppm in the 182W/184W of early segregated metals relative to that found in bulk carbonaceous chondrites. This deficiency, together with Hf-W internal isochrons for ordinary chondrites and theoretical predictions from modeling 182Hf as a product of the same processes that produce the actinide elements, all yield the same result, that the 182Hf/180Hf at the start of the solar syste