Any Old Iron

Abstract

EARTH SCIENCE The Hf-W system has been used as the basis for determining when iron cores formed in Earth and Mars and when the Moon may have formed. The idea is that 182Hf decays to 182W with a half-life of about 9 million years. Thus, essentially no 182Hf should persist 40 to 60 million years after the formation of the solar system. Chemically, tungsten but not hafnium tends to be incorporated into an iron core, so W crustal abundances are low. Thus, if iron cores formed before the time that all of the 182Hf became extinct—or similarly if the Moon formed because of a giant impact with Earth—an isotopic signal should be present in W isotope data from crustal rocks. An early analysis suggested that there was no difference between the isotopic values of W for Earth and chondritic meteorites, which provide a reference value, implying that Earth's core formed late. Three new and independent analyses by Schoenberg et al ., Yin et al ., and Kleine et al . instead indicate that the value for chondrites needs revision. A difference was observed between crustal values and those for carbonaceous chondrites, and crust samples from Earth and the Moon do contain evidence of the former presence of live 182Hf. Together the data imply that the planets of the solar system accreted rapidly, within a few tens of millions of years, and formed their cores rapidly by internal segregation of iron. This notion is consistent with recent numerical models and other chemical and isotopic data. — BH Geochim. Cosmochim. Acta 66 , 17 (2002); Nature 418 , 949; 952 (2002).