The cosmic molybdenum-ruthenium isotope correlation

Dauphas et al. [Astrophys. J. 565 (2002) 640-644] reported molybdenum isotopic anomalies in differentiated and bulk primitive meteorites. It is shown here that these isotopic anomalies correlate with one another in exactly the way expected from nucleosynthesis theory if different regions of the nebula received different contributions of s-process matter synthesized in low-mass AGB stars. Furthermore, when bulk measurements are grouped by meteorite class, the molybdenum isotopic anomalies correlate with the ruthenium isotopic anomalies recently reported by Chen et al. [Lunar Planet. Sci. XXXIV (2003) #1789], again indicating a heterogeneous distribution of s-process matter. As molybdenum is only moderately siderophile, most of the molybdenum presently in the mantle was delivered before the completion of core formation. In contrast, because ruthenium is highly siderophile, nearly all of the mantle ruthenium was delivered by a late veneer, after the end of core formation. Thus, the fact the silicate Earth lies on the Mo-Ru cosmic correlation supports the idea that the Earth accreted homogeneously. Stated otherwise, the feeding zone of the Earth did not change drastically with time, as both the bulk of the Earth and the late veneer accreted from material from the same Mo-Ru isotopic reservoir. (C) 2004 Elsevier B.V. All rights reserved.