Palladium–silver chronology of IAB iron meteorites

Abstract

The extinct 107Pd–107Ag decay system (half-life ∼6.5Ma) is a useful chronometer to constrain the thermal evolution of the IAB parent body. To this end, Pd/Ag concentrations and the Ag isotope compositions of metals separated from 6 different IAB iron meteorites were determined. The samples show ε107Ag variations between +0.1 and +15.8 with 108Pd/109Ag ratios between 38 and 200. The data can be divided into two groups based on their petrology, each defining an isochron: a graphite and troilite rich inclusion bearing group (A), with the IAB meteorites Toluca, Odessa and Canyon Diablo and a more silicate rich group (B), which includes Campo Del Cielo, Caddo County and Goose Lake. Using the initial abundance of 107Pd derived from carbonaceous chondrites, the corresponding age for the group (A) is 18.7 (+3.6/−5.0)Ma after the start of the solar system and 14.9 (+2.5/−4.9)Ma for the group (B). This suggests that the last thermal event to reach high enough temperatures to melt metal on the IAB parent body occurred within the first 15Ma of our solar system.