Palladium-silver chronology of IAB iron meteorites

The extinct Pd-107-Ag-107 decay system (half-life similar to 6.5 Ma) 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 epsilon Ag-107 variations between +0.1 and +15.8 with Pd-108/Ag-109 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 JAB 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 Pd-107 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 15 Ma of our solar system. (C) 2012 Elsevier B.V. All rights reserved.