FIGURE 6 Pd-Ag isochrons from various classes of meteorites. For extinct radioisotope systems, isochrons give not the age of the sample, but the abundance ratio of the extinct isotope at the time the isochron relationship was established (e.g. the ¹⁰⁷Pd/¹⁰⁸Pd ratio). The higher this ratio is, the older the sample, with the ratio decreasing by a factor of 2 every 6.5 My, the half-life of ¹⁰⁷Pd. The top panel shows the results for the volatile-depleted Group IVA iron meteorite Gibeon (Chen and Wasserburg 1990). Here the very high Pd/Ag ratios (x-axis) have created a wide range in ¹⁰⁷Ag/¹⁰⁹Ag ratios between metal (squares) and sulfides (diamonds). At much lower Pd/Ag ratios, and consequently much smaller variation in Ag isotope composition (now expressed as ¹⁰⁷Ag, i.e. the parts in 10,000 difference between¹⁰⁷Ag/¹⁰⁹Ag in the sample compared to that in a terrestrial Ag standard), the middle panel shows that the Group IAB iron meteorites Toluca and Canyon Diablo have correlated Pd/Ag ratios and Ag isotope compositions in metal (squares), but not in sulfides (diamonds). The middle panel also shows that MC-ICP-MS analyses are much more precise than the one thermal ionization analysis shown (Chen and Wasserburg 1990), whose error bars extend off the diagram. The lower panel shows that 6 out of 8 carbonaceous chondrites (also shown in the middle panel) lie on a Pd/Ag versus Ag isotope regression line with a slope of 5.9 x 10^-5, the highest value yet seen for any solar system material. Data sources include Chen and Wasserburg (1990) and Schönbächler et al. (2008) and references therein.