The isotopic composition of silver and lead in two iron meteorites: Cape York and Grant

Abstract

Silver in the metal phases of Cape York (IIIA) and Grant (IIIB) has been determined after an extensive surface cleaning process. The 107Ag 109Ag was found to be enriched over that found in terrestrial Ag by ~7%. to 19%., demonstrating the presence of excess 107Ag ( 107Ag ∗) in this class of meteorites. An effort was made to find schreibersite with a distinctive 108Pd/ 109Ag ratio in order to establish a three-point isochron, but the results are not markedly different from those obtained for the bulk metal. The Ag isotopic ratio of sulfides from the same meteorites were nearly normal in composition. These results demonstrate correlations of 107Ag 109Ag with 108Pd 109Ag between coexisting phases of two iron meteorites that are associated with planetary differentiation processes. The ratios 107Ag ∗ 108Pd were found to be 1.7 × 10 −5 and 1.2 × 10 −5 for Cape York and Grant, respectively. These observations are in support of the widespread presence of 107Pd in the early solar system. The difference in isotopic composition between metal and sulfide phases demonstrates that silver diffusion was small (over 6.5 × 10 6 y) indicating a cooling rate much greater than 150°C/my for meteorites which have been attributed to small planetary cores. Uranium determinations were carried out on the metal phases and concentrations of ~ 1 × 10 12 g U/g and 2 × 10 −10 g U/g were found for Cape York and Grant, respectively. The Pb in these meteorites was determined using the improved cleaning procedures and chemical separations with low blank levels. The results confirm the presence of variable proportions of radiogenic Pb in both the metal and sulfide phases of iron meteorites. No simple explanation for the presence of radiogenic lead is apparent; while terrestrial contamination may appear to be the obvious explanation, it is possible that this effect could result from relatively recent metamorphism in the meteorite parent body.