Silver isotope variations in chondrites: Volatile depletion and the initial 107Pd abundance of the solar system

The extinct radionuclide Pd-107 decays to Ag-107 (half-life of 6.5 Ma) and is an early solar system chronometer with outstanding potential to study volatile depletion in the early solar system. Here, a comprehensive Ag isotope study of carbonaceous and ordinary chondrites is presented. Carbonaceous chondrites show limited variations (epsilon(107) Ag = -2.1 to +0.8) in Ag isotopic composition that correlate with the Pd/Ag ratios. Assuming a strictly radiogenic origin of these variations, a new initial Pd-107/Pd-108 of 5.9 (+/- 2.2) x 10(-5) for the solar system can be deduced. Comparing the Pd-Ag and Mn-Cr data for carbonaceous chondrites suggests that Mn-Cr and Pd-Ag fractionation took place close to the time of calcium-aluminium-rich inclusion (CAI) and chondrule formation similar to 4568 Ma ago. Using the new value for the initial Pd-107 abundance, the revised ages for the iron-rich meteorites Gibeon (IVA, 8.5 +3.2/-4.6 Mal, Grant (IIIAB, 13.0 +3.5/-4.9 Ma) and Canyon Diablo (IA, 19.5 +24.1/-10.4 Mal are consistent with cooling rates and the closure temperature of the Pd-Ag system. In contrast to carbonaceous chondrites, ordinary chondrites show large stable isotope fractionation of order of 1 permil for Ag-107/Ag-109. This indicates that different mechanisms of volatile depletion were active in carbonaceous and ordinary chondrites. Nebular processes and accretion, as experienced by carbonaceous chondrites, did not led to significant Ag isotope fractionation, while the significant Ag isotope variations in ordinary chondrites are most likely inflicted by open system parent body metamorphism. (C) 2008 Elsevier Ltd. All rights reserved.