High Precision Ag Isotopic Measurements of Low Pd/Ag Meteorites

Introduction: The now extinct radionuclide Pd decays to Ag with a half life of 6.5Myr. Highly volatile element depleted (i.e. high Pd/Ag) Group IVA iron meteorites show good correlations between excess Ag and Pd/Ag. This strongly indicates the presence of live Pd at the start of the solar system [1]. Pd is both more refractory and more siderophile than Ag. Hence condensation/evaporation processes in the nebula, as well as planetary core formation, may have caused the requisite early Pd/Ag fractionation necessary for the system to be useful as a chronometer. Isochrons determined for iron meteorites generally yield initial Pd/Pd in the range 1.5 2.5 x 10 [1], and this was confirmed recently by Carlson and Hauri [2] who measured an initial Pd/Pd of (2.39 ± 0.26) x10 for the Group IA iron meteorite Canyon Diablo. Primitive chondritic meteorites have received only limited study with regard to Pd-Ag systematics. Early investigations using TIMS were hampered by lack of precision (typically 10-20ε, where ε = [(sample/ standard)-1]*10000, and showed that, within error, the ordinary chondrites Rose City (H5) and Floyd (L4) were identical in Ag isotopic composition to the Ag isotopic standard NIST SRM 978a [1]. Improved analytical precision (±1.3ε, 2σ) obtained through the use of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) [2] permitted analysis of the carbonaceous chondrite Allende (CV3). The results of their study [2] indicated that Allende is enriched in Ag by between +4 and +6.5ε relative to the NIST Ag standard.