Extending the Pd-107-Ag-107 chronometer to low Pd/Ag meteorites with multicollector plasma-ionization mass spectrometry

A new technique that uses multicollector plasma-ionization mass spectrometr y (MC-ICP-MS) for the determination of Ag isotopic composition is reported. By correcting for instrument-induced mass Fractionation of Ag by compariso n with an added "spike" of normal Pd, Ag-107/Ag-109 can be determined to an external precision of approximately +/-1.3 epsilon. This precision allows resolution of variations in the Ag isotopic composition of low Pd/Ag object s such as pallasites, group IA irons. chondrites, and the Earth. For subsam ples of the group IA iron Canyon Diablo, the group IIIB iron Grant, and the pallasite Brenham. variation in Ag isotopic composition correlates with Pd /Ag, suggesting contribution from the decay of now-extinct Pd-107. Canyon D iablo sulfide and metal provide a correlation corresponding to an initial P d-107/Pd-108 of (2.39 +/- 0.26) X 10(-5), essentially identical to the high est value previously seen for any meteorite. Data for the pallasite Brenham define a four-point correlation corresponding to Pd-107/Pd-108 of (1.65 +/ - 0.05) X 10(-5), indicating that the Pd-Ag system in Brenham closed 3.5 +/ - 0.3 Myr after that in Canyon Diablo. The initial Ag isotopic composition of Brenham is indistinguishable from that in Canyon Diablo. Thus. the paren tal materials of Brenham must have had low Pd/Ag, within the chondritic ran ge, during this 3.5-Myr interval. In contrast, a sulfide-metal tie line for the group IIIB iron, Grant, provides a similar Pd-107/Pd-108 ((1.61 +/- 0. 70) X 10(-5)) to Brenham, but has radiogenic initial Ap (+36 epsilon compar ed with the initial AE isotopic composition of Canyon Diablo), indicative o f evolution in a parent body with high Pd/Ag, perhaps similar to the bulk c omposition of Grant. The Ag isotopic composition measured for Canyon Diablo sulfide is the lowest yet reported for any solar system material (11 epsil on below the terrestrial standard). When this value is used to approximate the initial Ag isotopic composition of the solar nebula, Pd-Ag data for the CV chondrite Allende suggest a solar system initial Pd-107/Pd-108 of (39 /- 19) x 10(-5). Alternatively. the unexpectedly high Ag-107/Ag-109 for All ende could reflect either mass fractionation or a nuclear anomaly in the Ag contained in this primitive chondrite. Copyright (C) 2001 Elsevier Science Ltd.