EXTENDING THE 107PD-107AG CHRONOMETER TO LOW PD/AG METEORITES WITH MULTICOLLECTOR PLASMA-IONIZATION MASS SPECTROMETRY

Abstract

A new technique that uses multicollector plasma-ionization mass spectrometry (MC-ICP-MS) for the determination of Ag isotopic composition is reported. By correcting for instrument-induced mass fractionation of Ag by comparison with an added ''spike'' of normal Pd, 107Ag/109Ag can be determined to an external precision of approximately +/-1.3#. This precision allows resolution of variations in the Ag isotopic composition of low Pd/Ag objects such as pallasites, group IA irons, chondrites, and the Earth. For subsamples 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 107Pd. Canyon Diablo sulfide and metal provide a correlation corresponding to an initial 107Pd/108Pd of (2.39 +/- 0.26) x 10-5, essentially identical to the highest value previously seen for any meteorite. Data for the pallasite Brenham define a four-point correlation corresponding to 107Pd/108Pd 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 parental materials of Brenham must have had low Pd/Ag, within the chondritic range, during this 3.5-Myr interval. In contrast, a sulfide-metal tie line for the group IIIB iron, Grant, provides a similar 107Pd/108Pd ((1.61 +/- 0.70) x 10-5) to Brenham, but has radiogenic initial Ag (+36# compared with the initial Ag isotopic composition of Canyon Diablo), indicative of evolution in a parent body with high Pd/Ag, perhaps similar to the bulk composition of Grant. The Ag isotopic composition measured for Canyon Diablo sulfide is the lowest yet reported for any solar system material (11# 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 107Pd/108Pd of (39 +/- 19) x 10-5. Alternatively, the unexpectedly high 107Ag/109Ag for Allende could reflect either mass fractionation or a nuclear anomaly in the Ag contained in this primitive chondrite.