The isotopic composition and concentration of Ag in iron meteorites and the origin of exotic silver

Abstract

The isotopic composition of Ag and the concentration of Ag and Pd have been determined in Canyon Diablo (IA), Grant (IIIB), Hoba, Santa Clara, Tlacotepec and Warburton Range (IVB), Piñon and Deep Springs (anom.). Troilite from Grant and Santa Clara have also been analyzed. All of these meteorites, with the exception of Canyon Diablo, give 107Ag 109Ag in the metal phase that is greater than the terrestrial value with the enrichments of 107Ag ranging from ~2% to 212%. These data show that Ag of anomalous isotopic composition is common to all IVB and anomalous meteorites. The results on Grant suggest that the anomalies may be widespread including more common meteorite groups. There is a general correlation of 107Ag 109Ag with Pd Ag except for the data from FeS of Santa Clara. It is concluded that the excess 107Ag is the result of decay of 107Pd, a nuclide that is extinct at present with an abundance of 107Pd 108Pd of about 3 × 10 −5. The troilite in Grant exhibits normal 107Ag 109Ag to within errors, a high Ag concentration and a low ratio of 108Pd 109Ag ~0.17. Grant metal has 107Ag 109Ag that is ~2% greater than normal and a high ratio of 108Pd 109Ag ~ 10 3. The data from Grant appear to represent a 107Pd- 107Ag isochron and indicate that the cooling rate at elevated temperatures was sufficiently rapid to preserve substantial isotopic differences between metal and troilite. Troilite in Santa Clara was found to contain Ag with a very high 107Ag 109Ag ratio (108% above normal), an Ag concentration only a factor of three above the metal and a high value of 108Pd 109Ag ~1.3 × 10 4. The troilite has a higher 107Ag 109Ag than the metal. These data are not compatible with a simple model of in situ decay and subsequent local Ag redistribution between metal and troilite during cooling. These data suggest that Ag in Santa Clara and possibly other IVB meteorites is made up of almost pure 107Ag produced from 107Pd decay and 109Ag produced by nuclear reactions with only a small amount of “normal” Ag. This indicates an intense energetic particle bombardment history in the early solar system (~10 20 p/m 2) which occurred after the formation of small planetary bodies. We infer that a T-Tauri activity by the early sun contributed to some late stage “nucleosynthesis” and the heating of a dust cloud. In addition, implications on the early thermal evolution of iron meteorites are presented based on 107Pd decay and models of the cooling history.