The isotopic composition of silver in iron meteorites

Abstract

Microgram quantities of silver were extracted from the troilite of Toluca iron meteorite and the metal phases of Sikhote Alin and Canyon Diablo meteorites using anion-exchange techniques. Isotopic analysis of silver in iron meteorites indicates a 2–3 per cent higher Ag 107 Ag 109 ratio relative to the terrestrial silver. If this excess Ag 107 is attributed to the decay of the extinct radioactive nuclide Pd 107, an unreasonably short interval of time between the end of nucleosynthesis and the formation of iron meteorites is indicated. The silver isotopic anomaly in iron meteorites appears too large to be attributed entirely to Pd 107 produced in the galactic synthesis of elements, and may have to be interpreted in terms of a last-minute contribution of some amounts of short-lived radioactivities to the solar nebula, or by postulating that some Pd 107 was produced in the solar system in its early stages, after its separation from the galactic nucleosynthesis. In either case, other short-lived radioactive nuclides would be extant in the early history of the solar nebula, and extinct radioactivity as the heat source in the early history of meteorite parent bodies becomes a definite possibility.