Modeling closure of the Pd-Ag system in iron meteorites

Abstract:

J.A. Van Orman¹, M. Matthes², M. Fischer-Godde², M.J. Krawczynski³_, T. Kleine²

¹ Case Western Reserve University, Cleveland, OH 44106 (james.vanorman@case.edu)

² Westfalische Wilhelms-Universitat Muenster, 48149 Muenster, Germany

³ Washington University, St. Louis, MO 63130

The short-lived Pd-107/Ag-107 system can provide constraints on the timing of assembly and cooling of iron meteorite parent bodies, but to interpret the dates derived from this system it is necessary to understand the closure conditions. Palladium is strongly enriched in the metal phases, and sulphide (troilite) is the primary sink for radiogenic silver. Closure of the system hence depends primarily on the transfer of Ag-107 from metal to troilite. Because cation diffusion in troilite is extremely rapid, Ag-107 transfer is likely to be controlled by diffusion through the metal. Sugiura and Hoshino (2003) estimated a closure temperature of ~1100 K for the Pd/Ag system in iron meteorites under the assumption that the diffusion rate of Ag in the metal is similar to that of Ni diffusion in taenite. Here we consider the problem in more detail, utilizing constraints on Ag diffusion in taenite and kamacite from the metallurgical literature to numerically model diffusive exchange between metal and troilite with simultaneous radiogenic ingrowth. The process is complicated by exsolution of the metal into bcc kamacite and fcc taenite phases during cooling. We will discuss approaches to the treatment of this issue and their influence on the derived closure temperatures.

Sugiura N., Hoshino H. (2003) Meteorit. Planet. Sci. 38, 117-143.