Closure temperature of the Pd-Ag system and the crystallization and cooling history of IIIAB iron meteorites

Abstract

To better constrain the crystallization and cooling history of the IIIAB iron meteorite parent body, we report new 107Pd-107Ag data for metal and troilite samples from the IIIAB iron Cape York, and combine these data with a numerical model for the diffusive exchange of 107Ag between metal and troilite. We find that the Pd-Ag closure temperature for iron meteorites varies between 500 and 700 °C, and for most irons typically is between 550 and 650 °C. The closure temperature not only depends on cooling rate, grain size, and bulk Ni content, but also on the abundance and distribution of troilite nodules. Specifically, metal in direct contact to troilite has a lower closure temperature than more distant metal. Consistent with this, our new Pd-Ag data show that metals adjacent to troilites have lower Ag contents and plot on shallower Pd-Ag isochrons than more distant metals. These disparate Pd-Ag systematics in metal as a function of distance to troilite provide a new means to determine cooling rates for iron meteorites. Using this approach, we obtained a cooling rate of 67–202 °C/Ma for Cape York, which is in good agreement with metallographic cooling rates for IIIAB irons. This cooling rate combined with the precise Pd-Ag age of Cape York of 5.0 ± 0.4 Ma after solar system formation reveals that the IIIAB core completely solidified at 2.6 ± 1.3 Ma after solar system formation. This rapid crystallization was most likely facilitated by collisional disruption of the IIIAB parent body, which removed most of the insulating mantle and exposed its core.