Pd-Ag chronometry of iron meteorites: Correction of neutron capture-effects and application to the cooling history of differentiated protoplanets

Matthes M., Fischer-Gödde M., Kruijer T., Leya I., Kleine T.

Zusammenfassung

The short-lived 107Pd-107Ag system is a versatile tool for dating iron meteorites, but neutron capture reactions during cosmic ray-exposure might have modified Ag isotope compositions. These cosmic ray-induced effects would vary depending on the exposure time of a sample and its location within the parent meteoroid and, therefore, could bias the age information inferred from Pd-Ag isotope systematics. Our new combined Pd-Ag and Pt isotope data for iron meteorites in conjunction with model calculations reveal large cosmic ray-induced downward shifts of 107Ag/109Ag, which preclude the determination of Pd-Ag isochrons based on measured Ag isotope compositions. For the strongly irradiated iron meteorites Ainsworth (IIAB) and Carbo (IID) these shifts are similar to or even larger than the effects from radiogenic ingrowth resulting from 107Pd-decay. For the less strongly irradiated IIIAB iron meteorites Boxhole, Grant and Henbury, the cosmic ray-induced shifts are smaller than the radiogenic 107Ag excesses, but are nevertheless significant. We have developed a method to quantify the cosmic ray-induced Ag isotope shifts using a neutron capture model and Pt isotope compositions as the neutron dose monitor. After correction, Pd-Ag isochrons are obtained for all investigated iron meteorites, even for the most strongly irradiated samples. The Pd-Ag ages inferred from the isochrons are in good agreement with other chronological data for iron meteorites, indicating that our neutron capture model provides a reliable correction method for quantifying cosmic ray-induced shifts on measured Ag isotope compositions. The Pd-Ag ages for iron meteorites obtained in this and previous studies indicate rapid crystallization and cooling of the parental metal cores within a few Ma after core formation and solar system formation. Such rapid cooling can be attributed to either small parent body sizes or collisional erosion of the insulating silicate mantle from larger bodies. The collisions would have facilitated rapid cooling below Pd-Ag isotopic closure and so in this case the Pd-Ag ages would effectively date the time of the collisions.