Isotopic evidence for pallasite formation by impact mixing of olivine and metal during the first 10 million years of the Solar System.

ABSTRACT

Pallasites are mixtures of core and mantle material that may have originated from the core-mantle boundary of a differentiated body. However, recent studies have introduced the possibility that they record an impact mix, in which case an isotopic difference between metal and silicates in pallasites may be expected. We report a statistically significant oxygen isotope disequilibrium between olivine and chromite in main group pallasites that implies the silicate and metal portions of these meteorites stem from distinct isotopic reservoirs. This indicates that these meteorites were formed by impact mixing, during which a planetary core was injected into the mantle of another body. The impactor likely differentiated within ∼1-2 Myr of the start of the Solar System based on Hf-W chronology of pallasite metal, and we infer the age of the impact based on Mn-Cr systematics and cooling rates at between ∼1.5 and 9.5 Myr after Ca-Al-rich inclusions (CAIs). When combined with published slow subsolidus cooling rates for these meteorites and considering that several pallasite groups exist, our results indicate that such impacts may be an important stage in the evolution of planetary bodies.