The Pre‐disruption Hydrogen Budgets of the Tafassite and Brachinite Parent Bodies

Abstract Recent evidence from primitive achondrites and achondrites suggests that early‐formed melted planetesimals are depleted in hydrogen relative to the bulk silicate Earth. Nevertheless, evidence from angrites, the lone oxidized group of achondrites investigated to date, suggests that oxidized planetesimals may be H‐rich relative to their reduced counterparts. Additionally, we have limited constraints, derived from a few ungrouped samples, on the H budgets of outer solar system planetesimals that underwent melting. Therefore, in order to provide additional constraints on the H budgets of oxidized and outer solar system planetesimals that experienced melting, we measured the H contents of silicate minerals in brachinite and tafassite group meteorites, respectively. We found that olivine, pyroxene, and plagioclase across both groups are essentially devoid of H (<~2.6 μg/g H 2 O T ; total H as H 2 O equivalents). Based upon the thermal histories of the tafassite and brachinite parent bodies as well as their petrology and mineralogy, we argue that both bodies were essentially anhydrous prior to their disruption. This result is consistent with prior work on primitive achondrites and achondrites and requires that Earth's H budget be accounted for by accretion of thermally primitive materials, such as chondrites, comets, and ices or capture of nebular gas.