Hydrogen in Eucrite, Angrite, and Ureilite Pyroxene: Implications for Water During the Planetesimal-Embryo Stage of Earth’s Formation
<p>In consideration of new evidence that many of Earth’s precursors were highly processed materials, and that Earth’s volatiles were acquired during accretion, it’s plausible that differentiated materials played a large role in the evolution of Earth’s water. Achondrite meteorites are samples from differentiated planetesimals and can allude to the amount of water retained by the most processed materials during terrestrial planet formation. We used SIMS to measure 1H in pyroxene in 5 achondrites: eucrites Juvinas and Pasamonte, volcanic angrite D’Orbigny, plutonic angrite Northwest Africa 4590 (NWA 4590), and ureilite Pecora Escarpment 82506 (PCA 82506). We compare whole-chip mounting of Juvinas (3±3ppm wt H<strong><sub>2</sub></strong>O), D’Orbigny (10±2 ppm wt H<strong><sub>2</sub></strong>O), NWA 4590 (4±3ppm wt H<strong><sub>2</sub></strong>O) and PCA 82506 (12±2 ppm wt H<strong><sub>2</sub></strong>O) to grain mounting of Pasamonte (2±3 ppm wt H<strong><sub>2</sub></strong>O) and conclude that grain mounting is most appropriate for measuring hydrogen in nominally anhydrous minerals (NAMs) using SIMS. Our results confirm previous studies of eucrite and angrite H<strong><sub>2</sub></strong>O and include the first non-bulk determination of H<strong><sub>2</sub></strong>O in a ureilite. In addition, we note that plutonic angrite NWA 4590 has less H<strong><sub>2</sub></strong>O than volcanic angrite D’Orbigny, a likely result of the slower cooling rates of plutonic samples on the angrite parent body (APB). Using clinopyroxene-melt partition coefficients, batch melting equations, and literature models of meteorite genesis, we predict 0 to 90 ppm wt H<strong><sub>2</sub></strong>O in Vesta, 50 to 230 ppm wt H<strong><sub>2</sub></strong>O in the APB, and 200 to 500 ppm wt H<strong><sub>2</sub></strong>O in the ureilite parent body (UPB). We observe that each of these estimates overlap the H<strong><sub>2</sub></strong>O concentration of Earth’s upper mantle, and that differentiated materials retained Earth-like abundances of water post-processing. We propose that a mix of differentiated and undifferentiated materials shaped Earth’s current volatile budget during the planetesimal-embryo stage of Earth’s formation, and present new implications for an in-situ delivery and late-delivery of water.</p>