Seminar: Megan Newcombe

Speaker: MEGAN NEWCOMBE, UNIVERSITY OF MARYLAND

Title: Efficient Degassing of Planetesimals During Melting and Differentiation: Water Delivery to Earth via Unmelted Material

Abstract:The origin of water in the terrestrial planets is a topic of debate. Inner solar system bodies (e.g., Angrites, 4-Vesta, and the Moon) have D/H and volatile element ratios consistent with delivery of water by chondritic material, but uncertainties persist regarding the timing of that delivery, the ability of planetesimals to retain volatiles during accretion and differentiation, and the role of alternative volatile sources such as nebular ingassing and comets. We have measured water in nominally anhydrous minerals in ungrouped achondrite and primitive achondrite meteorites with both carbonaceous (CC) and non-carbonaceous (NC) affinities. These analyses allow us to constrain the behavior of volatiles during the earliest stages of planetary accretion and melting. We expected that CC achondrite parent bodies would be more water-rich than their NC counterparts, due to the fact that the CC bodies likely accreted in the ice-rich outer solar system. However, our results show that nominally anhydrous minerals (olivine, orthopyroxene, clinopyroxene and plagioclase) in both CC and NC ungrouped achondrites contain ≤2 ppm H2O. Several processes could account for the dry nature of the CC and NC achondrites studied thus far, including: (1) metamorphism of shallow unmelted chondritic material during internal heating, driven by decay of short-lived radionuclides; (2) magma ocean degassing; and (3) melting and degassing of near-surface material during impact events. Since the undifferentiated meteorites among our samples are as dry as the samples that have undergone differentiation, our results suggest that water is efficiently lost prior to differentiation, during the early stages of melting and metamorphism, such that water delivery to Earth likely occurred via unmelted material.