APATITE IN ASTROMATERIALS: A WINDOW INTO THE DISTRIBUTION AND BEHAVIOR OF VOLATILES WITHIN THE SOLAR SYSTEM (Invited Presentation)

The mineral apatite occurs in a wide array of planetary materials, and it is often the only that has F, Cl, and OH as essential structural constituents. Consequently, apatite could hold important clues as to the distribution and behavior of these volatile components in extraterrestrial systems. Numerous experimental studies of F, Cl, and OH partitioning between apatite and melt have indicated progress in calibrating apatite as a magmatic hygrometer. In the present study, we use apatite compositions from a broad array of planetary materials and mineral-melt partitioning data for F, Cl, and OH to glean information about the distribution of F, Cl, and OH resulting from magma ocean crystallization.

Our analysis indicates that magma ocean crystallization leads to mantle residues that are enriched in F and OH relative to Cl if trapped melt abundances are less than 1–2% because F and OH are preferred in pyroxene by more than an order of magnitude over Cl. In contrast, a body that did not experience melting would either retain a chondritic F:Cl:OH ratio, or Cl and OH would be decoupled from F through fluid processes given the hydrophilic nature of Cl and OH relative to F.

Apatite in ordinary chondrites have Cl-rich compositions that are linked to Cl-rich fluids on their respective parent bodies. In comparison, unaltered basaltic rocks from Earth, Moon, and 4 Vesta have Cl-depleted apatite that exhibit substantial F-OH variation, consistent with crystallization from melts that were derived from sources that underwent magma ocean crystallization. Basalts from Mars exhibit apatite compositions that are intermediate between ordinary chondrites and unaltered basalts from other bodies. Numerous models can explain differences between Mars and other differentiated bodies, and we continue to evaluate the efficacy of each model. Apatite from planetary materials exhibit systematic variations in F, Cl, and OH that could elucidate the distribution of volatiles during differentiation.