THE DISTRIBUTION OF FLUORINE, CHLORINE, AND HYDROGEN DURING PLANETARY DIFFERENTIATION: IMPLICATIONS FOR APATITE COMPOSITIONS IN PLANETARY MATERIALS (Invited Presentation)

Magmatic volatiles affect a wide range of physicochemical properties of geological materials, and their abundance and distribution within a parent body affects to what extent volatiles can influence geological processes. Of particular importance for the distribution of magmatic volatiles is the thermochemical evolution of a parent body, particularly whether it has undergone differentiation via magma ocean crystallization. The large-scale homogenization that occurs with planet-wide melting has important implications for the composition of geochemical reservoirs that form in planetary crusts and mantles, including those of volatiles such as H, F, and Cl. In particular, differences in geochemical compatibility of H, F, and Cl in minerals, fluids, and silicate melts will influence the ratios of F, Cl, and H within geochemical reservoirs. In the present study, I will consider (1) compositions of apatite in planetary materials, (2) relative geochemical compatibilities and partitioning behavior of F, Cl, and H in minerals, silicate melts, and fluids, and (3) the abundances of F, Cl, and H in chondrites to determine whether apatite can provide information about differentiation processes on their respective parent bodies.

The partitioning behavior of F, Cl, and H between nominally anhydrous magma ocean minerals and melt indicate that F and H are preferred in pyroxene more than an order of magnitude more than Cl. Consequently, magma ocean crystallization should lead to a mantle residue that is enriched in F and H and depleted in Cl relative to the bulk silicate values, provided the amount of trapped residual melt within the cumulates does not dominate the volatile signature. In contrast, partial melting of a mantle that did not form through a prior melting event would either retain its initial bulk silicate abundances of F, Cl, and H, or Cl and H would be decoupled from F where metasomatism occurred given the higher affinity for Cl and H in fluids compared to F. Apatite from unaltered mare basalts, eucrites, and terrestrial basalts all have F-OH rich compositions with limited Cl abundances. In contrast, ordinary chondrites are consistently Cl-rich. These results suggest that global datasets of apatite X-site chemistry may provide insights into differentiation processes, but a more comprehensive analysis is required.