Don R. Baker (Department of Earth and Planetary Sciences, McGill University)
Martian nakhlite MIL 03346: A tale of S and Cl in two crystals
The volatile concentrations of Martian magmas remains an important, outstanding question. The sulfur and chlorine concentrations, and their spatial distribution, in clinopyroxenes from the nakhlite MIL 03346 were measured to provide insight into the concentrations of these volatiles in the parental melts of nakhlites, as well as to constrain the volatile evolution of the nakhlite melts. Sulfur and chlorine in four clinopyroxene crystals from MIL 03346 were measured at crystal cores and rims by synchrotron X-ray fluorescence using beamline I18 at the Diamond Light Source. Portions of the two crystals from MIL 03346 were mapped for S and Cl, and a few reconnaissance analyses of Cl and F in MIL 03346 were made by ion microprobe. The clinopyroxene cores in MIL 03346 contain ~ 10 ppm S, ~ 5 ppm Cl and ~ 53 ppm F. Using these volatile concentrations combined with previously determined partition coefficients, the basaltic melt from which these clinopyroxenes crystallized contained 900 ± 400 ppm S, 500 ± 300 ppm Cl, and 160 to 360 ppm F. Mapping the spatial distribution of S and Cl in two clinopyroxene grains of MIL 03346 demonstrates that the concentrations of these volatiles in the evolving melts changed significantly from the core to the rim, as has been previously noted for major elements by other researchers. Increasing S and Cl concentrations and a changing Cl/S ratio in the region between the core and rim suggests incorporation of additional volatiles through assimilation. However, concentrations of Cl and S at the outer crystal rims dramatically decrease, most probably due to volatile degassing during the final stages of clinopyroxene growth. Thus, these two crystals provide a record of the igneous evolution of S and Cl in the nakhlite MIL 03346 from the crystallization of clinopyroxene cores at temperatures near 1200 °C to the quenching of the groundmass near 1000 °C.