Crystal-chemical control of Na substitution into Martian apatite

Apatite is an important reservoir for volatile and rare earth elements in Martian meteorites and can be used to evaluate volatile cycling in the Martian lithosphere. Sodium is a minor element in apatite and is typically considered to be a charge-balancing cation in the coupled substitution of trivalent cations for Ca on the apatite M sites. Using a compilation of new and published Martian apatite compositions spanning a wide range of Cl/F ratios, we have identified a positive correlation between Cl and Na in Martian apatites. Most shergottite apatites cluster at an approximately constant Na/Cl ratio of 0.07, whereas apatites from nakhlites and Martian regolith breccias have lower Na at a given Cl content compared to shergottite apatite. Published low- and high-pressure experimental data of apatite crystallized from basaltic shergottite melts reproduce the Na/Cl ratios of the natural shergottite apatites. In these experiments, the absence of rare earth elements (e.g., Ce3+, La3+) and sulfur, and the low melt Fe3+ concentrations indicate that substitution of Na into Cl-rich apatite is not due to coupled substitutions with cations derived from the melt. Likewise, Na in Martian apatite is not correlated with any of these potential charge-balancing cations. We therefore propose that Na incorporation into Cl-rich Martian apatite is primarily controlled by the crystal chemistry of the apatite: Cl-induced distortion of the anion column facilitates Na substitution for Ca on the M sites, which is charge-balanced by defects on the halogen X-site, including vacancies or O2– occupancy. We propose that Na incorporation occurs via two possible coupled substitutions: 2Na+ + Vacancy = 2Ca2+ + O2- and Na+ + Vacancy = Ca2+ + (Cl-, F-, OH-). Crystal-chemical control on Na substitution into Cl-rich Martian apatite is supported by a strong correlation between the apatite Cl content and the measured Na partition coefficient between apatite and melt reported in published experiments. Applying these experimentally-determined Na partition coefficients, apatite-saturated Martian shergottite melts are calculated to contain on average ∼2.1 wt% Na2O, overlapping with measurements of quenched glass inclusions in shergottite apatites. Lower apatite Na/Cl ratios in polymict breccias and in nakhlites likely reflect different parageneses, e.g., interaction between Na-poor and Cl-rich fluids and evolved melts, reaction of fluids with merrillite, or early apatite saturation in volatile-rich magmas. The inferred increase in anion-column defect concentrations in Na- and Cl-rich apatite might bias calculations of OH by difference using microprobe measurements of Cl-rich apatite and subsequent reconstruction of melt volatile contents (H2O, F, Cl) in Martian magmas and their mantle sources.