Effect of Cl on Near-Liquidus Crystallization of Olivine-Phyric Shergottite NWA 6234: Implication for Volatile-Induced Melting of the Martian Mantle

Abstract:

Martian magmas are thought to be rich in chlorine compared with their terrestrial counterparts. Consistent with other Martian meteorites, apatite grains in Martian meteorite NWA 6234 are dominantly Cl-apatite suggesting that the parental magma to NWA 6234 may have been rich in Cl. Here we experimentally investigate the effect of chlorine on liquidus depression and near liquidus crystallization of a synthetic composition of NWA 6234 and compare these results with previous experimental results on the effect of chlorine on near-liquidus crystallization.

Previous experimental results using two different starting synthetic Martian basalt compositions showed that the change of liquidus temperature is dependent on the bulk composition of the basalt. The effect of Cl on liquidus depression is greater for lower SiO₂, higher Al₂O₃ magmas than higher SiO₂, lower Al₂O₃ magmas, possibly due to an affinity for Cl to complex with network-modifying cations. The bulk composition for this study has an intermediate Al₂O₃ content; therefore, we can further constrain the effect of the bulk composition on the influence of chlorine on near-liquidus crystallization.

High pressure and temperature crystallization experiments were performed at 10 Kbar (1 GPa) on a synthetic basalt, of the bulk composition of NWA 6234, with 0 - 4 wt% Cl added to the sample as AgCl. The preliminary results are consistent with previous notions that with increasing wt. % Cl in the melt the crystallization temperature decreases. Importantly, our results have a liquidus depression ∆T (^oC) from added chlorine that is intermediate between the two previous results. This is consistent with the difference in bulk composition, i.e., this work has an intermediate Al-composition. This suggests that the addition of Cl to the Martian mantle may lower the solidus temperature and potentially aid in the petrogenesis of aluminous basalts.