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 of surface basalts Humphrey and Fastball. 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. The bulk composition for this study has lower Al2O3 and high FeO contents than previous work; 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 1 GPa (10 Kbar) on a synthetic basalt, of the bulk composition of NWA 6234, with 0 - 4 wt% Cl added to the sample as AgCl. The 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, consistent with the difference in bulk composition. This suggests that the addition of Cl to the Martian mantle may lower the magma genesis temperature and potentially aid in the petrogenesis of Martian magmas.

Dreibus and Wänke (1985) calculated a bulk composition for the interior of Mars (DW model) based on eight Martian meteorites. Since then, experiments on the Martian interior have used this model, or similar models, to understand processes that may influence basalt genesis within the Martian mantle. Recent experiments have shown that the DW model does not fit with the current, now greater number of Martian meteorites (132 including paired stones as of April 2014) and surface basalt compositions. One of the parameters that does not match is the Mg # (atomic [Mg2+/(Mg2+ + Fe2+)]*100); the current data show that Mars is not as iron rich as once thought and needs a higher bulk Mg # for the mantle to produce Martian basaltic compositions. This project involves experiments using a new bulk composition with an Mg # of 80 to update the compositions of a partially melted Martian mantle. A melt produced by this new bulk composition consists of MgO and FeO levels that match well with the primitive Martian meteorite Yamato-980459. Additionally, this composition was produced with 37% melt. Melts produced with this bulk composition match poorly with Na2O composition of Martian meteorites and match better, but still not particularly well, with CaO and Al2O3 compositions. However, lower-temperature partial melts of the Mg # 80 mantle match well with CaO, Al2O3, FeO, and Na2O compositions of Martian surface basalts. This shows that the source of the surface basalts is represented well by the model in this study.

The most widely used and accepted composition for the Martian mantle in experimental petrology is the Dreibus and Wänke (1985) proposed composition based on only eight SNC meteorites. This composition is enriched in iron with respect to the Earth, which follows what we see from samples of Mars. The magnesium number (Mg#=Mg/Mg+Fe) of the Dreibus and Wänke (1985) composition is Mg#75, which is iron rich compared to Earth’s Mg# of around 90. However, when experimentally melted as a source for generating Martian basalts, the melt concentrates iron further, higher than the Mars basalt compositions, and requires melting a large percentage of the mantle to reach a composition that is comparable to known Martian basalts. Partial melting experiments of an Mg# 80 mantle composition produced shergottite-like melts with a lower percentage of partial melting than with the Mg#75 compositions. This would be more likely since the Martian mantle would have cooled considerably by the time it would have produced the shergottites, which was only approximately 180 million years ago. The reprised composition is Mg#80 and less iron rich than the DW composition, but more iron-rich than Earth.