ET-THEN: A POSSIBLE MARTIAN MAGMA?

On Sol 86 of the Curiosity mission, the rover examined a rock named “Et-Then”. Due to other analyses, the APXS measurement was conducted at distance with no rock-surface preparation, leading to a high margin of error and possible dust contamination. The rock is fine-grained but does not show obvious textural indications of whether it is igneous or sedimentary. Despite these uncertainties, the APXS-measured composition resembles compositions predicted from calculations involving the MELTS algorithm, particularly in the FeO content.

Et-Then is a low-SiO₂ (43.6±4 wt. %), high-FeO (27.5±1.3 wt. %) rock. If we assume it is volcanic, it would be an alkali basalt classified as a tephrite. We tested crystallization paths for proposed martian parental magmas and found that there are multiple MELTS-calculated paths which produce compositions similar to Et-Then as natural products of fractional crystallization.

The Yamato 980459 meteorite composition produces a similar composition to Et-Then when crystallized dry in the lower martian crust. The calculated liquid reaches 27 wt. % FeO(TOT) and is within error of Et-Then in MgO, Na2O, SiO2, and TiO2. There are some differences: Al₂O₃ and CaO are too high in the calculated liquid, K₂O is very low as it is virtually absent in Y-98, and no volatiles are included, but overall the compositions are similar. High FeO/MnO could be produced by crystallization of garnet or Mn-rich olivine at pressure.

Stolper et al. (2013) argued that the rock Jake Matijevic could be produced by suppression of plagioclase during crystallization at crustal pressures. A similar mechanism could give rise to a magma resembling Et-Then, although suppression of olivine rather than plagioclase is required. Even though this rock cannot be guaranteed to be igneous, its chemistry is predicted as a normal product of martian crystallization by the MELTS algorithm. If CO₂or Cl were involved in the suppression of olivine, degassing of those elements could contribute to pyroclastic eruptions, which could produce fine grain sizes or sedimentary textures as well.

Et-Then could therefore represent the product of common martian magmatic processes such as crystallization in the lower crust or from magma rich in Cl or CO₂.

Stolper et al. (2013), 44^th LPSC, The Petrochemistry of Jake_M: A Martian Mugearite, Abs. #1685