ET-THEN: A POSSIBLE MARTIAN MAGMA?
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), 44th LPSC, The Petrochemistry of Jake_M: A Martian Mugearite, Abs. #1685