FORMATION OF FELSIC MARTIAN ROCKS AT GALE CRATER THROUGH FRACTIONAL CRYSTALLIZATION

The finding of felsic (SiO₂ > 55 wt.%) rocks at Gale crater by the Mars Science Laboratory (MSL) Curiosity rover prior to sol 800 revealed that martian igneous rocks are more diverse than previously thought. Gale crater evolved rocks contain up to ~68 wt % SiO₂ and ~14 wt % total alkalis and consist of abundant plagioclase, K-feldspar, and silica. The occurrence of felsic rocks at several localities at the martian surface (Gale Crater, Syrtis Major) and in meteorites confirms that such rocks are widespread on Mars. In addition, it was previously suggested that the Gale crater martian felsic rocks are similar to the terrestrial Archean tonalite-trondhjemite-granodiorite (TTG, the felsic building blocks of continental crust on Earth). In this study, we conduct thermodynamical models to understand the conditions of formation of the Gale felsic lithologies using different variables, such as starting compositions, water content, oxygen fugacity—ƒO₂, and pressure. We also compare the Gale crater felsic rocks to various terrestrial rocks, such as TTG and intraplate volcanic compositions (e.g., Hawaii, Iceland, and Canary Islands).

We show that fractional crystallization of magmas with compositions analyzed by the Spirit rover at Gusev crater and a clast from the regolith breccia meteorite Northwest Africa 7034, lead to formation of trachytic and rhyolitic compositions. In addition, the felsic Gale crater rocks could have formed through fractionation crystallization of a Gusev crater Backstay-like composition (SiO₂ = 50.7 wt.%, Na₂O = 4.1 wt.%) with various conditions, likely involving plagioclase fractionation. The major element compositions and mineralogies of these samples resemble more closely to Earth intraplate volcanoes, such as Iceland, with the exceptions of CaO (lower in the martian magmas) and FeO (higher in the martian magmas), than to the continental crust, represented by the TTG. For example, TTG suites contain quartz, sodic plagioclase, K-feldspar, and water-bearing phases. However, most of the felsic Gale crater rocks are not quartz-normative and hydrated minerals were not detected in these rocks. Thus we suggest that a restricted amount of martian felsic rocks formed at the martian surface from fractionation of basaltic magmas, similar to intraplate volcanoes on Earth.