Tridymite in Gale Crater: a Witness of Explosive Volcanism on Early Mars?

Tridymite is a silica polymorph that is stable in a hexagonal form at high-temperatures and low pressures (870<T<1470 °C;1) and is commonly found on Earth near impacts and in silicic volcanic complexes. During cooling, low-T metastable crystalline arrangements of tridymite occasionally form. Monoclinic tridymite formed from rearrangement of hexagonal tridymite at T<450°C, has only been reported in 5 volcanic provinces on Earth (2). Monoclinic tridymite has also been observed in meteorites and for the first time in situ on Mars, in the 3.8 Ga Gale crater (3-4). The X-ray diffractometer in the CheMin instrument onboard the Curiosity rover, identified monoclinic tridymite with feldspar, cristobalite, and a Si-rich amorphous component including opal and/or rhyolitic glass in a lacustrine mudstone from the Murray formation (5). The scarcity of monoclinic tridymite on Earth raise questions about how such a rare polymorph could have formed on Mars. Assuming monoclinic tridymite results from the crystalline rearrangement of hexagonal tridymite, impact, magmatic, ash, fumarolic, and hydrothermal crystallization are possible. Low-T crystallization processes that have never been observed on Earth are also considered. Using thermodynamical modeling (6-7) and literature on terrestrial analogs, this study deciphers the most reasonable formation pathway of tridymite. Because no quartz is observed and the amorphous component is too depleted in insoluble elements to represent a rhyolitic glass, the most likely scenario is that a silicic explosive eruption covered Gale crater's watershed and lake with a thin layer of hexagonal tridymite, cristobalite, feldspar and Si-glass ashes. This result would suggest that explosive volcanism on Mars is not only restricted to basaltic systems.

(1) Howie, R.A., et al. (1992). Longman; (2) Jackson et al. 96.1 (2011): 81-88; (2) Bridges et al. 30.6 (1995): 715-727; (4) Morris, et al. 113.26 (2016): 7071-7076; (5) Czarnecki et al. 125.3 (2020); (6) Payre et al. Accepted; (7) Gualda et al. (2012). 53(5), 875-890.