Payre, and 4 co-authors, reported in early 2022 that the unexpected detection of ∼16 wt.% monoclinic tridymite, a high-temperature silica polymorph, within an otherwise lacustrine mudstone in Gale crater on Mars, raises significant questions about its formation and the extent of magmatic evolution on that planet. The rock sample, analyzed by the X-ray diffractometer onboard the Curiosity Rover, also contained feldspar, cristobalite, and opaline silica (±Si-glass). Monoclinic tridymite is extremely rare on Earth, and has only been discovered in silicic volcanic environments, high-
temperature impact settings, and extraterrestrial rocks.
The authors reviewed the most common formation pathways of natural tridymite and conducted thermodynamical modeling to investigate possible formation mechanisms. The authors considered the broader context of the sample to propose a formation and transport mechanism based on:
1) the mineralogical assemblage of the mudstone and rocks in the vicinity,
2) the composition of the mudstone layer, and
3) the overall geological context.
Based on the large amount of tridymite, the high SiO2 and low Al2O3 concentration of the mudstone, and the low temperature context within distal lacustrine mudstone, teh authors proposed that an explosive eruption released Si-rich ashes, which were deposited into Gale crater’s watershed as a tridymite-rich ashfall along with cristobalite, feldspar, Ti-oxide, and Si-rich glass, when Gale was still a lake (Hesperian). The dissolution of Si-rich glass and
mineral sorting during transport would have concentrated tridymite, caused opaline silica precipitation, and relatively lowered the Al2O3 concentration. This scenario implies that explosive volcanism on Mars occurred during the Hesperian and might not be restricted to basaltic eruptions, revealing the complexity
of Mars magmatism.
The authors concluded that overall, their proposed model is in agreement with a silicic provenance and a detrital origin of tridymite as suggested by Mor-
ris et al. (2016) and Czarnecki et al., 2020a,b. A thorough re- view of natural settings and thermodynamic models suggests magmatic and/or vapor crystallization/devitrification forming tridymite, cristobalite, feldspar, and Ti-oxide.
Sedimentary observations argue against in-situ volcanics or hydrothermal alteration including fumaroles, and favor contemporaneous ashes falling into Gale crater lake and/or watershed, enhanced by the precipitation of opaline silica. Based on observations in Gale crater, there was at least one instance of explosive silicic volcanism on Mars during the Hesperian. Such explosive volcanism in the Hesperian provides both compositional and timing constraints revealing that Mars magmatism was not restricted to mafic eruptions and crystallization of mafic minerals as suggested by Baratoux et al. (2013). Felsic magmas, potentially similar to intraplate melts on Earth like Icelandic melts (Udry et al., 2018), have formed on Mars from the Noachian (e.g., Sautter et al., 2015) to the Hesperian, and perhaps later on.
Ongoing and future missions should seek evidence for additional evolved volcanism and additional constraints on the timing of volcanic eruptions; Mars is not only a basaltic world.
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