Explosive Volcanism on Early Mars: Explaining the Tridymite Layer in Gale Crater

Explosive volcanism has been widely identified on Mars from orbital observations, but the spectrally neutral character in VNIR spectra of Si-rich minerals like SiO₂ polymorphs limits their detections from orbit [1]. This led to most explosive volcanisms being attributed to basaltic plinian eruptions [1]. The Curiosity rover that landed in Gale crater analyzed mostly sedimentary rocks containing little silica except for one single 1m-thick layer of thinly laminated mudstone composed of up to 70 wt. % of SiO₂ and abundant tridymite (15% of the rock [2]). The igneous tridymite (td) along with plagioclase, Si-amorphous material, sanidine, and cristobalite comprise the most compositionally and mineralogically distinct bedrock observed in the entire mission [2-4]. The minerals are derived from rhyolitic ashes [2], but the provenance has not been identified and the presence of rhyolitic rocks is inconsistent with the surrounding strata. Using alphaMELTS models at low pressure (200 bars [5]), we show that tridymite can crystallize at >870°C within a rhyolitic melt by the same formation path as igneous minerals and rocks from the first 750 sols [6]: fractional crystallization of an anhydrous melt extracted from an adiabatic ascent of a primitive mantle composition. A rhyolitic explosive event from the same magma source could have spread td-bearing ashes, which could have been transported by wind and fluvial stream [2]. This helps solve the mystery of the td-bearing layer since fractional crystallization of a single magmatic source could have formed the whole range of plagioclase, pyroxene and tridymite analyzed in Gale. Silicic plinian eruption appears to be rare from orbit, but is unlikely to be exclusive to this region, and other in situ missions may discover widespread traces of these events.

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[2] Morris et al., (2016), Proc. Nat. Ac. Sc., 113(26), 7071-7076

[3] Morrison, S. M., et al., (2017), Am. Min., 03(6), 857-871

[4] Czarnecki, S., et al., (2019), Mars 9th Int. Conf., abs. 6451

[5] Smith, P. M., and P. D. Asimow, (2005), G3, 6(2)

[6] Payre, V., et al., (2019), Mars 9th Int. Conf., abs. 6231