An olivine cumulate outcrop on the floor of Jezero crater, Mars

Abstract

The geological units on the floor of Jezero crater, Mars, are part of a wider regional stratigraphy of olivine-rich rocks, which extends well beyond the crater. We investigate the petrology of olivine and carbonate-bearing rocks of the Séítah formation in the floor of Jezero. Using multispectral images and x-ray fluorescence data, acquired by the Perseverance rover, we performed a petrographic analysis of the Bastide and Brac outcrops within this unit. We find that these outcrops are composed of igneous rock, moderately altered by aqueous fluid. The igneous rocks are mainly made of coarse-grained olivine, similar to some Martian meteorites. We interpret them as an olivine cumulate, formed by settling and enrichment of olivine through multi-stage cooling of a thick magma body.

Aqueously altered igneous rocks sampled on the floor of Jezero crater, Mars

“…The millimeter-sized grains, when observed in abraded surfaces of Bastide (the Dourbes and Garde abrasion patches), are light toned and mostly equant, but some elongated grains are also present (Fig. 3C) ( 35 ); we observed no gradation in grain size in the abrasion patches. Less common than the light-toned grains are slightly smaller dark-gray or green grains that are sometimes angular but more often appear to fill the space between lighter grains.…”

Compositionally and density stratified igneous terrain in Jezero crater, Mars

“…2, E and F). Fine-scale observations in Séítah by the Planetary Instrument for X-ray Lithochemistry (PIXL) instrument show that olivine and pyroxene are intergrown in a pattern that clearly demonstrates they are cogenetic (29). The prevalence of olivine is consistent with a density-driven cumulate rather than classical coarse-grained igneous rocks that contain various crystals of similar sizes such as pyroxene, olivine, and plagioclase in a basalt.…”

“…The observed olivine crystals do not show evidence of intergrowth with each other, but rather are closely packed with abundant faceto-face contacts (Fig. 2F) (29), suggesting settling as a mechanism for concentrating olivine. Settling by gravity within a melt is also consistent with the narrow grain size distribution observed in most of the SuperCam RMI images of Séítah (table S1).…”

“…28 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. 29 University of Copenhagen, Copenhagen, Denmark. 30 McGill University, Montreal, QC, Canada.…”

“…Observed grains within Séítah are predominantly angular and monomineralic (Fig. 2F), consisting of olivine and, more sparsely, pyroxene and plagioclase within interstitial spaces (29). Although the mean grain size and narrow range of sizes of Séítah are consistent with dynamic sorting typical of sedimentary deposits, grains do not show any indication of preferential sorting within individual layers (e.g., graded bedding) that might be attributed to either waning flow or fallout, with no obvious rounding of grains.…”