The nature of igneous olivine cumulate rocks in Jezero crater, Mars: A PIXL perspective

The Perseverance rover explored the olivine- and carbonate-bearing unit exposed on the floor of Jezero crater, a 3.9 billion-year-old impact crater at the western edge of Isidis Planitia. This olivine-rich unit, named Séítah formation, displays massive, centimeter- to decimeter-scale layering with abundant dark, millimeter-scale angular grains in grain-to-grain contact (Farley et al., 2022, submitted). The Séítah formation is part of an extensive olivine-rich regional unit (circum-Isidus unit) identified from orbital data (e.g., Kremer et al., 2019; Mandon et al. 2020); before landing, hypotheses for the origin of this unit included impact melt (Mustard et al., 2009), an igneous intrusive complex (Hoefen et al., 2003), basaltic flows (Hamilton and Christensen, 2005; Tornabene et al., 2008), volcanic ash (Mandon et al., 2020, Kremer et al., 2019), and clastic material (Rogers et al., 2018). Three rock outcrops (Bastide, Brac and Issole) in the Séítah formation were abraded to generate 5 cm diameter patches (the patches, in turn, are named Garde on Bastide, Dourbes on Brac, and Quartier on Issole). PIXL collected only multispectral images of Garde but collected both multispectral images and XRF data on Dourbes and Quartier. The texture of Garde from the multispectral images is nearly identical to that of Dourbes. Based on the texture, grain orientation analyses and chemistry, we recognized that Brac is an olivine cumulate rock that experienced moderate aqueous alteration (Liu et al., 2022, Tice et al., 2022). The protolith of Brac consists mainly of coarse olivine grains (Fo55±1, up to 3.5 mm) with interstitial augite (Wo35-38En43-44Fs20-21, up to 8 mm) and mesostasis (alkali feldspar, plagioclase, Fe-Cr-Ti oxides, phosphate). Olivine grains are equant but rounded with possible melt inclusions. Olivine grains poikilitically enclosed in large augite display similar chemistry to those in the non-poikilitic regions. Bulk chemistry of the areas scanned by PIXL shows the rock is ultramafic with Mg# of ~55. The proportion of minerals suggests the protolith of Brac is an olivine-rich wehrlite. These Brac results indicate that the protolith likely formed through a multi-stage cooling history, involving initial relative fast cooling as indicated by large melt inclusions followed by relatively slow cooling to allow the accumulation of olivine and formation of large pyroxene grains, and homogenization of Fe/Mg in and between olivine and pyroxene. This finding is consistent with more than one emplacement mode, including a surface flow >100 m thick or a shallow subsurface sill of comparable thickness (Liu et al., 2022). The Quartier patch shows a similar cumulate texture, with abundant olivine and poikilitic textures, as well as the effects of aqueous alteration. However, the olivine in Quartier displays more variable morphology than that in Dourbes. Olivine grains enclosed by augite display chain habit and skeletal form respectively (Fig. 1), respectively, both indicators of rapid growth from a strongly undercooled melt. However, olivine grains in the non-poikilitic regions of Quartier are more tabular and prismatic than those in Dourbes with sizes up to mm. Olivine of both skeletal and tabular shapes in Quartier are more evolved than in Dourbes (~Fo45). Small melt inclusions are also present in Quartier olivine. Augite in Quartier is also slightly more Fe-rich than that in Dourbes (Wo32-35En39-40Fs26-28). The feldspar in Quartier appears to be only alkali feldspar. The record of rapidly grown olivine, the slowly grown olivine, and the evolved olivine composition suggests that Quartier/Issole is likely derived from a more fractionated melt which experienced an initial large thermal gradient followed with a smaller thermal gradient or experienced magma mixing. The olivine chemistry and sizes are consistent with those inferred from orbital observations for the regional (moderate Fo, 1-4 mm, Ody et al., 2013; Brown et al., 2020). All three rocks analyzed by PIXL suggest the Séítah formation likely formed through fractionation of a large magma body that may have been built incrementally. Such a formation model for the local outcrop area implies that the regional extensive olivine-rich unit cannot be easily explained by any single formation mechanism.

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