Fine-scale element mobility and alteration styles recorded in Jezero Crater floor rocks by PIXL

During the Crater Floor Campaign of the Mars 2020 Perseverance mission, the Planetary Instrument for X-ray Lithochemistry (PIXL) acquired eight X-ray fluorescence (XRF) map scans of abraded rock surfaces. Each scan consisted of ~3000 spatially-contiguous XRF analysis points separated by 125 μm, with scan dimensions of up to 12.5 x 4.0 mm. Four scans were collected in the Séitah formation with two each at the targets Dourbes and Quartier, and four were collected in the Máaz formation at Bellegarde, Montpezat, Guillaumes, and Alfalfa. To evaluate the degree of alteration of these igneous rocks, we use the chemical index of alteration (CIA) (Nesbitt & Young 1982) and mafic index of alteration (MIA-O, with total Fe as Fe2O3) (Babechuk et al. 2014), which are based on the molar ratios of immobile to mobile elemental oxides measured by PIXL. We corrected the data for X-ray diffraction, roughness, and accessory minerals including carbonate, halite, phosphate, sulfate, and titanomagnetite. These indices are typically applied to bulk rock compositions (e.g., Thorpe & Hurowitz 2021), but PIXL provides a high-resolution map from which we can evaluate differential element mobility at fine scale.

In Séitah, 7% of all points have high CIA values (typically indicative of chemical weathering) ranging from 50-100 (avg. = 58). Most >50 CIA points corresponded to the rims of feldspathic mesostasis (Dourbes) (Fig. 1) or a secondary silicate alteration phase identified as low (Fe, Mg) silicate (Quartier) in Tice et al. (in review). In Máaz, 5% of all points have CIA values >50, with high CIA points from 50-87 (avg.= 58). The MIA-O appears to be a more effective indicator of alteration in Máaz: 20% of all points have MIA-O values >50, ranging from 50-93 (ave. = 60). Most Máaz >50 CIA and MIA-O points correspond to an Fe-silicate alteration phase formed by replacement of olivine (Fig. 2), described by Tosca et al. (in prep). Since olivine is not expected to have high CIA, we interpret this as indicative of Al-mobilization into altered olivine, possibly under hydrothermal conditions. While Séitah is characterized by low temperature alteration phases that can be identified with high CIA, alteration in Máaz is more readily distinguished by the MIA-O index and points to Al-mobilization during olivine alteration.