291-3 Element mobility and alteration styles recorded by PIXL in the Jezero Crater Margin unit on Mars

Session: Mineralogy in the Solar System

Presenting Author:

An Li

Authors:

Li, An Y.¹, Ramo, Kirtee², Kizovski, Tanya³, Hurowitz, Joel A.⁴, Treiman, Allan H.⁵, Schmitz, Nicole⁶

(1) Earth and Space Sciences, University of Washington, Seattle, Washington, USA, (2) Stony Brook University, USA, (3) Brock University, Canada, (4) Stony Brook University, Stony Brook, NY, USA, (5) Lunar & Planetary Institute, Houston, TX, USA, (6) German Aerospace Center (DLR), Germany,

Abstract:

Characterized by high carbonate and silica content, the Margin unit in Jezero crater has an enigmatic origin with hypotheses ranging from in-situ alteration of an igneous cumulate to serpentinization of a clastic, olivine-rich rock. Using the Planetary Instrument for X-ray Lithochemistry on the Mars 2020 Perseverance rover, we apply elemental ratios to bulk sums and distinct mineral phases at fine scale (~120 μm spot size) to understand differences between the compositions of the Eastern and Western Margin. We find that altered olivine––defined as 0.6<(Fe+Mg)/Si<1.0, SiO2 wt%<50, and Ca/(Mg+Fe)<0.05––exhibits progressively higher Mg# than coexisting pristine olivine from the Eastern to the Western Margin, and this trend persists even after accounting for differences in the pristine olivine Mg#. This increase in altered-olivine Mg# coincides with progressively higher bulk CO₂ wt% concentrations toward the Western Margin and increasing elevation. Together, these observations are consistent with progressively greater Fe partitioning during alteration, suggesting that the Western Margin records a more advanced stage of serpentinization to carbonation pathway. The Si-rich (>70 wt%) interstitial phases Eastern and Western Margin also record distinct alteration histories. The Eastern Margin Si-rich phase is enriched in SiO₂, TiO₂, Al₂O₃, FeT, CaT, Na₂O, K₂O, and Cl relative to the Western Margin. This geochemical signature is consistent with diagenetic cementation of a porous clastic framework that incorporates external silica cement and mobile element concentration from pore fluid interaction. The co-enrichment of Na₂O, K₂O, and Cl in particular is difficult to explain by in-situ serpentinization of an olivine-dominated protolith, which provides no local source for these elements. In contrast, the Western Margin Si-rich phase is enriched in MgT. This enhanced MgT in both the altered olivine and Si-rich phase is consistent with more closed-system alteration of a less porous in-situ protolith with limited external fluid access. Whether the Western Margin represents an in-situ altered igneous cumulate or a less reworked clastic end-member (in which open-system Mg loss was limited) cannot be fully resolved with the current dataset and represents a priority for returned sample analysis. The differences in altered-olivine Mg#, CO₂ abundance, and Si-rich interstitial phase composition indicate that the Western and Eastern Margin record distinct alteration histories rather than representing a single, compositionally uniform alteration process. 

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