Iron Mobility During Diagenesis as Observed by ChemCam at Gale Crater, Mars

Abstract : The Curiosity rover reached a local topographic high known as Vera Rubin Ridge (VRR) near sol 1800, after covering 18 km across mostly fluvio-lacustrine deposits and local eolian erosion remnants since its landing in Gale crater. The rocks of this morphological structure appear to lie conformably on top of the lacustrine mudstones of the Murray formation and show continuity in sedimentary structures, suggesting comparable depositional environments. The uniqueness of the VRR resides in its topography and the strong hematite signature observed in CRISM orbital spectra, later confirmed in-situ by Mastcam multispectral and ChemCam passive spectra observations. ChemCam and APXS data on the VRR do not present any significant increase in Fe abundance in the bedrock, but instead highlighted sporadic, anomalously high Fe detections usually associated with diagenetic dark-toned features along light-toned Ca-sulfate veins, as well as grey patches within larger veins. High-Fe abundances are consistent with a Fe-oxide mineralogy as Fe is not associated with enrichments in other major (Si, Al, Ti, Mg, K and Na) or minor (Ba, Cr, Li, Sr, Rb and Zn) elements, except for a slight correlation with Mn, nor with detections of volatiles (P, S and Cl). ChemCam passive reflectance spectra lack ferric absorption features (535 nm absorption band, downturn after 750 nm) which hints at the presence of mixed valence Fe-oxide (e.g. magnetite). In addition, bleached halos with low-Fe (and low-Mn) content are observed in the surrounding host rock suggesting mobility of Fe (and Mn), likely during diagenesis. These observations are limited to shallow depressions on the upper part of the ridge, where the bedrock exhibits a lighter-toned grey color and lack the telltale ferric signature of other VRR bedrocks. In contrast, Ca-sulfate light-toned veins are also observed on the adjacent red and ferric bedrocks but do not show any association with high Fe features. Early cementation in oxidizing environment and reducing groundwater circulation could explain these lateral facies changes, which in turn allowed for the mobilization of Fe and formation of Fe-oxide in during subsequent diagenetic episode(s). These observations highlight the significant role played by diagenesis in the mobility and distribution of Fe at Gale Crater.
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Submitted on : Tuesday, November 5, 2019 - 10:43:31 AM
Last modification on : Wednesday, November 6, 2019 - 1:49:31 AM

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R. C. Wiens, J. l'Haridon, N. Mangold, A. Cousin, J. R. Johnson, et al.. Iron Mobility During Diagenesis as Observed by ChemCam at Gale Crater, Mars. AGU meeting, Dec 2018, New Orleans, United States. ⟨hal-02347208⟩

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