  The Curiosity rover investigated a topographic structure known as Vera Rubin
ridge, associated with a hematite signature in orbital spectra. There,
Curiosity encountered mudstones interpreted as lacustrine deposits, in
continuity with the 300 m-thick underlying sedimentary rocks of the Murray
formation at the base of Mount Sharp. While the presence of hematite
($\alpha$-Fe2O3) was confirmed insitu by both Mastcam and ChemCam spectral
observations and by the CheMin instrument, neither ChemCam nor APXS observed
any significant increase in FeO$_T$ (total iron oxide) abundances compared to
the Murray formation. Instead, Curiosity discovered dark-toned diagenetic
features displaying anomalously high FeO$_T$ abundances, commonly observed in
association with light-toned Ca-sulfate veins but also as crystal pseudomorphs
in the host rock. These iron-rich diagenetic features are predominantly
observed in "grey" outcrops on the upper part of the ridge, which lack the
telltale ferric signature of other Vera Rubin ridge outcrops. Their composition
is consistent with anhydrous Fe-oxide, as the enrichment in iron is not
associated with enrichment in any other elements, nor with detections of
volatiles. The lack of ferric absorption features in the ChemCam reflectance
spectra and the hexagonal crystalline structure associated with dark-toned
crystals points toward coarse "grey" hematite. In addition, the host rock
adjacent to these features appears bleached and show low-FeO$_T$ content as
well as depletion in Mn, indicating mobilization of these redox-sensitive
elements during diagenesis. Thus, groundwater fluid circulations could account
for the remobilization of iron and recrystallization as crystalline hematite
during diagenesis as well as color variations observed in the Vera Rubin ridge
outcrops.
