Diagenesis of Vera Rubin ridge, Gale crater, Mars from Mastcam multispectral images

Abstract

Images from the Mars Science Laboratory (MSL) mission of lacustrine sedimentary rocks of Vera Rubin ridge on "Mt. Sharp" in Gale crater, Mars, have shown stark color variations from red to purple to gray. These color differences crosscut stratigraphy and are likely due to diagenetic alteration of the sediments after deposition. However, the chemistry and timing of these fluid interactions is unclear. Determining how diagenetic processes may have modified chemical and mineralogical signatures of ancient Martian environments is critical for understanding the past habitability of Mars and achieving the goals of the MSL mission. Here we use visible/near-infrared spectra from Mastcam and ChemCam to determine the mineralogical origins of color variations in the ridge. Color variations are consistent with changes in spectral properties related to the crystallinity, grain size, and texture of hematite. Coarse-grained gray hematite spectrally dominates in the gray patches and is present in the purple areas, while nanophase and fine-grained red crystalline hematite are present and spectrally dominate in the red and purple areas. We hypothesize that these differences were caused by grain-size coarsening of hematite by diagenetic fluids, as observed in terrestrial analogs. In this model, early primary reddening by oxidizing fluids near the surface was followed during or after burial by bleaching to form the gray patches, possibly with limited secondary reddening after exhumation. Diagenetic alteration may have diminished the preservation of biosignatures and changed the composition of the sediments, making it more difficult to interpret how conditions evolved in the paleolake over time.Plain Language Summary Sedimentary rocks found in deserts on Earth often exhibit striking color differences from red and purple to white, which are caused by groundwater dissolving and reprecipitating iron oxides within the rocks. NASA's Mars Science Laboratory (MSL) mission has observed similar color differences on Mars within the sedimentary rocks of Vera Rubin ridge in Gale crater, which were laid down in an ancient lake. We use color images and spectral data from the Mastcam cameras on MSL to investigate the origin of these color differences and find that they are consistent with changes in iron oxides through the ridge. This variation in iron oxides suggests that groundwater flowed through and altered these rocks multiple times before and after they were buried by later sediments. The MSL mission has shown that habitable lake environments once existed in Gale crater, through detections of the building blocks of life, including organic molecules, and by showing that conditions that existed in the lake were clement for life. However, later alteration by groundwater may have diminished the preservation of organics and changed the composition of these rocks, making it more difficult to interpret the details of how conditions evolved in the lake over time.

Extensive Polygonal Fracture Network in Siccar Point group Strata: Fracture Mechanisms and Implications for Fluid Circulation in Gale Crater, Mars

“…Alteration haloes, however, are likely analogous to polygon ridges as they represent the utilization of pre‐existing fractures in the substrate by later‐diagenetic fluids, which differentially affect host materials surrounding the fracture pathways. The presence of later‐diagenetic fluid flow in the region of the Murray‐Stimson unconformity is consistent, as well, with observations of diagenetic concretions in the basal Stimson formation (Banham et al, ) and the color alteration of the Murray formation directly below the unconformity (Fraeman et al, ; Horgan et al, ). Together, these observations emphasize the need to decouple the formation mechanism of polygonal fractures in the SPg from one or more potential fluid event that may have utilized pre‐existing fractures and modified the host rock around the fractures.…”

“…At the terminus of the capping unit, strata occur as a thin (<1 m), dark‐toned, erosionally resistant, cross‐bedded caprock, similar to that observed within the Stimson formation (Bryk et al, ). The underlying Murray bedrock shows distinct variation in color that has been interpreted to represent fluid alteration of Murray formation bedrock (Fraeman et al, ; Horgan et al, ). No through going mineralized fractures or polygonal features have been observed, thus far, in rover images of the Greenheugh pediment capping unit.…”

Constraints on the Mineralogy and Geochemistry of Vera Rubin Ridge, Gale Crater, Mars, From Mars Science Laboratory Sample Analysis at Mars Evolved Gas Analyses

“…The X-ray amorphous material in the DU sample is Fe rich compared to samples from VRR, suggesting the presence of nanophase Fe oxides/oxyhydroxides in rocks below the ridge (Rampe et al, 2020). Notably, the gray color of the HF drill powder (as can be seen in the drill tailings surrounding the HF drill hole in Figure 2), and the spectral data from the gray Jura (Horgan et al, 2019) considered together with the CheMin detection of hematite in HF (Rampe et al, 2020), suggests the presence of relatively coarsely crystalline gray hematite in the gray Jura. There were also differences in the clay mineral abundances between DU and the VRR samples.…”

“…Red Jura seems generally more resistant than the gray Jura, which tends to occur in topographic lows. Boundaries between red and gray patches do not follow stratigraphy, supporting a role of diagenesis in their distribution (e.g., Frydenvang et al, 2019;Horgan et al, 2019). In addition to the red and gray patches, VRR shows a variety of other geological features that indicate diagenetic processes such as dark crystal molds surrounded by light-toned "bleached" areas, other crystal molds, concretions, and nodules (e.g., Bennett et al, 2018;L'Haridon et al, 2020).…”

Detection of Reduced Sulfur on Vera Rubin Ridge by Quadratic Discriminant Analysis of Volatiles Observed During Evolved Gas Analysis

“…Additional observations indicate that VRR experienced several diagenetic events. For example, the Jura member consists of red and gray patches whose boundaries are not stratigraphically defined, which is suggestive of alteration (Horgan et al, 2019). Variations in Li and Mn content on VRR also suggest the flow of diagenetic fluids through VRR at some point in its history (Frydenvang et al, 2019).…”