Exploring a diagenetic origin for X-ray amorphous material in Gale crater sedimentary rocks

Abundant volatile-bearing X-ray amorphous materials are found in billions of years old sedimentary rocks in Gale crater, Mars. The mechanisms and timing of their formation are poorly constrained, but a diagenetic origin has been hypothesized. Diagenetic cement compositions depend on the original fluid compositions and the evolution of the fluids through interactions with host lithologies and changing chemical and physical conditions (e.g., f_O2, P, T). Gale crater rocks show evidence for variable degrees of diagenesis: variable sulfate vein and concretion size and abundance, and diagenetic jarosite that is significantly younger than the sedimentation age (~2 Ga). Here we test the diagenetic origin hypothesis by exploring relationships between bulk amorphous component (AmC) abundances and compositions, primary mineralogy, and independent evidence for diagenesis (e.g., minerals, textures, chemistry).

We combine bulk XRD mineralogy from CheMin with bulk chemical compositions from the Alpha Particle X-ray Spectrometer instrument to estimate the compositions of the AmCs in Gale crater sedimentary rocks. We then compare these compositions to observations of diagenetic features in the surrounding rocks as seen in ChemCam, Mastcam, and MAHLI datasets.

Of the thirteen AmC compositions calculated thus far, we find significant variations between and within stratigraphic units.All AmCs are mixtures of silicates, iron-bearing phases, and sulfates, and are depleted in Al₂O₃compared to their respective crystalline counterparts. The AmCs in Bradbury and Siccar Point group rocks (not including alteration halos) show depleted SiO₂compared to their crystalline counterparts. In the Mt. Sharp group, the AmCs in all mudstone rocks are slightly enriched in SiO₂, while the AmC of Oudam, a cross-stratified sandstone, is significantly depleted in SiO₂ compared to its crystalline counterpart. MgO, FeO_Tand SO₃are highly variable among the samples, but most AmCs are enriched in SO₃compared to their respective crystalline counterparts. Thus, the AmC compositions are complicated and are likely dependent on local factors, as would be expected if the materials were formed through diagenetic processes. Further analyses will explore potential correlations with rock mineralogy, diagenetic minerals, and concretions.