Characterizing Multiple Episodes of Fluid Alteration within Stimson Fracture Halos, Gale Crater, Mars

Abstract:

The Stimson formation is an eolian sandstone perched unconformably above the Murray formation near the base of Mount Sharp in Gale crater. The Stimson sandstones exposed at the Emerson and Naukluft Plateaus contain a network of fractures surrounded by decimeter-scale light-toned fracture halos of rock affected by fluids moving through the fractures. The Mars Science Laboratory rover Curiosity analyzed the chemistry and mineralogy of the unaltered dark basaltic sandstones and the apparently bleached altered fracture halos. Multiple hypotheses have been proposed to describe the fluids that flowed through the fractures: Yen et al. (2017, EPSL) proposed an acidic bleaching followed by more alkaline solutions; Frydenvang et al. (2017, GRL) proposed dominantly silica addition; and Hausrath et al. (2018, EPSL) modeled the solution chemistries and found that an acidic solution could account for most of the major element and mineralogical changes.

Here, we utilize the mineralogical data collected from Curiosity’s Chemistry and Mineralogy X-ray diffraction instrument (mineralogy of two altered and two unaltered sites) and the chemical data from the Alpha Particle X-ray Spectrometer (chemistry of sixteen altered and sixteen unaltered sites) of the samples to identify characteristics of the fluids and distinguish between aforementioned hypotheses. Sandstone compositions were plotted in MATLAB to track changes related to the alteration episode. Some element trends were best defined by multiple trendlines, in agreement with multiple fluid events altering the sandstone. During the alteration process, TiO₂, SO₃, SiO₂ and P₂O₅ relatively increased, MgO, Zn, Ni, Na₂O, MnO, FeO_T, and Al₂O₃ decreased, and K₂O, Cl, CaO and Br remained constant. Previous work has not explained the constant K₂O. A few altered targets have more P₂O₅ and less FeO_T than other targets for the same SiO₂. Our research aims to further characterize each of the fluids that passed through the fracture halos.