Senior Thesis Defense
Student: Madison Morris
Department: Earth, Environmental and Planetary Sciences
Defense Date: Tuesday, May 4, 2021
Time: 4:00 p.m.
Characterizing Multiple Episodes of Fluid Alteration within Stimson Fracture Halos, Gale Crater, Mars
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 twenty-six altered and thirty-seven 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, FeOT, and Al2O3 decreased, and K2O, Cl, CaO and Br remained constant. A relative increase in Ti, along with quickly depleted trace elements, implies dissolution within the rock. The relative increases in CaSO₄ and silica provide evidence for later addition to the sandstone. We conclude that the Sitmson experienced distinct acidic dissolution and addition events.