Using Fluid-Mobile Elements to Decipher an Aqueous History Preserved in the Sedimentary Rocks of Gale Crater, Mars

Fluid-mobile elements preserve a record of the aqueous history of Mars in the sedimentary rocks of Gale Crater, and they are traceable with the rover Curiosity’s Alpha Particle X-ray Spectrometer (APXS). This is the fundamental principle behind the scientific questions addressed in the four main chapters of this dissertation. First, we conducted a field study at Hawai’i, an established Mars analogue, to investigate sedimentary processes and alteration in rocks comparable to those in Gale Crater. We conclude that open system, circumneutral weathering is very limited in Gale rocks; acid sulfate alteration and sedimentary mixing of diverse rocks is likely. Second, the Hawaiian samples were analyzed by Particle Induced X-ray Emission spectroscopy (PIXE) to evaluate the technique as an analogue for APXS. Third, we developed a novel, non-standard APXS calibration to determine the composition of a very thin layer of airfall dust on the rover. The global dust is enriched in the fluid-mobile elements sulfur and chlorine, but at a ratio found to be constant around the planet. This enables deconvolution of modern dust from underlying rock in APXS measurements to determine inherent sulfur and chlorine content in ancient rocks. Fourth, we used fortuitous zinc and germanium enrichments in Gale Crater rocks to establish a geochemical tracer pair for fluid processes in Gale. We propose a model in which the two trace elements were enriched together by hydrothermal fluids in the sediment source region, transported into the crater, and then fractionated by low temperature diagenetic fluids. The work presented here constrains fluid events affecting Gale sediments and can aid in unraveling fluid histories as Curiosity’s traverse continues