Phase Equilibria Modeling of Low-Grade Metamorphic Martian Rocks

Hydrous phases have been identified to be a significant component of Martian mineralogy. Particularly, prehnite, zeolites, and serpentine are evidence for low-grade metamorphic reactions at elevated temperatures in mafic and ultramafic protoliths. Their presence suggests that at least part of the Martian crust is sufficiently hydrated for low-grade metamorphic reactions to occur. A detailed analysis of changes in mineralogy with variations in fluid content and composition along possible Martian geotherms can contribute to determine the conditions required for subsurface hydrous alteration, fluid availability, and rock properties in the Martian crust. In this study, we use phase equilibria models to explore low-grade metamorphic reactions covering a pressure-temperature range of 0-0.5GPa and 150-450 degrees C for several Martian protolith compositions and varying fluid content. Our models replicate the detected low-grade metamorphic/hydrothermal mineral phases like prehnite, chlorite, analcime, unspecified zeolites, and serpentine. Our results also suggest that actinolite should be a part of lower-grade metamorphic assemblages, but actinolite may not be detected in reflectance spectra for several reasons. By gradually increasing the water content in the modeled whole-rock composition, we can estimate the amount of water required to precipitate low-grade metamorphic phases. Mineralogical constraints do not necessarily require an elevated geothermal gradient for the formation of prehnite. However, restricted crater excavation depths even for large impact craters are not likely sampling prehnite along colder gradients, suggesting either a geotherm of similar to 20 degrees C/km in the Noachian or an additional heat source such as hydrothermal or magmatic activity. Plain Language Summary There is evidence that greater amounts of water were present on a younger Mars as compared to the dry conditions observed today. Water not only shapes the surface and forms river beds, deltas, and lakes but also reacts with the existing rocks to form characteristic minerals. Understanding how these minerals formed gives us useful information about how much water may be present in the Martian crust, which is also important for possible habitable environments. Our knowledge about Mars has increased greatly with the data from rovers and orbiters, but we still only have a limited amount of rocks available. Therefore, we use computer models to simulate how rocks from Mars would behave if they were exposed to higher temperatures, pressures, and water. We can estimate how much water is needed to form certain minerals, and that helps us to understand the conditions on Mars over geologic timescales and how and why it is different from Earth.