Date of Award
Master of Science
Iron (oxyhydr)oxide concretions in the Navajo Sandstone of southern Utah have been extensively researched as Martian analogues. However, the discovery of calcium carbonate concretions in areas such as Coyote Gulch, Utah, has encouraged recent studies to understand the relationship between calcium carbonate spheroidal concretions as possible precursors to iron (oxyhydr)oxide concretions, and to determine the fluid chemistries involved in diagenesis. This is important because nucleation and precipitation mechanisms of these spheroidal calcium carbonate and iron (oxyhydr)oxide concretions and fluid mechanisms in iron rich environments could affect the preservation of possible biosignatures in other subsurface features on Mars. The elemental and mineralogical compositions of the concretions were examined in order to determine physical and chemical features shared by the two types of concretions and did show that they share similar morphologies; however, the Coyote Gulch concretions are calcite cemented (~30 wt.%), with secondary iron (oxyhydr)oxide precipitation and decreases in calcite in transects away from the calcium carbonate concretions. Several chemical and mineralogical differences exist between the two separate populations of concretions, possibly due to regional variability of reacting phases in fluid systems. Spring fluids emanating from the Navajo Sandstone in Coyote Gulch were tested to determine the fluids responsible for the development of any of the concretion mineralogies in the study area which could form in distinctive geochemical systems. Geochemical modeling performed in this research explored the question of fluid chemistry involved in concretion formation in the Navajo Sandstone and findings suggest that the calcite concretions formed prior to the precipitation of secondary iron (oxyhydr)oxides and may have provided a localized buffering environment for the precipitation of iron (oxyhydr)oxides. Paleofluid circulation, redox processes, and elemental mobility are examined using the geochemistry of Navajo Sandstone concretions and host rock. Various simulations applicable to diagenetic fluids in the studied concretions show the importance of salinity and pH in paleoaquifers in order to precipitate mineral assemblages similar to those found in the Navajo Sandstone. Widespread dissolution features, major and trace element distributions, and geochemical modeling identified feasible fluid-rock interactions in paleofluids, including the importance of limited H2S gas and the limited feasibility of hydrocarbon rich fluids in concretion formation using current data. A universal mechanism for calcium carbonate to iron (oxyhydr)oxide concretion formation could be applied on other planets and provide exciting implications in the search for carbon rich redox gradients which could support life in the subsurface of otherwise inhospitable planets.
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