Date of Award

December 2020

Degree Type

Thesis

Degree Name

Master of Science

Department

Geosciences

First Advisor

Lindsay McHenry

Committee Members

Tim Grundl, Barry Cameron

Keywords

Geochemistry, Geology, Geothermal, Hydrothermal, Iceland, Mars

Abstract

Silica- and sulfate-rich deposits observed by Mars Exploration Rover (MER) Spirit near Home Plate, Gusev crater, Mars, indicate alteration of Mars basalt by a diverse array of hydrothermal fluids and processes. Constraining the precise fluid conditions present at the time of deposition for these deposits on Mars relies on investigations of terrestrial hydrothermal systems that produce similar mineral assemblages. Alteration products and fluids collected from the Ölkelduháls, Nesjavellir, and Geysir hydrothermal areas in southern Iceland cover a wide range of end-member and intermediate fluid and alteration environments, and provide a means to compare the secondary minerals produced from different hydrothermal processes. Altered sediments, precipitates, and thermal fluids were collected from hot springs with pH ranging from 2.15 – 8.70 and one active fumarole and analyzed for their geochemical and mineral composition. Pyrite was abundant throughout all three field sites indicating highly reducing conditions up to very shallow depths. Phyllosilicates were dominated by smectite and kaolinite, and smectite was associated with both acidic and alkaline hot springs, demonstrating that its formation may not be impeded by acidic conditions. Patterns of enrichment in leached rocks indicate that TiO2 and Zr follow a nearly linear trend of residual accumulation in rocks subjected to acidic fluids, consistent with their low solubility. This relationship was used to assess the behavior of trace elements in altered solids. Vanadium and Cr were found to be most resistant to acid leaching, with other elements likely incorporated into secondary minerals that could lead to depletion or enrichment depending on the mineral assemblage. In surface waters, Mg, Fe, Al, Ca, Mn, Cr, Ni, Ti, V, and Zr were found to be mobile only at very low pH, with a downward exponential trend at pH > 3. Chloride, K, Na, Ge, B, As, and Rb displayed the opposite trend and were scavenged from rocks at depth and incorporated into neutral to alkaline hot springs at the surface. These results support the findings of other studies indicating that the sulfur-rich Paso Robles class soils observed by MER Spirit near Home Plate, Gusev crater likely formed through fumarolic processes. The extensive diagenetic maturation and advanced alteration observed in Iceland in relatively young (~ 10,000 yrs) hydrothermal deposits has implications for the length of time hydrothermal processes may have been active near Home Plate, Mars, where evidence of diagenetic maturation and long-term accumulation of alteration products is minimal.

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