Magnesium Carbonates in Ultramafic Rocks: Carbon Dioxide Transformations in Crustal Environments

Advisors

Dennis K. Bird, primary advisor
Katharine Maher,primary advisor
Gordon E. Brown, Jr., advisor
Norman H. Sleep, advisor

Abstract

Magnesium carbonate mineralization in Earth's crust occurs in a variety of geologic settings, including hydrothermal and mineral ore systems to sedimentary, weathering, and biogenic environments. Tectonic, chemical, and isotopic descriptions of magnesium carbonates and host ultramafic lithologies constrain the geologic conditions of the interaction between CO₂-rich fluids with magnesium silicate minerals. Studying carbonate mineralization in ultramafic rocks is important not only for fundamental understanding of Earth processes, but also for assessing the potential of sequestering anthropogenic CO₂ in ultramafic rocks. Mineral carbon sequestration, considered to be the longest-lasting mechanism for subsurface storage of anthropogenic CO₂, harnesses the reactivity of ultramafic rocks with CO₂-rich fluids to form magnesium carbonates. In my thesis, I study natural analogues of mineral carbon sequestration with the objective of determining the geologic parameters that govern magnesium carbonate mineralization in ultramafic rocks. This objective is reached by utilizing (1) equations of state of H₂O-CO₂ fluid mixtures; (2) hydrogen, carbon, oxygen, and neodymium isotope systematics; and (3) clumped-isotope thermometry to determine the tectonic and geochemical parameters related to the emplacement of magnesite (MgCO₃) vein ores in peridotite of the Del Puerto ophiolite within the California Coast Ranges. My study of the transformations of CO₂-rich fluids to magnesium carbonates through interaction with ultramafic rocks has applications to other worldwide magnesium carbonate deposits, provides constraints on CO₂ sequestration in ultramafic rocks, and illuminates fundamental questions regarding geochemical pathways in weathering, hydrothermal, and mineral ore systems.