Abstract
Dolomite is common in sedimentary sequences yet its formation is ambiguous. This 'dolomite problem' is comprised of four aspects that remain problematic; 1) occurrence in a variety of sedimentary environments; 2) rarity in modern settings; 3) petrographic data enabling multiple interpretations; 4) difficulty of laboratory synthesis at near-surface conditions. This study addresses these issues through a combination of modelling and field work.Assuming calcite-dolomite equilibrium a new dolomite solubility constant (valid 0 – 200°C) is developed by evaluating log₁₀(ᵃCa²⁺/ᵃMg²⁺) values of US subsurface fluids. This assumption appears valid as >90% of fluids appear to be at calcite-dolomite equilibrium suggesting ubuiqity of these minerals in the subsurface. Variations in log₁₀(ᵃCa²⁺/ᵃMg²⁺) values appear to correlate to changes in dolomite compositions. Modelling the carbon capture and storage potentials of these subsurface US fluids we develop strategies to enhance cap rock integrity and optimize injection.
The source of the Mg-rich fluids for dolomitization remains enigmatic. Basinal fluids are commonly invoked yet the aforementioned meta-analysis demonstrates that these fluids are at dolomite equilibrium and thus have limited dolomitizing potential. We present and test a new conceptual model for generating Mg-rich fluids through the carbonation of ultramafic rocks, forming what are known as listwanites. This process has the potential to generate both high-temperature dolomitesin precursor limestones and, through fluids escaping into lake basins, contribute to formation of Mg-clays and dolomites. Lacustrine carbonates in the South Atlantic pre-salt sequences overly ultramafic rocks but the formation of these minerals has remained poorly understood. This model is field tested in Atlin, British Columbia (Canada) where a listwanite, forming at the base of a thick ultramafic section of ophiolite, is in contact with a fault breccia with diagenetic reactions correlated between the two units suggesting that Mg is released during listwanitization.
| Date of Award | 25 Jan 2022 |
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| Original language | English |
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| Supervisor | Fiona F Whitaker (Supervisor), Maurice E Tucker (Supervisor), Cathy Hollis (Supervisor) & Hilary J Corlett (Supervisor) |