Carbonate precipitation during anoxic water-rock interaction, with implications for the formation of the Martian carbonates

Thesis

Despite over a Martian decade (20 years) of surface-based observations, reconciling Mars’ water- altered landscapes with a detailed reconstruction of its former atmosphere has remained problematic. Carbon dioxide is widely considered the main early atmospheric constituent based on expected outgassing behaviour and observed atmospheric isotope ratios, yet carbonate minerals — the expected sinks for this carbon — are only abundant in scarce isolated localities across the surface. While orbital spectroscopic investigations continue to yield an increasing inventory of these deposits, their relative rarity has prompted suggestions that the Noachian atmosphere was commonly characterised by low pCO2. However, without a thorough investigation of how exactly carbonates form under Mars-like conditions, their absence cannot be definitively linked to the absence of an atmosphere. To supply this key information, and to answer the more fundamental issue of carbonate precipitation mechanisms under anoxic, aqueous conditions, this thesis examines the controls on Fe(II)-carbonate precipitation. Anoxic experiments were conducted that varied initial pH, rock:water ratio, and pCO2. These were run with the minerals fayalite, a combination of fayalite/forsterite (Chapter 3), followed by rock common to Mars (basalt; Chapter 4). In these experiments no substantive Fe(II)-carbonate precipitation occurred despite significant supersaturation with respect to siderite. Reaction path models indicate Fe(II)-carbonate supersaturation thresholds would have been commonly met in low rock:water ratio systems within the subsurface as opposed to high-pCO2, high water-rock systems exposed at the near-surface. This suggests that the rarity of Martian carbonates may be partly controlled by a lack of deep crustal exposures, rather than insufficient atmospheric pCO2. Later, at hydrothermal temperatures of 75 °C (Chapter 5), siderite formed in appreciable quantities as rhombs and hexagonal platelets. This is consistent with carbonate formation in a warmer, sheltered subsurface. Together, the experiments within this thesis support wider orbital data by other researchers suggesting that carbonates may comprise a large component of the deep crust, potentially representing a geologically significant carbon sink and offering a suggested fate for a once substantial (~1 bar) atmosphere.

Authors: Kissick, LE

• DOI: 10.5287/ora-eq6dwgryo
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Noachian; Mars; fluid-rock interaction; carbonate; olivine; geochemistry
• Subjects: Planetary science