Magnesium and calcium carbonate precipitation in serpentinite-hosted alkaline environments: natural and experimental constraints

Abstract

In continental settings, the interaction ofmeteoric water with ultramafic rocks generates waters of variable physicochemical characteristics owing to serpentinization and weathering. The discharge of these waters forms aerial alkaline to hyperalkaline spring systems where waters mix, undergo evaporation, and take up atmospheric CO2, leading to the formation of carbonate minerals. The understanding of natural carbonation taking place in such serpentinite-hosted alkaline environments is critical for assessing the role of this potentially significant sink in the global carbon cycle, and the viability of CO2 sequestration techniques for safe carbon storage. Serpentinization-driven, alkaline environments provide critical insights into the natural conditions regarding the capture of atmospheric carbon dioxide through carbon mineralization. The main objective of this Ph.D. thesis is to advance our understanding of serpentinization-related alkaline spring systems and the associated precipitation of carbonate minerals under alkaline conditions. To contribute to this main research goal, the present Ph.D. thesis aims to (i) provide an additional account of how water composition, mixing, and mineral precipitation and textures co-evolve in serpentinization-driven alkaline spring systems in ophiolites, (ii) investigate alkaline spring sites in subcontinental mantle peridotites and associated mineralizations formed by the interaction between hyperalkaline fluids and river waters, and (ii) experimentally investigate the crystallization sequence and morphologies of hydrated magnesium carbonates, and define the conditions under which their nucleation, crystal growth, and transformation take place. These aims have been addressed through the study of natural alkaline springs hosted in exposed oceanic (Samail Ophiolite, Oman) and subcontinental mantle serpentinized peridotites (Ronda peridotites, Spain), and through carbonate crystallization experiments, to fill gaps in our current knowledge on the mechanisms and the conditions characterizing carbonate mineral precipitation in such systems.