The formation of carbonate minerals during alteration of ultramafic rocks represents a geological analogue of mineral carbon sequestration. In the Oman Ophiolite, these carbonation reactions are manifested in (1) active, on-going low-temperature systems involving meteoric water, which result in serpentinization, carbonate vein formation, and travertine precipitation at alkaline springs, and (2) older, higher- temperature systems, which resulted in completely carbonated peridotite, known as listvenite. Employing electron microprobe analysis, x-ray diffraction, stable and clumped isotope thermometry, Sr isotope geochemistry, and geochemical modeling, this study seeks to constrain the conditions under which natural carbonation has occurred in the Oman ophiolite, with the broader goal of understanding what factors and feedbacks control efficient carbonation of peridotite. Near low-temperature alkaline springs emanating from peridotite in Oman, networks of young carbonate veins are prevalent in highly serpentinized peridotite. A notable feature in some carbonate-veined serpentinite samples is the coexistence of Fe- rich serpentine and quartz. At a given pressure, the formation of iron-rich serpentine at the expense of magnetite should be favored at lower temperatures. Calculations of thermodynamic equilibria in the MgO-SiO2-H2O-CO2 system show that serpentine + quartz is stable assemblage at sufficiently low temperatures (e.g., less than ~15-50°C), and is stabilized to higher temperatures by preferential cation substitutions in serpentine over talc. Thus, the observed serpentine + quartz assemblages could result from serpentinization at near-surface temperatures. Clumped isotope thermometry of carbonate veins yields temperatures within error of the observed temperatures in Oman groundwater, while the !18O of water calculated to be in equilibrium with carbonate precipitated at those temperatures is within error of the observed isotopic composition of Oman groundwater. As groundwater geochemistry suggests that carbonate precipitation and serpentinization occur concomitantly, this indicates that both hydration and carbonation of peridotite are able to produce extensive alteration at the relatively low temperatures of the near-surface weathering environment in Oman.

The formation of carbonate minerals during alteration of ultramafic rocks represents a geological analogue of mineral carbon sequestration. In the Oman Ophiolite, these carbonation reactions are manifested in (1) active, on-going low-temperature systems involving meteoric water, which result in serp...