Title: Carbonation of Peridotite in The Oman Ophiolite
Description: 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℃), 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 d18O 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.

Along some locations near the basal thrust of the ophiolite, hydrothermal alteration of peridotite in the Samail Ophiolite of Oman has resulted in the formation of listvenite, characterized by complete carbonation, in which all of the Mg and much of the Fe has been incorporated into carbonate minerals, resulting in a rock composed primarily of magnesite (and/or dolomite where Ca has been added) + quartz. Mineral parageneses and clumped isotope data from magnesite and dolomite suggest that carbonate phases within the listvenite formed at peak temperatures ~100℃. CO2-enriched fluids were likely derived from underlying calcite-bearing sediment during emplacement of the ophiolite. Initial 87Sr/86Sr values in the listvenite vary from 0.7085 to 0.7135, mostly significantly higher than seawater values, and are consistent with values within the underlying allochthonous and autochthonous metasediments. An internal Rb- Sr isochron from one listvenite sample yields an age of 97 ± 29 Ma, consistent with the timing of emplacement of the ophiolite.

Release of pore fluid during compaction of subducted sediments may result in similar carbonation of peridotite in the shallow hanging wall of other subduction/obduction environments. These natural systems demonstrate that significant carbonation of peridotite may occur even at low temperatures, but can be much more efficient at higher temperatures. Furthermore, complete carbonation of peridotite may be achieved, in spite of the potential for armoring of reactive surfaces and reduction of permeability, as demonstrated by the formation listvenite. These natural processes of hydrothermal alteration and weathering could potentially be accelerated to provide a permanent storage solution for the disposal of CO2 via the in situ formation of solid carbonate minerals in peridotite.