Petrologic Study of Peridotite Carbonation in Oman: Temperature, Timing and Fluid Composition

Columbia University, New York NY

Investigators

Abstract

This project is a study of hydrothermal alteration and weathering of large, subaerial peridotite massifs that form the mantle section of the Samail ophiolite in Oman. Peridotite is a rock comprised mainly of the mineral olivine (gemstone name, 'peridot'), and it comprises most of the Earth's upper mantle but is normally shielded from reaction with surface rocks by the oceanic and continental crust. Plate tectonic collisions coupled with erosion expose peridotite at the surface, where it reacts rapidly with surface waters. It is proposed to investigate formation of hydrated minerals and solid carbonate minerals via reaction of peridotite with near-surface waters in (a) active, ongoing low-T systems, probably at 30 to 60°C and involving meteoric water, and (b) older, fossil high-T systems which may have reached ~ 200°C, formed completely carbonated peridotites - in which all Mg and Ca are in solid carbonate minerals - called listwanites. The high-T system may have involved fluids from carbonate bearing metasediments beneath the ophiolite. They will constrain the temperatures, timing, and fluid composition of both the active low-T systems and fossil high-T systems through chemical and isotopic analysis. The goal is to constrain factors that control efficient carbonation of peridotite, expand thermodynamic models of phase equilibria incorporating solid solution models, and provide cross-calibration of isotope thermometers. It is hypothesized that complete carbonation of peridotite occurred at pressures, temperatures and fluid compositions close to the conditions at which olivine carbonation rates are maximized, and that rapid rates facilitate a positive feedback between volume change, reaction-induced cracking, permeability, and reaction rate. A key step in testing this hypothesis is to confirm preliminary inferences by the group about the conditions of peridotite carbonation. One might expect decreasing permeability and armoring of reactive surfaces to limit the extent of reaction, but natural peridotite carbonation can and does go to completion, in which all Mg and Ca (and much of the Fe) are incorporated in carbonate minerals. Similarly, addition of H2O commonly produces 100% hydration of Mg in large volumes of rock. This study will characterize the chemical and physical processes that lead to rapid, extensive peridotite hydration and carbonation. Reaction of CO2 from ground water or seawater with peridotite forms abundant carbonate minerals, in processes that are driven by a vast reservoir of available, chemical potential energy. Carbonation of the abundant mineral, olivine, is faster than for other abundant, rock-forming minerals. Recently, several papers have emphasized the potential for in situ carbonation of peridotite, either via reaction with injected, CO2-rich fluids, or simply via enhanced reaction with surface-derived seawater. Optimal peridotite carbonation conditions could yield CO2 uptake of ~ 1 billion tons/km3 of peridotite/yr, and could provide an enormous reservoir for up to ~ 100 trillion tons of CO2. Results of this study will facilitate future design of engineered, in situ techniques for in situ geological CO2 capture and storage (CCS). In particular, we need to learn how natural peridotite carbonation processes avoid potential limitations due to decreasing permeability and loss of reactive surface area during precipitation of carbonate minerals in pore space. If these negative feedbacks can be overcome in an engineered system, in situ storage in solid carbonate minerals may be cost competitive with more conventional injection of CO2 into subsurface pore space for CCS. In situ mineral carbonation poses fewer property problems and leakage hazards than injection of fluid into pore space, providing stable, inert, non-toxic storage.