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.
We studied natural uptake of water and CO2 by reaction between rocks from the Earth’s deep interior – the mantle – with surface water and the atmosphere. Mantle rocks ("peridotite") are thrust toward the surface during the collision of tectonic plates, and then exposed by erosion. Peridotite is far from equilibrium with water and CO2, and reacts readily to form carbonate minerals and hydrous silicate minerals. We focused on the world’s largest exposure of peridotite, in a block of oceanic crust and mantle – the Samail ophiolite – that was thrust onto the Arabian continental margin about 100 million years ago. Our goal was to determine the pressure, temperature and physical conditions conducive to hydration and carbonation of peridotite, with the secondary aim of emulating natural systems to design engineered methods for CO2 removal from air and surface waters, and for CO2 storage in subsurface, solid carbonate minerals. We found that ongoing reaction of peridotite in Oman, at 20 to 60°C and depths of one or two kilometers, forms subsurface carbonate veins plus large carbonate terraces on the surface. Mineral carbonation in Oman takes up 10,000 to 100,000 tons of CO2 per year, in a process that has continued for at least 50,000 years. A significant finding for metamorphic petrologists is that this low temperature reaction forms intergrown chrysotile serpentine + quartz instead of talc. We also found that a similar, higher temperature process occurred at 80 to 120°C and 10 to 20 kilometers depth about 100 million years ago, as the peridotite was thrust onto the Arabian continent. Metamorphic petrologists will be interested to know that this formed antigorite serpentine + quartz ± talc in a narrow reaction zone. CO2 was dissolved from underthrust sediments in aqueous fluids, that rose into the overlying peridotite. Reaction of these fluids with peridotite led to full carbonation, in which every magnesium and calcium atom in the peridotite combined with CO2 to form carbonate minerals, plus quartz. This observation is important because full, subsurface carbonation is a goal for engineered CO2 storage mechanisms. One 5 km long ridge in Oman, where fully carbonated peridotite is exposed, contains more than one billion tons of CO2 in solid carbonate minerals. The observation that CO2 was transferred from underthrust sediments into overlying mantle peridotite inspired us to review the carbon cycle in subduction zones, where oceanic crust and sediments are thrust into the mantle beneath continental plates. We compiled data on fluxes of carbon into and out of subduction zones. We also made new calculations of (a) solubility of carbon in water-rich fluids at the high pressure conditions of subduction zones and (b) formation of diapirs – like rising blobs in a lava lamp – from buoyant sediments that, when they get hot enough, ascend through the mantle to the base of the overlying plate. We found that the amount of CO2 extracted from subducting sediments and crust and returned to the overlying plate may be approximately equal to the amount of CO2 that is subducting. If so, then outgassing of CO2 from volcanoes is not balanced by recycling into the Earth’s deep interior, and the carbon content of the surface- the plates, oceans and atmosphere – must have been rising for much of Earth history. Addition of H2O and CO2 to solid minerals causes increases in volume. This can have two outcomes. Newly formed hydrous silicates can clog pore space, stop fluid flow and armor reactive mineral surfaces, causing hydration and carbonation to cease. Alternatively, volume changes can cause differential pressure and fracturing, allowing continued access of fluids to reactive surface area, in a positive feedback mechanism achieving full carbonation. We call this "reaction-driven cracking". Reaction-driven cracking could be important for geothermal power generation, solution-mining of important commodities like Uranium, and/or extraction of oil and gas from low permeability reservoirs, as well as CO2 capture and storage. In other settings, such as cap rocks overlying storage reservoirs for CO2 liquid or nuclear waste, it is important to avoid reaction-driven cracking. However, this process is poorly understood and has received little study until now. Our current research is delineating the conditions that favor or suppress reaction-driven cracking, via field work, laboratory experiments, theoretical calculations, and numerical modeling. Thermodynamic data and fracture densities that the chemical potential energy inherent in far-from-equilibrium peridotite + H2O or peridotite + CO2 if converted to mechanical energy can and does generate thousands of atmospheres of differential pressure, more than sufficient to break rocks. Continuing these investigations is the main focus of our ongoing work. We also calibrated a new clumped 13C-18O-16O thermometer for the carbonate mineral magnesite.
