The rate of natural carbonation of tectonically exposed mantle peridotite during weathering and low-temperature alteration can be enhanced to develop a significant sink for atmospheric CO 2. Natural carbonation of peridotite in the Samail ophiolite, an uplifted slice of oceanic crust and upper mantle in the Sultanate of Oman, is surprisingly rapid. Carbonate veins in mantle peridotite in Oman have an average 14 C age of Ϸ26,000 years, and are not 30 -95 million years old as previously believed. These data and reconnaissance mapping show that Ϸ10 4 to 10 5 tons per year of atmospheric CO 2 are converted to solid carbonate minerals via peridotite weathering in Oman. Peridotite carbonation can be accelerated via drilling, hydraulic fracture, input of purified CO 2 at elevated pressure, and, in particular, increased temperature at depth. After an initial heating step, CO 2 pumped at 25 or 30°C can be heated by exothermic carbonation reactions that sustain high temperature and rapid reaction rates at depth with little expenditure of energy. In situ carbonation of peridotite could consume >1 billion tons of CO 2 per year in Oman alone, affording a low-cost, safe, and permanent method to capture and store atmospheric CO 2.alteration and weathering ͉ carbon capture ͉ exothermic ͉ carbon sequestration ͉ mineral R ecognition that anthropogenic CO 2 input to the atmosphere has substantially increased atmospheric CO 2 concentration, and that increased CO 2 may drive rapid global warming, has focused attention on carbon capture and storage (1). One storage option is conversion of CO 2 gas to stable, solid carbonate minerals such as calcite (CaCO 3 ) and magnesite (MgCO 3 ) (2). Natural carbonation of peridotite by weathering and lowtemperature alteration is common. Enhanced natural processes in situ may provide an important, hitherto neglected alternative to ex situ mineral carbonation ''at the smokestack.'' In this article, we evaluate the rate of natural carbonation of mantle peridotite in the Samail ophiolite, Sultanate of Oman, and then show that under certain circumstances exothermic peridotite alteration (serpentinization, carbonation) can sustain high temperature and rapid reaction with carbonation up to 1 million times faster than natural rates, potentially consuming billions of tons of atmospheric CO 2 per year. In situ mineral carbonation for CO 2 storage should be evaluated as an alternative to ex situ methods, because it exploits the chemical potential energy inherent in tectonic exposure of mantle peridotite at the Earth's surface, does not require extensive transport and treatment of solid reactants, and requires less energy for maintaining optimal temperature and pressure.Tectonically exposed peridotite from the Earth's upper mantle, and its hydrous alteration product serpentinite, have been considered promising reactants for conversion of atmospheric CO 2 to solid carbonate (3). However, engineered techniques for ex situ mineral carbonation have many challenges. Kinetics is slow unless olivine and serpentine ...

No abstract

CO 2 capture and storage (CCS) has the potential to develop into an important tool to address climate change. Given society’s present reliance on fossil fuels, widespread adoption of CCS appears indispensable for meeting stringent climate targets. We argue that for conventional CCS to become a successful climate mitigation technology—which by necessity has to operate on a large scale—it may need to be complemented with air capture, removing CO 2 directly from the atmosphere. Air capture of CO 2 could act as insurance against CO 2 leaking from storage and furthermore may provide an option for dealing with emissions from mobile dispersed sources such as automobiles and airplanes.

The Samail ophiolite in Oman is undergoing modern hydration and carbonation of peridotite and may host a deep subsurface biosphere. Previous investigations of hyperalkaline fluids in Oman have focused on fluids released at surface seeps, which quickly lose their reducing character and precipitate carbonates upon contact with the O 2 /CO 2-rich atmosphere. In this work, geochemical analysis of rocks and fluids from the subsurface provides new insights into the operative reactions in serpentinizing aquifers. Serpentinite rock and hyperalkaline fluids (pH >10), which exhibit millimolar concentrations of Ca 2+ , H 2 and CH 4, as well as variable sulfate and nitrate, were accessed from wells situated in mantle peridotite near Ibra and studied to investigate their aqueous geochemistry, gas concentrations, isotopic signatures, mineralogy, Fe speciation and microbial community composition. The bulk mineralogy of drill cuttings is dominated by olivine, pyroxene, brucite, serpentine and magnetite. At depth, Fe-bearing brucite is commonly intermixed with serpentine, whereas near the surface, olivine and brucite are lost and increased magnetite and serpentine is detected. Micro-Raman spectroscopy reveals at least two distinct generations of serpentine present in drill cuttings recovered from several depths from two wells. Fe K-edge x-ray absorption near-edge spectroscopy (XANES) analysis of the lizardite shows a strong tetrahedral Fe coordination, suggesting a mixture of both Fe(II) and Fe(III) in the serpentine. Magnetite veins are also closely associated with this second generation serpentine, and 2-10µm magnetite grains overprint all minerals in the drill cuttings. Thus we propose that the dissolved H 2 that accumulates in the subsurface hyperalkaline fluids was evolved through low temperature oxidation and hydration of relict olivine, as well as destabilization of pre-existing brucite present in the partially serpentinized dunites and harzburgites. In particular, we hypothesize that Fe-bearing brucite is currently reacting with dissolved silica in the aquifer fluids to generate late-stage magnetite, additional serpentine and dissolved H 2. Dissolved CH 4 in the fluids exhibits the most isotopically heavy carbon in CH 4 reported in the literature thus far. The CH 4 may have formed through abiotic reduction of dissolved CO 2 or through biogenic pathways under extreme carbon limitation. The methane isotopic composition may have also been modified by significant methane oxidation. 16S rRNA sequencing of DNA recovered from filtered hyperalkaline well fluids reveals an abundance of Meiothermus, Thermodesulfovibrionaceae (sulfate-reducers) and Clostridia (fermenters). The fluids also contain candidate phyla OP1 and OD1, as well as Methanobacterium (methanogen) and Methylococcus sp. (methanotroph). The composition of these microbial communities suggests that low-temperature hydrogen and methane generation, coupled with the presence of electron acceptors such as nitrate and sulfate, sustains subsurface microbial life within the O...

No abstract

5Carbonate formation at hyperalkaline springs is typical of serpentinization in 6 peridotite massifs worldwide. These travertines have long been known to exhibit large 7 variations in their carbon and oxygen isotope compositions, extending from apparent 8 equilibrium values to highly depleted values. However, the exact causes of these 9 variations are not well constrained. We analyzed a suite of well-characterized fresh 10 carbonate precipitates and travertines associated with hyperalkaline springs in the 11 peridotite section of the Samail ophiolite, Sultanate of Oman, and found their clumped 12 isotope compositions vary systematically with formation environments. Based on these 13 findings, we identified four main processes controlling the stable isotope compositions of 14 these carbonates. These include hydroxylation of CO2, partial isotope equilibration of 15 dissolved inorganic carbon, mixing between isotopically distinct carbonate end-members, 16 and post-depositional recrystallization. Most notably, in fresh crystalline films on the 17 surface of hyperalkaline springs and in some fresh carbonate precipitates from the bottom 18 of hyperalkaline pools, we observed large enrichments in Δ47 (up to ~0.2‰ above 19 expected equilibrium values) which accompany depletions in δ 18 O and δ 13 C, yielding 20 about 0.01‰ increase in Δ47 and 1.1‰ decrease in δ 13 C for every 1‰ decrease in δ 18 O, 21 relative to expected equilibrium values. This disequilibrium trend, also reflected in 22 preserved travertines ranging in age from modern to ~40,000 years old, is interpreted to 23 arise mainly from the isotope effects associated with the hydroxylation of CO2 in high-24 pH fluids and agrees quantitatively with our theoretical prediction. In addition, in some 25 fresh carbonate precipitates from the bottom of hyperalkaline pools and in subsamples of 26 one preserved travertine terrace, we observed additional enrichments in Δ47 at 27 intermediate δ 13 C and δ 18 O, consistent with mixing between isotopically distinct 28 carbonate end-members. Our results suggest that carbonate clumped isotope analysis can 29 be a valuable tool for identifying and distinguishing processes not readily apparent from 30 the carbonate bulk stable isotope compositions alone, e.g., kinetic effects or mixing of 31 different carbonate end-members, which can significantly alter both the apparent 32 formation temperatures and apparent radiocarbon ages. The isotope trends observed in 33 these travertine samples could be applied more broadly to identify extinct hyperalkaline 34 springs in terrestrial and extraterrestrial environments, to better constrain the formation 35 conditions and post-depositional alteration of hyperalkaline spring carbonates, and to 36 extract potential paleoclimate information. 37 38

CO2 sequestration in the form of carbonate minerals via alteration of oceanic crust and upper mantle is an important part of the global carbon cycle, but the annual rate of CO2 mineralization is not well quantified. This study aimed to constrain groundwater ages within the Samail ophiolite, Sultanate of Oman. Such ages could provide upper bounds on the time required for ongoing low temperature CO2 mineralization. While we were able to estimate apparent groundwater ages for modern waters, results from hyperalkaline boreholes and springs were disappointing. Waters from boreholes and hyperalkaline springs within the ophiolite were characterized using multiple environmental tracers including tritium (3 H), noble gases (He, 4 He, Ne, Ar, Kr, Xe), stable isotopes (δ 18 O, δ 2 H), and chemical parameters (pH, Ca, Mg, DIC, etc.). Shallow peridotite groundwater and samples from boreholes near the mantle transition zone have a pH < 9.3, are 4-40 years old, have little to no non-atmospheric He accumulation, NGTs (noble gas temperatures) equivalent to the modern mean annual ground temperature, and stable isotopes within the range of current local precipitation. In contrast, hyperalkaline springs and deeper samples from peridotite boreholes have pH > 10, are pre-H-bomb (older than 1952), have significant non-atmospheric helium accumulation (30-70% of dissolved helium), often are isotopically heavier (enriched in δ 18 O), and can have NGTs 6-7 o C lower than the modern ground temperature. These differences suggest that groundwater in deep (> 50 m) peridotite aquifers is considerably older than shallow groundwater in peridotite and water in deeper aquifers near the mantle transition zone. Unfortunately, how much older remains an open question. The low NGT of groundwater from one deep (300 m) peridotite borehole indicates it is probably glacial in origin. If so, it must date back to at least the late Pleistocene, the most recent glacial period; He accumulation suggests it could be from 20-220 ka. The inefficacy of this suite of environmental tracers to quantitatively estimate apparent groundwater age for hyperalkaline fluids necessitates the use of different techniques. Future work to constrain groundwater ages should utilize a packer system to isolate discrete depth intervals within boreholes and less common environmental tracers such as 39 Ar and 81 Kr.

No abstract