Weathering is a natural geological process whereby atmospheric CO2 dissolved in rainwater attacks rocks, partly dissolving them. The CO2 is converted into alkalinity or carbonate minerals that securely store carbon on timescales of >10,000 years. Modelling studies show that if weathering rates can be increased (by selecting the most reactive rocks, increasing reactive surface area), up to an additional 2 Gt CO2 yr-1 could be removed from the atmosphere, ~40% of the amount required by 2100 to meet the Paris Agreement target.The mining industry extracts gigatonnes of rock each year, generating large amounts of freshly exposed, reactive surface area that could be used as a feedstock for enhanced weathering. Ore deposits with the highest CO2 removal capacity are those mined in high quantities that have an abundance of calcium- and magnesium-bearing silicate minerals [1]. Here, we report the results of an investigation into the reactivity of serpentinised peridotite samples  from the Sakatti Cu-Ni-PGE deposit (Finland). The deposit consists of both disseminated and massive sulphides hosted within a large olivine-cumulate  body [2]. The material consists primarily of serpentine [Mg3Si2O5(OH)4] and olivine [Mg2SiO4] minerals that have a high potential for CO2 removal via enhanced weathering. We have conducted a series of laboratory experiments whereby crushed peridotite samples were reacted with CO2-enriched solutions at 25 °C and 50 °C, and 1 bar CO2 (100% CO2).  Surface area normalised dissolution rates were determined for various grain sizes,  temperatures, mineral compositions (degree of serpentinization) and solution chemistry (with/without a chelating agent).

This paper provides an overview of research on core from Oman Drilling Project Hole BT1B and the surrounding area, plus new data and calculations, constraining processes in the Tethyan subduction zone beneath the Samail ophiolite. The area is underlain by gently dipping, broadly folded layers of allochthonous Hawasina pelagic sediments, the metamorphic sole of the Samail ophiolite, and Banded Unit peridotites at the base of the Samail mantle section. Despite reactivation of some faults during uplift of the Jebel Akdar and Saih Hatat domes, the area preserves the tectonic “stratigraphy” of the Cretaceous subduction zone. Gently dipping listvenite bands, parallel to peridotite banding and to contacts between the peridotite and the metamorphic sole, replace peridotite at and near the basal thrust. Listvenites formed at less than 200°C and (poorly constrained) depths of 25 to 40 km by reaction with CO2-rich, aqueous fluids migrating from greater depths, derived from devolatilization of subducting sediments analogous to clastic sediments in the Hawasina Formation, at 400-500°. Such processes could form important reservoirs for subducted CO2. Listvenite formation was accompanied by ductile deformation of serpentinites and listvenites – perhaps facilitated by fluid-rock reaction – in a process that could lead to aseismic subduction in some regions. Addition of H2O and CO2 to the mantle wedge, forming serpentinites and listvenites, caused large increases in the solid mass and volume of the rocks. This may have been accommodated by fractures formed as a result of volume changes, perhaps mainly at a serpentinization front.

The transition from the gabbroic oceanic crust to the residual mantle harzburgites of the Oman ophiolite has been drilled at Holes CM1A and CM2B (Wadi Tayin massif) during Phase 2 of the International Continental Scientific Drilling Program (ICDP) Oman Drilling Project (OmanDP) (Nov. 2017-Jan. 2018). In order to unravel the formation processes of ultramafic rocks in the Wadi Tayin massif (CM) crust-mantle transition zone and deeper in the mantle sections beneath oceanic spreading centers, our study focuses on the whole rock major and trace element compositions (together with CO2 and H2O concentrations) of these ultramafic rocks (56 dunites and 49 harzburgites). Despite extensive serpentinization and some carbonation, most of the trace element contents (REE, HFSE, Ti, Th, U) record high temperature, magmatic process-related signatures. Two major trends are observed, with good correlations between (1) Th and U, Nb and LREE on one hand, and between (2) HREE, Ti and Hf on the other hand. We interpret the first trend as the signature of late melt/peridotite interactions as LREE are known to be mobilized by such processes (‘lithospheric process’), and the second trend as the signature of the initial mantle partial melting (‘asthenospheric process’), with little or no overprint from melt/rock reaction events.

The Oman Drilling Project “Multi-Borehole Observatory” (MBO) samples an area of active weathering of tectonically exposed peridotite. This paper reviews the geology of the MBO region, summarizes recent research, and provides new data constraining ongoing alteration. Host rocks are partially to completely serpentinized, residual mantle harzburgites and replacive. Dunites show evidence for “reactive fractionation”, in which cooling, crystallizing magmas reacted with older residues of melting. Harzburgites and dunites are 65-100% hydrated. Ferric to total iron ratios vary from 50 to 90%. In Hole BA1B, alteration extent decreases with depth. Gradients in water and core composition are correlated. Serpentine veins are intergrown with, and cut, carbonate veins with measurable 14C. Ongoing hydration is accompanied by SiO2 addition. Sulfur enrichment in Hole BA1B may result from oxidative leaching of sulfur from the upper 30 m, coupled with sulfate reduction and sulfide precipitation at 30-150 m. Oxygen fugacity deep in Holes BA3A, NSHQ14 and BA2A is fixed by the reaction 2H2O = 2H2 + O2 combined with oxidation of ferrous iron in serpentine, brucite and olivine. fO2 deep in Holes BA1A, BA1D and BA4A is 3-4 log units above the H2O-H2 limit, controlled by equilibria involving serpentine and brucite. Variations in alteration are correlated with texture, with reduced, low SiO2 assemblages in mesh cores recording very low water/rock ratios, juxtaposed with adjacent veins recording much higher ratios. The proportion of reduced mesh cores vs oxidized veins increases with depth, and the difference in fO2 recorded in cores and veins decreases with depth.

The transition from the Semail ophiolite mantle to the underlying metamorphic sole was drilled at ICDP OmanDP Hole BT1B. We analyzed the bulk major, volatile and trace element compositions of the mantle-derived listvenite series and metamorphic rocks, with the aim to constrain chemical transfers associated to peridotite carbonation along the ophiolite basal thrust. The listvenite series comprise variously carbonated serpentinites and (fuchsite-bearing) listvenites. They have high CO2 (up to 43.2 wt.%) and variable H2O (0-12.1 wt.%). Yet, they have compositions close to that of the basal banded peridotites for most major and lithophile trace elements, with fuchsite-bearing listvenites overlapping in composition with amphibole-bearing basal lherzolites (e.g., Al2O3= 0.1-2.2 wt.%; Yb= 0.05-1 x CI-chondrite), The protolith of the listvenite series was likely similar in structure and composition to serpentinized banded peridotites which immediately overlie the metamorphic sole elsewhere in Oman. The listvenite series are enriched in fluid mobile elements (FME) compared to Semail peridotites (up to ~103-104 x Primitive Mantle), with concentrations similar to the underthrusted metabasalts and/or metasediments for Cs, Sr and Ca and sometimes even higher for Pb, Li, As, and Sb (e.g., Li up to 130 ppm; As up to 170 ppm). We also observe a decoupling between Sr-Ca enrichments and other FME, indicating interactions with several batches of deep CO2-rich fluids transported along the basal thrust. These results suggest that peridotite carbonation could represent one of the major trap-and-release mechanisms for carbon, water and FME along convergent margins.