Ongoing carbon mineralization and peridotite alteration at the Oman Drilling Projects Multi-Borehole Observatory

Studies of weathering and alteration inform design of engineered systems for CO2 management via carbon mineralization. We summarize relevant results from the Wadi Tayin massif of the Samail ophiolite, in Oman. 3 cored and 4 rotary boreholes 330 to 400 m deep were sited near older observation well NSHQ14. Core was described onboard DV Chikyu. Results are reported at [1] and papers submitted to the JGR Special Issue on Ophiolites and Oceanic Lithosphere [2-7 with preprints available at ESSOAr]. Carbonate veins in core are restricted to the upper 100 m, mostly the upper 30 m. There is measurable 14C in 2/3 of carbonate veins from initial sampling [3], with a more detailed textural and isotopic study [7] revealing several generations. Distinctive waxy serpentine veins are intergrown with and locally cut 14C-bearing carbonate veins. Waxy veins comprise a few percent of all cores. Inferred rates are ~ 1%/10kyr, or 3E-14/s, in the upper 30 m for carbon, and over 400 m for serpentinization. Approximate fO2 estimated from Eh, pH and temperature in borehole waters is within error of the H2O-H2 limit in BA2A, BA3A and NSHQ14; other Holes approach constant fO2 ~ 4 log units higher. Silicate, oxide, sulfide and metal assemblages in core record similarly low fO2. These observations indicate ongoing (a) conversion of brucite to serpentine via reaction with relatively SiO2-rich waters, (b) oxidation of Fe in brucite and serpentine solid solutions around all Holes, and (c) H2 production and perhaps magnetite formation around the most reduced Holes. These processes are accompanied by biological methanogenesis, limiting carbon storage. Dissolved H2 in deep waters is 10% of saturation, perhaps due to drilling disturbance or periodic convection and degassing. Sulfide-enriched core from 30 to 150 m in BA1B could form via supergene enrichment, with oxidative leaching of S from shallower depths (including rocks now eroded away), coupled with precipitation in reduced conditions at greater depth. Sulfate-reducing bacteria are among the microbes detected in borehole samples. 1 http://publications.iodp.org/other/Oman/OmanDP.html 2 Eliison ea JGR 126 2021 3 Kelemen ea JGR 2021 submitted 4 & 5 Nothaft ea JGR 202 a & b submitted 6 Templeton ea JGR 2021 submitted 7 Ternieten ea JGR 2021 submitted