POTENTIAL FOR CARBON MINERALIZATION AND CRITICAL MINERAL RECOVERY IN THE TWIN SISTERS DUNITE, NORTHWESTERN WASHINGTON, UNITED STATES

Geologic carbon storage via in situ mineralization offers an opportunity to permanently sequester atmospheric CO₂ in the subsurface as carbonate minerals, resulting in the reduction in atmospheric CO₂ to mitigate climate change. In addition to basalt provinces, ultramafic rocks offer opportunities for carbon mineralization. Ultramafic rocks are enriched in critical minerals, like nickel, and may offer opportunities for integrated carbon mineralization and critical mineral recovery processes (Wang & Dreisinger, 2022). The Twin Sisters Dunite is an ultramafic body in NW Washington comprised of olivine, orthopyroxene, and clinopyroxene (Onyeagocha, 1973), all minerals that can react with CO₂ to form carbonate minerals. We investigate the potential of the Twin Sisters Dunite for coupled carbon mineralization and critical mineral recovery processes. The sample is 91.4% forsterite olivine, 7.0% enstatite, 1.5% chrysotile, and 0.1% chromite. The olivine is forsterite-rich (Mg# 85-93). Nickel in olivine content ranges from 3133-5166 ppm. This forms the groundwork for dissolution and carbonation experiments to evaluate the Twin Sisters Dunite as a resource for carbon mineralization and critical mineral recovery. In the dissolution experiments, olivine is reacted with an aqueous fluid containing various additives for 7 days. Multiple fluid samples are taken throughout to evaluate which additives increase olivine dissolution at pH = 4, 90°C. Results from these dissolution experiments will be utilized to fine tune additives for carbonation experiments and developing dissolution kinetics which can parameterize reactive transport reservoir models. In carbonation experiments, olivine is reacted with H₂O, additives, and CO₂ at 90°C and 90 bar for 30+ days. Results from the carbonation experiments will be used to understand the carbonation and critical mineral recovery potential of the Twin Sisters Dunite. Finally, we present a resource estimate for the total amount of carbon dioxide that can be mineralized and nickel produced from the Twin Sisters Dunite. Taken together, these results highlight how ultramafic rocks, in addition to basalt reservoirs, can be sites for the in-situ storage of carbon dioxide.