Greenhills Dunite Subsurface Reaction Kinetics in a Global Mafic–Ultramafic Context

The strong impetus to reduce emissions from fossil fuels has led to the development of abatement technologies, such as in situ carbon storage via mineralization in mafic and ultramafic rocks. While field-scale implementation of carbon storage in basaltic reservoirs is currently ongoing in Iceland, only pilot-level injections have been conducted in highly reactive ultramafic deposits in the Middle East. The South Island of New Zealand hosts numerous and extensive ultramafic rock bodies, including the intrusively deposited Greenhills Dunite within the Greenhills Ultramafic Complex. In this study, we evaluate the potential for carbon storage via in situ mineralization in Greenhills Dunite. An experimental matrix of batch reactions at subsurface temperatures and pressures shows a high carbonation extent across different conditions and size fractions, with the reaction products largely consisting of magnesite with evidence for iron incorporation. A 10-day high-pressure in situ X-ray diffraction experiment shows that the incipient stage of the reaction is dominated by brucite carbonation followed by the slower kinetics of olivine carbonation. The kinetics of these reactions were then compared to those in the broader (ultra)mafic carbonation literature that includes basalt and basaltic glass carbonation experiments. Overall, these results show potential for field-scale in situ mineralization deployment in the Greenhills Ultramafic Complex and highlights the potential for CO₂ storage in mafic-ultramafic rocks around the globe.