Carbon capture and storage (CCS) provides a solution towards decarbonization of the 21global economy. The success of this solution depends on the ability to safely and 22 2 permanently store CO 2 . This study demonstrates for the first time the permanent 23 disposal of CO 2 as environmentally benign carbonate minerals in basaltic rocks. We 24 find that over 95% of the CO 2 injected into the CarbFix site in Iceland was mineralized 25 to carbonate minerals in less than two years. This result contrasts with the common 26 view that the immobilization of CO 2 as carbonate minerals within geologic reservoirs 27 takes several hundreds to thousands of years. Our results, therefore, demonstrate 28 that the safe long-term storage of anthropogenic CO 2 emissions through 29 mineralization can be far faster than previously postulated. 30 31The success of geologic CO 2 storage depends on its long-term security and public 32 acceptance in addition to regulatory, policy, and economical factors (1). CO 2 and brine 33 leakage through a confining system above the storage reservoir or through abandoned 34 wells is considered as one of the major challenges associated with geologic CO 2 storage 35 [e.g. (2, 3, 4)]. Leakage rates into the atmosphere of ≤0.1% are required to ensure 36 effective climate change mitigation [e.g. (5, 6)]. To avoid CO 2 leakage, caprock integrity 37 needs to be evaluated and monitored (7). Leakage risk is further enhanced by induced 38 seismicity, which may open fluid flow pathways in the caprock (8). Mineral 39 carbonatization (i.e. the conversion of CO 2 to carbonate minerals) via CO 2 -fluid-rock 40 reactions in the reservoir minimizes the risk of leakage and thus facilitates long-term 41 and safe carbon storage and public acceptance (9). The potential for carbonatization is, 42 however, limited in conventional CO 2 storage reservoirs such as deep saline aquifers, 43 and depleted oil and gas reservoirs in sedimentary basins due to the lack of calcium, 44 3 magnesium and iron rich silicate minerals required to form carbonate minerals (10, 11). 45An alternative is to inject CO 2 into basaltic rocks, which contain up to 25% by weight of 46 calcium, magnesium and iron. Basaltic rocks are highly reactive, and one of the most 47 common rock types on Earth, covering ~10% of continental surface area and most of the 48 ocean floor [e.g. (12, 13)]. 49 50The CarbFix pilot project was designed to promote and verify in situ CO 2 mineralization 51 in basaltic rocks for the permanent disposal of anthropogenic CO 2 emissions (14). Two 52 injection tests were performed at the CarbFix injection site near the Hellisheidi 53 geothermal power plant: Phase I: 175 tons of pure CO 2 from January to March 2012, and 54Phase II: 73 tons of a CO 2 -H 2 S gas mixture in June to August 2012, of which 55 tons were 55 CO 2 . Note that H 2 S is not only a major constituent of geothermal gases but also of CO 2 -56 rich sour gas. Since the cost of CCS is dominated by capture and gas separation, the 57 overall cost could be lowered substan...

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