玄武岩質帯水層内でのＣＯ２地化学トラッピング：ＣＯ２‐水‐玄武岩反応実験からの示唆

Abstract

Quite recently, a basaltic aquifer receives particular attention as a suitable candidate for CO2 aquifer storage because basaltic rocks contain high concentrations of Ca, Mg, and Fe that can enhance CO2 geochemical trapping via acid neutralization and carbonate mineral formation. Despite the increasing interest in basaltic aquifer CO2 storage, there are few experimental researches on CO2-water-basalt interaction. In addition, the influence of rock alteration on the geochemical trapping processes has not been considered. In this study, therefore, we conducted CO2-water-basalt interaction experiments using three types of fresh to altered basaltic rocks.The fresh basalt (non-altered basalt) is composed of olivine, plagioclase, and basaltic glass. On the other hand, the basic schist (high-T altered basalt) completely consists of secondary greenschist minerals (albite, epidote, chlorite, actinolite, quartz) , and the seamount basalt (low-T altered basalt) is mainly composed of secondary clay minerals (celadonite, smectite, Fe-oxyhydroxide) . In our experiments, the cations eluted from fresh basalt and basic schist are mostly Mg and Na, while the fluids reacting with seamount basalt contain large amount of Na and K with noticeably higher pH. This suggests that Na and K eluted from clay minerals play an important role in acid neutralization. Our experiments further demonstrate that, regardless of alteration types, a Na-dominant condition emerges at an early stage of the CO2 storage in a basaltic aquifer, leading to dawsonite (NaAlCO3 (OH) 2) precipitation. The present findings lead us to propose a new perspective on carbonate formation sequence of dawsonite → siderite → calcite/dolomite in basaltic aquifers.