[S-CG46] 岩石―流体相互作用の新展開：表層から沈み込み帯深部まで

[SCG46-P17] 花崗岩の風化における反応表面積の評価：広島ががら山の例

*岡村 光¹、横山 正²、海堀 正博³ (1.広島大学　総合科学部、2.広島大学　大学院先進理工系科学研究科、3.広島大学　防災・減災研究センター)

キーワード：花崗岩、風化、反応-輸送モデリング、反応表面積

Granites are widely distributed in Chugoku district, Japan. Saprolite is formed as a result of weathering of granite. Granites are mainly composed of quartz, plagioclase, K-feldspar and biotite, and each of which has different resistance to weathering. In this study, we examined how the reactive surface area (surface roughness) of minerals in granite from Mt. Gagara, Hiroshima, changes as weathering progresses. Borehole cores of Mt. Gagara, drilled to a depth of 20 meters from the ground surface, were analyzed. Dissolution experiments were conducted on unweathered (UNW) and strongly weathered (W-3B) rock samples and concentrations of dissolved elements were measured. In addition, numerical simulations of rock dissolution were performed using PHREEQC (Parkhurst and Appelo, 2013). By comparing the calculated results with the experimental results, the surface roughnesses of minerals were evaluated. The results showed that the surface roughnesses of plagioclase and K-feldspar in W-3B were significantly greater than those in UNW. The interior of neither Plagioclase nor K-feldspar is homogeneous, and there are differences in composition within a single grain. The resistance to weathering depends on their composition. In W-3B, areas of low resistance to weathering inside plagioclase and K-feldspar may have already been lost due to weathering. Therefore, water may have penetrated inside the plagioclase and K-feldspar in W-3B, causing dissolution, resulting in greater surface roughness in W-3B compared to those in UNW.