[S-CG54] New Insights of Fluid-Rock Interactions: From Surface to Deep Subduction Zone

[SCG54-13] Changes in the surface roughness of minerals associated with the weathering of granite: an example of Mt. Gagara, Hiroshima

*Hikaru Okamura¹, Tadashi Yokoyama¹, Masahiro Kaibori² (1.Hiroshima University Graduate School of Advanced Science and Engineering, 2.Hiroshima University, Resilience Research Center)

Keywords:Granite, Weathering, Reactive transport modeling, Reactive surface area

Granite is mainly composed of quartz, plagioclase, K-feldspar, and biotite, and each has different resistance to weathering. The purpose of this study is to clarify how the reactive surface area and surface roughness of each mineral change with the progress of the weathering of granite. The granite borehole core, drilled up to depth of 20 meters in Mt. Gagara on Hiroshima Univ. Campus, was used. A series of changes from the unweathered part to the weathered part can be seen in this core. Experiments were conducted on three samples, unweathered (UNW), moderately weathered (W-2) and strongly weathered (W-3) rock. Each sample was reacted with pure water in a flow-through type reactor, and the amount of dissolved elements was measured. We also used PHREEQC (Parkhurst and Appelo, 2013) to simulate the reactive-transport process inside rocks, and the reactive surface area and the surface roughness of each mineral were evaluated by searching for conditions in which the amounts of eluted elements obtained in the experiment and the calculation results match. As a result, for plagioclase, the surface roughness increased as the degree of weathering increased from UNW to W-2, and then decreased as the degree of weathering increased from W-2 to W-3. For K-feldspar, the surface roughness increased as the degree of weathering increased from UNW, W-2, and W-3. The results show that dissolution is most pronounced in W-2 for plagioclase and W-3 for K-feldspar, and is consistent with the well-known fact that plagioclase is less resistant to chemical weathering than K-feldspar. Both plagioclase and K-feldspar are not uniform in internal composition. The reason for the increase or decrease in the surface roughness of each mineral may be that the areas that dissolve quickly in terms of composition disappear as weathering progresses, increasing the exposed area inside, and that the reactivity of the exposed surface inside also decreases as weathering progresses.