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Solute distribution in porous rhyolite as evaluated by sequential centrifugation

Abstract : Pore water in a porous rhyolite, having a porosity of 27% and pore radii ranging from >25 μm to 0.008 μm, was centrifugally extracted stepwise with increasing centrifugal speed to examine the potential variations of the compositions of pore water and their relationships to reaction and transport occurring in the rock. The rock was soaked for from 1 h to 7 days in an aqueous solution prior to centrifugation. To evaluate the effect of adsorption under minimum effect of dissolution, Li+ and Br− were added to the solution as tracer ions. As centrifugal speed increased, water was extracted in order of large to small pores and the thickness of residual water film became thinner. The concentrations of ions dissolving from the rock (Na+, K+, Ca2+, etc.) after 7 days of immersion were relatively constant in pores of 1−10 μm radii and exponentially increased by 3−100 fold with decreasing pore radius to 0.1 μm. These ions are dissolved from the rock and transported toward the exterior of the rock by diffusion. The calculation using a reactive-transport equation showed that the observed concentration changes reflect the change in solute distribution profile with pore size. The concentration of Si after 7 days of immersion was approximately constant or slightly decreased with increasing centrifugal speed, which appears to be controlled by the solubility. The concentration of Li+ decreased with increasing centrifugal speed after 1 h of immersion but the trend changed after 7 days of reaction. Initial behaviour of Li+ is explained by adsorption on pore walls, and the change of trend is explained by desorption of that previously adsorbed, slight amounts of dissolution, and inflow from the outside of the rock. The change in concentration of Br− with increasing centrifugal speed was small, probably because Br− was not adsorbed on the surfaces. The sequential centrifugation thus provides information on the solute distribution associated with reaction and transport occurring in rock pores.
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Submitted on : Thursday, June 23, 2011 - 11:37:48 AM
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Tadashi Yokoyamaama, Satoru Nakashima, Takashi Murakami, Lionel Mercury, Yusuke Kirino. Solute distribution in porous rhyolite as evaluated by sequential centrifugation. Applied Geochemistry, Elsevier, 2011, 26 (8), pp.1524-1534. ⟨10.1016/j.apgeochem.2011.06.017⟩. ⟨insu-00602697⟩

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