Direct determinations of the rates of rhyolite dissolution and clay formation over 52,000 years and comparison with laboratory measurements

Four porous, glass-dominated rhyolites from Kozushima Island, different in age and extent of weathering, were studied. Because the four rhyolites are homogeneously weathered to considerable depth, and because their initial chemical compositions were equal, the different rock characteristics can provide information about rates of rhyolite dissolution and clay mineral formation over similar to52,000 yr. Because glass surfaces retreat without surface roughening, surface area (measured by Brunauer-Emmett-Teller method; BET) was assumed to be approximately constant over time. The field dissolution rate, as inferred from the rate of loss of Si, was similar to6 x 10(-19) mol cm(-2) s(-1). The estimated clay mineral formation rate was similar to1 x 10(-19) mol cm(-2) s(-1). About 20% of dissolved Si precipitated as clays. In order to investigate the factors affecting the field dissolution rate, dissolution experiments that used powdered and block rhyolite samples were conducted. Under relevant field conditions (20degreesC and pH 6similar to7), the rates were similar to5 x 10(-17) and similar to5 x 10(-18) mol cm(-2) s(-1) for powdered rhyolite and blocks, respectively. The dissolution rates obtained in this study decrease in the order powder > block > field. Because all surface areas were directly measured by BET, the differences are not attributable to the errors in surface area. The most plausible explanations of the slower rates are the lower degree of flushing and resultant high-solution saturation states in the pores (both in the field and in the rhyolite blocks used in experiments) plus the formation of alteration/hydrated layers at the glass surface. Copyright (C) 2002 Elsevier Science Ltd.