閉鎖系実験における水 : 岩石相互反応の初期反応速度の温度依存性

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A dissolution experiment was carried out to examine a temperature dependence of reaction rate and characteristics of water-rock interaction using fifteen principal rock-types: four kinds of sedimentary rock (limestone, chert, sandstone, and tuff), five kinds of plutonic rock (dunite, gabbro, diorite, granodiorite, and granite), four kinds of volcanic rock (basalt, andesite, dacite, and rhyolite), and two kinds of metamorphic rock (serpentinite and hornfels). For each rock type, a small block sample (5.0×5.0×40.0 mm^3) was treated with distilled water of 250.0 mL at 5, 50, and 75 ℃. Electric conductivity (EC) of the aqueous solution was measured as an index of total concentration of dissolved materials at several-hour and day interval during 2164 hours (ca. 90 days). The EC of aqueous solution was larger at higher temperature for all rock-types, although there was an exception for sandstone. Change in EC with time was parabolic at an initial stage of experiment. The changing curve was adapted to a parabolic rate law: EC = kt^<1/2>, where k is a rate constant of reaction. The duration in which the curve followed the parabolic rate law, i.e. a period of initial reaction, was shorter, and the k-value was larger at higher temperature in the same rocktype. The k-value in limestone was the highest, and chert and granite had a lower k-value than the other rock-types under all thermal conditions. The temperature dependence of k is represented by the Arrhenius equation: k=A exp (-E_<app>/RT), where E_<app> is the apparent activation energy, R is gas constant and T is absolute temperature. The E_<app> of initial reaction were calculated to be from the lowest value of 11.4 kJ/mol for tuff to the highest value of 27.7 kJ/mol for dunite. These values were lower than the values of mineral dissolution according to the data collected from literature. This indicates that the rate-determining step of the overall reaction at the initial stage of water-rock interaction is mainly a mass transport process.
日本地形学連合の論文
2006-04-25

閉鎖系実験における水 : 岩石相互反応の初期反応速度の温度依存性

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