Variations of Color and Leachate Contents of Volcanic Ashes from Sakurajima Volcano, Japan(＜Special Section＞Sakurajima Special Issue)

Abstract

To understand magma degassing processes near the top of volcanic conduit, we investigated a series of volcanic ashes from the Sakurajima volcano, Japan. We describe temporal changes in the color and the amount of leachates (Cl, F, S) of ash erupted from 1981 to 2011. Based on the amount of leachates present, ash samples are classified into two major groups: one is with a molar S/Cl ratio of〜10, and another with S/Cl of〜1 and that is relatively depleted in S. Ash samples that were erupted during 1981-1991 from the Minamidake summit crater belong to the latter group. Ashes erupted from the Showa crater in early 2008 belong the former group, of which the S content was found to decrease systematically through time, although in 2011, ashes of the latter group erupted for the first time from this crater. Based on coloration, the ash samples in this study are classified into two groups: one with a yellowish color, another with a less yellowish color. The coloration of the former group can be explained by the existence of yellowish native sulfur as well as other hydrothermally altered minerals. We observed positive correlations between the interval of successive eruptions and both the yellowness and the amount of ash leachates. Our interpretation is that the observed temporal changes in volcanic ash result from a transition in the amount of fumarolic sulfur accumulation in partly solidified magma near the top of volcanic conduit, which we interpret to reflect the mean residence time of the magma. The magma probably periodically renewed in response to ash eruption and/or magma convection near the top of the volcanic conduit.