IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3P1_2G-O18

Room B2

Date/Time: July 23 16:00-16:15

Magma degassing, oxidation, and gas adsorption processes during a series of ash
eruptions in progress at the Sakurajima volcano, Kagoshima, Japan
Isoji MIYAGI, Hiroshi SHINOHARA, Jun’ichi ITOH
Geological Survey of Japan, Japan
E-mail: miyagi.iso14000@aist.go.jp

In order to understand magmatic processes near the top of a volcanic conduit, we examined more than 100 ash
samples from the Sakurajima volcano erupted during 1981 to 2012. We describe variations in the color and theamount of leachates for ash samples that were collected shortly after an eruption without having been exposed torain. Chemical analysis of ash leachates (Cl, F, and SO4 contents) was performed using an ion chromatographon aqueous solutions prepared from the ash and deionized water. The color analysis was performed using acolorimeter on bulk ash, on separated finer ash fractions, and on finely ground coarser ash fractions. We also
obtained SEM images, XRD on finer fractions, and EPMA data on polished ash sections, as well as time and dateof each eruption.

The amount of leachates extracted from ash sample varies by nearly three orders of magnitude over theentire range of sampled eruptions through time and exceeds, especially the sulfur, range of the same chemicalcompounds dissolved within the original melt. The long-term temporal change was observed as follows. Based on
the amount of leachates present, ash samples are classified into two major groups: one is with a molar S/Cl ratioof ∼10, and the other with S/Cl of ∼1 and that is relatively depleted in S. Ash samples erupted during 1981-1991
from the Minami-dake summit crater belong to the latter group. Ashes erupted from the Showa crater in early2008 belong the former group, of which the S content was found to decrease systematically through time, and in
2011, ashes of the latter group erupted for the first time from this crater. Based on the coloration, ash samples inthis study are classified into two groups: one is yellowish color, the other is less yellowish. The coloration of theformer group can be explained by the existence of yellowish native sulfur as well as other hydrothermally alteredminerals that were subsequently confirmed using X-ray diffractometer. Short-term temporal change was also
defined based on the following observations: (1) positive correlations between the interval of successive eruptions
and both the yellowness and the amount of ash leachates were observed; (2) the lower the total amount of ash
leachates present and the lower the yellowness of the ash; and (3) the ratio of altered to the unaltered ash particles
is higher in cases where the ash deposits resided for a longer repose time before the eruption, and this ratio drops
correspondingly (and rapidly) with a drop in the repose time.

Our interpretation is that the observed temporal changes in volcanic ash result from a transition in the amount offumarolic sulfur accumulation in partly solidified magma near the top of volcanic conduit, which may reflect themean residence time of the magma. The magma probably periodically renewed in response to ash eruption and/ormagma convection near the top of the volcanic conduit.

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