Origin of chemical variation of the magmas formed Aso caldera

Aso caldera has been considered to form by four large scale pyroclastic flow eruptions, Aso 1-4, small scale Plinian eruption, Aso 2/1-4/3, and lavas. The volcanic rocks consist basalt, trachyte, dacite and rhyolite. These volcanic rocks are characterized by high alkaline contents compared with volcanic rocks occur on the volcanic front elsewhere. There are many evidences that geochemical characteristics of Aso caldera can not be able to explain cooling process in single magma chamber (Yasuhara and Yokose, 1998, 1999): (1) LILE-HFSE have a positive correlations, (2) incompatible elements (K, Y, Zr, Nb and REE) / SiO2 ratios decrease through eruption cycles, (3) incompatible trace element abundances pattern on spider diagram does not similar to island arc signature, (4) chemical variations cannot indicate as a single trend, (5) volcanic glass in essential fragments does not change through eruption cycles and have a similar composition systematically, (6) there are geochemical differences between pumice from pyroclastic flow deposits and fall deposits. These chemical variations in the magmas of Aso caldera can not be explained by AFC or magma mixing models. In order to understand the contribution of fluids on magma genesis, we have analyzed on chlorine and sulfur contents in the pumices. Chlorine contents indicates positive correlation with SiO2 and LILE, and negative correlation with Sr. This positive correlations imply that increasing of volatile elements in a melt correspond to increasing with degree of melting. The negative correlation may be linked with stability of plagioclase. The ratios of HFSE / Cl have wide variation and become small to the order of lava, scoria, pyroclastic fall deposit and pyroclastic flow deposits. As eruption cycle becomes new, the ratio decrease gradually. It suggests that volatile components represented by Cl may control the degree of melting and abundance of incompatible elements. Based on the above observations, geochemical variation in volcanic rocks from Aso caldera is not created in the magma chamber, but in the source region by progressive dehydration melting. It is conceivable that there were few volatile components in the partial melting at the early stage of eruption cycles, and the volatile component increased in the partial melting at the late stage of eruption cycles. Concentrations of the volatile elements play an important role in the reaction.