[SVC41-P16] Eruption processes of the 3rd stage of Niigata-Yakeyama Volcano, Japan, inferred from groundmass textural analysis
Keywords:Niigata-Yakeyama, Maeyama lava flows , Hayakawa pyroclastic flows, plagioclase microlite, FeTi oxide microlite
Maeyama lava flows and Hayakawa pyroclastic flow deposits consist of andesite and dacite lava blocks. Microlite number density (MND) and crystallinity of plagioclase and FeTi oxide in andesite and dacite lava samples were obtained from groundmass textural analysis. Our results indicated that andesite and dacite lavas contain microlites with different features. As for plagioclase microlite, andesite lavas showed lower MND and higher crystallinity, while dacite lavas showed higher MND and lower crystallinity. FeTi oxide microlites in andesite and dacite lavas showed higher and lower MND, respectively.
MND and crystallinity of microlite are useful indicator of undercooling of erupted magma (e.g. Hammer et al., 2000). Generally speaking, FeTi oxides can be crystallized under lower temperature than plagioclase, suggesting different crystallization depths at the conduit. Our results suggested that degree of undercooling differ with the depth at the conduit. Maeyama samples did not show different results with Hayakawa pyroclastic flows, suggesting Hayakawa pyroclastic flow may be derived from non-explosive lava dome eruption.