Fluorine Contents of Rocks from Aoso Volcano, Northeast Japan Arc

Abstract

The fluorine contents of rocks from the Aoso volcano as well as mafic inclusions are determined. The volcanic activity of the Aoso volcano can be classified into two stages, earlier and later stages divided by a caldera collapse. The fluorine contents of earlier stage rocks are 105-180 ppm, while later stage rocks have a wider range (44-298 ppm). Mafic inclusions have significantly higher fluorine contents (315-1564 ppm) compared with their host rocks. The observed fluorine contents of mafic andesitic and andesitic rocks (except for the mafic inclusions) for the Aoso volcano are consistent with these for other volcanoes of the Aoso-Osore volcanic zone, and are also in the range observed for the Nasu (Sekiryo) volcanic zone. The fluorine contents of andesitic and dacitic rocks of the Aoso-Osore volcanic zone do not show any systematic increases with silica contents, and are lower than those of any other volcanic zone in the northeast Japan arc. Some of the mafic inclusions from the Aoso volcano show far higher fluorine contents than any rocks from other volcanoes of the northeast Japan arc, which may provide a useful key for understanding the origin of these mafic inclusions.