Fluorine and chlorine contents in the lavas and ejecta of the 1986 eruption of Izu-Oshima volcano

Fluorine and chlorine in 61 rock samples effused by the 1986 eruption of Izu-Oshima volcano were determined to make clear the behavior of volatiles in the magma. Their F—Cl variation patterns differ from each other according to the features of the eruption. The rocks erupted from the central cone crater (A-group) have very uniform F contents (82-90 μg/g), while Cl contents are variable (200-300 μg/g) due to selective degassing of Cl after effusion. The lavas and ejecta of fissure eruptions on the caldera floor (B-group) have distinctly higher F (92-136 μg/g) and Cl (290-400 μg/g) contents than the A-group rocks with larger variations. Strong correlations are seen among their F, Cl and SiO ₂ contents. These facts can be explained by separation of volatile-free and silica-poor minerals from the magma forming the A-group rocks. The products of fissure eruptions on the outer slope of the somma (C-group) have intermediate F (70-126 μg/g) and Cl (240-390 μg/g) contents between those of the A- and B-group, but their variations are as large as the B-group rocks. Contrary to the B-group samples, no correlations are seen among F-SiO ₂ or Cl-SiO ₂. Only F-Cl correlation is significant. This indicates that the loss of volatiles during magma migration is predominant. Andesitic to dacitic pumices ejected from the B-craters generally have high F contents (148-179 μg/g). But, they can be subdivided into two classes according to the Cl content. The high-Cl samples (490-560 μg/g) might be derived from the A-magma by strong crystallization fractionation. On the other hand, the low-Cl samples (40-160 μg/g) must have another origin, e.g. the basement rocks.