Fluorine and Chlorine Contents of Volcanic Ash from the 2004 Eruption of Asama Volcano, Japan

Abstract

Asama volcano in central Honshu, Japan, commenced four-month-long eruptive activity on September 1, 2004, with a phreatic explosion after approximately 20 years of quiescence. According to the mode of eruption, the activity is divided into three phases : Phase I, Phase II and Phase III, which are characterized by a phreatic explosion and plume activity, nonexplosive eruptions with swarms of BH-type earthquakes and explosions at short intervals, respectively. In Phase I, the ash samples are mainly composed of altered volcaniclastic material. Meanwhile, the ash samples in Phase II and Phase III are pristine and originated from new magma. The fluorine and chlorine contents of the ash samples in these phases are determined, and their behavior corresponding to the dramatic changes in the mode of eruption is examined. The ash samples in Phase I characteristically contain large amounts of fluorine and chlorine in watersoluble and water-insoluble forms. The prolonged reactions of HF, HCl and SO2 in volcanic gases with old volcaniclastic material within the crater during the noneruptive period prior to Phase I produced extraordinary amounts of water-soluble and water-insoluble F and Cl compounds. The intense degassing in Phase I and Phase II corresponding to the ascent of the magma near the surface sharply decreased the volatile contents of the uppermost part of the magma column; however, the water-insoluble F contents of the ash samples in Phase IIa are higher than those of the ash in Phase IIb. These results strongly suggest that volatiles were greatly concentrated in the uppermost part of the ascending magma prior to Phase I. Meanwhile, the water-insoluble Cl contents of the ash in Phase IIa are significantly lower than those of the ash in Phase IIb. Chlorine was degassed from the uppermost part of the ascending magma column far more intensely than fluorine because chlorine is more volatile than fluorine in magma, which probably decreased the chlorine content of the ash in Phase IIa significantly. Chlorine was degassed intensely, whereas fluorine was less degassed between Phase IIb and Phase IIIa. The intense degassing of chlorine from magma corresponded to the dramatic change in the mode of eruption from nonexplosive to explosive eruptions. The transition of the water-insoluble Cl contents of the ash samples in Phase III demonstrates steady degassing of chlorine from the effused lava. The significant increases in the water-soluble F and Cl contents of the ash samples in Phase III demonstrate that the effused lava gradually reacted with volcanic gases, although the ash was not subjected to obvious alteration. Water-soluble F compounds formed on the surface of the lava were partially converted to water-insoluble F compounds, which significantly increased the water-insoluble F contents of the ash samples in Phase III. Determination of the water-soluble and waterinsoluble F and Cl contents of volcanic ejecta enables clarification of their behavior through emanation from magma and reaction with volcaniclastic material and inference of the vertical gradients for these volatiles in the ascending magma column.