DISCHARGE RATE OF SO2 FROM UNZEN VOLCANO, KYUSHU, JAPAN

The discharge rate of SO2 from Unzen volcano, which started erupting i n November 1990. has been measured with a correlation spectrometer (CO SPEC) at a fixed ground-based station about 7.3 km SSW from the summit lava dome. The discharge rate of SO, increased greatly after the expl osive eruptions on June 8th and 11th, 1991. High discharge rates of SO 2 (an average of 180 ton/day) were observed from July 1991 to January 1992, followed by a general decrease to an average of 110 ton/day duri ng March 1992 and May 1994. The discharge rate of SO2 correlates with the extrusion rate of magma. Judging from the sulfur inventory, i.e., 130-450 g of discharged SO2 per ton of erupted magma, and the sulfur c ontent of the extruded lava, the proportion of sulfur degassed from th e magma is estimated to be ca. 80 %, suggesting an efficient? volatile degassing. The discharge I ate of the total volatiles is calculated t o be 7,700 ton/day by combining the SO2 discharge rate with the chemic al composition of volcanic gases.