Observation and investigation for the formation SPM originates in high temperature volcanic gas

高温的火山气体是活火山喷出SPM的来源。为了探讨火山岩高温气体化学成分的控制作用，根据天山地区的火山岩高温气体观测资料，建立了火山岩岩浆脱气模型。云善富含CO_2和SO_2的火山气可能在距地表-8km处与岩浆分离。当深度小于-1km时，脱气后的岩浆再次溶出富含H_O和CH_1的气相，SPM的量与SO_2气体的流量成正比。以三宅岛火山为例，研究了SO_2的脱气机理。用化学捕集法在火山侧翼采集了扩散的SO_2气体。在该方法中，KOH溶液在露天下暴露数月。用KOH溶液吸收火山SO_2气体。在吸收，动力学同位素分馏的发生证实了实验室实验。利用实验测定的同位素分馏，估算了<34>三宅岛火山岩浆中S的δ^ S值与日本第四纪火山的典型值相同。SO_2气体δ^ S随时间的变化<34>是一个单调递减的过程<34>，这可能是岩浆开放系统脱气的结果。氧逸度决定了气相和岩浆之间的同位素分馏。产生与岩浆开放脱气相容的同位素分馏所需的氧逸度估计比Ni-NiO氧逸度缓冲区高+0.6至+1.2个对数单位。

The high temperature fumarolic gasi is the source of SPM emitted from active volcanoes. In order to investigate the control on the chemical composition of high temperature volcanic gas, a degassing model of magma was constructed based on the data observed at Mt. Unzen. The volcanic gas rich in CO_2 and SO_2was estimated to be separated from magma at depth -8 km from surface. The degassed magma again exsolved a gas phase rich in H_O and CH1 at depth shallower than -1 km.The amount of SPM would be proportional to the flux of SO_2 gas. As a case study, the mechanism of SO_2 degassing was investigated at Miyakejima volcano. A diffused SO_2 gas was sampled at the flank of volcano by using the so called chemical trap method. In the method, a KOH solution was exposed over several months under open air. The volcanic SO_2 gas was absorbed into the KOH solution. At the absorption, the occurrence of a kinetically isotopic fractionation was confirmed by an laboratory experiment. Employing the experimentally determined isotopic fractionation, the δ^<34>S of S in magma of Miyakejima volcano was estimated to be identical to the value typical to the Quarternary volcanoes in Japan. The temporal change in the observed δ^<34> S of SO_2 gas, a monotonically decreasing, δ^<34> S, could be attributed to the open system degassing of magma. The isotopic fractionation between gas phase and magma depends on oxygen fugacity. The oxygen fugacity necessary to produce the isotopic fractionation compatible to the open degassing of magma was estimated to be +0.6 to +1.2 log unit higher than Ni-NiO oxygen fugacity buffer.