マグマの上昇・噴火における揮発性成分の役割  [in Japanese] Role of volatiles during ascent of magmas and eruptions  [in Japanese]

マグマの発泡は, マグマの物性に大きな影響をあたえる。特に0.1 wt%以上のCO_2を含むマグマは飽和圧力が2kbに達し, 10 km以浅では発泡している可能性がある。マグマから発泡分離した揮発性成分のCO_2/H_2O比は圧力依存性が非常に大きいため脱ガス圧力計として応用可能である。玄武岩質マグマでは1kb以上の高圧化でもCO_2の脱ガスにより無視しえないマグマの体積膨脹が生じること, H_2Oの脱ガスは100 bar以下の圧力下で重要なことがわかった。玄武岩質マグマは通常, 表層2-3 kmの地殻よりも密度が高いため, 浅所では浮力を持ちえない。したがって, 気泡の濃集等によりマグマのバルク密度が低下しないかぎり, 地表へ噴出することは難しい。伊豆大島の玄武岩質マグマの場合では, マグマ中のCO_2量が0.3 wt%以上含まれないと浮力を持てないことがわかった。マグマ溜り内でCO_2に富んだ気泡が上昇し, マグマ溜り上部に濃集すれば, マグマ溜りの一部はこの浮力を持つ条件が満たしうる。マグマ溜り内のCO_2が濃集したマグマだけが噴火し, 残りはマグマ溜り内に留まると推察される。噴火様式, 揮発性成分量および噴火時の噴出率等の関係を空気-水系などでこれまで得られている流動様式線図を用い考察した。空気-水系の流動様式線図を用い, ガス-メルト系に応用する際に, 粘性, 表面張力, 密度の違いを考慮した結果, 伊豆大島1986年噴火の様式変化を説明しうることを示した。

Vesiculation of volatile-bearing magmas largely affects physical properties of magma. Saturation pressure of CO_2-bearing magma (>0.1 wt%) easily exceeds 2kb, due to the low solubility of CO_2 in magmas. Most of the magma containing CO_2 would be saturated with volatiles under the condition of a shallow magma reservoir (<10 km). The CO_2/H_2O ratio of a vapor phase exsolved from a magma strongly depends on pressure. This ratio can be used as a geobarometer for degassing conditions. Calculation of volume of volatile-bearing magmas indicated that CO_2 decreases the density of magma at high pressure and the effect of water exsolution is limited only on low pressure; less than 100 bars for basalts. Buoyancy causes magma ascent. Less dense magmas such as dacite and rhyolite are generally buoyant in the crust, however, basaltic magmas are generally heavier than the crust at shallower than a few kilo meters. Therefore, eruptions of basaltic magmas may require accumulation of CO_2-rich bubbles to decrease their bulk densities to smaller than that of the crust. For the basaltic magma of Izu-Oshima volcano, CO_2 enrichment to 0.3 wt% is estimated to be necessary for positive buoyancy. The CO_2 enrichment can be due to bubble floatation in the magma reservoir. And the vertical profile of CO_2 content, that CO_2 was concentrated at the roof of the magma reservoir, is likely to be made just before the beginning of magma ascent. Driving pressure of eruption at magma reservoir is proportional to the difference between the crustal load pressure and the pressure due to the weight of magma column in the conduit. Because the density of a magma is a function of volatile contents, the driving pressure is thus a function of volatile content. As magma transportation to the surface occurs formed driving pressure, only a portion of a magma which is enriched in CO_2 can erupt. Relationship among eruption styles, volatile content, and the discharge rates of magma was discussed using a flow pattern map for air-water two phase flows with an adjustable parameter of physical properties for the application to the gas-melt flows.