地殻内におけるマグマシステムの進化 : 思考実験とアナログ実験による考察  [in Japanese] Evolution of magma system in the crust : Investigations based on thought experiments and analogue experiments  [in Japanese]

大陸地殻中のマグマの熱物質進化は, 地殻の溶融とマグマの対流によって大きな影響を受ける。高温液と低温固体間の熱移動について数値計算を行った結果, 活発な対流による溶融が進行している場合, それ以外の条件にくらべて, 液の冷却速度が著しく大きくなることが分かった。マグマ溜まりの上面と下面において地殻の溶融やマグマの対流が起こる条件を決定するために, NH_4Cl-H_2Oの二成分共融系を用いてアナログ実験を行った。その結果, 上面では, 急速な溶融と活発な熱対流が起こるのに対し, 下面では, 熱が効果的に輸送されず, 相対的にゆっくりした溶融と組成対流が起こることがわかった。また, 上面の溶融によって生成した密度の小さい液は, その下の液とは混合せずに分離した液層を形成するのに対し, 下面の溶融で生成した液は上の液と混合する, という違いが観察された。これらの結果を大陸地殻中に貫入した玄武岩質マグマに応用すると, (1)貫入直後に, 上面固体の溶融によって玄武岩マグマとは化学的に分離した珪長質マグマが急速にできる, (2)両方のマグマは, 地殻の融点に達するまでは急冷するが, その後, 冷却速度が遅くなる, (3)玄武岩マグマは下面での溶融液と混合するため分化経路が変化する, などの熱物質進化がおこることが予想される。

Thermal and compositional evolution of magmas in the continental crust are profoundly affected by the melting processes of the crust and convection in magmas. The results of calculations on heat transfer between a hot liquid and a cold solid suggest that the cooling rate of the liquid significantly increases when the solid/liquid interface moves due to melting driven by vigorous convection. Analogue experiments have been carried out using NH_4Cl-H_2O binary eutectic system to determine the conditions under which melting and/or convection occur at the roof and the floor of a magma chamber. When a cold solid mixture with the eutectic composition is placed at the top of a hot solution of higher NH_4Cl content, vigorous thermal convection occurs in the solution, which results in rapid melting of the solid roof to form a stable melt layer with negligible mixing of the underlying liquid. On the other hand, when the cold solid mixture is placed at the bottom of the hot solution, the convection is driven by compositional gradient due to floor melting as well as crystallization just above the floor. Because the compositional convection carries a low heat flux, the rate of melting and the temperature profile around the floor do not differ greatly from those that would be observed due to conductional one. Unlike the roof melting, the melt generated by the floor melting efficiently mixes with the overlying solution. The implication for the thermal and compositional evolution of a magma system is that, when a basaltic magma is emplaced in the continental crust, a silicic magma is rapidly formed by the roof melting, and that the magmas evolve very slowly after the temperatures of the magmas become as cool as the fusion temperature of the crust. The major effect of the floor melting would be that the liquid line of descent of the basaltic magma can be greatly modified by mixing with the melt generated at the floor.