メルト生成の場について : 沈み込み帯を例として(<特集>火山学の『夢』を語る)

概要

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To understand the field that causes melting within the Earth, the relationship between styles of mantle convection and melting processes in subduction zones is discussed as an example, with special reference to the heat source of magmatism. Numerical experiments on mantle convection show that the subduction zone can be effectively cooled (e. g., down to 600 degree C under a steady state), if a closed convection cell in the upper mantle with standard heat sources is assumed. Additional heat sources such as radionuclides of high concentrations (40 times compared to a depleted mantle) or heat supply from the lower mantle are required to sustain a long-term (e. g., more than 100 m. y.) magmatism in subduction zones. In any case of these experiments, involving large variations in heat balance and mechanical boundary conditions, a downward flow along the subducting plate is dominant, which is compensated by a gently upward flow from the back arc side. In the presence of a small amount of H_2O (i. e., undersaturated), the absence of melting in a pressure range along the upward flow occurs owing to a negative dT/dP of the solid us with amphibole, whereas compression melting occurs in the same pressure range along the downward flow. This behaviour causes the melt to be produced in several separate regions at different depths, and may account for the existence of volcanic chains.
特定非営利活動法人日本火山学会の論文
1995-12-25