Rates of Thermal and Chemical Evolution of Magmas in a Cooling Magma Chamber: a Chronological and Theoretical Study on Basaltic and Andesitic Lavas from Rishiri Volcano, Japan

摘要

Rates of magmatic processes in a cooling magma chamber were investigated for alkali basalt and trachytic andesite lavas erupted sequentially from Rishiri Volcano, northern Japan, by dating of these lavas using 238U–230Th radioactive disequilibrium and 14C dating methods, in combination with theoretical analyses. We obtained the eruption age of the basaltic lavas to be 29·3 ± 0·6 ka by 14C dating of charcoals. The eruption age of the andesitic lavas was estimated to be 20·2 ± 3·1 ka, utilizing a whole-rock isochron formed by U–Th fractionation as a result of degassing after lava emplacement. Because these two lavas represent a series of magmas produced by assimilation and fractional crystallization in the same magma chamber, the difference of the ages (i.e. ～9 kyr) is a timescale of magmatic evolution. The thermal and chemical evolution of the Rishiri magma chamber was modeled using mass and energy balance constraints, as well as quantitative information obtained from petrological and geochemical observations on the lavas. Using the timescale of ～9 kyr, the thickness of the magma chamber is estimated to have been about 1·7 km. The model calculations show that, in the early stage of the evolution, the magma cooled at a relatively high rate (>0·1°C/year), and the cooling rate decreased with time. Convective heat flux from the main magma body exceeded 2 W/m2 when the magma was basaltic, and the intensity diminished exponentially with magmatic evolution. Volume flux of crustal materials to the magma chamber and rate of convective melt exchange (compositional convection) between the main magma and mush melt also decreased with time, from ～ 0·1 m/year to ～ 10?3 m/year, and from ～ 1 m/year to ～ 10?2 m/year, respectively, as the magmas evolved from basaltic to andesitic compositions. Although the mechanism of the cooling (i.e. thermal convection and/or compositional convection) of the main magma could not be constrained uniquely by the model, it is suggested that compositional convection was not effective in cooling the main magma, and the magma chamber is considered to have been cooled by thermal convection, in addition to heat conduction.