Journal of Petrology Advance Access published online on May 28, 2007
Journal of Petrology, doi:10.1093/petrology/egm018
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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
1The Pheasant Memorial Laboratory for Geochemistry & Cosmochemistry, Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori 682-0193, Japan
2Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Beppu, Oita 874-0903, Japan
Received August 18, 2006; Revised typescript accepted March 27, 2007
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Abstract |
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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.
KEY WORDS: convection; magma chamber; heat and mass transport; timescale; U-series disequilibria
*Corresponding author. Present address: Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan. E-mail: kuritani{at}mail.tains.tohoku.ac.jp
Present address: Department of Geology, University of Maryland, College Park, MD 20742, USA
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