HR: 1340h
AN: V43A-1113    [Abstracts]
TI: Rates of Thermal and Chemical Evolution of Magmas in a Cooling Magma Chamber Beneath Rishiri Volcano, Japan
AU: * Kuritani, T
EM: kuritani@mail.tains.tohoku.ac.jp
AF: Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Noguchibaru, Beppu, Oita, 874-0903, Japan
AU: * Kuritani, T
EM: kuritani@mail.tains.tohoku.ac.jp
AF: Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Aoba 6-3, Aramakiaza, Aoba, Sendai, Miyagi, 980-8578, Japan
AB: Rates of magmatic processes in a magma chamber were investigated for alkali basalt and trachytic andesite lavas from Rishiri Volcano, northern Japan. Pre-eruption magmatic history of the lavas has been investigated by detailed petrology and geochemistry (e.g. Kuritani et al., 2005), and it has been shown that these lavas represent a series of magmas evolved by assimilation and fractional crystallization in the same magma reservoir. In addition, the eruption ages of the basalt and andesite lavas have also been estimated to be 29.3 ka and 20.2 ka, respectively (Kuritani et al., 2006; 2007), suggesting that the timescale for the parental basaltic magma to have evolved to the daughter andesitic magma was ~9.1 kyr. In this study, thermal and chemical evolution of the Rishiri magma chamber was modeled using mass and energy balance considerations, as well as quantitative constraints obtained from petrologic and geochemical observation on the lavas, in order to estimate timescales of magmatic processes. A model developed in this study considers a thermal structure of a magma chamber to be determined by conductive heat balance between the surrounding crust and the magma chamber, in which the main molten part of the magma chamber (main magma) is cooled by thermal convection. The model assumes a sill-like magma body, in which heat loss from the vertical walls of the reservoir is negligible (i.e. one-dimensional problem). The magma body is divided into three regions: the roof boundary layer, the main magma body, and the floor boundary layer. Heat transfer through the boundary layers and the surrounding crust is assumed to occur by conduction. The main magma is cooled solely by thermal convection, and is homogeneous in temperature and chemical composition. Following e.g. Turner (1979) and Kerr et al. (1990), the Nusselt-Rayleigh relationship is utilized to express the dimensional heat flux from a convecting liquid layer. The heat flux is further constrained by an observed relationship between estimated magmatic temperatures and bulk-rock chemical composition (K2O contents) of the lavas. In order to close the governing equations, the K2O contents are linked to parameters used in the equations by utilizing a mass balance model of Kuritani et al. (2005). Using the timescale of the magmatic evolution of ~9.1 kyr, the thickness of the magma body was estimated to have been ~1.7 km. The calculations show that, in the early stage of the evolution, the magma cooled at relatively high rate (>0.1 �E#8249;C/year), and the cooling rate decreased with time. The rate of chemical evolution has also decreased progressively 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. Rate of convective melt exchange (compositional convection) between the main magma and mush melt also decreased progressively with time, from ~1 m/year to ~0.01 m/year, as the magmas evolved from basaltic to andesitic compositions.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting