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

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doi: 10.1093/petrology/egm018
Authors:Kuritani, Takeshi; Yokoyama, Tetsuya; Nakamura, Eizo
Author Affiliations:Primary:
Tohoku University, Department of Earth and Planetary Materials Science, Sendai, Japan
Other:
University of Maryland, United States
Institute for Study of the Earth's Interior, Japan
Volume Title:Journal of Petrology
Source:Journal of Petrology, 48(7), p.1295-1319. Publisher: Oxford University Press, Oxford, United Kingdom. ISSN: 0022-3530
Publication Date:2007
Note:In English. 74 refs.; illus., incl. 3 tables, geol. sketch map
Summary: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.
Sections:Age determination; Petrology
Subsections:Volcanology
Subjects:Absolute age; Andesitic composition; Basaltic composition; Cenozoic; Chemical composition; Cooling; Dates; Fractional crystallization; Geochemistry; Isochrons; Lava; Lithogeochemistry; Magma chambers; Magmas; Pleistocene; Quaternary; Th/U; Theoretical studies; Thermal history; Upper Pleistocene; Asia; Far East; Hokkaido; Japan; Rishiri
Coordinates:N450500 N450500 E1410000 E1410000
Abstract Numbers:07M/2823
Record ID:2008000859
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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