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
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
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发表时间：
2007-07
影响因子：
3.9
通讯作者：
T. Kuritani;T. Yokoyama;E. Nakamura
中科院分区：
文献类型：
--
作者：
T. Kuritani;T. Yokoyama;E. Nakamura
采用 U^Th 放射性不平衡法和 C 测年方法，结合理论分析，研究了日本北部利尻火山依次喷发的碱性玄武岩和粗面安山岩熔岩在冷却岩浆室中的岩浆过程速率。通过木炭C测年，我们得出玄武岩熔岩的喷发年龄为29 3 0 6 ka。利用熔岩侵位后脱气导致的 U^Th 分馏形成的全岩等时线，估计安山岩熔岩的喷发年龄为 20 2 3 1ka。由于这两个熔岩代表了同一岩浆房中通过同化和分异结晶产生的一系列岩浆，因此年龄之差（即 9 kyr）是岩浆演化的时间尺度。利尻岩浆房的热和化学演化是使用质量和能量平衡约束以及从熔岩岩石学和地球化学观测中获得的定量信息来建模的。采用9 kyr的时间尺度，估计岩浆房的厚度约为1 7 km。模型计算表明，在演化早期，岩浆以较高的速率冷却（40 18 C/年），并且冷却速率随着时间的推移而降低。当岩浆为玄武质时，来自主岩浆体的对流热通量超过2W/m，且强度随着岩浆演化呈指数衰减。随着岩浆从玄武岩成分演化为安山岩成分，地壳物质进入岩浆房的体积通量以及主岩浆和糊状熔体之间的对流熔体交换（合成对流）速率也随着时间的推移而降低，分别从 0 1 m/年减少到 10 3 m/年，从 1m/年减少到 10 2 m/年。尽管模型不能唯一地约束主岩浆的冷却机制（即热对流和/或合成对流），但表明合成对流对主岩浆的冷却并不有效，并且除了热传导之外，岩浆房被认为还通过热对流进行了冷却。
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 U^Th radioactive disequilibrium and C dating methods, in combination with theoretical analyses. We obtained the eruption age of the basaltic lavas to be 29 3 0 6 ka by C dating of charcoals. The eruption age of the andesitic lavas was estimated to be 20 2 3 1ka, 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 (40 18C/year), and the cooling rate decreased with time. Convective heat flux from the main magma body exceeded 2W/m 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 1m/year to 10 3 m/year, and from 1m/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.