Magma systems of the Kutcharo and Mashu volcanoes (NE Hokkaido, Japan): Petrogenesis of the medium-K trend and the excess volatile problem

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• 作者：Isoji Miyagi ; ^{miyagi.iso14000@aist.go.jp} ; Jun'ichi Itoh ; Nguyen Hoang ; Yuichi Morishita
• 关键词：Caldera ; Magmatic volatiles ; Medium-K ; Low-K
• 刊名：Journal of Volcanology and Geothermal Research
• 出版年：2012
• 期刊代码：53_03770273
• 类别：ear
• 出版时间：15 June, 2012
• 卷：231-232
• 期：Complete
• 页码：50-60
• 文件大小：1448 K

摘要

Despite the existence of a growing database of the amount of H₂O in melt inclusions, it is difficult to calculate the bulk H₂O content for vapor-saturated magma because the observable H₂O concentration in melt is limited by its solubility. To overcome this problem, we focus on K₂O, which shows similar geochemical behavior to H₂O but is largely retained within melt in the case of volatile saturation and magma degassing. As an initial trial of this approach, we document the evolution of K₂O content for the Kutcharo and Mashu volcanoes in northeastern Hokkaido, Japan, which are located close to each other and have similar timing of eruptive activity, but are chemically distinct, with Kutcharo being medium-K in composition and Mashu being low-K. Despite the contrasting bulk-rock compositions of the two volcanoes, the melt inclusions show a smooth compositional trend that is continuous from low-K basalt to andesite, rhyolite, medium-K rhyolite, and finally high-K rhyolitic melt. The observed medium-K series probably is made by mixing of low-K basalt and medium-K rhyolite which derive from solidified low-K basalt. Thermodynamic computations using MELTS show that the observed chemical variation in melt inclusions can be explained by fractional crystallization and subsequent re-melting of low-K basalt. In addition, the similar Sr, Nd, and Pb isotopic compositions of the basalt and rhyolite, combined with the progressive enrichment of trace elements from basalt to rhyolite, support the interpretation that the two rock types are genetically related. The observed evolution of K₂O contents suggests that the total volume of basalt supplied intermittently beneath the volcanoes was approximately 10 times the volume of erupted rhyolite magma. The solidified basalt, which contains H₂O 鈮?#xA0;5 wt.%, S 鈮?#xA0;0.3 wt.%, and Cl = 鈭?#xA0;0.01 wt.%, supplied a much larger amount of magmatic volatiles to the overlying felsic magma than could be dissolved.