第131回ジオダイナミクスセミナー
　　Geodynamics　Seminar
　　　　　　　　＆
　　地球科学教室特別談話会

　 　　　　　　　　

　　　　　　　 "A radical view of andesite genesis"
　　　　　　　　
　　　　　講師：巽　好幸（IFREE/JAMSTEC 領域長）
　　　　　　　　　　　　　　　　
　　　　　　　　　　　　　　
　　　　　　主催　：　愛媛大学理学部地球科学教室
　　　 　 　日時　：　２００５年６月２４日（金）17：00〜
　　　　　　場所　：　愛媛大学理学部講義棟　１０１教室　
要　旨
　Andesite erupts in the majority of arc volcanoes and through time has contributed to over 20% of terrestrial magmatic products. In addition to this volumetric importance, the continental crust, a geochemical reservoir for light elements and the most differentiated end-member among components within the solid Earth, has a bulk composition equivalent to andesite. Thus elucidating andesite genesis will provide key constraints on the differentiation and evolution processes of the Earth. Two distinctive differentiation trends, tholeiitic (TH) and calc-alkalic (CA), are recognized in andesites, denoting the presence or absence of relative iron-enrichment during magmatic differentiation. These two types of andesites are often present within a single volcanic system, suggesting the simultaneous production of two different magma series. To improve our understanding of andesite genesis, detailed descriptions of both TH and CA rocks from a single volcanic system is required.
　Precise Sr-isotope micro-analyses were conducted on plagioclase phenocrysts from CA and TH andesites from the Quaternary Zao Volcano. 87Sr/86Sr of plagioclase occurring in TH rocks is constant at 0.70428±0.00005, whereas that for CA rocks varies from 0.70337 to 0.70418 with decreasing the anorthite component. These results may not be consistent with a general consensus that the major mechanisms of TH and CA andesite production are fractional crystallization of a mantle-derived TH basaltic magma and mixing between TH basaltic and crust-derived felsic magmas, respectively. Instead, two isotopically distinct basaltic magmas could form beneath the volcano, one for the TH series produced by anatexis of the preexisting, hence more radiogenic mafic lower crust, and other for the CA series by partial melting of less radiogenic mantle wedge peridotite. A mantle-derived CA basaltic magma will mix with crustal TH melts to form CA andesites. This hypothesis, which proposes a crustal-origin for the TH magmas and a mantle-origin for CA basaltic magmas, requires our stereotypes on arc magma genesis to be discarded and revised.


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　　　 　　　　　　　　　　　　　　　E-mail　takut@sci.ehime-u.ac.jp