著者

佐藤 暢 熊谷 英憲 根尾 夏紀 中村 謙太郎

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.117, no.1, pp.124-145, 2008-02-25 (Released:2010-02-10)

参考文献数

60

被引用文献数

3 5

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Mid-ocean ridge basalt (hereafter, MORB) is a final product of melt generated from the partial melting of mantle peridotite, following reaction with mantle and/or lower crustral rocks, fractionation at a shallower crust and other processes en route to seafloor. Therefore, it is difficult to estimate melting processes at the upper mantle solely from any investigations of MORB. In contrast to the restricted occurrence of peridotite of mantle origin in particular tectonic settings (e.g., ophiolites, fracture zones, or oceanic core complexes), the ubiquitous presence of MORB provides us with a key to understanding global geochemical variations of the Earth's interior in relation to plate tectonics. In fact, MORB has been considered to show a homogeneous chemical composition. In terms of volcanic rocks from other tectonic settings (e.g., island arc, continental crust, ocean island), this simple concept seems to be true. However, recent investigations reveal that even MORB has significant chemical variations that seem to correspond to location (Pacific, Atlantic, and Indian Oceans). These observations suggest that the mantle beneath each ocean has a distinct chemical composition and an internally heterogeneous composition. In this paper, global geochemical variations of MORB in terms of major and trace element compositions and isotope ratios are examined using a recently compiled database. The compilation suggests that MORB has heterogeneous compositions, which seem to originate from a mixture of depleted mantle and some enriched materials. Coupled with trace element compositions and Pb-isotope ratios, there seems to be at least two geochemical and isotopic domain of the upper most mantle: equatorial Atlantic-Pacific Oceans and southern Atlantic-Indian Ocean. Material (melt and/or solid) derived from plume, subducted slab, subcontinental crust, or fluid added beneath an ancient subduction zone is a candidate to explain the enrichment end-member to produce heterogeneous MORB. Because MORB is heterogeneous, using a tectonic discrimination diagram that implicitly subsumes homogeneous MORB or its mantle sources should be reconsidered. Further investigations, particularly of off-axis MORB, are needed to understand the relationship between heterogeneous compositions of MORB and geophysical parameters (e.g., degree of melting, temperature, spreading rate, crustal thickness, etc). In addition, the role of the MOHO transitional zone should be investigated to interpret the chemical characteristics of MORB.

著者

町田 嗣樹 折橋 裕二 マルコ マニアーニ 根尾 夏紀 サマンサ アンスワース 谷水 雅治 玉木 賢策

出版者

一般社団法人日本地球化学会

雑誌

日本地球化学会年会要旨集

巻号頁・発行日

vol.58, pp.318, 2011

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中央インド洋海嶺の南緯20°-18°付近は、典型的な中央海嶺-ホットスポット相互作用の場所である。しかし、南緯20°から15°にかけての中央海嶺より採取された玄武岩の化学組成の変化は、レユニオンマントルプルームによる上部マントルの汚染とは関連しないことが判明した。つまり、上部マントル中のリサイクルされた過去のプレート物質が、ホットスポットからの熱によって大量に融解している場所であると解釈される。

著者

根尾 夏紀 宮下 純夫

出版者

日本地質学会

雑誌

日本地質学会学術大会講演要旨

巻号頁・発行日

vol.111, 2004-09-10