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 4

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.

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中央海嶺玄武岩は太平洋ではマグネシウムが低く,鉄が多い試料が認められる。ナトリウムに関しては,インド洋の試料が太平洋や大西洋に比べて,同じマグネシウム含有量で比較したときに高い値を示す傾向がある。 その他の元素ではTiO2 が太平洋で高く,大西洋・インド洋で低いといった特徴が挙げられる。 アイスランドのようなホットスポット近傍などでは,通常の中央海嶺玄武岩よK2O やTiO2 に富む玄武岩 ...
サンディエゴに行ってこられたのですか。いいですね。 これが中央海嶺玄武岩(MORB)ですか。本当に真っ黒なのですね。私は岩石学が専門ではありませんが、近くに岩石学の専門家がいたので、MORBという言葉はよく聞いていました。沈み込み帯の玄武岩とはどのような成分が違うのかということはしょっちゅう話を聞いていたのですが、頭が悪くて何も残っていません。 https://www.jstage.j ...

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