34 0 0 0 OA 蛇紋岩化作用における水素の発生に対する岩石学的制約条件
- 著者
- 野坂 俊夫
- 出版者
- 一般社団法人 日本鉱物科学会
- 雑誌
- 岩石鉱物科学 (ISSN:1345630X)
- 巻号頁・発行日
- vol.41, no.5, pp.174-184, 2012 (Released:2012-11-30)
- 参考文献数
- 74
- 被引用文献数
- 2 2
Serpentinization of peridotites involves the production of hydrogen, which is a source of vital energy for chemosynthetic communities and abiotic methane or other hydrocarbons. Serpentinite-hosted hydrothermal vent fields that discharge fluids with hydrogen have been widely noticed as a possible environment for the generation of life on the early Earth and other terrestrial planets. In this context, it is important for us to understand petrological constraints on serpentinization processes related to hydrogen production. Magnetite formation by oxidation of iron in olivine is the most effective process for hydrogen production during serpentinization. Recent petrological studies have revealed that the magnetite formation is controlled by silica activity and Fe-Mg diffusion rate in olivine crystal, as well as temperature and water/rock ratio during serpentinization. Without local elevation of silica activity via fluid infiltration, magnetite forms at temperatures ranging approximately from 150 to 350 °C with most favorable condition at around 300 °C, but fails to form because of increasing diffusion rate in olivine crystal at higher temperatures and Fe-serpentine or Fe-brucite formation at lower temperatures. It should be kept in mind, however, that the formation of oxidized serpentine could produce hydrogen as well.
1 0 0 0 OA 海洋下部地殻および上部マントルの変質作用と変質鉱物
- 著者
- 野坂 俊夫
- 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.253-267, 2008-02-25 (Released:2010-02-10)
- 参考文献数
- 51
- 被引用文献数
- 2 2
Gabbroic rocks recovered from deep holes in the oceanic crust significantly vary in the abundance and assemblage of alteration minerals, showing a close association with the original lithology and distribution of dikes and veins. The mineralogical variation is considered to reflect the durability of primary minerals, accessibility and composition of alteration fluids, and alteration temperature. Textural relationships of alteration minerals suggest a common cooling history of oceanic gabbros from granulite or pyroxene hornfels facies to zeolite facies conditions. It is considered that regardless of spreading rate, the static formation of upper greenschist- to lower amphibolite-facies minerals is the dominant alteration process at the lower crust near oceanic ridges, whereas subgreenschist-facies alteration represents the exhumation histories of gabbroic masses from depth. High-temperature plastic shear zones with almost anhydrous recrystallization of primary minerals develop locally at slow-spreading ridges, and possibly provide pathways for later hydrothermal fluids. In contrast to the gabbroic rocks, oceanic peridotites have a monotonous mineralogy formed during low-temperature serpentinization processes, making it difficult for us to depict their cooling histories or in-situ alteration processes at the upper mantle. The hypothesis that oceanic Moho represents a serpentinization front in peridotites is suitable for the uniformity of crustal thickness inferred from seismological observations, but lacks a rationale for supplying a constant amount of water to the upper mantle or for the cessation of serpentinization at a constant degree. Alternatively, preferential alteration of pyroxene at relatively high-temperature conditions might form the oceanic crust of uniform thickness.
南西インド洋海嶺のアトランティス・バンクにおいてIODP 第360次航海で掘削されたHole U1473Aから採取された斑れい岩類と,高知コア研究センターに保管してあったODP Hole 735Bのコア試料の一部について,研究期間内に計画していた分析のほとんどを完了した。今年度は補足的な顕微鏡観察,電子線マイクロアナライザー分析,レーザーラマン分光分析,レーザーアブレーション誘導結合プラズマ質量分析を行った。特に電子線マイクロアナライザー分析とレーザーラマン分光分析の結果から,黒雲母と緑泥石混合層の存在を確定することができ,この混合層が広範囲にわたって生じていることが明らかになった。さらにコンピューターによる熱力学的解析を行い,黒雲母と緑泥石,および両者の混合層の生成条件を求めた結果,それらの鉱物はほぼ同程度の温度条件(700℃前後)で,シリカとカリウムの濃度の異なる流体が関与する変質作用によって生じたことが推定された。黒雲母とそれを含む混合層の形成は剪断帯や割れ目に沿って浸透した珪長質含水メルトと関連しており,そのようなメルトの浸透は超低速拡大海嶺周辺の海洋下部地殻に特徴的な現象である可能性が高い。以上の研究成果は,海洋下部地殻における水と岩石の相互作用を理解するうえで重要な新知見を提供するものである。この成果は日本鉱物科学会で発表し,論文にまとめて国際学術誌に投稿した。また現世の海洋リソスフェアと比較するために,過去の海洋リソスフェアであるオフィオライトの低温変質作用,特に蛇紋岩化作用についても並行して研究を進めてきた。その結果,蛇紋岩化作用に伴ってかんらん石の鉄とマンガンの含有量が系統的に変化すること,およびモンチセリかんらん石を生じる場合があることを明らかにした。これらも海洋底構成岩の変質作用に関わる重要な新知見であり,国内学会と国際学術誌上で発表した。
1 0 0 0 蛇紋岩化作用における水素の発生に対する岩石学的制約条件
- 著者
- 野坂 俊夫
- 出版者
- 日本鉱物科学会
- 雑誌
- 岩石鉱物科学 (ISSN:1345630X)
- 巻号頁・発行日
- vol.41, no.5, pp.174-184, 2012
- 被引用文献数
- 2
Serpentinization of peridotites involves the production of hydrogen, which is a source of vital energy for chemosynthetic communities and abiotic methane or other hydrocarbons. Serpentinite-hosted hydrothermal vent fields that discharge fluids with hydrogen have been widely noticed as a possible environment for the generation of life on the early Earth and other terrestrial planets. In this context, it is important for us to understand petrological constraints on serpentinization processes related to hydrogen production. Magnetite formation by oxidation of iron in olivine is the most effective process for hydrogen production during serpentinization. Recent petrological studies have revealed that the magnetite formation is controlled by silica activity and Fe-Mg diffusion rate in olivine crystal, as well as temperature and water/rock ratio during serpentinization. Without local elevation of silica activity via fluid infiltration, magnetite forms at temperatures ranging approximately from 150 to 350 °C with most favorable condition at around 300 °C, but fails to form because of increasing diffusion rate in olivine crystal at higher temperatures and Fe-serpentine or Fe-brucite formation at lower temperatures. It should be kept in mind, however, that the formation of oxidized serpentine could produce hydrogen as well.<br>
1 0 0 0 OA 中国山東省・江蘇省のエクロジャイト調査
- 著者
- 石渡 明 平島 崇男 野坂 俊夫 坂野 昇平
- 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.99, no.4, pp.382-387, 1990-08-25 (Released:2009-11-12)
- 参考文献数
- 15
- 被引用文献数
- 1 1