Hidenori Kumagai

地球化学的基準としてしばしば用いられる中央海嶺玄武岩(MORB)内部の不均質の起源を検討するため、海嶺軸の急冷玄武岩ガラスに加えて、近傍の断裂帯等で産する海洋底かんらん岩や海洋コアコンプレックスのはんれい岩についてヘリウム同位体分析を行った。南西インド洋海嶺東経１０度付近の顕著な同位体異常のみられる海域を含め、海嶺軸の玄武岩ガラスとかんらん岩、はんれい岩試料の<SUP>3</SUP>He/<SUP>4</SUP>Heは非常によい一致を示す。一方で、オフリッジや海嶺軸沿いのマグマ過剰な場所では、玄武岩ガラスにややエンリッチした微量元素組成とやや低い<SUP>3</SUP>He/<SUP>4</SUP>Heが対応してみられ、そのような活動はおおよそ数km<SUP>3</SUP>の規模である。

K-Ar系によるレーザーを用いた微小鉱物や岩石薄片などの年代測定は、Ar-Ar年代測定法に限られていた。原子炉による照射が必要なAr-Ar年代測定では、照射後の試料取り扱いに法的な管理制限があり、また中性子照射に伴う核反応により、初生希ガス同位体情報は失われてしまう。そこで、照射を要しないレーザー溶融K-Ar年代測定を試みた。高知コアセンターに導入されたレーザー溶融希ガス同位体測定システムを用いて、Ar同位体とHe同位体の、一粒の岩石鉱物もしくは薄片の面分析を行った。年代決定に必要なカリウム濃度については、JAMSTEC横須賀のEPMAを使用した。年代標準試料と火山岩試料について、予察的なレーザー溶融K-Ar年代測定を試みたので報告する。

We propose a hypothesis that Ultra-H3 (Ultramafic rock, Hydrothermal activity, Hydrogen, and Hyper SLiME) linkage was responsible for the development of primitive life on the early Earth. In order to establish the Ultra-H3 linkage hypothesis, we plan experimental investigations of hydrothermal reactions among hydrothermal solution, ultramafic rocks, and microorganisms using newly designed hydrothermal apparatus with both flexible reaction-cell and fluid-flow systems. The hydrothermal experiments will be performed to resolve the following issues: 1) relationships between HyperSLiME activity and hydrogen concentration of hydrothermal fluid, 2) hydrothermal hydrogen-generation under the specific condition of the early Earth (e.g., low-O2, low-SO4, high-CO2, low-pH, and komatiitic rocks), and 3) probability of formation of organic compounds such as amino acids that are essential to cellular life during the hydrothermal reaction.

We present the result of precise determination of rare-earth elements (REE) in leucogranite from an Ocean Core Complex at the Central Indian Ridge. The sample was recovered by the deep-diving submersible SHINKAI 6500 during the YK05-16 Cruise. In order to obtain high precision REE data which is critically important to discuss lanthanite tetrad effect, a new method of ICP-MS analysis was developed using a combined acid treatment with Li2B4O7 fusion digestion. Reproducibility obtained from separate dissolutions and measurements of geochemical reference materials was generally better than 3%. REE abundances of the leucogranite are noticeably high, indicating that the rock is highly evolved. Chondrite-normalized REE pattern of the sample clearly shows M-type lanthanide tetrad effect with conspicuous negative Eu anomaly. However, the Eu anomaly is significantly smaller than other reported highly evolved granites such as Naegi granite in Central Japan. This may suggest different process of fluid-silicate interaction during evolution of granitic rocks in oceanic crust.

Dating of frictional rocks are very difficult to analyze, because the closure temperature is different from each mineral what any rock is composed such minerals. Radiometric age should be detected as cooling age (=closing age), and thermal history for geologic event, which is calculated from temperature ascent and its temporary variation, is connecting and investigating quantitative analysis. It is expected as a good tool that a sensitivity mineral of thermal rising should be applied such geological event as fault movement for frictional thermal.