Hydrothermal alteration in lower crustal rocks from Atlantis Bank, Southwest Indian Ridge

2019 Fiscal Year Final Research Report

• Project/Area Number: 16K05611
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Research Field: Petrology/Mineralogy/Economic geology
• Research Institution: Okayama University
• Principal Investigator: Nozaka Toshio 岡山大学, 自然科学研究科, 准教授 (80252948)
• Project Period (FY): 2016-04-01 – 2020-03-31
• Keywords: 海洋リソスフェア / 下部地殻 / 斑れい岩 / 変質作用 / アトランティス・バンク

Outline of Final Research Achievements

Biotite was found in drill cores of lower oceanic crustal gabbros recovered from Atlantis Bank, SW Indian Ridge. This is the first discovery of such an alkali-rich mineral from the lower oceanic crust. Detailed petrological analyses have revealed that the biotite was formed by reactions of primary olivine and plagioclase with Si- and K-rich aqueous fluids derived from differentiated melts. This finding suggests widespread chemical modification of the lower oceanic crust and provides significant implications for the global-scale geochemical cycle.
In addition, serpentinization conditions in gabbroic rocks from Hess Deep Rift near the East Pacific Rise and in peridotites from the Oeyama ophiolite, Southwest Japan were analyzed and compared with that of the Atlantis Bank gabbros. From these studies, it has been revealed that the physico-chemical conditions of low-temperature alteration of the oceanic lithosphere are variable between localities and between lithologies.

Free Research Field

岩石学

Academic Significance and Societal Importance of the Research Achievements

海洋下部地殻を構成する斑れい岩は火成作用による分化の程度が小さく，一般にアルカリ元素に乏しい。海洋下部地殻中に黒雲母のようなアルカリに富む鉱物が産出することは，世界で初めての発見である。これは海洋下部地殻が形成後に広範囲にわたって流体による化学的改変を受けている可能性を示唆するものである。また海洋リソスフェアの高温ならびに低温変質作用は，地域間あるいは岩相間で差異があることが明らかになった。これらのことはグローバルな物質循環および地球環境の変遷のプロセスを理解する上で重要な新知見である。