Origin and formation environment of deep mantle diamonds and the role of fluid

2022 Fiscal Year Final Research Report

• Project/Area Number: 20H02009
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Review Section: Basic Section 17040:Solid earth sciences-related
• Research Institution: Tohoku University
• Principal Investigator: Ohfuji Hiroaki 東北大学, 理学研究科, 教授 (80403864)
• Co-Investigator(Kenkyū-buntansha): M Satish‐Kumar 新潟大学, 自然科学系, 教授 (50313929) ; 鍵 裕之 東京大学, 大学院理学系研究科(理学部), 教授 (70233666)
• Project Period (FY): 2020-04-01 – 2023-03-31
• Keywords: ダイヤモンド / C-H-O流体 / マグネサイト / 炭素同位体

Outline of Final Research Achievements

Analysis of nano-scale inclusions in natural samples indicates that the formation of “deep diamonds” is most likely to occur in a fluid-saturated environment. High-pressure experiments in the presence of carbonate and a CH4-rich C-H-O fluid revealed that the diamonds produced by the reaction between them have a very wide range of carbon isotopic compositions, suggesting that similar carbon isotopic compositions (-25 to -15‰) found in some E-type diamonds and deep diamonds are likely the result of carbon isotope assimilation through the reaction of subducted carbonate and reduced C-H-O fluids derived from biogenic materials. Mixing of the CO2 fluid produced by carbonate decomposition with the surrounding CH4-rich reducing C-H-O fluid may be the key to the formation of “deep diamonds”.

Free Research Field

鉱物学

Academic Significance and Societal Importance of the Research Achievements

本研究の結果，起源・成因のよく分かっていないマントル深部起源のダイヤモンドの形成にCH4やH2Oなどの混合からなるC-H-O流体が密接に関与し，そのような流体と沈み込み帯から地球深部へ供給される炭酸塩との反応が深部起源ダイヤの起源となり得ることが初めて明確に示された．ダイヤモンドは，地球の深部に渡る炭素の大循環の主役であり，その成因を理解することは，地球における物質循環とその時間的変動・進化を捉える上でも意義は大きい．