Presentation Information
[R2-05]Crystal structure of hydroxide spinel in a mélange chromitite with microdiamonds at Saikai, Nagasaki, Japan.
*Akira YOSHIASA1, Satoko Ishimaru1, Makoto Tokuda1, Tadao Nishiyama1, Shoji Arai2, Kazumasa Sugiyama3 (1. Kumamoto University. Sci, 2. Kanazawa University. Sci, 3. Tohoku University. Material)
Keywords:
hydroxide spinel with microdiamonds,multi-defects structure,single crystal diffraction,water in the mantle
We have found euhedral single crystals of hydroxide spinel (3-8 wt% H2O) in a diamond-bearing serpentinite mélange exhumed from depths greater than 90 km. The cation distribution of a certain hydroxide spinel is T(Fe2+0.48Fe3+0.17Si0.08Mg0.03Mn0.02Zn0.0Vacant0.21) M(Cr0.92Fe2+0.34Fe3+0.31Ni0.03Ti0.02Ca0.01Al0.01Vacant0.36) [O2.47(OH)1.53]. A significant H is incorporated into the spinel structure along with the formation of cation vacancies. The substitution by tetravalent Si or Ti stabilizes the hydroxide spinel structure. The formation of this high configuration-entropy multi-defects hydroxide spinel indicates an abundant supply of hydrogen, therefore must have occurred under water-saturated conditions. The occurrence of the hydroxide spinel provides compelling evidence that the cold subduction plates transport water deep into the mantle, hydrating the transition zone reservoir.
