Ultradeep diamond genesis at Redox conditions of slab-mantle boundary
Jing Gao1,2, Bin Chen1, Xiang Wu3
1 Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI, 96822, USA
2 Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing 100871, China
3 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan), Wuhan 430074, China
Diamond genesis is an intriguing issue for diamond resources and Earth Carbon Cycle. Besides kimberlitic diamonds, many characteristic ultradeep diamonds hosting inclusions with phase assemblages with a sublithospheric origin have been exploited throughout the world. Ultradeep diamonds with their chemical and inclusion compositions not only record a history of oceanic lithosphere subduction and upward transport at a depth of >250 km to 660 km or deeper, but also indicate their genesis pertinent to mantle-carbonate melts in a Fe0-bufferred reduced condition. In our pilot experiment, the formation of diamonds from MgCO3-Fe0 system was evidenced in a diamond anvil cell device at ~25 GPa and ~1800 K, the conditions of the depth of ~660 km in the mantle. A detailed study of redox mechanism of MgCO3-Fe0 coupling has been conducted using large volume press along the oceanic lithosphere subduction paths in the
pressure-temperature range of 6-25 GPa and 1200-2000 K, covering the formation region of most ultradeep diamonds. The clear reaction boundaries around Fe0-foil in contact with MgCO3 strongly support the redox reaction between carbonatitic slab and Fe0-bearing metals under mantle conditions. Our study experimentally documents the possibility of ultradeep diamond genesis at Redox conditions of carbonateitic slab and Fe0-bearing metals. Furthermore, we will discuss the rates of diamond formation as a function of pressure-temperature conditions.