4.1 YANG Jingsui
Diamond in ophiolitic mantle rocks: A new observation on deep mantle carbon
Jingsui Yang1, Dongyang Lian1, Paul T. Robinson1，Weiwei Wu1, Xiangzhen Xu1, Fahui Xiong1, Michael Wiedenbeck2
1 CARMA, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China, (yangjsui@163.com)
2 SIMS Laboratory of Helmholtz-Zentrum Potsdam, GeoForschungsZentrum, Potsdam, Germany （michawi@gfz-potsdam.de）
It is known that carbon can be transported into the mantle by subducted slabs and recycled back to the surface by degassing of volcanoes at mid-ocean ridges, arc volcanoes, oceanic islands and plumes. Diamond from the mantle rocks and chromitite in ophiolites may provide a new observation on this process of carbon recycling.
Ophiolites are fragments of ancient oceanic lithosphere tectonically accreted into continental margins, which usually contain mantle peridotite along with significant podiform chromitites. Recently, ultrahigh-pressure (UHP) and super-reducing mineral groups have been discovered in peridotites and/or chromitites in 15 ophiolites around the world, including China, Russia, Myanmar, Albania, Turkey and India. Diamond is a typical and common ultrahigh-pressure mineral in these locations, and other minerals include coesite, pseudomorphic stishovite, qingsongite (BN) and Ca-Si perovskite. The most important native and highly reduced minerals recovered to date include moissanite (SiC), Ni-Mn-Co alloys, Fe-Si and Fe-C phases. These mineral groups collectively confirm extremely high pressures (300 km to ≥660 km) and super-reducing conditions in their environment of formation in the mantle.
Secondary ion mass spectrometric (SIMS) analysis shows that the ophiolite-hosted diamonds have distinctive 13C-depleted isotopic composition (δ13C from −18 to −28‰, n=70), comparable to the ophiolite-hosted moissanite (δ13C from −18 to −35‰, n=36); both are much lighter than the main carbon reservoir in the upper mantle (δ13C near −5‰). The compiled data from moissanite from kimberlites and other mantle settings share the characteristic of strongly 13C-depleted isotopic composition. This suggests that the diamonds and moissanite originated from a distinct carbon reservoir in the mantle or that their formation involved strong isotopic fractionation. Subduction of biogenic carbonaceous material could potentially satisfy both the unusual isotopic and redox constraints on diamond and moissanite formation, but this material would need to stay chemically isolated from the upper mantle until it reached the high-T stability field of diamond and moissanite.
The carbon isotopes and other features of the ultrahigh-pressure and super-reduced mineral groups point to previously subducted surface material as their source material. Recycling of subducted crust in the deep mantle may proceed in three stages: Stage 1 – Carbon-bearing fluids and melts may have been formed in the mantle transition zone (MTZ, ≥410 km), or even deeper. Stage 2 – Fluids or melts may rise along with deep plumes. Some minerals, such as diamond, stishovite, qingsongite and Ca-silicate perovskite can precipitate from these fluids or melts in the deep mantle during their ascent. Material transported to the MTZ would be mixed with highly reduced and UHP phases, presumably derived from zones with extremely low fO2, as required for the formation of moissanite and other native elements. Stage 3 – Continued ascent of peridotites above the transition zone would carry chromite and ultrahigh-pressure minerals to shallow mantle depths, where they would be incorporated in peridotites and chromitites of oceanic lithosphere, and eventually emplaced on land as ophiolites. The widespread occurrence of ophiolite-hosted diamonds and associated UHP mineral groups suggests that they may be a common feature of in-situ oceanic mantle, and that they can act as a new window into the life cycle of deeply subducted oceanic and continental crust.