Geophysical investigations and laboratory experiments show evidence for possible subduction of ancient oceanic crust.Geological and mineralogical observations suggest that subducted oceanic crust is recycled into the ...Geophysical investigations and laboratory experiments show evidence for possible subduction of ancient oceanic crust.Geological and mineralogical observations suggest that subducted oceanic crust is recycled into the upper mantle.The subduction is supported by the recovery of superdeep diamonds from kimberlites and the presence of crustal materials in ophiolitic chromitites and their host peridotites.What is the mechanism?Here we report the new discovery of ophiolite-hosted diamonds in the podiform chromitites within the Skenderbeu massif from the Mirdita ophiolite in the western part of Neo-Tethys(Fig.1).The diamonds are characterized by exceedingly light C isotopes(δ13CPDB^-25‰),which can be interpreted as evidence for subduction of organic carbon from Earth’s surface.The diamonds are also characterized by an exceptionally large range inδ15Nair(-12.9‰to+25.5‰),accompanied by a low N aggregation state(Fig.2).On the other hand,materials sparsely included in diamonds include amorphous material,Ni-Mn-Co alloy,nanocrystals(20 nm×20 nm)of calcium silicate with an orthorhombic perovskite structure(Ca-Pv),and fluids(Fig.3).We consider that the Skenderbeu diamonds nucleated and grew from a C-saturated,NiMnCo-rich melt derived from a subducted slab of ocean crust and lithosphere in the deep mantle environment.The environment is in the diamond stability field or near the top of the mantle transition zone.The new discovery of diamonds from the Mirdita ophiolite provides a valuable opportunity to understand deep cycling of subducted oceanic crust and mantle(i.e.,composition and process).展开更多
The diamond-to-graphite transformation at diamond-stable conditions is studied by temperature gradient method (TGM) under high pressure and high temperature (HPHT), although it is unreasonable from the view of thermod...The diamond-to-graphite transformation at diamond-stable conditions is studied by temperature gradient method (TGM) under high pressure and high temperature (HPHT), although it is unreasonable from the view of thermodynamic considerations. It is found that, at diamond-stable conditions, for example, at 5.5 GPa and 1550 K, with fine diamond grits as carbon source and NiMnCo alloy as metal solvent assisted, not only large diamond crystals, but metastable regrown graphite crystals would be grown by layer growth mechanism, and the abundance of carbon source in the higher temperature region is indispensable for the presence of metastable regrown graphite crystals. From this transformation, it is concluded that, with metal solvent assisted, although the mechanism of crystal growth could be understood by the macro-mechanism of solubility difference between diamond and graphite in metal solvents, from the point of micro-mechanism, the minimum growth units for diamond or graphite crystals should be at atomic level and unrelated to the kinds of carbon source (diamond or graphite), which could be accumulated free-selectively on the graphite with sp2π or diamond crystals with sp3 bond structure.展开更多
基金funded by grants from the International Geoscience Programme(IGCP 649,2015–2020)the Natural Science Foundation of China(41802055 and 41802034)+3 种基金the project from the China Postdoctoral Science Foundation(2018M632942)the Natural Science Foundation of Jiangsu(BK20180349)the Research Founding from Chinese Academy of Geological Sciences(J1903)the fund from the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources
文摘Geophysical investigations and laboratory experiments show evidence for possible subduction of ancient oceanic crust.Geological and mineralogical observations suggest that subducted oceanic crust is recycled into the upper mantle.The subduction is supported by the recovery of superdeep diamonds from kimberlites and the presence of crustal materials in ophiolitic chromitites and their host peridotites.What is the mechanism?Here we report the new discovery of ophiolite-hosted diamonds in the podiform chromitites within the Skenderbeu massif from the Mirdita ophiolite in the western part of Neo-Tethys(Fig.1).The diamonds are characterized by exceedingly light C isotopes(δ13CPDB^-25‰),which can be interpreted as evidence for subduction of organic carbon from Earth’s surface.The diamonds are also characterized by an exceptionally large range inδ15Nair(-12.9‰to+25.5‰),accompanied by a low N aggregation state(Fig.2).On the other hand,materials sparsely included in diamonds include amorphous material,Ni-Mn-Co alloy,nanocrystals(20 nm×20 nm)of calcium silicate with an orthorhombic perovskite structure(Ca-Pv),and fluids(Fig.3).We consider that the Skenderbeu diamonds nucleated and grew from a C-saturated,NiMnCo-rich melt derived from a subducted slab of ocean crust and lithosphere in the deep mantle environment.The environment is in the diamond stability field or near the top of the mantle transition zone.The new discovery of diamonds from the Mirdita ophiolite provides a valuable opportunity to understand deep cycling of subducted oceanic crust and mantle(i.e.,composition and process).
基金Supported by the National Natural Science Foundation of China (Grant No. 50572032)Foundation of He’nan Educational Committee (Grant No. 2009A430014)Open Research Fund Program of State Key Laboratory of Superhard Materials of Jilin University (Grant No. 200801)
文摘The diamond-to-graphite transformation at diamond-stable conditions is studied by temperature gradient method (TGM) under high pressure and high temperature (HPHT), although it is unreasonable from the view of thermodynamic considerations. It is found that, at diamond-stable conditions, for example, at 5.5 GPa and 1550 K, with fine diamond grits as carbon source and NiMnCo alloy as metal solvent assisted, not only large diamond crystals, but metastable regrown graphite crystals would be grown by layer growth mechanism, and the abundance of carbon source in the higher temperature region is indispensable for the presence of metastable regrown graphite crystals. From this transformation, it is concluded that, with metal solvent assisted, although the mechanism of crystal growth could be understood by the macro-mechanism of solubility difference between diamond and graphite in metal solvents, from the point of micro-mechanism, the minimum growth units for diamond or graphite crystals should be at atomic level and unrelated to the kinds of carbon source (diamond or graphite), which could be accumulated free-selectively on the graphite with sp2π or diamond crystals with sp3 bond structure.
