Diamond is an evidence for carbon existing in the deep Earth. Some diamonds are considered to have originated at various depth ranges from the mantle transition zone to the lower mantle. These diamonds are expected to carry significant information about the deep Earth. Here, we determined the phase relations in the MgCO 3 -SiO 2 system up to 152 GPa and 3,100 K using a double sided laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction. MgCO 3 transforms from magnesite to the high-pressure polymorph of MgCO 3, phase II, above 80 GPa. A reaction between MgCO 3 phase II and SiO 2 (CaCl 2 -type SiO 2 or seifertite) to form diamond and MgSiO 3 (bridgmanite or post-perovsktite) was identified in the deep lower mantle conditions. These observations suggested that the reaction of the MgCO 3 phase II with SiO 2 causes formation of super-deep diamond in cold slabs descending into the deep lower mantle.
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