The carbon-bearing phases in the subducted slabs under the lower mantle condition

Carbon, which is one of the most important volatile elements in the solar system, is suggested to be stored in the deep part of the Earth. The evidence for the deep carbon is found in super-deep diamonds or estimations of carbon fluxes between the surface and interior of the Earth. The candidates of a carbon-source into the mantle are subducting slabs. Therefore, it is important for the studying of Deep Carbon Cycle to reveal the reactions related to carbon-bearing phases in the slabs descending into the lower mantle. The MgCO₃-SiO₂ system is considered to constrain the carbon-bearing phases in the slabs since following reactions can occur under the lower mantle conditions:

MgCO₃ (magnesite) + SiO₂ (stishovite) → MgSiO₃ (perovskite) + CO₂

CO₂ → C (diamond) + O₂

The phase boundary in the MgCO₃-SiO₂ system has ambiguity because of the contradiction between the previous studies. We aimed to reconcile this contradiction and determine the potential carbon-bearing phases in the deep subducting slabs.

We have investigated the reaction between MgCO₃ and SiO₂ up to about 100 GPa and 3000 K using the double sided laser heated diamond anvil cell combined with the in-situ synchrotron XRD technique and Raman spectroscopy. The starting material was a powered 1:1 (in mole fraction) mixture of natural magnesite (Brazil, Bahia) and reagent α-quartz. XRD patterns of high P-T samples and recovered samples were acquired at BL10XU, SPring-8. We measured the Raman spectra of the samples at high-P and room temperature and those recovered at an ambient condition.

Diamond and MgSiO₃ perovskite were observed at 70 GPa and temperatures above 1750 K. The high P-T XRD patterns showed the appearance of MgSiO₃ perovskite at 50-60 GPa and around 2000 K. Our study revealed that magnesite could decarbonate to form diamond in cold slabs at the depths greater than 1800- km depth due to the above reactions in the MgCO₃-SiO₂ system.