A viable mechanism to form boron-bearing diamonds in deep Earth

Boron is considered extremely depleted inside Earth's mantle. It is therefore a great challenge to elucidate the prevalence of boron impurity seen in sublithospheric diamonds, especially in identifying the boron source and the mechanism for its incorporation into these enigmatic diamonds. Here, we unveil a pathway for the crystallization of boron-bearing diamonds via redox reactions of carbonates and borides at pressure-temperature conditions relevant to the Earth's lower mantle. We present computational results along with pertinent experimental evidence for a genesis of boron-bearing diamonds via the redox reaction of CaCO3 and FeB at 22.5 GPa and 2100 K, corresponding to the geological conditions at the top of the lower mantle. The present findings offer a viable mechanism for the formation of boron-bearing diamonds deep inside the Earth's mantle.