Genesis of diamonds in the lower mantle

Abstract

The “forbidden” assemblage (ferropericlase + enstatite) as inclusions in diamonds has been taken as evidence to imply that these inclusions and their host diamonds formed initially in the lower mantle. Magnesite is probably the only stable carbonate at depths greater than ∼220 km. Like dehydration reactions, the reaction boundary for the decarbonation of magnesite has a positive dT/dP slope at lower pressures, which becomes negative at higher pressures, if no other phase intervenes. This reaction boundary probably intersects the geotherm between ∼900 and ∼1100 km, below which magnesite decomposes into an assemblage periclase + diamond + oxygen. Thus, ferropericlase is the most likely inclusion in diamond formed in the lower mantle. The high frequency of sole occurrence of ferropericlase in diamonds from Sao Luiz, Brazil seems to substantiate the present speculation.