From the lithosphere to the lower mantle: An aqueous-rich metal-bearing growth environment to form type IIb blue diamonds

A study of five diamonds containing mineral and fluid inclusions, selected among forty-nine specimens from the Cullinan Mine, South Africa, was carried out to better document origin formation N-absent B-poor (type IIb) diamonds. The combination several in-situ non-destructive techniques used identify mineralogy chemical composition primary secondary inclusions. These include breyite, larnite, graphite, Fe-Ni-Cu native metallic alloys, sulfides pyrrhotite group, Ni-rich oxide potential hydrous ferric sulfates. common abundant H2O + CH4 also identified. From various observations, we suggest that these type IIb grew in an aqueous oxidized reacting with a reduced mantle characterized by low oxygen fugacity. Remnant pressures recorded breyite Raman shifts XRD measurements enabled calculation minimal entrapment inclusions using elastic geothermobarometry. Applying pressure corrections caused relaxation, minimum trapping 4.9 GPa 5.6 were calculated, suggesting lithospheric depths consistent occurrence numerous graphite association which is often considered as indicator sublithospheric origin, occurs at 6 or lower H2O-rich carbonate/Ca-rich environment. features do not require classic subduction-related model their formation. Whereas high-pressure minerals would host boron cold subducting slabs, slabs are important carriers nitrogen into deep mantle, this latter element mostly absent In our alternative model, proposed source boron, whereby alloys N speciation between melt still prevent incorporation nitrogen, leading expression blue, boron-related observed mineralogical assemblage neither proves nor does it exclude for Hence, propose form continuum, depths.