Flotation of olivine and diamond in mantle melt at high pressure: Implications for fractionation in the deep mantle and ultradeep origin of diamond

The density of the three ultramafic melts, PHN1611, IT8720, and MA at high pressure was determined by the sink-float method using olivine and diamond as density markers. Using these data, we obtain K and K’ of the Birch-Murnaghan equation of state for the compression curves of the silicate melts. The density relation between the mantle minerals and the partial melt provides a possible fractionation mode in the deep mantle; accumulation of olivine and pyroxene could occur in the region just above the transition zone, and separation of the partial melt and garnet could provide a potential mechanism for enrichment of the basaltic component in the transition zone. The density relation between diamond, the mantle minerals, and the melt suggests a possibility for accumulation of diamond in the transition zone. Recent discovery of the high-pressure minerals as inclusions in diamond may be consistent with the present model of accumulation of diamond in the deep mantle. The compositional features of the high-pressure mineral inclusions in diamond are discussed on the basis of the mineral-melt partition coefficients determined in the laboratory. The present analysis implies that the compositions of the high-pressure phases in diamond, especially a high concentration of REE in Ca-perovskite cannot be explained by a simple chemical equilibrium at high temperature above 2000°C, but can be explained by (1) chemical equilibrium at low temperature around 1000-1200°C perhaps in the hydrous slab conditions, or (2) multiple events of partial melting and fractional crystallization, that preclude chemical equilibrium among mineral inclusions.