Spectroscopic 2D-tomography: Residual pressure and strain around mineral inclusions in diamonds

Abstract

We have studied high-pressure inclusions (Ca-silicates, coesite, graphite) in three large diamonds, one from the Kankan district, Guinea, and the other two from the Panda kimberlite, Ekati diamond mines, Canada. Using the in situ point-by-point mapping technique with a confocal Raman system, the mineralogy of the inclusions, as well as their area distribution pattern ( e.g. , of different Ca-silicate phases) and their order-disorder distribution pattern (shown for graphite/disordered carbon), were determined. Raman mapping of the host diamonds yielded 2D-tomographic pressure and strain distribution patterns and provided information on the residual pressure of the inclusions (∼ 2.3 GPa for a coesite inclusion and ∼ 2.6 GPa for a graphite inclusion). The inclusions are surrounded by haloes of significantly enhanced pressure, several hundred σm across. These haloes exhibit complex pressure relaxation patterns that consist of micro-areas affected by both compressive and dilative strain, with the latter being intensive enough to result in apparent “negative pressures”.