Inclusions under remnant pressure in diamond: a multi-technique approach

Measurement of the remnant pressure sustained by a mineral inclusion within a diamond can allow calculation of the source pressure and temperature conditions of the diamond formation. While Raman spectroscopy (point analyses and 2D mapping) has been the most commonly used non-destructive method for measuring this remnant pressure, two new techniques (quantitative birefringence analysis and in situ X-ray diffraction) have recently been developed. In this paper we apply all of these techniques to the study of two diamonds. The first is a diamond twin (made) from Shandong (China), containing two olivine inclusions. Analysis of the largest inclusion by the birefringence and X-ray techniques returned compatible values for the remnant pressure of 0.1-0.2 GPa. However, while 2D Raman mapping of the diamond host qualitatively confirmed the presence of a radial stress field, the low remnant pressures values were too small to be detected by pressure shifts in the olivine's Raman spectrum. The second sample studied was a rounded dodecahedral diamond (unknown origin) containing several coesite inclusions. Its morphology prevented quantitative birefringence analysis or Raman mapping being performed but Raman analyses on three inclusions (2.2-2.5 GPa) provided similar results to those obtained by X-ray diffraction from a single inclusion (2.7 GPa); all of these values lie within the range of previous published remnant pressures for coesite in diamond. Calculation of the source pressure and temperature conditions for both diamond samples returned anomalously low values, both below the diamond stability field for a range of mantle temperatures (950-1350 degrees C). The reasons for this are due to violations of the fundamental assumptions upon which such studies of these elastic effects in diamond are based. Although previous studies showed that olivine inclusions have the potential to record reasonable formation pressures under favourable conditions, the present study re-iterates the serious concerns of using the coesite-in-diamond geobarometer. It is inferred that, as the remnant pressures in the coesite-diamond pair are so high, there is a high probability that the diamond will undergo some plastic deformation, thus reducing the elastic behaviour and leading to severe underestimation of entrapment pressure.