A mineralogical Raman spectroscopy study on eclogitic garnet inclusions in diamonds from Argyle

In an attempt to better define the depths of formation of eclogitic-paragenesis diamonds from the Argyle lamproite pipe, we have employed a Laser Raman microprobe to determine the Raman peak shift of a garnet inclusion (extracted from diamond) with pressure in a diamond-anvil pressure cell. On the basis of these data, we further found that the in situ garnet inclusions record near-atmospheric pressures within the limits of experimental uncertainty. Data on the compressibility and thermal expansivity of both diamond and garnet were used to define a P-T curve for the entrapment of garnet in diamond. A window within the range 47 kbar at 1100° C (150 km) to 93 kbar at 1500° C (280 km) for the formation of syngenetic garnet inclusions in diamond is defined by the intersection of the continental geotherm with the diamond-graphite boundary and the entrapment curve determined in the present study. This P-T window is consistent with the constraints imposed by other petrological studies of co-existing inclusions. Most of eclogitic-paragenesis diamonds from Argyle are estimated to have formed at a depth less than 250 km, if temperature estimates from petrological study are used.