Diamond – garnet elastic geobarometry applied to an eclogitic system

The investigation of mineral inclusions in diamonds represents a unique tool to better understand the mineralogy and composition of hidden portions of Earth's mantle and, hence, determine conditions of pressure and temperature at the time of diamond formation. Using a combination of experimental techniques and different geothermobarometric approaches, we characterized a natural diamond from Udachnaya kimberlite pipe entrapping nine inclusions; the inclusions are five garnets, three clinopyroxene and one sulfide and represent an eclogitic paragenesis. Here, we adopted, for the first time, the elastic geobarometry method to the garnet-diamond inclusion-host system to calculate the entrapment conditions for the diamond-garnet pair, resulting in 5.7(±0.3) GPa at 1154 °C. These P-T data are compared with estimates obtained through chemical geothermobarometry, employing T projection onto the local geotherm, a common approach used for eclogite xenoliths in absence of robust calibrated barometers. Our data demonstrate that elastic geobarometry for the garnet-diamond pair results to be a very reliable tool to determine the diamond formation also for eclogitic systems and this will allow to expand our knowledge on eclogitic diamonds in terms of depth of formation.