Towards the real diamond formation conditions: insights and perspective

Pressure and temperature conditions of diamond formation provide valuable insights for reconstructing paleo-geotherms and the deep carbon cycle. These conditions can be determined by investigating mineral inclusions in diamonds using elastic geobarometry, which can be applied to any mineral inclusion in a diamond. However, it assumes purely elastic host-inclusion behaviour throughout exhumation. In reality, almost all mineral inclusions in diamonds are surrounded by cracks. Here, we combine in situ high-temperature micro-Raman spectroscopy with thermo-elastic modelling to evaluate the pressure-release process in one of the most common diamond inclusions, olivine [(Mg,Fe)2SiO4]. Our results indicate that a diamond can sustain pressures of ~2.5 GPa before undergoing minor fracturing, which induces only partial loss of inclusion pressure rather than a complete reset. The estimated pressure loss can lead to an underestimation of up to 2.5 GPa in the entrapment pressure at 1150 °C, a typical temperature for diamond formation. This study shows that olivine in diamond can undergo partial release of pressure during kimberlite eruptions due to fast adiabatic decompression, leading to cracked olivine inclusions and entrapment pressures underestimated by up to 2.5 GPa at 1150 °C