Olivine inclusions in diamond: towards real entrapment conditions

Diamond is an extraordinary material of the Earth’s deep interior, characterized by remarkable thermo-elastic properties and chemical stability. However, pure diamond itself does not provide definitive information about the pressure and temperature at which it forms but in principle, these conditions can be determined by measuring the stress state of minerals trapped as inclusions at the time of diamond growth (Angel et al. 2022). Indeed, inclusions and defects in diamond have the potential to provide fundamental constraints on the mechanisms of plate tectonics and carbon and volatile cycles in the Earth if the depth and temperature of diamond growth are known. Olivine is one of the most common mineral phases found within diamonds. However, most olivine inclusions entrapped in diamonds are surrounded by cracks, show evidence of fluid rims (Nimis et al. 2016) and the calculated residual pressures are so low that they indicate diamond growth and olivine entrapment outside the diamond stability field (Angel et al. 2022). This is clearly unrealistic and indicates the need to investigate the mechanisms responsible for the release of residual inclusion pressure, which are not accounted for by the simple elastic geobarometry model that has been successfully applied to inclusions in garnets from ultra-high-pressure metamorphic rocks (Murri et al. 2018, 2022).In this work we will therefore (i) review what is currently known about the effects of cracking and plastic deformation in diamond, as well as other factors that may contribute to the reduction in inclusion pressures; and (ii) discuss possible approaches to identify and quantify the key mechanisms responsible for low inclusion pressures that will then allow the entrapment conditions of the majority of inclusions in diamond to be determined. References: Angel, R. J., Alvaro, M., & Nestola, F. (2022). Crystallographic methods for non-destructive characterization of mineral inclusions in diamonds. Reviews in Mineralogy and Geochemistry, 88(1), 257-305.Murri, M., Mazzucchelli, M. L., Campomenosi, N., Korsakov, A. V., Prencipe, M., Mihailova, B. D., ... & Alvaro, M. (2018). Raman elastic geobarometry for anisotropic mineral inclusions. American Mineralogist, 103(11), 1869-1872.Murri, M., Gonzalez, J. P., Mazzucchelli, M. L., Prencipe, M., Mihailova, B., Angel, R. J., & Alvaro, M. (2022). The role of symmetry-breaking strains on quartz inclusions in anisotropic hosts: Implications for Raman elastic geobarometry. Lithos, 422, 106716.Nimis, P., Alvaro, M., Nestola, F., Angel, R. J., Marquardt, K., Rustioni, G., ... & Marone, F. (2016). First evidence of hydrous silicic fluid films around solid inclusions in gem-quality diamonds. Lithos, 260, 384-389. AcknowledgmentsThis work has been supported by the InROAD+ 2025 - Fostering ERC talents @UNIPV assigned to M. Murri and by the Fondazione Cariplo grant agreement #2023-2431 assigned to M. Alvaro.