Diamonds formation revealed by their internal structure: a case study from Pohorje, Eastern Alps, Slovenia

Abstract

The metasedimentary rocks from the Pohorje Mountains (Slovenia) are predominantly gneisses, which represent parts of the Austroalpine metamorphic units of the Eastern Alps. The peak P-T conditions experienced during subduction in the Cretaceous orogeny reached UHPM in the diamond stability field [1, 2 and references therein]. The garnet porphyroblasts contain numerous fluid, solid and polyphase inclusions, among which diamonds have also been found. To better understand the conditions of diamonds formation, their internal structure was investigated. Advanced high-resolution (scanning) transmission electron microscopy (HR(S)TEM) techniques were applied, including high-angle annular dark-field (HAADF-STEM), combined with EDS and electron energy loss spectroscopy (EELS). The TEM lamella was prepared with FIB/SEM technique. Atomic scale resolution was achieved, allowing thorough analysis of diamond. The investigated lamella contained three diamond grains, which was also confirmed by EELS and EDS analysis. Contacts between the diamond grains and the host garnet are closely intergrown but show no crystallographic relations. Selected area electron diffraction patterns (SAED) of individual diamond grains show that one of the grains is monocrystalline, while the others are polycrystalline. The latter exhibit some preferential orientation of the crystallites. The high-resolution imaging allowed to recognise the structures of the polycrystalline grains at the atomic scale. They consist of numerous nanocrystallites ranging in size from a few to several tens of nanometres in size. Some of them exhibit an undisturbed internal structure, while the others show some imperfections in the form of stacking faults, twins, and lattice dislocations. In addition, a detailed HRTEM study of the polycrystalline diamonds revealed the presence of carbon domains in the form of graphite, which was confirmed with the SAED. Furthermore, in certain areas between the nanocrystalline grains and host garnet, an amorphous layer with a composition close to garnet was identified. Monocrystalline diamond appears without internal defects or graphitic domains and is completely crystalline. The presence of both monocrystalline and polycrystalline grains in a single inclusion is puzzling in terms of their crystallisation conditions. A change in physical (pressure, temperature) and/or chemical parameters (interaction of the trapped COH fluid with garnet) in the inclusions during metamorphism might induce the precipitation of diamonds with different structures. We hypothesise that the diamonds precipitated in response to changing P-T conditions during the prograde phase of metamorphism, when the solubility of C in COH fluids decreases. Absence of standalone prograde graphitic grains indicates that the C saturation was achieved in the diamond stability field. At first, before reaching peak metamorphism, monocrystalline diamond was formed. After reaching peak metamorphism, though still in the diamond stability field, the COH fluid began to interact with the walls of the garnet enabling precipitation of the amorphous phase, after which polycrystalline diamonds formed. The graphitic domains within the diamond nanocrystallites were formed due to transition from the diamond to the graphite stability field during exhumation of the rock. To draw more precise conclusions, further investigations need to be carried out.