Diamond precipitation from COH fluids: a case study from Pohorje, Eastern Alps, Slovenia

The metamorphic rocks from the Pohorje Mountains represent parts of the Austroalpine metamorphic units of the Eastern Alps, which are thought to experience the peak P-T in the ultrahigh-pressure zone during the Cretaceous orogeny, mainly near to or in the diamond stability field (Vrabec et al., 2012; Janák et al., 2015 and references therein). Gneisses are the most common occurring lithologies. They host garnet porphyroblasts, which contain numerous fluid and solid inclusions, among which diamonds have also been identified. To better understand their precipitation from COH fluids, thermodynamic modelling was performed and to elucidate the structure of diamond-bearing inclusions, they were investigated at the atomic scale using a plethora of TEM techniques.

The C-O-H fluid system undergoes evolution during metamorphism as a result of variations in pressure and temperature. The saturation of the C-O-H fluid with carbon depends on the P-T-fO₂ conditions. Carbon saturation of the C-O-H fluid can be represented with carbon saturation isopleths. Assuming that the fluids, trapped in the metapelitic rock share common P-T path, the relationship between the carbon saturation isopleths and the P-T path is essential.

P-T diagrams were calculated at various logfO₂ values to determine the threshold at which the system's behaviour changes. The modelling revealed that it is highly sensitive to the change of fO₂. Due to the shape of the P-T path of metapelite, graphite precipitates in all cases up to approximately 700 °C and 2.5 GPa, where the P-T path changes its slope. Up to the fO₂ value of QFM –1.5, the P-T path is parallel to the isopleths; therefore, the carbon doesn’t precipitate. When the fluid is more reduced, reaching QFM –1.6, the P-T path intersects the isopleths throughout its range. If the C-O-H fluid is even more reduced, the P-T path crosses more isopleths, resulting in even more abrupt precipitation of carbon. This means that ~ QFM –1.6 was the maximum fO₂  value at which the diamonds from the Pohorje metapelites could have precipitated.

The behaviour of the COH fluids predicts dissolution of the diamonds during the retrograde metamorphism, which didn’t occur. TEM analysis of the diamond-bearing inclusion revealed the presence of an amorphous phase, which enclosed the diamonds and prevented dissolution processes. Furthermore, the amorphous phase influenced the internal structure of the diamonds, which precipitated after the amorphous solid. Electron diffraction and high-resolution TEM showed that some diamonds are polycrystalline, composed of numerous nanocrystallites. The crystallisation of metamorphic diamonds suggests complex dynamics within the microsized system, which was partially revealed by our study. However, further studies are needed to draw more precise conclusions.

Janák, M., Froitzheim, N., Yoshida, K., Sasinková, V., Nosko, M., Kobayashi, T., Hirajima, T. & Vrabec, M. Diamond in metasedimentary crustal rocks from Pohorje, Eastern Alps: A window to deep continental subduction. J. Metamorph. Geol. 33, 495–512 (2015).

Vrabec, M., Janák, M., Froitzheim, N. & De Hoog, J. C. M. Phase relations during peak metamorphism and decompression of the UHP kyanite eclogites, Pohorje Mountains (Eastern Alps, Slovenia). Lithos 144, 40–55 (2012).