Crystallization from a melt and crystallization at subsolidus conditions: comparison from crystal size distribution study on Gennargentu Rocks (Sardinia, Italy)

Plagioclase crystal size distribution (CSD) has been investigated in a quartz-diorite body, in the leucosome of migmatites and in the melanosome of un-melted contact metamorphic rocks from Gennargentu Complex (Sardinia, Italy). During the crystallization of the dioritic magma, a variety of competing kinetic processes determine the evolution of the igneous microstructure, but the relative contribution of each process remains elusive. Our approach was aimed to study the plagioclase crystallisation from a liquid (quartz-diorites and migmatite leucosomes), comparing it to a crystallisation at subsolidus conditions. CSD indicates that plagioclase in the quartz-diorite nucleated and grew in a cooling system at a constant cooling rate, producing straight-line CSD in a diagram of ln of population density vs. size range. The plagioclase crystallisation continued until the latent heat was available and the temperature was high enough to allow the plagioclase growing. This can occur only when a crystal is held at temperature close to its liquidus for a long period of time. Under these conditions, the plagioclase nucleation rate is zero, but growth rate is high for crystal larger than the critical size. This does not necessarily mean that the temperature was held constant, just that the undercooling remained small (Ostwald ripening process). The aggregated small crystals, due to their high surface energy per unit volume, to minimise energy in the system dissolved and“fed” the growth of larger crystals. This process occurs because small grains have a higher surface energy per unit volume than do larger grains. The crystallisation temperature (~900 °C, 100 MPa) allows the formation of plagioclase as liquidus phase. From CSD measurements we calculated the different cooling ages for the different sample types.