The effect of heterogeneous nucleation site availability on rock microstructure: a new perspective from EBSD investigation

Abstract In this study, we investigate the effect of the availability of sites for heterogeneous nucleation on the microstructure of moderately to highly undercooled synthetic trachybasalts under anhydrous and hydrous (2 wt% H₂O) conditions. Experiments were performed at 400 MPa by cooling melts from 1300 °C to target temperatures of 1150 °C, 1100 °C, and 1050 °C, followed by isothermal holds of 5 or 30 min. In one experimental series, 0.5 wt% Cr₂O₃ was added to increase the liquidus temperature of spinel and promote heterogeneous nucleation of clinopyroxene on spinel. All Cr-doped anhydrous samples comprise a high number of small skeletal crystals of clinopyroxene and spinel. Electron backscatter diffraction maps (EBSD) reveal the presence of crystallographic orientation relationships (CORs) at the interfaces shared between clinopyroxene and spinel grains situated in their centers, indicating heterogeneous nucleation of clinopyroxene on spinel. On the contrary, the Cr-free anhydrous samples are characterized by fewer large dendritic clinopyroxene crystals, which grew without significant competition from their neighbors because of the low amount of pre-existing spinel grains acting as heterogeneous nucleation sites. Our findings confirm that the availability of heterogeneous nucleation sites and the timing and sequence of nucleation can play a dominant role in shaping microstructures, more than compositional variations and nominal undercooling. Early formed mineral phases may act as seeds for heterogeneous nucleation and can potentially induce fast crystallization of magmatic systems that experience thermal or constitutional undercooling and increase magma viscosity, influencing its stagnation or fragmentation, depending on the crystallinity of the system.