Kinetics of cooling- and decompression-induced crystallization in hydrous mafic-intermediate magmas

The crystallization of a hydrous basaltic-andesite magma was investigated using cooling and decompression experiments. These experiments were designed so that the driving force for crystallization of feldspar (the undercooling) is equal whether in the direction of cooling or decompression. After initial equilibration steps at conditions T = 1025 °C and PH2O = 150 MPa, charges were either cooled or decompressed near-instantaneously to final conditions corresponding to undercoolings ΔT⁎ = 52-154 °C, with dwell times of 12, 24 and 48 h at the final P-T. This 'single-step' experimental strategy (SSD for decompression experiments, SSC for cooling) allowed derivation of kinetic parameters such as nucleation and growth rates as a function of undercooling. Even though our setup was primarily designed to investigate feldspar crystallization, we report kinetic and geochemical data for all phases present (glass, feldspar, clinopyroxene, olivine and amphibole). At low to intermediate undercoolings, the solidification behavior of mafic-intermediate magmas is very similar texturally and compositionally whether the crystallization mechanism is cooling or decompression. At high degrees of undercooling, feldspar and clinopyroxene nucleation rates are nevertheless higher in SSD compared to SSC runs, perhaps due to slower diffusivities or larger nucleation barriers in the cooling runs. Minerals transition from interface-limited growth to diffusion-limited growth with increasing undercooling, while nucleation regimes shift from homogeneous to heterogeneous. Crystal morphologies are highly variable but can be used qualitatively to assess the extent of undercooling. Relative to more silicic melts with similar crystal contents, feldspar formed in andesitic melts nucleate at a lower rate but grow to larger sizes.