Bimodal distribution of amphibole-melt apparent dihedral angles in the product of the early crystallization of hydrous basaltic melts

Abstract

• Amp-Melt-Amp apparent dihedral angle distribution shows bimodality. • Bimodality becomes obvious after annealing for more than 1 h at 0.6 GPa and 970 °C. • Median-standard deviation evolution obeys no longer a unidirectional change. • The high anisotropy of amphibole is the main cause of the bimodality. The dihedral angle of mineral-melt is a very important parameter for further understanding the textural evolution of magma crystallization. In this study, we have investigated the amphibole-melt dihedral angle distribution of four previous experimental samples at 0.6 GPa and 970 °C with annealing time of 1 h, 10 h, 60 h, and 100 h, as well as two additional experiments with annealing time of 5 h and 30 h. In this experiment, we have achieved the assemblage of amphibole + granodioritic melt ± traces of other minerals in the process of melt crystallization under the above experimental conditions. The apparent dihedral angles of amphibole-melt of the experimental products were obtained by randomly measuring the dihedral angles in the backscattered images. The results indicate that the apparent dihedral angle presents a gradually obvious bimodal distribution with the increasing annealing time. The high anisotropy of amphibole is believed to be the main cause of the bimodal characteristics of the apparent dihedral angle distribution. Consequently, two Gauss functions are used to fit one distribution. As the annealing time increases, the median-standard deviation relationships of the apparent dihedral angle for these two Gauss functions show great differences. Therefore, our experiments proved that in the amphibole-melt system, the median-standard deviation with the growth of textural equilibrium in the early crystallization is no longer a unidirectional change. Furthermore, combining the results of this study and the previous work, whether the dihedral angle distribution in the early crystallization is unimodal or bimodal may have great relationship with the anisotropy of minerals.