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First in situ observation of crystallization processes in a basaltic-andesitic melt with the moissanite cell

¹ Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany

The evolution of texture and crystal size distribution of igneous rocks is mainly determined by processes such as nucleation, grain growth, and Ostwald ripening. However, the role played by these processes on the final crystal size distribution is difficult to quantify with conventional methods. Here we present the first direct in situ observation of a crystallizing supercooled basaltic-andesitic liquid in order to directly assess the role of grain-scale processes occurring during magma solidification. The experiments were conducted at 900 °C and 1 bar with a new moissanite cell specifically designed for long-term high-temperature experiments. The method allowed the direct continuous observation of the crystallizing melt until the crystal fraction approached 0.7. Investigation of the evolving texture revealed that individual crystals followed distinct growth histories characterized by intervals of relatively uniform free growth, abrupt size increase due to grain coalescence, and growth reduction due to impingement; the latter played a dominant role at higher crystal fractions. Despite this growth dispersion, average growth rates calculated for single grains showed only little variation (4.9 x 10^–6 to 1.9 x 10^–6 mm/s). No clear relation between growth rate and crystal size was observed. Crystal coarsening through a combination of overgrowth and coalescence can also account for the counterclockwise rotation and the drop-off at smaller size fractions of the recovered crystal size distributions in our experiments. These results suggest that crystal size distributions of natural rocks do not record the original nucleation rates.

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