In-situ observation of the crystallization kinetics and texture evolution of basalts

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Crystal size distribution (CSD) theory has been used extensively to interpret the textures of igneous rocks and to extract information on the time scales of crystallization. In a series of experiments using a newly developed high-temperature cell with moissanite (SiC) anvils, it was possible to directly observe the crystallization of magmas under the microscope. The data show that classical CSD theory is fundamentally flawed and does not properly describe the textural evolution of a crystallizing magma. In particular, nucleation of crystals is limited to short events and does not continue once crystals of a phase started to grow. CSD curves are not shifted parallel to themselves with time, but they are rotated due to the effects of proportionate growth, grain coalescence and Ostwald ripening. In the continuation of this project, experiments shall clarify (1) whether crystal nucleation is always limited to short events in the crystallization of a magma, (2) under which conditions proportionate growth occurs (i.e. the growth rate is proportional to the size of a crystal), and (3) whether information on time scales can be reliably extracted from CSD diagrams. To this end, and to provide a quantitative calibration of igneous textures, crystallization experiments with variable cooling rates are planned for basaltic to andesitic melts.