In-situ observations of crystal growth in a basalt melt and the development of crystal size distribution (CSD) in igneous rocks.

Igneous rocks are formed by the solidification of silicate melts. From a microscopic perspective, melt solidification is known to involve a series of physico-chemical processes, including crystal nucleation, crystal growth, Ostwald ripening, and others. A good understanding of these processes and their collective operation is therefore fundamental in deciphering magma history from rock texture. To understand the solidification processes of natural magma and the texture evolution of igneous rocks, we have studied the crystallization of a high-K basaltic melt cooling from ~1240 °C by in-situ observation in a moissanite cell. In a series of experiments with different thermal history, olivine or clinopyroxene (cpx) appeared as the liquidus phase before the formation of plagioclase. Nucleation of olivine occurred only in a narrow temperature and time interval below the liquidus. During cooling at 100 °C/hr, the morphology of olivine and cpx transited from tabular to hopper habit. To first order approximation, crystal grow rate is proportional to crystal size. This causes a counterclockwise rotation of the crystal size distribution (CSD) curves, whereas Ostwald ripening reduces the population density of the smaller crystals. Our experiments provide the first observation of proportionate crystal growth and suggest that conventional analyses of crystal size distributions of igneous rocks may be in error – the slope of the CSD cannot be interpreted in terms of a uniform growth rate and the intercept with the vertical axis does not correspond to a nucleation density. In reality, nucleation of each mineral is probably limited to a short event, which is followed by a long period of crystal growth (with growth rates proportional to crystal size) and annealing.