CRYSTAL SIZE DISTRIBUTIONS IN IGNEOUS AND METAMORPHIC ROCKS (IGNEOUS PETROLOGY, VOLCANOLOGY, CRYSTALLIZATION)

A quantitative method of analyzing the size distribution of crystals in a crystallizing liquid has been developed from the study of aqueous brines (e.g. Randolph and Larson, 1971). The number of crystals in a given size class (population density n) is related to crystal size (L) by n = n('o)exp(-L/G(tau)) where G is the average crystal growth rate, (tau) is the average crystal growth time, and n('o) is the population density of nuclei (n as L (--->) 0). For systems that are adequately described by this equation, a plot of ln (n) vs. L gives a straight line with slope -1/G(tau) and intercept n('o). Crystal size distributions (CSDs) have been determined for selected suites of igneous and metamorphic rocks. CSD plots of both plagioclase and ilmenite from Makaopuhi lava lake define straight lines, implying that crystal nucleation and growth were continuous throughout the crystallization interval. Known temperature-depth distributions for the lava lake provide an estimate of the growth time ((tau)), and growth rates may be determined. Plagioclase growth rates are 5-10 x 10('-11) cm/sec; rates decrease with increasing crystallization. Plagioclase nucleation rates are 2-34 x 10('-3)/cm('3)sec. Comparison of these rates with those determined experimentally suggests that crystallization occurs at small undercoolings ((DELTA)T) throughout the crystallization interval. Plagioclase growth and nucleation rates were determined for phenocrysts and microlites in 1980-1986 Mount St. Helens dacite lavas. Linear CSDs and time brackets of 30-150 years yielded phenocryst growth and nucleation rates of 3-10 x 10('-12) cm/sec and 4-15 x 10('-6)/cm('3)sec, respectively. Metamorphic rocks from contact aureoles have linear CSDs. Using growth rate estimates of Wood and Walther (1983), growth times were found to be short, and temperature oversteps small. Regional metamorphic rocks have bell-shaped CSDs, a form which originates from initial continuous nucleation and growth of crystals and later modification by annealing. The large crystals may be used to obtain quantitative information on original conditions of crystal nucleation and growth. These studies of crystallization kinetics in both igneous and metamorphic rocks indicate that (1) crystal nucleation and growth is continuous throughout the crystallization interval, (2) crystallization in natural systems occurs at small values of (DELTA)T, and (3) while growth rates vary by only a few orders of magnitude, nucleation rates vary enormously and appear to be much more sensitive to conditions of (DELTA)T.