On the Interpretation of Crystal Size Distributions in Magmatic Systems

Abstract

holocrystalline under a wide spectrum of cooling regimes implies batch system. Instead, the CSDs of each system reflect a combination that cooling and crystallization can be uncoupled and considered of kinetic and dynamic influences on crystallization. Heterogeneous separately. This is tantamount to realizing that the Avrami number nucleation and annexation of small crystals by larger ones, enis large in most igneous systems. Crystallization automatically trainment of earlier grown and ripened crystals, rate of solidification adjusts through nucleation and growth to the cooling regime, and front advance, and protracted transit of a well-established mush all aspects of the ensuing crystal population reflect the relative roles column are some of the eVects revealed in the observed CSDs. There of nucleation and growth, which reflect the cooling regime. The may be an overall CSD evolution, reflecting the maturity of characteristic scales of crystal size, crystal number, and crys- the magmatic system, from simple straight nonkinked CSDs in tallization time are intimately tied to the characteristic rates of monogenetic systems to multiply kinked, piecewise continuous CSDs nucleation and growth, but it is the crystal size distributions (CSDs) in well-established systems such as Hawaii and Mount Etna. This that provide fundamental insight on the time variations of nucleation is not unlike the evolution of CSDs in some industrial systems. and growth and also on the dynamics of magmatic systems. Crystal Finally, the fact that comagmatic CSDs are not often captured size distributions for batch systems are calculated by employing evolving systematically through large changes in nucleation rates, the Johnson‐Mehl‐Avrami equation for crystallinity related to even in low crystallinity systems, may suggest that magma is always exponential variations in time of both nucleation and growth. The laced with high population densities of nuclei, supernuclei, and slope of the CSD is set by the diVerence a ‐ b, where a and b crystallites or clusters that together set the initial CSD at high are exponential constants describing, respectively, nucleation and characteristic population densities. Further evolution of the CSD growth. The batch CSD has constant slope and systematically occurs through sustained heterogeneous nucleation and rapid anmigrates to larger crystal size (L) with increasing crystallinity. The nealing at all crystallinities beginning at the liquidus itself and diminution in nucleation with loss of melt is reflected in the CSD operating under more or less steady (not exponentially increasing) at late times by a strong decrease in population density at small rates of nucleation. crystal sizes, which is rarely seen in igneous rocks themselves. Observed CSDs suggest that a ‐ b ~6‐10 and that b ~0. That is, growth rate is approximately constant and nucleation rate apparently increases exponentially with time. Correlations among CSD slope, intercept, and maximum crystal size for both batch and open systems suggest that certain diagnostic relations may be useful in interpreting the CSD of comagmatic sequences. These systematics are explored heuristically and through the detailed