Crystal size distribution (CSD) in rocks and the kinetics and dynamics of crystallization II: Makaopuhi lava lake

Abstract

Crystal size distribution (CSD) theory has been applied to drill core samples from Makaopuhi lava lake, Kilauea Volcano, Hawaii. Plagioclase and Fe-Ti oxide size distribution spectra were measured and population densities (n)were calculated and analyzed using a steady state crystal population balance equation: n=n ⁰ exp(-L/Gτ). Slopes on ln(n) versus crystal size (L) plots determine the parameter Gτ, a. product of average crystal growth rate (G) and average crystal growth time (τ). The intercept is J/G where J is nucleation rate. Known temperature-depth distributions for the lava lake provide an estimate of effective growth time (τ), allowing nucleation and growth rates to be determined that are independent of any kinetic model. Plagioclase growth rates decrease with increasing crystallinity (9.9−5.4×10⁻¹¹ cm/s), as do plagioclase nucleation rates (33.9−1.6×10⁻³/cm³ s). Ilmenite growth and nucleation rates also decrease with increasing crystallinity (4.9−3.4 ×10⁻¹⁰ cm/s and 15−2.2×10⁻³/cm³ s, respectively). Magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus (G =2.9×10⁻¹⁰ cm/s, J=7.6×10⁻²/cm³ s). Moments of the population density function were used to examine the change in crystallization rates with time. Preliminary results suggest that total crystal volume increases approximately linearly with time after ∼50% crystallization; a more complete set of samples is needed for material with <50% crystals to define the entire crystallization history. Comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling. This interpretation is consistent with proposed thermal models of magmatic cooling, where heat loss is balanced by latent heat production to maintain equilibrium cooling.