Determination of crystallization temperatures in fractional crystallization series by nickel partitioning equations

Abstract

Representative crystallization temperatures for five basaltic series are calculated by use of Ni partitioning equations using the Neuman equation, which expresses the bulk partition coefficient in terms of mineral-liquid coefficients and mineral weight fractions. Bulk partition coefficients are calculated using the Rayleigh distribution law. Mineral compositions of the precipitating solid are determined from a computer program which allows us to calculate the mineral composition of the solid subtracted from the liquid, and from judgements based on the petrography of the samples. We use the empirical equations for Ni mineral-liquid partitioning with temperature for restricted SiO 2 compositions calculated by Hart and Davis (1978) , Leeman (1974) , D.J. Lindstrom (unpublished, 1976) and Hakli and Wright (1967) and Hakli (1968) . These are substituted into the Neumann equation in place of the mineral-liquid partition coefficients, and this equation is then solved to determine the temperature. These calculations have been made for volcanic series in Terceira (1200–1270°C) and Flores (1050–1225°C) Islands in the Azores, Miocene plateau basalts in northwest Iceland (1180–1270°C), Cape Verde Islands (970–1180°C) and Grenada Island (1200–1240°C), Lesser Antilles arc. The extremes of the temperatures in the brackets represent the differences between the sets of equations used; however, the calculated error on each temperature is only about ±2% or 20–25°C despite much larger errors on the calculated bulk partition coefficient and mineral fractions. The results have been counterchecked for Terceira and Flores by calculating plagioclase temperatures based on the method of Drake (1976) . The plagioclase temperature for Terceira (1270°C) agrees almost exactly with the Hart-Leeman-Lindstom calculations. The plagioclase temperature for Flores (1130°C) lies in between those calculated by the partitioning equations, but this may be due to the error in the orthopyroxene equation.