Measurements of nucleation rates and real growth rates of plagioclase in a solution of basaltic composition.

Abstract

In all previous experimental studies on silicate systems, only nucleation densities and apparent growth rates were measured. Since growth of crystals in magma is regarded as solution growth, the nucleation density divided by time (N/t) and the apparent growth rate (R*) do not directly represent the nucleation rate I and the 'real' growth rate R, respectively. In this study, a series of experiments on the isothermal crystallization of plagioclase was carried out to determine the nucleation and 'real' growth rates using a natural basalt from Maegatake, Adatara-yama, Japan. The results indicate that the nucleation curves and the 'real' growth rates vs DELTA T are different from the nucleation density curve and the apparent growth rate vs DELTA T. Thus, care should be taken in interpreting textures of igneous rocks. Changes in morphology depending on DELTA T are also illustrated and discussed. (Authors' abstract)-M.Ak.