Decompression-induced crystallization in hydrated silica-rich melts: Empirical models of experimental plagioclase nucleation and growth kinetics
Abstract
Isothermal and isobaric crystallization of plagioclase in a water-saturated synthetic rhyolitic melt is investigated through time series of decompression experiments. The experimental variables are the rate at which samples are initially decompressed (30, 150, and 1200 MPa/h) from 200 MPa and 875°C, final pressure (25 to 160 MPa), and holding time at final pressure (up to 17 days). Through textural measurements of the crystals, plagioclase crystallization kinetics has been characterized in terms of nucleation lag and rates of nucleation and growth. Plagioclase crystallization is markedly dependent on effective undercooling, Teff, and holding time at crystallization pressure. With Teff increasing from 55 to 110°C, (i) nucleation lag decreases from 1-2 days to ~15 min, (ii) maximum nucleation rates increase from ~10-3 to 10-2 mm-2.s-1, and (iii) maximum growth rates decrease from ~10-6 to 5x10-7 mm.s-1. The initial decompression rate (30, 150, and 1200 MPa/h) has no systematic control on crystallization at final pressure, except for the 1200-MPa/h series in which samples show nucleation difficulties. From the experimental data of Teff-constrained plagioclase number density, proportion, and morphology, we provide means to assess conditions of nucleation and growth of natural plagioclase microlites from rapidly-ascended rhyolitic melts, through the determination of plagioclase liquidus curve and Teff prevailing during crystallization.
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