Decompression-induced Crystallization in Hydrated Silica-rich Melts: Empirical Models of Experimental Plagioclase Nucleation and Growth Kinetics
Decompression-induced Crystallization in Hydrated Silica-rich Melts: Empirical Models of Experimental Plagioclase Nucleation and Growth Kinetics
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DOI：
10.1093/petrology/egs031
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发表时间：
2012-08
影响因子：
3.9
通讯作者：
E. Mollard;C. Martel;J. Bourdier
中科院分区：
文献类型：
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作者：
E. Mollard;C. Martel;J. Bourdier
通过时间序列的减压实验，研究了斜长石在水饱和流纹质熔体中的等温等压结晶过程。实验变量是样品从200 MPa和875 ℃初始减压（30、150和1200 MPa/h）的速率、最终压力（25至160 MPa）和在最终压力下的保持时间（最多17天）。通过晶体的织构测量，斜长石结晶动力学的特点是成核滞后和成核和生长速率。斜长石结晶明显依赖于有效过冷度、过冷度Teff和在结晶压力下的保持时间。随着温度Teff从55 °C增加到110°C，（i）成核滞后从1-2天减少到~15分钟，（ii）最大成核速率从~10-3增加到10-2 mm-2.s-1，以及（iii）最大生长速率从~10-6减少到5x 10 -7 mm-2.s-1。初始减压速率（30，150和1200 MPa/h）没有系统的控制结晶在最终压力，除了1200-MPa/h系列中的样品显示成核困难。通过测定斜长石液相线曲线和结晶过程中普遍存在的斜长石数密度、比例和形态，从斜长石数密度、比例和形态的实验数据中，我们提供了从快速上升的流纹质熔体中评估天然斜长石微晶成核和生长条件的方法。
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.