PHASE TRANSFORMATION BOOSTER IN WATER DURING ULTRAHIGH-PRESSURE METAMORPHISM
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摘要: 据静水压力梯度计算, 榴辉岩形成深度至少要70km, 含柯石英榴辉岩的形成深度至少要12 0km, 而同时含有柯石英和金刚石的榴辉岩形成深度至少要145km, 超高压变质岩要从如此深的部位折返到地表是不可想象的.大量资料表明水参与了超高压变质作用过程.通过矿物的脱水行为、水的相变和地温曲线等研究, 指出由于存在水的相变增压等多种增压因素, 大大提高了超高压变质作用过程中的地压梯度, 据静水压力计算出的超高压变质岩形成深度只是最大深度, 其形成时的实际深度要比该最大深度小得多.Abstract: The calculation in the hydrostatic pressure gradient shows that the eclogites were formed in the depth of at least 70 km, and the coesite-bearing eclogites in the depth of at least 120 km. In addition, the eclogites containing both the diamond and coesite in the depth of at least 145 km. It is unimaginable that the ultrahigh-pressure metamorphic rocks are uplifted to the earth surface from such depths. Many references show that the water is indispensable to the ultrahigh-pressure metamorphism. Based on the studies of the mineral dehydration, the geothermal curves and the phase change diagram of water, this paper indicates that such supercharges as the phase transformation booster in water greatly increase the lithostatic gradient in the process of the ultrahigh-pressure metamorphism. The depth for the formation of the ultrahigh-pressure metamorphic rocks calculated with hydrostatic pressure is only the greatest depth. However, the actual depth for the formation of the ultrahigh-pressure metamorphic rocks was much smaller than the theoretical depth as calculated with hydrostatic pressure.
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Key words:
- ultrahigh-pressure metamorphism /
- water /
- phase change /
- pressure
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图 1 岩石圈内水和一些矿物的相图及地温曲线
SiO2相图引自文献[4];Dmd→Gr和Jd+Qz→Ab引自文献[6];前寒武纪地温和大洋地温据Clark和Ringwood据文献[5];秦岭中部地温线, 引用伊川-宜昌地学断面岩石圈地温分布图之栾川至西峡段地温, 据文献[8]绘制; A为水的临界点, tc为374℃, pc为0.022 1 GPa, 为了清晰, A点的压力未按比例标出, 引自文献[3];tm为地幔固相线, 在0.85 tm~tm之间温、压数据不准; 压力与深度的对应关系是据初步地球参考模型(PREM), 引自文献[7].相变温度是温度区间.不同的实验者结果有差异, 没有考虑体系中各组分相互作用对相变界线的影响, 此图不足于精确定p和t.St.斯石英; Coe.柯石英; Qz.石英; α-Qz.α石英; β-Qz.β石英; Dmd.金刚石；Gr.石墨; Jd.硬玉
Fig. 1. Phase diagram of water and some minerals and geothermal curves in the lithosphere
表 1 不同温度和压力下气态水的密度和摩尔体积
Table 1. Density and molar volume of vapor at various pressures and temperatures
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