非静水压力下的流体-矿物质平衡:分子动力学的启示

IF 1.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY
M. Mazzucchelli, E. Moulas, B. Kaus, Thomas Speck
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引用次数: 0

摘要

对地质材料中的相平衡和反应的解释是以标准热力学为基础的,而标准热力学假定的是静水和均质应力条件。然而,岩石圈中的岩石和矿物可以承受应力梯度和非静水压力。目前,还没有一个公认的宏观热力学理论来包括非静水压力对矿物反应的影响,而且在受压地质系统中使用几种热力学势的问题仍有争议。在非静水压力下进行的实验中,由于可能会产生压力梯度,因此往往难以解决差应力对相平衡的直接影响。这种梯度会影响局部尺度的变质平衡。在此,我们利用分子动力学模拟研究了非零压力和高温条件下均质、非静水压力场对固液平衡的直接影响。我们的结果表明,对于恒温下的简单单组分系统,受压系统的平衡流体压力总是大于静水压力条件下的流体压力值。与固体晶体中的差应力水平相比,流体压力平衡值的位移大约小一个数量级。因此,以流体压力作为平衡压力的代表,可以准确预测相平衡。相反,在处于热力学平衡状态的受压系统中,固体的平均应力可能与流体的压力有很大偏差。这对在地球动力学模型中使用热力学压力产生了影响,因为流体压力是预测变质反应位置的更准确的替代物,而固体的平衡密度则必须根据其平均应力来确定。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Fluid-mineral Equilibrium Under Nonhydrostatic Stress: Insight From Molecular Dynamics
The interpretation of phase equilibria and reactions in geological materials is based on standard thermodynamics that assumes hydrostatic and homogeneous stress conditions. However, rocks and minerals in the lithosphere can support stress gradients and nonhydrostatic stresses. Currently, there is still not an accepted macroscopic thermodynamic theory to include the effect of nonhydrostatic stress on mineral reactions, and the use of several thermodynamic potentials in stressed geological system remains under debate. In experiments under nonhydrostatic stress, it is often difficult to resolve the direct effect of differential stress on phase equilibria because pressure gradients may be developed. Such gradients can affect the metamorphic equilibria at the local scale. Here, we investigate the direct effect of a homogeneous, nonhydrostatic stress field on the solid-fluid equilibrium using molecular dynamics simulations at non-zero pressure and elevated temperature conditions. Our results show that, for simple single-component systems at constant temperature, the equilibrium fluid pressure of a stressed system is always larger than the value of fluid pressure at hydrostatic stress conditions. The displacement of the equilibrium value of the fluid pressure is about an order of magnitude smaller compared to the level of differential stress in the solid crystal. Thus, phase equilibria can be accurately predicted by taking the fluid pressure as a proxy of the equilibration pressure. On the contrary, the mean stress of the solid can deviate substantially from the pressure of the fluid in stressed systems at thermodynamic equilibrium. This has implications on the use of thermodynamic pressure in geodynamic models since the fluid pressure is a more accurate proxy for predicting the location of metamorphic reactions, while the equilibrium density of the solid has to be determined from its mean stress.
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来源期刊
American Journal of Science
American Journal of Science 地学-地球科学综合
CiteScore
5.80
自引率
3.40%
发文量
17
审稿时长
>12 weeks
期刊介绍: The American Journal of Science (AJS), founded in 1818 by Benjamin Silliman, is the oldest scientific journal in the United States that has been published continuously. The Journal is devoted to geology and related sciences and publishes articles from around the world presenting results of major research from all earth sciences. Readers are primarily earth scientists in academia and government institutions.
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