Quantifying the Effects of Non‐Hydrostatic Stress on Multi‐Component Minerals
Quantifying the Effects of Non‐Hydrostatic Stress on Multi‐Component Minerals
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DOI：
10.1029/2022jb025201
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发表时间：
2022-09
期刊：
影响因子：
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通讯作者：
Benjamin L. Hess;J. Ague;P. Voorhees
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文献类型：
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作者：
Benjamin L. Hess;J. Ague;P. Voorhees
矿物成分可用于推断地质过程的压力、温度和时间尺度。这些推论所依据的热力学技术假定了一个均匀的、恒定的压力。尽管如此，收敛边缘产生显著的非流体静力学（不相等）应力，违反了均匀压力假设并产生了不确定性。材料科学家F. Larché和J. Cahn推导出了一个适用于非流体静力学应力地质环境的方程，该方程将弹性多组分晶体中的应力和平衡成分联系起来。然而，先前的工作已经表明，对于具有理想混合行为的二元固溶体，需要数百MPa到GPa级的应力才能使矿物成分改变百分之几的摩尔分数，这限制了方程的适用性。在这里，我们将Larché和Cahn方程应用于石榴石，单斜辉石和斜长石固溶体，首次纳入非理想混合行为和两个以上的端元。我们表明，非理想混合增加预测的应力引起的组成变化高达一个数量级。此外，加入额外的固溶体端元改变了所考虑的其他端元的预测应力诱导成分变化。最后，我们证明了Larché和Cahn的方法产生正熵产生，这是任何真实的过程发生的要求。我们的研究结果表明，几十和几百兆帕之间的应力可以改变矿物成分的百分之几的摩尔分数。因此，矿物成分可以合理地用于推断应力状态。我们认为，应力成分效应可以在任何高级变质环境中通过晶内扩散发展，但最有可能在热，干燥和坚固的岩石中，如下地壳麻粒岩。
Mineral compositions are used to infer pressures, temperatures, and timescales of geological processes. The thermodynamic techniques underlying these inferences assume a uniform, constant pressure. Nonetheless, convergent margins generate significant non‐hydrostatic (unequal) stresses, violating the uniform pressure assumption and creating uncertainty. Materials scientists F. Larché and J. Cahn derived an equation suitable for non‐hydrostatically stressed geologic environments that links stress and equilibrium composition in elastic, multi‐component crystals. However, previous works have shown that for binary solid solutions with ideal mixing behavior, hundreds of MPa to GPa‐level stresses are required to shift mineral compositions by a few hundredths of a mole fraction, limiting the equation's applicability. Here, we apply Larché and Cahn's equation to garnet, clinopyroxene, and plagioclase solid solutions, incorporating for the first time non‐ideal mixing behavior and more than two endmembers. We show that non‐ideal mixing increases predicted stress‐induced composition changes by up to an order of magnitude. Further, incorporating additional solid solution endmembers changes the predicted stress‐induced composition shifts of the other endmembers being considered. Finally, we demonstrate that Larché and Cahn's approach yields positive entropy production, a requirement for any real process to occur. Our findings reveal that stresses between tens and a few hundred MPa can shift mineral compositions by several hundredths of a mole fraction. Consequently, mineral compositions could plausibly be used to infer stress states. We suggest that stress‐composition effects could develop via intracrystalline diffusion in any high‐grade metamorphic setting, but are most likely in hot, dry, and strong rocks such as lower crustal granulites.