Reactive fluid flow can control the mineralogical, mechanical and chemical evolution of the Earth’s crust. When rocks are exposed to differential stresses (i.e., vertical stress ≠ horizontal stress ≠ pore-fluid pressure (Pf)) during reactive fluid flow, effective pressure is usually assumed to control the overall reaction process. Here, we show that fluid pressure can play an important role in mineral replacement reactions. We conducted experiments in which calcite (CaCO3) grains (fraction size 53–150 µm) reacted with a Mg-rich solution at ~ 200 °C both in a closed system and under reactive fluid flow conditions with different fluid flow rates and fluid pore pressures, but with similar confining pressure (σn = 10 or 20 MPa) and effective pressure (Pe). Under closed system, vapor-saturated pressures, the magnesite formed with large pores between the magnesite and the calcite. In the open system flow-through experiments, however, brucite (Mg(OH)2) or magnesite (MgCO3) formed, depending on pore-fluid pressure. The main reaction product was brucite at low pore-fluid pressure (0.2 MPa), but magnesite at higher pore-fluid pressures (≥ 1 MPa). Calcite dissolution and precipitation of the product mineral increased concomitantly with flow rate, but the flow rate did not affect the nature of the products. The permeability of the reacting rock was related to the reaction pathway, i.e. the nature of the products. Magnesite replaced the pristine calcite in a pseudomorphic manner, and mantled the pristine calcite with 10–100 µm wide pores. In contrast, tabular and/or platy brucite blocked the porosity and resulted in a decrease in permeability. Our results show that the pore-fluid pressure can be a significant parameter controlling the reaction products and reaction processes in volatile-rich (e.g., CO2, HCl, H2S and SO2) systems at conditions close to phase separation; these conditions occur for example in epithermal and porphyry hydrothermal systems, and in carbonate-replacement and some metamorphic environments.

中文翻译：

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通过流体压力控制的方解石置换形成碳酸镁和氢氧化镁

反应性流体流动可以控制地壳的矿物、机械和化学演化。当岩石在反应流体流动过程中受到不同应力（即垂直应力≠水平应力≠孔隙流体压力（Pf））时，通常假设有效压力控制整个反应过程。在这里，我们表明流体压力可以在矿物质置换反应中发挥重要作用。我们进行了实验，其中方解石 (CaCO3) 颗粒（粒度为 53–150 µm）与富镁溶液在 ~ 200 °C 下在封闭系统中和在具有不同流体流速和流体孔隙压力的反应流体流动条件下发生反应，但具有相似的围压（σn = 10 或 20 MPa）和有效压力 (Pe)。在封闭系统、蒸汽饱和压力下，菱镁矿在菱镁矿和方解石之间形成了大孔隙。然而，在开放系统流通实验中，水镁石 (Mg(OH)2) 或菱镁矿 (MgCO3) 的形成取决于孔隙流体压力。主要反应产物在低孔隙流体压力（0.2 MPa）下为水镁石，而在较高孔隙流体压力（≥ 1 MPa）下为菱镁矿。产物矿物的方解石溶解和沉淀随着流速的增加而增加，但流速不影响产物的性质。反应岩的渗透率与反应途径有关，即产物的性质。菱镁矿以假形方式取代了原始方解石，并覆盖了具有 10-100 µm 宽孔隙的原始方解石。相比之下，板状和/或板状水镁石阻塞了孔隙并导致渗透率降低。我们的结果表明，在接近相分离的条件下，孔隙流体压力是控制富挥发物（例如，CO2、HCl、H2S 和 SO2）系统中反应产物和反应过程的重要参数；这些条件发生在例如超热液和斑岩热液系统、碳酸盐置换和一些变质环境中。