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首页> 外文期刊>Contributions to Mineralogy and Petrology >Formation of Mg-carbonates and Mg-hydroxides via calcite replacement controlled by fluid pressure
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Formation of Mg-carbonates and Mg-hydroxides via calcite replacement controlled by fluid pressure

机译：通过流体压力控制通过方解石更换形成Mg-碳酸盐和Mg-氢氧化物

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摘要

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 not equal horizontal stress not equal pore-fluid pressure (P-f)) 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 mu m) reacted with a Mg-rich solution at similar to 200 degrees 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 (sigma(n) = 10 or 20 MPa) and effective pressure (P-e). 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 mu m 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.
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