Deformation of fluid inclusions in fluorite under confining pressure

Abstract

A high-pressure heating-freezing stage constructed at our laboratory permits continuous microscopic observations of fluid inclusions during experimental runs in pressure-temperature paths (patent No. 85.025.12). Conditions currently attainable are − 180° to + 450°C and 1 to 900 bar. Runs were performed on fluid inclusions in fluorite to study the mechanism of stretching and deformation of the cavities by overheating under confining pressure. Confining pressures of 200, 410 and 620 bar were applied on aqueous (H 2O, 15 eq.wt.% NaCl) cogenetic secondary inclusions and hydrocarbon primary inclusions, having volumes between 10 3 and 10 5 μm 3 in Tunisian fluorite from Djebel Guebli. In most cases leakage (deformation with loss of content, sometimes observed at 1 bar) was avoided under confining pressure and stretching (yielding with volume change) is the main mechanism observed. Combining our experiments with the law of stretching at 1 bar, we have tried to derive an empirical law of the pressure effect: P I s=P I s1+KPc (mean value of K=07) Confining pressures permit to reach very high temperatures of stretching. The values of internal pressure at the stretching temperatures calculated from the isochore curves of the H 2ONaCl system are between 850 and 1100 bar. The values of ΔT h increase regularly with the amount of overheating ΔT oh; on the curves ΔT h = f( ΔT oh) we observed two parts: in the first part, the value of T h is constant, that means that the internal pressure is not sufficient to induce a deformation of the mineral; and in the second part, the function increases in relation to plastic deformations of the mineral; the latter deformations are a function of the temperature. When an external overpressure is applied on a preliminary stretched inclusion, we observed a decrease of the new homogenization temperature ( T h), that means a decrease of the inclusion volume. This reversible effect could be related to a mechanism of relaxation of the deformation. We have observed that decrepitation of fluid inclusions could also occur by rapid releasing of pressure as well as by overheating. This should be taken into account for interpretation of experiments in blind autoclaves. Those observations could be explained by a microfracturing effect connected with dislocations in a matrix with elasto-plastic deformation under change of temperature, and the possibility of strain relaxation.