High-temperature impregnation of helium into fluid inclusions in minerals: Experiments with quartz and nepheline

Abstract

ratio in comparison to background values for the section as a whole [2]. The possibility of helium penetration into quartz grains and its accumulation in fluid inclusions therein has been shown in experiments. We performed experiments on the saturation of quartz samples with different contents of CO 2 inclusions at T = 208 ° C, P He = 58 kgs/cm 2 , and a run time ranging from 120 to 598 h. The amount of He impregnated into the inclusions under these conditions was five orders of magnitude higher than the initial He content in quartz and correlated with the quartz-hosted CO 2 content conditionally accepted as the total volume of inclusions [3]. We performed further experiments on He impregnation into fluid inclusions according to a new scheme. The runs were carried out with quartz and nepheline samples in a helium atmosphere at normal pressure and temperatures ranging from 150 to 1000 ° C. The experimental assembly comprises a quartz glass tube with a vessel containing the charge of the studied mineral. The tube was placed into an electric furnace and blown out with helium at a rate of 10‐15 cm 3 /min. Experiments with quartz were conducted on the same three samples used in the runs of the first series [3] (the samples were taken from granite pegmatites of the Kola Peninsula with different contents of CO 2 inclusions). The nepheline sample was taken from alkali pegmatite of the Khibiny pluton (Kola Peninsula). We chose this mineral for continuation of the investigation of helium migration into natural mineral crystals, because Khibiny nepheline usually contains numerous inclusions of hydrocarbon gases (HCG) with methane as the major component [4]. In the studied nepheline sample, the HCG content extracted by crushing was 26.7 cm 3 /g · 10 –3 . CH 4 accounts for 98 vol %.The remainder is composed of methane homologs C 2 ‐C 4 . By analogy with previous experimental data on quartz [2], we assumed that helium can also impregnate the nepheline-hosted gas inclusions. The results of new experiments are described below. Quartz. Several charges with grain sizes ranging