The intrinsic effects of using rhenium gaskets in hydrothermal diamond anvil cell experiments on background fluorescence, contamination, and redox control

Abstract

Because of the great stiffness and high melting point of rhenium (Re) metal, Re gaskets are widely used in hydrothermal diamond anvil cell (HDAC) experiments that involve aqueous fluids and even hydrous melts under high pressure–temperature (P–T) conditions. This metal was usually considered chemically inert in hydrothermal experiments, but some concerns have been raised about its reactivity with the fluids. Therefore, this communication addresses the intrinsic effects of using Re gaskets in HDAC experiments, including background fluorescence in Raman spectroscopy, contamination, and redox control. A series of hydrothermal experiments involving rhenium were performed in pure water, H2O-NaCl, and SiO2-COH systems using the HDAC up to 28.9 kbar and 850 °C and fused silica capillary capsule (FSCC) up to 87 bar and 300 °C, and Raman spectroscopy was employed for in situ analysis. The results indicate that (1) strong background fluorescence, often occurring between 200 and 700 °C, is caused by the dissolution of Re gaskets and the formation of aqueous species of Re with a low oxidation state, such as a Re2+cation and eventually, its hydroxide complexes; (2) in addition to the fluorescent Re species and unwanted carbon species produced from the diamond-water reaction catalyzed by Re, a considerable amount of ReO4−, which does not cause fluorescence and shows a dominant Raman peak at ∼970 cm−1, is detected in the fluids either in situ at high temperatures above 700 °C or under cooling conditions, and it can reduce the activity of H2O and the pH of the fluids and interfere with the assignments of Raman peaks of other species in the samples; and (3) the production of oxidizing aqueous species at high temperatures, including ReO4− and CO2, is attributed to the substantial loss of H2 and the corresponding increase in oxygen fugacity. Because of these intrinsic effects of using Re gaskets in HDAC experiments, implications for hydrothermal experiments are discussed with several previously reported examples such as the discrepancy in the solubility of rutile in water under elevated P–T conditions. Based on the results of the current and previous studies, four suggestions have also been made to minimize the unwanted effects of using Re gaskets, including the generation of a reduced experimental condition, a pre-heating treatment of Re gaskets, a gasket pre-indentation approach, and a gold-lined gasket method. Armed with the knowledge of these intrinsic effects as well as the advantages of using different HDAC metal gaskets, the researchers will be able to create appropriate experimental designs.