Redox control and measurement in low-temperature (<450°C) hydrothermal experiments

• Published in: The American mineralogist Vol. 106; no. 8; pp. 1333 - 1340
• Main Authors: Fang Jing, Fang Jing, Chou I Ming, Chou I Ming
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 01.08.2021; Walter de Gruyter GmbH
• Subjects: buffers; capillarity; Containers; Control; Equilibrium; experimental studies; Experiments; Fugacity; Fused silica; fused silica capillary; Geochemistry; Hydrogen; hydrogen fugacity sensor; hydrothermal conditions; hydrothermal experiments; instruments; Low temperature; measurement; Membrane permeability; Membranes; Oxidoreductions; Palladium; Permeability; Redox buffer and control techniques; redox reactions; Silica; Silicon dioxide; Silver base alloys; Sulfur; Sulphur; techniques; temperature
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-2021-7687

Redox control in hydrothermal experiments is routinely achieved through double-capsule and Shaw membrane techniques. These techniques control oxygen fugacity (fO2) by imposing a defined hydrogen fugacity (fH2) on a studied sample enclosed, together with H2O, in a hydrogen membrane capsule made of Pt or Ag-Pd alloys. However, due to the low permeability of these membranes to H2 at low temperatures (T), these techniques do not work efficiently below 450°C. Here, we tested fused silica as a new hydrogen membrane and successfully applied it to monitor and control the redox states of studied samples at T down to 200°C in hydrothermal experiments. Our results showed that 3, 8, 16, 36, 96, and 216 h are sufficient for a fused silica capillary capsule (FSCC) to reach osmotic equilibrium with the externally imposed 1 bar of H2 at 350, 300, 250, 200, 150, and 100°C, respectively, and H2 pressures inside a FSCC was very close to the externally imposed values after osmotic equilibrium. By using FSCC as a hydrogen fugacity sensor, equilibrium H2 pressures for Ni-NiO-H2O and Co-CoO-H2O redox buffer assemblages at 250-400°C and 1000 bar total pressure were measured. The equilibrated fO2 calculated are consistent with those derived from previous literature. Besides, FSCC can be used as a sample container, where fH2 and fO2 of enclosed samples can be continuously controlled. Furthermore, FSCC is an ideal container for sulfur-bearing samples, and its transparency allows spectroscopic analyses of the sample. Our work extended the low-T limit of previously well-developed redox control techniques and may open up a new research avenue in low-T hydrothermal experiments.