Abstract

Three fluid compositions in the C-O-H-N system, each with nominal mole fraction of H2O = 0.25, were used for comparative determinations of the solidus of synthetic albite at 4.4kbar. The fluids were generated by the decomposition of guanidine nitrate CH5N3.HNO3 ammonium oxalate (NH4)2C2O4·H2O, and from a l:2 molar mixture of oxalic acid dihydrate and anhydrous oxalic acid. Hydrogen fugacity during the experiments was not buffered and was found to lie between Ni-NiO-H2O and MnO-Mn3O4-H2O buffers. Excess H2 produced by the decomposition of the C-O-H-N solid compounds diffuses out of the sample capsule, resulting in fluids consisting essentially of the species H2O, CO2 and N2. The ternary nature of these fluids, which was predicted by thermodynamic equilibrium calculations, is confirmed by quantitative gas chromatographic analysis of the quenched fluids. Reversals confine the albite solidus between 954°C and 958°C (±2°C) and within experimental limits there is no significant difference between solidus temperatures for each of the three fluid compositions. This suggests that activity coefficients for H2O are similar in each case and are independent ofthe CO2/N2 ratio in the fluid. These data place an upper bound on the activity coefficient for H2O at 960° and 4.4 kbar of approximately 1.8 in H2O-CO2-N2 fluids with the mole fraction of H2O = 0.25

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