Unexpected behaviour of fluid inclusions synthesized from silver oxalate and an aqueous NaCl solution

Fluid inclusions were synthesized in quartz from silver oxalate (Ag2C2O4) a nd a 6-wt.% NaCl aqueous solution to produce a CO2-H2O-NaCl fluid, followin g the method of previous workers. Synthesis conditions of 500-650 degreesC and 1500-4500 bar were chosen with the intention of trapping a homogeneous, ternary fluid. The resulting inclusions were analyzed by microthermometry and by Raman spectroscopy. Solid residues in the experimental capsules were analyzed after the syntheses by XRD and SEM-EDS. Several properties of the synthetic inclusions were unexpected: Tm-Cln and Th-tot vary over significant ranges and they correlate negatively; the calc ulated total molar Volumes deviate significantly from data on the same bulk composition derived by Gehrig [Gehrig, M., 1980. Phasengleichgewichte und pVT-Daten ternarer Mischungen aus Wasser, Kohlendioxid und Natriumchlorid b is 3 kbar und 500 degreesC. Hochschul Sammlung Naturwissenschaft, Chemie, B and 1. Hochschul Verlag, Freiburg] in an optical cell, and from data derive d from our fluid inclusions synthesized for comparison using the gas-loadin g technique of Frantz et al. [Frantz, J.D., Zhang, Y.-G., Hickmott, D.D., H oering, T.C., 1989. Hydrothermal reactions involving equilibrium between mi nerals and mixed volatiles: 1. Techniques for experimentally loading and an alyzing gases and their application to synthetic fluid inclusions. Chem. Ge ol. 76, 57-70]; all fluid inclusions contain a chlorargyrite (AgCl) daughte r crystal; and HCO; was detected in the aqueous phase of the inclusions by Raman spectroscopy, presumably reflecting high concentrations of NaHCO3(aq) . All these anomalous features can be traced back to effects caused by precip itation of insoluble AgCl and simultaneous formation of sodium oxalate upon loading the experimental capsules with silver oxalate and NaCl solution. R econstruction of the chemical reactions that occurred during the hydrotherm al syntheses suggests that incomplete redissolution of the AgCl during prog ressive quartz precipitation and fluid trapping yields inclusions with vari able bulk properties. Nevertheless, quite accurate total homogenisation tem peratures can be recovered by regressing the Th-tot-Tm-Cla correlations, bu t considerable effort is required. We conclude that silver oxalate should be used in combination with chloride solutions only with extreme caution, whereas the gas-loading technique pro vides an excellent alternative for the synthesis of CO2-H2O-NaCl fluid incl usions. (C) 2001 Elsevier Science B.V. All rights reserved.