Y. Kruger et Lw. Diamond, Unexpected behaviour of fluid inclusions synthesized from silver oxalate and an aqueous NaCl solution, CHEM GEOL, 173(1-3), 2001, pp. 159-177
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.