Composition Of Aqueous Fluid Research Articles

Experimental study in the system albite-andalusite-quartz-NaCl-HCl-H 2O at 600°C and 400 to 2000 bars

The composition of aqueous fluid equilibrated with a mineral assemblage of quartz, albite, and andalusite was experimentally obtained at 600°C, with pressure ranging from 400 to 2000 bars and NaCl concentrations ranging from 0.05 to almost 100 m. This allowed us to determine 1. (1) the pressure dependence of the apparent equilibrium constant (Ka) of the following reaction under critical and subcritical conditions, 1 2 Al 2SiO 5 + 5 2 SiO 2 + 1 2 H 2O + NaCl = NaAlSi 3O 8 + HCl 2. (2) the composition of coexisting vapor and liquid phases in the NaCl-HCl-H 2O system. The Ka was obtained as the HCl NaCl concentration ratio in dilute fluid (〈 2 m). The equilibrium constant ( K) of the reaction, which is equivalent to the Ka with an assumption of a unit activity coefficient ratio of HCl and NaCl, varies from 0.013 at 2000 bars to 1.7 at 400 bars with ( ∂ lnK/ ∂P) being greater at lower pressure. The large pressure dependence of the equilibrium constant is attributed to the large difference in partial molar volumes of HCl and NaCl in the aqueous phase at high temperature and low pressure. The compositions of the coexisting vapor and liquid phases are graphically estimated on a diagram showing concentrations of NaCl and HCl by regarding the above reaction as a buffer reaction. The concentration ratio of HCl in vapor to liquid phase ( m HCl vapor m HCl liquid ) ranges from 1.5 at 1000 bars to 7 at 400 bars. The addition of HCl into the NaCl-H 2O system increases the critical pressure and also increases the concentration of NaCl in the vapor phase at low pressure. These results indicate that the effects of pressure are comparable but inverse to temperature effects and must be considered in modeling the geochemical structure of hydrothermal systems.

Partitioning of fluorine and chlorine between apatite and aqueous fluids at high pressure and temperature: implications for the F and Cl content of high P-T fluids

Experiments in a piston-cylinder apparatus equilibrated natural fluorapatite with aqueous HCl, NaCl, NaOH, Na 2CO 3 and CO 2 H 2O mixtures at 1–2.0 GPa and 950–1050°C. Post-experiment apatite compositions were determined by electron microprobe and fluid compositions were calculated from mass-balance. This enabled calibration of the fluorine and chlorine contents of apatite coexisting with a variety of fluids at high P and T, and provides better constraints on the halogen composition of aqueous fluids which may exist in the deep crust and upper mantle. Results obtained at 2.0 GPa and 1050°C indicate that the ratio X ClAp/X OHAp in apatite ( X FAp , X ClAp and X OHAp are the mole fractions of fluor-, chlor- and hydroxyapatite) is proportional to the chlorine content of the fluid up to ∼ 1 (H 2O CO 2), ∼ 5 (H 2O HCl) and ∼ 15 (H 2O NaCl) wt% Cl. In contrast, the ratio X FAp/X OHAp was found to vary in proportion to the fluorine content of the fluid only at concentrations of ≤ 0.15 (H 2O HCl) and ≤ 0.4 (H 2O NaCl) wt% F. Arising from changes in the activities of aqueous HCl and HF, the results of this study demonstrate that fluid composition, in addition to the absolute abundances of F and Cl in the fluid, will determine the amounts of these elements incorporated into coexisting apatite. Specifically, relatively low Cl and F concentrations in acidic (H 2O HCl) fluids are required to obtain high ratios of X ClAp/X OHAp and X FAp/X OHAp in apatite, whereas much higher abundances of Cl and F in basic fluids (i.e., Na-bearing) are required to achieve the same result. In addition, resulting from dilution of the aqueous phase with CO 2 and the subsequent reduction in the activity of H 2O, apatites with high ratios of X ClAp/X OHAp and X FAp/X OHAp may be produced by CO 2 H 2O fluids with relatively low levels of Cl or F. Application of these results to apatites from a suite of ultramafic xenoliths from southeastern Australia indicates chlorine abundances of 500–3500 ppm in the fluid phase and points to chlorine as a potentially important component of some mantle fluids.

Composition of aqueous fluid from serpentinite in the subducted lithosphere : Tatsumi, Yoshiyuki and Noboru Nakamura, 1986. Geochem. J., 20(4): 191–196. Dept. of Geol. and Mineral., Faculty of Sci., Kyoto Univ., Kyoto 606, Japan

Composition of aqueous fluid from serpentinite in the subducted lithosphere : Tatsumi, Yoshiyuki and Noboru Nakamura, 1986. Geochem. J., 20(4): 191–196. Dept. of Geol. and Mineral., Faculty of Sci., Kyoto Univ., Kyoto 606, Japan

Composition of aqueous fluid from serpentinite in the subducted lithosphere.

High pressure experiments up to 4.0GPa on natural serpentinite, one of constituent members of the hydrated subducted oceanic lithosphere, show that it decomposes totally at temperatures below 700°C. The results suggest that the serpentinite-derived aqueous fluid can be supplied to the mantle wedge only beneath the fore-arc region. The concentration of Rb, Sr and Yb in the aqueous fluid released through the dehydration process of serpentinite are estimated on the basis of open-system dehydration experiments under 1.5 and 3.0GPa, in which the fluid is lost through perforations on a gold sample container; the aqueous fluid transports a larger amount of an element with a larger ionic radius under higher pressures.