The solubility of chlorite solid solutions in 3.2 wt% NaCl fluids from 300-400°C, 500 bars

Peter J. Saccocia, William E Seyfried

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Abstract

Solubility equilibria for the assemblage chlorite-albite-paragonite-quartz in 3.2 wt% NaCl fluids has been investigated as a function of chlorite composition from 300-400°C, 500 bars. Natural samples of a low Fe-clinochlore and high Mg-chamosite were employed as reactants representing a range of Fe2+/ (Fe2+ + Mg) ratios from 0.09 to 0.52, respectively. At each temperature, reaction of low Fe-clinochlorealbite-paragonite-quartz in NaCl solutions resulted in relatively Mg-rich and Fe-poor fluids compared to reaction of the high Mg-chamosite bearing assemblage. In both cases, Mg concentrations decrease from 300 to 350°C and then increase to 400°C. Iron concentrations increase sharply with increasing temperature. Thus, the aqueous Fe/Mg concentration ratio of NaCl solutions coexisting with chlorite increase with temperature and mole fraction chamosite. Activity-composition models for chlorite solid solution (both ideal and regular) have been evaluated in the context of our solubility data. For this analysis, the enthalpy interaction parameter (WH) was assumed to be independent of temperature and the excess free energy (ΔGex) was symmetric with respect to composition. The results show that the measured dependence of chlorite solubility on composition is consistent with ΔGex values from 0 (ideal) to 200 cal/mol. A larger departure from ideality (ΔGex > 200 cal/mol) is inconsistent with the solubility data. An analysis of fluid-mineral equilibria in this chemical system shows that thermodynamic data for clinochlore and chamosite retrieved from high temperature-pressure phase equilibria experiments are consistent with the aqueous Fe Mg ratios measured as a function of temperature and chlorite composition. Systematic discrepancies were noted, however, between measured and predicted Fe and Mg concentrations in equilibrium with chlorite-albite-paragonite-quartz assemblages. It is suggested that these discrepancies result from uncertainties in the thermodynamic data for albite and paragonite. Requisite adjustments to the ΔGof and ΔHof of the Na-Al silicates are derived that bring into agreement measured and predicted chlorite solubility. The results of this study are relevant to a wide variety of hydrothermal environments where chlorite solid solution is a common phase and should better constrain the chemical consequences of water-rock interaction at temperatures previously unsupported by experimental data.

Original languageEnglish (US)
Pages (from-to)567-585
Number of pages19
JournalGeochimica et Cosmochimica Acta
Volume58
Issue number2
DOIs
StatePublished - Jan 1994

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