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H (sub 2) O activity in H (sub 2) O-N (sub 2) fluids at high pressure and temperature measured by the brucite-periclase equilibrium

Andreas Haefner, Leonid Y. Aranovich, James A. D. Connolly and Peter Ulmer
H (sub 2) O activity in H (sub 2) O-N (sub 2) fluids at high pressure and temperature measured by the brucite-periclase equilibrium
American Mineralogist (July 2002) 87 (7): 822-828

Abstract

We present experimental results constraining water activity in H (sub 2) O-N (sub 2) fluids containing 40-90 mol% water at pressures of 6-13 kbar and temperatures of 680-840 degrees C. In the experiments, the displacement of the brucite dehydration equilibrium was used as a gauge of water activity. The experiments were performed in a conventional piston-cylinder apparatus, with NaCl pressure medium and silver azide, AgN (sub 3) , as a source of nitrogen. Reversals of the dehydration reaction were used to bracket the equilibrium fluid compositions within 3 mol% H (sub 2) O. Water activities were computed from the equilibrium brucite dehydration conditions in pure H (sub 2) O as determined by Aranovich and Newton (1996) using thermodynamic data of Holland and Powell (1998). The experimentally derived activities were fit to a van Laar-type equation that reproduces our compositional data with a standard error of 1.6 mol% H (sub 2) O: RT lngamma (sub 1) = (X (sub 2) ) (super 2) W{V (super 0) (sub 1) (V (super 0) (sub 2) ) (super 2) /[(V (super 0) (sub 1) +V (super 0) (sub 2) ) (X (sub 1) V (super 0) (sub 1) +X (sub 2) V (super 0) (sub 2) ) (super 2) ]} where gamma (sub 1) is the activity coefficient of H (sub 2) O, X (sub i) is the mole fraction of end-member i (1 = H (sub 2) O and 2 = N (sub 2) ), V (super 0) (sub i) is the molar volume of the pure end-member at the pressure (P) and temperature (T) of interest, and W is analogous to a regular solution parameter. The parameter W was fit as a function of pressure and temperature by the expression W = (A-BT)[1-expt(-20P)]+C . P (super 0.3) T, with A = 40005 J, B = 51.735 J/K, C = 14.848 J/(K . kbar (super 0.3) ), P in kbar and T in K. With these expressions, activity-concentration relations in H (sub 2) O-N (sub 2) fluids can be reconstructed in a broad P-T-X range using any equation of state (EOS) for pure H (sub 2) O and N (sub 2) . The activity-concentration relations are similar to the semi-empirical EOS of Duan et al. (2000) and the theoretical EOS of Churakov and Gottschalk (2002a), although the former somewhat underestimates activities within the experimental pressure range whereas the latter appears to overestimate activities of the components at pressure above 20 kbar.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 87
Serial Issue: 7
Title: H (sub 2) O activity in H (sub 2) O-N (sub 2) fluids at high pressure and temperature measured by the brucite-periclase equilibrium
Affiliation: ETH-Zentrum, Institute for Mineralogy and Petrography, Zurich, Switzerland
Pages: 822-828
Published: 200207
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 42
Accession Number: 2002-064801
Categories: General geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Secondary Affiliation: Russian Academy of Sciences, RUS, Russian Federation
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200220
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