Other
Scientific paper
Dec 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.5503m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 24, pp.5503-5523
Other
3
Scientific paper
Metamorphic black shales are characterized by the association of graphite and ammoniumrich micas and by N 2 -(CH 4 , CO 2 )-rich fluid inclusions. Model compositions of mixed fluids in equilibrium with graphite and NH 4 + -bearing biotite as part of natural mineral assemblages, were calculated for T = 450-600° C , P = 1-3 kb , and for 1 molal total chloride. As a starting point, the equilibrium constant for the reaction K - phlogopite + NH 4 + = NH 4 - phlogopite + K + has been determined on basis of the experimental data of Bos et al. (1988). The effects of the main controls of the relative proportions of N 2 , CH 4 , CO 2 , and H 2 O have been quantified. The N 2 content of the fluid strongly decreases with increasing temperature. By contrast, the effect of varying pressure is much less dramatic. Increasing the NH + 4 content of biotite results in an increase of X N 2 / X CO 2 in the fluid which trends toward N 2 -CH 4 -(H 2 O) compositions. Thus, the NH 4 + content of biotite constrains oxygen fugacity to a narrow range where a very efficient mineral buffer is lacking. The K + / H + activity ratio, determined by the mineral assemblage, exerts a major control on the fluid-mineral equilibria. Increasing this ratio has the same effect as increasing NH 4 + concentration of biotite by the same factor. Therefore, high values of a K + / a H + characteristic, among others, of K-feldspar + calc-silicate assemblages promote the formation of N 2 -CH 4 -(H 2 O) fluids in equilibrium with biotite of common composition (<3000 ppm NH 4 + ) whereas N 2 -CO 2 -(H 2 O) fluids would be produced in metapelitic rocks with lower a K + / a H + .
Gibert François
Guillot Christophe
Moin Bernard
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