Statistics – Applications
Scientific paper
May 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.2277b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 10, pp.2277-2288
Statistics
Applications
2
Scientific paper
Three parametric (nonthermodynamic) equations have been developed to calculate final equilibration temperatures for natural, quasibinary paragonite-muscovite (Pg-Ms) pairs (mole fraction margarite in Pg < 0.05, Si/formula unit 6.2 and/or (Mg + Fe 2+ + Fe 3+ ) 0.35 for coexistent Ms). The first two equations are paragonite-based and muscovite-based, because they yield equilibration temperatures that depend on the Na-K compositions of K-saturated Pg and Na-saturated Ms, respectively. The third equation is closure-based, because it yields equilibration temperatures that vary with the width of the gap between the Na-K compositions of coexisting Pg and Ms (i.e., the degree of solvus closure). Equilibration temperatures can be estimated, using just one of the geothermometers, or by averaging the temperatures obtained from any combination of the three equations. Comparison of the three calculated temperatures yields information on the mutual consistencies of the Na-K compositions of coexisting Pg and Ms with respect to their utility in Pg-Ms solvus thermometry. Geothermometric inconsistencies in the Na-K compositions of quasibinary Pg-Ms pairs can be caused by (1) inaccuracies in determinations of the Na-K compositions of the coexisting micas, (2) H 3 O + K + and vacancy K + substitution in muscovite and/or paragonite (3) crystallization and preservation of quasibinary Pg-Ms pairs with metastable Na-K compositions. Due to the effects of pressure on the stability relations of Pg-Ms pairs and the K/Na ratios of Nasaturated muscovites, practical applications of Pg-Ms solvus thermometry are restricted to quasibinary Pg-Ms pairs that equilibrated at pressures between approximately 2 and 8 kbar. Within this pressure range, the utility of Pg-Ms solvus thermometry is limited thermally to approximately 300 T 700° C by (1) the decomposition of K-saturated paragonite at temperatures between approximately 580 and 700°C, and (2) steepening of the solvus limbs at temperatures below 300°C. Graphical and calculated Pg-Ms solvi based on solvus data for synthetic, binary Pg-Ms micas are inconsistent with solvus data for natural, quasibinary Pg-Ms micas. Consequently, these solvi should not be used to estimate equilibration temperatures for natural, quasibinary Pg-Ms pairs.
Blencoe James G.
Guidotti Charles V.
Sassi Francesco P.
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