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Trial to establish muscovite-paragonite solvus by synthesis experiments

Huijuan Li , Hugh O'Neill, Jörg Hermann and Ulli Troitzsch

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Since the pioneering work of Eugster and Yoder (1955), numerous efforts have been made to study the nature of Ms-Pa solvus and its application as geothermometry by experimental work (Flux & Chatterjee, 1986 and references therein). Guidotti (1994) stated: "Unfortunately, field, experimental and thermodynamic investigations of Pg-Ms equilibria have yielded conflicting results".

Comparison of results in this study with the solvi calculated at 2, 6 and 15 kbar using Model D (continuous lines), Model A (dashed lines) and chatterjee & Flux (1986) (dotted lines). The rectangular boxes and circles are compositions for natural micas from Grambling (1984) and Guidotti et al. (1994). (from Roux & Hovis, 1996).

In this study, we planned to obtain Ms-Pa solvus brackets at various T and 20 kbar by hydrothermal treatment of a variety of gel starting materials using Piston-Cylinder apparatus. Paragonite can be stable at a wider temperature interval at 20 kbar and high pressure will contribute to a faster and closer approach to the equilibrium.

The XRD spectra of all the runs accord with the spectra of 2M1 polytype mica. Through a calibration curve expressing cell volume V (Å) as a function of Xms, we can obtain the compositions of predominant K-rich mica which are plotted in Fig 1. For run C3170 at 700°C, 20kbar, single phase mica was formed (blue circle in Fig 1), of which similar compositions were got from probe (Xms=0.645) and XRD (Xms=0.622). For runs at 650°C, 20kbar with 50%Ms+50%Pa as starting material, two phases of micas coexist, and the composition for K-rich mica is around Xms=0.58. K-rich mica with Xms=0.6587 was formed and Na-rich mica decomposed to Jadeite and Kyanite in run D1004 at 600°C, 20kbar. We obtain a graphical Ms limb of the solvus which locates at lower temperature than it should be according to the models in Chatterjee & Flux (1986) and Roux & Hovis (1996).

Eugster HP, Yoder HS (1995) Micas: The join muscovite-paragonite. Carnegie Inst. Washington Yearbook 54, 124-126
Guidotti CV, Sassi FP, Blencoe JG, Selverstone J (1994) The paragonite-muscovite solvus: I P-T-X limits derived from the Na-K compositions of natural, quasibinary paragonite-muscovite pairs. Geochimica et Cosmochimica Acta 58, 2269-2275
Chatterjee ND, Flux S (1986) Thermodynamic mixing properties of muscovite-paragonite solid solutions at high temperatures and pressures, and their geologic applications. Journal of petrology 27, 677-693
Roux J, Hovis GL (1996) Thermodynamic mixing models for muscovite-paragonite solutions based on solution calorimetric and phase equilibrium data. Journal of petrology 37, 1241-1254