Untitled Document
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