HR: 08:15h
AN: V51E-02    [Abstracts]
TI: Calculated Phase Relations in the System KFMASH Between 6 and 16 GPa
AU: * Massonne, H
EM: h-j.massonne@mineralogie.uni-stuttgart.de
AF: Institut fuer Mineralogie und Kristallchemie, Universitaet Stuttgart, Azenbergstrasse 18, Stuttgart, D-70174 Germany
AU: Brandelik, A
V51E-02 AF: Institut fuer Mineralogie und Kristallchemie, Universitaet Stuttgart, Azenbergstrasse 18, Stuttgart, D-70174 Germany
AB: To better understand the modal compositions of deeply buried metagranitoids and metapelites, phase relations in the model system K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O (KFMASH) with SiO₂ in excess were calculated applying thermodynamic principles. We used the software package PTGIBBS, published data, and thermodynamic data (e.g. for phase egg (AlSiO₃OH), K-hollandite (KAlSi₃O₈)) newly derived on the basis of former high-pressure (HP) experiments. Non-ideal mixing was considered for the solid solution series of garnet (components: pyrope, majorite, almandine) and potassic white mica (components: muscovite, MgAl-celadonite, FeAl-celadonite). For phases such as HP-clinoenstatite ((Mg,Fe)SiO₃), Si-spinel ((Fe,Mg)₂SiO₄), and beta phase ((Mg,Fe)₂SiO₄) only binary solid solutions, assuming ideal mixing, were taken into account. On the basis of the above data, we constructed petrogenetic grids mainly for the P-T range 6 to 16 GPa and 600 to 1600 ° C. Typical features of these grids are, for instance, the disappearance of K-cymrite (KAlSi₃O₈ H₂O) with rising pressure close to 10 GPa and the occurrence of phase egg above 12 GPa. In KMASH potassic white mica reacts with OH-topaz at about 11 GPa (1000-1200 ° C) to form pyrope + K-hollandite. The content of majorite component in pyrope is less than 1 mol% which is systematically so for all garnets coexisting with an Al-silicate at least up to 16 GPa. Potassic white mica, which is virtually pure MgAl-celadonite, finally breaks down at pressures close to 12 GPa. Decomposition assemblages are K-hollandite + HP-clinoenstatite + H₂O (T < 1100 ° C) or K-hollandite + Si-wadeite (K₂Si₄O₉) + majorite-bearing pyrope + H₂O. According to these results, metapelitic rocks at pressures above 12 GPa should consist of the assemblage stishovite + K-hollandite + (virtually majorite-free) garnet + Al-silicate. The latter phase is either OH-topaz (Al₂SiO₄(OH)₂) or phase egg or kyanite also depending on the availability of H₂O. Metagranitoids should be composed of shishovite + K-hollandite + majorite-bearing garnet + (enstatite-rich) clinopyroxene. Si-spinel is an important additional phase in this assemblage. This phase shows increasing amounts by approaching to 16 GPa.
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3654 Ultra-high pressure metamorphism
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005