Effect of Sodium on Cpx+Opx+Grt Equilibrium: Experiments in NCMAS System to 5 GPa Pressure

Vadim K. Bulatov (ryabchikov @em.uni-frankfurt.de)¹, Igor D. Ryabchikov (ryabchikov@em.uni-frankfurt.de)² & Gerhard P. Brey (brey@em.uni-frankfurt.de)³

¹ Vernadsky Institute of Geochemistry, 19 Kosygin Str., Moscow, Russia

² Institute for Geology of Ore Deposits, Staromonetny P., 35, Moscow 109017, Russia

³ Institute for Mineralogy, J.W. Goethe University, Senckenberganlage 28, Frankfurt/Main, D60054, Germany

Experimental data and thermodynamic modelling of sodic clinopyroxene+orthopyroxene equilibrium were previously obtained for enstatite-jadeite join (Gasparik, 1992). In order to assess interaction of sodium-rich clinopyroxenes with orthopyroxene and pyrope-rich garnet in calcium-bearing system we performed experimental investigation of phase equilibria in Na₂O-CaO-MgO-Al₂O₃-SiO₂system. Mixtures of presynthesized pyroxenes and garnet with Na₂CO₃ were run between 2 and 5 GPa 800 - 1300°C in piston-cylinder and Belt apparatus. Na₂O contents in clinopyroxenes varied between 2.3 and 6.5 wt%. Coexisting orthopyroxenes contained between 0.4 and 1.4 wt% Na₂O. Even at 5 GPa sodium contents of garnets were negligible. Thermodynamic modelling indicated significant increase in activity coefficient of En component in clinopyroxene from the addition of jadeite: estimated asymmetric Margules parameters for En-Jd interaction are WGMg-Na = 20860 + 0.74P; WGNa-Mg = -18390 + 0.74P (J, bar), which yields excess free energy values for diluted solution of En in Jd approximately 2 times higher by comparison with Gasparik's (Gasparik, 1992) fitting for Jd - En system and is also higher than the values for En-Di interaction parameters in CMS and CMAS systems (Brey et al., 1986; Nickel and Brey, 1984; Nickel et al., 1985). The modelling of the thermodynamics of aluminous orthopyroxenes revealed that they are most likely characterized by the intermediate state of cation disordering on M1 and tetrahedral sites. Therefore, excess entropy terms should be introduced if completely ordered ("molecular") model is used for calculations (WGMg-Al = 8300 - 15T - 0.05P - symmetrical Margules parameter). For clinopyroxene in CMAS system we also accepted WSMg-Al = 15 J/K/mol and estimated symmetrical WHMg-Al to be equal to 33300 J/mol. Present experimental data show, that the variations of Na₂O and Al₂O₃ contents in pyroxenes may affect estimations of P and T by two pyroxene - garnet geothermobarometers (e.g. Brey and Koehler, 1990).

Brey G & Koehler T, Journ. Petrology, 31, 1353-1358, (1990).

Brey G, Nickel KG & Kogarko LN, Contrib. Mineral. Petrol, 92, 448-455, (1986).

Gasparik T, Contrib. Mineral. Petrol, 111, 283-298, (1992).

Nickel KG & Brey G, Contrib. Mineral. Petrol, 87, 35-42, (1984).

Nickel KG, Brey G & Kogarko LN, Contrib. Mineral. Petrol, 81, 44-53, (1985).

Stability of Dolomite and Solid Solution between Calcium and Magnesium Carbonates at High Pressures and Temperatures

Alessandra Buob (buob@erdw.ethz.ch) & Peter Ulmer (pulmer@erdw.ethz.ch)

Department of Earth Sciences, ETH-Zurich, Sonneggstr.5, CH-8092 Zurich, Switzerland

Carbonates are important candidates as potential hosts for carbon in the Earth's mantle and their stability has implication on the oxygen fugacity and therefore on the stability of other important minerals (e.g. carbon/iron-bearing species) in the mantle.

A series of high-pressure experiments have been performed on a multi-anvil apparatus in order to delimit the stability of dolomite and its high-pressure breakdown products aragonite plus magnesite. The solubilities of Mg and Ca in magnesite, dolomite and aragonite, and the structural state of dolomite are further objectives of the study. The starting materials consisted of variable mixtures of synthetic calcite, natural dolomite and magnesite. The experiments were conducted between 5 and 8 GPa and 800 to 1400°C and run for 24 and 72 hours. The run products were analyzed with Raman spectroscopy, X-ray powder diffraction and electron microprobe analysis techniques.

The experimental results constrain the breakdown reaction of dolomite to aragonite and magnesite between 5 and 6 GPa at 800°C and between 6 and 7 GPa at 1000°C in accordance with the recent results of Luth (1999). Between aragonite and CaMg(CO₃)₂ complete solubility occurs above 1100°C. The breakdown reaction of dolomite ends in a point at 7 GPa and 1100°C where the solvus crest intersects the reaction curve. At higher pressures and temperatures disordered CaMg carbonate exists. In bulk-composition between CaMg(CO₃)₂ and MgCO₃ disordered, CaMg carbonate coexists with calcian magnesite to temperatures exceeding 1200°C.

Luth RW, EOS, Transactions, AGU, Suppl., 80, 350, (1999).

Phengite Chemistry in a S-type Granitic System ­ Evidence for a New Mica Substitution

Michael Burchard (michael.burchard@ruhr-uni-bochum.de)

Ruhr-Universität-Bochum, Institut für Geologie, Mineralogie und Geophysik, 44780 Bochum, Germany

Experiments were performed on a natural S-type granitic starting material at pressures of 20 to 45 kbar, temperatures of 675 to 900°C with 1.9 wt.% to 9.9 wt.% water content. The run products of the 240 experiments consist mainly of quartz/coesite, phengite, jadeitic clinopyroxene, potassium feldspar / potassium feldsparhydrate and a fluid phase. Possible accessories are garnet, epidote, titanite, rutile and apatite. With increasing pressure, decreasing temperature and decreasing water content the phengites become richer in silicon and aluminum. Phengites from the experimental run at 40 kbar, 725°C and 3.8 wt.% water content have the formula K_0.97Na_0.03Ti_0.02Al_1.45Fe²⁺ _0.38Mg_0.11[Al_0.46Si_3.54O₁₀](OH)_1.98F_0.02. Due to the high aluminum content these phengite compositions cannot be described by a solid solution of muscovite, Al-celadonite, AlFe-celadonite, annite and phlogopite. This makes it necessary to define a new mica component with the ideal formula KAl_1.66[]_0.66[Si₄O₁₀](OH)₂ (mono-di-octahedral mica, MDO). The new component can be derived from the muscovite endmember by the substitution: 1/3[]^[6] + Si^[4] <=> 1/3Al^[6] + Al^[4] (1). The classical phengite substitution (inverse Tschermak) Al^[6] + Al^[4] <=> (Mg,Fe²⁺)^[6] + Si^[4] (2) between muscovite and Al-celadonite / AlFe-celadonite can be described by a combination of the new substitution (1) and the muscovite-biotite substitution 2Al^[6] + []^[6] <=> 3 (Mg,Fe²⁺)^[6] (3). This means that all phengites in the system K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O can be described by the tow substitutions (1) and (3). Probably the new MDO mica is particularly important for rock compositions with low magnesium and iron contents, for example granitic rocks. Analyses of phengites of the biotite-phengite-gneisses from the high pressure unit of the Dora Maira massif, western Italy, show relatively high amounts of the MDO mica component. Thermodynamic calculations of low temperature, high pressure rocks involving phengites could be inaccurate when then activity calculations do not take the MDO mica into consideration.