Zoisite/Clinozoisite Phase Relations: An Experimental Study in CFASH

Gerhard Franz (gerhard.franz@tu-berlin.de)¹, Axel Brunsmann (axelhpgh@mailszrz.zrz.tu-berlin.de)¹ & Mathias Gottschalk (mgott@gfz-potsdam.de)²

¹ FG Petrologie, Sekr. EB 15, TU Berlin, Str. d. 17 Juni 135, 10623 Berlin, Germany

² GeoForschungsZentrum Potsdam, 14473 Potsdam, Germany

Zoisite/clinozoisite (general formula = Ca₂Al₂(Al,Fe³⁺) [Si₃O₁₁(O/OH)]) phase relations in CFASH were determined, performing hydrothermal and Piston-Cylinder synthesis experiments at 0,5 GPa/500-650°C and 2,0 GPa/650-800°C. Starting materials were oxide/hydroxide mixtures with bulk compositions ranging from 0 to 40% Al₂Fe [Al₂Fe% = Fe/(Fe+Al-2)*100%]. To overcome kinetic problems and to enhance growth rate, a 1 molal CaBr₂-solution was added and natural zoisite and clinozoisite crystals served as seed-crystals. Oxygen fugacity was buffered at HM with a solid state hematite/magnetite buffer by conventional double capsule technique. All run products were analysed by SEM, EMP and XRD.

Run products consist of quartz, anorthite, zoisite and clinozoisite in variable modal amounts. Both zoisite and clinozoisite seed-crystals show orthorhombic and/or monoclinic overgrowths of up to 150 µm length, that could easily be analysed by EMP. From coexisting zoisite and clinozoisite overgrowths, the zoisite/clinozoisite phase relations have been constructed. Zoisite is the iron poor, high temperature and high pressure polymorph. Iron contents in coexisting zoisite and clinozoisite both increase with increasing P and T and can be calculated as Al₂Fe%_max ^zoi = 0.024*T + 2.614*P - 7.21 and Al₂Fe%_min ^czo = 0.044*T + 3.903*P - 11.521 (T in °C, P in GPa). Errors are < ± 1% Al₂Fe for zoisite and < ± 2% Al₂Fe for clinozoisite.

Extrapolation of the data to the iron free system yields P/T conditions for the endmember reaction Fe-free clinozoisite = Fe-free zoisite of 230 ± 30°C at 0.5 GPa and 90 ± 20°C at 2.0 GPa. The reaction has a negative slope of -11 ± 5 MPa/°C. Assuming ideal mixing in zoisite and clinozoisite, ln K vs. 1/T plots yield S_czo-zoi = 2.01 ± 0.04 J/mol*K and H_czo-zoi = 0.86 ± 0.14 kJ/mol.

Water Loss from Gold-Palladium Capsules During Piston-Cylinder Experiments

Carmela Freda (lillif@hotmail.com)¹, Don R. Baker (donb@eps.mcgill.ca) & Luisa Ottolini (ottolini@crystal.unipv.it)²

¹ Earth and Planetary Sciences, McGill University, 3450 rue University, Montréal, QC H3A 2A7, Canada

² CNR-CS per la Cristallochimica e la Cristallografia, Via Ferrata 1, 27100 Pavia, Italia

Water loss from Au₇₅Pd₂₅ capsules by hydrogen diffusion in NaCl-pyrex glass-crushable alumina-pyrophyllite assemblies has been measured during piston-cylinder experiments at 1200°C and 1.0 GPa using as starting material a glass of rhyolitic composition. These experimental conditions are the same used in a previous work (unpublished data) in which we documented severe water loss from Au₇₅Pd₂₅ capsules during experiments in NaCl-pyrex glass-crushable alumina assemblies. In that study we recorded variable water loss, in some cases up to 70% of the initial water added, but no correlation with either temperature or experimental duration. It has long been known that surrounding capsules with pyrophyllite powder reduces water loss, but the efficacy of this technique has not previously been tested. The present study was performed in order to quantify the amount of water retained in the experimental capsules when they are surrounded by pyrophyllite. Our results confirmed that by using pyrophyllite in the assembly the loss of water can be significantly reduced. When about 6 H₂O wt% is initially added to the sample, the loss of water is reduced from more than 50% to 15% (relative) if pyrophyllite is used in the assembly; when about 2 H₂O wt% is initially added to the sample, the loss of water is reduced from more than 70% to 7% (relative).

Crystal Growth Experiments in the Undercooled Orthoclase-Quartz-H₂O System

Carmela Freda (lillif@hotmail.com) & Don R. Baker (donb@eps.mcgill.ca)

Earth and Planetary Sciences, McGill University, 3450 rue University, Montréal, QC H3A 2A7, Canada

The relationship between undercooling and textures formed during crystallization of the eutectic composition in the system Orthoclase-Quartz-H₂O has been studied experimentally. Experiments were performed at 500 MPa, 50, 100, and 200°C undercooling, variable duration, and different water concentrations (4, 8, and 14 wt%). A small amount of Rb was added to investigate the effects of crystallization rate on Rb partitioning between feldspars and melt and to measure the relative times of K-feldspar crystallization. At 100°C undercooling the textural development commences with the formation of subhedral, skeletal, and dendritic quartz which occurred in 25 hours. Tabular K-feldspar nucleated and grew between quartz in the 50-hour experiments. In longer duration experiments these minerals continue to crystallize with only minor changes in shape and texture. The most common texture is spherulitic with quartz-K-feldspar intergrowths varying from submicron to microns in size; set within this intergrowth are either individual crystals or intergrowths of quartz and K-feldspar with dimensions of 10's to 100's of microns. No significant differences were seen between experiments that were initially water saturated (14 wt% H₂O) and those with 8 wt% H₂O to the system. Experiments with 4 wt% H₂O were also performed at an initial undercooling of approximately 200°C, then, as crystallization proceeded the increasing water concentration in the melt caused the undercooling to decrease to about 100°C. Experiments at 200°C undercooling are characterized by spherulitic quartz-K-feldspar intergrowths terminating in cavities or in pools of residual melt and by quartz-K-feldspar graphic intergrowths. Modal quartz in the spherulites decreases from core to rim and the Rb₂O concentration in the K-feldspar increases. Rb₂O concentrations in K-feldspars of the graphic intergrowth are invariable and suggest that these intergrowths crystallized before the spherulites from a melt of approximately constant composition.

Experimental Phase Relations in Natural Phonolites from the Kerguelen Archipelago. The Effect of Water and fO₂

Marcus Freise (m.freise@mineralogie.uni-hannover.de)¹, Francois Holtz¹, Jürgen Koepke¹, Dimitri Damasceno² & Herve Leyrit³

¹ Institut für Mineralogie, Universität Hannover, Welfengarten 1, D-30167, Germany

² Department of Earth & Environmental Sciences, CP160/02, Université Libre de Bruxelles, Avenue F.D.Roosevelt 50, B-1050 Brussels, Belgium

³ Institut Geologique Albert-de-Lapparent, Institut Polytechnique Saint Louis, 13, boulevard de l´Hautil, F-95092 Cergy-Pontoise cedex, France

Phase relations in a phonolite (LVLK 81) and a tephrite (LVLK 90) from the Upper Miocene lavas in the Southeast Province of the Kerguelen Archipelago (Weis et al.,1993) have been investigated in the p/T range 100 - 500 MPa and 760 - 890°C at two fO₂ conditions (~NNO and ~NNO+2.3) to clarify the differentiation and pre-eruptive conditions of these magmas. Equilibrium crystallization experiments were performed using dry glasses, water and Ag₂C₂O₄ (source for CO₂) as starting materials. Various X_H2O were used to change the water activity. Experiments under reducing and oxidizing conditions were conducted in cold-seal pressure vessels pressurized with water and argon, respectively. Under reducing conditions, the resulting phase assemblage for LVLK 81 was: titanomagnetite, nepheline, alkali feldspar, clinopyroxene and biotite; under oxidizing conditions, the assemblage was: magnetite, plagioclase, alkali feldspar, nepheline, titanite (minerals given in the order of appearance with decreasing T at 200 MPa for 4 wt% water in the melt). The compositions of experimental biotites and pyroxenes obtained under oxidizing conditions in LVLK 90 is similar to the natural minerals (i.e. Fe-rich). In LVLK 81, phenocrysts of clinopyroxene, amphibole, biotite and alkali feldspar occur in the natural sample. However, nepheline is only present in the groundmass, which is not consistent with the results under reducing conditions. Thus, both the differentiation and pre-eruptive conditions must have been relatively oxidizing (f_O2 slightly lower than ~NNO+2.3). Under oxidizing conditions, the stability field of feldspars (plagioclase and alkali feldspar) and of clinopyroxene (only found at p>300 MPa) suggests that magmatic pressure was above 250 MPa prior to eruption of the phonolite. Amphibole, one of the main mafic phases observed in the sample, has not yet been synthesised experimentally. Experiments are currently under progress to constrain the stability field of amphibole in phonolitic lavas from the Kerguelen Archipelago.

Weis et al, Earth and Planetary Science Letters, 118, 101-119, (1993).