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

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

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

Experimental textures formed during crystallization of the eutectic composition in the system Orthoclase-Quartz-H₂O at 500 MPa and 50, 100, and 200°C undercoolings (Freda and Baker, this volume) have been simulated using a two-dimensional Ising model formulation. Simulations, performed to complement and to interpret the experimental products, investigate what combinations of crystal growth rates (G) and diffusion in the melt (D) can yield crystal shapes and textures found in experiments. The simulations, despite their limitations, provide textures similar to the experimental ones. We obtained intergrowths at different scales simply by changing the G/D ratio in the simulations. In particular, simulations produced quartz/K-feldspar intergrowths when G=D and singular, large quartz and K-feldspar crystals when G<D. To obtain a simulation resembling the spherulitic textures observed in experiments we set G=D for the first 20,000 sweeps and then G=0.1D. The growth of large crystals in a fine-grained matrix was simulated by setting G=D except in one region of the simulation where G=0.5D. Experiments and simulations demonstrate that changes in G/D of only a factor of 10 have large effects on crystal shape and texture in severely undercooled melts. These changes in the G/D ratio, in the experiments and in natural pegmatites, can be produced by an increasing volatile content in the melt due to crystallization. Based on these results we suggest that during pegmatite formation the foot wall line rock crystallizes when G almost equals D (probably in the absence of a fluid phase), while the hanging wall giant crystals form at lower G/D ratio (later in time when the water concentration in the melt has increased). The quartz core possibly grew at the contact between fluid and melt, and the fluids remaining after quartz crystallization produced gem pockets.

Fluid-magmatic Interaction and Oscillation Phenomena at Melt Crystallization

Victor Balashov (balashov@iem.ac.ru)¹, George Zaraisky¹ & Reimar Seltmann²

¹ 142 432 Institute of Experimental Mineralogy, Chernogolovka, Moscow Distr., Russia

² Natural History Museum, London, UK

A new model of crystallization from a quasi two-component melt is presented which takes into account an interaction with fluid (as third component) in conjugation with unequilibrium process of fluid gain and loss. The fluid-magmatic interaction leads to a shift of crystallization eutectic (Epelbaum, 1980). In this way the crystallization process is sensitive to content of fluid component in melt. Under definite regime of fluid accumulation in melt the system is moved into unequilibrium region and the theoretical calculations show the possibility of oscillation regime for melt devolatilization. The changes of fluid concentration, the temperature and the crystallization rates get the oscillation character. The oscillation change of fluid concentration in melt produces the swing motion of eutectic position in phase diagram and causes the rhythmic crystallization of mineral components. This type of process is considered on the basis of data for system alb-qtz-H₂O-NaF (Johannes and Holtz, 1996). A comparison of the model patterns with the natural samples of line rocks from Etyka and Orlovka (Transbaikaliea) is made.

Epelbaum, MB, Silicate melts with volatiles, Moscow, 256p, (1980).

Johannes W & Holtz F, Petrogenesis and experimental petrology of granitic rocks, Springer Verlag, 335p, (1996).

Investigation of Diffusion Metasomatic Interaction on the Basis of Theoretical and Experimental Modelling

Victor Balashov (balashov@iem.ac.ru)¹, George Zaraisky¹ & Marina Lebedeva²

¹ 142432 Institute of Experimental Mineralogy, Chernogolovka, Moscow Distr., Russia

² 142432 Instutute of Chemical Physics, Chernogolovka, Moscow Distr., Russia

We have generalized a theoretical macrokinetic model (Zaraisky, Balashov and Lebedeva, 1989; Balashov and Lebedeva, 1991)and applied it to diffusion experimental zoning in multicomponent systems. A numerical methods of finite differences were developed for solution of a multicomponent reactive diffusion stiff problem with high degree of nonlinearity. The experimental (Zaraisky, 1998; Zaraisky et al., 1986) and theoretical metasomatic columns are compared with eachother for greisen and skarn metasomatism in system Na₂O-K₂O-MgO-CaO-Al₂O₃-Fe₂O₃-SiO2-H₂O-HCl-CO₂ at 400-600°C, P = 100 MPa. The analysis of theoretical and experimental data allows for a new possibilities in correction of thermodynamic and kinetic constants in hydrothermal fluid-rock interaction.

Balashov VN & Lebedeva MI, Progress in Metamorphic and Magmatic Petrology, Cambridge, 167-195, (1991).

Zaraisky GP, Proceedings of the Ninth Quadrennial IAGOD Symposium, 151-164, (1998).

Zaraisky GP, Balashov VN & Lebedeva MI, Geokhimiya, 10, 1386-1395, (1989).

Zaraisky GP, Zharikov VA, Stoyanovskaya FM & Balashov VN, Experimental Investigation of Skarn Formation, Moscow, Nauka, 380p, (1986).