Superliquidus Cluster Differentiation of Fluid Magmatic Melts as Mechanism of Layered Massifs Formation

Nickolai Bezmen (bezmen@iem.ac.ru)

Institute of Experimental Mineralogy of RAS, Chernogolovka Moscow District, 142432, Russia

Experimental researches have shown that interaction of nearest to natural complex hydrogen-bearing magmatic fluids in superliquidus conditions with silicate melts causes their depolymerization resulted in of the formation of fluctuated micromolecular groups - clusters of 6 nm (60 Å) and higher in sizes (TEM data). Clusters are intermediate state between liquid and crystals. According to up-to-date idea (Schmid, 1985) the clusters consist of a central ordering core of molecules surrounded by electrons cloud ("jellium" model) or ligands, such as Cl, F, Cr₂O₃, P₂O₅, OH, etc (Tredoux et al., 1995). The clusters are stabilized by the presence of appropriate ligands. In comparison with the melt, the clusters are open, fluctuating systems which exchange energy and groups of molecules with the melt. The atoms in the ligand layer are highly mobile, so that the structure as a whole develops a solid-like core with a liquid-like surface. The outcome is that these strongly ordered, so-called dissipative, structures develop and exit for long periods of time (Glensdorff and Prigogine, 1971). The formation of clusters is an irreversible process which occurs under disequilibrium conditions once the fluid phase corresponds with a certain critical value. As it was experimentally shown under certain critical thermodynamical conditions the clusters or aggregates of clusters are capable to the gravitational movement, as consequence the cryptic and contrasting liquid layering of melts forms. The different basic-ultrabasic layering, ijolite urtite-nepheline syenite, peraluminous granite-silicic granite superliquidus magmatic layering and apatite, chromite, ilmenite, quartz liquid separation from the silicate melts has been simulated under pressure of H-O-C-S fluid system under control of gas species fugacities. Depending on the composition of the fluid dissolved in the melt more basic or ore liquids may get accumulated either at the top or bottom of the samples.

Glensdorff H & Prigogine I, Thermodynamic theory of structure, stability and fluctuations, London, 280, (1971).

Schmid G, Structure and Bonding, 62, 52-85, (1985).

Tredoux M, Lindsay NM, Davies G & McDonald I, S. Afr. J. Geol., 98, 157-167, (1995).

Solidi of Silicate-Water-Hydrogen Systems

Nickolai I. Bezmen (bezmen@iem.ac.ru) & Vilen A. Zharikov

Institute of Experimental Mineralogy of RAS, Chernogolovka Moscow District, 142432, Russia

The vapor-saturated solidi of NaAlSi₃O₈-H₂O-H₂ and SiO₂-NaAlSi₃O₈-H₂O-H₂ systems at 2 kbar over the range of gas phase composition from pure water to X(H₂O)v=0.1 were studied in the internally heated gas-media pressure vessel. Various H₂O/H₂ fluid compositions were controlled directly rather than by using solid buffers. The results show that the melting temperatures decrease in the X(H₂O)v range from ~1 to 0.7 compared to H₂O-saturated curves under relevant water fugacities. At X(H₂O)v=0.91 for albite and 0.953 for Qz-Ab eutectic the solidus curves have a pronounced minimum with the temperature depression of ~30°C. The further addition of H₂ to the gaseous mixture leads to the increase of melting temperatures. In the region of WI-buffer the temperatures of solidi are about 50°C higher than hydrous Ab and Qz-Ab under the pure water fugacity equal the partial one in the H₂O-H₂ mixture. The melting point of 1078°C for Ab and 987°C for Qz-Ab in pure hydrogen has been calculated by an extrapolation of H₂O-H₂ data.

The Differentiation of a Tephritic Melt: An Experimental Study

Ulrich Bläß (blaess@petro1.min.uni-muenchen.de) & Thomas Kunzmann (kunzmann @petro1.min.uni-muenchen.de)

Institut für Mineralogie, Petrologie und Geochemie, Ludwig-Maximilians-Universität München, Theresienstr. 41/III, D-80333 München, Germany

Phonolitic rocks are assumed to be late fractionation products of nepheline basanitic or tephritic melts in magma reservoirs close to the surface at low pressure. To elucidate the genesis of phonolites a low pressure fractionation model is presented. The fractionation model was studied experimentally at a pressure of 0.2 GPa at liquidus and near liquidus temperatures. The chemical compositions of near liquidus phases are determined for tephritic, phonotephritic and tephriphonolitic compositions to evolve a three stage differentiation model. The starting materials for the experiments are synthetic pure oxide mixtures of the eight major elements: SiO_2, TiO₂, Al₂O₃, Fe₂O₃, MgO, CaO, Na₂O and K₂O. For all runs 2-3% H₂O was added. Experiments were carried out using an internally heated gas media apparatus. Samples were sealed in silver/palladium capsules. Run products, glasses and near liquidus phases, were characterized using optical microscope, X-ray diffraction and electron microprobe. The amount of crystallized phases and glasses are calculated using a least square fit computer program.

For the differentiation from tephritic bulk composition to a phonotephritic melt, olivine, magnetite and clinopyroxene are the near liquidus phases and will therefore be fractionated. In a further step clinopyroxene, magnetite, ± feldspar and ± amphibole must be removed from the phonotephritic melt to get a tephriphonolitic composition. In a last step clinopyroxene, feldspar, ± amphibole and ± mica are the fractionated phases to produce a phonolitic melt. Different fractionation trends are discussed considering the variable amount and chemical compositions of crystallized phases.

The Influence of Composition and Cation Order on the High Pressure Phase Transition in the Cummingtonite - Grunerite System

Tiziana Boffa Ballaran (tiziana@esc.cam.ac.uk)¹, Ross J. Angel² & Michael A. Carpenter¹

¹ Dept. of Earth Sciences, Downing St., Cambridge CB2 3EQ, UK

² Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany

Single-crystal X-ray diffraction experiments were performed at room temperature in a diamond-anvil pressure cell on four natural cummingtonite amphiboles with Fe/(Fe + Mg) = 0.45, 0.69, 0.89 and 0.97 respectively. Two of these samples were also annealed to change the state of cation order, and then restudied at high pressures. With increasing pressure the crystals undergo a phase transition from C2/m to P2₁/m symmetry as indicated by the appearance of super-lattice reflections with h+k = 2n+1. The evolution of the intensities of these super-lattice reflections shows that the transformation is reversible with no significant hysteresis and close to second order in character for all compositions and degrees of Mg/Fe non-convergent order. The evolution of the spontaneous strain arising from the transition does not depend on the Mg/Fe substitution or ordering, but it is sensitive to the Al, Na and Ca content. The transition pressure increases with both increasing Fe content and cation order of the crystals. Moreover there is a plateau in the variation of transition pressure with composition at the grunerite end of the solid solution whose magnitude correlates inversely with the magnitude of the strain fields arising around the atoms of Mg substituted into the grunerite structure.