The Pressure Dependence of the Rheological Properties for Depolymerised Silicate Melts

Joy E Reid (joy.reid@uni-bayreuth.de)¹, Brent T Poe (brent.poe@uni-bayreuth.de)¹, David C Rubie (dave.rubie@uni-bayreuth.de)¹, Akio Suzuki (a-suzuki@mail.cc.tohoku.ac.jp)², Eiji Ohtani (ohtani@mail.cc.tohoku.ac.jp)² & Kenichi Funakoshi (funakosi@sp8sun.spring8.or.jp)³

¹ Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, Germany

² Institute of Mineralogy, Petrology, and Economic Geology, Tohoku University, Sendai 980-857, Japan

³ Japan Synchrotron Radiation Research Institute, 1-1-1, Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan

A deep, magma ocean is thought to have formed in the early Earth due to the impact of a planetary-sized body (Tonks and Melosh, 1993). The resulting magma ocean, which may have been up to 1000 km in depth, would have profoundly influenced the resulting structure of the Earth's Mantle on cooling. An understanding of the pressure dependence of the rheology of the magma in necessary in order to model processes such as rates of convection and crystal settling in the magma ocean. The modelling of modern day mantle processes, such as melt migration, is also dependent on an understanding of the rheology of relevant melt compositions at pressure. In this study, the pressure dependence of the rheologies of simple silicate systems relevant to the Mantle have been measured to allow a better understanding of the dynamics of the melt structure.

The viscosity and the silicon and oxygen self-diffusivities in diopside (CaMgSi₂O₆) liquid (NBO/T = 2) have been measured at high pressure. The viscosity has been measured using in-situ, falling sphere viscometry with a SPEED1500 multianvil apparatus in conjunction with synchrotron X-ray imaging (Spring8, Japan). The self-diffusivities of silicon and oxygen have been measured through isotope tracer diffusion with subsequent analysis using an ion probe. Including previously measured lower pressure data, the validity of the Eyring equation to relate silicon and oxygen self-diffusion and viscosity in diopside liquid has been examined over the pressure range from atmospheric pressure to 17 GPa at 2000°C. The silicon and oxygen self-diffusivities have also been measured in liquid compositions: CaMgSi₂O₆ - 15 wt% SiO₂ (NBO/T = 1.5) and Na₂O.2SiO₂ (NBO/T = 1) in order to compare the effect of the degree of polymerization on the pressure dependence of self-diffusion. An alumina-free system was maintained in order to eliminate the effects of Al coordination changes on the melt structure.

Tonks WB & Melosh HJ, Jour. Geophys. Res, 98 (E3), 5319-5333, (1993).

Water and the Physical Properties of Magmas

Pascal Richet (richet@ipgp.jussieu.fr), Alan Whittington & M. Ali Bouhifd

Institut de Physique du Globe, 4 place Jussieu, 75252 Paris cedex 05, France

The strong and volcanologically important influence of water on the physical properties of magmas is poorly known because measurements are problematic at the high-pressures required to dissolve significant amounts of volatiles. Thanks to the slowness of water exsolution, however, it is possible to perform accurate measurements in the supercooled liquid state just above the glass transition. The usefulness of the method has been first demonstrated through a determination of the viscosity of a series of hydrous andesite melts. As a continuation of this work, we have measured the viscosities of other hydrous synthetic melts whose composition approximate rhyolite, basalt, phonolite or tephrite melts. The important conclusion is that the depressing effects of water on the viscosity strongly depend on silicate composition. Aa a matter of fact, they increase so much with the degree of polymerization of the melt that basalts become more viscous than rhyolite liquids for water contents higher than 3%. To complement these observations, we have measured the density of the quenched glasses, the thermal expansion coefficient of supercooled liquids and the heat capacity of the materials up to the glass transition. These results also show that the contribution of the water component to melt properties is not generally an additive function of composition. As will be discussed, this has important consequences for volcanology and also interesting implications for water solubility mechanisms.

Ternary Analcite-Pollucite Solid Solutions: Phase Equilibrium Data (1-5 kbar, 600°C)

Elizabeth Rodrigues (beth@cnrs-orleans.fr)¹, Jacques Roux (jroux@cnrs-orleans.fr)¹ & Guy Hovis (hovisguy@lafvax.lafayette.edu)²

¹ ISTO, CNRS-Université d'Orléans, 1A Rue de la Férollerie, 45071 Orléans Cedex 2, France

² Department of Geology and Environmental Geosciences, Lafayette College, Easton, Pa 18042-1708, USA

The aim of this study is to measure thermodynamic properties of Analcite-Leucite solid solution analogues (K, Rb, Cs). These solid solutions are characterised by the substitution of small Na cations and water by a large alkali cations (K, Rb, Cs). These minerals can also accommodate large variations in the Si to Al ratio (and water content), reflecting P-T-X conditions which are well known for pure Na-Analcites. The purpose of this work is to provide thermodynamic and stability field data of these complex solid solutions using complementary techniques: phase equilibria, volumetric measurements, HF solution calorimetry and IR spectroscopy.

It was necessary to investigate the solid solution stability fields as a function of P and T to determine the widest range of solid solutions within the main compositional range. These experiments established the conditions required for synthesis of pure end-member phases and helped to constrain the mixing- parameter models.

The topology of the water-saturated system CsAlSiO₄-NaAlSiO₄-SiO₂ is controlled by the divariant assemblages: (1) Quartz, Albite, Analcite-Pollucite; and (2) Nepheline, Albite, Analcite-Pollucite. In both cases the composition of the Analcite-Pollucite is strongly pressure dependent. At 600°C from 1 to 5 kbar, X=Cs/(Cs+Na) varies between 0.12-0.85 and Y=Si/Al can reach 2.32 for pollucite rich compositions. Pure sodic analcite with Y between 1.5 and 3 is known. The main result of this study is that analcime-pollucite series must be considered as ternary solution, even close to rich Cs compositions, in agreement with the compositional range of natural pollucites described by Teerstra et al., (1994).

Teerstra DK, Sheriff B, Xu Z, Cerny P, Can. Mineral. 32, 69-80, (1994).