Rheology of Synthetic Plagioclase-clinopyroxene Aggregates

Alexandre Dimanov (dima@gfz-potsdam.de) & Georg Dresen

GeoForschungsZentrum, Telegrafenberg, D424, Germany

We investigated the effect of a second phase (diopside) on the rheological properties of fine grained anorthite aggregates. The samples were fabricated by hot isostatic pressing of fine grained powders of synthetic glasses and/or natural crystalline diopside at 300 MPa. The grain size of the matrix and the second phase was ~ 3.5 µm and 1.5 - 15 µm, respectively. The starting material contained traces of water (0.03 - 0.06 wt.%) and < 1 vol.% of melt contained in three and four grain junctions. Gain boundaries were found to be melt free. The deformation tests were performed between 1000 and 1300°C at 0.1 MPa and 300 MPa confining pressure, and at flow stresses between 5 and 450 MPa. Below 250 MPa the stress exponent for the end members and the two-phase aggregates was close to 1. Mechanical results and microstructures indicate grain boundary diffusion controlled creep.

At 0.1 MPa diopside is about an order of magnitude stronger than anorthite. Dry anorthite, diopside and two-phase samples (< 500 ppm H/Si) have a thermal activation of ~ 500 ± 50 kJ/mol. End members and two-phase samples containing traces of water (~ 2500 ± 1000 ppm H/Si) have an activation energy of ~ 400 ± 50 kJ/mol. The strength increases with increasing diopside content. At 300 MPa wet end members and two-phase aggregates (5000 - 10000 ppm H/Si) have an activation energy of ~ 250 ± 50 kJ/mol. No systematic hardening was observed whith increasing diopside content. Wet two-phase aggregates were always weaker than pure anorthite with a minimum flow strength for 50 vol.% anorthite. The weakening can only partly be accounted for by grain size refinement observed in the two-phase aggregates.

Magma Properties and Volcanic Eruptions

Donald B. Dingwell (don.dingwell@uni-bayreuth.de)

Bayerisches Geoinstitut, Uni Bayreuth, D-95502 Bayreuth, Germany

The properties of magmas are a powerful control on the nature of volcanic eruptions. The wide variation in the physico-chemical state of volcanic materials erupted at the Earth's surface indicates the scale of the challenge we face in accurating describing magma properties for simulating igneous and especially eruptive processes. Further, enormous changes in magma properties are precipitated by degassing and associated phenomena immediately prior to and during an eruption. This latter point is underlined by the fact that magma properties can evolve from liquid-like to solid-like (!) during the course of an eruption. Our improved understanding of eruptive systems has benefited from better observations, new concepts, more powerful simulations AND experimental innovation. A major constraint on the modelling of such dynamic systems is provided by careful experimental investigation of the physical properties of magma. Experimental mineralogists, petrologists and geochemists are well-prepared to address this challenge. In fact, our physico-chemical view of the magmatic system involved in an eruption has improved greatly in the past decade due to their efforts. Major steps forward have been made in the experimental investigation of the equation-of-state, thermal conductivity, rheology, permeability, surface properties, and degassing kinetics of magmas. Challenges which remain as high priorities for the future include the kinetics of bubble nucleation microlite growth, the strength of magma, the rheology of magmatic foams, and the seismic and acoustic properties of magmatic systems. Some of these investigations have begun, others await experimental innovations that lie ahead. I will provide an overview of challenges met and challenges remaining in this rapidly developing field.

Viscosity and Diffusion in Liquid Fe and FeS Mixtures at High Pressures and Temperatures: Towards Outer Core Properties

David Dobson (d.dobson@ucl.ac.uk), Lidunka Vocadlo (l.vocadlo@ucl.ac.uk), John Brodholt (j.brodholt@ucl.ac.uk) & Wilson Crichton (w.crichton@ucl.ac.uk)

Department of Geological Sciences, University College London, Gower Street, London, WC1E 6BT, UK

The transport properties of the outer core play a fundamental role in controlling the Earth's cooling and geomagnetic history. Despite their importance, the viscosity and diffusivity of the outer core are very poorly known, and even experimental studies on candidate materials are scarce.

We present results of experiments to measure viscosity and diffusivity in pure liquid Fe-FeS mixtures and self-diffusivity in liquid Fe. Viscosity was measured using the in situ falling sphere method in a multi-anvil-press installed on BM13D of the APS (Dobson et al, 1996), and diffusivity was measured using a horizontal furnace arrangement (Dobson, 2000) to eliminate convection.

We find the viscosity of Fe-FeS liquids to be close to metallic values (around 10^-3 PaS), contrary to previous studies (LeBlanc & Secco, 1996), with a systematic increase in viscosity and activation energy (41 kJ/mol in Fe (in Beer, 1972); 100 kJ/mol in FeS eutectic; 200 kJ/mol in FeS) as S increases. S and Fe diffusivity in Fe-FeS eutectic are consistent with the viscosity measurements (D = 10^-4 to 10^-5 cm²/s), with similar activation energies, confirming the validity of the Stokes-Einstein relation for this system). Initial experiments to measure pressure dependences suggest that diffusivity is constant along a homologous path to 20 GPa, consistent with earlier suggestions (Poirier, 1988). This is supported by the remarkable agreement between the current experimental results at relatively low pressures and the results of ab-initio simulations at the P-T conditions of the inner core boundary (Alfe & Gillan, 1998).

Dobson DP et al, Earth Planet. Sci. Lett, 143, 207-215, (1996).

Dobson DP, Phys. Earth Planet. Inter, in press, (2000).

LeBlanc GA & Secco RA, Geophys. Res. Lett, 23, 213-216, (1996).

Beer SZ, Liquid metals, (1972).

Poirier JP, Geophys. J, 92, 99-105, (1988).

Alde D & Gillan MJ, Phys. Rev. B, 58, 8248-8256, (1998).