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3.1 d. Effects of the olivine-modified spinel transformation on the rheological properties of (Mg,Fe)2SiO4 (S. Karato, D.C. Rubie, C. Dupas and T.G. Sharp)

The viscosity-depth profile of the Earth's mantle has an important influence on mantle convection and other related geodynamical and geochemical processes. Phase transformations in mantle minerals are among the most important factors that affect the viscosity-depth profile. However, because of the difficulties in performing high-pressure, high- temperature deformation experiments, almost nothing is known about the effects of phase transformations on rheological properties of mantle minerals.

The principal aims of this new research project are (i) to establish a new method of studying rheological properties under high pressure and temperature conditions (P = 15 - 20 GPa, T = 1500 - 2000 K), (ii) to apply it to the determination of rheological contrasts between olivine and ß-phase as a function of grain-size and, as a by-product, (iii) to get experimental data on the dependence of grain-size of transformed ß-phase on the temperature at which the phase transformation occurs.

Previous efforts to investigate rheological properties under high pressures and temperatures have focused on deriving rheological constitutive relations (i.e., viscosity as a function of pressure, temperature and stress). However, due to the difficulties in estimating differential stresses and in applying differential stresses in a controlled manner, these studies have not been able to provide any definitive conclusions concerning rheological stratification in the Earth's mantle.

In this research project, we are developing a new strategy for the study of rheological properties under high pressures and temperatures. A simple shear-deformation geometry with two thin slices of specimen sandwiched between thoriated tungsten pistons will be used to determine the relative strength (viscosity) of the two specimens under high pressures and temperatures. Upon moving the ram of the multi-anvil press while the specimen is under high pressure and temperature, homogeneous compaction of a sample assembly will create differential stresses to cause shear strain of the specimen. Strains of two co-existing specimens will be measured from the rotation of strain markers to provide a measure of the relative strength. We adopt this strategy because determination of relative strength is more feasible than the determination of absolute strength and because relative strength often controls the dynamics of the Earth's interior. Furthermore, once the rheological properties of some standard material is well characterized under high pressures and temperatures, one can obtain quantitative data on the rheology of a specimen by measuring its strength relative to that of the standard material.

We shall first investigate the deformation properties of ß-phase as a function of grain-size using specimens prepared by transforming olivine at various P-T conditions. Two specimens with different grain-size will be inserted together into a the shear assembly and deformed simultaneously under the same P/T conditions. After establishing the transition conditions between grain-size sensitive and insensitive creep in ß-phase, we shall determine the relative strength of olivine and modified spinel for a given grain-size.

The results will provide a first data set to constrain the rheological stratification of the Earth's transition zone from mineral physics studies.

Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
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