Activation volume for dislocation creep of forsterite and of iron-free enstatite
Abstract
A good knowledge of the mechanical behavior of olivine and enstatite at high pressure and high temperature is essential to model Earth upper mantle dynamics. In this study, we have performed deformation experiments on forsterite and on iron-free enstatite polycrystalline aggregates at upper mantle pressures and temperatures. Fine-grained forsterite powders were obtained by crushing a commercial forsterite in WC or zirconia grinders and dried at high temperature. Enstatite powders were synthesized by solid state reaction between mixed fine-grained powders of silica and forsterite in a conventional furnace. The powders were sintered by Spark Plasma Sintering (SPS) at 1000-1300°C and 100 MPa. We obtained aggregates with very low porosities (>99% dense), low water content and well equilibrated microstructures with mean grain sizes of a few microns. Compression deformation experiments were conducted on both types of aggregates in a D-DIA apparatus coupled with synchrotron white X-ray beam at the X17-B2 beamline at the National Synchrotron Light Source (Brookhaven National Laboratory, NY, USA). Strain and stress were measured in situ during deformation. Macroscopic strains were determined by measurements of sample shortening on X-ray radiographies. Stress and pressure were determined from the analysis of 5 to 8 diffraction peaks in the X-ray diffractograms of forsterite or enstatite collected on detectors arranged in different orientations with respect to the maximum principal stress. Experiments were performed at pressures between 3 and 11 GPa and temperatures ranging from 1100 to 1300°C. 14 different samples were deformed to total strains of up to 30% with deformation rates ranging from 8 10-6 to 6 10-5 s-1. Microstructures analyzed using high resolution SEM showed features characteristic of dislocation creep. Analysis of the deformation data for forsterite at 5-7 GPa yielded a stress exponent of 2.5 to 3 at different temperatures, similar to values obtained at room pressure (Relandeau, 1981). The apparent activation energy appears lower than for forsterite or San Carlos olivine at low pressures but is subject to large errors due to the diffficulty to control temperature precisely. An activation volume V* of 8 cm3/mol was determined at constant temperature (1100°C and 1200°C). Extrapolation of the data using V* to low pressures gives strengths that are consistent with those calculated using low pressure flow laws for olivine. Interestingly, this V* value is intermediate between the activation volumes for forsterite single crystals oriented for [100](010) slip, which dominates at low pressures and high temperatures, and those oriented for [001](010) slip, which becomes increasingly active at high pressures (15 and 0 cm3/mol, respectively; Raterron et al. 2011). Iron-free enstatite aggregates were deformed in the ortho- and high clino- enstatite fields. Results indicate that orthoenstatite has a stress exponent of 3-4. It tends to be stronger than forsterite, but the strength contrast between the two phases depends on pressure and temperature conditions. We will discuss the effect of pressure on the rheology of enstatite and on the relative viscosities of enstatite and forsterite.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMMR41A2335B
- Keywords:
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- 3902 MINERAL PHYSICS Creep and deformation;
- 3924 MINERAL PHYSICS High-pressure behavior;
- 3954 MINERAL PHYSICS X-ray;
- neutron;
- and electron spectroscopy and diffraction;
- 3621 MINERALOGY AND PETROLOGY Mantle processes