Microstrain mechanical testing of ultramafic materials: the onset of anelastic relaxation, its grainsize sensitivity, and the role of orthopyroxene
Creators: name: Qu, Tongzhang
Publisher: name: The Australian National University
Year: 2022

description: Forced-oscillation methods have previously been used to explore the anelastic/viscoelastic relaxation in polycrystalline olivine and its sensitivities to variation of grain sizes, partial melting, prior deformation, fugacities of water and oxygen through simultaneous measurement of shear modulus and strain-energy dissipation at seismic periods (1 - 1000 s), high temperature (up to 1300 degree C) and pressure (200 MPa). Because anelastic relaxation reflects the presence of crystal defects and prevailing physico-chemical conditions, such laboratory studies are critical to interpretation of seismic wave dispersion and attenuation in the Earth's upper mantle. Such studies in our ANU laboratory have been confined to a limited range of chemical compositions and been conducted with progressively further refined experimental methods. Accordingly, the previous studies have left largely unexplored role of orthopyroxene during viscoelastic relaxation and the mechanical properties of olivine polycrystals during the onset of anelastic relaxation. However, these two subjects are of particular importance, mainly because the content of orthopyroxene can reach to 30% in the shallow upper mantle, and most of the upper mantle only departs modestly from elastic behavior. In this study, fine-grained polycrystalline specimens were prepared by solution-gelation method followed by hot pressing the precursor powder at 1200 degree C and 300 MPa within Ni70Fe30 capsules. The resulting specimen, again, enclosed within a Ni70Fe30 liner, was sandwiched between alumina torsion rods, jacketed usually within a mild-steel sleeve, and tested by torsional forced oscillation and microcreep. Refinement of the experimental methodology involved replacement of polycrystalline alumina control sample with single-crystal sapphire and removal of interfacial NiFe foils from the specimen assembly. To avoid complication caused by phase transition of conventional mild-steel jacket during staged cooling, stainless-steel jacket was employed in one of the experiments, requiring a complementary study of viscoelastic relaxation in stainless steel. Specimens of larger grain size were hot-pressed under more oxidizing and hydrous conditions within a Pt-lined mild-steel jacket at 1200-1300 degree C, followed by firing within CO/CO2 atmosphere of 1:1 partial pressure to restore the appropriate redox state before mechanical testing. Torsional oscillation tests, whether conducted within mild-steel or the alternative stainless-steel jacket, consistently reveal the onset of anelastic relaxation in dry and melt-free olivine polycrystals as a mild dissipation peak with relaxation strength of 0.02 +/- 0.01, probably associated with elastically accommodated grain-boundary sliding. The grain-size (d) sensitivity of viscoelastic relaxation, newly constrained in this study for specimens ranging in mean grain size from 4 to 22 micron, is markedly stronger than previously observed with relaxation times varying approximately as d**3 in accord with theoretical models for grain-boundary sliding. Comparison of results from orthopyroxene-rich samples with the model fitted to the data for polycrystalline olivine and evaluated at the appropriate grain sizes, suggests that adding 30% orthopyroxene into an olivine aggregate causes little variation in shear modulus and dissipation within the observational window. By overcoming some of the experimental limitations on previous measurements of anelastic relaxation in ultramafic materials, this study helps to establish a more robust model for upper-mantle seismic wave dispersion and attenuation.