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Pdf/7/1/40/3341343/40.Pdf by Guest on 26 September 2021 High P, T Tomography on Deformed Composites In situ high-pressure and high-temperature X-ray microtomographic imaging during large deformation: A new technique for studying mechanical behavior of multiphase composites Yanbin Wang1, Charles Lesher2, Guillaume Fiquet3, Mark L. Rivers1, Norimasa Nishiyama1,*, Julien Siebert3, Jeffery Roberts4, Guillaume Morard3, Sarah Gaudio2, Alisha Clark2, Heather Watson4, Nicolas Menguy3, and Francois Guyot3 1Center for Advanced Radiation Sources, University of Chicago, 5640 S. Ellis Avenue, Chicago, Illinois 60637, USA 2University of California at Davis, Department of Geology, One Shields Avenue, Davis, California 95616, USA 3Institut de Minéralogie et de Physique des Milieux Condensés, Institut de Physique du Globe de Paris, 140 rue de Lourmel, 75015 Paris, France 4Lawrence Livermore National Laboratory, P.O. Box 808, L-184, Livermore, California 94551, USA ABSTRACT static stress, migrated into silicate grain static stress, whereas weaker materials are more bound aries, and propagated in a manner susceptible to fl ow. Although researchers have We have examined the microstructural similar to melt inclusions in a deforming solid investigated the effects of LPO on fl ow proper- evolution of a two-phase composite (olivine matrix. The grain size of the silicate matrix ties and seismic anisotropy for several mantle + Fe-Ni-S) during large shear deformation, was signifi cantly reduced under large strain minerals (e.g., Bascou et al., 2002; Mainprice using a newly developed high-pressure X-ray deformation. The strong shape-preferred et al., 2004; Bystricky et al., 2006; Long et al., tomography microscope. Two samples were orientation thus developed can profoundly 2006; Warren et al., 2008), the effects of SPO examined: a load-bearing framework–type influence a composite’s bulk elastic and have received little attention, especially for texture, where the alloy phase (Fe-Ni-S) was rheological properties. High-pressure–high deep Earth dynamic processes. present as isolated spherical inclusions, and temperature tomography not only provides In studying the fl ow laws of multiphase com- an interconnected network–type texture, quantitative observations on textural evolu- posites, Handy (1994a) showed that two end- where the alloy phase was concentrated tion, but also can be compared with simula- member texture types can be defi ned for the along the silicate grain boundaries and tion results to derive more rigorous models plastic regime: (1) a load-bearing framework tended to form an interconnected network. of the mechanical properties of composite (LBF) texture, where the stronger phase sur- The samples, both containing ~10 vol% alloy materials relevant to Earth’s deep mantle. rounds isolated pockets of the weaker phase, inclusions, were compressed to 6 GPa, fol- and (2) interconnected layers of the weaker lowed by shear deformation at temperatures INTRODUCTION phase (IWL) separating boudins and clasts of up to 800 K. Shear strains were introduced the stronger phase. Under large deformation, by twisting the samples at high pressure and The entire rocky interior of Earth is com- bulk composite materials can undergo a tran- high temperature. At each imposed shear posed of multiphase composites. Volumetric sition from the initial LBF texture to IWL by strain, samples were cooled to ambient tem- fractions of the constituents and the spatial developing SPO (e.g., Handy, 1994a; Holyoke perature and tomographic images collected. relation of one phase relative to another have and Tullis, 2006; Takeda and Griera, 2006). The The three-dimensional tomographic images profound effects on the physical properties of materials science, mechanical engineering, and were analyzed for textural evolution. We the bulk constituents. It is well known that for structural geology literature provides a wealth found that in both samples, Fe-Ni-S, which a composite undergoing large deformation, two of information on models of the mechanical is the weaker phase in the composite, under- types of preferred orientations may develop. behaviors of composite materials (e.g., Guoan went signifi cant deformation. The resulting In cases where the constituents are deformed and Castañeda, 1993; Lee and Paul, 2005; lens-shaped alloy phase is subparallel to within the dislocation creep regime, a lattice- Tandon and Weng, 1986; Fletcher, 2004; Handy, the shear plane and has a laminated, highly preferred orientation (LPO) can develop (e.g., 1994a; Kanagawa, 1993; Treagus, 2002). anisotropic interconnected weak layer tex- see Karato, 1998; Wenk, 2006). Since deform- Experimental efforts have also yielded valua- ture. Scanning electron microscopy showed ing a weaker phase is energetically more ble information on the mechanical behavior of that many alloy inclusions became fi lm-like, favorable than deforming a stronger one, a crustal rocks and rock analogs (e.g., Jordan, with thicknesses <1 μm, suggesting that Fe- shape-preferred orientation (SPO) is also inevi- 1987; Shea and Kronenberg, 1993; Holyoke and Ni-S was highly mobile under nonhydro- table (Handy, 1994a, 1994b). Here we use the Tullis, 2006), by relating stress-strain curves terms “strong” and “weak” to indicate relative with textural information. *Present address: Geodynamics Research Center, rheological property contrast: stronger materi- Because of the technical challenges, virtu- Ehime University, Matsuyama, Ehime, Japan. als are more resistant to fl ow under nonhydro- ally no rheological data are available under Geosphere; February 2011; v. 7; no. 1; p. 40–53; doi: 10.1130/GES00560.1; 12 fi gures; 1 table; 3 animations. 40 For permission to copy, contact [email protected] © 2011 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/1/40/3341343/40.pdf by guest on 26 September 2021 High P, T tomography on deformed composites deep-mantle conditions for rock composites cor- copy, to examine microstructures beyond the with homogeneous composition. Complex den- responding to the mineralogies in the transition current resolving power of the HPXTM. In sity contrast was observed in the darker silicate zone and the lower mantle. Theoretical bound- this report we present results on composites regions in backscattered electron micrographs ing analyses (e.g., Handy, 1994a; Takeda, 1998) consisting of Fe-Ni-S and silicates and dis- (labeled m in Fig. 1; also see inset). EPMA provide limits on the composite fl ow stress. cuss the potential implications for the rheol- showed that the compositions in these areas These studies indicate that the lower bound ogy and dynamic processes of Earth’s mantle. varied from stoichiometric olivine to more Mg corresponding to a homogeneous strain rate The current HPXTM setup does not allow us rich [i.e., with the (Mg + Fe)/Si ratio >2]. We approaches the strength of the composite with to measure stress and strain directly. Hence, we interpret these regions as quenched melt. Many an ideal IWL texture, whereas the upper bound concentrated on characterizing texture during fi ne Fe-Ni-S inclusions were present in these associated with homogeneous stress approaches large strain deformation. In the future we plan regions (a few are indicated by the arrows). that of an ideal LBF texture. to combine high-pressure tomography with dif- Most important, essentially all the Fe-Ni-S Quantitative analyses of the effects of texture fraction analyses using a multielement detector inclusions were spherical in shape, with no con- on rheological properties are diffi cult, if not so that we can relate textural evolution directly nection to the nearby neighbors (Fig. 1). The impossible, to determine using analytical solu- to stress-strain measurements. nature of this complex texture is still not well tions. One approach is to conduct numerical understood. It was probably due to an unex- simulations on a mixture of phases with known EXPERIMENTAL PROCEDURE pected power surge during sample synthesis, physical properties and a prescribed spatial and causing the temperature to fl uctuate and olivine shape distribution. So far, numerical models Sample Preparation and Characterization to melt incongruently to enstatite and liquid, as of large shear deformation have been limited has been reported at high pressure by Ohtani to two dimensions (e.g., Takeda and Griera, For this study we chose a mixture of olivine et al. (1998). An excursion into the melting 2006). The applicability of these results to more and Fe-Ni-S as the model composite for three region is further supported by the spherical realis tic three-dimensional (3D) situations may reasons: (1) silicate and iron-nickel sulfi de shape of Fe-Ni-S inclusions dispersed through- be limited because the effects of geometry and do not have a strong chemical interaction, so out the sample. While melting was not intended, mechanical interactions cannot be accounted for we can focus on the physical aspects of large this serendipitous event created an LBF texture, in the third dimension. Madi et al. (2005) con- deformation; (2) the two phases have a large the initial mechanical response of which should ducted a 3D fi nite-element study to model the X-ray absorption contrast, allowing us to read- be dominated by the strong phase (in this case behavior of the two-phase lower mantle miner- ily separate domains within the tomographic the silicate matrix). alogical assemblage during deformation. These images; and (3) the strength contrast between Sample B was synthesized in a graphite cap- simulations were limited to strains of a few olivine and Fe-Ni-S, expressed as a ratio of sule at a more stable temperature, where only percent because modeling large deformation shear stresses, ranges from 500:1 to 1000:1 (see the Fe-Ni-S alloy was molten. The resulting is time consuming and costly. Although their Discussion herein), similar to that between the texture consisted of an olivine matrix separated preliminary high strain calculations suggested a predicted strengths of silicate perovskite and by interconnected channels of quenched Fe- trend toward a transition from LBF-like texture ferropericlase (Yamazaki and Karato, 2001), Ni-S melt (Fig. 2). This texture approaches the to IWL, no quantitative results were given.
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