Plastic deformation experiments to high strain on mantle transition zone minerals wadsleyite and ringwoodite in the rotational Drickamer apparatus

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Abstract

We report the results of plastic deformation experiments on polycrystalline wadsleyite and ringwoodite performed at 15–23 GPa and 1300–2100 K conducted using the rotational Drickamer apparatus (RDA). Wadsleyite was synthesized from fine-grained (∼2 μm) San Carlos olivine in a Kawai-type multianvil apparatus; the average grain size of the resulting wadsleyite was 1.2 μm. The initial water content of the undeformed wadsleyite was 24,000–26,000 H/106 Si but the final water content is variable and less than the initial water content. Ringwoodite was synthesized from wadsleyite in situ in the RDA. Both strain and stress were measured in situ using a synchrotron x-ray facility. Determinations of strains and strain rates were made from x-ray radiographs of the sample, using a Mo foil strain marker in the sample assembly. The state of stress was determined from the observed d-spacing of multiple lattice planes as a function of azimuth angle. Samples were deformed at various strain rates at around 10−4–10−5 s−1. Deformation mechanisms were inferred from the stress exponent and the microstructures. We determined the stress exponent n for wadsleyite to be 6±3, suggesting dislocation creep was the dominant deformation mechanism in wadsleyite. At grain sizes of ∼1 μm, our samples were still deforming primarily by dislocation creep. However, small dislocation-free grains are also observed suggesting that diffusion creep may operate in some parts of our samples.

Highlights

► Reports deformation experiments on wadsleyite and ringwoodite at mantle conditions. ► Ringwoodite was synthesized from wadsleyite in situ in the RDA. ► Determined stress exponent n=6±3 for wadsleyite. ► At grain sizes of∼1 μm samples still deforming primarily by dislocation creep.

Introduction

The mantle transition zone (MTZ, a layer between 410 km and 660 km depth) is the bottommost region of the upper mantle, just above the lower mantle, and has important influences on mantle convection due to complicated changes in the physical properties of materials caused by a series of phase transformations. High-resolution tomographic images show that subducted slabs are deformed in the MTZ, although the nature of deformation varies from one subduction zone to another (e.g., van der Hilst et al., 1991; Kárason and van der Hilst, 2000, Fukao et al., 2001). Among various physical properties, viscosity is a critical parameter that controls convective patterns as it varies with depth (e.g., Davies, 1995, Bunge et al., 1996, Bunge et al., 1997). Viscosity of the mantle is determined by the rheological properties of its constituent minerals. Some previous studies showed that the viscosity of the MTZ will determine whether subducted slabs will penetrate into the lower mantle or be deflected in the MTZ (Davies, 1995, Bina et al., 2001, Fukao et al., 2001, Karato et al., 2001, Richard et al., 2006). In addition, the degree of energy dissipation by slab deformation is an important factor that might control the rate of mantle convection. Consequently, quantifying the viscosity of the MTZ by determining the rheological properties of transition zone minerals is fundamental in our efforts to understand the evolution of the Earth, patterns of convection in the mantle, and the energy budget of the planet. Therefore, to better understand and model convection of the mantle, knowledge of the quantitative rheological properties of wadsleyite and ringwoodite, the primary components of the MTZ (Ringwood, 1991), is needed.

In this study we use the Rotational Drickamer apparatus (RDA) to conduct deformation experiments under the conditions of the MTZ. The RDA, described by Yamazaki and Karato (2001) and Xu et al. (2005), is capable of reaching the pressure and temperature conditions of the mantle transition zone and subsequently performing deformation experiments to large strains using torsion deformation geometry. Previous experimental studies using the RDA have reported the results of plastic deformation experiments on olivine (Nishihara et al., 2008, Kawazoe et al., 2009) and wadsleyite (Kawazoe et al., 2010) up to P∼17 GPa and T∼2000 K.

The rheological properties of ringwoodite have not been explored in the RDA previously, though the rheological properties of ringwoodite have been investigated by others in some preliminary studies. Karato et al. (1998) performed stress relaxation tests on Fe-rich ringwoodite samples (Fe/(Fe+Mg)=0.4), rather than a typical mantle value of Fe/(Fe+Mg)=0.1, in a multianvil apparatus at P∼16 GPa and T=1400–1600 K (the larger proportion of Fe altered the ringwoodite stability field, allowing the synthesis of ringwoodite at lower pressures). They inferred two deformation mechanisms from microstructural observations and proposed a preliminary deformation mechanism map. Xu et al. (2003) performed stress relaxation tests on ringwoodite at 20 GPa and 1623 K using a two-stage T-cup. However, results from stress-relaxation tests have limitations in obtaining quantitative results on a flow law at steady-state deformation, and in Karato et al. (1998), there were no constraints on the magnitude of stress, and the stress estimates by Xu et al. (2003) have large uncertainties caused by the use of x-ray peak broadening to determine stress.

Continued refinement of the sample assembly for the RDA has increased the upper limit of the pressures achievable, while improvements in multi-element detectors installed at synchrotron facilities have increased the resolution of stress estimates in the samples (Weidner et al., 2010). Most critical to our improved stress resolution has been the installation of a ten-element detector, allowing for the collection of energy-dispersive x-ray diffraction (EDXRD) patterns over a half-circle. These recent developments have allowed us to use the RDA to explore the rheological properties of mantle minerals under the entire transition zone conditions (14–23 GPa, 1500–2100 K). Here we report the results of plastic deformation experiments on single-phase wadsleyite, ringwoodite, and mixed phases of wadsleyite and ringwoodite, deformed at pressures up to 23 GPa and 2100 K, with stress resolution of 0.1 GPa.

Section snippets

Sample preparation

Samples of wadsleyite were synthesized from San Carlos olivine, (Mg0.9,Fe0.1)2SiO4. The preparation of the olivine powders has been described in detail previously (Kawazoe et al., 2009, Kawazoe et al., 2010); we will provide only a brief overview here. The olivine starting material was ground and filtered to a grain size of less than 2 μm. The filtered olivine powders were packed in capsules of Ni foil and then loaded into a Kawai-type multianvil press inside a 14/8 assembly. Wadsleyite was

Strain and strain rate

An example of the x-radiographs collected to measure strain in the samples during the course of the experiments is depicted in Fig. 4, where a series of chronological images show the progressive deformation of the strain marker. Strain rates for the samples were determined from these radiographs by fitting the accumulated strain (both uniaxial compression and simple shear) measured from the deflection of the Mo strain marker against time. Equivalent strain for each sample, defined as ɛE=ɛU2+43ɛS

Deformation mechanisms

One of our goals has been to identify the operating deformation mechanisms and determine the flow law for each mechanism for wadsleyite and ringwoodite. To that end, we performed strain-rate stepping experiments to constrain the stress exponent, n.

The challenge in these strain rate stepping experiments is the identification of a steady-state stress. Once a steady state is judged to have been achieved, the strain rate is changed and the evolution of stress towards a steady state value is

Conclusions

In this study, we have conducted quantitative in situ rheological measurements up to P=23 GPa and T=2100 K using the RDA. While wadsleyite has been deformed in the RDA before (see Kawazoe et al., 2010), this new data benefited from additional detectors installed at the BNL NSLS X17B2 (see Weidner et al., 2010), resulting in higher resolution of stresses, and some constraints on the flow law were obtained.

Combined with the microstructural observations by TEM and a comparison with creep models, we

Acknowledgments

We thank Michael Vaughan, Liping Wang, and Donald Weidner for technical assistance at NSLS. William Durham is thanked for discussions on the analysis of x-ray observations. This work was supported by NSF and COMPRES and a Bateman fellowship at Yale to JH.

References (31)

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    Wadsleyite and ringwoodite constitute the main volumetric fraction (up to ∼60%) of the TZ, while the remaining ∼40% consists of majorite garnet (Ita and Stixrude, 1992). Most recent high pressure (P) and temperature (T) deformation experiments of the high-P polymorphs of olivine (Nishihara et al., 2008; Farla et al., 2015; Hustoft et al., 2013; Kawazoe et al., 2010, 2013; Meade and Jeanloz, 1990; Kavner and Duffy, 2001; Nishiyama et al., 2005; Miyagi et al., 2014) reveal high flow stresses during quasi steady state deformation at typical laboratory strain rates of ∼10−5 s−1. It was demonstrated that the flow stress under quasi steady state conditions for both wadsleyite and ringwoodite remains generally ∼2-4 GPa at the appropriate P, T conditions of the TZ.

  • An experimental study of grain-scale microstructure evolution during the olivine–wadsleyite phase transition under nominally “dry” conditions

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    However, the study by Karato et al. (2001) was based only on theoretical estimation of grain-size, and the flow law parameters were based on the scaling law because experimental studies on plastic deformation at the transition zone conditions were missing at that time. In the mean time, there has been major progress in the experimental studies of plastic deformation of minerals at high-pressures (e.g., Farla et al., 2015; Hustoft et al., 2013; Kawazoe et al., 2010a, 2010b; Nishihara et al., 2008) and of diffusion (e.g., Shimojuku et al., 2004). These studies provide better constraints on the flow laws of olivine, wadsleyite and ringwoodite, and also show that a substantial grain-size reduction would reduce the creep strength of these minerals.

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    Solid-state flow in the lowermost part of the mantle transition zone (between 520 km and 660 km depth), is determined by the rheological properties of its primary constituent phase (Mg,Fe)2SiO4 ringwoodite, a high pressure polymorph of olivine. Deformation experiments have been developed to investigate the plasticity of ringwoodite (Meade and Jeanloz, 1990; Chen et al., 1998; Karato et al., 1998; Cordier and Rubie, 2001; Kavner and Duffy, 2001; Thurel, 2001; Yamazaki and Karato, 2001; Cordier et al., 2002; Cordier, 2002; Wang et al., 2003; Xu et al., 2003; Wenk et al., 2004; Nishiyama et al., 2005; Wenk et al., 2005; Kawazoe et al., 2010; Hustoft et al., 2013; Miyagi et al., 2014). Although considerable progress in deformation techniques have been achieved over the last twenty years, it still remains difficult to conduct experimental deformation at high pressure (∼20 GPa) and high temperature (∼1800 K) conditions of the lowermost transition zone.

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    We performed two deformation runs, one where a step test was performed at two constant anvil rotation rates corresponding to equivalent strain rates of 5.8 × 10−5 and 12.3 × 10−5 s−1 and a second run at a constant anvil rotation rate corresponding to an equivalent strain rate of 12.8 × 10−5 s−1. As the sample assembly for the RDA has been previously described (Nishihara et al., 2008; Kawazoe et al., 2009; Hustoft et al., 2013), only relevant details will be discussed here. In the previously mentioned works all sample assembly parts (alumina confining rings, insulations, heaters and electrodes, etc.) were cut using an ultrasonic cutter.

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Now at: GeoSoilEnviroCARS, University of Chicago, Argonne National Laboratory, Argonne, IL, USA.

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