Morales Etal2018 Olivine-Antigorite Orientation Relationships.Pdf
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Tectonophysics 724–725 (2018) 93–115 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Olivine-antigorite orientation relationships: Microstructures, phase T boundary misorientations and the effect of cracks in the seismic properties of serpentinites ⁎ Luiz F.G. Moralesa, , David Mainpriceb, Hartmut Kernc a Scientific Center for Optical and Electron Microscopy (ScopeM), ETH Zürich, Auguste-Piccard-Hof 1, HPT D9, 8093 Zürich, Switzerland b Géosciences Montpellier, Université Montpellier 2, Place Eugène Bataillon, Batîment 22, 34095 Montpellier, France c Institut für Geowissenschaften, Universität Kiel, 24098 Kiel, Germany ARTICLE INFO ABSTRACT Keywords: Antigorite-bearing rocks are thought to contribute significantly to the seismic properties in the mantle wedge of Serpentinite subduction zones. Here we present a detailed study of the microstructures and seismic properties in a sample of EBSD antigorite-olivine schist previously studied by Kern et al. (1997, 2015). We have measured crystallographic Olivine-antigorite transformation orientations and calculated the seismic properties in three orthogonal thin sections. Microstructures indicate that Phase boundary misorientation deformation is localized in the bands with high antigorite fractions, resulting in strong crystallographic preferred Effect of cracks in seismic properties orientations (CPOs) with point maxima of poles to (100) parallel to lineation and poles to (001) to the foliation Subduction normal. Olivine CPO suggests deformation under high temperature and low stress, with a [100] fiber texture. The CPO strength varies with grain size, but is strong even in fine-grained antigorite, and larger grains tend to display higher internal misorientation. Orientation relationships between olivine and antigorite are evident in phase boundary misorientation analysis, (100)ol||(001)atg being more frequent than [001]ol||[010]atg. Two new orientation relationships between olivine and antigorite have been documented. Seismic velocities decrease while anisotropy increases with increasing antigorite modal content. Antigorite grain shape has a weak effect on seismic velocities, but it is important on anisotropy. Comparison between CPO-derived seismic velocities using Voigt, Reuss, Hill averages and geometric mean only showed good agreement in 1/3 of experimental velocities. If the crack porosity of 1.63% measured experimentally at 600 MPa was used in the self-consistent model with two crack orientations with planes normal to Z and Y, good match with all experimental velocities was achieved. The self-consistent model implies important crack porosity in the foliation plane at 600 MPa that reduces Vp normal to the foliation by 0.3 km/s. 1. Introduction et al., 2013; Scambelluri and Philippot, 2001). In particular, antigorite is stable up to pressures of 5 GPa and temperatures around 600 °C In the last 15 years, the minerals of the serpentine group (chrysotile, (Ulmer and Trommsdorff, 1995), it may contain up to 13 wt% of water lizardite and antigorite) have been receiving increasing attention due to in its crystal structure, and its dehydration and subsequent water re- their importance in subduction zone dynamics (e.g. Peacock and lease play an important role in triggering partial melting in the mantle Hyndman, 1999; Hacker et al., 2003a, b; van Keken, 2003; Hyndman wedge (e.g. Ulmer and Trommsdorff, 1995; Green II, 2007). and Peacock, 2003; Faccenda et al., 2009; Hilairet and Reynard, 2009; Currently, there is a debate over the mechanisms responsible for Katayama et al., 2009; Bezacier et al., 2010; Boudier et al., 2010; deformation of antigorite in the mantle wedge (e.g. Escartin et al., Chernak and Hirth, 2010; Plümper et al., 2012; Reynard, 2013; 1997; Hilairet et al., 2007; Chernak and Hirth, 2010; Auzende et al., Amiguet et al., 2014; Auzende et al., 2015; Guillot et al., 2015; Nagaya 2015; Amiguet et al., 2014). Much of this discussion is related to the et al., 2016). Serpentine polymorphs are stable over a wide range of P “brittle-ductile” transition in antigorite, as brittle features such as and T conditions, and are found on multiple levels in subduction zones fractures, kinking, and grain crushing have been observed at high P-T as a direct expression of mantle wedge hydration (e.g. Ulmer and experimental conditions (e.g. Auzende et al., 2015; Amiguet et al., Trommsdorff, 1995; Hacker et al., 2003a, b; Evans, 2004; Schwartz 2014). Nevertheless, the relative mechanical strength of antigorite at ⁎ Corresponding author. E-mail address: [email protected] (L.F.G. Morales). https://doi.org/10.1016/j.tecto.2017.12.009 Received 26 June 2017; Received in revised form 5 December 2017; Accepted 11 December 2017 Available online 10 January 2018 0040-1951/ © 2018 Elsevier B.V. All rights reserved. L.F.G. Morales et al. Tectonophysics 724–725 (2018) 93–115 geological strain rates is much lower than olivine, and for this reason it Table 1 is expected that deformation will localize in antigorite, rather in the Area fractions of olivine and antigorite determined by EBSD in thin sections YZ, XZ and mantle phases (Hirth and Guillot, 2013). This is evident when one looks XY. at the deformation features in serpentinized mantle rocks, where de- ⊥X section YZ ⊥Y section XZ ⊥Z section XY Average formation is mainly accommodated by antigorite (e.g. Hirauchi et al., 2010; Soda and Takagi, 2010; Padrón-Navarta et al., 2012; Soda and Olivine 0.32 Olivine 0.25 Olivine 0.09 Olivine 0.22 Wenk, 2014; Brownlee et al., 2013; Morales et al., 2013; Nagaya et al., Antigorite 0.68 Antigorite 0.75 Antigorite 0.91 Antigorite 0.78 2014). δ Large magnitude shear wave-splitting delay times ( t = 0.2 to 1.4 s) We have performed microstructural analysis and crystallographic with trench parallel Vs1 polarization directions have been observed in preferred orientation (CPO) determinations on three orthogonal thin many subduction zones (e.g. Long and Silver, 2008; Long, 2013). One sections cut parallel and normal to macroscopic fabric elements X, Y generally accepted hypothesis is that these shear wave-splitting ob- and Z. In this reference frame, X is parallel to lineation as defined by servations are related to a strong preferred orientation of antigorite in magnetite elongation and shape preferred orientation of antigorite the mantle wedge and its highly anisotropic single crystal elastic flakes, Y is normal to lineation (X) within the foliation plane, and Z is properties (e.g. Katayama et al., 2009; Bezacier et al., 2010, 2013; normal to foliation, defined by the alignment of platy antigorite grains. Mookherjee and Capitani, 2011; Marquardt et al., 2015). In fact, recent Morales et al. (2013) – Tectonophysics, 594, their Fig. 5) speculated advances in the determination of elastic properties of antigorite, cou- that due to the topotactic olivine-antigorite orientation relationship in pled with antigorite CPO determinations have led to a plethora of pa- two different planes and consequent control of olivine on the antigorite pers about the seismic properties of antigorite bearing rocks (e.g. CPO, the actual lithospheric mantle foliation and lineation in partially Hirauchi et al., 2010; Jung, 2011; Morales et al., 2013; Nagaya et al., serpentinized mantle rocks is at 90° from the antigorite foliation. In this 2014; Watanabe et al., 2014). These data have been complemented case, the mantle foliation is a vertical N-S plane in the pole figures, and ff with ultrasonic measurements in di erent types of serpentine-rich ag- the lineation is a horizontal line, also N-S. Therefore, the mantle gregates via pulse transmission methods (Christensen, 1989, 2004; lineation is parallel to the pole of antigorite foliation in the pole figures, Kern, 1993; Kern et al., 1997, 2015; Ji et al., 2013; Shao et al., 2014). and the mantle foliation is orientated at 90° to the antigorite foliation ff These two methods di er in how the seismic properties are determined. reference frame. The “mantle reference frame” is indicated in red in the For CPO-derived seismic properties (e.g. Mainprice, 1990, 2007), the olivine pole figures. crystal orientations, single crystal elastic constants, the volume of each The CPO measurements of antigorite and olivine were acquired phase and their densities are taken into account in the calculation of the using automatic indexation of EBSD patterns in a scanning electron aggregate elastic tensor. The computation of the wave speeds Vp, Vs1 microscope (e.g. Adams et al., 1993; Prior et al., 1999). The thin sec- and Vs2, with Vp > Vs1 > Vs2, and orthogonal polarization direc- tions were polished via standard methods. Final chemical-mechanical ff tions are calculated using the Christo el equation for each propagation polishing was performed for 2 h with an alkaline solution of colloidal direction. In the ultrasonic method, an elastic wave of known frequency silica on a neoprene-polishing pad. The EBSD measurements were ff is propagated in di erent directions through the volume of rock, and conducted with a FEI Quanta 3D FEG SEM equipped with an EDAX-TSL the travel time is measured by transducers (e.g. Kern et al., 1997; Ji EBSD Digiview camera and the OIM/TSL version 5.31 software (Adams et al., 2013). The experimental data (Vp, Vs1, Vs2 and their polariza- et al., 1993). The measurements were conducted on uncoated thin tions) describe the seismic properties of the rock as a whole, which sections with the SEM operating at low-vacuum conditions (10 Pa of results from the