EBSD-measured lattice-preferred orientations and seismic properties of eclogites

Abstract

measured lattice-preferred orientations and seismic properties of eclogites. Tectonophysics, Elsevier, 2001, 342, pp.61 -80. <10.1016/S0040-1951 AbstractWe investigated the deformation mechanisms and the seismic properties of 10 eclogite samples from different localities (Alps, Norway, Mali and eastern China) through the analysis of their microstructures and lattice-preferred orientations (LPO). These samples are representative of various types and intensity of deformation under eclogitic metamorphic conditions. Omphacite and garnet LPO were determined from electron backscatter diffraction (EBSD) technique. Garnet appears to be almost randomly oriented whereas omphacite develops strong LPO, characterized by the [001]-axes concentrated sub-parallel to the lineation, and the (010)-poles concentrated sub-perpendicular to the foliation. In order to analyze the deformation mechanisms that produced such omphacite LPO, we compare our observations to LPO simulated by viscoplastic selfconsistent numerical models. A good fit to the measured LPO is obtained for models in which the dominant slip systems are 1/2h110i{110}, [001] {110} and [001] (100). Dominant activation of these slip systems is in agreement with TEM studies of naturally deformed omphacite. Seismic properties of eclogite are calculated by combining the measured LPO and the single crystal elastic constants of omphacite and garnet. Although eclogite seismic anisotropies are very weak (less than 3% for both P-and S-wave), they are generally characterized by a maximum P-wave velocity sub-parallel to the lineation and by a minimum velocity approximately normal to foliation. The mean P-and S-wave velocities are high (respectively, 8.6 and 4.9 km/s). The S-wave anisotropy pattern displays complex relationships with the structural frame but the fast polarization plane generally tends to be parallel to the foliation. Calculated reflection coefficients show that an eclogite/crust interface is generally a good reflector (Rc > 0.1), whereas an eclogite body embedded in the upper mantle would be hardly detectable. D

Mantle hydration and Cl-rich fluids in the subduction forearc

“…Petrophysical models and acoustic measurements along different directions give access to anisotropy. Seismic velocities obtained from petrophysical modeling (Bascou, et al 2001;Bezacier, et al 2010a;Bezacier, et al 2010b;Cao, et al 2013) are in good agreement with measurements on rock aggregates (Ji, et al 2013;Shao, et al 2014). They provide a basis for estimating the proportion of water stored in minerals.…”

Crystallographic preferred orientations of exhumed subduction channel rocks from the Eclogite Zone of the Tauern Window (Eastern Alps, Austria), and implications on rock elastic anisotropies at great depths

“…Barruol and Kern, 1996;Pros et al, 2003;Ullemeyer et al 2010;Llana-Fúnez and Brown, 2012). Elastic data for subduction channel rocks were provided by Mauler et al (2000), Bascou et al (2001), and Worthington et al (2013), who investigated mineral CPOs of eclogites using EBSD (electron backscatter diffraction) and bulk rock elastic anisotropies calculated from the CPO data. However, these studies neither include metasedimentary rocks in the subduction channel nor retrogressed eclogites, which yield information on the processes and changes in state during exhumation.…”

The effect of cation order on the elasticity of omphacite from atomistic calculations

“…The proportion of clinopyroxene in eclogite, the 31 principal component of subducted oceanic crust, is even higher, reaching up to ~65% by volume, 32 although this proportion drops rapidly above 10 GPa, where clinopyroxene dissolves into garnet, 33 the other major component of eclogite (Irifune et al 1986;Irifune and Ringwood 1987;Irifune and 34 Ringwood 1993). In natural eclogites, such as those that occur in the mantle and subducted oceanic 35 crust, garnet exhibits weak crystallographic preferred orientation (CPO) and is essentially elastically 36 isotropic (Bascou et al 2001;Mainprice et al 2004). In contrast, omphacite has pronounced shear-37 induced CPO, with the [001] axis parallel to the direction of greatest elongation (Mauler et al 2001;38 Bascou et al 2002).…”

“…In contrast, omphacite has pronounced shear-37 induced CPO, with the [001] axis parallel to the direction of greatest elongation (Mauler et al 2001;38 Bascou et al 2002). Consequently, seismic anisotropy in eclogitic rocks is dominated by the 39 behaviour of omphacite (Mauler et al 2000;Bascou et al 2001;Zhang and Green 2007). It has 40 balance considerations.…”