Antigorite crystallographic preferred orientations in serpentinites from Japan

Abstract

Foliated antigorite serpentinites are contributing to seismic anisotropy in subduction zones. However, uncertainty remains regarding the development of antigorite crystal preferred orientations (CPO). This study analyzes the CPOs of antigorite and olivine in variably serpentinized samples of antigorite serpentinite from three localities in Southwest Japan, using U-stage, EBSD, and synchrotron X-rays. In all samples (010) poles show a maximum parallel to the lineation, and (001) poles concentrate normal to the foliation with a partial girdle about the lineation. (100) poles are less distinct and scatter widely. The three measurement methods give similar results, though orientation distributions from X-ray analysis which averages over larger sample volumes are more regular and weaker than U-stage and EBSD which focuses on local well-crystallized grains. In olivine-bearing serpentinite, the olivine CPO does not appear to control the antigorite CPO, though there are some topotactic relationships. The antigorite CPO formed contemporaneously with serpentinization and shear deformation. Based on our analyses, in a subduction zone mantle wedge, the (010) pole of antigorite is parallel to the direction of the subduction, and (001) lattice planes are parallel to the interface of the subducting oceanic plate. By averaging single crystal elastic properties over orientation distributions wave velocities have been calculated. Fast P velocities are parallel to the subduction direction and slow velocities are perpendicular to the subducting plate with an anisotropy of 10-15%.

In Situ Quantitative Tensile Testing of Antigorite in a Transmission Electron Microscope

“…Antigorite serpentinites from different geological settings have been characterized by this technique. Some have been interpreted as resulting from dislocation creep involving various slip systems (Auzende et al, 2015;Bezacier et al, 2010;Brownlee et al, 2013;Katayama, 2009;Padrón-Navarta et al, 2012;Soda & Wenk, 2014), although the relationship with well-established intracrystalline deformation mechanisms is still poorly understood (Auzende et al, 2006(Auzende et al, , 2015. Other interpretations can be considered, however, like inherited textures from topotactic transformation (Boudier et al, 2010;Morales et al, 2013;Plümper et al, 2012).…”

Lattice-preferred orientation of olivine found in diamond-bearing garnet peridotites in Finsch, South Africa and implications for seismic anisotropy

“…Trench-parallel seismic anisotropy has been observed in the mantle wedge above subducting slabs (Long, 2013;Long and Silver, 2008;Smith et al, 2001), as well as below subducting slabs at a deeper portion of the upper mantle (Long andSilver, 2008, 2009;Russo and Silver, 1994;Tian and Zhao, 2012). Proposed mechanisms for the source of this trench-parallel seismic anisotropy include waterinduced B-type LPO of olivine in the mantle wedge (Jung, 2009;Jung and Karato, 2001a;Karato et al, 2008;Katayama and Karato, 2006;Kneller et al, 2008;Mizukami et al, 2004), LPO of serpentine in serpentinite altered from peridotite (Ji et al, 2013;Jung, 2011;Katayama et al, 2009;Soda and Wenk, 2014;Watanabe et al, 2011), trench-parallel mantle flow due to slab roll back (Long andSilver, 2008, 2009;Russo and Silver, 1994), rapid toroidal flow around slab edge (Jadamec and Billen, 2010), pressure-induced B-type LPO of olivine due to slip transition at high pressure greater than P ¼ 3 GPa Ohuchi et al, 2011;Raterron et al, 2011), aligned faults by hydration in subducting oceanic plate (Faccenda et al, 2008) and an effective orthorhombic symmetry for the oceanic asthenosphere, which is translated to the depth beneath the subducting slab (Song and Kawakatsu, 2012). However, the origin of seismic anisotropy remains poorly understood.…”

Seismic anisotropy of serpentinite from Val Malenco, Italy

“…for Happo, Watanabe et al ., ; and 6–20 m.r.d. for Saganoseki, Soda and Wenk , ]. While these samples are all strongly textured, many serpentinites, including in the Alpine nappes, even in Val Malenco, and the California Coast Ranges and Sierra Nevada foothills, are largely isotropic.…”

“…Serpentinites are formed at the top of subducting plates when fluids are liberated and transform olivine‐bearing rocks [e.g., Bostock et al ., ; Boudier et al ., ; Tibi et al ., ]. During subduction, serpentinites may deform and develop crystal preferred orientation as has been documented for samples from the Alps [e.g., Jung , , ], Southern Spain [ Padrón‐Navarta et al ., ], Cuba [ Van de Moortèle et al ., ], Japan [e.g., Nishii et al ., ; Soda and Takagi , ; Soda and Wenk , ], and China [ Shao et al ., ]. Because of the high elastic anisotropy of antigorite crystals [e.g., Bezacier et al ., ], representing the high‐temperature type of the serpentine minerals, crystal alignment produces significant anisotropy of serpentinite slabs and may be responsible for the observed trench‐parallel seismic anisotropy in subduction zones [e.g., Long and Van der Hilst , ; Smith et al ., ].…”