Abstract
Various rock phases, including those in subducting slabs, impact seismic anisotropy in subduction zones. The seismic velocity and anisotropy of rocks are strongly affected by the lattice-preferred orientation (LPO) of minerals; this was measured in retrograded eclogites from Xitieshan, northwest China, to understand the seismic velocity, anisotropy, and seismic reflectance of the upper part of the subducting slab. For omphacite, an S-type LPO was observed in three samples. For amphibole, the <001> axes were aligned subparallel to the lineation, and the (010) poles were aligned subnormal to foliation. The LPOs of amphibole and omphacite were similar in most samples. The misorientation angle between amphibole and neighboring omphacite was small, and a lack of intracrystalline deformation features was observed in the amphibole. This indicates that the LPO of amphibole was formed by the topotactic growth of amphibole during retrogression of eclogites. The P-wave anisotropy of amphibole in retrograded eclogites was large (approximately 3.7–7.3%). The seismic properties of retrograded eclogites and amphibole were similar, indicating that the seismic properties of retrograded eclogites are strongly affected by the amphibole LPO. The contact boundary between serpentinized peridotites and retrograded eclogites showed a high reflection coefficient, indicating that a reflected seismic wave can be easily detected at this boundary.
Highlights
Subduction zones are known to have varying seismic velocity structures and anisotropies caused by subducting slabs [1,2,3,4]
The results of the present study provide a valuable opportunity to understand the seismic velocity, anisotropy, and reflectance of retrograded eclogites and to identify the boundary between the subducting slab and mantle wedge in various subduction zones
The samples had a high proportion of amphibole and plagioclase, and half of the clinopyroxene grains exhibited a symplectite structure of omphacite, diopside, and plagioclase, indicating that our samples were retrograded
Summary
Subduction zones are known to have varying seismic velocity structures and anisotropies caused by subducting slabs [1,2,3,4]. Eclogites that are formed at the deep part of the subducting slab are known to have a faster seismic velocity—similar to that of the upper mantle (VP ~ 8.0 km/s)—than the surrounding rocks, and they have a strong seismic anisotropy, affecting various seismic velocities and anisotropies in subduction zones [30,31]. Amphibole exists in subducting slabs under lower-pressure and lower-temperature conditions than eclogite stability conditions, and in the middle to lower crust [33], whereas the LPO development of amphibole is known to produce a strong seismic anisotropy [18,34,35,36]. Deformed retrograded eclogites may represent seismic velocity and anisotropy in relatively shallow subducting slabs [5,6,20,31,37,38,39,40,41,42,43,44,45,46,47,48,49]
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