Formation and deformation mechanisms of pyroxene-spinel symplectite in an ascending mantle, the Horoman peridotite complex, Japan: An EBSD (electron backscatter diffraction) study

Abstract

Symplectites, fine vermicular intergrowth (< 10 μm) of orthopyroxene, clinopyroxene, and spinel, and the sur- rounding lenticular coarser-grained (100-200 μm) aggregate (seam) with the same mineral assemblage of the symplectites define remarkable foliation and lineation in spinel lherzolites of the Horoman complex, northern Japan. They are inferred to be products of reaction between garnet and olivine during decompression of the host peridotite accompanying deformation. Microstructure of a symplectite was investigated with automated electron backscattered diffraction (EBSD) analysis using a field -emission gun SEM (FE-SEM) in order to clarify reaction and deformation mechanisms and thereby better constraining mechanical interaction between the Earth's upper mantle and lower crust. The symplectite is composed of two segments with a large misorien- tation angle of ~ 60° only for spinel, and the two spinel crystals are in mirror symmetry with the segment boundary approximately parallel to the mirror plane. The segment boundary is interpreted as spinel law twin formed during phase transition from garnet. Each segment is further subdivided into several sectors with grad- ual lattice distortion smaller than a few degrees/mm and intra-sector misorientation mostly smaller than 25° for all constituent minerals and with misorientation axes nearly perpendicular to the lineation and parallel to the foliation. The sector boundaries are inferred to be subgrain boundaries formed by dislocation creep of py- roxenes and spinel in the spinel stability field. The spinel twin suggests that a garnet was decomposed directly into entangled aggregate of pyroxenes and spinel, which grew from an embryo nucleated on the surface of the reactant garnet. The symplectite minerals in each sector show systematic crystallographic orientations (topo- taxy) with each other. The topotaxial relationship in the fine intergrowth with subgrain structure demonstrates that the systematic crystallographic orientations were acquired when the crystals grew by decomposing the garnet and were later modified by deformation during the consecutive ascent of the complex.