Two-dimensional distribution of trace elements in a reaction texture in plagioclase lherzolite from the Horoman complex, northern Japan, and its implication in ascent processes of the upper mantle

Many reaction textures, which are usually noticed as domains of finer grain size than the main part, are often found in mantle peridotites. They are principally attributable to pressure and temperature changes induced by motion of the mantle and are regarded as a record of a part of large-scale mantle dynamics. Reaction texture can provide us a clock recast from microstructural or chemical variations in the reaction products. Such variations could provide us a useful tool to understand processes that could have taken place contemporaneously with the reaction, such as partial melting, ductile deformation, and introduction of hydrous fluid. In this study, we have examined trace element distribution in plagioclase-rich seam in plagioclase lherzolite from the Horoman peridotite complex in order to constrain timing of plagioclase formation during the ascent/deformation of the Horoman complex. A plagioclase lherzolite sample collected from the Lower Zone of the Horoman complex was examined with EPMA and SIMS. The sample consists of olivine, orthopyroxene, clinopyroxene, plagioclase-rich seams, and rare isolated chromite spinel. The seams, which are composed of olivine, plagioclase, and chromite spinel with minor pyroxenes, were derived from garnet through two-pyroxenes and spinel aggregate by consecutive decompression from the garnet to plagioclase stability fields via the spinel stability field (Ozawa and Takahashi, 1995; Ozawa, 2004). The grain size of seam minerals is much smaller than the size of the host part. The seams are flattened parallel to the foliation and strongly elongated defining remarkable lineation. SIMS analyses of plagioclase in a thin section parallel to the lineation and vertical to the foliation shows spatial variation in trace elements. Particularly remarkable distribution is noticed for Sr increasing from the middle of a seam towards the both ends. The maximum concentration contrast is a factor of two. There is no detectable variation in Sr within each plagioclase grain even if it shows marked Na/Ca zoning. The systematic increase of Sr toward the tips of seam is most plausibly attributed to strong elongation of plagioclase-rich seam during the formation of plagioclase. When plagioclase first appeared by reaction among two-pyroxenes and spinel, Sr was strongly partitioned into plagioclase from clinopyroxene grains occurring near the reaction site by rapid diffusion because of higher temperature. When temperature dropped rapidly (Ozawa, 2004) diffusive transportation was hampered but plagioclase formation proceeded due to continuing decompression. Newly formed plagioclase inherited the low Sr concentration of clinopyroxene originally derived from garnet. Elongation of seam (deformation) took place contemporaneously with formation of plagioclase over the period of initiation of reaction at high temperature to complete consumption of clinopyroxene at low temperature. The Sr distribution can be used as a tool to correlate thermal history and deformation history in the upper mantle materials.