Native copper included by anorthite from the island of Miyakejima : implications for arc magmatism

Dark-brown micro-minerals scattering in the wine red part of lava-coated anorthite megacryst, from the island of Miyakejima, were identified as native copper by using the conventional EPMA equipment and the enhanced X-ray microdiffractometer of a new type developed by the authors. Tiny lamellae of these native coppers are distributed on ( ?? 04) of anorthite. Their chemical compositions are near pure copper metal, whereas the anorthite around their coppers is slightly rich in silica and contains a small amount of chlorine. Comparison with the X-ray CuLα and CuLβ spectra, measured by EPMA, of native and metallic coppers, and cuprite and tenorite, brings out that the peak shift and band shape of CuLα radiation from this Cu inclusion agree well with those of native and metallic coppers. The present microscopical investigation implies that after exsolution of the H3CuSi3O8 feldspar endmember possibly forming a solid solution of the primary anorthite, decomposition of this Cu-bearing feldspar in the process of descent temperature conduces to yielding native coppers and the anorthite slightly rich in SiO2. The present paragenesis of euhedral anorthite megacrysts including native coppers with fused olivines asserts that there is some high temperature region (>1400°C) underneath Miyakejima, namely beneath a volcanic front and over the subduction zone, where such an idiosyncratic anorthite can crystallize.