Case study of zircon from a pegmatite in the Tanakami Granite pluton, central Japan: Occurrence, morphology, texture and chemical composition.

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Zircon grains were collected from the graphic and large-crystal zones of a pegmatite dyke in the medium-grained porphyritic biotite granite within the Tanakami Granite pluton. Type I (relatively large tabular zircons of 0.05-1 mm in width and 3 mm-1 cm in length with considerable shape variation) and Type II (finer string zircons of 0.01-0.05 mm in width and 1-3 mm in length) zircons coexist in the graphic zone and large-crystal zone. BSE images and element maps show the morphological, textural and chemical differences between the Type I and Type II zircons. Many Type I zircons consist of clear interiors with zoning textures and spongy rims with mottled textures having many micropores, although some Type I zircons are clear almost over a grain. Type II zircon strings are generally spongy and show mottled textures. These textures are mainly controlled by the distribution patterns of Zr, Hf, (U+Th), REE and P. The totals of chemical compositions (wt. %) of Type I clear zircon are around 100 wt. % with stoichiometry, but those of Type I and Type II spongy zircon, are often below 95 wt. % approximately with stoichiometry. Especially, the totals of chemical compositions of Type II zircons are often below 90 wt. %. These textural and compositional data suggest that Type I clear zircons preserve primary compositions in spite of metamictization, and that spongy zircons of Type I and Type II underwent secondary hydrothermal reactions. It is compositionally noted that, differently from the Type II zircons, the Type I zircons are richer in UO2 among the Japanese pegmatite zircons.

Case study of zircon from a pegmatite in the Tanakami Granite pluton, central Japan: Occurrence, morphology, texture and chemical composition.

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