Chromian pumpellyite and associated chromian minerals from Sangun metamorphic rocks, Osayama, southwest Japan

抄録

Chromian pumpellyite and associated chromian minerals occur in basic schists in the Osayama ultramafic body in Okayama Prefecture, southwest Japan. The minerals were investigated to examine their chromium contents and mode of occurrence. The chromian minerals occur in basic schist blocks within the ultramafic body, but have not been found in basic schists adjacent to the ultramafic body. Three mineral assemblages were recognized in the basic schist blocks from the ultramafic body: (1) tremolite + chromian pumpellyite + chromian phengite + chromian chlorite + relic chromite; (2) amphibole + chromian chlorite + chromian omphacite + chromian pumpellyite + titanite; (3) diopside + chromian chlorite + chromian vesuvianite + chromian garnet + stilpnomelane + titanite + ilmenite ± relic chromite. Chromian pumpellyite, chromian phengite, chromian chlorite and relic chromite in assemblage (1) form domains in a matrix consisting of fine prismatic tremolite crystals. In assemblage (2), chromian pumpellyite occurs within chromian chlorite domains, and chromian omphacite is distributed along their margins. In assemblage (3), chromian vesuvianite, chromian garnet and stilpnomelane form veinlets along fine fractures within chromian chlorite domains.
     Chromian pumpellyite is reddish gray and twinned. The Cr distribution is extremely heterogeneous, and Cr₂O₃ content is highest around chromite, reaching 13.3 wt%. However, Fe distribution is relatively uniform, and is not related to the distribution of chromite. On the basis of the distribution coefficient of Cr between the X and Y sites determined by Nagashima and Akasaka (2007), the chemical formula of the chromian pumpellyite with greatest Cr₂O₃ content is calculated to be (K_0.02Na_0.19Ca_7.86)_Σ8.07(Mn²⁺_0.05Fe²⁺_0.35Ni_0.01Mg_1.71Cr_1.13Al_0.76)_Σ4.01(Cr_2.34V_0.01Al_5.65)_Σ8.00(Al_0.16Si_11.84)_Σ12.00O_41.66(OH)_14.34.
     The chromian mineral domains often contain relic chromite, and regions rich in Cr are confirmed to the vicinity of these chromite grains. These features suggest that chromite is the source of the Cr in the chromian metamorphic minerals, and that metamorphic fluid caused dissolution of chromite and diffusion of Cr into the surrounding metamorphic minerals. The bulk compositions of the basic schist blocks in the ultramafic body suggest their protoliths were ultrabasic rocks of picrobasaltic composition.