Manganese contents in volcanic pyroxenes in island arcs: case study from the South Yatsugatake Volcanic area, Japan

摘要

Examination by electron microprobe analysis (EMPA) of the volcanic pyroxenes from the Japanese island arc reveals that there are high MnO contents (up to 0.71 wt%) in the pyroxenes from andesites and andesitic tuffs of South Yatsugatake Volcanic area (abbr. SYVA) in central Japan. This is found to be common to the pyroxene phenocrysts from the Japanese volcanic rocks, compared with the great majority of volcanic pyroxenes of worldwide occurrences. These pyroxenes in our compilation were divided into high-Ca and low-Ca types to discuss the substitution mechanism of Mn in pyroxene structure. Detailed examination into crystal chemistry of the Mn-bearing volcanic pyroxenes suggests that negative correlations exist between the Ca2+ and Mn2+ content in the high-Ca pyroxenes and between the Mn2+ and Mg2+ contents in the low-Ca pyroxenes. End- member analysis of the former pyroxenes suggests the substitution of Mn2+ MgSi2O6 (kanoite) for CaMgSi2O6 (diopside) and that of the latter indicates the one of Ca2+ MnSi2O6 (Johannsenite) for CaMgSi2O6 (diopside). The Mn enrichment characteristic of the pyroxene phenocrysts from Japanese quaternary volcanic rocks is very similar to that of the allanites in granitic rocks from the Japanese island arc (hoshino et al. 2007, 2008). The Mn-rich pyroxene phenocrysts are an example of "island arc mineralogy", because the Mn enrichment in these two minerals may be derived from the assimilation of Mn-rich crust materials accreted from the subducting plate under the Japanese island arc.