A new occurrence of okhotskite in the Kurosegawa belt, Kyushu, Japan: the okhotskite + Mn–lawsonite assemblage as a potential high–pressure indicator

Abstract

We present the first report of okhotskite in a lawsonite–blueschist–subfacies metachert of the Hakoishi subunit, Kurosegawa Belt, Kyushu, Japan, which was metamorphosed at peak temperatures and pressures of 200–300 °C and 0.6–0.8 GPa. This okhotskite–bearing assemblage is particularly notable because it formed at higher pressures than that of previously documented okhotskite with available pressure estimations. Textural relationships indicate that okhotskite formed during peak metamorphism in equilibrium with piemontite, Na pyroxene, mag-nesioriebeckite, braunite, and hematite. Okhotskite shows a significant variation in Fe:Mn ratio (Fetot/Mntot = 0.13–0.56) and a following average empirical formula; (Ca7.62Mn2+0.16)Σ7.78(Mn2+2.71Mg1.29)Σ4.00(Mn3+4.13Fe3+2.26Al1.36 V3+0.23Ti0.02)Σ8.00Si11.86O44.02(OH)16.98. Raman spectra of okhotskite are reported for the first time and show characteristic peaks at 362, 480, and 563 cm-1. The stability relationships between okhotskite and other Mn–bearing minerals, such as piemontite, sursassite, spessartine, braunite, and Mn–bearing lawsonite, are examined using a revised Schreinemakers’ analysis. The obtained petrogenetic grid provides tight constraints on the P–T relationship of natural mineral assemblages observed in Mn–bearing cherts within epidote–blueschist–grade and law-sonite–blueschist–grade. Furthermore, this petrogenetic grid predicts that the assemblage of okhotskite and Mn– bearing lawsonite should be stable at higher pressures. The higher–pressure stability suggests that highly oxidized Mn–bearing metacherts can transport water and buffer oxygen in the deeper parts of subduction zones, given that okhotskite and Mn–bearing lawsonite contain high water contents (6.9 and 11.3 wt% H2O, respectively) and trivalent manganese.