Ilvaite–manganilvaite series minerals in jasper and iron–manganese ore from the Northern Chichibu belt, central Shikoku, Japan

抄録

Ilvaite, a hydrous calcium mixed–valent iron silicate, is found in red hematitic chert (jasper) lenses in altered basalt and an iron–manganese ore from the Sumaizuku unit (Middle Jurassic accretionary complex) of the Northern Chichibu belt, SW Japan. The jasper lenses are composed of ilvaite, andradite, stilpnomelane, riebeckite, hematite and quartz. Ilvaite in the jasper lenses shows an unusual spherical morphology (~ 100 µm in diameter) and each ilvaite spherule exhibits a foam–like microstructure with recrystallized quartz grains. Ilvaite also occurs as discrete euhedral crystals (up to 500 µm in length) in quartz segregation veins or monomineralic ilvaite veins. Ilvaite in the jasper lenses is close to the endmember composition, but the material in the iron–manganese ore is rich in ^[6]Mn²⁺ (up to 0.84 atoms per formula unit) corresponding to manganilvaite. Thermodynamic modeling shows that the ilvaite–andradite–hematite–quartz association (ilvaite + O₂ = hematite + andradite + quartz) is stable under the peak P–T conditions of the Sumaizuku unit (~ 0.35 GPa, 230–250 °C) and oxygen fugacity slightly above the hematite–magnetite buffer (log f_O2 = ~ –38). Ilvaite spherules are intimately associated with colloform–textured andradite, suggesting that they probably originated from a Ca–Fe–Si colloidal precursor deposited along hydrothermal fluid conduits within the Early Permian oceanic crust, and recrystallized during subduction–related low–grade metamorphism.