Presentation Information
[S2-02]Nanoscale mineralogy of Devonian layered ironstone in the Ultra-Tamba Belt: Significance of unusual mineral assemblages formed during diagenesis
*Nana Suzuki1, Tatsuki Tsujimori1, Ryo Fukushima2, Sayako Inoue3, Tatsuhiko Kawamoto4, Yoshiaki Sugamori5, Shizuo Takemura6 (1. Tohoku Univ. Sci., 2. JAMSTEC Kochi, 3. Ehime Univ. GRC, 4. Shizuoka Univ. Sci., 5. Tottori Univ. Sci., 6. Hyogo Univ. Teacher Edu. Sci.)
Keywords:
ironstone,very low-grade metamorphism,nanoscale mineralogy,Fe-Mn olivine,Famennian (Late Devonian)
The behavior of iron in seawater reflects the redox evolution of the atmosphere–ocean system, making marine ironstones valuable geochemical archives. However, their primary mineralogical signatures are commonly modified by diagenesis and low-grade metamorphism. We investigated Devonian layered ironstone from the Kozuki Unit of the Ultra-Tamba Belt, SW Japan, which experienced only zeolite-facies metamorphism, to constrain the early mineralogical evolution of iron-rich marine sediments. The studied ironstone consists of millimeter-scale graded beds alternating between magnetite-rich and silicate-rich layers. The latter preserve well-defined radiolarian fossils. Associated basalt contains albite, chlorite, calcite, and anatase, but lacks prehnite and pumpellyite, confirming the very low metamorphic grade. FE-SEM, Raman spectroscopy, FIB sampling, and FE-TEM observations reveal that the apparently homogeneous basal parts of magnetite-rich layers consist predominantly of magnetite grains as small as ~200 nm, accompanied by nanoscale apatite. Silicate-rich layers contain submicrometer Ca–Mn clinopyroxene, caryopilite, and quartz, together with clot-like aggregates rich in Fe–Mn olivine. At the nanoscale, these aggregates are polyphase and polycrystalline, comprising at least six mineral phases within a single FIB foil. Olivine occurs in association with ilvaite, which itself forms nanoscale polyphase and polycrystalline aggregates.. These nanoscale mineral assemblages are unlikely to have formed as colloidal particles in seawater and been deposited directly on the seafloor. Instead, they probably formed through localized reactions during diagenesis or very low-grade metamorphism. Decomposition of precursor materials may have produced transient supersaturation and high nucleation densities, generating the observed nanopolycrystalline aggregates. Their disequilibrium textures provide a record of spatially localized low-temperature reactions. Such observations are important not only for reconstructing past ocean conditions, but also for understanding abiotic formation of polyphase nanocrystalline materials under low-temperature, nonequilibrium conditions.