11:00 〜 13:00
[BGM02-P03] 秋田県北鹿地域における12 Maの海底熱水起源鉄酸化物の地質学的、鉱物学的研究
キーワード：ペペライト、ヘマタイト、ノントロナイト、コロフォーム状組織
Archean and Proterozoic banded iron formations (BIFs) were formed by using submarine hydrothermal Fe and Si. On the other hand, there exist uncertainties as to how large amounts of Fe and Si were extracted from seafloor volcanic rocks, and if microorganisms were involved in Fe and Si precipitations. Limited preservation of ancient BIFs made difficult to approach the above problems. Fe-rich jaspers occur in Phanerozoic ages. Those are considered as analogues of BIFs.
Fe-rich jaspers often accompanied with black shales and footwalls of submarine hydrothermal alteration. Therefore, Phanerozoic Fe-rich jaspers provide opportunities to illustrate entire scheme of Fe mobilization from footwall to seafloor and role of microbial activities in Fe precipitations.
Fe-rich jaspers, often called tetsu-sekiei, occur in the Hokuroku area, Akita, Japan. Such Fe-rich jaspers in the Hokuroku area occur before, on the top or after the Kuroko deposits, which were formed at 15 Ma. In the present study, filed surveys were performed at Koyukizawa, Hitorizawa, Koshigenaizawa, and Nittobezawa in Odate city. 12 Ma Fe-rich jaspers and associated rocks, including footwall rocks, occur at each locality. The purpose of this study is to investigate (1) the migration of Fe in the footwall rock during submarine hydrothermal alterations, (2) the precipitation process of Fe on the seafloor and in the footwall rocks, and (3) the relationship between the microstructure and organic matter in the Fe oxides.
Geological surveys revealed the following time sequences: initial submarine rhyolitic activities, formation of peperite, submarine hydrothermal alteration and Fe mobilization, and Fe oxide formation in subseafloor and on the seafloor. Collected samples were examined by polarizing microscope, SEM-EDS, H-XRF, and XRD.
It is found that the peperite, thus the mixture of rhyolite lavas and mudstones, were enriched in Si and K in the peperite matrix. Ilmenite in rhyolite and mudstone parts lost Fe, allowing Fe2+ migration. K in the peperite matrix in rhyolite in particular, enrichment sites of K were further altered and Fe-rich clays (nontronite) were formed. Such Fe-rich clays formed veinlets with organic matter. Those observation suggests that organic matter were reductant of Fe in rhyolites and mudstones, and reduced Fe migrated in peperite zones. A part of migrating Fe formed Fe-rich clays in footwall. Finally, Fe-rich hydrothermal fluids discharged and formed Fe-rich jasper on the seafloor.
Fe oxide-rich rocks appear in subsurface altered peperites. Those peperites contain hematite-quartz colloform, and various forms of hematite. The colloform structure appears in hydrothermal veins, and some of them brecciated by later hydrothermal processes. Such observation suggests that colloforms with other hematite were formed at subsurface regions. Fe and Si, or often clays, were not segregated at the core of colloforms. But Fe and Si formed separated microscopic layers at the middle or edge of colloforms. Such segregation of Fe and Si may represent redox conditions of injected hydrothermal fluids at each layer-forming stage. Organic matter were often found in quartz layers of colloforms, and some layers were formed only by organic matter. These organic matters were transported with Fe and Si by submarine hydrothermal fluids. Those organic matter were most likely transported organic matter from mudstones, and not from in site microbial life on hematite. This imply hematite in colloforms were inorganic in origin rather than the products of microbial oxidation.
Fe-rich jaspers often accompanied with black shales and footwalls of submarine hydrothermal alteration. Therefore, Phanerozoic Fe-rich jaspers provide opportunities to illustrate entire scheme of Fe mobilization from footwall to seafloor and role of microbial activities in Fe precipitations.
Fe-rich jaspers, often called tetsu-sekiei, occur in the Hokuroku area, Akita, Japan. Such Fe-rich jaspers in the Hokuroku area occur before, on the top or after the Kuroko deposits, which were formed at 15 Ma. In the present study, filed surveys were performed at Koyukizawa, Hitorizawa, Koshigenaizawa, and Nittobezawa in Odate city. 12 Ma Fe-rich jaspers and associated rocks, including footwall rocks, occur at each locality. The purpose of this study is to investigate (1) the migration of Fe in the footwall rock during submarine hydrothermal alterations, (2) the precipitation process of Fe on the seafloor and in the footwall rocks, and (3) the relationship between the microstructure and organic matter in the Fe oxides.
Geological surveys revealed the following time sequences: initial submarine rhyolitic activities, formation of peperite, submarine hydrothermal alteration and Fe mobilization, and Fe oxide formation in subseafloor and on the seafloor. Collected samples were examined by polarizing microscope, SEM-EDS, H-XRF, and XRD.
It is found that the peperite, thus the mixture of rhyolite lavas and mudstones, were enriched in Si and K in the peperite matrix. Ilmenite in rhyolite and mudstone parts lost Fe, allowing Fe2+ migration. K in the peperite matrix in rhyolite in particular, enrichment sites of K were further altered and Fe-rich clays (nontronite) were formed. Such Fe-rich clays formed veinlets with organic matter. Those observation suggests that organic matter were reductant of Fe in rhyolites and mudstones, and reduced Fe migrated in peperite zones. A part of migrating Fe formed Fe-rich clays in footwall. Finally, Fe-rich hydrothermal fluids discharged and formed Fe-rich jasper on the seafloor.
Fe oxide-rich rocks appear in subsurface altered peperites. Those peperites contain hematite-quartz colloform, and various forms of hematite. The colloform structure appears in hydrothermal veins, and some of them brecciated by later hydrothermal processes. Such observation suggests that colloforms with other hematite were formed at subsurface regions. Fe and Si, or often clays, were not segregated at the core of colloforms. But Fe and Si formed separated microscopic layers at the middle or edge of colloforms. Such segregation of Fe and Si may represent redox conditions of injected hydrothermal fluids at each layer-forming stage. Organic matter were often found in quartz layers of colloforms, and some layers were formed only by organic matter. These organic matters were transported with Fe and Si by submarine hydrothermal fluids. Those organic matter were most likely transported organic matter from mudstones, and not from in site microbial life on hematite. This imply hematite in colloforms were inorganic in origin rather than the products of microbial oxidation.