6:15 PM - 7:30 PM
[MIS46-P02] Depositional process of Fe-Mn oxide minerals at an active submarine volcano, in the Izu-Bonin Arc
Keywords:ferromanganese crust, low-temperature hydrothermal activity, bayonaise knoll, manganese mineral, vernadite, biomineralization
We carried out an experimental approach to investigate on-site metal-oxide precipitation on artificial substrates. Mn-free (plastic, glass, ceramic) substrates were deployed for 12 years on the Bayonaise Knoll of a possible hydrothermal area in the Izu-Bonin arc, at depths of 918-920 m. The mineralogical and chemical characrerization to the experimental surfaces were documented by field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS).
FE-SEM observations of the surfaces revealed the presence of ball-, doughnut- and rod- shaped structure contain substantial amount of Fe-Mn oxides. These size and shape of the precipitates looks like microorganisms or bacterial mats. The average growth rate of the objects is calculated ave. 0.05-2.79 mm/m.y. The X-ray element mapping of the precipitate showed Mn-Fe enrichment with Si and Ca (without sediments or calcareous planktons) and may be associated with coprecipitation.
This finding was the first evidence of modern active precipitation of initial Fe-Mn oxides from hydrothermal/ normal sea waters in the ocean floors. The Fe-Mn oxides are probably hydrogenetic precipitate of ferruginous vernadite, a major constituent of Fe-Mn crusts. Ferruginous vernadite is the only one major iron-hosting marine authigenic manganese mineral (Usui & Terashima, 1997) and is characterized by low crystallinity caused by randomly-stacked sheets of manganese and iron hydroxides (Ostwarld, 1984). Thus, our data suggests that the precipitation initiates from the formation of Fe-Mn oxide even in hydrothermal areas.