[S-EM16] 地磁気・古地磁気・岩石磁気・環境磁気

[SEM16-P12] 北部沖縄トラフ冷湧水サイトに分布する強い磁化をもつ炭酸岩

*川村 紀子¹、青木 智² (1.海上保安大学校　基礎教育講座、2.海上保安庁　海洋情報部)

キーワード：炭酸塩岩、生物起源磁鉄鉱、生物起源グリグ鉱

The Okinawa Trough is located on the western Pacific Ocean and is an active back-arc basin. The sediment and carbonate crust samples were collected from cold seep in the northern Okinawa Trough. Rock magnetic parameters of the samples; low-field magnetic susceptibility, anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) were measured. IRM component analysis and thermomagnetometry were also conducted. Thermomagnetometry results suggest that magnetic carriers of the sediments are magnetite (Fe₃O₄), greigite (Fe₃S₄) and/or pyrrhotite (Fe₇S₈). Results of X-ray diffraction spectrums indicate that calcite is a main mineral in carbonate crust samples. Although calcite is a diamagnetic mineral, magnetic susceptibility of all carbonate crusts indicates positive values. The thin sections of crust samples were observed using a polarizing microscope, the diffusions of terrestrial materials and pyrite were recognized. The presence of a clear Verwey transition and low coercivity components were recognized in carbonate crusts. It implies that a magnetic carrier of carbonate crusts is biogenic magnetite and/or greigite in terrestrial materials.