Elucidation of interaction between lithosphere/hydrosphere and biosphere based on iron isotopic compositions

本研究的目的是建立现代和古代沉积物和岩石的铁同位素轮廓，并在此基础上阐明岩石圈/水圈与生物圈之间的相互作用。我们获得了以下科学成果。(1)测定了群马铁矿沉积铁矿的主要元素、微量元素和稀土元素含量。通过生物诱导矿化形成的铁矿石（BIM矿石）优先吸收P、Sc、Cu、Mo、Ba、Th和轻稀土。这些元素的独特富集是古沉积铁矿生物矿化的一个有希望的指标（Kato et al.,2002, Resource Geology）。利用等离子体质谱联用激光烧蚀取样技术测量了这些铁矿和陨石铁的57Fe/54Fe、56Fe/54Fe和57Fe/56Fe比值。BIM矿石分选明显，57Fe/54Fe分选率为2 ~ 7‰，56Fe/54Fe分选率为2 ~ 5‰，57Fe/56Fe分选率为1 ~ 3%，具有质量依赖分选特征，同位素分选程度不同。(2)在长崎佐世保附近保存了大量藻类化石的铁矿中也存在较大的铁同位素分异。(3) 3.46 Ga含铁雪壳被解释为源自洋中脊的高t热液的原位沉淀（Kato和Nakamura， 2003，前寒武纪研究）。(4)采用激光烧蚀-多重收集器-电感耦合等离子体质谱法（LA-MS-ICP-MS）对固体样品进行了原位铁同位素比测量。利用激光样品导入技术实现的干等离子体条件，可以成功地减少质谱干扰（Hirata和Ohno， 2001， JAAS）。

The aim of our study was to establish the outline of Fe isotopic for modern and ancient sediments and rocks and to elucidate interactions between lithosphere/hydrosphere and biosphere based on these Fe isotopes. We have gained scientific results as follows.(1)Major, trace and rare earth element contents were determined for sedimentary iron ores from the Gunma iron deposit. Preferential uptake of P,Sc,Cu, Mo, Ba, Th, and light REEs occurs in the iron ores (BIM ores) formed through biologically induced mineralization. Unique enrichment of these elements is a promising indicator of biomineralization for 〓〓cient sedimentary iron deposits (Kato et al.,2002, Resource Geology). The 57Fe/54Fe, 56Fe/54Fe and 57Fe/56Fe ratios for these iron deposits and meteoritic irons have been measured using a plasma mass spectrometry coupled with laser ablation sampling technique. Those of BIM ore are fractionated significantly, about 2〜7‰ for 57Fe/54Fe, 2〜5‰ for 56Fe/54Fe and 1〜3% for 57Fe/56Fe, indicating the mass-dependent fractionation feature with different degree of isotopic fractionation.(2) There is also a large Fe isotope fractionation in the iron ores preserving numerous algal fossils, near Sasebo (Nagasaki).(3) 3.46 Ga ferruginous cherls are interpreted to have been an in situ precipitate derived from a high-T hydrothermal solution emanating from a mid-oceanic ridge (Kato and Nakamura, 2003, Precambrian Research).(4) In-situ Fe isotopic ratio measurements for solid samples have been carried out using laser ablation-multiple collector-inductively couples plasma mass spectrometry (LA-MS-ICP-MS). Using a dry plasma condition achieved by laser sample introduction technique, mass spectrometric interferences could be successfully reduced (Hirata and Ohno, 2001, JAAS).