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Fe同位素的轮廓，并根据这些Fe同位素阐明岩石圈/水圈和生物圈之间的相互作用。我们取得了如下科学成果。（1）测定了群马铁矿存款中沉积铁矿石的常量、微量和稀土元素含量。通过生物诱导矿化作用形成的铁矿石（BIM矿石）优先吸收P、Sc、Cu、Mo、Ba、Th和轻稀土元素。这些元素的独特富集是古沉积铁矿床生物矿化的一个有希望的指标（Kato等人，2002年，《资源地质学》）。用等离子体质谱结合激光烧蚀取样技术测量了这些铁沉积物和陨铁的~（57）Fe/~（54）Fe、~（56）Fe/~（54）Fe和~（57）Fe/~（56）Fe比值。BIM矿石中57 Fe/54 Fe分馏明显，约为2. 77 ‰，56 Fe/54 Fe约为2. 55 ‰，57 Fe/56 Fe约为1. 33%，具有不同程度同位素分馏的质量分馏特征。(2)在佐世保（长崎）附近的铁矿石中保存着大量的藻类化石，其中也有大量的铁同位素分馏。(3)3.46 Ga的含铁燧石被解释为源自大洋中脊的高温热液的原位沉淀物（Kato和中村，2003年，前寒武纪研究）。(4)采用激光烧蚀-多收集器-电感耦合等离子体质谱（LA-MS-ICP-MS）对固体样品进行了原位铁同位素比值测量。使用通过激光进样技术实现的干燥等离子体条件，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).