Mineralogical and Geochemical Study of Soils Developed on Osuzawa Limestone Cave in Saitama Prefecture

本研究では埼玉県の鍾乳洞石灰岩上に発達する土壌を採取し、鉱物組成および元素・Sr同位体組成分析を行い、土壌形成過程および土壌鉱物の起源について検討を行った。X線回折の結果から鍾乳洞上に発達する土壌層からは石英、長石、粘土鉱物が同定された。石灰岩試料で検出された方解石は土壌中では見られなかった。また土壌のSr同位体比は石灰岩のそれとはわずかながら違う値であった。これらの結果から、土壌下部の石灰岩とは起源が異なる母材から土壌が形成されたことが示唆される。深度50cm以浅の土壌上層のSr同位体比は石灰岩よりわずかに低く、低いSr同位体比を示す火山噴出物の寄与が考えられる。深度50cm以深の土壌下層は石灰岩に含まれない粘土鉱物や長石に由来するAl, Ti, Fe, K, Baが濃集しており、そのSr同位体比は土壌上層や石灰岩よりもわずかに高い。これらの結果から、風化および土壌生成過程において大気起源物質,例えば降水や大気降下物、の寄与が考えられる。This study was undertaken to determine mineralogical, elemental, and Sr isotopic compositions of soil collected in Saitama prefecture, eastern Japan and to evaluate the provenance and formation of soils developed on a limestone cave. X-ray diffraction analysis showed that soil samples developed on the limestone cave were composed of quartz, feldspar and clay minerals. The soils contained no calcite minerals, although the underlying limestone was composed mainly of calcite. Moreover their 87Sr/86Sr ratios were slightly different from those of the limestone. These results suggest that the minerals in the soils were not derived from the underlying carbonate rock. The upper soil samples (depth: 50 cm) were enriched in Al, Ti, Fe, K, and Ba, probably mainly from clay mineral and feldspar, and had higher 87Sr/86Sr ratios than those of the upper soils and the limestone. These results implicate contribution of atmospheric input, such as rainwater and/or eolian minerals, during soil weathering and formation process

Determination of lead isotope ratios in rainwater by inductively coupled plasma mass spectrometry

誘導結合プラズマ質量分析計(SHIMADZU ICPM-8500)を用いた降水中のPb同位体比の高精度測定方法と条件を報告する.積分時間は2秒で100回積分を繰り返すことで,精度の良いデータを得ることができ,鉛同位体の標準試料(NIST-SRM 981)の測定値は保証値と一致した.測定値の精度は,鉛濃度が0.5 ppb以上で高いほど良かった.標準試料と未知試料を交互に5回ずつ測定することで,精度の良いデータを得ることができた.Pb同位体比の測定に,内標準元素の添加は影響しないことがわかった.We describe an analytical method on the high-precision determination of lead stable isotope ratios in rainwater samples by inductively coupled plasma mass spectrometry (SHIMADZU ICPM-8500). At a scanning mode, dwell time of 2 s and 100 times of scan gave the best precision to the lead isotope ratio measurements. The analytical lead isotope ratios of the standard reference material for lead isotope (NIST-SRM 981) corresponded with their recommended values. Precision of the measurement was better at a high concentration of lead in sample whose concentration was more than 0.5 ppb. Five times of the set of repeating measurements of the standard and unknown samples gave satisfactory results with good precision level. The analytical results were not affected by the internal standard substance