砂岩層に産する方解石団塊における,酸素同位体比の改変一新第三系からの数例

摘要

Measurements of the oxygen and carbon isotopic compositions of calcite in nodules occurring in medium- to coarse-grained sandstone were conducted to verify the possibility that minerals comprising the stratum are isotopically altered due to the passage of groundwater of rainwater origin. The strata studied are the following Neogene formations: Oshimojo, Tamugigawa, Ronji, Abuzuri, Ichinose and other Formations. Except for the Oshimojo Formation, δ~(18)O for nodule calcite in sandstone ranges from 17 to 24‰ vs. SMOW, which is 8 to 15‰ lower than the initial value (approximately 32‰) set during the early diagenesis. The nodule calcite that occurs in sandstone is oxygen-isotopically in equilibrium with groundwater of rainwater origin. Except for the calcite from the Oshimojo Formation, δ~(13)C for nodule calcite ranges from -9 to +3‰. The observed δ~(13)C is markedly higher than that for organic matter (ca. -25‰). It is considered that δ~(13)C for nodule calcite was also determined by equilibration with carbonate ions dissolved in groundwater. The isotopic ratios of calcite nodules in the Oshimojo Formation indicate incomplete equilibrium with groundwater. The isotopic alteration in nodules resulted from the following two conditions:1)the host rock of nodules is medium- to coarse-grained sandstone of Neogene in age with high permeability, and 2) the mineral comprising the nodules is calcite that easily re-equilibrates with aqueous solutions.