2022 Volume 76 Issue 3 Pages 111-127
The carbon and oxygen isotope ratios of calcite in calcite nodules with or without apatite from the Miocene Tomikusa Group in the south of Nagano Prefecture were measured. The isotope ratios are considerably scattered in the δ13C versus δ18O diagram. However, the isotopic ratio of calcite from each sampling locality shows a linear trend with different positions and slopes on the δ13C versus δ18O diagram. The trends converge to a narrow region of δ13C = −5~+1‰ and δ18O = 15~16‰, when the trends are extended to the low δ18O direction. The convergence zone was named PC (Position of Convergence), which corresponds to the isotopic ratio of calcite in equilibrium at 45°C with meteoric water of δ18O = −9‰ in the southern part of Nagano Prefecture. Calcite nodules of the Tomikusa Group were formed by early diagenesis of sulfate reduction to methane fermentation stages, and were subjected to ground water infiltration after the Tomikusa Group uplifted in the late Miocene to the Pliocene. Consequently, carbon and oxygen isotope ratios shifted toward the PC to varying degrees. Clinoptilolite and opal-CT occur in the rocks of the Tomikusa Group. It is inferred that the isotopic alteration of calcite nodules occurred during burial metamorphism with the formation and infiltration of silicic acid-rich aqueous solutions.
