Hydrogen and oxygen isotope ratios of shales and characteristics of formation waters in sedimentary complexes accreted at different times, Kinki district, southwest Japan

Abstract

Oxygen and hydrogen isotope analyses were made of shale samples from southwest Japan to investigate water circulation in sedimentary complexes during burial and low-grade metamorphism. The sedimentary complexes of the Kinki district were accreted to the Asian continent at two different times, and the geology is characterized by a clear zonal structure. Hydrogen isotope ratios of the bulk shales become heavier from the older northernmost terrain ( δD SMOW = −105 to − 90‰ ) to the younger southernmost terrain (−60 to −40‰). Hydrogen isotope ratios of illite show two distinctive ranges for the two different accretionary complexes: −100 to −60‰ for the Jurassic to early Cretaceous complexes and −65 to −45‰ for the Cretaceous to Paleogene complexes. These hydrogen isotope characteristics of the bulk shales and illite suggest that deeply circulating meteoric water interacted with the Jurassic sedimentary formations, whereas the original sedimentary isotope ratios were likely retained in the Cretaceous to Paleogene terrains. Oxygen isotope ratios of the bulk shales and the constituent minerals are similar for the two accretionary complexes, indicating that sediments were constantly supplied from the same hinterland of the Asian continent. Chlorite and illite with normal to high δ 18 O values (+ 5 to + 17‰ and +10 to + 21‰, respectively) may have formed during burial diagenesis and low-grade metamorphic reactions, although a fraction of the minerals are detrital in origin. The similar oxygen isotope values of fine quartz (+17 to + 23‰) to those previously reported for detrital quartz in shales and soils suggest homogeneity of detrital quartz in the Northern Hemisphere.