ABSTRACT

The strontium isotope ratio (⁸⁷Sr/⁸⁶Sr) is often used to identify the origin of agricultural products and the movement of ancient people, and a nationwide ⁸⁷Sr/⁸⁶Sr distribution map would greatly assist such studies. The Geological Survey of Japan (GSJ), AIST has already created nationwide element distribution maps using the <180 μm fraction of stream sediments, but this grain size fraction may not necessarily be suitable for mapping the ⁸⁷Sr/⁸⁶Sr isotopic ratio. Therefore, this study investigated the effect of grain size on elemental concentrations and ⁸⁷Sr/⁸⁶Sr ratios in stream sediments and compared their values with those of the source rocks. The stream sediments studied were collected from the granitic drainage basin of the Yahagi and Yada rivers in Aichi Prefecture, central Japan. The elemental concentrations and ⁸⁷Sr/⁸⁶Sr ratios of five fractions in the size range 1000–75 μm from the stream sediments of the Yahagi River tributaries varied with the grain size, and the variations corresponded to the heterogeneity of the mineral compositions in each fraction. The difference in the elemental concentrations and ⁸⁷Sr/⁸⁶Sr ratios among the sampling points was smallest in the 300–75 μm fractions, which showed ⁸⁷Sr/⁸⁶Sr and ⁸⁷Rb/⁸⁶Sr values that were closest to those of the source rocks. The coarser (>300 μm) and the fine (<75 μm) fractions of the stream sediments showed systematically higher ⁸⁷Rb/⁸⁶Sr and lower ⁸⁷Sr/⁸⁶Sr ratios than the Rb-Sr mineral isochron for the source rock, and the result suggests that 1) these fractions are enriched in K- and Rb-rich minerals such as K-feldspar, biotite, and hornblende; and 2) Sr is lost relative to Rb because of weathering processes. Furthermore, temporal variations in ⁸⁷Sr/⁸⁶Sr of the <180 μm stream sediments collected at a fixed sampling site in the Yada River were 0.001, which is smaller than the variations associated with grain size seen in the ⁸⁷Sr/⁸⁶Sr values in the granitic study area. Consequently, we concluded that, in granite areas, the <180 μm fraction of the stream sediments can be used for ⁸⁷Sr/⁸⁶Sr mapping, implying that a nationwide ⁸⁷Sr/⁸⁶Sr map can be made using sediment samples with a grain size of less than 180 μm.