Grain-size variations in 87Sr/86Sr and elemental concentrations of stream sediments in a granitic area: Fundamental study on 87Sr/86Sr spatial distribution mapping

The strontium isotope ratio (Sr-87/Sr-86) is often used to identify the origin of agricultural products and the movement of ancient people, and a nationwide Sr-87/Sr-86 distribution map would greatly assist such studies. The Geological Survey of Japan (GSJ), AIST has already created nationwide element distribution maps using the < 180 mu m fraction of stream sediments, but this grain size fraction may not necessarily be suitable for mapping the Sr-87/Sr-86 isotopic ratio. Therefore, this study investigated the effect of grain size on elemental concentrations and Sr-87/Sr-86 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 Sr-87/Sr-86 ratios of five fractions in the size range 1000-75 mu m 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 Sr-87/Sr-86 ratios among the sampling points was smallest in the 300-75 mm fractions, which showed Sr-87/Sr-86 and Sr-87/Sr-86 values that were closest to those of the source rocks. The coarser (> 300 mu m) and the fine (< 75 mu m) fractions of the stream sediments showed systematically higher Sr-87/Sr-86 and lower Sr-87/Sr-86 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 Sr-87/Sr-86 of the < 180 mu m 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 Sr-87/Sr-86 values in the granitic study area. Consequently, we concluded that, in granite areas, the < 180 mu m fraction of the stream sediments can be used for Sr-87/Sr-86 mapping, implying that a nationwide Sr-87/Sr-86 map can be made using sediment samples with a grain size of less than 180 mu m.