Geochemical Journal, Vol. 51 (No. 6), pp. 469-484, 2017
Masayo Minami,1* Yuka Jomori,1 Kazuhiro Suzuki1 and Atsuyuki Ohta2
1Institute for Space-Earth Environmental Research, Nagoya University, Chikusa, Nagoya 464-8601, Japan
2Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8567, Japan
(Received July 6, 2016; Accepted March 8, 2017)
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.
Key words: stream sediment, geochemical map, grain size, 87Sr/86Sr, granite