Strontium-isotope mapping of the Kyushu islands, southwestern Japan, using stream sediments

A geochemical mapping study was conducted to investigate the spatial distribution of 87Sr/86Sr ratios in Kyushu. The study aimed to understand the factors controlling the geochemical distribution of Sr isotopes in stream sediments more completely. Using a thermal ionization mass spectrometer, we analyzed the 87Sr/86Sr of 307 stream sediments collected for geochemical mapping in Japan. The results showed that systematic variations in 87Sr/86Sr ratios correlate with surface geology. Stream sediments derived from mafic volcanic and pyroclastic rocks displayed low 87Sr/86Sr ratios (0.704–0.708), whereas those derived from felsic volcanic, granitic, and metamorphic rocks showed intermediate Sr isotope ratios (0.705–0.711). In the northwestern and southeastern areas underlain with accretionary complex and non-accretionary sedimentary rock, high 87Sr/86Sr ratios (0.705–0.721) were observed in the stream sediments. Based on the 87Sr/86Sr data of stream sediments derived from these rock types, we found a predominant supply of clastic materials originating from the Asian continent to the Paleogene back-arc basin in northwest Kyushu and the Paleogene subduction zone in southwest Kyushu. This suggests that the Sr isotope map is useful for the provenance analysis of clastic particles in sedimentary rocks. Furthermore, we present a map of the 87Sr/86Sr ratio in the exchangeable fraction of stream sediments. This fraction, which dissolves easily in water and is accessible for plants, is important for provenance studies of agricultural products. The 87Sr/86Sr ratio in the exchangeable fraction was predominantly affected by the presence of plagioclase in stream sediments and exhibits a positive correlation with that in the bulk fraction.