Changes in the heavy mineral content and element concentration of Tarumae-a tephra with distance from the source volcano

Tarumae-a (Ta-a) is a young tephra that erupted from Mt Tarumae in ad 1739. Tarumae-a was collected at various distances from Mt Tarumae. Sand-size fractions of the tephra were divided into minerals with a specific gravity (ρ) higher than 2.9 (heavy minerals [HM]) and lower than 2.9 (light minerals [LM]), and the concentrations of 19 elements in the HM and LM were examined. The ratio of HM to the total sand-size Ta-a was 38-58% within 42 km, 21% at 113 km and 5.8-8.7% at 290 km from Mt Tarumae. The HM contained Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Zn and Cd at higher concentrations and Na, Al, Si, K, Se, Sr, Ba and Pb at lower concentrations than the LM. The Ca and Cu concentrations did not differ much between the HM and LM. The element concentrations of the total sand-size Ta-a were calculated from those for the HM and LM. The Mg and Fe concentrations in the total sand-size Ta-a decreased with increasing distance from the volcano because of a decrease in HM. Mg and Fe were the first and the second most dominant elements among those showing much higher concentrations in the HM than in the LM. The Na, Al, Si and Ca concentrations in the total sand-size Ta-a tended to increase with distance from Mt Tarumae because of increases in LM. Among the minor elements, Co decreased and Sr increased with increasing distance from Mt Tarumae. Co and Sr were highly concentrated in the HM and LM, respectively, with lower concentration variations in each mineral fraction.