Distribution of boron and ammonia in soil around Usu Volcano, Japan, prior to the 2000 eruption

Concentrations of B and NH 3 in soil at 150 sites in and around the summit area of Usu volcano were measured in September 1998, 18 months before the 2000 eruption. B concentrations reached 1,300 μg.kg -1 and NH 3 up to 14 mg.kg -1 . The spatial distribution of B is characterized by high B contents (> 900 μg.kg -1 ) on the NW flank of the Usu volcanic edifice near the site of the March 2000 eruption and at the summit region of Usu volcano where high-temperature fumarolic activity occurs. High NH 3 contents occur at the same places as the B anomalies and also at Showa-Shinzan volcanic dome. The B and NH 3 anomalies correlate with high soil CO 2 , suggesting a common source for all three species. Our results show that, in addition to CO 2 , B and NH 3 provide useful tools to define a volatile flow in a volcanic system.