Zeta potential estimation of volcanic rocks on 11 island arc‐type volcanoes in Japan: Implication for the generation of local self‐potential anomalies

Abstract

From streaming potential measurements, we deduced the zeta potential of 73 volcanic rock samples collected in 11 volcanoes where self‐potential (SP) surveys had also been conducted. Experiments with crushed rock samples and 0.001 mol/L NaCl solution showed a large variation in streaming potential coefficient, which ranged from −2860 to 2280 mV/MPa (deduced zeta potential ranged from −45.1 to 37.2 mV). Among the 73 samples, 9 samples showed positive values, and 11 samples showed small absolute values less than 10 mV. Even in a volcano, the zeta potential values of rocks were sometimes highly variable. To investigate the reason for the variation in zeta potential, we performed X‐ray fluorescence (XRF) measurements on the rock samples. However, no significant correlations between zeta potential (or isoelectric point) and weight percentage of major elements were found, implying that the zeta potential of the volcanic rocks is not controlled by chemical composition but by mineralogical composition or crystalline structure. In volcanic areas, rocks with positive or small absolute values of zeta potential, which are considered to be unusual, frequently exist in areas along with local positive SP anomalies. Although positive SP anomalies are usually interpreted as an indication of hydrothermal upflow, this correlation suggests that in addition to the groundwater flow pattern, the polarity of zeta potential should also be considered for interpretation of SP data. Therefore, in cases where detailed zeta potential measurements have not been conducted, local positive SP anomalies should be interpreted with caution.