Hydrogeological structure of a seafloor hydrothermal system deduced from a pair of positive and negative self-potential anomalies observed at the Oomuro-dashi hydrothermal field in the Izu-Ogasawara Arc, south of Japan

Abstract This paper presents and interprets two new self-potential data measured over a hydrothermally active field associated with a Quaternary rhyolitic volcano, Oomuro-dashi, in the northern Izu-Ogasawara Arc, south of Japan. The measured data show a pair of positive and negative anomalies of the order of one millivolt at 5 m above the seafloor. The observation of a positive self-potential near a seafloor hydrothermal system is new, in spite that negative self-potential anomalies have been regularly reported in various studies for different locations. Determining the dominant mechanism(s) is therefore key to further understanding the subseafloor structure of seafloor hydrothermal systems. To this end, we also conducted long-term observations of subseafloor temperatures at two sites in the area of the self-potential anomaly to estimate the Darcy velocity. We found a downward fluid flow of the order of tens of metres per year at both sites. The flow in the area of the negative self-potential anomaly is stronger than in the area of the positive anomaly. Based on these observations, we propose two end-member models to explain the paired self-potential anomaly. The first model considers a horizontal geo-battery, in which part of a subhorizontal electrically conductive body is crossed by a subvertical redox front. In this model, the oxidised part of the geo-battery causes a negative self-potential anomaly, as in the previous observations, while the reduced counterpart of the geo-battery, which is normally buried, is exposed near the seafloor and causes a positive anomaly. In this case, a conductive body is expected to lie beneath both anomalies, and we could access the reduced part of the geo-battery. This model is consistent with the results of the Darcy velocity estimation if the strong hydrothermal circulation would cause the redox horizon to deepen. The second model is a combination of the thermal and streaming potentials causing both positive and negative self-potential anomalies. This model does not necessarily require a buried conductive body beneath the self-potential anomalies. These end-member models could be distinguished by resistivity imaging, which identifies the distribution of conductive bodies beneath self-potential anomalies, although they would overlap in natural systems. Graphical Abstract