Heat source for an amagmatic hydrothermal system, Noto Peninsula, Central Japan

Abstract

Although there is no evidence of volcanism in the Noto Peninsula since the late Miocene, the peninsula has long been known to be unusual and atypical of non‐volcanic regions, as indicated by high‐temperature hot springs and a geothermal gradient greater than 50 K/km. In order to provide geochemical constraints on the heat source for amagmatic hydrothermal activity, the chemical and isotopic compositions of 14 gas and water samples from hot springs were measured. The observed 3He/4He ratios of most hot spring gases range from 0.03 to 1.2 Ra (Ra denotes the atmospheric 3He/4He ratio of 1.4 × 10−6). In these samples, mantle helium composes less than 10% of the total helium, indicating an insignificant contribution of mantle‐derived volatiles from, for example, newly ascending magma and/or aqueous fluid generated by dehydration of the subducting slab. The Noto Peninsula mainly consists of Neogene volcaniclastic and sedimentary rocks overlying Paleozoic to Mesozoic basement rocks associated with uranium‐bearing granite pegmatite containing elevated concentrations of heat generating elements such as uranium, thorium, and potassium. A plausible heat source for the amagmatic hydrothermal activity can be attributed to this distinctive geological environment where high heat producing granitic rocks are buried under Neogene sedimentary rocks with low thermal conductivities that act as thermal blankets. Most Noto Peninsula hot springs are found in areas of active faulting, where stress concentrations since the late Miocene could open an existing fault pathway because the fluid pressure is close to the lithostatic pressure. Meteoric waters circulating through hot basement rock come to the surface along these permeable conduits with minimal mixing with shallow groundwater, resulting in emanation of high‐temperature hot springs with low 3He/4He ratio gases along active fault zones.