Multiple lines of evidence for crustal magma storage beneath the Mesozoic crystalline Iide Mountains, northeast Japan

Abstract

It has been recognized that the Iide Mountains, consisting of Mesozoic sedimentary rocks and Late Cretaceous to Paleogene granitic rocks, northeast Japan, are unique. Although they occur in a nonvolcanic region, hot springs in the Iide Mountains have anomalously high heat discharge values similar to those from hot springs in volcanic regions. In order to provide geochemical constraints on the heat source for the hydrothermal activity, the chemical and isotopic compositions of 10 gas and water samples were determined. The 3He/4He ratios determined range from 0.22 to 7.9 Ra, and the highest ratio is similar to MORB‐type helium, indicating a significant contribution of primordial mantle helium. Considering the relationship between the magnitude of the 3He/4He ratios and the distance of the sample locations from the Kitamata‐dake, the peak in the Iide Mountains where the largest ratio occurs, it is apparent that there is a sharp decrease in 3He/4He ratios laterally away from the peak. Furthermore, the peak is also the location where geophysical anomalies such as an anomalous conductive body and zones with low Vp and low Vs have been detected in the middle‐lower crust. These high helium ratios are considered to indicate the likelihood of mantle‐derived materials supplying MORB‐type helium beneath the Kitamata‐dake and the possibility that mantle‐derived helium has been diluted by atmospheric and/or crustal components with lower helium ratios away from the peak. In order to examine whether or not ancient magma of Middle Miocene age is a possible source of the high 3He/4He ratios of the hot spring gases, we calculated the evolution in 3He/4He ratios of the ancient magma with time. As a result, it is concluded that the anomaly beneath the Iide Mountains is due to newly ascending magmas in the present‐day subduction system rather than hydrothermal fluids related to late remnant magmatism of Middle Miocene age.