Tracing extinct spreading center in SW Japan by helium-3 emanation

Abstract

In the Kinki district of southwestern Japan, anomalously high 3He/ 4He ratios (high Ra) are observed in the frontal arc region. This globally unique phenomenon is called the “Kinki Spot.” These high- Ra values are not attributable to dehydration of the young, warm subducting Philippine Sea slab based on the magma aging effect. A recent study reported that the source melt generated below the plate may penetrate a large slab tear fissure and arrive at the shallow crust by upwelling flow. To verify this hypothesis, we collected 25 gas and water samples from hot springs in the northern district and measured the 3He/ 4He and 4He/ 20Ne ratios of these samples. The geographical distribution of high- Ra values revealed not a spot but a region extending parallel to the extinct spreading center in the Shikoku Basin. More precisely, the distribution is shaped like a backwards letter “L,” suggesting that the subducted spreading center may have been a mid-ocean ridge with a transform fault. This hypothesis is supported by seismic tomography results and is consistent with the geometry of the subducting slab deduced from seismic isodepth contours as well as that of a young magnetic anomaly in the region. Thus, the high 3He emanation may trace an extinct ridge–transform–ridge type spreading center in a subduction zone.