Koji Umeda

Variations in the 3He/4He ratios of hot springs on Shikoku Island, southwest Japan

New helium isotopic data from hot springs on Shikoku Island were obtained from a larger number of sampling points than previous studies in order to elucidate the geographical distribution of 3He/4He ratios in the fore-arc regions of southwest Japan. Systematic, regional variations in 3He/4He ratios were observed on both Shikoku Island and the Kii Peninsula. The hot springs on Shikoku Island have 3He/4He ratios between 0.21 RA and 2.5 RA. On the other hand, the majority of hot springs in the Kii Peninsula have significantly higher 3He/4He ratios than those on Shikoku Island. It has been suggested that the anomalously high 3He/4He ratios in the fore-arc region of southwest Japan are related to the migration of MORB-type helium with aqueous fluids generated by dehydration of the subducting Philippine Sea plate and mixing with terrestrial waters (e.g., Matsumoto et al., 2003). According to recent seismological observations of crustal microearthquakes and their focal mechanisms, the geotectonic environment beneath Shikoku Island is subject to a stress field with no normal faulting and with extremely low crustal seismicity. This argues against migration of aqueous fluids from the subcrustal lithosphere, resulting in a relatively small influence of mantle-derived helium at the surface. As a consequence, the regional variation in 3He/4He ratios of hot springs on Shikoku Island is interpreted as a three-component mixture of mantle-derived helium associated with magmatism of middle Miocene of age (<3.4 RA), crustal, radiogenic helium and atmospheric helium.

Magnetotelluric imaging of crustal magma storage beneath the Mesozoic crystalline mountains in a nonvolcanic region, northeast Japan

Mesozoic crystalline mountains (Iide Mountains) in a nonvolcanic region of the Northeast Japan Arc were imaged by wide-band magnetotelluric soundings. A two-dimensional model shows that an anomalous conductive body (<10 Ωm) is clearly visible beneath the Iide Mountains. The conductor widens with increasing depth and extends from the near-surface down to the base of the crust and perhaps into the upper mantle. The location of the conductive body correlates with high-temperature hot springs with high 3He/4He ratio, thinning of the brittle seismogenic layer, and anomalies of low seismic velocity. We conclude that the conductor reflects the presence of partial melts in the crust, related to renewed magmatism in the present-day subduction system.

Electrical Resistivity Structure and Helium Isotopes around Naruko Volcano, Northeastern Japan and Its Implication for the Distribution of Crustal Magma

The two-dimensional electrical resistivity structure beneath Naruko volcano was determined using magnetotelluric soundings. The resulting model shows that a prominent conductor exists through the middle crust to the uppermost mantle beneath the volcano. The location of the conductor agrees closely with a seismic low-velocity zone. Low-frequency microearthquakes occur near the conductor around the Moho depth. The cutoff depth of crustal earthquakes is coincident with the upper boundary of the conductor, implying that the conductor has a temperature appreciably higher than 400∘C. Furthermore, new helium isotope data from hot springs around the volcano were obtained. The spatial distribution of the observed 3He/4He ratios reveals the extent of mantle-derived materials beneath Naruko volcano. Consequently, it is apparent that the conductor determined beneath the volcano reflects the presence of high-temperature mantle-derived materials such as magmas and/or related fluids derived from active magmatism in the northeastern Japan subduction zone.

THERMAL CONSTRAINTS ON CLAY GROWTH IN FAULT GOUGE AND THEIR RELATIONSHIP WITH FAULT-ZONE EVOLUTION AND HYDROTHERMAL ALTERATION: CASE STUDY OF GOUGES IN THE KOJAKU GRANITE, CENTRAL JAPAN

In order to elucidate the process of mineralization of clay minerals in fault gouge and its spatial-temporal relationship with fault-zone evolution and hydrothermal alteration, X-ray diffraction (XRD) analysis and K-Ar dating were performed on clay samples from the Kojaku Granite of central Japan, including fault gouge along an active fault. The area studied is suitable for understanding thermal constraints on clay mineralization because the wall rock is homogeneous and its thermal history well defined. The results from XRD indicated that the clay minerals in the gouge samples are dioctahedral smectite, kaolinite, and 1Md illite, whereas clay fillings in fractures and joints in the intact granite (clay vein) include 2M1 illite in addition to dioctahedral smectite and 1Md illite. The evolution of clay mineralization is reconstructed as follows: (1) high-temperature hydrothermal alteration of feldspar and biotite produced 2M1 illite in clay veins; and (2) alteration accompanied by shearing at a lower temperature resulted in the formation of 1Md illite in the gouges. This scenario is consistent with the cooling history of the granite constrained by fission-track, U-Pb, and K-Ar dating methods. K-Ar dating of the clay samples separated into multiple particle-size fractions indicated that the low-temperature alteration leading to the production of 1Md illite was dated to ~40 Ma. Based on the cooling history of the granite, the 1Md illite formed at temperatures of 60–120°C. This temperature range was at the lower limit of the range reported in previous studies for faults. The spatial and geometrical relation of the faults studied and their K-Ar ages infer evolution which can be described as extensive development of small-scale faults at ~40 Ma followed by coalescence of the small-scale faults to form a larger, recently reactivated, active fault. The K-Ar ages have not been reset by the recent near-surface fault activity.

Three-dimensional magnetotelluric inversion using a heterogeneous smoothness-constrained least-squares method

This paper presents a fast algorithm for electromagnetic data inversion to three-dimensional (3D) resistivity models. The algorithm is distinctive for the level of accuracy it attains while bypassing the sensitivity matrix update. A common sensitivity matrix for homogeneous half-space is used in all iterations. Instead of updating the sensitivity matrix, the smoothness filter coefficients at each model element are updated, based on the spatial variations in resistivity in the model derived from the latest iteration. This substitution is expected not only to reduce the computation time required for large-scale inversions, such as those for 3D surveys, but also to allow the resolution of sharp boundaries in resistivity structures. Our algorithm was applied to 3D magnetotelluric inversion in order to confirm its effectiveness. Using synthetic examples under several conditions, we demonstrated that the method can reduce the number of forward calculations required to reduce data misfits to noise level, and that the method is robust for constructing target models even with sharp boundaries without generating fatally false resistivity structures or boundaries under noisy conditions.

Current R & D Activities in the Study on Geosphere Stability

The Japanese islands are located in a tectonically active zone. The scientific base is required for assessing the geosphere stability for long-term isolation of radioactive waste in Japan. JAEA is promoting the establishment of investigation method for geotectonic events affecting geosphere stability and prediction model for the future changes of geological environments, that is necessary for site selection and safety assessment of the HLW geological disposal. For seismicity and faulting, detection techniques for active faults without topographic surface expression, such as using helium isotope ratios in hot spring gases or detection of hydrogen gas, and studies on the assessment of fault evolution have been developed. For volcanism and geothermal activity, heat sources for anomalous geothermal activity in non-volcanic regions are considered. Detection techniques for high-temperature fluids and magma deep underground, using geophysical and geochemical approaches, were constructed. For uplift, denudation and climatic/sea-level changes, a methodology to predict the future topographic change was developed. Also, for dating techniques as an essential part to proceed on these studies, C-14 and Be-10 dating using AMS and (U-Th)/He dating using QMS and ICP-MS have been developed. We are planning the establishment of assessment methods for geosphere stability including assessment of the activity of faults encountered in underground excavations, development of long-term prediction model of volcanism and hydrothermal activities, and hydrogeological analyses considering topographic change.© 2010 ASME