Hisatoshi Ito

Annealing kinetics of fission tracks in zircon: an experimental study

A series of laboratory annealing experiments on zircon fission tracks has been carried out under heating conditions of 350–750°C for 10−1–103 hr (i.e. 4.5 min-∼40 days). Variation in the confined lengths of spontaneous fission tracks was determined using zircon grains from Nisatai Dacite. The fading contours of normalized mean track length (r) on the Arrhenius diagram showed as sets of straight lines. We performed a series of model fittings, called the parallel and fanning models, in order to describe the decrease in r with increasing temperature or heating time. The lowest temperature limit of the zircon partial annealing zone (ZPAZ) was defined as r ≈ 0.95, and the highest as r ≈ 0.4, which approximately corresponded to the total fading of surface tracks. Extrapolation of the results of the laboratory experiments to the geological timescale gives, for a heating duration of 106 yr, estimated values of the ZPAZ of ∼210–320°C (± 60°C, 2σ) with the parallel model; ∼190–350°C (± 50°C, 2τ) with the fanning model (critical temperature, T0 = ∞); and ∼170–390°C (±50°C, 2σ) with the fanning model (T0 ≠ ∞). The temperatures of the ZPAZ decrease by ∼20°C for an annealing duration that is an order of magnitude longer. Because the estimated closure temperature of zircon fission-track analysis approximately corresponds to the middle of the ZPAZ, these results support the previously estimated closure temperature of ∼240°C. By varying the etching time it was revealed that significant removal of α-radiation damage occurs at r ≈ 0.93.

Thermal annealing characteristics of spontaneous fission tracks in zircon

Abstract One-hour isochronal annealing experiments were carried out on zircon crystals KT06, separated from a rhyolitic welded tuff of the Koto Rhyolites, southwest Japan, in order to study thermal annealing characteristics of spontaneous fission tracks in zircon. Variations of track density, horizontal confined track length and track angular distribution were measured at various temperatures between 400° and 750°C. Track density and track length begin to decrease around 450°C and reach zero around 750°C. Both time and anisotropy of track etching increase drastically for the samples annealed above 500°C because of the loss of accumulated radiation damage derived from α-decay of U and Th. Angular distributions of etched tracks in those samples suggest no significant apparent reduction of etching efficiency due to increased etching anisotropy, so long as adopted etching criteria are strictly maintained. Angular distribution measurements also reveal anisotropic annealing characteristics of tracks, which may produce an additional sampling bias in track length measurement.

Grounded electrical-source airborne transient electromagnetic (GREATEM) survey of Mount Bandai, north-eastern Japan

Airborne electromagnetics (AEM) is a useful tool for investigating volcanic structures because it can survey large and inaccessible areas. Disadvantages include lower accuracy and limited depth of investigation. The Grounded Electrical Source Airborne Transient Electromagnetic (GREATEM) survey system was developed to increase the depth of investigation possible using AEM. The method was tested in a survey at Mount Bandai in north-eastern Japan. Mount Bandai is an andesitic stratovolcano that rises 1819 m above sea level. An eruption in July 1888 left a hoof-shaped collapsed wall in its northern crater and avalanche debris at its base. Previous surveys of Mount Bandai allow for comparisons of data on its structure and collapse mechanism as obtained by GREATEM and other geophysical methods. The results show resistive structures in recent volcanic cones and conductive structures in the collapsed-crater area. Conductive areas around the collapsed wall correspond to an alteration zone resulting from hydrothermal activity, supporting the contention that a major cause of the collapse associated with the 1888 eruption was hydrothermal alteration that structurally weakened the interior of the volcanic edifice.

Tectonic history of granitic bodies in the South Fossa Magna region, central Japan: new evidence from fission-track analysis of zircon

Abstract A fission-track (FT) analysis of zircons has been performed to clarify tectonic history of the Tertiary granitic bodies in the northern part of the Izu—Bonin arc and the South Fossa Magna region of central Japan. A total of 30 FT ages was obtained from the Kaikoma, Tsuburai, Ashigawa, Tokuwa and Tanzawa granitic bodies. Confined FT lengths were also measured on 11 zircon samples from the granitic rocks to obtain more information on the thermal history of the region. The FT ages and confined FT length data from the Kaikoma indicate that this granitic body cooled below the isotherm of zircon (240 °C) at 9.4± 0.4 Ma (2 σ ). The Tsuburai, Ashigawa, Tokuwa and Tanzawa granitic bodies, which have a similar lithology and tectonic setting, lie from northwest to southeast, respectively. The FT ages for these bodies are: 15.0 ± 0.6Ma for the Tsuburai; 11.9 ± 0.4 Ma for the Ashigawa; 12.5 ± 1.4 Ma for the Tokuwa (which is comparable with the FT age of the Ashigawa); 7.1 ± 2.0 Ma for the Tanzawa (which is the youngest). The data indicate a differential uplift of the granitic bodies in the South Fossa Magna region. In general, the FT ages become younger as one proceeds southeastwards approaching the Izu-Bonin arc. The FT age data provide constraints on the tectonic effects of the collision of the Izu-Bonin arc with the central Honshu of the Japan arc.

Earth's youngest exposed granite and its tectonic implications: the 10-0.8 Ma Kurobegawa Granite.

Although the quest for Earths oldest rock is of great importance, identifying the youngest exposed pluton on Earth is also of interest. A pluton is a body of intrusive igneous rock that crystallized from slowly cooling magma at depths of several kilometers beneath the surface of the Earth. Therefore, the youngest exposed pluton represents the most recent tectonic uplift and highest exhumation. The youngest exposed pluton reported to date is the Takidani Granodiorite (~ 1.4 Ma) in the Hida Mountain Range of central Japan. Using LA-ICP-MS and SHRIMP U-Pb zircon dating methods, this study demonstrates that the Kurobegawa Granite, also situated in the Hida Mountain Range, is as young as ~ 0.8 Ma. In addition, data indicate multiple intrusion episodes in this pluton since 10 Ma with a ~ 2-million-year period of quiescence; hence, a future intrusion event is likely within 1 million years.

Grounded electrical-source airborne transient electromagnetics (GREATEM) survey of Aso Volcano, Japan

Grounded electrical-source airborne transient electromagnetics (GREATEM), a type of semi-airborne electromagnetics, was used to examine Aso Volcano in south-west Japan, to verify its applicability to surveying deep subsurface resistivity structures. Comparison of the GREATEM resistivity values with those of ground-based transient electromagnetics (TEM) data, repeated GREATEM survey results at the same and different flight heights, and lithologic descriptions indicated that GREATEM can successfully identify underground structures as deep as ~800 m in rugged mountainous areas. An active volcanic region (Naka-Dake crater) was mapped as a low-resistivity zone from the surface to a depth of 100 m. This low-resistivity zone extended to the west-north-west, implying future volcanic activity in this area. Therefore, the GREATEM method is useful for surveying deep structures in large, inaccessible areas, such as volcanic provinces, in a quick, cost-effective way.

Insights on the thermal history of the Valles caldera, New Mexico: evidence from zircon fission-track analysis

Abstract The zircon fission-track dating method was applied to the VC-2B core obtained from the active hydrothermal system at Sulphur Springs, Valles caldera, New Mexico. Four samples were analyzed to obtain both zircon ages and track length data from Permian strata to Precambrian quartz monzonite, the present temperatures of which are from 222 to 294 °C. Zircon ages obtained from the deeper three samples indicate partially annealed ages of about 450–600 Ma. Thus, zircons from Permian strata probably originated from Precambrian rocks. Contrary to the normal relationship between FT ages and temperatures of the sampling points, non-correlation between them is probably due to hot fluid flow and/or hot vein emplacement associated with recent volcanic activity. Zircon ages of the shallowest sample were much younger than the Permian, the reason for which is unknown. The closure temperature of the zircon fission-track dating method seems to be higher than that previously determined from geological data.

Magmatic tempo of Earth’s youngest exposed plutons as revealed by detrital zircon U-Pb geochronology

Plutons are formed by protracted crystallization of magma bodies several kilometers deep within the crust. The temporal frequency (i.e. episodicity or ‘tempo’) of pluton formation is often poorly constrained as timescales of pluton formation are largely variable and may be difficult to resolve by traditional dating methods. The Hida Mountain Range of central Japan hosts the youngest exposed plutons on Earth and provides a unique opportunity to assess the temporal and spatial characteristics of pluton emplacement at high temporal resolution. Here we apply U-Pb geochronology to zircon from the Quaternary Kurobegawa Granite and Takidani Granodiorite in the Hida Mountain Range, and from modern river sediments whose fluvial catchments include these plutons in order to reconstruct their formation. The U-Pb data demonstrate that the Kurobegawa pluton experienced two magmatic pulses at ~2.3 Ma and ~0.9 Ma; whereas, to the south, the Takidani pluton experienced only one magmatic pulse at ~1.6 Ma. These data imply that each of these magmatic systems were both spatially and temporally distinct. The apparent ~0.7 Myr age gap between each of the three magmatic pulses potentially constrains the recharge duration of a single pluton within a larger arc plutonic complex.

International Symposium on Airborne Geophysics

Airborne geophysics can be defined as the measurement of Earth properties from sensors in the sky. The airborne measurement platform is usually a traditional fixedwing airplane or helicopter, but could also include lighter-than-air craft, unmanned drones, or other specialty craft. The earliest history of airborne geophysics includes kite and hot-air balloon experiments. However, modern airborne geophysics dates from the mid-1940s when military submarine-hunting magnetometers were first used to map variations in the Earths magnetic field. The current gamut of airborne geophysical techniques spans a broad range, including potential fields (both gravity and magnetics), electromagnetics (EM), radiometrics, spectral imaging, and thermal imaging.