Hideki Iwano

An improved system for measuring refractive index using the thermal immersion method

Abstract An improved refractive index measuring system using thermal immersion method has been developed. The main improvements are: (1) precise control of temperature using Peltier modules, (2) a well sealed cell, which can prevent evaporation of immersion oil, and (3) automatic measurement using an image memory board. With this system, refractive index of each single grain of glass or mineral can be feasibly determined with an accuracy as good as ± 2 × 10−4.

Fission-fragment registration and etching properties of diallyl phthalate with reference to its use as an external detector in fission-track dating

Abstract The properties of diallyl phthalate (DAP) resin were studied with reference to its use as an external detector in fission-track (FT) dating. The etching characteristics and bulk etch rates of DAP in 30% KOH in water at 90 °C (PW-etch) and 15% KOH, 65% ethanol and 20% H 2 O at 60 °C (PEW-etch) are different. In the case of the PW-etch, high background densities of recoil tracks were observed, that are produced by collisions of fast neutrons with C and O in the detector during irradiation. The PEW-etch does not reveal the background tracks as a result of the high bulk etch rate. The PEW-etch is recommended for use in FT dating because of the absence of a recoil-track background and the shorter etching time. The optimum etching time for both solutions was determined in step-etching experiments. For both etching solutions, the track density in DAP is 4–5% higher than in muscovite mica, as a result of the lower track registration threshold. Automated track counts confirm that image analysis enables accurate determinations of the track density in DAP etched in the PEW-etch.

New age constraints on the Miocene tectonic evolution of southwestern Japan: fission track ages from the Shitara district, Aichi Prefecture

西南日本の時計回り回転や垂直運動などの事件に年代制約を与える目的で，愛知県設楽地域の北設亜層群と設楽火成複合岩体から32個の岩石試料を採取しジルコンのFT年代を測定した．北設亜層群上部の凝灰岩は17.5 Ma前後のFT年代を示し，これは生層序年代と調和する．他方，設楽火成複合岩体の珪長質噴出物のFT年代は15 Ma前後に集中し，これは膨大な噴出物が約15 Maの短期間に定置したことを示す．こうした結果と地質，古地磁気，古応力場に関するデータから，筆者らは次のようなモデルを提案する：（1）北設亜層群は西南日本の主要な時計回り回転の直前にあたる17.5 Ma頃に，西南日本マイクロプレートが沈降しながら南方へ移動していた状況下で堆積した；（2）西南日本の時計回り回転は15 Maには既に終了していた；（3）西南日本と四国海盆の接合，およびそれによって生じた広域的な地殻隆起は，15 Maより少し前に起きた．

福島県東棚倉地域に分布する中新世海成層に挟在するKt-7凝灰岩の フィッショントラック年代

The fission track dating was applied for the Kt-7 Tuff, interbedded in the uppermost part of the Miocene Kubota Formation in the eastern Tanagura area, N ortheast J apan. The fission track age of the Kt-7 Tu任 (10.6:t0.3Ma; 1σerror) is almost equal to the K-Ar and fission track ages of the Kt -1 Tu旺 , interbedded in the lowest part of the Kubota Formation. These radiometric ages show good agreement with the previously established calcareous and siliceous microfossil biostratigraphy. The integrated stratigraphy based on microfossils as well as K-Ar and fission track ages indicate a high sedimentation rate of the Kubota Formation.

Ascent of migmatites of a high-temperature metamorphic complex due to buoyancy beneath a volcanic arc: a mid-Cretaceous example from the eastern margin of Eurasia

ABSTRACT Mixtures of melt and residue in a high-T metamorphic complex have a lower density and viscosity than the surrounding host crust, and the mixtures should ascend due to buoyancy. The mixtures are recognized as migmatites in the high-T metamorphic complex. To confirm ascent of migmatites, we conducted numerical simulations of ascent of a model migmatite (buoyant viscous fluid). The numerical simulations show that the model migmatite could rise to shallow levels of a model crust so long as it is continuously produced at the bottom of the model crust. Otherwise it ceases to rise at depth due to loss of buoyancy by cooling. The numerical simulations also show that the model migmatite experiences vertical thinning during the ascent. The ascent mechanism proposed in this paper requires the continuous production of partially melted rocks at the base of the crust, which is provided by a continuous input of energy into the crust from the mantle. Given that high-T metamorphic complexes are associated with igneous activity beneath a volcanic arc, the igneous activity reflects the energy input into the lower crust from the mantle. A high-grade part (migmatites) of a high-T metamorphic complex in the Omuta district of northern Kyushu, southwest Japan, experienced thinning during ascent. Large amount of igneous rocks, such as plutonic and volcanic rocks, are also distributed in northern Kyushu. Zircon U–Pb ages of igneous rocks from northern Kyushu reveal that igneous activity continued from 115 to 93 Ma, and that peak igneous activity at 110–100 Ma was synchronous with the ascent of migmatites of the high-T metamorphic complex in northern Kyushu. Therefore, the numerical simulations may provide an appropriate model of the ascent of migmatites of the high-T metamorphic complex beneath a volcanic arc, at the eastern margin of Eurasia in the mid-Cretaceous. GRAPHICAL ABSTRACT