Tohru Danhara

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.

Correlation of widespread tephra deposits based on paleomagnetic directions: Link between a volcanic field and sedimentary sequences in Japan

Abstract Magnetic measurements were made of the Yabakei and the Imaichi pyroclastic-flow deposits of the late Matuyama Chron in central Kyushu and their correlative co-ignimbrite ashes: the Pink and the Azuki tephra deposits in the Kinki district, and the Ku6C and the O7 tephra deposits in Boso Peninsula. Site mean magnetic directions of the Imaichi deposit are all of reversed polarity with declinations deflected slightly to the east. These directions are consistent with those of the Azuki and the Ku6C ashes. Magnetizations of the Yabakei, the Pink, and the O7 ashes are also consistent with one another. All are characterized by shallow inclinations (about 30°) of normal polarity with slightly westerly declinations. The identical paleomagnetic directions of these widespread tephra confirm correlations that have been proposed mainly on the basis of glass and mineral chemistry, and provide a tephrochronological linkage between the source volcanic area and the lacustrine or marine sequences over distances of at least 1000 km. This linkage allows us to correlate the oxygen isotope record of the Kazusa Group in Boso Peninsula with the alternating marine/non-marine lithology of the Osaka Group in the Kinki district. Radiometric dates from the Yabakei and the Imaichi pyroclastic-flow deposits are concordant with the astronomically-calibrated geomagnetic polarity time scale.

Juvenile volcanic glass erupted before the appearance of the 1991 lava dome, Unzen volcano, Kyushu, Japan

Fugen-dake, the main peak of Unzen volcano, began to erupt on 17 November 1990 after 198 years of dormancy. Following a period of steam-dominated tephra eruptions, a new lava dome appeared at the bottom of Jigokuato Crater in the summit region on 20 May 1991. Non-hydrated vesiculated glass was detected in the precursory tephra eruptions, beginning 12 February 1991. Determination of whether glass pyroclasts are hydrated or not appears to be an effective means of ascertaining whether the tephra contains juvenile material from ascending magma. The detection of juvenile pyroclasts at Fugen-dake more than 3 months before the appearance of the lava dome suggests that monitoring of early tephra emissions is an effective way to help forecast impending magmatic eruptions.

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．

Identification of a pair of co-ignimbrite ash and underlying distal plinian ash in the Early Pleistocene widespread tephra in Japan

Abstract The Early Pleistocene Azuki and the Ku6C tephras in Japan have been correlated with a compositionally zoned ignimbrite, the Imaichi pyroclastic-flow deposit, using ratios of accurately determined refractive indices of volcanic glass shards and phenocrysts and paleomagnetic directions. Thin, fine-grained, white ash layers at the base of the Azuki and the Ku6C tephras have been correlated with the Imaichi plinian-fall deposit, which underlies the Imaichi pyroclastic-flow deposit near the source Shishimuta caldera on central Kyushu. This indicates that the Azuki and the Ku6C tephras comprise a pair of compositionally zoned co-ignimbrite ash and underlying distal plinian ash. Thus, the initial sequence of physical and chemical changes at the source of the pyroclastic eruption has been retained in the distal widespread tephras, even though the ashes were blown downwind by westerlies as much as 900 km. Identification of a distal plinian ash at the base of a co-ignimbrite ash ensures the stratigraphic correlation of the distal tephra with the source ignimbrite.

Single grain ESR analysis of zircon: Comparison with fission-tracks

Abstract Electron spin resonance (ESR) signals from small zircon single crystals having various amounts of natural radiation damage (evaluated from fossil fission-track density) have been examined. ESR signals were normally detectable if a crystal had a weight more than 10 μg. ESR signals of Gd-centers at g = 1.993 and of radiation-related centers at g = 2.011 were commonly found in the zircons with the c-axis parallel to the magnetic field. The latter signal is probably due to a hole-center [H3O4]0, which is produced in association with the hydroxylation process SiO4→(OH)4. The ESR intensity of the hole-centers is proportional to fission-track density up to 1.5 × 106 cm−2. The ESR intensity sharply drops to zero as the spontaneous fission-track density becomes greater than 4–5 × 106 cm−2. Single grain ESR analysis of zircon may be useful not only for Quaternary geochronology but also for the fingerprinting of tephras containing zircons.