Kuniyuki Furukawa

In this study, Raman spectra (3700–10 cm ^–1 ) and attenuated total reflection infrared–far-infrared (ATR-IR/FIR) spectra (4000–50 cm ^–1 ) including low-frequency region were measured for amorphous rocks, which were five types of obsidians whose formation ages and sources are different and pitchstone to clarify the differences in water content (free and bound water species), their Si–O bonds and possible linkage with a metal ion, and the mean atomic volume. In order to explore these points, we focused on infrared (IR) absorptions of hydroxyl (OH) groups that is observed in the 4000–3000 cm ^–1 region, those of Si–O bond that is identified in the 1300–850 cm ^–1 region and a Boson peak that appears in a low-frequency region of Raman spectra, respectively. IR absorption of Si–O stretching was detected for all samples and that of OH stretching and H–O–H bending was also detected in some rocks. Therefore, using IR spectroscopy was useful to discriminate each rock based on the water content and the environment of Si–O bonds. On the other hands, a Boson peak could be detected for the low-frequency region below 60 cm ^–1 of Raman spectra, which appears in amorphous solids. This study is the first finding that the Raman shift of Boson peak was different among similar natural glassy rocks from multiple sources and it means that the mean atomic volume of samples was different. In addition, sharp bands of Raman scattering which came from inorganic substances such as feldspar helped to identify ingredients in samples. As a results, we made clear that using both IR and Raman including low-frequency regions is effective to identify the same types of natural amorphous rocks.

Abstract To test the hypothesis that a Cretaceous hairpin turn is absent in the apparent polar wander path (APWP) of the inner arc of southwestern Japanese island (southwest Japan), we refined a mid-Cretaceous (100 Ma) paleomagnetic pole from southwest Japan. Red mudstone samples from the 100 Ma Hayama Formation were collected for paleomagnetic analysis from eight sites in the Hayama area in the central part of southwest Japan. A high-temperature remanent magnetization component carried by hematite was isolated from these sites and was found to be of primary mid-Cretaceous origin. The primary nature of the magnetization is supported by the detrital character of the magnetic carrier. The primary directions provided a paleomagnetic pole (35.0°N, 209.6°E, A₉₅ = 6.1°, N = 8), which represented southwest Japan at 100 Ma. This pole falls into a cluster of Cretaceous poles in southwest Japan. An APWP for southwest Japan between 110 and 70 Ma was updated to ascertain the stationarity of the pole positions for this region. Therefore, it is unlikely that the APWP for southwest Japan experienced a hairpin turn during the Cretaceous.

<title>SUMMARY</title> A palaeomagnetic study has been conducted to examine the deformation of thick crusts of rhyolite lava while its inner portions continue to flow. The Sanukayama rhyolite lava, which erupted in the Pleistocene in Kozushima Island, Japan, was chosen as the investigation site because of its well-exposed vertical lithofacies variations classified into three distinct zones (pumiceous, obsidian and crystalline). The targets of this study are the pumiceous and obsidian zones, which constitute the crust of the lava. Thermal demagnetization reveals three remanent magnetization components from the pumiceous and obsidian samples but only a single magnetization component from the inner crystalline rhyolite samples. Alternating field demagnetization is ineffective in isolating the magnetization components in the pumiceous and obsidian samples. The multiple components of remanent magnetization of the crust are interpreted to have been acquired during cooling as thermoremanent magnetizations. We suspect intermittent lava transport of the inner portions, the primary mode of rhyolite lava advancement, to be responsible for the presence of multiple components in pumice and obsidian of the lava crust. When the inner portions of the lava retain mobility to flow out of the crust, the solidified crust of the lava surface below the magnetite Curie temperature remains susceptible to deformation. Analysis of palaeomagnetic directions from the crust allows the deformation of the crust to be described in terms of rotation. Although the mode of rhyolite lava advancement is not well understood, because of its infrequent occurrence, our observations offer an important insight on how the mobile part of the lava is associated with the deformation of the crust during continued lava advance.

<p>The Hachiya Formation of the Kani Group in southern Gifu Prefecture is the oldest sediment in the eastern part of the Setouchi Geologic Province, and is composed mainly of volcaniclastic rocks deposited in a nonmarine basin. We present the results of new laser ablation-inductively coupled plasma-mass spectrometry U-Pb analyses of zircons separated from a sample of the Tochibora Welded Tuff Member in the lowest horizon of the Hachiya Formation. The analyses yielded a weighted mean ²³⁸U-²⁰⁶Pb age of 22.38±0.17 Ma (2σ), which constrains the time of initial sedimentation of the Hachiya Formation. The whole-rock major- and trace-element compositions of the welded tuff sample, analyzed by X-ray fluorescence, are also reported.</p>

Volcanic activity in the Aso area of central Kyushu, SW Japan, is divided into three stages based on caldera formation: a pre-caldera stage, a caldera-forming stage, and a post-caldera stage. We established the stratigraphy, and analyzed the whole-rock Sr and Nd isotopic compositions of volcanic products distributed in the NW wall of Aso Caldera to investigate the eruption style and magma-plumbing system of pre-caldera volcanism. Multiple andesitic lavas with minor dacitic lava make up a large part of the caldera wall. This eruption style of pre-caldera volcanism is difference from that of the caldera-forming stage, which is characterized by gigantic pyroclastic eruptions. The cause of this transition in eruption style from the pre-caldera to caldera-forming stages is probably a decrease in the rate of extensional crustal strain in the area. The whole-rock isotopic compositions of the pre-caldera volcanic products are largely similar to those of the caldera-forming volcanic products, indicating that the magmas of these volcanic products had a common origin. Previous petrological studies have suggested that the silicic magma of the caldera-forming stage originated from crustal melting; consequently, it is also considered that the magma of the Pre-caldera stage formed via this process. The pre-caldera volcanic products show a wide range of ⁸⁷Sr/⁸⁶Sr values (0.7042-0.7045), whereas the products of the caldera-forming stage show a lower and narrower range of values (0.7040-0.7042). This observation indicates isotopic heterogeneity of the crust and differences in the composition of the crust that melted during the pre-caldera and caldera-forming stages.

Pre-caldera volcanism in the Aso area (2.2–0.43 Ma), central Kyushu, is characterized by effusive eruptions of multiple lava flows. To clarify the chemical evolution of the magma chamber beneath the Aso area, we investigated the petrological characteristics of these lavas where exposed in the caldera wall.<br>The pre-caldera lavas are divided into eight types with distinct petrographic and compositional characteristics: A. cpx-ol basalt, B. ol-2px andesite, C. ol-hb-2px andesite, D. 2px andesite, E. hb-2px andesite, F. hb andesite, G. 2px-hb dacite, and H. bt-hb rhyolite. Incompatible trace element modeling demonstrated that these eight types did not originate via simple fractional crystallization.<br>The phase assemblages and abundances of phenocrysts of the pre-caldera andesite-rhyolite differ from those of the caldera-forming and post-caldera andesite-rhyolite. In addition, the pre-caldera andesite-rhyolite contain relatively low concentrations of incompatible trace elements compared to the caldera-forming and post-caldera andesite-rhyolite. These observations may indicate that the physical conditions and/or chemical compositions of the source materials that gave rise to the andesite-rhyolite magmas differed between the pre-caldera and caldera-forming stages.

Planetary aerosol laboratory experiments for science education were carried out in a curriculum of Kyoto University. Our goal is to reproduce "the blue sunset" on Mars which are reported from NASA's Mars Pathfinder. In reproducing the rays scattered by Martian atmosphere (dust storm) in a laboratory, the number density of scattering particles has to be as large as possible. Three experiments were conducted in the air and water. Although we were not able to reproduce Martian blue sunset, we elucidated its spectrum. Converting this spectrum to a color in the RGB system, we obtained R = 114, G = 122, B = 192. Though the experiment, we proved that planetary aerosol laboratory experiments are significantly fruitful for science education as well as for science studies. We propose that researchers and lecturers should make active use of planetary aerosol laboratory experiments for science education.

This study describes volcanic structures of four drilling cores obtained from the Takanoobane rhyolite lava distributed in Aso caldera, Japan and discuss on development of the structures of the lava. The Takanoobane lava is divided into three parts : Alternation of the pumiceous layers and the obsidian layers (the upper part), the crystalline rhyolite layer (the central part), and the obsidian layer (the lower part). This variation of internal structures is interpreted to have been formed as a result of cooling and degassing processes during flowage. The central crystalline rhyolite layer contains many minute cavity parts defined as crystalline with many minute cavities. The minute cavity parts were formed immediately before or after the extrusion. At distal drilling site, the minute cavity parts were flattened and show flow layering. The flow layering shows high tilt angle in upper part of the crystalline rhyolite layer, suggesting a ramp structure.

The drilling core (501.9 m in depth) has been obtained from Okogashima Island located in southern margin of the Aira caldera at northern end of Kagoshima Bay in southern Kyushu. The core reveals detailed eruption history prior to the Aira pyroclastic eruptions about 25,000 years ago by descriptions of lithofacies and fissiontrack dating. The three types of lithofacies are identified in the drilling core. They are (1) 2-8 m in depth: pyroxene andesite; (2) 8-253.2 m in depth: the Okogashima rhyolite lava flow; (3) 253.2-501.9m in depth: pyroclastic materials. This study focuses mainly on pyroclastic materials, which have not been described yet. The pyroclastic materials in lower levels are composed mainly of tuffaceous sand with cross lamination, containing shell fossils. They also contain pumice-concentration zone and welded tuff, indicating explosive eruption. The pumice-concentration zone occurs in 326-340 m (unit A), 354-360 m (unit B), 425-427 m (unit D), 449-450 m (unit E), and 478-497 m (unit F), while the welded tuff occurs in 360-380 m (unit C). The fission-track ages of around 0.4 Ma from the units B and C indicate that the deposits were erupted prior to the Aira pyroclastic eruptions. Therefore, this drilling core shows that at least sixth pyroclastic eruptions are identified in southern margin of the caldera prior to the Aira pyroclastic eruptions.