Volcanic processes revealed by geophysical observations of the 1977–1982 activity of USU volcano, Japan

Volcanic processes of Usu volcano, manifested for about 5 years in dacitic activity, are deduced from the results of long- and short-term observations of earthquakes and deformations. At the beginning stage of the activity, earthquakes and deformations appear abruptly and develop hyperbolically with time, resulting in the first outburst, whereas at the terminating stage, they disappear as abruptly.A qualitative model of visco-plasto-elastic property is proposed to interpret the volcanic processes, especially of the beginning and terminating stages. From the standpoint of the prediction of eruptions of such volcanoes, we should observe any nonmechanical forerunning phenomena for long-term prediction, and add mechanical ones such as earthquakes and deformations to these observations for short-term prediction.     

Petrographic features of a high-temperature granite just newly solidified magma at the Kakkonda geothermal field, Japan

A 3729-m-deep geothermal research well, WD-1a, provides us with a unique opportunity to study initial petrographic features of a high-temperature granite just after solidification of magma. The well succeeded in collecting three spot-cores of the Kakkonda Granite that is a pluton emplaced at a shallow depth and regarded as a heat source of the active Kakkonda geothermal system. The core samples were collected at the present formation temperatures of 370, 410 and over 500°C. These samples are granodiorite to tonalite consisting mainly of plagioclase, quartz, hornblende, biotite and K-feldspar. A sample collected at a formation temperature of over 500°C possesses the following remarkable petrographic features compared to the other two samples. Interstitial spaces are not completely sealed. K-feldspar exhibits no perthite by the exsolution of albite lamella. Quartz includes glassy melt inclusions without devitrification. Hornblende is less intensively altered to actinolite, and biotite is not altered. This study directly confirmed that perthite in K-feldspar is a recrystallization texture formed at 410–500°C based on a comparison of the in situ temperatures of the samples. Chemical compositions of minerals were analyzed to compare temperatures determined from geothermometers in several publications to the in situ temperatures of the samples.     

Mid- to- late Holocene hydroclimatic changes on the Chinese Loess Plateau: evidence from <Emphasis Type="Italic">n</Emphasis>-alkanes from the sediments of Tianchi Lake

We have reconstructed the history of mid-late Holocene paleohydrological changes in the Chinese Loess Plateau using n-alkane data from a sediment core in Tianchi Lake. We used P_aq (the proportion of aquatic macrophytes to the total plant community) to reflect changes in lake water level, with a higher abundance of submerged macrophytes indicating a lower water level and vice versa. The P_aq-based hydrological reconstruction agrees with various other lines of evidence, including ACL (average chain length), CPI (carbon preference index), C/N ratio and the n-alkane molecular distribution of the sediments in Tianchi Lake. The results reveal that the lake water level was relatively high during 5.7–3.2 ka BP, and decreased gradually thereafter. Our paleohydrological reconstruction is consistent with existing paleoclimate reconstructions from the Loess Plateau, which suggest a humid mid-Holocene, but is asynchronous with paleoclimatic records from central China which indicate an arid mid-Holocene. Overall, our results confirm that the intensity of the rainfall delivered by the EASM (East Asian summer monsoon) is an important factor in affecting paleohydrological changes in the region and can be considered as further evidence for the development of a spatially asynchronous “northern China drought and southern China flood” precipitation pattern during the Holocene.     

A migration model of magmatism explaining a ridge subduction, and its details on a statistical analysis of the granite ages in Cretaceous Southwest Japan

A migration model of magmatism based on the granite ages in Southwest Japan is proposed to explain the ridge subduction beneath the Eurasia continent as the cause of the along-arc and across-arc youngings of the granite ages and the very high activity of the magmatism in the Cretaceous. For the construction of the magmatic model, the localities of the granite age samples are denoted by the cartesian coordinates X and Y, which are measured along and normal to the Median Tectonic Line (MTL), respectively, and their ages are set corresponding to the coordinate Z vertical to the X–Y plane. The age trend is then formulated by a regression plane of Z on X and Y, which inclines in both directions along and normal to the MTL, and approximates the ages with the very high multiple correlation coefficient 0.91. Evaluating the magmatic trend by such a method, various characteristics of the activities can be taken easily; for example, the isochronous line of the magmatism, which is an intersection of the regression plane and an arbitrary horizontal plane, is found to extend landward obliquely across the continental margin. The migrating rate of the isochronous line along the MTL is also taken to be 2.8 cm/year as a reciprocal of the inclination of the along-arc younging. The isochronous line is speculated to be the out-cropped manifestation of the subcrustal linear heat source. Such a migrating linear heat source is probably due to the subduction of an active ridge, the Kula (or Izanagi)–Pacific ridge in the Cretaceous. The migration model of magmatism harmonizes very well with the plates and the ridge motions in the East Asia area during the late Mesozoic. The ridge subduction is one of the important phenomena that explain the unusually active arc magmatism and the migrating slab window; it is important to grasp dynamically the geological messages issued from the system.     

Fluvial to bay sequence stratigraphy and seismic facies of the Cretaceous to Paleogene successions in the MITI Sanriku‐oki well and the vicinities,the Sanriku‐oki forearc basin,northeast Japan

This paper describes the significant depositional setting information derived from well and seismic survey data for the Upper Cretaceous to Lower Eocene forearc basin sediments in the central part of the Sanriku‐oki basin, which is regarded as a key area for elucidating the plate tectonic history of the Northeast Japan Arc. According to the results of well facies analysis utilizing cores, well logs and borehole images, the major depositional environments were of braided and meandering fluvial environments with sporadically intercalated marine incursion beds. Seismic facies, reflection terminations and isopach information provide the actual spatial distributions of fluvial channel zones flowing in a north–south trending direction. The transgression and regression cycles indicate that the Upper Cretaceous to Lower Eocene successions can be divided into thirteen depositional sequences (Sequences SrCr‐0 to SrCr‐5, and SrPg‐1 to SrPg‐7). These depositional sequences demonstrate three types of stacking patterns: Types A to C, each of which shows a succession mainly comprising a meandering fluvial system, a braided fluvial system with minor meandering aspects in the upper part, and major marine incursion beds in the middle part, respectively, although all show an overall transgressive to regressive succession. The Type C marine incursion beds characteristically comprise bay center and tidal‐dominated bay margin facies. Basin‐transecting long seismic sections demonstrate a roll up structure on the trench slope break (TSB) side of the basin. These facts suggest that during the Cretaceous to Eocene periods, the studied fluvial‐dominated forearc basin was sheltered by the uplifted TSB. The selective occurrences of the Type C sequences suggest that when a longer‐scale transgression occurred, especially in Santonian and early Campanian periods, a large bay basin was developed, creating accommodation space, which induced the deposition of the Cretaceous Kuji Group along the arc‐side basin margin.     

Constraining sediment provenance for tsunami deposits using distributions of grain size and foraminifera from the Kujukuri coastline and shelf,Japan

Tsunami deposits preserved in the geological record provide a more comprehensive understanding of their patterns of frequency and intensity over longer timescales; but recognizing tsunami deposits can prove challenging due to post-depositional changes, lack of contrast between the deposits and surrounding sedimentary layers, and differentiating between tsunami and storm deposition. Modern baseline studies address these challenges by providing insight into modern spatial distributions that can be compared with palaeotsunami deposits. This study documents the spatial fingerprint of grain size and foraminifera from Hasunuma Beach and the Kujukuri shelf to provide a basis from which tsunami deposits can be interpreted. At Hasunuma Beach, approximately 50 km east of Tokyo, the spatial distribution of three common proxies (foraminiferal taxonomy, foraminiferal taphonomy and sediment grain size) for tsunami identification were mapped and clustered using Partitioning Around Medoids cluster analysis. Partitioning Around Medoids cluster analysis objectively discriminated two coastal zones corresponding to onshore and offshore sample locations. Results show that onshore samples are characterized by coarser grain sizes (medium to coarse sand) and higher abundances of Pararotalia nipponica (27 to 63%) than offshore samples, which are characterized by finer grain sizes (fine to medium sand), lower abundances of Pararotalia nipponica (2 to 19%) and Ammonia parkinsoniana (0 to 10%), higher abundances of planktonics (15 to 58%) and species with fragile tests including Uvigerinella glabra. When compared to grain-size and foraminiferal taxonomy, foraminiferal taphonomy; i.e. surface condition of foraminifera, a proxy not commonly used to identify tsunami deposits, was most effective in discriminating modern coastal zones (identified supratidal, intertidal and offshore environments) and determining sediment provenance for tsunami deposits at Kujukuri. This modern baseline study assists the interpretation of tsunami deposits in the geological record because it provides a basis for sediment provenance to be determined.     

Effects of alkali and alkaline-earth cations on the high-pressure sound velocities of aluminosilicate glasses

Physics and Chemistry of Minerals - A black tourmaline sample from Seagull batholith (Yukon Territory, Canada) was established to be a schorl with concentrations of Fe2+ among the highest currently...     

Separation of Heavy Lanthanoids by Flash Column Chromatography for Precise Determination of Er and Yb Isotope Compositions in Rock Samples

We present a new column chemistry technique for the quantitative separation of heavy lanthanoids by an ultra‐fine‐grained LN resin (20–50 µm) with a specific emphasis on the purification of Er and Yb for their isotopic analysis. To achieve the quantitative separation of Er and Yb within a reasonable timescale, flash column chromatography was applied, where the column was attached to a newly designed vacuum box system, thus accelerating the elution speed by ten times compared with that of the normal column procedure operated by gravity flow. The recovery yields of Er and Yb were confirmed to be approximately 100%, which is important to suppress the effect of the mass‐independent fractionation of the Er and Yb isotopes during chromatography. Additionally, we have developed precise Er and Yb isotope measurements by thermal ionisation mass spectrometry (TIMS) using multistatic and/or dynamic methods. Moreover, in most cases, the Er and Yb isotope compositions of the measured four terrestrial rock samples were indistinguishable from those of the commercially available Er and Yb Alfa Aesar solutions. The new method presented in this work will be useful for future studies on heavy lanthanoids in various geological materials.