Microboudin structures of piemontite along the Sambagawa metamorphic belt, Japan: implications for lateral variation of differential stress during regional metamorphism

Evidence is presented of a lateral variation in differential stress during metamorphism along a regional metamorphic belt on the basis of the proportion of microboudinaged piemontite grains (p) in a quartz matrix in metacherts. It is proposed that p is a practical indicator of relative differential stress. Analysis of 123 metacherts from the 800 km long Sambagawa metamorphic belt, Japan, reveals that p‐values range from < 0.01 to 0.7 in this region. Most samples from Wakayama in the mid‐belt area have p‐values of 0.4–0.6, whereas those from western Shikoku have p‐values of < 0.1. This difference cannot be explained by variations in metamorphic temperature, and is instead attributed to a regional, lateral variation in differential stress during metamorphism.     

Zircon U–Pb ages of granitic and mafic dikes associated with granitoids and metamorphic rocks of the Ryoke Belt in the eastern Kii Peninsula,southwest Japan

Zircon U–Pb dates for felsic and intermediate to mafic dikes intruding into the Ryoke granitoids and metamorphic rocks at selected outcrops in the Takamiyama area of the eastern Kii Peninsula, southwest Japan, were determined along with their geology and petrography to reveal the history of Cretaceous magmatism. At each outcrop, the felsic and intermediate to mafic dikes exhibit specific structures that are indicative of magma intermingling and have coeval intrusion ages of ca. 81–77 Ma. Our zircon U–Pb data complement previously published data, suggesting that the mafic magmatism continued intermittently from 83 to 76 Ma in the Takamiyama area and that magmatism migrated eastward within the Ryoke Belt. A comparison of intrusion ages between a dike and a host Ryoke granitoid at one outcrop indicates that the host rock experienced ductile deformation at ~88 to ~83 Ma. Judging from the small number of zircons and the concordant date distributions, we didn't recognize the evidence suggesting the partial melting of the host rocks, as Nakajima et al. (Journal of the Geological Society of Japan, 2021, 127, 69–78) reported.     

Thermal-chemical instability in a pre-galactic gas cloud

The growth of thermal-chemical instability in a pre-galactic medium is followed by numerical simulation of gas dynamics. Results show that a primordial gas cloud breaks into self-gravitating subcondensations with the mass of normal stars.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Bar dissolution in non-spherical halos

Dynamical evolution of N-body bars embedded in spherical and prolate dark matter halos is investigated. In particular, the configuration such that galactic disks are placed in the plane perpendicular to the equatorial plane of the prolate halos is considered. Such a configuration is frequently found in cosmological simulations. N-body disks embedded in a fixed external halo potential were simulated, so that the barred structure was formed via dynamical instability in initially cool disks. In the subsequent evolution, bars in prolate halos dissolved gradually with time, while the bar pattern in spherical halos remained almost unchanged until the end of simulations. The e-folding time of bars suggest that they could be destroyed in a time smaller than a Hubble time. This revised version was published online in August 2006 with corrections to the Cover Date.     

Oxygen isotope characteristics of chondrules from the Yamato‐82094 ungrouped carbonaceous chondrite: Further evidence for common O‐isotope environments sampled among carbonaceous chondrites

High‐precision secondary ion mass spectrometry (SIMS) was employed to investigate oxygen three isotopes of phenocrysts in 35 chondrules from the Yamato (Y) 82094 ungrouped 3.2 carbonaceous chondrite. Twenty‐one of 21 chondrules have multiple homogeneous pyroxene data (?¹⁷O 3SD analytical uncertainty: 0.7‰); 17 of 17 chondrules have multiple homogeneous pyroxene and plagioclase data. Twenty‐one of 25 chondrules have one or more olivine data matching coexisting pyroxene data. Such homogeneous phenocrysts (1) are interpreted to have crystallized from the final chondrule melt, defining host O‐isotope ratios; and (2) suggest efficient O‐isotope exchange between ambient gas and chondrule melt during formation. Host values plot within 0.7‰ of the primitive chondrule mineral (PCM) line. Seventeen chondrules have relict olivine and/or spinel, with some δ¹⁷O and δ¹⁸O values approaching ?40‰, similar to CAI or AOA‐like precursors. Regarding host chondrule data, 22 of 34 have Mg#s of 98.8–99.5 and ?¹⁷O of ?3.9‰ to ?6.1‰, consistent with most Acfer 094, CO, CR, and CV chondrite chondrules, and suggesting a common reduced O‐isotope reservoir devoid of ¹⁶O‐poor H₂O. Six Y‐82094 chondrules have ?¹⁷O near ?2.5‰, with Mg#s of 64–97, consistent with lower Mg# chondrules from Acfer 094, CO, CR, and CV chondrites; their signatures suggest precursors consisting of those forming Mg# ~99, ?¹⁷O: ?5‰ ± 1‰ chondrules plus ¹⁶O‐poor H₂O, at high dust enrichments. Three type II chondrules plot slightly above the PCM line, near the terrestrial fractionation line (?¹⁷O: ~+0.1‰). Their O‐isotopes and olivine chemistry are like LL3 type II chondrules, suggesting they sampled ordinary chondrite‐like chondrule precursors. Finally, three Mg# >99 chondrules have ?¹⁷O of ?6.7‰ to ?8.1‰, potentially due to ¹⁶O‐rich refractory precursor components. The predominance of Mg# ~99, ?¹⁷O: ?5‰ ± 1‰ chondrules and a high chondrule‐to‐matrix ratio suggests bulk Y‐82094 characteristics are closely related to anhydrous dust sampled by most carbonaceous chondrite chondrules.     

The concept of tidal exchange and the tidal exchange ratio

The principal character of the tidal exchange process is neither diffusion nor advection, but a third category of transport, “Massenaustausch”, which appears in the space/time averaged transport. The exchange process can be divided into four fluxes: the flux of standing eddies, the flux of tidal exchange, the flux of tidal eddies and the flux of local eddies. The results of observations at the entrance channel of Kumihama-Bay show a typical example of transport dominated by tidal exchange. The tidal exchange ratio defined by Parkeret al. (1972) applies to the process of exchange between the outflowing watermass and the surrounding watermass outside of the bay mouth, but this should also be considered as being coupled with the ratio for the process of exchange between the inflowing watermass and the surrounding watermass inside of the bay mouth. These two exchange ratios can be combined into a single exchange ratio which describes the exchange process between the outer watermass and the bay water.     

Tidal residual circulation produced by a tidal vortex

“TIDAL VORTEX” is a term for a kind of starting vortex formed as a pair of vortices at the head of a tidal jet emanating from a narrow entrance into a bay. In this study, its formation and movement have been investigated by means of a hydraulic experiment and an analytical model. A tidal vortex is formed as a result of flow separation at the abrupt widening of a channel entrance followed by rolling up of the discontinuity surface around its free end. The vortex shows three types of life-history (type I, II and III), which are characterized by the Strouhal number and the aspect ratio of the horizontal shape of the entrance channel. In the case of type-I, the tidal vortex proceeds toward the inner region of the bay and there amalgamates with successive vortex cores into a core of tidal residual circulation. In the case of type-II, the tidal vortex core flows out into the entrance channel on the ebb but returns back into the bay on the subsequent flood. And, in the case of type-III, the tidal vortex core which was formed on the bay-side opening of the entrance channel flows out to the open sea and never comes back, whereas the core which was formed on the open-sea side of the entrance flows into the bay and never flows out. The circulation of a tidal vortex core is proportional to the reciprocal of the Strouhal number. The movement of the core near the bay entrance is determined by interaction between the cores and transportation due to the irrotational component of the tidal current. There are three types of tidal residual circulation, corresponding to three life-history types of tidal vortices. In the case of type-I, a strong tidal residual circulation is formed, but in type-II a small and weak circulation is formed. While, in type-III, the circulation having an inverse sense of rotation to that of type-I is formed.     

A pair of Langmuir cells in two laboratory tanks (II) on generation mechanism

Vertical and cross-wind profiles of mean currents were measured systematically in vertical cross-sections of two wind-wave tanks with aspect ratios of order one to study the secondary flow in the tanks. A pair of Langmuir cells turned out to be driven by a close combination of the pressure gradient along the tank and the side-wall effects. That is, part of the adverse pressure gradient produced a parabolic cross-wind profile with the smallest downwind current at the centerline and the largest current along the two sidewalls. As a result, upwelling occurred in the center zone where the return flow was strongest, probably because of the entrainment action of the wind-driven current. In order to compensate for this upwelling, downwelling occurred along the two side-walls from the flow continuity. The resulting vertical circulation formed a pair of Langmuir circulations across the span and served to maintain the parabolic profile formed by the pressure gradient. A positive feedback mechanism is thus found between the primary and secondary circulations through upwelling of the return flow in the center zone. Vertical shears of the span-averaged downwind current measured in two tanks were found to be systematically different from each other. This difference seems to depend on the magnitude of the advective Reynolds stresses in the two tanks.     

Spatial and temporal patterns of seagrass habitat use by fishes at the Ryukyu Islands,Japan

To investigate whether or not regional–temporal patterns of seagrass habitat use by fishes existed at the Ryukyu Islands (southern Japan), visual surveys were conducted in seagrass beds and adjacent coral reefs in northern, central, and southern Ryukyu Islands, in November 2004, and May, August, and November 2005, the northern region having less extensive seagrass beds compared with the central and southern regions. During the study period, the seagrass beds were utilized primarily by 31 species, the densities of some of the latter differing significantly among regions. With the exception of Apogonidae and Holocentridae, all species were diurnal and could be divided into 6 groups based on seagrass habitat use patterns; (1) permanent residents A (10 species, e.g. Stethojulis strigiventer), juveniles and adults living in seagrass beds as well as other habitats; (2) permanent residents B (5 species, e.g. Calotomus spinidens), juveniles and adults living only or mainly in seagrass beds; (3) seasonal residents A (4 species, e.g. Cheilodipterus quinquelineatus), juveniles living in seagrass beds as well as other habitats; (4) seasonal residents B (6 species, e.g. Lethrinus atkinsoni), juveniles living only or mainly in seagrass beds; (5) transients (5 species, e.g. Parupeneus indicus), occurring in seagrass beds in the course of foraging over a variety of habitats; and (6) casual species (1 species, Acanthurus blochii), occurring only occasionally in seagrass beds. Regarding temporal differences, juvenile densities in each group were high in May and August compared with November in each region, whereas adult densities did not differ drastically in each month. For regional differences, juvenile and adult densities of permanent residents A and B were higher in the southern and central regions than in the northern region. Moreover, some seasonal residents showed possible ontogenetic habitat shift from seagrass beds to coral reefs in each region. These results indicated that seagrass habitat use patterns by fishes changed temporally and regionally and there may be habitat connectivity between seagrass beds and coral reefs via ontogenetic migration in the Ryukyu Islands.