The superstructure of plagioclase feldspars

A modulation function representing the position and density of (Na, Ca) atoms in the superstructure of the e-plagioclase has been derived from the average structures of different plagioclase and a general modulation theory. Based on this function the superstructure of bytownite (An₇₃) has been studied with the single crystal X-ray method. The cell dimensions by Megaw's axes are a=7.946(3)A, b=67.09(2)A, c=12.236(4)A, α=39.03(1)°, β=45.63(1)° and γ=59.63(1)°. Z=18(Na, Ca) Al(Al, Si)Si₂O₈. The initial phase factor of the modulation function for bytownite has been obtained from the intensity data of the satellite reflections. This modulation function indicates a coherent small-scale alternation of the Na-rich and Ca-rich bands in the superstructure. This superstructure has been refined by applying the albite and anorthite structures to the Na-rich and Ca-rich bands, respectively. The change of the superstructure of the e-plagioclase due to the compositional change has been described based on the movements of the satellites in reciprocal space. The direction of the coherent small-scale intergrowth of the anorthite-like and albite-like bands is perpendicular to the t vector. The thickness of the intergrowth is 1/|t|. Both direction and thickness change regularly from An₇₅ to An₂₅.     

Rare gases in a phlogopite nodule and a phlogopite-bearing peridotite in South African kimberlites

Rare gas isotopes in a phlogopite nodule and a phlogopite-bearing peridotite nodule in South African kimberlites were studied to examine the state of rare gases in the deep interior of the kimberlite region.Within the experimental error of 1 ～ 2%, rare gas isotopic compositions are atmospheric except for radiogenic⁴He and⁴⁰Ar. No excess¹²⁹Xe was observed.In phlogopite, Ne is more depleted, whereas the heavier rare gases are more enriched than the atmospheric rare gases relative to³⁶Ar.Together with other data these results suggest that the state of rare gases in the upper mantle of the South African kimberlite region might have been changed from the typical primitive mantle by a process such as mixing of crustal materials.     

Rare gas isotopic compositions in natural gases of Japan

Isotopic and elemental compositions of rare gases in various types of gas samples collected in the Japanese Islands were investigated. Excess³He was found in most samples. Many samples showed a regionally uniform high³He/⁴He ratio of about 7 times the atmospheric ratio. The He concentrations varied from 0.6 to 1800 ppm, and they were low in CO₂-rich gases and high in N₂-rich gases. Ne isotopic deviations from the atmospheric Ne were detected in most volcanic gases. The deviations and the elemental abundance patterns in volcanic gases can be explained by a mixing between two components, one is mass fractionated rare gases and the other is isotopically atmospheric and is enriched in heavy rare gas elements. Ar was a mixture of mass fractionated Ar, atmospheric Ar and radiogenic Ar, and the contribution of radiogenic⁴⁰Ar was small in all samples. Except for He, elemental abundance patterns were progressively enriched in the heavier rare gases relative to the atmosphere. Several samples were highly enriched in Kr and Xe relative to the abundance pattern of dissolution equilibrium of atmospheric rare gases in water. The component which is highly enriched in heavy rare gases may be released from sedimentary materials in the crust.     

Theoretical analysis of luminosity drop in red giant branch evolution

Recent evolutionary computations of stars in the globular clusters show that the star ascending up the red giant branch decreases its luminisity temporarily when the outward advancing hydrogen-burning shell passes over the discontinuity in the chemical compositions, which is the trace of maximum inward penetration of the convective envelope in the previous subgiant stage. To understand the physical circumstance of this luminosity drop, the semi-analytical approach using the classical red giant model was applied. The difference of the present analysis from those with usual models of the red giant is that the gradient and the discontinuity in the hydrogen profile, which were usually neglected in the classical red giant model, in the region into which the hydrogen-burning shell advances were taken into account explicitly. It was found that the luminosity would always drop at the critical stage, the stage when the hydrogen-burning shell just passes over the discontinuity. It was also pointed out that the luminosity drop depends on the time step chosen for constructing the stellar evolutionary models and computations with crude time step would fail to catch up this phenomenon.     

The 1998 Papua New Guinea Earthquake and its Fault Plane Estimated from Relocated Aftershocks

— The 1998 Papua New Guinea earthquake of M _w 7.0 occurred near the Wewak trench where the North Bismarck plate is subducting beneath the Australian plate. Its mechanism is thrust-type, and one of the nodal planes is almost parallel to the plate interface. To determine which of the two nodal planes of the main shock is the fault plane, we relocated the main shock and aftershocks using a method of modified joint hypocenter determination. We combined and employed two types of data in this study. Firstly, we used data reported by the National Earthquake Information Center (NEIC) of the U.S. Geological Survey (USGS), which includes three stations at the northeastern edge of Irian Jaya and one station in northern Papua New Guinea, from which the epicentral distances are less than 2 degrees. Secondly, in addition to the above permanent-station data, we used data from temporary aftershock observations near the epicentral area around the Sissano Lagoon carried out by Tsuji et al. (1998). Using three-component seismometers, they carried out observations from August 2 to October 2, 1998 at three sites. Although the network did not record the main shock and immediate aftershocks, the data obtained by temporary observation sites can clearly assist in identifying their absolute locations, since it is possible to apply the joint hypocenter determination (JHD) method. Hypocenters were relocated between the coastline and the Wewak trench, distributed along a nodal plane dipping shallowly to the southwest. Therefore, we can conclude that this nodal plane is the main shock fault and that the 1998 Papua New Guinea earthquake was an interplate earthquake between the North Bismarck and Australian plates.     

A 100 m laser strainmeter system installed in a 1 km deep tunnel at Kamioka, Gifu, Japan

We have installed a laser strainmeter system in a deep tunnel about 1,000 m below the ground surface at Kamioka, Gifu, Japan. The system consists of three types of independent interferometers: (1) an EW linear strainmeter of the Michelson type with unequal arms, (2) an NS-EW differential strainmeter of the Michelson type with equal arms and (3) a NS absolute strainmeter of the Fabry–Perot type. These are configured in L-shaped vacuum pipes, each of which has a length of 100 m. (1) and (2) are highly sensitive (order of 10⁻¹³ strain) and have wide dynamical range (10⁻¹³–10⁻⁶). Observations with strainmeters (1) and (2) started on June 11, 2003. (3) is a new device for absolute-length measurements of the order of 10⁻⁹ of a long-baseline (100 m) Fabry–Perot cavity by the use of phase-modulated light. This third strainmeter will be ready for operation before the end of 2004. The laser source of strainmeters (1) and (2) is a frequency-doubled YAG laser with a wavelength of 532 nm. The laser frequency is locked onto an iodine absorption line and a stability of 2 × 10⁻¹³ is attained. The light paths of the laser strainmeter system are enclosed in SUS304 stainless steel pipes. The inside pressure is kept to be 10⁻⁴ Pa. Consequently, quantitative measurement of crustal strains of the order of 10⁻¹³ can be attained by employing the laser strainmeter system of (1) and (2) at Kamioka. This resolving power corresponds to that of a superconducting gravimeter. Using the laser strainmeter system, we expect to determine parameters related to fluid core resonance, core modes and core undertone as well as other geodynamic signals such as slow strain changes caused by silent earthquakes or slow earthquakes.     

Timing of the cataclastic deformation along the Akaishi Tectonic Line,central Japan

This paper describes K-Ar ages of cataclasites and fault gouges from the Akaishi Tectonic Line (ATL), central Japan. Petrological and mineralogical features of these rocks are also examined. Based on the results, we test the hypothesis that the K-Ar ages of these rocks represent the age of hydrothermal alteration associated with the fault movement. Intensity of deformation and alteration increase passing from host rock into cataclasite and finally into fault gouge. This increase corresponds to an increase of the value of crystallinity index (Kübler index) of the micaceous minerals contained in these rocks. Furthermore, the degree of rejuvention of K-Ar ages of the micaceous minerals increases in the same sense. A correlation of K-Ar ages (plotted on the y axis) versus Kübler index (plotted on the x axis) yields a concave curve asymptotically parallel with the x axis at approximately 15 Ma. This curve is interpreted to represent the mode of decrease of the relative amounts of inherited argon in the K-Ar system of the micaceous minerals, corresponding to an increase in the intensity of deformation and alteration. Inclination of the curve becomes zero if all the inherited argon is lost from the K-Ar system of the micaceous minerals. Thus, it is concluded that the hydrothermal alteration occurred at approximately 15 Ma. The strike-slip basin along the ATL formed the middle Miocene. The K-Ar dating of ATL gouges indicates that the ATL was active simultaneously with formation of the strike-slip basin along its trace during the middle Miocene.     

A trial prediction of the values of some surface-layer turbulence constants using Schumann's method for promoting realizability

Based on a suggestion by Yamada (1986), values of the unknown constants in a particular second-order closure model are determined by applying Schumann's method (Schumann, 1977), using empirical data under neutral conditions. It is found that this closure model predicts favorably the Monin-Obukhov universal functions on both the stable and the unstable sides. It is likely that Schumann's method can be used as an expedient means of determining values of unknown model constants.     

Electron microscopy of clinoenstatite from a boninite and a chondrite

Clinoenstatite crystals from a boninite and the Yamato-74191 chondrite have been studied with an analytical electron microscope. (100) twins and cracks perpendicular and parallel to the c axis are characteristic of their submicroscopic textures. The frequency in appearance along the c axis and widths of the cracks have been explained by the dimensional change of the c axis in the direct transformation of protoenstatite to clinoenstatite and by the cooling rate around the transformation temperature. The cracks in the crystals from the boninite are filled with fibrous crystals of talc, while those from the chondrite are open or filled with glass in which fine crystals of plagioclase are common.