Exsolution textures in orthopyroxene in aluminous granulites as indicators of UHT metamorphism: New evidence from the Eastern Ghats Belt, India

Highly aluminous orthopyroxene, coexisting with sapphirine, cordierite, sillimanite, quartz and garnet in various combinations, constitute granoblastic mosaic peak metamorphic assemblages in aluminous granulites from three localities in the Eastern Ghats Belt, India. Orthopyroxene contains four types of intergrowths: (a) involving sapphirine with or without cordierite, (b) involving spinel, but without sapphirine, (c) involving cordierite, but without sapphirine and spinel, and (d) involving garnet, without sapphirine, spinel or cordierite. On the basis of textural and compositional data, origin of the intergrowths is ascribed to breakdown of Mg-Tschermak component, locally also involving Fe- and Ti-Tschermak. An attempt is made to compute the “pre-breakdown” compositions of orthopyroxene by image analysis, which shows maximum Al₂O₃ content of 13.4 wt.% in the pristine orthopyroxene. Geothermometry, phase equilibria consideration and application of existing experimental data on alumina solubility in orthopyroxene coexisting with sapphirine and quartz, collectively indicate extreme thermal conditions of metamorphism (> 1000 °C) for the studied assemblages. This re-affirms the notion that Al₂O₃ solubility in orthopyroxene is the most powerful indicator of UHT metamorphism (Harley, S.L., 2004. Extending our understanding of ultrahigh temperature crustal metamorphism. J. Mineral. Petrol. Sci. 99, 140–158). The intergrowths are considered to have formed due to cooling from the thermal peak spanning a temperature range of approximately 150 °C. Appearance of diverse types of intergrowths is probably related to subtle differences in bulk composition, particularly Fe:Mg ratios.     

Determination of Ti, K, Sm and Gd Values in Geological Survey of Japan Reference Materials by Prompt Gamma Neutron Activation Analysis

Prompt gamma neutron activation analysis was applied to the determination of the titanium, potassium, samarium and gadolinium contents of nine Geological Survey of Japan (GSJ) reference materials (JB-1, 1a, 2, 3; JA-1, 2, 3; JR-1, 2). Firstly, the values in JB-1 were determined by the standard addition method: pressed powder disks of JB-1 were used for neutron irradiation and gamma-ray measurements, after known quantities of standard reagents had been added. Secondly, the contents of eight other reference materials were determined by comparison methods using JB-1 as the comparative standard. The precision of analyses were obtained by replicate determinations on these samples. The relative standard deviation was generally less than 5%. For most samples, analysed values agreed well (< 5%) with the recommended values.     

Speciation of Chromium in Artificially Contaminated Soil Reference Material GSJ JSO-2 Using XANES and Chemical Extraction Methods

The oxidation states of chromium in GSJ JSO-2 (artificially contaminated soil) and three other geochemical reference materials (GSJ JSO-1, JLS-1 and JMS-1) were observed using X-ray near edge structure (XANES). For comparison, other artificially contaminated soil materials (mimic-JSO-2) were prepared by adding Cr(VI) into JSO-1. Their oxidation states of chromium were determined using XANES. The chromium contents were 1118 μg g^-1 for JSO-2, 1352 μg g^-1 for mimic-JSO-2 and 69-113 μg g^-1 for the other reference materials. Most chromium was present as hexavalent in mimic-JSO-2. No hexavalent species were detected in other samples. These results for chromium oxidation state in JSO-2 and mimic-JSO-2 obtained with XANES resembled those obtained from a chemical extraction method. The present JSO-2 has no trace of Cr(VI), although Cr(VI) was added as a major species during preparation. On the other hand, the content of Cr(VI) obtained in mimic-JSO-2 agreed with the original Cr(VI) content. A time-elapse study showed that Cr(VI) contents in mimic-JSO-2 decreased gradually to 70% of the original abundance during 240-day preservation in dry conditions. Moreover, the abundance of Cr(VI) decreased markedly to 15% after 240 days in the wet mimic-JSO-2 containing 20% m/m of water. These experiments suggested that soil humidity enhanced the reduction of Cr(VI) and that Cr(VI) was reduced even in dry conditions. Consequently, it is reasonable to infer that Cr(VI) doped into JSO-2 was completely reduced to Cr(III) during the preservation period of 5 years. The certification of the long-term stability of the chemical form in reference materials will be much more important in future.     

Middle Miocene to Quaternary primary basalt and high magnesian andesite magmas of North Hokkaido, Japan: Source mantle characteristics and degrees of partial melting

Abstract   Middle Miocene to Quaternary primitive basalts and high magnesian andesite (HMA) in North Hokkaido resulted from three periods of intense volcanism; early-stage (12–10 Ma), middle-stage (9–7 Ma) and late-stage (3–0 Ma). Based on the chemical compositions of olivines and chromian spinels and bulk chemistry of the primitive rocks, we examined depths of segregation of the calculated primary magmas and the degrees of partial melting of the source mantle. In the context of asthenospheric mantle upwelling, petrological data from the present study can be accounted for by the secular change in the depth of magma segregation from the upwelled asthenospheric mantle, which is composed of fertile peridotite. Thus, the early-stage primary magmas were generated by higher degrees of partial melting of the shallower part of hot asthenospheric mantle, whereas the middle- and late-stage primary magmas resulted from lower degrees of partial melting of a deeper part of the asthenospheric mantle. The early-stage HMA magma was generated by partial melting of the remnant subcontinental lithospheric mantle composed of refractory peridotite. This melting might have resulted from an increased geothermal gradient caused by upwelling of hot asthenosphere.     

Geochemical characteristics of Tertiary Sagara oil from an active forearc basin, Shizuoka, Japan

Abstract   The Sagara oil field is located in the Neogene Kakegawa Basin, close to the Izu collision zone at the junction between the main Japanese Islands and the Izu–Bonin Arc. The Sagara oil field is one of the few oil fields situated in a forearc basin on the Pacific side of Japan and is present in a sedimentary basin with poor oil-generating potential. Several crude oils from Sagara oil field were investigated to infer their origin. Organic geochemical characteristics of Sagara oils showed the influences of light biodegradation, migration-contamination, and migration-fractionation. The maturity levels of Sagara oils evaluated based on abundant alkylnaphthalenes corresponded to 0.9–1.2% vitrinite reflectance. Sagara oils were characterized by significant amounts of higher plant biomarkers, a high pristane/phytane ratio and an absence of organic sulphur compounds, suggesting a siliciclastic source rock deposited under nearshore to fluvial–deltaic environments. Numerous faults and fractures in the active forearc basin provided excellent conduits and facilitated upward migration of light hydrocarbons generated at greater depth in the Kakegawa Basin.     

Short-range prediction experiments of the Kuroshio path variabilities south of Japan

Assimilation and prediction experiments of the Kuroshio path variability south of Japan were conducted to investigate the predictability of the Kuroshio path. The assimilation and prediction system is composed of an eddy-resolving model and a three-dimensional variational analysis scheme with vertical coupled temperature–salinity empirical orthogonal function modes. The sea surface height (SSH) variability and the variations of the Kuroshio path of the assimilation fields are in good agreement with those observed. The results of the assimilation are then used as the initial conditions for 138 cases of 90-day prediction experiments conducted from 1993 to 2004. The predictive limit of our system is assessed by the SSH anomaly in the assimilation field and is found to be around 40–60 days, which is much longer than that of the persistence. The prediction results show good performance in the transition stage from a straight to a meandering path. For example, a large meandering event that occurred in August 2004 is successfully predicted in a 2-month forecast. Two types of failure cases are investigated. One is a case where the eastward propagation speed of the meander is faster than a real state. The dynamical response of the model to the assimilation revealed that an initial shock, caused by the dynamically unbalanced initial condition, induces the fast eastward propagation of the meander. The other case exhibits an unrealistic meander. In this case, a cold anomaly at an intermediate layer in the initial condition grows rapidly and results in the unrealistic meander. This implies that the Kuroshio path south of Japan has a chaotic nature. These facts revealed by the failure cases give us some insight for improving the predictive skill of the Kuroshio path variability.     

Temporal Variations of Seismic Wave Velocity Associated with 1998 M6.1 Shizukuishi Earthquake

—?We attempt to detect temporal variations of seismic wave velocity before and after 1998 M6.1 Shizukuishi, northeastern Japan, earthquake by using waveform data from explosions and earthquake doublets spanning the period immediately before and after the earthquake. Direct P waves of the second explosion are delayed by ～20 ms at observation stations with epicentral distances less than 15 km. This tendency does not change if the analysis frequency band is changed. Our result suggests that the P-wave velocity decreased by at least 1% in the extremely shallow region of the hanging wall of the M6.1 thrust event after its occurrence. On the other hand, there was the frequency dependence of the coda wave delays for both artificial sources and for natural events. At 5–10 Hz, immediate sharp increases by more than 20 ms in time delays and lower coherency were observed at several stations. We estimated the region in which P-wave velocity might have decreased after the M6.1 earthquake. Maximum depth of the region is 13 km. The region includes the aftershock area of the M6.1 earthquake, but is eccentric to the earthquake in the west. Considering the frequency band analyzed (5–10 Hz), the scale of the spatial inhomogeneity which led to the coda wave delay is several hundreds meters. We investigated candidates for the cause of the direct P-wave and coda wave delay. Observed direct P-wave delay can be partly explained by the stress changes caused by coseismic fault slip. However, the coda wave delay cannot be explained by the stress changes that are limited to the superficial area. Crustal heterogeneity should have changed at coseismic time in the deeper area where aftershocks of the M6.1 earthquake occurred.     

Cloning and sequencing of the allophycocyanin genes fromSpirulina maxima (Cyanophyta)

The genes coding for the α-and β-subunit of allophycocyanin (apcA andapcB) from the cyanophyteSpirulina maxima were cloned and sequenced. The results revealed 44.4% of nucleotide sequence similarity and 30.4% of similarity of deduced amino acid sequence between them. The amino acid sequence identities betweenS. maxima andS. platensis are 99.4% for α subunit and 100% for β subunit.

     

Transposable genetic elements inSpirulina and potential applications for genetic engineering

Transposable elements in cyanobacteria are briefly reviewed. Evidence is presented to show that transposable elements inSpirulina platensis is actually reflected on the phenotype change, i e., helical to straight filaments. Transposition intermediates of DNA were isolated from the extrachromosome and the transposition was related to helical variations inSpirulina. Uses of transposable elements for microalgal recombination are discussed based on the transposition mechanism.