In EPMA analysis of plagioclase in common igneous rocks, measured Fe content can be over-estimated by 15-50 % due to the effect of secondary fluorescence in Fe-bearing adjoining phase such as tholeiitic glass and fayalite, when distance between the analyzed point in plagioclase and adjoining phase is less than 40 μm. The present study reports empirical correction equation for measurements of Fe content in the plagioclase based on new experimental data. The magnitude of the over-estimation of Fe in plagioclase is larger than that of Ca in olivine. It is necessary to analyze carefully with respect to the chemical composition of adjoining phases and distance from the interface for quantitative measurements of Fe in plagioclase and Ca in olivine.
Hahajima, Bonin archipelago, is located approximately 1050 km away to the south-south east of Tokyo. Hahajima is the Tertiary volcano mainly composed of evolved island arc andesites and primitive volcanics also occur. The primitive volcanics can be classified into tholeiitic basalts and calc-alkaline basaltic andesites. Their genetical relationships were considered. Primitive tholeiites (TH-P) and calc-alkaline rocks (CA-P) show magnesian compositions (FeO*/MgO<1, Cr>700ppm, Ni>150ppm). They can be distinguished, however by assortment of groundmass pyroxenes. Euhedral olivines with Mg#>90 exist in the TH-P. The olivine should be the phenocrysts which crystallized from the liquid. Olivines in the CA-P could not be analyzed because of the serpentinized alteration. Their augite and Cr-spinel compositions are significantly magnesian. The CA-P is not distinguished from the TH-P by the Sr isotopic ratio. Cr-spinels in olivines in CA-P are systematically enriched in Cr than those in the TH-P. Therefore, the TH-P and CA-P would have originated from different primitive magmas. Judging from Cr-spinel compositions and REE modelings, the CA-P's parental mantle might be more depleted than the TH-P's. Hydrous partial melting experiments of lherzolite imply that the CA-P would have been produced under more hydrous condition than the TH-P. This inference is not contradictory to the CA-P's alkali and alkali-earth elements enrichment. The TH-P might have been produced at the deeper region of oceanic mantle where was not so refractory, while the CA-P might be produced at the shallower region where is relatively deplete. Incompatible trace elements characteristics of the TH-P and CA-P are very similar to those of the tholeiite and calc-alkaline volcanics from Funagata volcano (North east Japan arc). This feature might imply that the geochemical characteristics of relatively unevolved island arc volcanics could be explained by immutable process not according to the age or region of subduction.
In order to explain why Tl-silicate minerals are extremely rare in natural occurrences, Tl-polyhedral comparisons between crystal structures of Tl-minerals and their isotypes were attempted qualitatively using the eccentricities and sphericities defined by the polyhedral-distortion analysis of the software IVTON. The structure data for Tl-minerals usually confirm the presence of Tl 6s2 lone pair electrons causing distortion of the polyhedron: Tl+ exhibits stereoactive lone pair behavior. Inasmuch as the silicate structures afford no accommodation for extremely distorted polyhedra, the cations with the lone pair electrons responding as incompatible elements during the crystallization of silicate magmas can be finally crystallized as the sulphide minerals where these cations require no extreme distortion. The Hard-Soft Acid and Base (HSAB) principle holding for more stable complexes facilitates an effective explanation for the reason that thalliums prefer sulfurs to oxygens. Furthermore the rule in the valence shell electron pair repulsion (VSEPR) model, that a nonbonding pair occupies more space on the “surface” of the central atom than a bonding pair, gives a significance to the orientation of lone pair electrons in Tl restricted by the software IVTON. The lone pair electrons responsible for extremely distorted Tl-polyhedra, namely stereoactive lone pair behavior of Tl+, offer a substantial evidence for creating structural instability of the silicate minerals, leading to rare occurrences of these minerals containing the cation as a major component, resulting in about forty species of Tl-sulphide minerals. The presence of lone pair electrons imposing considerable constraints upon the kinds of Tl-minerals may provide a comprehensive explanation for mineralizing not as silicates but mainly as sulphides or sulphates in nature.
Yamamoto et al. (1999) obtained a low-temperature geotherm typical of continental shield from a “garnet peridotite” xenolith in the Arato-yama alkali basalt. Their geotherm is, however, possibly erroneous. Their data of the “garnet peridotite” suggest discrepancies from the chemical equilibration of minerals, and the pressure obtained is too large for the depth of xenolith entrainment by alkali basalt magma. The lack of thin section descriptions is also a cause of uncertainty.
In order to determine their Sr isotopic ratios, we have manually selected clinopyroxene grains taken from mantle peridotite xenoliths. The Arato-yama harzburgite sample was all crushed for hand-picking because it was too small to allow preparation of thin section. The garnet grains in this sample were found under a binocular microscope. The clinopyroxene of the above Arato-yama sample is relatively low in CaO and appears to have relatively high equilibrium temperature within the low temperature group peridotites (Fig. 2 of Yamamoto et al., 1999). The Arato-yama mantle peridotites as a whole, however, still belong to the low temperature group in the same way as do the Kurose, On-yama and Megata peridotites (Fig. 3 of Yamamoto et al., 1999). This result is consistent with the temperatures estimated by Arai and Hirai (1983) and Abe and Arai (2001) who applied the same geothermometer to the Noyama-dake (high temperature group), Kurose and Arato-yama mantle peridotites. Orthopyroxenes of the low temperature group peridotites (Arato-yama and other peridotites) are characteristically low in Al2O3. This suggests a different geothermal gradient from that of the Fukue-jima peridotites which include high Al2O3 orthopyroxenes even in shallow seated origin.