Chichijima, Bonin archipelago, is a Tertiary volcanic island approximately 1000 km south-south east of Tokyo, and has been known as the type locality of boninite. Two types of boninite are found in the Bonin archipelago: one is high-Ca boninite from the Mikazukiyama Formation (Mk-HCB) and the Marubewan Formation (Mr-HCB) in Chichijima, and another is low-Ca boninite from the Marubewan Formation (Mr-LCB), Anijima (Ani-LCB), and Mukojima (Muko-LCB). High-Ca boninite is distinguished by CaO content from dominant low-Ca boninite, and contains augite phenocrysts. The bulk MgO, Ni and Cr contents and the magnesian compositions of olivines and pyroxenes in Mk-HCB and Mr-LCB indicate their primitive nature. Bronzite phenocrysts in Mk-HCB and Mr-HCB are highly enriched in Wo molecule, implying existence of high-Ca primitive boninite magmas. Mr-HCB is a mixing product between a high-Ca boninite magma and a differentiate from Mr-LCB. Sr isotopic ratios and incompatible element ratios of Mr-HCB, Mr-LCB, Ani-LCB and Muko-LCB are nearly the same and therefore they could have been produced from the common source. In contrast, Mk-HCB has different Sr isotopic and incompatible element ratios from the others and then it may have been produced from different parental materials. Both REE model calculations and Cr-spinel compositions imply that the parental mantle of the Mr-LCB is more depleted than that of the Mk-HCB. The large enrichment of LILE and LREE in Mr-LCB compared to Mk-HCB suggests that Mr-LCB has been generated under more hydrous condition than Mk-HCB.
Whole-rock K-Ar dating for a basaltic andesite lava from the Uegusukudake Formation in Kume-jima, Central Ryukyu Arc, yields age of 2.13±0.19 Ma. The sample dated is from the middle horizon of the Formation. This age is compatible with recently published K-Ar ages of 2.76±0.48 and 2.24±0.10 Ma for a picrite basalt from the lowest part and an andesite from the upper part of the Formation, respectively. The age, however, is inconsistent with the previously reported K-Ar ages of 5.5 and 4.6 Ma. The result is also concordant with ages estimated from Sr isotopic ratios for molluscan fossils (∼3.2-3.1 Ma) for the Aka Formation underlying the Uegusukudake Formation. This result suggests that tholeiitic basalts and basaltic andesites erupted after the eruption of picrite basalts with a short time interval at the late Pliocene.
A micro-Raman spectrometer is a very useful device to identify and characterize small crystals in natural and synthetic samples. Raman spectra are taken also at high pressure using mainly a diamond anvil cell. However, it should be careful to select diamond in order to avoid fluorescence. This article describes some technical information and several examples of Raman spectroscopy.
Extinction angle can be calculated with using spherical trigonometry and Biot-Fresnel's law. The variation curves of extinction angle are useful for the determination of twinning laws, estimation of angle between crystallographic axis and optic elastic axis, and analyses of petrofabric. This paper describes how to draw the variation curves of extinction angle using spread sheet-type calculation soft ware in a personal computer.