We analyzed the Sr-Nd isotopic and geochemical compositions of 112 late Cenozoic volcanic rocks from the northern Fossa Magna, central Japan. The volcanic rocks are divided, on the basis of Nd isotopic compositions, into a high-Nd group (HND) and a low-Nd group (LND). The HND has higher Zr/Nb and Hf/Nb ratios than the LND volcanic rocks. The isotopic ratios and high field strength element (HFSE) ratios indicate that the HND volcanic rocks were derived from a relatively depleted mantle compared with the LND rocks. The relationship between the HND and LND rocks, in terms of their isotopic and Zr/Nb, Hf/Nb ratios, is similar to that observed between Quaternary rear-arc and frontal-arc volcanic rocks of the NE Japan arc. The rear-arc rocks from NE Japan have higher Nd isotopic and HFSE ratios than the frontal-arc rocks, indicating the injection of depleted asthenospheric mantle beneath the rear-arc, associated with Miocene opening of the Sea of Japan. The HND rocks occur in the central part of the Fossa Magna region in the form of a NW-trending wedge. This wedge shape of the HND area is consistent with the wedge-shaped rifting that occurred perpendicular to the strike of the arc and that opened the Fossa Magna contemporaneously with the opening of the Sea of Japan. The Sr and Nd isotopic compositions of the HND and LND rocks fall within the mantle array and extend into the high-Sr isotopic field. Some of the LND rocks contain lower-crustal mafic xenoliths with Sr-Nd isotopic compositions that overlap those of the LND rocks. In the Sr-Nd isotope diagram, the trend of the HND rocks can be extrapolated to the most radiogenic Sr isotopic compositions obtained for the xenoliths. This result suggests that the lower-crustal materials, which have the most radiogenic Sr isotopic compositions, contributed to the Sr isotopic enrichment of the LND and HND magmas.
The chemical compositions and oxidation states of Fe in olivine phenocrysts in scorias from Kami-Kometsuka, Aso, Kumamoto Prefecture, Japan, were analyzed to evaluate effect of high temperature oxidation on the scoria. The rocks in the interior of the Kami-Kometsuka scoria cone are red-brown and weakly welded, whereas black scoria occurs on the upper zone of the cone. Olivine phenocrysts within the black scoria lack precipitate minerals, but those exposed on the voids contain small amounts of precipitates at their rims. Olivine phenocrysts in the red-brown scoria contain abundant cryptocrystalline precipitates including hematite, magnetite and enstatite. Olivines in the black scoria have normal zoning with Fo87 cores and Fo68 rims and reverse zoning with Fo66 cores and Fo70 rims, whereas those in the red-brown scoria reach 99 mol% Fo. By applying the relationship between FeLβ/FeLα-intensity ratios and Fe3+/∑Fe, the Fe3+/∑Fe of the olivine phenocrysts in the black scoria were determined to be 0-1(2)% at the cores and 3(2)% at the rims. The Fe3+ at the rims of the olivine phenocrysts in the black scoria and the cryptocrystalline precipitates and extremely high Fo contents within olivine phenocrysts in the red-brown scoria are due to high temperature oxidation.
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April 11, 2015
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