Abukuma plutonic rocks in the central to western Abukuma plateau have been divided into ‘older' (weakly foliated and intermediate) and ‘younger'(massive and felsic) granitic rocks, as well as small amounts of gabbroic to dioritic rocks, based on field occurrences and their lithofacies. Although many radiometric ages have been reported for these rocks, it remains unclear whether the gabbroic to dioritic rocks represent the first stage of granitic magmatism or are pre-Cretaceous basement rocks. Moreover, the geochronological discrepancy in K-Ar biotite ages between ‘older' and ‘younger' granitic rocks are still ambiguous. These problems are expected to be resolved by the U-Pb dating of zircon, which has a significantly higher closure temperature, making this one of the best ways to estimate the crystallization age of the plutonic rocks.
  We have determined U-Pb zircon ages for six samples of Abukuma plutonic rocks using laser-ablation ICP mass spectrometry. We found that these ages were 104.9 ± 0.9 Ma for the Utsushiga-take gabbroic body, 100.4 ± 0.7 Ma for the Nagaya body, 113.4 ± 0.5 Ma for the Shikayama body, 106.7 ± 0.8 Ma for the Ishimori body, 118.0 ± 0.7 Ma for the Miharu body and 101.9 ± 1.6 Ma for the Hatsumori body.
  These results suggest that the Utsushiga-take gabbroic body did not result from first stage granitic magmatism or from magma contaminated by pre-Cretaceous basement rocks, but rather that gabbroic magmatism in this district occurred during the same stage as granitic magmatism. Although our results showed clear geochronological contrast among each granitic rock, there was no significant difference between ‘older' and ‘younger' granitic rocks. These findings indicate that the previous classification system, based only on lithofacies, should be re-examined based on other criteria, such as further field observations of intrusive relationships and/or U-Pb dating of zircon.
  The cooling histories of each granitic rock were also estimated by K-Ar, Ar-Ar and U-Pb age. We found that the minimum cooling rate of the Utsushiga-take gabbroic rock at relatively higher temperatures (750-530 °C) was more rapid (~ <200 °C/m.y.) than at lower temperature (530-310 °C) and the other granitic samples (∼ 10-70 °C/m.y.).

View full abstract

The local structure around zinc atoms in Cretaceous-Tertiary (K-T) boundary clay from Stevns Klint, Denmark, was studied by Zn K-edge XAFS spectroscopy. XAFS measurements were performed at the BL-12C and BL-9C beamlines at the Photon Factory in the High Energy Accelerator Research Organization (KEK), Japan. The local structure around Zn in the K-T clay resembles the framework structure of the tetrahedral ZnO₄ site in the zinc silicate willemite, judging from first shell Zn-O and second shell Zn-cation distances [1.953(3) and 3.51(2) Å, respectively], X-ray absorption near edge structure (XANES) spectra, and radial structure function. The Debye-Waller factor σ² of the K-T clay sample is similar to those for the tetrahedral sites in crystalline phases, where Zn occupies mainly tetrahedral sites. The local structure of Zn in the K-T clay is peculiar and differs from that found in many clay minerals.

View full abstract

High-pressure and high-temperature phase relations for Mg₃Cr₂Si₃O₁₂ have been studied at pressures from 8 to 16 GPa and temperatures of 1200-1800 °C using a Kawai-type multianvil apparatus. The low-pressure phase assemblage of MgSiO₃ pyroxene + Cr₂O₃ eskolaite was found to transform into a high-pressure phase assemblage of majoritic knorringite + eskolaite with a negative phase boundary of dP/dT = -0.010 GPa/°C. No pure Mg₃Cr₂Si₃O₁₂ knorringite garnet was observed over the entire range of the P-T conditions in the present study, suggesting that pure knorringite garnet may not be a stable phase under these pressure and temperature conditions. This result is inconsistent with earlier studies, where knorringite was reported to be stable at pressures higher than ∼ 10 GPa. On the other hand, the present result agrees well with recent results for the synthesis of majoritic knorringite at pressures of 11 and 14 GPa and at temperatures of 1500-1600 °C. Majoritic knorringite becomes more Cr-deficient with increasing pressure, which is similar to the Al-deficient nature of majoritic garnet in the MgSiO₃-Al₂O₃ system.

View full abstract

Pyrrhotite (Po) occurs as inclusions and as isolated crystals in pumice from the 1914-15 eruption of the Sakurajima volcano, Kyushu, Japan. The Po crystals have partly reacted to form spongy Fe oxides. A similar texture has been reported in some previous studies (Hattori, 1993), but the mineral phases and formation processes of the spongy Fe oxides have not been clarified. Our quantitative and compositional map analyses with electron probe microanalysis (EPMA) reveal that the spongy Fe oxides are mostly magnetite (Mt), with a thin rim (<3 μm) of hematite on rare occasions. The spongy texture includes unreacted regions of Po, mesh-like pores, and S-rich spots, showing that it was formed by desulfidation of Po. No Ti was detected, even in the outermost rim; this indicates that the reaction occurred syn-eruptively. According to diffusion calculations, the spongy Mt was formed during the 4 h preceding quenching. Thermodynamic calculations showed that Po is stable at log f_O2 < NNO + 2 at a pressure of 1 bar and magmatic temperature, which is 1-2 log units higher than the usual magmatic f_O2. These constraints on the timing and oxidation condition of desulfidation lead to the conclusion that the reaction was caused by oxidation of the magma in a shallow volcanic conduit, not in magma chamber processes. The pumice groundmass consists mostly of glass, indicating that the rate of the desulfidation reaction is faster than the decompression-induced crystallization of microlites in the andesitic magma. Therefore, the desulfidation reaction of Po has the potential to be used as a geospeedometer for very fast magma ascent in vigorous explosive eruptions.

View full abstract