Journal Article Experimental Study of SXES: Determination of Iron Oxidation State in Silicate Minerals Get access Takaomi Yokoyama, Takaomi Yokoyama JEOL Ltd., Akishima, Tokyo, Japan Search for other works by this author on: Oxford Academic Google Scholar Shogo Koshiya, Shogo Koshiya JEOL Ltd., Akishima, Tokyo, Japan Search for other works by this author on: Oxford Academic Google Scholar Kenichi Tsutsumi, Kenichi Tsutsumi JEOL Ltd., Akishima, Tokyo, Japan Search for other works by this author on: Oxford Academic Google Scholar Terumi Ejima, Terumi Ejima Shinshu University, Matsumoto, Nagano, Japan Search for other works by this author on: Oxford Academic Google Scholar Yoshiaki Kon Yoshiaki Kon National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 26, Issue S2, 1 August 2020, Pages 1018–1021, https://doi.org/10.1017/S1431927620016682 Published: 01 August 2020
We measured the LA-ICP-MS U-Pb age distribution of detrital zircons in three psammitic schist samples of Hitachi and Nishidohira medium P/T metamorphic rocks from the southern part of Abukuma Belt, Northeast Japan. It has been proposed that these medium P/T metamorphic rocks mark the eastern extension of the Triassic collisional suture between the North China and South China blocks. Therefore, we aim to obtain the age of sedimentation, stratigraphy, and provenance of their protolith from the measurements, and evaluate the above proposition. The psammitic schist sample (DIO-9) of Hitachi Metamorphic Rocks, originating from quartzose sandstone at the lowest part of the Daioin Formation, contains detrital zircons of the youngest age clustered around 410 Ma and the youngest zircon at 395 ± 20 Ma (206Pb/238U age; 2σ). Considering that the upper part of the Daioin Formation contains Visean (Lower Carboniferous) corals and that the formation intercalates abundant felsic tuff layers, the lowest part of the Daioin Formation is likely to be correlated with Devonian Nakasato or Lower Carboniferous Hikoroichi Formation of South Kitakami Belt, Northeast Japan. Nishidohira Metamorphic Rocks lie beneath ultramafic rocks along the base of Hitachi Metamorphic Rocks, and consist of mafic, siliceous, calcareous, pelitic, and psammitic schists or gneisses. Because the siliceous schist of Nishidohira Metamorphic Rocks is meta-pelagic chert, the metamorphic rocks presumably originated from an accretionary complex. The ages of detrital zircons in two psammitic schist samples (ND-12 and -13) of Nishidohira Metamorphic Rocks mostly fall between ca. 300 Ma and 200 Ma, with the youngest two zircons at 154 ± 6 Ma (ND-12) and 175 ± 3 Ma (ND-13) (206Pb/238U age, 2σ). The protolith age of the psammitic schists must be ca. 154 ± 6 Ma (Kimmeridgian of Late Jurassic) and ca. 175 ± 3 Ma (Aalenian of Middle Jurassic) or younger, suggesting that Nishidohira Metamorphic Rocks originated from a Jurassic accretionary complex. In the Hitachi area three tectonostratigraphic units superpose, i.e., in ascending order, (1) the Jurassic accretionary complex of Nishidohira Metamorphic Rocks, (2) ultramafic rocks, and (3) Hitachi Metamorphic Rocks that are at least partly correlated with the Paleozoic sequence of South Kitakami Belt. The tectonostratigraphy is similar to that of non-metamorphic rocks of Kitakami Mountains, Northeast Japan, where (1) the Jurassic accretionary complex of North Kitakami Belt is overlain by (2) the Hayachine mafic–ultramafic complex, which in turn is overlain by (3) the Paleo–Mesozoic succession of South Kitakami Belt.
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We have developed new analytical procedures to measure precise and accurate 238 U– 206 Pb and 235 U– 207 Pb ages for young (~ 1 Ma) zircons using laser ablation‐ ICP ‐mass spectrometry. For young zircons, both careful correction for the background counts and analysis of very small Pb/U ratios (i.e., 206 Pb/ 238 U < 0.00016 and 207 Pb/ 235 U < 0.0001 for 1 Ma zircons) are highly desired. For the correction of the background, the contribution of the background signal intensities for the analytes, especially for the residual signal intensities for 206 Pb and 207 Pb, was defined through laser ablation of synthesised zircons (ablation blank) containing negligible Pb. The measured signal intensities for 202 Hg, 206 Pb and 207 Pb signals obtained by the ablation blank were slightly higher than those obtained by data acquisition without laser ablation (gas blank). For the wider dynamic range measurements on Pb/U ratios, an attenuator device for the ion detection system was employed to extend the capability to monitor high‐intensity signals (i.e., > 3 Mcps). Through the attenuator device, the ion currents were reduced to 1/450 of the signal intensity without the attenuator. Because the switching time for the attenuator was shorter than 1 ms, signal intensities for only specific isotopes could be reduced. With attenuation of the 238 U signal, counting statistics on 206 Pb and 207 Pb isotopes could be improved and counting loss on the 238 U signal could be minimised. To demonstrate the reliability of this new analytical technique, 238 U– 206 Pb and 235 U– 207 Pb ages for three young zircon samples (collected from Osaka Group Pink Volcanic Ash, Kirigamine and Bishop Tuff) were measured. The data presented here demonstrate clearly that the present technique could become a major analytical tool for in situ U–Pb age determination of young zircons (~ 1 Ma).
