Incorporation of U, Pb and Rare Earth Elements in Calcite through Crystallisation from Amorphous Calcium Carbonate: Simple Preparation of Reference Materials for Microanalysis

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  • Yusuke Miyajima
    Geochemical Research Center Graduate School of Science The University of Tokyo 7‐3‐1 Hongo, Bunkyo‐ku Tokyo 113‐0033 Japan
  • Ayaka Saito
    Geochemical Research Center Graduate School of Science The University of Tokyo 7‐3‐1 Hongo, Bunkyo‐ku Tokyo 113‐0033 Japan
  • Hiroyuki Kagi
    Geochemical Research Center Graduate School of Science The University of Tokyo 7‐3‐1 Hongo, Bunkyo‐ku Tokyo 113‐0033 Japan
  • Tatsunori Yokoyama
    Tono Geoscience Center Japan Atomic Energy Agency (JAEA) Jorinji, Izumicho Toki 509‐5102 Japan
  • Yoshio Takahashi
    Department of Earth and Planetary Science Graduate School of Science The University of Tokyo 7‐3‐1 Hongo, Bunkyo‐ku Tokyo 113‐0033 Japan
  • Takafumi Hirata
    Geochemical Research Center Graduate School of Science The University of Tokyo 7‐3‐1 Hongo, Bunkyo‐ku Tokyo 113‐0033 Japan

説明

<jats:p>Uncertainty for elemental and isotopic measurements in calcite by LA‐ICP‐MS is largely controlled by the homogeneity of the reference materials (RMs) used for calibration and validation. In order to produce calcite RMs with homogeneous elemental and isotopic compositions, we incorporated elements including U, Pb and rare earth elements into calcite through heat‐ and pressure‐induced crystallisation from amorphous calcium carbonate that was precipitated from element‐doped reagent solution. X‐ray absorption spectra showed that U was present as U(VI) in the synthesised calcite, probably with a different local structure from that of aqueous uranyl ions. The uptake rate of U by our calcite was higher in comparison with synthetic calcite of previous studies. Variations of element mass fractions in the calcite were better than 12% 2RSD, mostly within 7%. The <jats:sup>207</jats:sup>Pb/<jats:sup>206</jats:sup>Pb ratio in the calcite showed < 1% variations, while the <jats:sup>238</jats:sup>U/<jats:sup>206</jats:sup>Pb ratio showed 3–24% variations depending on element mass fractions. Using the synthetic calcite as primary RMs, we could date a natural calcite RM, WC‐1, with analytical uncertainty as low as < 3%. The method presented can be useful to produce calcite with controlled and homogeneous element mass fractions and is a promising alternative to natural calcite RMs for U‐Pb geochronology.</jats:p>

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