Japan Geoscience Union Meeting 2019

Presentation information

[J] Oral

M (Multidisciplinary and Interdisciplinary) » M-TT Technology & Techniques

[M-TT48] Frontiers in Geochemistry

Sun. May 26, 2019 1:45 PM - 3:15 PM 201A (2F)

convener:Hirochika Sumino(Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo), Tetsuya Yokoyama(Department of Earth and Planetary Sciences, School of Science, Tokyo Institute of Technology), Hajime Obata(Marine inorganic chemistry division, Atmosphere and Ocean Research Institute, University of Tokyo), Chairperson:Hirochika Sumino, Hajime Obata(Atmosphere and Ocean Research Institute, University of Tokyo), Tetsuya Yokoyama(Tokyo Institute of Technology)

2:45 PM - 3:00 PM

[MTT48-05] Synthesis of calcite reference materials for in situ U–Pb dating through crystallization from amorphous calcium carbonates

*Yusuke Miyajima1, Ayaka Saito1, Hiroyuki Kagi1, Tatsunori Yokoyama2, Takafumi Hirata1 (1.Geochemical Research Center, Graduate School of Science, The University of Tokyo, 2.Tono Geoscience Center, Japan Atomic Energy Agency (JAEA))

Keywords:Amorphous calcium carbonate, LA-ICP-MS, Reference material, U–Pb geochronology

Calcium carbonates are ubiquitously present throughout the Earth history as animal shells, speleothems, fault-related vein fillings, and hydrothermal or cold-seep precipitates. Direct dating of carbonates provides valuable information on paleoenvironmental change, tectonics, and fluid and material cycling. Uranium–lead (U–Pb) dating using high spatial-resolution laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a key technique to date natural carbonates such as fault-hosted calcite that cannot be dated by biostratigraphy or Sr isotope stratigraphy (Roberts and Walker, 2016; Beaudoin et al., 2018). In situ U–Pb dating by LA-ICP-MS needs matrix-matched reference materials to correct matrix-dependent elemental fractionation in LA-ICP-MS. Roberts et al. (2017) demonstrated that a natural calcite cement WC-1 (254.4 ± 6.4 Ma) is suitable as a calcite reference material. However, the WC-1 calcite has an inhomogeneous distribution of U and Pb and lacks concordance in the U–Pb system (Roberts et al., 2017). In this study, we synthesized novel calcite reference materials with homogeneous U and Pb concentrations and isotope ratios. Incompatible elements such as Sr can be incorporated into calcite through pressure- or heat-induced crystallization from amorphous calcium carbonate (ACC) precipitated from element-doped reagent solution (Matsunuma et al., 2014; Saito et al., 2017). We incorporated U and Pb into calcite through heat-induced crystallization from U, Pb-doped ACC. The homogeneity of the U/Ca and Pb/Ca ratios in the synthetic calcite examined with LA-quadrupole (Q)-ICP-MS was generally better than 8% and 13%, respectively, in relative standard deviation (2SD, n = 10). The 207Pb/206Pb ratio of the synthetic calcite measured with LA-multiple-collector (MC)-ICP-MS was homogeneous within ~1% errors, whereas the 238U/206Pb ratio was less homogeneous (3%–11% errors). The inhomogeneity in the U/Pb isotope ratio would be propagated into the analytical precision for unknown samples. In order to test the usability of the synthetic calcite, we dated WC-1 using the synthetic calcite for correction of elemental fractionation. We calculated the nominal 238U/206Pb ratio in the synthetic calcite from its U and Pb concentrations measured by LA-Q-ICP-MS using Ca as an internal standard. We then obtained the fractionation factor to correct bias between the isotope ratios measured by LA-MC-ICP-MS and the nominal value. Using the fractionation factor, we could accurately date WC-1 with an ~3% precision (246.6 ± 7.3 Ma). If the isotopic compositions of the synthetic calcite are certified by isotope-dilution technique, we could date natural carbonates with <10% precisions using the synthetic reference calcite.