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Miyajima, Yusuke*; Saito, Ayaka*; Kagi, Hiroyuki*; Yokoyama, Tatsunori; Takahashi, Yoshio*; Hirata, Takafumi*
Geostandards and Geoanalytical Research, 45(1), p.189 - 205, 2021/03
Times Cited Count：6 Percentile：32.95(Geochemistry & Geophysics)Uncertainty for elemental and isotopic analyses of calcite by LA-ICP-MS is largely controlled by the homogeneity of the reference materials (RMs) used for normalization 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 crystallization 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 synthesized 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 to synthetic calcite of previous studies. Variations of element mass fractions in the calcite were better than 12% 2RSD, mostly within 7%. The 
Pb/
Pb ratio in the calcite showed 
1% variations, while the 
U/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.
Miyajima, Yusuke*; Saito, Ayaka*; Kagi, Hiroyuki*; Yokoyama, Tatsunori; Hirata, Takafumi*
no journal, ,
no abstracts in English
Miyajima, Yusuke*; Saito, Ayaka*; Kagi, Hiroyuki*; Yokoyama, Tatsunori; Hirata, Takafumi*
no journal, ,
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. U-Pb dating using high spatial-resolution LA-ICP-MS is a key technique to date natural carbonates. 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 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. In this study, we synthesized novel calcite reference materials with homogeneous U and Pb concentrations and isotope ratios. We incorporated U and Pb into calcite through heat-induced crystallization from U, Pb-doped amorphous calcium carbonate (ACC). The homogeneity of the U/Ca and Pb/Ca ratios in the synthetic calcite was generally better than 8% and 13%, respectively, in relative standard deviation. The Pb/Pb ratio of the synthetic calcite was homogeneous within ~1% errors, whereas the U/Pb ratio was less homogeneous (3%-11% errors). 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 U/Pb ratio in the synthetic calcite from its U and Pb concentrations. We then obtained the fractionation factor to correct bias between the isotope ratios and the nominal value. We could accurately date WC-1 with an ~3% precision. 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.
Miyajima, Yusuke*; Saito, Ayaka*; Kagi, Hiroyuki*; Yokoyama, Tatsunori; Hirata, Takafumi*; Roberts, N. M. W.*; Horstwood, M.*
no journal, ,
Direct age determination of carbonate minerals provides valuable insight to paleoenvironmental change, tectonics, and sub-surface fluid-flow. The U-Pb method using 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. A natural calcite cement WC-1 (254.4 
6.4 Ma) is suitable as a reference material to correct elemental fractionation with LA-ICP-MS. However, the resulting uncertainty on a sample date is limited by the heterogeneity of U and Pb distribution in the reference material. Our objective is to synthesize calcite with homogeneous U and Pb concentrations and isotope ratios to be used as novel reference materials. To achieve this, we incorporate incompatible U and Pb into calcite through heat-induced crystallization from amorphous calcium carbonate precipitated from U, Pb-doped reagent solution. LA-ICP-MS analyses of results so far revealed that the U/Ca and Pb/Ca ratios in the synthetic calcites were generally homogeneous with relative standard deviation (2SD, n = 10) better than 4% and 7%, respectively. The Pb/Ca homogeneity of calcites doped with 1 
g/g Pb was up to ~25%. The Pb/Pb ratio of the synthetic calcites was very homogeneous with 1% variation, whereas the U/Pb ratio was less homogeneous (2%
7% variation). To evaluate the suitability of the synthetic calcite as a reference material, we measured the age of WC-1 using the synthetic calcite for elemental fractionation correction; we could date WC-1 with a precision and accuracy of ~3% (246.6 7.3 Ma). This method is a promising alternative in addition to characterising natural carbonate materials for elemental and isotopic composition.
