Incorporation of U, Pb and Rare Earth Elements in Calcite through Crystallisation from Amorphous Calcium Carbonate: Simple Preparation of Reference Materials for Microanalysis
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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DOI：
10.1111/ggr.12367
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发表时间：
2020-12-10
影响因子：
3.8
通讯作者：
Hirata, Takafumi
中科院分区：
文献类型：
--
作者：
Miyajima, Yusuke;Saito, Ayaka;Hirata, Takafumi
用LA-ICPMS测量方解石中元素和同位素的不确定度在很大程度上受用于校准和验证的标准物质(RMS)的均一性控制。为了得到元素和同位素组成均一的方解石RMS，我们从掺元素试剂溶液中析出的无定形碳酸钙，通过加热和压力诱导结晶，在方解石中掺入了U、铅和稀土元素。X-射线吸收光谱表明，U(VI)以U(VI)的形式存在于方解石中，可能具有与铀酰离子不同的局域结构。与以往研究合成的方解石相比，我们的方解石对铀的吸收速率更高。方解石中元素质量分数的变化优于12%~2RSD，大部分在7%以内。方解石中的Pb207/Pb206比值随元素质量分数的不同而变化1%，U-238/Pb206比值变化3%~24%。用合成的方解石作为初级RMS，我们可以测得天然方解石Rm，WC-1的分析不确定度低至<；3%。该方法可用于生产元素质量分数可控且均匀的方解石，是一种很有前途的天然方解石有效值的U-Pb年代学替代方法。
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 Pb-207/Pb-206 ratio in the calcite showed < 1% variations, while the U-238/Pb-206 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.