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首页> 外文期刊>Geochemistry, geophysics, geosystems >Isotope dilution analysis of Ca and Zr in apatite and zircon (U-Th)/He chronometry
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Isotope dilution analysis of Ca and Zr in apatite and zircon (U-Th)/He chronometry

机译：磷灰石和锆石中Ca和Zr的同位素稀释分析（U-Th）/ He计时

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摘要

Because radiation damage influences He diffusivity, correlations between (U-Th)/He ages and effective uranium (eU, eU=U+0.235 x Th) concentrations of single apatite and zircon grains are important for understanding thermal histories. Here we describe a method for quantifying eU concentrations in apatite and zircon grains using isotope dilution ICP-MS measurements of Zr and Ca and stoichiometry of zircon (ZrSiO4) and apatite (Ca-5(PO4)(3)F) to obtain grain masses. Combined with independent U and Th measurements, these yield eU concentrations not based on the traditional morphologic measurements and assumptions. Additional benefits of this method include correct identification of an apatite or zircon and volume estimates for crystal shards. In some cases, this method gives eU concentrations consistent with those calculated with the morphologic approach, but often significant differences are observed between concentrations calculated from the two methods. Differences in eU concentrations for our apatite grains are greater and less than morphology estimates, and the majority are between 0.7 and 31%. With the exception of two grains, all of our zircon grains have differences between 3 and 34% less than morphology estimates. These differences could result from incorrect grain width measurements, mischaracterized grain shape, or incorrect volume calculations of the pure mineral phase due to inclusions. These morphologic errorscombined with evidence for the accuracy of our isotope dilution method from analyses of reference materialssuggest that eU concentrations calculated from morphology may often be significantly inaccurate. Finally, we demonstrate that differences between the two measurements of eU cause age-eU correlation variations for representative thermal histories.
机译：由于辐射损伤会影响He的扩散率，因此（U-Th）/ He年龄与单个磷灰石和锆石的有效铀（eU，eU = U + 0.235 x Th）浓度之间的相关性对于理解热历史很重要。在这里，我们描述了一种使用同位素稀释ICP-MS测量Zr和Ca以及锆石（ZrSiO4）和磷灰石（Ca-5（PO4）（3）F）的化学计量来量化磷灰石和锆石晶粒中eU浓度的方法，以获得晶粒质量。结合独立的U和Th测量，这些结果不会基于传统的形态学测量和假设得出eU浓度。该方法的其他好处包括正确鉴定磷灰石或锆石以及晶体碎片的体积估计。在某些情况下，该方法给出的eU浓度与使用形态学方法计算出的eU浓度一致，但通常在两种方法计算出的浓度之间观察到显着差异。我们的磷灰石颗粒的eU浓度差异大于或小于形态估计，且大多数介于0.7％和31％之间。除两种晶粒外，我们所有的锆石晶粒均比形态学估计值低3％至34％。这些差异可能是由于不正确的晶粒宽度测量，错误表征的晶粒形状或由于夹杂物导致的纯矿物相体积计算不正确所致。这些形态学错误与参考材料分析中我们的同位素稀释方法的准确性的证据相结合，表明从形态学计算得出的eU浓度通常可能非常不准确。最后，我们证明了两次eU测量之间的差异会导致代表eU历史的eU相关性变化。

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