CHIME dating of monazite, xenotime, zircon and polycrase: Protocol, pitfalls and chemical criterion of possibly discordant age data
CHIME dating of monazite, xenotime, zircon and polycrase: Protocol, pitfalls and chemical criterion of possibly discordant age data
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DOI：
10.1016/j.gr.2008.01.005
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发表时间：
2008-12
影响因子：
6.1
通讯作者：
Kazuhiro Suzuki;Takenori Kato
中科院分区：
文献类型：
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作者：
Kazuhiro Suzuki;Takenori Kato
本文概述了 CHIME（化学 Th-U-总 Pb 等时线法）测年方法，该方法基于对独居石、磷钇矿、锆石和聚合酶等含 Th 和 U 副矿物中的 Th、U 和 Pb 进行精确电子探针分析。还描述了适用于年轻独居石和锆石的年龄测绘技术。 CHIME 测年包括分析同质年龄域内显示出足够组成变化的多个点，然后使用这些数据构建“伪等时线”，通过回归可以从中获得年龄。该方法与通过化学标准（例如独居石的 (Ca+Si)/(Th+U+Pb+S) 比率以及锆石的 Ca 和 S 含量）区分可能一致的年龄数据时，具有显着精度的潜在优势，并且能够处理具有显着初始共同 Pb 成分的矿物。该技术可以识别两个或多个同质域，这些域之间的年龄差距小于单个斑点年龄分析的误差。许多在主量元素分析中不重要的特征可能对痕量元素数据的采集产生重大影响。影响精度的关键因素包括准直器狭缝、检测器气体、背景估计、加速电压、探头电流、X 射线干扰和计数率的作用，以及在不使用纯 Th 和 U 氧化物或合成纯 ThSiO4 的情况下应用 Th 和 U 干扰校正的方法。年轻独居石和锆石的年龄测绘程序包括使用多个光谱仪获取各个像素的 PbMα（或 PbMβ）强度，使用根据测量前确定的强度关系从测量的背景强度计算出的背景图校正背景，使用标准校准强度，并根据 Th、U 和 Pb 浓度计算年龄。该技术提供的年龄图显示了 20 Ma 数量级的年龄域差异，而独居石的年龄域则低至 100 Ma。还描述了强烈和长时间的探针测量辐照对样品损坏的影响。
This paper outlines the CHIME (chemical Th–U-total Pb isochron method) dating method, which is based on precise electron microprobe analyses of Th, U and Pb in Th- and U-bearing accessory minerals such as monazite, xenotime, zircon and polycrase. The age-mapping technique that is applicable to young monazite and zircon is also described. CHIME dating consists of analyzing multiple spots within homogeneous age domains that show sufficient compositional variation, and then these data are used to construct a “pseudo-isochron” from which an age can be obtained via regression. This method, when coupled with discrimination of possibly concordant age data by chemical criteria such as the (Ca+Si)/(Th+U+Pb+S) ratio for monazite and Ca and S contents for zircon, has the potential advantage of significant precision, and the ability to work with minerals that have a significant initial common Pb component. This technique can identify two or more homogeneous domains that are separated by age gaps smaller than the error on individual spot age analysis. Many features that are insignificant in major element analysis can have major impact in the acquisition of trace element data. Critical factors include the roles of collimator slit, detector gas, background estimation, accelerating voltage, probe current, X-ray interferences and count rate in affecting the accuracy, and a way to apply the Th and U interference correction without pure Th- and U-oxides or synthesized pure ThSiO4. The age-mapping procedure for young monazite and zircon includes acquiring PbMα (or PbMβ) intensity of individual pixels with multiple spectrometers, correcting background with background maps computed from a measured background intensity by the intensity relationships determined in advance of the measurement, calibrating of intensity with standards and calculating of ages from the Th, U and Pb concentrations. This technique provides age maps that show differences in age domains on the order of 20 Ma with in monazite as young as 100 Ma. The effect of sample damage by irradiation of intense and prolonged probe measurement is also described.