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Age discordance and mineralogy

机译：年龄不景气和矿物学

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Observations of discordant ages, meaning that an age given by one mineral geochronometer is different from the age given by another geochronometer from the same rock, began in the early days of geochronology. In the late 1950s and 1960s, discordant U-Pb zircon ages were unquestioningly attributed to Pb diffusion at high temperature. Later, the mineralogical properties and the petrogenesis of the zircon crystals being dated was recognized as a key factor in obtaining concordant U-Pb ages. Advances in analytical methods allowed the analysis of smaller and smaller zircon multigrain fractions, then the analysis of individual grains, and even pieces of grains, with higher degrees of concordancy. Further advances allowed a higher analytical precision, a clearer perception of accuracy, and a better statistical resolution of age discordance. As for understanding the cause(s) of discordance, belief revision followed the coupling of imaging, cathodoluminescence (CL), and backscattered electrons (BSE), to in situ dating by secondary ion mass spectrometry (SIMS) or by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Discordant zircon and other accessory minerals (e.g., monazite, apatite, etc.) often consist of young rims accreted onto/into older cores. Age gradients are sharp, and no Pb diffusion gradients are observed. As U-Pb discordance in crystalline, non-radiation damaged grains is caused by diachronous, heterochemical mineral generations, interpretations of mineral ages, based on the exclusive role of diffusion, are superseded, and closure temperatures of zircon and monazite are irrelevant in geological reality.
机译：一种不安的年龄的观察，这意味着一个矿物地球射流器的年龄与来自同一岩石的另一个地球节计不同的年龄不同，开始在地理学的早期。在20世纪50年代后期和20世纪60年代，不和谐的U-PB锆石年龄毫无疑问地归因于高温下的PB扩散。后来，矿物学性质和氧化锆晶体的纤维化被认为是获得高效U-Pb年龄的关键因素。分析方法的进步允许分析较小且较小的锆型多基团分数，然后分析个体颗粒，甚至是颗粒的分析，具有较高程度的一致性。进一步的进步允许更高的分析精度，更清晰的准确性感知，以及更好的年龄不间断的统计决策。至于理解不景气的原因，信仰修订遵循成像，阴极发光（CL）和反向散射电子（BSE）的耦合，以通过二次离子质谱（SIMS）或激光烧蚀电感耦合等离子体质谱（La-ICP-MS）。不和谐的锆石和其他配件矿物质（例如，Monazite，磷灰石等）通常由凹陷的年轻轮辋组成为较旧的核心。年龄梯度是尖锐的，没有观察到Pb扩散梯度。随着u-Pb在结晶中的不间断的作用，非放射性受损晶粒是由二次辐射的，异种化学矿物代来引起的，矿物年龄的解释，基于扩散的独家作用，被取代，锆石和单一的闭合温度在地质现实中无关紧要。

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