Reconsidering initial Pb in titanite in the context of in situ dating
Reconsidering initial Pb in titanite in the context of in situ dating
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DOI：
10.2138/am-2020-7274
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发表时间：
2020-10
影响因子：
3.1
通讯作者：
C. Bonamici;T. Blum
中科院分区：
文献类型：
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作者：
C. Bonamici;T. Blum
摘要 近年来，钛矿原位 U-Pb 定年法得到了广泛应用，它不仅可以保存各种岩石学过程的微量元素记录，而且还可以包含重要的初始 Pb。广泛使用钛矿数据来构建构造 P-T-t 路径，需要仔细评估可用的测年技术，并关注支撑 U-Pb 数据分析的假设。这项贡献首次对原位 U-Pb 钛矿定年的两种主要分析方法 [SHRIMP (SIMS) 和 LA-ICP-MS] 进行了直接比较。通过敏感高分辨率离子微探针 (SHRIMP) 和 LA-ICP-MS，沿着相同的交叉晶粒横断面分析了来自纽约州哈里斯维尔阿迪朗达克山脉的一组经过充分表征的钛矿晶粒的 U-Th-Pb 同位素。 LA-ICP-MS 和 SHRIMP 数据集在 Tera-Wasserburg Concordia（半全 Pb/U）图上定义了近似线性阵列，并且通常被解释为表示具有较小分散性的单个日期总体。然而，之前的研究表明，阿迪朗达克钛矿实际上记录了两个区域明确的热事件，约 50-100 m.y。分开。当详细处理钛矿数据阵列时，尝试通过稳健的三维线性回归来确定协和截距年龄会产生很大的不确定性和/或拟合不良的统计数据，这表明数据实际上不是等时的。 U-Pb 钛矿数据的逐粒分析表明，不同的钛矿子集（由额外的地球化学和微观结构数据确定）显示出不同的 U-Pb 数据模式。通过将 Tera-Wasserburg 图中铅向内生长演化路径的预测与观测数据进行比较，可以识别阿迪朗达克钛矿 U-Pb 数据集中初始铅成分的变化和铅损失。这项研究提供了如何从大型原位 U-Pb 钛矿数据集中提取更多地质年代学细节的示例。即使无法恢复精确的日期，也可以通过使用 Tera-Wasserburg 图分析 U-Pb 数据模式来识别导致数据分散的地质过程和事件。
Abstract In situ U-Pb dating of titanite, which can preserve trace-element records of various petrologic processes but also incorporates significant initial Pb, has proliferated in recent years. The widespread use of titanite data to construct tectonic P-T-t paths warrants careful assessment of the available dating techniques, as well as attention to the assumptions that underpin the U-Pb data analysis. This contribution provides the first direct comparison of the two major analytical methods [SHRIMP (SIMS) and LA-ICP-MS] for in situ U-Pb titanite dating. A set of well-characterized titanite grains from Harrisville, New York, in the Adirondack Mountains were analyzed for U-Th-Pb isotopes along the same cross-grain traverses by Sensitive High Resolution Ion Microprobe (SHRIMP) and LA-ICP-MS. Both LA-ICP-MS and SHRIMP data sets define approximately linear arrays on the Tera-Wasserburg Concordia (semi-total Pb/U) diagram and would commonly be interpreted as representing a single date population with minor scatter. However, previous studies have suggested that Adirondack titanite actually records two regionally well-defined thermal events, ~50–100 m.y. apart. When titanite data arrays are treated in detail, attempts to determine concordia-intercept ages by robust three-dimensional linear regression produce large uncertainties and/or poor fit statistics that suggest that the data are not, in fact, isochronous. Grain-by-grain analysis of U-Pb titanite data shows that different subsets of titanite (determined by additional geochemical and microstructural data) show different patterns of U-Pb data. By comparing predictions for Pb-ingrowth evolution paths in Tera-Wasserburg diagrams with observed data, it is possible to recognize both a change in initial Pb composition and Pb loss in the Adirondack titanite U-Pb data set. This study provides an example of how greater geochronologic detail can be extracted from large in situ U-Pb titanite data sets. Even when precise dates are not recovered, geological processes and events that cause data scatter can be recognized through analysis of U-Pb data patterns using the Tera-Wasserburg diagram.