Electron-microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites
Electron-microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites
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DOI：
10.2138/am.2005.1303
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发表时间：
2005-04
影响因子：
3.1
通讯作者：
A. Cocherie;E. B. Mezeme;O. Legendre;-C.;Mark Fanning;M. Faure;P. Rossi
中科院分区：
文献类型：
--
作者：
A. Cocherie;E. B. Mezeme;O. Legendre;-C.;Mark Fanning;M. Faure;P. Rossi
摘要 通过电子探针微量分析仪 (EPMA) 对独居石进行高空间分辨率测年，可以对小矿物进行系统而详细的研究。与锆石一样，独居石记录了主岩经历的复杂历史。最近对许多现场数据的统计处理的改进现在使得破译相关的热事件成为可能，从而获得可靠和精确的年龄。我们的工作表明，需要进行大量的单独点分析才能获得如此精确的信息（即超过 30.40 个数据）。通过使用来自三种混合岩和一种花岗岩的独居石示例，我们展示了如何根据我们正在尝试测定的颗粒或颗粒域的 U 和 Th 地球化学选择最有效的年龄计算方法。可能会遇到三种情况：（1）独居石表现出显着的 Th/U 比率变化，（2）独居石表现出相当恒定的 Th/U 比率，但显着的 U + Th 异质性，以及（3）独居石具有恒定的 U 和 Th 浓度。对于第一种情况，可以使用Th/Pb = f(U/Pb)图中数据缩减的方法计算出精确的平均年龄，通常可以达到±5−10 Ma(2σ)的精度。对于第二种情况，可以根据Pb=f(Th*)方法计算等时线年龄，一般精度在20Ma(2σ)左右，而对于第三种情况，可以计算简单的加权平均年龄。使用这些方法，再加上背散射电子图像研究，我们证明独居石的遗传可能与锆石一样常见。此外，高空间分辨率和精确年龄测定的结合表明独居石中铅扩散的程度有限。最后，来自法属圭亚那南部的混合岩的一个例子证明了独居石中 Th-U-Pb 系统的特别强大的行为。该系统在后期的混合作用期间以及随后的锆石结晶和原岩锆石的锆石过度生长期间保持封闭。独居石的年龄与原岩锆石的年龄完全相同。
Abstract High spatial resolution dating of monazite by the electron-probe microanalyzer (EPMA) enables systematic and detailed studies of small minerals. Like zircon, monazite records the complex history undergone by the host rocks. Recent improvements in the statistical treatment of many in situ data now make it possible to decipher the related thermal events and so obtain reliable and precise ages. Our work shows that a significant number of individual spot analyses is required to reach such precise information (i.e., more than 30.40 data). Using the examples of monazites from three migmatites and one granite, we show how to select the most efficient method of age calculation according to the U and Th geochemistry of the grains, or grain domains, that we are trying to date. Three situations may be met: (1) monazites exhibiting significant Th/U ratio variation, (2) monazites exhibiting a fairly constant Th/U ratio, but significant U + Th heterogeneity, and (3) monazites of constant U and Th concentrations. For the first case, a precise mean age can be calculated using a method of data reduction in the Th/Pb = f(U/Pb) diagram, whereby a precision of ±5−10 Ma (2σ) is commonly achieved. For the second case, an isochron age can be calculated according to the Pb = f(Th*) method, with a common precision of around 20 Ma (2σ), whereas for the third case, a simple weighted average age can be calculated. Using these approaches, coupled with a back-scattered electron image study, we demonstrate that inheritance is probably as common for monazite as for zircon. In addition, the combination of high spatial resolution and precise age determination show the limited extent of Pb diffusion in monazite. Finally, an example from a migmatite from southern French Guiana demonstrates the especially robust behavior of the Th-U-Pb system in monazite. This system remains closed during late migmatization and during the subsequent zircon crystallization and zircon overgrowth of protolith zircons. The monazite yielded exactly the same age as the protolith zircons.