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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Zircon (U-Th)/He thermochronometry in the KTB drill hole, Germany, and its implications for bulk He diffusion kinetics in zircon
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Zircon (U-Th)/He thermochronometry in the KTB drill hole, Germany, and its implications for bulk He diffusion kinetics in zircon

机译：德国KTB钻孔中的锆石（U-Th）/ He热计时法及其对锆中整体He扩散动力学的影响

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摘要

This study presents new down-hole zircon (U-Th)/He (ZrHe) ages, laboratory He diffusion measurements, and numerical thermal modeling of ZrHe ages from the Continental Deep Drilling Project (KTB) in Germany to investigate He diffusion kinetics in zircon in nature over geologic timescales and to test to applicability of laboratory-derived He diffusion kinetics. Single-grain laser (U-Th)/He ages, calculated using standard alpha-ejection correction procedures assuming homogenous parent-nuclide distribution, display a systematic decrease in ZrHe ages from ~112 to <1Ma with increasing depth. Down-hole ZrHe results display consistent ages of ~85±15Ma down to ~4.7km, in agreement with rapid Cretaceous cooling documented by previous thermochronometric studies from the KTB drill hole. Below ~5km, ZrHe ages systematically decrease in age and are completely reset (<1Ma) below ~7.2km. The temperature range (~130-200°C) in which ZrHe ages systematically decrease defines a well-behaved zircon helium partial retention zone (HePRZ). In addition, this study presents new, cycled step-heating experiments on zircon samples from the KTB drill hole. Results from these new KTB zircon diffusion experiments indicate an activation energy (E_a) of 160kJ/mol and a frequency factory (Do) of 0.03cm~2s~(-1) with an estimated closure temperature (T_c) of 181°C, which are in excellent agreement with published He diffusion kinetics for zircon. To compare the ZrHe results and bulk diffusion kinetics, we modeled diffusion parameters using the well-established thermal history of the KTB drill hole. The computed zircon HePRZ for the KTB drill hole is consistent with observed down-hole ZrHe ages and published and KTB-specific laboratory-derived He diffusion kinetics. Our results from ZrHe analysis from the KTB drill hole suggest that He diffusion of zircon in nature may not be controlled by anisotropic diffusion behavior, but rather behaves in accordance with laboratory-derived diffusion kinetics. The observed ZrHe ages from the KTB drill hole are in excellent agreement with predicted ZrHe age data and underscore the validity and applicability of ZrHe dating as a reliable thermochronometer.
机译：这项研究提供了德国大陆深部钻探项目（KTB）的新井下锆石（U-Th）/ He（ZrHe）年龄，实验室He扩散测量以及ZrHe年龄的数值热模型，以研究He在锆石中的扩散动力学在自然界中跨越地质时间尺度，并测试实验室衍生的He扩散动力学的适用性。假设同质的亲核素分布，使用标准的α喷射校正程序计算的单晶激光（U-Th）/ He年龄随着深度的增加，显示ZrHe年龄从〜112到<1Ma逐渐降低。 ZrHe井下结果显示一致的年龄约85±15Ma，低至约4.7km，这与先前通过KTB钻孔进行的热年代学研究证明的白垩纪快速冷却相符。在〜5km以下，ZrHe年龄会系统地降低，并在〜7.2km以下完全重置（<1Ma）。 ZrHe的年龄会系统降低的温度范围（〜130-200°C）定义了行为良好的锆石氦气部分保留区（HePRZ）。此外，本研究还针对来自KTB钻孔的锆石样品提出了新的循环式步进加热实验。这些新的KTB锆石扩散实验的结果表明，活化能（E_a）为160kJ / mol，频率工厂（Do）为0.03cm〜2s〜（-1），估计闭合温度（T_c）为181°C，与已发表的锆石的He扩散动力学非常一致。为了比较ZrHe结果和整体扩散动力学，我们使用公认的KTB钻孔热历史对扩散参数进行建模。计算得出的KTB钻孔的锆石HePRZ与观察到的井下ZrHe年龄以及已发表的和KTB特定于实验室的He扩散动力学一致。我们从KTB钻孔进行的ZrHe分析得出的结果表明，锆石在自然界中的He扩散可能不受各向异性扩散行为的控制，而是根据实验室衍生的扩散动力学来控制。从KTB钻孔中观察到的ZrHe年龄与预测的ZrHe年龄数据非常吻合，并强调了ZrHe测年作为可靠的温度计时器的有效性和适用性。

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