Abstract The radiogenic ingrowth and diffusive loss of 4He in accessory minerals is used to determine the timing of low-temperature thermal processes such as shallow tectonic, magmatic, hydrothermal, and geomorphic events. While the thermal history experienced by a sample cannot be directly constrained by a single (U Th)/He total gas age, 4He concentration profiles, normalized against irradiation-induced spallogenic 3He, have been shown to reliably recover continuous thermal history constraints if the parent nuclide distribution is either spatially uniform or can be determined. As such, 4He/3He thermochronometry has been successfully applied to apatite. However, individual zircon grains can contain complex parent nuclide zonation. Using moderately zoned, euhedral zircon grains from the rapidly cooled, distal Fish Canyon Tuff, we investigated the potential of the zircon 4He/3He thermochronometer through combining 4He/3He diode laser fractional loss step-heating with detailed analysis of parent element zonation. We present a routine method for the quantitative characterization of parent nuclide zonation by LA-ICP-MS depth-profiling. This study also demonstrates the importance of thermal control and avoidance of intra-sample thermal gradients during laser step-heating fractional loss experiments. Accurate and stable thermal control was achieved with an optimized sample-holder design, laser defocusing, and PID control loops to minimize temperature overshoot, failure to reach set-point, and temperature drop-offs. These analytical developments illustrate both the pitfalls and promise of zircon 4He/3He thermochronometry and create a reproducible methodology for 4He/3He thermochronometric applications to zoned and slowly cooled zircon.

中文翻译：

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锆石 4He/3He 分数损失阶梯加热和母核素分布表征

摘要 利用副矿物中 4He 的放射成因向内生长和扩散损失来确定浅层构造、岩浆、热液和地貌事件等低温热过程的时间。虽然样品经历的热历史不能直接受单个 (U Th)/He 总气体年龄的约束，但 4He 浓度分布，针对辐照诱导的 3He 进行归一化，已被证明可以可靠地恢复连续热历史约束，如果母体核素分布要么是空间均匀的，要么是可以确定的。因此，4He/3He 测温法已成功应用于磷灰石。然而，单个锆石颗粒可能包含复杂的母体核素分带。使用来自快速冷却的远端鱼峡谷凝灰岩的适度分区的自形锆石颗粒，我们通过将 4He/3He 二极管激光器分数损耗阶梯加热与母元素分带的详细分析相结合，研究了锆石 4He/3He 温度计的潜力。我们提出了一种通过 LA-ICP-MS 深度剖面定量表征母体核素分带的常规方法。该研究还证明了在激光步进加热分数损失实验中热控制和避免样品内热梯度的重要性。通过优化的样品架设计、激光散焦和 PID 控制回路实现了准确和稳定的热控制，以最大限度地减少温度过冲、未能达到设定点和温度下降。