Neutron-induced ³⁷Ar recoil ejection in Ca-rich minerals and implications for ⁴⁰Ar/³⁹Ar dating

The ⁴⁰Ar/³⁹Ar dating technique requires the transformation of ³⁹K into ³⁹Ar by neutron activation. Neutron activation has undesirable secondary effects such as interfering isotope production, and recoil of ³⁹Ar and ³⁷Ar atoms from their (dominant) targets of K and Ca. In most cases, the grains analysed are large enough (&gt;50 μm) such that the amount of target atoms ejected from the grains is small and has a negligible effect on the ages obtained. However, increasing needs to date fine-grained rocks requires constraining, and in some cases correcting for, the effect of nuclear recoil. Previous quantitative studies of recoil loss focus mostly on ³⁹Ar. However, ³⁷Ar loss can affect the ages of Ca-rich minerals via interference corrections on ³⁶Ar (and, to a lesser extent, ³⁹Ar), yielding lower ⁴⁰Ar*/³⁹Ar_K and, thus, an age spuriously too young. New results focused on ³⁷Ar recoil by measuring the apparent age of multi-grain populations of Ca-rich minerals including Fish Canyon plagioclase (FCp) and Hb3gr hornblende, with discrete sizes ranging from 210 to &lt;5 µm. We use previous result on sanidine grains to correct for the ³⁹Ar recoil loss. For the finest fractions, FCp and Hb3gr apparent ages are younger than the ³⁹Ar recoil-corrected ages expected for these minerals, with a maximum deviation of −40% (FCp) and −21% (Hb3gr) reached for grains below 5 μm. We calculate ³⁷Ar-depletion values ranging from approximately 30 to 91% and from approximately 28 to 98% for plagioclase and hornblende, respectively. This results in <em>x</em>₀ values (mean thickness of the partial depletion layer) of 3.3±0.4 μm (2σ; FCp) and 3.6±1.4 μm (Hb3gr), significantly higher than suggested by current models. The reason for the substantial ³⁷Ar loss is not well understood, but might be related to the radiation damage caused to the mineral during irradiation. <em>x</em>₀ (³⁹Ar) and <em>x</em>₀ (³⁷Ar) values obtained in this study, along with crystal dimensions, can be used for correcting ⁴⁰Ar/³⁹Ar ages from ³⁹Ar and ³⁷Ar recoil loss. We also discuss the relevance of our results to vacuum-encapsulation studies and isotopic redistribution in fine-grained minerals.

⁴⁰Ar/³⁹Ar测年技术（⁴⁰Ar/³⁹Ar dating technique）需要通过中子活化将³⁹K转化为³⁹Ar。中子活化存在诸多不利副效应，例如干扰同位素生成，以及³⁹Ar和³⁷Ar原子从其主要靶矿物钾（K）与钙（Ca）中发生反冲。多数情况下，所分析的矿物颗粒粒径足够大（>50 μm），从颗粒中逸出的靶原子总量极少，对所得年龄的影响可忽略不计。然而，细粒岩石测年需求的日益增长，要求对核反冲效应进行约束，在部分场景下还需对其进行校正。过往针对反冲损失的定量研究大多聚焦于³⁹Ar的损失，但³⁷Ar的损失可通过对³⁶Ar的干扰校正（在较小程度上也会影响³⁹Ar），进而对富钙矿物的年龄产生影响，导致⁴⁰Ar*/³⁹Ar_K比值降低，最终得到虚假偏年轻的年龄结果。本研究聚焦³⁷Ar反冲效应，通过测量多组富钙矿物多颗粒组分的视年龄开展相关研究，所选用的矿物包括鱼峡谷斜长石（Fish Canyon plagioclase, FCp）与Hb3gr角闪石，其离散粒径范围为210 μm至<5 μm。我们借助既往关于透长石颗粒的研究结果，对³⁹Ar的反冲损失进行校正。对于最细的粒级组分，FCp与Hb3gr的视年龄低于这些矿物经³⁹Ar反冲校正后的预期年龄，其中粒径小于5 μm的颗粒最大偏差分别为-40%（FCp）与-21%（Hb3gr）。我们计算得到斜长石与角闪石的³⁷Ar损耗值分别约为30%~91%与28%~98%。由此得到的部分损耗层平均厚度（x₀）分别为3.3±0.4 μm（2σ；FCp）与3.6±1.4 μm（Hb3gr），显著高于当前模型所提出的数值。目前对于如此显著的³⁷Ar损失的成因尚不明确，但可能与辐照过程中矿物遭受的辐射损伤有关。本研究得到的x₀(³⁹Ar)与x₀(³⁷Ar)数值，结合晶体尺寸参数，可用于校正由³⁹Ar与³⁷Ar反冲损失导致的⁴⁰Ar/³⁹Ar测年偏差。此外，我们还讨论了本研究结果对细粒矿物真空封装研究与同位素再分布研究的参考价值。