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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 >Empirical evidence for cosmogenic He-3 production by muons
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Empirical evidence for cosmogenic He-3 production by muons

机译：uONs的宇宙中的宇宙中的经验证据

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Cosmic ray muons penetrate deeply into rock where they interact with atoms to produce cosmogenic nuclides. Incorporation of the muon contribution to the production rates of cosmogenic nuclides increases the accuracy of exposure dates, burial ages, and erosion rates inferred from measured nuclide concentrations. In the absence of empirical evidence, it is generally assumed that muons do not produce He-3, a cosmogenic nuclide commonly used for exposure dating. Here we assess whether muons produce He-3 by measuring He isotope concentrations in pyroxene and ilmenite from a similar to 300 m deep drill core and other subsurface samples of the mid-Miocene Columbia River Basalt in Washington, USA. He-3 concentrations in our samples exhibit an exponential decline with depth with an e-folding length of 32.4 m, which corresponds to an attenuation length for He-3 production of 8780 g cm(-2). The deeply penetrating exponential is diagnostic of He-3 production by cosmic ray muons. Assuming no erosion, we constrain the minimum surface muonogenic production rate to be 0.23 atom g(-1) pyroxene yr(-1), whereas when incorporating erosion the production rate is 0.45 atom g(-1) pyroxene yr(-1). He-3 concentrations in samples deeper than similar to 100 meters are consistent with model-based estimates of depth-independent nucleogenic production from the capture by Li-6 of neutrons produced by alpha particle reactions on light elements. Measurements in other subsurface samples indicate that muon-produced He-3 is prevalent across the Columbia Plateau. The penetration depth of muonogenic He-3 production is substantially deeper, and the ratio of muon- to spallation-produced He-3 is substantially lower, than found for other cosmogenic nuclides. Our results provide the first definitive empirical evidence for He-3 production by muons, which has several implications for quantifying the timing and rates of Earth surface change and interpreting He isotope ratios. Importantly, despite the low production rates, landforms in the Channeled Scablands, which were formed by incision of the Columbia River Basalt by the late-Pleistocene Missoula floods, have high concentrations of He-3 inherited from post-Miocene muon exposure. Hence He-3 production by muons must be considered, particularly when dating rapid erosional events in old bedrock. Our findings indicate samples with less than several tens of meters of shielding by overlying rock will contain cosmogenic He-3 that elevates He-3/He-4 ratios. Hence caution should be used when using He-3/He-4 ratios from samples at shallower depths to infer mantle sources of basalt. (C) 2021 Elsevier B.V. All rights reserved.
机译：宇宙射线μ子深入岩石，在岩石中与原子相互作用，产生宇宙核素。μ介子对宇宙成因核素产生率的贡献增加了暴露日期、掩埋年龄和根据测量的核素浓度推断的侵蚀率的准确性。在缺乏经验证据的情况下，一般认为μ介子不会产生He-3，这是一种通常用于暴露测年的宇宙成因核素。在这里，我们通过测量辉石和钛铁矿中的He同位素浓度来评估μ介子是否产生He-3，这些辉石和钛铁矿来自美国华盛顿中中新世哥伦比亚河玄武岩的类似300米深的钻芯和其他地下样品。我们样品中的He-3浓度随深度呈指数下降，e折叠长度为32.4米，这相当于氦-3产量的衰减长度为8780 g cm（-2）。深穿透指数是宇宙线μ子产生He-3的诊断。假设没有侵蚀，我们将最小表面产铀率限制为0.23原子g（-1）辉石yr（-1），而当考虑侵蚀时，产铀率为0.45原子g（-1）辉石yr（-1）。深度大于100米的样品中的He-3浓度与基于模型的估算一致，即Li-6捕获轻元素上的α粒子反应产生的中子，从而产生与深度无关的核物质。对其他地下样品的测量表明，μ子产生的He-3在哥伦比亚高原普遍存在。与其他宇宙成因核素相比，μ介子生成的He-3的穿透深度要深得多，μ介子与产生的He-3层裂的比率要低得多。我们的结果为μ介子产生He-3提供了第一个明确的经验证据，这对量化地球表面变化的时间和速率以及解释He同位素比率具有若干意义。重要的是，尽管生产率较低，但由晚更新世密苏拉洪水切割哥伦比亚河玄武岩形成的沟渠化粗糙地带的地形，具有从中新世后μ介子暴露中继承的高浓度He-3。因此，必须考虑μ介子产生的He-3，尤其是在测定旧基岩中快速侵蚀事件的年代时。我们的发现表明，被上覆岩石屏蔽不到几十米的样品将含有宇宙成因的He-3，这会提高He-3/He-4比率。因此，当使用较浅深度样品的He-3/He-4比值推断玄武岩的地幔源时，应谨慎行事。（c）2021爱思唯尔B.V.保留所有权利。

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