A Test of Global and Antarctic Models for Cosmogenic-nuclide Production Rates using High-precision Dating of 40Ar/39Ar Lava Flows from Mount Erebus

非技术描述：地球表面岩石和土壤的年龄可以帮助回答对人类福祉至关重要的多个问题，包括：火山何时爆发以及它们是否可能再次爆发？ 冰川是什么时候形成的？它们告诉我们关于气候的什么？ 滑坡、洪水和泥石流等灾害发生的频率是多少？ 土壤形成需要多长时间？土壤侵蚀是否会使农业无法持续？ 有一种方法每年都被用于解决这些问题，这种方法被称为“宇宙成因表面暴露定年”。 这种方法利用了宇宙射线，这是强大的质子和中子产生的超新星，不断轰击地球的大气层。一些宇宙射线到达地球表面并产生核反应，产生稀有同位素。 测量稀有同位素的数量可以计算岩石或土壤暴露在大气中的时间长度。 宇宙射线在地球仪周围的分布取决于地球磁场，如果要获得有用的暴露年龄，就必须准确地知道这种分布。 目前有两个剩余的理论，从许多缩小，如何计算这个分布。 需要在高海拔和高纬度（靠近两极）的地点进行测量，以检验这两种理论。 这项研究涉及实地和实验室研究，包括博士学位。学生和本科生。 研究小组将从南极洲一座名为埃里伯斯山的活火山上收集熔岩流中的岩石，并测量两种稀有同位素：36 Cl和3 He的含量。 样本的喷发年龄将使用一种不依赖于宇宙射线的高精度方法来确定，称为40 Ar/39 Ar测年。 这两种宇宙射线理论将被用来计算样本的年龄，使用36 Cl和3 He浓度，然后将与40 Ar/39 Ar测年计算的年龄进行比较。 准确的宇宙射线理论将是一个与40 Ar/39 Ar测年相同的年龄。 准确理论的确定将使宇宙成因表面测年方法在地球上任何地方都能使用。技术说明：宇宙射线在地球表面岩石中产生的核素广泛用于第四纪地质年代学和地貌研究，并且其使用量逐年增加。最近完成的CRONUS-地球项目（宇宙射线在地球上产生的核素）系统地评估了宇宙成因核素的生产率和理论基础。然而，CRONUS-Earth项目无法区分两种主要的理论方法：原始的Lal模型（St）和新的Lifton-Sato-Dunai模型（LSD）。数学模型用于将核素的产生作为地球上的位置、海拔和磁场配置的函数来缩放，这是这种测年方法的重要组成部分。无法区分这两种模型是因为预测的生产率在校准点的位置没有足够的差异。两种模型预测的南极埃里伯斯火山宇宙成因核素产生率存在显著差异。因此，埃里伯斯山是测试哪种生产模式最能描述地球仪上实际宇宙成因核素生产变化的绝佳地点。研究小组最近测量了从埃里伯斯熔岩流中提取的矿物分离物中的3 He和36 Cl。每种核素的暴露年龄在每个流内都是可重复的（~2%标准偏差），并且3 He和36 Cl年龄之间非常一致。然而，由于生产率对该纬度和海拔的比例的敏感性，通过St和LSD比例方法计算的年龄相差约15-25%。这些结果使人相信，埃里伯斯有资格作为一个合适的高纬度/高海拔校准网站。仍然缺乏的其余组件是准确和可靠的独立（即，然而，由于熔融包裹体中含有过量的氩，已公布的40 Ar/39 Ar年龄过于不精确，并且通常偏向于更老的年龄。研究小组的新40 Ar/39 Ar数据表明，现代质谱法和更好的样品制备克服了Erebus斜长岩地质年代学以前的问题。 这表明该提议在确定准确的全球缩放模型方面很有可能成功。尽管令人鼓舞，但仍有许多工作要做。 该项目将对海拔3公里以上的熔岩流进行采样，并确定其40 Ar/39 Ar和暴露年龄。这些综合数据将区分这两种定标方法，从而为全球宇宙成因地质年代学提供一个首选的定标模型。LSD方法包含两个子方法，“普通”LSD对所有核素进行相同的标度，而LSDn对每个核素单独进行标度。该项目可以使用不同海拔熔岩流的3 He和36 Cl数据区分这些模型，因为第一个模型预测这两种核素的生产率将不随海拔变化，第二个模型预测在采样的海拔范围内应该有10%的差异。最后，该项目将为南极洲的宇宙生成核素调查提供一个当地的有限年龄校准场地。

Nontechnical Description: The age of rocks and soils at the surface of the Earth can help answer multiple questions that are important for human welfare, including: when did volcanoes erupt and are they likely to erupt again? when did glaciers advance and what do they tell us about climate? what is the frequency of hazards such as landslides, floods, and debris flows? how long does it take soils to form and is erosion of soils going to make farming unsustainable? One method that is used thousands of times every year to address these questions is called 'cosmogenic surface-exposure dating'. This method takes advantage of cosmic rays, which are powerful protons and neutrons produced by supernova that constantly bombard the Earth's atmosphere. Some cosmic rays reach Earth's surface and produce nuclear reactions that result in rare isotopes. Measuring the quantity of the rare isotopes enables the length of time that the rock or soil has been exposed to the atmosphere to be calculated. The distribution of cosmic rays around the globe depends on Earth's magnetic field, and this distribution must be accurately known if useful exposure ages are to be obtained. Currently there are two remaining theories, narrowed down from many, of how to calculate this distribution. Measurements from a site that is at both high altitude and high latitude (close to the poles) are needed to test the two theories. This study involves both field and lab research and includes a Ph.D. student and an undergraduate student. The research team will collect rocks from lava flows on an active volcano in Antarctica named Mount Erebus and measure the amounts of two rare isotopes: 36Cl and 3He. The age of eruption of the samples will be determined using a highly accurate method that does not depend on cosmic rays, called 40Ar/39Ar dating. The two cosmic-ray theories will be used to calculate the ages of the samples using the 36Cl and 3He concentrations and will then be compared to the ages calculated from the 40Ar/39Ar dating. The accurate cosmic-ray theory will be the one that gives the same ages as the 40Ar/39Ar dating. Identification of the accurate theory will enable use of the cosmogenic surface dating methods anywhere on earth. Technical Description: Nuclides produced by cosmic rays in rocks at the surface of the earth are widely used for Quaternary geochronology and geomorphic studies and their use is increasing every year. The recently completed CRONUS-Earth Project (Cosmic-Ray Produced Nuclides on Earth) has systematically evaluated the production rates and theoretical underpinnings of cosmogenic nuclides. However, the CRONUS-Earth Project was not able to discriminate between the two leading theoretical approaches: the original Lal model (St) and the new Lifton-Sato-Dunai model (LSD). Mathematical models used to scale the production of the nuclides as a function of location on the earth, elevation, and magnetic field configuration are an essential component of this dating method. The inability to distinguish between the two models was because the predicted production rates did not differ sufficiently at the location of the calibration sites. The cosmogenic-nuclide production rates that are predicted by the two models differ significantly from each other at Erebus volcano, Antarctica. Mount Erebus is therefore an excellent site for testing which production model best describes actual cosmogenic-nuclide production variations over the globe. The research team recently measured 3He and 36Cl in mineral separates extracted from Erebus lava flows. The exposure ages for each nuclide were reproducible within each flow (~2% standard deviation) and in very good agreement between the 3He and the 36Cl ages. However, the ages calculated by the St and LSD scaling methods differ by ~15-25% due to the sensitivity of the production rate to the scaling at this latitude and elevation. These results lend confidence that Erebus qualifies as a suitable high- latitude/high-elevation calibration site. The remaining component that is still lacking is accurate and reliable independent (i.e., non-cosmogenic) ages, however, published 40Ar/39Ar ages are too imprecise and typically biased to older ages due to excess argon contained in melt inclusions.The research team's new 40Ar/39Ar data show that previous problems with Erebus anorthoclase geochronology are now overcome with modern mass spectrometry and better sample preparation. This indicates a high likelihood of success for this proposal in defining an accurate global scaling model. Although encouraging, much remains to be accomplished. This project will sample lava flows over 3 km in elevation and determine their 40Ar/39Ar and exposure ages. These combined data will discriminate between the two scaling methods, resulting in a preferred scaling model for global cosmogenic geochronology. The LSD method contains two sub-methods, the 'plain' LSD scales all nuclides the same, whereas LSDn scales each nuclide individually. The project can discriminate between these models using 3He and 36Cl data from lava flows at different elevations, because the first model predicts that the production ratio for these two nuclides will be invariant with elevation and the second that there should be ~10% difference over the range of elevations to be sampled. Finally, the project will provide a local, finite-age calibration site for cosmogenic-nuclide investigations in Antarctica.