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许多对人类福祉很重要的问题，包括：火山是什么时候爆发的，它们有可能再次爆发吗？冰川是什么时候前进的，它们告诉我们气候的什么信息？诸如滑坡、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不可持续？一种每年被使用数千次来解决这些问题的方法被称为“宇宙表面暴露测年”。这种方法利用了宇宙射线，宇宙射线是超新星不断轰击地球大气层产生的强大的质子和中子。一些宇宙射线到达地球表面并产生核反应，产生稀有的同位素。测量稀有同位素的数量可以计算出岩石或土壤暴露在大气中的时间长度。宇宙射线在地球上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，关于如何计算这种分布，从众多理论中缩小出来，剩下两种理论。测试这两种理论需要在高海拔和高纬度（接近两极）的地点进行测量。这项研究包括实地和实验室研究，包括一名博士生和一名本科生。研究小组将从南极洲一座名为埃里伯斯火山的活火山的熔岩流中收集岩石，并测量两种稀有同位素：36Cl和3He的含量。样品的喷发年龄将使用一种不依赖于宇宙射线的高度精确的方法来确定，这种方法被称为40Ar/39Ar测年法。这两种宇宙射线理论将被用来计算样品的年龄，使用36Cl和3He浓度，然后将与40Ar/39Ar测年计算的年龄进行比较。精确的宇宙射线理论将给出与40Ar/39Ar年代相同的年龄。确定准确的理论将使在地球上任何地方使用宇宙表面测年方法成为可能。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学研究，其用途每年都在增加。最近完成的CRONUS-Earth项目（地球上宇宙射线产生的核素）系统地评估了宇宙产生核素的产生速度和理论基础。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）。用于按比例计算核素产量的数学模型是地球上的位置、海拔和磁场结构的函数，是这种测年方法的重要组成部分。无法区分这两种模型的原因是，在校准地点的位置上，预测的生产率没有足够的差异。两种模型预测的宇宙成因核素产生率在南极洲埃里伯斯火山上差异很大。因此，埃里伯斯山是测试哪种生产模型最能描述全球实际宇宙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3He和36Cl。每种核素的暴露年龄在每次流动中都是可重复的（约2%的标准偏差），并且3He和36Cl的年龄非常吻合。然而，由于该纬度和高程的产量对结垢的敏感性，St和LSD结垢方法计算出的年龄差异约为15-25%。这些结果使人们相信埃里伯斯有资格成为一个合适的高纬度/高海拔校准地点。仍然缺乏的剩余成分是准确和可靠的独立（即非宇宙成因）年龄，然而，已发表的40Ar/39Ar年龄太不精确，并且由于熔体内含物中含有过量的氩而通常偏向于更古老的年龄。研究小组新的40Ar/39Ar数据表明，以前的ererebus斜长石地质年代学问题现在已经被现代质谱法和更好的样品制备所克服。这表明该建议在定义精确的全球标度模型方面成功的可能性很高。虽然令人鼓舞，但仍有许多工作有待完成。该项目将对海拔3公里以上的熔岩流进行采样，并确定其40Ar/39Ar和暴露年龄。这些合并的数据将区分两种标度方法，从而产生全球宇宙成因年代学的首选标度模型。LSD方法包含两个子方法，“普通”LSD对所有核素进行相同的测量，而LSDn对每个核素进行单独的测量。该项目可以使用来自不同海拔的熔岩流的3He和36Cl数据来区分这两种模型，因为第一个模型预测这两种核素的产生比将随海拔不变，而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核素调查提供一个局部的、有限年龄的校准点。

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.