Fundamentals of Surface Exposure Dating: Latitude, Matrix and Geometric Effects

宇宙成因核素为确定岩石在地球表面暴露于宇宙射线的时间以及确定地貌表面的侵蚀速率提供了重要工具。已经证明了几个重要的应用：通过冰碛上岩石的暴露年龄来确定过去冰川时期的年代，确定火山爆发的年代，以及确定陨石坑撞击的年代。虽然这些研究已经成功地产生了半定量的结果，但可靠的解释需要更好地了解宇宙核素产生机制的基本原理。地表暴露定年研究得最少的三个方面是宇宙射线通量对纬度的依赖性、生产速率对岩石基质的依赖性以及生产速率对岩石几何形状的依赖性。关于宇宙射线通量的有限现有数据不包括空气/地面边界的重要影响。该提案要求资助使用加速器质谱法在同一年代、同一地点、不同岩性的几块岩石中测量10Be、26Al和36Cl的深度剖面。纬度校正将使用来自不同纬度但年龄相同的花岗岩进行研究。所有岩石的年代都将独立确定，并将显示冰川抛光，以排除侵蚀。将使用蒙特卡罗粒子输运代码建立一个生产率模型，并将测量值与理论值进行比较。开发的产量模型可用于预测其他不同岩性、不同几何形状和不同纬度岩石的地表产量。

Cosmogenic nuclides provide an important tool for determining the time a rock has been exposed to cosmic rays on the Earth's surface and also for determining erosion rates of geomorphic surfaces. Several important applications have already been demonstrated: dating past glacial episodes from exposure ages of rocks on moraines, dating volcanic eruptions, and dating meteor crater impacts. Although these studies have been successful in producing semi-quantitative results, reliable interpretation requires a better understanding of the fundamentals of production mechanisms of cosmogenic nuclides. The three aspects of surface exposure dating that have been studied least are the dependence of the cosmic ray flux on latitude, the dependence of the production rates on the rock matrix, and the dependence of production rates on the geometry of the rock. The limited data available on the cosmic ray flux does not include important effects of the air/ground boundary. This proposal requests funding to measure 10Be, 26Al, and 36Cl depth profiles in several rocks of same age and in the same location but of different lithology using accelerator mass spectrometry. The latitude correction will be investigated using granitic rocks of same age, but from different latitudes. All rocks will be dated independently and will show glacial polish to rule out erosion. A production rate model will be developed using Monte Carlo particle transport codes and measurements will be compared with theoretical values. The production rate model developed will then be available for others to predict surface production rates in other rocks of different lithology, other geometries, and other latitudes.