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

宇宙成因核素为确定岩石暴露在地球表面的宇宙射线时间和确定地貌表面的侵蚀速率提供了重要工具。已经展示了几个重要的应用：从冰川上岩石的暴露年龄确定过去的冰川时期，确定火山喷发的日期，以及确定陨石坑撞击的日期。尽管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理解宇宙源核素产生机制的基本原理。表面曝光测年研究最少的三个方面是宇宙射线通量与纬度的关系、产生率与岩石基质的关系以及产生率与岩石几何形状的关系。现有的关于宇宙射线通量的有限数据不包括空中/地面边界的重要影响。这项提案要求提供资金，使用加速器质谱仪测量相同年龄、相同地点但不同岩性的几块岩石中的10Be、26Al和36Cl深度剖面。纬度校正将使用年龄相同但来自不同纬度的花岗岩进行研究。所有岩石都将独立测定年代，并将显示冰川抛光，以排除侵蚀。将使用蒙特卡罗粒子输运程序开发一个生产率模型，并将测量结果与理论值进行比较。然后，开发的产量模型将可用于其他人预测不同岩性、其他几何形状和其他纬度的其他岩石的地面生产率。

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.