岩石暴露年龄的光释光测年方法探索

A reliable numerical chronology is central to understanding environment changes, geomorphological processes, and major hazards associated with tectonic activity such as fault movement and landslides triggered by earthquakes. Robust rock exposure dating techniques are essential for this purpose, and luminescence provides an exciting new approach to this. The latent luminescence signal in the solid rock surface is reset (or bleached) by prolonged exposure to daylight. The longer the exposure time, the further into the rock is the luminescence signal reset; this provides the basic principle of rock surface luminescence dating.The main purpose of this project is to test and develop this new surface exposure dating method. The reliability of the new method will be calibrated and tested against a series of known ages samples, including modern normal fault samples, archaeological samples of historically known age and glacial boulder samples with cosmogenic nuclide ages. Two key problems will be addressed by this project. First, we will determine the most appropriate target mineral (quartz, K-feldspar or Na-feldspar) or, as an alternative, make use of whole rock slices, to provide the most appropriate stable dosimetric signal for dating. The estimation of model parameters (such as the photoionisation cross section, photon flux and attenuation coefficient of light) required for calculating an exposure age will be derived by fitting an existing model to luminescence/depth profiles from known age samples. The main outcome of this project will be the testing and proving of a novel rock-surface dating method with broad application in many fields in Quaternary geology, geomorphology and archaeology.

发展岩石暴露年代测定技术对于环境变化、构造活动、地貌过程、重大地质灾害等研究都有重大意义。岩石暴露表面一定深度的释光信号可能被阳光晒退，岩石表层被晒退的深度随着岩石暴露时间的延长而增加，这就是应用释光技术测定岩石暴露年代的基本原理。本项目旨在探索测定岩石暴露年代的释光方法和技术。为此，对不同时段的已知年龄(如现今的、有历史文献记载的、原地宇宙成因核素测定的)岩石暴露面，进行表层岩石薄片和矿物颗粒释光信号特征等的研究，遴选出适合释光测定的矿物及其稳定的释光信号，研究各个实例的岩石释光信号晒退速率、光透系数，以及其他影响或制约因素，建立释光信号与光晒退深度的函数关系，估计岩石暴露年代，并与已知年龄进行比较，借以评估所应用的释光技术测定岩石暴露年代的可行性、可靠性和适用性。

相对沉积物矿物颗粒光释光测年，岩石暴露面光释光测年除研究不同岩性的岩石样品对测年的影响程度外，还需获取不同暴露方向、不同表面条件、不同海拔特征下光晒退速率和光透系数值，这是成熟使用这一方法的基础。此外，利用宇宙成因核素10Be测年方法获得的已知年龄的漂砾样品来推算剥蚀速率对暴露年龄测定的影响，且与岩石释光测年方法测得的经历暴露、埋藏历史的漂砾样品的年龄作对比，分析判断粒级砾石释光暴露年龄在地貌面定年中的适用性和准确性。基于理论研究结果，探索此方法是否满足实际地质岩石样品古河道、河流阶地砾石的埋藏测年条件，选择实验测试现代河床砾石（砂岩、花岗岩）在河流搬运、磨蚀等的条件下光释光信号特征，从而分析讨论岩石光释光测年用于埋藏测年的可靠性。.课题研究结果表明：1) 结合漂砾与滑坡体砾石表面光释光测年方法与10Be暴露测年方法，首次通过释光信号计算得出了岩石的风化剥蚀速率；通过对比大、小漂砾的埋藏面不同的信号晒退特征，提出了用释光信号指示漂砾搬运、埋藏过程的新方法。2) 岩石岩性、不同岩石暴露方向、不同表面条件、不同海拔对暴露年龄的测定都有不同程度的影响； 3) 沿现代河床的上游至下游，其河流砾石信号快速的晒退归零过程发生在距离源头约60 km内，由于砾石搬运、沉积过程中，不同砾石形状上的差异，稳定停留会存在方向的优先选择花岗岩砾石朝上暴露表面的晒退归零深度大于埋藏面的深度。.本研究的实验结果为准确获得岩石样品定年所需的光晒退速率和光透系数值这两个重要参数奠定了基础，为了解不同岩石样品的光释光信号晒退速率、测年对象的合理选择以及岩石样品采集等提供了实验数据支持；证明了河流花岗岩砾石表层的光释光信号在河流阶地砾石的埋藏年龄测定上有巨大的潜力，为河流搬运动力学提供了研究新思路。