岩石释光埋藏测年中砾石晒退程度的快速评估

信号晒退是释光测年的前提。最近几年发展起来的岩石释光埋藏测年技术，可以依据释光-深度曲线判断砾石样品释光信号晒退程度，这是该技术的一大优势之一。已有的测年实践显示，在不均匀晒退的沉积环境中，可能仅有少数砾石释光信号晒退良好，而大部分砾石年代高估。因此，非常有必要发展快速判断砾石释光信号晒退程度的方法，以提高测量效率。本文介绍了砾石释光信号晒退的几种常见情景及其筛选的理论基础，基于砾石释光信号由表及里晒退的事实，以及砾石表层剂量率变化大、测试深度需要、岩片平整度等的考虑，尝试将砾石表面约2 mm的释光信号与饱和释光信号进行对比，并结合统计分析方法(文中称“表_{~2 mm}/饱和信号法”)，判断来自青藏高原东部横断山脉雀儿山北麓当子沟(32.1°N，98.9°E) 的典型不均匀晒退环境——冰川沉积的159个花岗岩冰水砾石的释光信号晒退程度。通过建立39根岩芯的完整释光-深度曲线，验证此方法可用于快速识别沉积前释光信号晒退良好的砾石。在此基础上，初步探讨了砾石释光信号晒退程度与沉积环境、砾石大小、磨圆度、球度、结构、砾石ab面方向间的关系，刻划释光信号晒退良好砾石的表观特征，以辅助岩石释光埋藏测年野外采样。

Rapid assessment of bleaching of cobbles in rock luminescence burial dating

Bleaching is a precondition in luminescence dating. Rock luminescence burial dating technique developed in recent years is able to assess the degree of bleaching of cobbles according to their luminescence-depth profile. This is one of the greatest advantages of the technique. Previous dating works showed that in heterogeneously bleaching environments, only a few cobbles were well bleached, most of the ages could be overestimated. Therefore, it is necessary to develop a rapid method to assess the degree of bleaching of cobbles, to improve measuring efficiency. This paper introduced common scenarios of cobble bleaching and the theoretical basis of bleaching screening. Base on the fact that cobble is bleached from surface to interior, considering the significant variation of dose rate at the surface layer of cobble, requirement of bleaching depth, flatness of slice, a rapid screening method(surface~2mm/saturated signal method)in the laboratory was designed. We try to compare luminescence signal of surface ca. 2 mm in depth with the saturated luminescence signal of cobbles to obtain the surface~2mm/saturated signal ratio. The surface~2mm/saturated signal ratios of all cores are divided into different components using the finite mixture model(FMM). The lowest component was classified as well bleached, the other components were poor bleached, among which with ratio ≥ 70 % were classified as saturated. One hundred and fifty-nine cobbles collected from nine sites at different glaciofluvial terraces and moraines(typical heterogeneously bleaching environments)with different chronologies in the Dangzi Valley(32.1°N, 98.9°E), located on the northern slope of Que'er Shan Mountain, eastern Tibetan Plateau, were explored. The statistical result shows that about 1/4 of the cores are well bleached, and the remaining 3/4 of the cores are poor bleached, of which about 1/4 were saturated. Cobbles that were well bleached prior to burial were able to be quickly identified, as verified by the established luminescence-depth profiles of selected 39 cores. The relationships between the degree of bleaching of cobble and depositional environment, size, roundness, sphericity, texture and ab plane directions of cobbles were investigated based on the screening results. For granite cobbles with an average diameter of about 40~94 mm, the size has little relationship with the bleaching degree, and there is no obvious relationship between the grain size, relative grain size and the bleaching degree as well. Both the upper and lower ab surfaces of the cobble have the opportunity to be exposed and thus bleached, and there is no obvious difference in bleaching degree between the two sides. The cobbles with poor roundness and low sphericity have better bleaching. The characteristics of well bleached cobbles provide guidelines for sampling in the field for rock luminescence burial dating.