岩石暴露年龄释光测定的若干影响因素研究

摘要

近年来,光释光测年技术已被成功应用于岩石暴露年龄的测定,在地质、地貌、气候和考古等研究领域显示出广泛的应用前景.该技术能否准确并广泛应用于经历各种地质活动而暴露的基岩、砾石定年,了解影响其表层光释光信号晒退速率的因素至关重要.文章采用红褐色长石石英砂岩、灰白色黑云母花岗岩和表面附着有岩石漆的花岗角砾岩,针对样品岩性、采样面朝向、表面覆盖物(岩石漆)3个因素对红外释光(IRSL)50度测量信号的影响程度进行了晒退实验.实验结果显示:岩石表层一定深度IRSL信号可被太阳光快速晒退,且随着曝光时间增长,砂岩和花岗岩释光信号的晒退深度均逐渐增大,与深色砂岩相比,浅色的花岗岩有更快的晒退速率;在阳光下曝光约5114 h后,花岗岩朝上一面的信号晒退深度大于朝北一面,表现为拟合朝上一面的晒退曲线所获—σφ0值和曲线拐点深度值皆大于朝北一面,揭示在该暴露时间内(如约5114 h)不同采样面朝向对信号晒退有一定程度的影响;阳光下暴露相对较短的时间(如40 h),样品表面覆盖物(岩石漆)可能因对光线的屏蔽作用而使光释光信号晒退速率减小,无岩石漆覆盖的岩块具有更深的信号晒退深度,但随着暴露时间的延长(如约2416 h),这种影响程度渐趋减弱.该研究为准确获得岩石样品定年所需的光晒退速率百—σφ0和光透系数μ这两个重要参数奠定了基础,也为了解不同岩性岩石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.%Optically Stimulated Luminescence(OSL)age-dating has been successfully applied to determine the exposure ages of rock surfaces.This technique is widely applicable in the fields of geology,geomorphology,climatology,and archaeology.The OSL dating method can accurately measure the age of bedrock and gravel exposed after a particular geological event.However,it is important to study factors that may affect the bleaching rate of the Infrared Stimulated Luminescence(IRSL) at 50 ℃ signal from a rock surface,including sample lithology,sampling surface orientation,and whether the rock surface has a patina (rock varnish).To analyze the effects of these factors,we performed a series of rock surface bleaching experiments on reddish-brown argillaceous feldspar sandstone,grey biotite granite,and granite breccia.The results show that the IRSL signal at certain depths beneath sandstone or granite rock surfaces can be quickly reset by exposure to sunlight.The longer the bleaching time,the deeper the bleaching depth.Furthermore,the bleaching rate of the IRSL signal in lighter granite is much higher than in darker sandstone.The sampling surface orientation (i.e.,upward or northward facing)has no significant effect on the resetting depth of the IRSL signal.After sunlight exposure for ca.5114 hours,however,the IRSL luminescence-depth profiles within granite show that the effective luminescence decay rate(—σφ0) and the inflection depth from the rock surface of bleaching inflection are higher for an upward-facing surface than for a north-facing surface,thereby indicating that rock surface orientation exerts an influence on age-dating results after ca.5114 h of exposure.For a short exposure time (40 h),the presence of a rock varnish may have a shielding effect and significantly reduce the bleaching rate of the IRSL signal,whereas a rock surface without rock varnish has a deeper bleaching depth.The shielding effect of the rock varnish decreases when the surface exposure time increases to ca.2416 h.This study advances our understanding of IRSL signal bleaching within rock surfaces,and provides a useful theoretical basis for the selection of appropriate rock surfaces for sampling to improve the accuracy of OSL age-dating.