研究论文 正式出版 版本 2 Vol 9 (3) : 223-229 2018
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砂岩和花岗岩岩石表层光释光信号晒退速率对比及测年意义
The bleaching rate of IRSL signal of granite, sandstone and the significance for rock surface dating
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: 2018 - 05 - 10
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摘要&关键词
摘要:光释光信号已被成功地应用于岩石表层暴露、埋藏年龄的测定以及判别岩石经历的曝光和埋藏历史。本文选择红褐色长石石英砂岩和灰白色黑云母花岗岩进行了岩块和岩片晒退不同时间的实验研究,结果显示,岩石光释光信号(IR50;IRSL)可被光(阳光和全光谱模拟太阳灯灯光)快速晒退。曝光一定时间后,岩块表层一定深度的光释光信号可以归零,且随曝光时间的延长,自暴露面向内的光释光信号归零深度也随之加深。砂岩和花岗岩的光晒退对比实验结果显示,与深色砂岩相比,浅色的花岗岩有更高的光释光信号晒退速率。实验同时揭示,本研究所采用的红外光源激发获得的光释光信号来自岩片(1.2 mm)表层一定深度,并没有完全穿透该岩片。该研究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、前处理和测试等提供了实验数据支持。
关键词:岩石暴露年龄;光释光测年;光释光信号;晒退速率;砂岩;花岗岩
Abstract & Keywords
Abstract: Background, aim, and scope Optically Stimulated Luminescence (OSL) signal has been successfully used to determine the exposure and burial age of rock surfaces and discriminate the history of rocks experienced. OSL dating of sediments determines the time elapsed since the last sunlight exposure event of the mineral (quartz or feldspar), i.e. the depositional time. Recently, this approach is used to measure luminescence signal with depth into rock surfaces that exposed to light, the valuable bleaching information on the rock surfaces can directly obtain from luminescence-depth profiles. And thus, it’s important to know the infrared stimulated luminescence (IRSL) at 50℃ signal (IR50) bleaching rate of different rock lithologies, and whether the IRSL signal could be completely bleached after exposing by daylight. Materials and methods We selected Reddish-brown argillaceous feldspar sandstone and hoary biotite-granite as samples and designed a series of rock surface bleaching experiments. The first set of experiments was that sandstone and granite rocks were exposed to sunlight (it started at July) for different periods of time (0.5 h, 3 h, 6 h, 40 h, 736 h); the second set was that these signal saturation rock slices from inner part of the rock (previously unexposed to light) were bleached under both a sunlight simulator lamp (SOL2) and daylight (on July) for different periods of time (0.0083 h, 0.083 h, 0.5 h, 1 h, 3 h). Luminescence measurements of all samples were taken by cutting pillars from the rock block and the slices with one millimeter thickness were then cut from these pillars using a water-cooled low-speed wafering saw. For all luminescence signal, we used IRSL signal of potassium-rich feldspar fractions from the rock. Results The results show that the IRSL signal of rocks can be quickly reset by exposing in light (sunlight and SOL2). For the rock surface experiment, it is obvious that the rock surface residual IRSL signal from light-colored granite are less than dark-colored sandstone for exposing same time, and the bleaching depth of granite rocks is deeper. while rock slices bleaching under daylight or SOL2, the bleaching level of granite and sandstone rock slices is same. IRSL signal could be bleached at or near zero level after exposing 0.5 h. However, for sandstone slices exposed under SOL2 for different time, it was found the residual luminescence signal intensity from the back side of slice is obviously stronger than the front side. Discussion Godfrey-Smith et al (1998) testified that the OSL signal from quartz is easier to bleach by light. The IRSL signal from rock can also be fast bleached by light in this study. The IRSL signal is bleached in certain depth into the rock surface after a given exposure time, and the longer the bleaching time is, the deeper the bleaching depth. The IRSL signal of granite rock surface can be rapidly bleached during a short time. However, the signal from the second slice (1-2 mm) of sandstone rock has not been completely bleached after exposing 736 h, and the first slice has extremely low signal value. By contrast with the measurement result of rock slices, it can prove that the light could not penetrate through the whole sandstone rock slice (with 1.2 mm of thickness) with exposing for a short time and the infrared light can only penetrate a certain depth of rock surface. Conclusions The light bleaching experiment results show that the bleaching rate of light-colored granite is much higher than that of the opaque dark-colored sandstone. This study also reveals that IRSL at 50℃ signal is just originated from a certain depth of the slice (1.2 mm thickness) and the slice is not fully penetrated by infrared light. Recommendations and perpection Predecessors had proved that OSL dating of rock surface is a robust and reliable dating technique. It is superior to conventional sediment dating. This study is significant for determining the bleaching rate of different lithological rocks and for selecting proper dating samples, pretreatment and measurement in practical application.
Keywords: rock surface dating; IRSL signal; bleaching rate; sandstone; granite
光释光测年(Optically Stimulated Luminescence dating,OSL dating)已被广泛用于测定各类第四纪沉积物的年代。光释光年龄是指松散沉积物中石英或长石等矿物颗粒自最后一次曝光后被沉积埋藏至今所经历的时间,即埋藏年龄(Aitken,1998)。最近,光释光信号被用来尝试测量岩石表层一定深度内的辐射剂量变化特征(Habermann et al, 2000; Simms et al,2011;Sohbati et al,2011,2012a,2012b,2012c;Chapot et al,2012),进而通过建立释光信号自岩石表面向内的释光信号晒退曲线与晒退时间的关系,来估算岩石的暴露年龄(Sohbati et al,2012b)。岩石形成后,经过漫长的地质时期,其积累的释光信号已经饱和,如果经历断层错动、滑坡、崩塌、河流搬运或人类移动等活动而暴露于地表,在阳光晒退作用下,岩石外层的石英和长石颗粒晶格缺陷内俘获的电子会陆续逃逸,也即积累的释光信号逐渐被晒退,直至达到接近或完全归零程度。这个释光信号归零深度将随着暴露时间的延长而加深。岩石释光残余信号强度L与暴露时间t存在如下关系(Sohbati et al,2012b):
L0,饱和释光信号强度;σ,光解离截面积(cm2);φ0,岩石暴露表面光子通量(cm-2·s-1);μ,光透系数(mm-1);\(\stackrel{-}{{\text{σ}\text{φ}}_{\text{0}}}\),岩石的光晒退速率(刘进峰,2014),其中,\(\stackrel{-}{{\text{σ}\text{φ}}_{\text{0}}}\)和μ与岩石类型和特征相关。可通过测量已知暴露年龄的岩石不同深度的释光信号,建立并拟合深度-释光信号强度-时间关系曲线,获得岩石光晒退参数\(\stackrel{-}{{\text{σ}\text{φ}}_{\text{0}}}\)和μ值;然后,测量未知暴露年龄岩石不同深度的释光信号,将上述获得的光晒退参数\((\stackrel{-}{{\text{σ}\text{φ}}_{\text{0}}}\)和μ值)代入该测年模型进而推算岩石的暴露年龄。理论模拟计算结果显示,释光暴露年龄方法的可测年龄上限可达100 ka(Sohbati et al,2012c)。当岩石暴露面被沉积埋藏后,其表层光释光信号又重新积累,通过测试岩石埋藏面表层岩片的光释光信号强度或者自表层向内释光信号随深度的变化规律,可测定埋藏面的埋藏年龄以及判别该面所经历的曝光和埋藏历史(Sohbati et al,2015;Freiesleben et al,2015)。
岩石暴露年龄释光测年方法在地质、地貌、气候和考古等研究领域有广泛的应用前景,但在地质样品测定的具体应用中还存在一些基本问题。比如具体研究对象,特别是岩性选择、释光信号测量条件等,尚需通过使用具体的地质样品进行室内模拟实验等进行深入研究。因此,我们选择常见岩石中颜色差别明显的深色砂岩和浅色花岗岩为研究对象,通过测定曝光不同时间后其表层一定深度范围内的释光信号变化规律,了解不同特征类型岩石的释光信号晒退情况。
1   样品制备和实验流程
1.1   样品选择和制备
样品为购于北京石材城的红褐色砂岩和灰白色花岗岩大块石板(长、宽、厚尺寸约为130 cm×60 cm×10 cm)。通过偏光显微镜观察砂岩和花岗岩岩石薄片,鉴定其分别为泥质胶结长石石英砂岩和黑云母花岗岩。砂岩石英、长石的粒径为0.1-0.3 mm,花岗岩石英、长石的粒径为0.5-2 mm;用显微镜目估含量法测定砂岩矿物组成约为石英(50%)、长石(30%)、方解石(10%)和泥质胶结物(10%),花岗岩矿物组成约为石英(40%)、长石(55%)和云母(5%)(图1)。大块岩板在水冷却环境下切割成小岩块(12 cm×10 cm×6 cm),置于实验室中避光保存。为了避免岩石表层被光晒退,切割时保证在避光或暗光条件下进行,且新鲜切割面立刻用锡箔纸包裹后避光保存。自岩石表层向内的样品,前人通常采用水钻钻取直径约10 mm的岩芯(Sohbati et al,2012a;Liu et al,2016)。本研究通过切割岩柱的方式获得自表层向内的系列样品,岩柱尺寸为7 mm×7 mm×60 mm(图2)。暗室内,将岩柱在切片机上连续切割成厚度为0.7 mm的岩片备测,存在0.3 mm厚度的损耗。为了减少单根岩柱测试数据的不确定性,同一样品均测试三根岩柱的岩片。


图1   (a)红褐色泥质胶结长石砂岩薄片的正交偏光照片,其中左下角插图为砂岩石板原样;(b)灰白色黑云母花岗岩薄片的正交偏光照片,其中左下角插图为花岗岩石板原样。
Fig.1 (a) Cross-polarized images of Reddish-brown argillaceous feldspar sandstone thin section, with the sandstone rock plate photo shown in the left-bottom. (b) Cross-polarized images of hoary biotite-granite thin section, with the granite rock plate photo shown in the left-bottom.


图2   载样盘、岩柱和岩片。
Fig.2 Sample holder, rock pillars and rock slices.
1.2   实验仪器和流程
光释光信号的测量仪器为丹麦RisøTL/OSL-DA-20型释光测量仪。激发光源是红外光(波长:875nm,最大功率:130mWcm-2),测量时激发光源光强为最大功率的90%,均采用50℃下红外激发IRSL信号(IR50),测试流程采用红外IRSL的实验流程(Hütt et al,1988;Wallinga et al,2000)(表1),光释光信号通过EMI 9235QA型光电倍增管(PMT)检测,在激发光源和PMT之间附加Schott BG3和BG39滤光片组合。辐照源为Risø TL/OSL-DA-20型自动测量系统机载β源。岩片直接放在释光测量仪的载样盘上进行测量,岩片朝向实验晒退面的一侧在切片时做了标记,有标记面统一朝上放置(图2)。
表1   红外激发(IRSL)的实验流程(Hütt et al,1988;Wallinga et al,2000)
序号Set处理方法Treatment输出值 Output
1预热(250℃,100 s)
Preheat (250℃, 100 s)
2激发(红外光激发,50℃,200 s)
Stimulate (IR LED, 50℃, 200 s)
Lx (IR50)
3附加实验剂量
Give test dose
4再次预热(250℃,100 s)
Preheat ( 250℃,100s)
5激发(红外光激发,50℃,200 s)
Stimulate (IR LED, 50℃, 200 s)
Tx (IR50)
2   光晒退实验结果与讨论
2.1   岩块信号阳光晒退实验
分别取五块切好的砂岩和花岗岩岩块,岩块四周用锡纸和胶带包裹避光,仅剩新鲜切割面暴露,将切割面朝上,放置于中国地震局地质研究所(北京)九楼屋顶平台上(四周无遮挡),置于阳光下晒退不同的时间(图3)(2016年8月开始)。实验开始前,先检测岩块新鲜切割面的IR50释光信号,自岩块新鲜切割面向内的变化趋势显示(图4),砂岩和花岗岩岩块的新鲜面均处于IR50释光信号饱和状态,同时也表明前处理过程未影响原始光释光信号。


图3   阳光下岩块的晒退实验
Fig.3 The solar bleaching experiment of rocks


图4   IR50释光信号自岩块切割面(实验晒退面)向内的变化。(a)砂岩;(b)花岗岩。
Fig.4 The variation of the sensitivity-corrected IRSL signal with depth into the rock. (a) Sandstone; (b) Granite.
阳光晒退不同时间后,自暴露面向内的释光信号变化趋势如图5a(砂岩)和5b(花岗岩)所示。红褐色长石砂岩和灰白色黑云母花岗岩表层释光信号对阳光均有较快的晒退反应,随着晒退时间延长,信号晒退曲线逐渐向岩块内部加深;相比此类深色砂岩,曝光相同时间,浅色花岗岩表层岩片残余信号值更小,晒退归零深度更大。比如,晒退半小时后,砂岩第一个测片(0-1 mm深度)的信号强度为饱和值的7%;花岗岩第一个测片为饱和值3%,信号几乎归零,其第四个测片(3-4 mm)部分信号也已经开始被晒退,约为饱和值的72%。晒退3小时后,砂岩第二个测片(1-2mm深度)的信号也受到光照影响,降至饱和值的70%;而花岗岩的第五个测片(4-5mm深度)约为饱和值的70%。晒退约736 h后,砂岩表层信号晒退的影响深度(定义为自岩石表面到信号饱和的深度)不足4 mm,而花岗岩在自表面向内7 mm的深度内,其信号已经基本归零,直到20mm处信号仍未达到饱和(图5b)。


图5   阳光晒退不同时间后,砂岩(a)和花岗岩(b)自暴露面向内的IR50释光信号强度变化。
Fig.5 Variation of IRSL intensity with depth after different sunlight bleaching time into sandstone (a) and granite (b).
砂岩岩块晒退实验结果显示(图5a),晒退约736 h (60 d)后,第二个测片(1-2 mm深度)的释光信号仍未完全归零(饱和信号值的7%),第四个测片(~4 mm深度)的释光信号基本为饱和状态,说明受到的光晒退影响很小。既然砂岩释光信号有如此慢的光晒退速率,为什么在晒退很短的时间内(0.5 h),岩块表层第一个测片(0-1 mm深度)也显示极低的残余信号量(饱和信号强度的2%)?其可能的原因是释光信号仅来自岩片表层一定的深度,并非来自整个岩片。另外,即使岩块在阳光下暴露约736 h后,岩块1 mm深度内(即第一个测片)是否被完全晒透而信号完全归零,尚无法根据该实验结果判断。因此设计了不同光源对岩片IR50释光信号影响实验,其中通过测试岩片正反面的方法对此进行了分析。
2.2   岩片正、反面信号晒退实验
将取自砂岩和花岗岩岩块内部、释光信号饱和的岩片(1.2 mm厚)各分为2组,一组岩片在阳光下晒退不同时间(北京,2016年8月中午时间)(0.0083 h、0.083 h、0.5 h、1 h、3 h),另一组岩片置于模拟全光谱太阳灯(SOL2)之下约70 cm距离处晒退不同时间(0.0083 h、0.083 h、0.5 h、1 h、3 h)。为保证和岩块晒退实验条件一致,晒退时,将砂岩岩片和花岗岩岩片分别放在砂岩岩块和花岗岩岩块上。将太阳灯下晒退后的砂岩和花岗岩岩片再各分为2组,一组岩片晒退面朝上置于载样盘内测量,另一组岩片将晒退面朝下置于载样盘内测试。
经太阳灯晒退不同时间后,砂岩和花岗岩岩片正、反面对比测试结果如图6a所示。晒退0.0083 h (30 s)后,花岗岩岩片反面就明显受到了光照影响,信号衰减了~35%,但此时整个岩片的信号并未完全归零,因为岩片反面信号强度为正面的3倍。直至晒退0.5 h后,岩片正、反面信号强度才基本一致,未随晒退时间的延长而变化,说明整个岩片IR50释光信号已基本晒退到本底状态。砂岩岩片则不同,即使在晒退3 h后,其反面信号强度仍是正面的10倍,说明太阳灯晒退3 h,还不足以将1.2 mm厚的砂岩岩片完全晒透。


图6   (a)砂岩和花岗岩岩片的正面(虚心符号)、反面(实心符号)释光信号强度随太阳灯晒退时间的变化;(b)砂岩和花岗岩岩片释光信号强度随阳光(N)和太阳灯(SOL2)晒退时间的变化(三角形代表砂岩,圆形代表花岗岩)。
Fig.6 (a) Comparison of IRSL intensity from two sides of sandstone and granite slices after different exposing time under SOL2, (hollow symbol represent the upper side; solid symbol represent the back side). (b) Variation of IRSL intensity from sandstone and granite slices with different sunlight and SOL2 light exposing time (triangle and circular represent sandstone and granite, respectively).
然而,太阳灯晒退不同时间后,岩片正面的释光信号测试结果显示(图6a中虚心符号),无论岩片是否被完全晒透,砂岩和花岗岩岩片正面的信号衰减变化趋势基本一致。刚开始短时间晒退,信号衰减速率较快,比如晒退0.0083 h(30 s)后,岩片的释光信号已经衰减了初始饱和信号值的80%,光照1 h后,信号几乎接近本底,达到一个稳定值。由此揭示,红外激发光源激发200 s仅激发了岩片上部一定厚度内的光释光信号,并没有穿透整个岩片。
岩片在阳光和太阳灯下晒退不同时间后,其信号衰减趋势基本一致(图6b),对比图6a可知,晒退时间超过0.5 h后,信号均已基本晒退到本底状态,且未随晒退时间的延长而变化,说明阳光和太阳灯这两者的晒退深度应均大于红外光源200 s激发所达到的深度,否则同一晒退时间点,不同测片间的光释光信号应有差异。
3 结论及测年意义
矿物颗粒沉积埋藏前,其积累的释光信号被光或者热激发而完全归零是释光测年的基本前提之一(Aitken M J,1998)。然而,对于不同类型的沉积物而言,检验这一基本前提也并非易事。相比长石而言,石英矿物的光释光信号更易被光快速晒退(如Godfrey-Smith et al,1988)。然而,并不是所有类型的岩石都能提取出符合我们要求的单矿物,而且在复杂的物理破碎及化学前处理过程中,我们也无法准确计算因粒径变化而导致的剂量改变量。一般认为,红外激发光源激发的信号主要来自于钾长石(如Baril and Huntley,2003),并应用于岩石岩片测试中(Liu et al,2016)。
本研究中的岩石晒退实验结果揭示,岩石IR50释光信号也可被光快速晒退。与深色砂岩相比,浅色的花岗岩有更高的晒退速率,在相对较短的暴露时间内(比如3 h),花岗岩表层~3 mm深度内的释光信号基本归零。因此认为此类深色砂岩适于暴露时间较长的岩石定年,而花岗岩更适于年轻样品,较短的暴露时间即可满足埋藏年龄的测试要求。如果能从岩石中(比如砂岩)提取出石英颗粒,或者从岩片蓝光激发中分离出来自石英的释光信号,则在相同的晒退时间内,其晒退深度应更大于IR50信号晒退深度。另外,也可结合岩石埋藏面自表层向内的释光信号随深度的变化规律,对该埋藏面在埋藏前是否经历充分曝光历史进行进一步判别(Sohbati et al,2015;Freiesleben et al,2015)。因此,相比颗粒矿物,利用岩石表层进行光释光定年,有其独特的优势。
本文结果同时提示,在野外采样和室内处理岩石样品时,要注意样品的避光问题。另外,红外光源激发200 s不能穿透1.2 mm厚的砂岩和花岗岩岩片。这就要求在室内测试自岩石暴露面表层向内的一系列岩片时,应统一将岩片朝向暴露面的一面朝上放置于测量仪器载样盘上。
致谢:
陈岳龙教授、李大鹏副教授和包创博士在岩石成分分析中给予帮助,杨会丽工程师、罗明博士生和覃金堂副研究员在释光实验中给予协助和有益讨论。丹麦奥胡斯大学Andrew Murray教授、Reza Sohbati博士,以及卢演俦、周力平、张培震、马胜利和陈杰等先生在岩石释光测年探索中给予积极鼓励、支持和帮助。王旭龙研究员、康树刚副研究员对文章初稿提出有益建议。两位审稿人提出了很好的修改意见。在此一并感谢。
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稿件与作者信息
崔富荣
刘进峰
国家自然科学基金(41472161),地震动力学国家重点实验室课题(LED2013A09)和中央级公益性院所基本科研业务专项(IGCEA1417)
National Natural Science Foundation of China (41472161), State Key Laboratory of Earthquake Dynamics (LED2013A09) and the Special Fund of Seismic Research (IGCEA1417)
出版历史
出版时间: 2018年5月10日 (版本2
参考文献列表中查看
地球环境学报
Journal of Earth Environment