研究论文 正式出版 版本 3 Vol 9 (6) : 599-606 2018
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高温烘烤作用对石英ESR信号特征的影响——以大同火山烘烤层为例
The effect of high temperature baking on the characteristics of quartz ESR signal— taking Datong volcanic baking layer as an example
: 2018 - 05 - 23
: 2018 - 08 - 24
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摘要&关键词
摘要:在第四纪地质学中,ESR信号特征可用于物源示踪、判断受热时间及受热历史等研究,具有广阔的应用前景,因此对ESR信号特征的研究具有重要意义。本文通过对大同火山熔岩烘烤层和未烘烤湖相层样品的石英不同ESR信号特征进行研究,并总结了高温烘烤作用对不同石英ESR信号特征的影响。研究发现:高温烘烤作用显著增强了石英Ti-Li心ESR信号灵敏度;石英Al心ESR信号灵敏度无明显变化;E'心信号灵敏度小幅度增加。另外,高温烘烤之前无Ge心信号,在高温烘烤作用之后出现了Ge心信号。光晒退实验结果显示:经过高温烘烤作用之后,石英Al心的不可晒退部分减小,且石英Al心和Ti-Li心信号光晒退“回零”速度更快。
关键词:ESR;册田水库;高温;烘烤层;灵敏度;地质年代学
Abstract & Keywords
Abstract: Background, aim, and scope Electron Spin Resonance (ESR) dating method is a new dating method developed rapidly in Quaternary chronology in recent decades and has received extensive attention from the Quaternary scholars. The characteristics of ESR signal can be used for source tracing, judging heating time and heating history, and has broad application prospects in Quaternary geology studies. Many studies on ESR dating are mainly focused on the determination of sediment age and dating reliability, but there are less studies on the characteristics of quartz E' center, Ge center, Al center, Ti-Li center, which were directly related to dating. Especially for the signal characteristics of samples subjected to special effects such as high temperature and high dose radiation. In fact, the essential characteristics of the ESR signal directly determine the dating range, experimental procedure and application of the method. Therefore, it is great significance to research the characteristics of ESR signals. Materials and methods Datong volcanic group is in the Datong Basin in the northern part of China. Many researchers have studied this area, the volcanic eruptions in this area lie between Quaternary loess and lacustrine layers. The lacustrine sediment was baked by volcanic lava overlain, and the lava baked lacustrine sediment is ideal for us to study the thermal characteristics of quartz ESR signals in nature. In this paper, the ESR signatures of the lava-baked and unfired lacustrine layers in Datong volcano were studied, and the effects of high-temperature baking on the characteristics of ESR signals and the characteristics of sunlight bleaching in different quartz were summarized. Results It was found that high-temperature baking significantly enhances the sensitivity of the quartz Ti-Li center ESR signal. There is no significant change in the sensitivity of the quartz Al center ESR signal, the sensitivity of the E' center signal increases slightly, and there is no Ge center signal before high-temperature baking. Ge center signal appeared after high temperature baking. The results of the sunlight bleaching experiment showed that after the high-temperature baking effect, the unbleaching part of the Al center was reduced, and the ESR signals back to zero faster. Discussion The increase in the sensitivity of the ESR signals will reduce the relative error of the signal measurement when use the ESR dating. Therefore, the samples which have been baked at high temperatures in the past, the ESR dating may have better results. Conclusions The sensitivity of quartz Ti-Li and E' centers ESR signals enhanced after baked by volcanic lava. The quartz Ge center ESR signal appears after baked by volcanic lava. The Al center sunlight unbleached part decrease after baked by volcanic lava. Recommendations and perspectives When using ESR dating, we must pay attention to the characteristics of various signals and select the appropriate dating signals.
Keywords: ESR; bleaching; heat; volcanic lava; lacustrine sediment
电子自旋共振(electron spin resonance,简称ESR)测年法是近几十年来在第四纪年代学研究中迅速发展起来的一种新的测年方法,受到第四纪学者的广泛关注。目前,关于ESR测年的大量研究主要围绕测定沉积物年龄及测年可靠性展开(Odom and Rink,1989;Toyoda and Ikeya,1991;Toyoda et al,1992;Toyoda,2005;Usami et al,2009;Rink et al,2007;Liu et al,2014;刘春茹等,2016;李文朋等,2018),但是,对于与测年直接相关的石英E'心、Ge心、Al心、Ti-Li心等ESR信号的特征研究较少。这里的信号特征主要是指信号的有无、信号的稳定性、灵敏度、光晒退特征等,特别是灵敏度和光晒退特征的研究尤为重要。国内外许多学者对ESR信号特征的研究主要是用来进行物源示踪(Ye et al,1993;魏传义等,2017;Wei et al,2017)、光晒退特征研究(Gao et al,2009)以及判断受热时间及受热历史(Fukuchi,1989;Toyoda et al,1993;Winotai et al,2000)。但现在几乎没有对于ESR信号灵敏度研究,而与ESR原理相近的释光测年的研究中,对于灵敏度的研究却有很多并且取得了比较可信的成果(Derbyshire,1995;Chithambo et al,2007;Lai and Wintle,2006;Li et al,2007;Zheng et al,2009;Lü and Sun,2011;郑辰鑫和周力平,2012)。
火山作用是自然界常见的地质作用,火山活动喷发大量高温岩浆,实验室研究表明高温作用对石英ESR信号特征有明显影响(Toyoda et al,1993;Winotai et al,2000),但是还没有关于自然界中的高温作用对石英ESR信号特征的系统研究。大同火山玄武岩覆盖在第四纪湖湘沉积物之上,当时喷发的岩浆对烘烤层有高温烘烤作用,且在烘烤层之下存在未烘烤的湖相层沉积,为研究高温作用对ESR信号特征的影响提供了良好的素材。
因此,本研究以大同火山烘烤的湖湘沉积物为切入点,研究高温烘烤前后其石英ESR信号变化特征,进而总结高温烘烤作用对石英ESR信号特征的影响。
1   野外概况及样品采集
大同盆地位于山西省的东部及河北省的西北部(东经112°15′—114°15′,北纬39°00′—40°30′),东西长约150 km,南北宽约60 km。大同火山群包括火山碎屑堆积的火山锥和玄武岩,在这一范围内,火山锥29座,火山玄武岩出露点有十几处,出露面积约15 km2。大同火山为第四纪时期火山作用的产物。剖面位置如图1所示。


图1   于家寨剖面位置简图
Fig.1 Location of Yujiazhai profile, Datong Basin
本实验样品全部采集自于家寨册田水库旁边的大同火山玄武岩下的湖相层剖面。采样点均位于粒度均匀、分选良好、石英含量高的细粉砂层,采集的样品用遮光布密封包裹,避免阳光的直接照射。
剖面自上而下分为黄土层、玄武岩层和湖相层,湖相层又分为烘烤层和未烘烤层,根据本研究目的,在烘烤层距离剖面顶面10 cm、30 cm和70 cm依次采集3个ESR样品CT01、CT02和CT03,在距离玄武岩层150 cm的未烘烤层采集一个湖相层ESR样品CT04,样品采集如图2所示。其中:① 高温烘烤湖相层,由于高温烘烤作用,岩层呈砖红色,厚约0.15—0.2 m;② 高温烘烤湖相层,青灰色,厚约0.25—0.3 m;③ 低温烘烤湖相层,淡红色,厚约0.5—0.6 m;④ 湖相层,灰绿色,厚约12 m。


图2   于家寨剖面、岩性柱及采样位置简图
Fig.2 A picture of the excavation profile and the sketch map showing the lithological column and sampling of the Yujiazhai
2   实验
2.1   石英提取
样品的前处理工作均在中国地震局地质研究所地震动力学国家重点实验室的ESR前处理实验室完成。首先将样品低温烘干,利用化学方法提纯石英颗粒(Liu et al,2010,2011)。筛取粒径105—200 μm的样品颗粒;分别用30%的H2O2浸泡24 h去除有机质;用40%的HCl浸泡24 h去除碳酸盐类物质,蒸馏水洗至中性后低温烘干;用磁选仪去除磁性矿物;然后用多钨酸钠进行重液分离,低温烘干;用40%的HF浸泡40 min去除长石矿物和石英颗粒外部α辐射贡献的表层部分。所有样品在相同条件下处理,将提纯好的石英颗粒样品称相同重量200 mg进行ESR信号测量。
2.2   光晒退实验
为了考察高温烘烤前后样品的石英ESR信号光晒退特征,选取了烘烤层样品CT03和湖相层样品CT04进行了光晒退实验,晒退时间为0—1078小时,在中国地震局地质研究所新构年代学ESR实验室完成。模拟太阳光晒退的仪器规格为:仪器尺寸43 cm × 43 cm × 60 cm,型号为Honle生产的SOL 2/500S型太阳光模拟器;仪器运行参数为:230 V,50 Hz,3.4 A/400 W;灯光强度为120,000 lux 和 910 W/m2,是自然太阳光的6.5倍;光谱(UVA+VIS+IR)介于320 nm—3.0 mm。所有的塑料小碟放置于灯管下方30 cm处,温度控制在30℃以内(内置散热系统)。具体实验步骤如下:将提纯后的CT03和CT04石英样品均匀放置在直径为8 cm的玻璃皿中,样品距灯管的距离为45 cm。将样品CT03和CT04分别在不同晒退时间取样,并进行石英Ti-Li心、Al心ESR信号测量,建立晒退曲线。
2.3   ESR测量
ESR 信号测量采用德国Bruker公司生产的X波段EMX-6型ESR谱仪,测试工作在中国地震局地质所地震动力学国家重点实验室的ESR实验室完成。低温测试时利用液氮在指状杜瓦中冷却样品,测试的温度约为77 K,测量石英Al心、Ti心ESR信号(图3),石英Al心ESR信号的测量位置为其超精细线结构的第1个峰的峰顶至第16个峰的峰底(Liu et al,2010);石英Ti-Li心ESR信号的测量位置为g=1.979的峰顶至g=1.913的峰底(Rink et al,2007)。低温ESR信号测量参数:微波频率9.46 GHz,微波功率5 mW,调制幅度0.1 mT,时间常数40.96 ms,扫描时间40.96 s。在常温条件下测量石英E'心和Ge心ESR信号(图4),E'心信号的测量位置为g=2.001(Toyoda and Naruse,2002),Ge心信号的测量位置为g=1.997(Ye et al,1993)。常温ESR信号测量参数:微波频率9.87 GHz,调制幅度0.1 mT,微波功率0.1 mW(Toyoda and Hattori,2000;Toyoda et al,2000),时间常数为40.96 ms,扫描时间10.49 s,扫描次数3次。考虑到石英晶体的各向异性,所有样品均在相同实验室条件下测量6遍,取其平均值作为该点的ESR信号强度。


图3   CT02自然样品低温(液氮,77 K)条件下的石英Al心和Ti-Li心ESR信号图谱
Fig.3 ESR spectrum showing the Al center and Ti-Li center of natural sample CT02 observed at low temperature (liquid nitrogen, 77 K )


图4   CT02自然样品常温条件下的石英E'心和Ge心ESR信号图谱
Fig.4 ESR spectrum showing the E' center and Ge center of natural sample CT02 observed at room temperature
3   结果和讨论
研究结果表明,高温烘烤前后石英ESR信号有较为明显的变化,主要为信号的有无、信号强度以及信号灵敏度的变化。
3.1   常温石英E'心和Ge心ESR信号变化特征
CT01、CT02、CT03和CT04四个样品的常温自然信号谱图如图所示(图5)。
E'心ESR信号:CT01、CT02、CT03和CT04自然样样品都有E'心ESR信号。CT01、CT02、CT03和CT04的E'心ESR信号强度相对值分别为1.8、1.9、1.9和1.5,三个烘烤层样品E'心相对大小基本相等,未烘烤层信号较其他三个要小。而且CT04未受高温作用,可能还残留了一定量的E'心信号,但是它的信号强度依然小于其他三个,这表明E'心ESR信号在高温烘烤作用之后信号灵敏度增加了,这可能是其他一些信号发生了转变。
Ge心ESR信号:CT04样品无Ge心,CT01、CT02和CT03自然样样品有Ge心。CT01、CT02和CT03的Ge心ESR信号强度相值都为0.3。湖相层样品没有积累Ge信号,而烘烤层中的样品经过高温烘烤后在埋藏过程中积累了Ge心信号。


图5   四个样品自然样常温ESR信号谱图
Fig.5 ESR spectrum showing the E' center and Ge center of four samples observed at room temperature
3.2   低温信号Al心、Ti-Li心ESR信号变化特征
四个自然样品的Al心、Ti-Li心ESR信号谱如图所示(图6),其变化特征如下:Al心ESR信号:CT01、CT02、CT03和CT04的Al心ESR信号强度相对值分别为2.9、2.3、2.3和2.8,相对大小CT01>CT04>CT02=CT03,但烘烤层和未烘烤的Al心ESR信号强度变化无规律性,初步判断高温烘烤作用对石英Al心ESR信号无明显影响。
Ti-Li心ESR信号:CT01、CT02、CT03和CT04的Ti-Li心ESR信号强度相对值分别为2.2、1.6、1.6和0.15,相对大小CT01> CT02=CT03>CT04,可以明显看出烘烤层Ti-Li心信号强度要明显大于未烘烤层,高温烘烤作用可以使石英Ti-Li心ESR信号的敏感度明显增加。
ESR信号灵敏度的增加对于ESR测年来说,会减小信号测量的相对误差,所以,对于历史曾高温烘烤过的样品,运用ESR测年时,可能测年结果会更好。


图6   四个样品自然样低温ESR信号谱图
Fig.6 ESR spectrum showing the Al center and Ti-Li center of natural sample observed at low temperature
有学者认为CT04样品所在地层可能也会受到玄武岩的烘烤作用,但是CT04与其他三个高温烘烤层样品具有明显不同的信号特征,主要表现为高温烘烤样品的Ti-Li心ESR信号敏感性增加,Al心和Ti-Li心信号比值减小。而且,CT04样品的ESR信号特征与在邻近地区泥河湾盆地湖湘沉积物的信号特征类似(Liu et al,2010,2013)。因此,本文认为CT04样品未受热或受热温度不够,以至于没有影响或改变湖相沉积物的ESR信号特征。
3.3   光晒退特征变化
为了观测高温烘烤前后湖相沉积物石英ESR信号光晒退特征的变化,对比了烘烤层样品CT03和未烘烤层样品CT04。两个样品的Al心和Ti-Li心ESR信号强度随光晒退时间的变化如图所示(图7)。


图7   CT03与CT04的Al心和Ti-Li心信号晒退特征
Fig.7 The light bleaching characteristics of quartz Ti-Li and Al centers ESR signals for CT03 and CT04
从图中可以看出,烘烤层样品CT03与未烘烤样品CT04的Ti-Li心ESR信号都在光照50 h变为零,这与前人研究结果相似(Yoshida,1996;Tanaka et al,1997;Toyoda et al,2000;Tissoux et al,2007;Gao et al,2009;Liu et al,2015;Duval et al,2016;李文朋等,2018)。但是通过光照32 h的ESR信号强度点可以看出CT03的回零比CT04更快,这说明经过高温烘烤作用之后石英Ti-Li心ESR信号更易晒退归零,对于历史曾高温烘烤过的样品,运用ESR测年时回零速度更快,可能测年可靠性会更好。烘烤层样品CT03与未烘烤样品CT04的Al心ESR信号在前200 h内随光照时间增加信号减小,在200 h左右到达稳定残留值,与前人研究结果相似(Yokoyama et al,1985;Nie,1992;Walther and Zilles,1994;Tanaka et al,1997;Toyoda et al,2000;Voinchet et al,2003;Tissoux et al,2007;Duval et al,2016;李文朋等,2018)。但烘烤层样品CT03晒退到达的稳定残留值明显要比未烘烤样品CT04小,分别为50%和15%。这说明高温烘烤作用使Al心的非光敏信号(Al心ESR信号分为可晒退部分和不可晒退部分,不可晒退部分又叫非光敏信号)所占比例减小,表现为晒退到达的稳定残留值要小。
4   结论
(1)高温烘烤作用增加了Ti-Li心和E'心ESR信号的灵敏度,特别是Ti-Li心信号灵敏度增加显著。
(2)高温烘烤作用之后,Al心信号敏感度无明显变化,但出现了Ge心信号。
(3)高温烘烤作用使Al心的非光敏信号部分所占比例减小,表现为晒退到达的稳定残留值要小;同时高温烘烤过的样品石英Al心和Ti-Li心信号光晒退回零速度更快,可能更适用于ESR测年。
致谢:
感谢中国地震局地质研究所韩非副研究员在文章写作过程中给予的指导和帮助;感谢中国地震局地质研究所杨会丽工程师在环境剂量测试及数据分析等方面给予的帮助,感谢审稿专家和编辑部老师建设性的修改意见。
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稿件与作者信息
李文朋1,2
LI Wenpeng1,2
刘春茹1*
LIU Chunru1*
liuchunru0821@126.com
尹功明1
YIN Gongming1
魏传义1
WEI Chuanyi1
李建平1
LI Jianping1
国家自然科学基金项目(41372178)
National Natural Science Foundation of China (41372178)
出版历史
出版时间: 2018年8月24日 (版本3
参考文献列表中查看
地球环境学报
Journal of Earth Environment