【作者】 贾丽；

【导师】 尹功明；

【作者基本信息】 中国地震局地质研究所 ， 地球化学， 2007， 硕士

【摘要】 方解石脉是断层中极为常见的物质，常与构造活动相关，沿断裂、节理等有空间的裂缝生长发育。因此，研究方解石脉的形成时代，可以有助于断层活动时代的研究。而对于断层活动时代的研究是活动构造、新构造和地震危险性研究中的重要内容之一。重大生命线工程如：大型水库坝址、核电站等。因安全考虑，必须要对工程场址作出地震安全性评价，确定工程场址和一定范围内各断层最后一次活动的时代，鉴定是否是活动断层（距今十万年内有过活动的断层）。在基岩区的断层，有时很难找到适合定年的与断层活动相关的样品，有时只能发现与断层活动相关的方解石脉。因此，方解石脉的年代学测定有时在核电站等重大建设项目中起关键作用。方解石脉测年可行性研究，对于断层活动时代的确定有着实际的应用意义。对于方解石脉测年，除铀系外，还没有其它成熟方法，但是，铀系方法对铀封闭条件、样品的纯度等要求较高，且测年范围有限（小于距今45万年，Schwarcz et al．，2001）。所以本文是针对方解石脉ESR信号特征进行较深入的研究，探讨ESR技术测定方解石脉年龄的可行性。电子自旋共振（Electron Spin Resonance，简称ESR）方法是近几十年来在第四纪年代学的研究中迅速发展起来的一种新的测年方法。ESR技术首先应用于物理、化学和生物学的研究中。20世纪60年代初，ESR方法被逐步引入地学研究中，成为地学研究的一个重要手段。目前，世界上许多国家在地学研究中都普遍使用了ESR方法，该方法已经成为地学研究中的重要技术之一，它在地学中的应用方兴未艾，其前景十分广阔。这种方法测年的年限较长，可以从几千年到几百万年。可测试的样品较多，如各种生物化石，海、陆相碳酸盐，海、湖相石膏，火山岩，沉积物中的石英等，远比¹⁴C、铀系、热释光、裂变径迹和氨基酸外销旋等测年方法可供测年的样品种类多。ESR测年可应用于古海洋学、石油地质学、灾害地质学（断层、滑坡、泥石流）、环境地质学、经济地质学（矿金的成矿年代）、工程地质学和第四纪地质的研究。获得可靠的古剂量是得到准确ESR年龄的重要前提条件之一。本论文是针对方解石脉ESR测年中古剂量的求取进行研究的，在实验室要获得一个准确的古剂量，将涉及以下几个方面的问题：1、ESR信号强度与辐照剂量的关系。2、方解石脉样品辐照后ESR信号的稳定性。3、ESR信号测量参数的选择。4、ESR顺磁中心的平均寿命。5、ESR顺磁中心辐照剂量饱和情况。其中4和5这两方面决定ESR顺磁中心测年范围的下限。本论文从上述几个方面着手，研究怎样才能获得可靠准确的古剂量，以便可以得到可靠的方解石脉样品ESR年龄。本论文研究的样品采自云南洱海东断裂带中的方解石脉，方解石脉样品ESR谱图显示除了Mn²⁺产生的六个峰之外至少还有三个ESR信号，通过对三个信号g值的标定，它们的g值分别为2.0032、2.0025和2.0003。通过与纯碳酸盐的谱图比较，这三个信号可能为含碳基团离子产生的信号。本论文把由CO₃^—产生g=2.0032（Ikeya，1993）和由CO₂^-产生的g=2.0003（Barabas et al，1992）信号作为重点研究的信号。对方解石脉中g=2.0032和g=2.0003信号辐照不同的剂量，发现这两个信号强度与辐照剂量大小相关，并且随辐照剂量的增加，信号强度增大，同时，这二个信号在1500Gy剂量范围内，信号强度呈线性增长。通过对这两个信号辐照后稳定性研究发现，样品辐照后，在室温条件下放置一周，基本可以消除辐照时产生的短寿命不稳定信号。影响ESR谱图测量参数有很多，例如：微波频率、调制幅度，调制频率、时间常数，扫场时间等。其中微波功率影响较大（马礼敦，2002）。本论文中通过人工辐射已知剂量方法，研究结果表明要获得可靠的古剂量，g=2.0032和g=2.0003的信号所选择的微波功率是不同的。对于g=2.0032信号，测量时选择合适的微波功率应在0.8-2mW之间；对于g=2.0003信号，微波功率应选择2-5mW。影响ESR测年范围的下限因素主要有三个，顺磁中心的平均寿命、辐照剂量饱和特性以及环境剂量率的大小，对于后者，本论文不展开具体的研究。平均寿命研究：采用等时和等温退火实验来得到顺磁中心的平均寿命，根据实验结果，方解石脉中g=2.0032信号室温下的平均寿命为6.541×10⁶ka，g=2.0003信号室温下的平均寿命为1.077×10⁶ka。因此，可以满足第四纪时间范围内的测年需要。辐照剂量饱和特性研究：对方解石脉样品中g=2.0032和g=2.0003信号回零后，辐照不同的剂量，获得信号增长曲线，实验结果表明，方解石脉中g=2.0032和g=2.0003信号辐照剂量饱和值是不同的，分别大约为35136Gy和40993Gy。如果按一般地质条件，方解石脉的环境剂量率为2-4Gy／ka，则这两个信号测年范围已覆盖第四纪时间段。ESR测年上限主要取决于所用的ESR谱仪的灵敏度，近年来，由于ESR谱仪性能的提高，尤其是使用了波谱累加技术，使的仪器的灵敏度大大提高，以至电子自旋共振测定年代的上限对某些样品可达到几千年。用方解石脉中g=2.0003信号，测量参数为：室温，X波段，中心磁场3520Gs，扫场宽度50Gs，微波功率2mW，调制频率100KHz，调制幅度0.5Gs，时间常数40.96ms，扫场时间10.24s，得到样品号为6065和6067方解石脉样品的古剂量值分别为169Gy和82Gy。用论文研究确定的ESR测年信号和测量条件，附加剂量法线性拟合得到方解石脉样品的古剂量分别是169Gy和82Gy，小于200Gy，与测年信号在1500Gy剂量范围内信号强度呈线性增长相呼应。这两个样品的ESR年龄小于一百万年，在方解石脉ESR测年信号的测年范围之内。因此，从实验的角度来看，我们得到的古剂量具有一定的可靠性。由于样品并非是我们自己现场采集的，样品提供者在样品采集时，没有同时采集周围岩石样，而方解石脉中的铀、钍、钾含量较低（样品6065铀、钍和钾分别为0.142ppm、0.114ppm、0.24％；样品6067的分别为0.106ppm、0.228ppm、0.26％。铀、钍含量通过中子活化分析确定的，而钾则采用火焰光度计技术），有可能周围岩石和宇宙射线的外部环境剂量率)会占很大的比例。因此，我们目前还不能给出具体的ESR年龄值。但是，从这二个样品按生长顺序而言，样品6065比样品6067要老，我们实际得到的古剂量也是样品6065比样品6067大。这也表明，我们获得的古剂量具有一定的可靠性。同时，这二个样品的古剂量分别为169Gy和82Gy，可以肯定这二个年龄不会大于距今100万年（仅宇宙射线至少为0.15Gy／ka，样品采自云南高原地区的地表），即方解石脉形成于距今100万年以内，这与野外判定的海东断裂可能在中晚更新世有过活动是一致。总之，用ESR方法可以对方解石脉进行测年。用本论文研究的信号和实验参数可以得到可靠的方解石脉样品的古剂量。方解石脉中g=2.0032和g=2.0003的信号的测年范围可以覆盖整个第四纪。更多还原

【Abstract】 Calcite is a common mineral in faults. It is often related with structural activity and grows along faults and joints. Therefore, the study of calcite formation age may help research of fault activity times. Active fault ages have important significance for studying active tectonics, neo-tectonics and earthquake risk research. Considering the Safety some significant projects, for example, large-scale reservoir sites, nuclear power stations and so on, we must make earthquake assessment including whether or not active faults （age less than 100ka） in the certain range of these project fields are active. Sometimes it is very difficult to find suitable samples for dating fault age, and we can only find calcite related with faulting. So calcite formation age is a key for some significant project sites. Therefore, the study of calcite chronology has application significance for dating of active faults.Besides U dating, there are no other maturement methods for calcite dating. But the U dating method requests good closed condition and higher purity, and the measured year range of this method is limited, smaller than 450ka （Schwarcz et al.,2001）. This thesis aims studying the ESR signal characteristic calcite to determine feasibility of ESR technology for measurement of calcite age.Electron Spin Resonance, called the ESR method is a new dating Quaternary chronology which has quickly developed in the recent several dozens years. First the ESR technology was applied to physics, chemistry and biology. Since the early 1960s the ESR method is gradually introduced in geology and becomes an important for geology. At present, in many countries the ESR method is widely used. This method has increasing already become one of important branches of earth sciences. Its application in geology is increasing and its prospect is extremely broad.The span of measured years of the ESR method is very large ranging from several ka to several thousand ka. This method can measure many samples, for example, biological fossil, carbonate in sea and on land, gypsum in sea and lake, volcanic rock, deposited quartz and so on, much more than ¹⁴C dating, U dating, and OSL. ESR method may be used in paleocean, petroleum geology, disaster geology（faults, landslides, mud-rock flow）, environmental geology, economical geology （formation ages of gold）, engineering geology and Quaternary geology.Obtaining a reliable total dose is one of important conditions for getting ESR ages. This thesis content is focused on studying how to get a reliable total dose. It isinvolve following several aspects:1: Relations between of ESR signal intensity and exposure dose.2: ESR signal stability after exposure,3: Parameter choice for measuring ESR signal.4: Mean lifetime of ESR paramagnetic center.5: Exposure dose saturated situation of ESR paramagnetic enter.The items 4 and 5 decide under the lower limit of the measured year scope of the ESR paramagnetic center. This work is to study how to obtain the reliable total dose from above several aspects, so that the reliable ESR ages of calcite samples.ESR spectrogram of the calcite sample demonstrates that it has six peaks produced by Mn²⁺, besides there are at least three ESR signals. Through demarcating g values of three signals, their g values are respectively 2.0032, 2.0025 and 2.0003. Comparing with the pure carbonate spectrogram, it is inferred that these three signals may be produced by carbonic radicals, the g=2.0032 signal produced by CO₃^- （Ikeya, 1,993） and g=2.0003 signal produced by CO₂^- （Barabaset al, 1992）.These are the focused objects of this study. It is found that g=2.0003 and g=2.0032 signals are correlated in intensity trength and the exposure dose after exposuring different dosage, and the signal intensity increases with the increasing exposure dose. Simultaneously, these two signal intensities assume a linear growth in the 1500Gy dose scope.Through reseaching of the intensity of these two signals, I discoveried that the short-lift and unsteady signal can be eliminated when cacite samples lay aside a week in the room temperature condition after exposuring.ESR spectrogram has many affecting parameters. Such as, microwave power, microwave frequency, modulation amplitude, modulation frequency, time constant, sweep time and so on. Among these parameters, the influence of microwave power is larger. Through artificially assigning the known total dose, I find that we should choose different microwave powers when using g=2.0032 and g=2.0003 signals to get total dose. Regarding g=2.0032 signal, the appropriate microwave power should be selected in between 0.8-2mW, regarding the g=2.0003 signal, the microwave power should be chosen in 2-5mW.There are three factors affecting the lower limit of relate with under the measured year scope of ESR paramagnetic center. They are the mean lifetime of ESR paramagnetic center, exposure dose saturated situation of ESR paramagnetic enter and environment exposure dosage, for the latter, the present work does not involve.For mean lifetime research, use isothermal annealing experiment at different times and isochronism annealing experiment at different temperatures. The obtained mean lifetime of g=2.0032 signal is 6.541×10⁶ka at room temperature, the mean lifetime of g=2.0003 signal is 1.077×10⁶ ka at room temperature. These two signals may satisfy the need for the dating of Quaternary.As for research of saturation characteristics, by returning signal intensity to zero and exposing the different dosage, a signal intensity growth line is yielded, indicate that g=2.0032 and g=2.0003 signals of calcite have different saturation dose values is probably 35136Gy and 40993Gy. According to the general geological condition, environment dose of calcite is 2-4Gy/ka, the year scopes of these two signals have covered the Quaternary period.The upper limit of measured year scope mainly depends on sensitivity of the used ESR spectrometer. In recent years, as the sensitivity of ESR spectrometer is enhanced, especially using technology of spectrum super-imposition, sensitivity of ESR spectrometer has been greatly enhanced. It makes the upper limit of the measured year scope achieve several ka.This work concludes that the ESR method can be used in dating calcite. If g=2.0023 signal is used, parameters are center field 3520 Gs, sweep width 50 Gs, modulation amplitude 0.5Gs, modulation frequency 100KHz, time constant 40.96ms, sweep time 10.24s, microwave power 0.8-2mW. If using g=2.0023 signal the choiced parameters are center field 3520 Gs, sweep width 50 Gs, modulation amplitude 0.5Gs, modulation frequency 100KHz, time constant 40.96ms, sweep time 10.24s and microwave power 2-2mW.更多还原