A Study For ESR Signal Characteristics Of Calcite

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.