Radiation-induced defects in quartz. III. Single-crystal EPR,ENDOR and ESEEM study of a peroxy radical

The X- and W-band single-crystal electron paramagnetic resonance spectra of an electron-irradiated natural quartz permit quantitative analysis of a ²⁹Si hyperfine structure (A ~12.6 MHz) and an ²⁷Al hyperfine structure (A ≤ 0.8 MHz) for a previously reported hole-like center. The ²⁹Si hyperfine structure arises from interaction with two equivalent Si atoms and is characterized by the direction of the unique A axis close to a Si–O bond direction. The ²⁷Al hyperfine structure, confirmed by pulsed electron nuclear double resonance and electron spin echo envelope modulation spectra, is characterized by the unique A axis approximately along a twofold symmetry axis. These ²⁹Si and ²⁷Al hyperfine data, together with published theoretical results on peroxy radicals in SiO₂ as well as our own density functional theory (DFT) calculations on model peroxy centers, suggest this hole-like center to have the unpaired spin on a pair of oxygen atoms linked to two symmetrically equivalent Si atoms and a substitutional Al³⁺ ion across the c-axis channel, a first peroxy radical in quartz. The nuclear quadrupole matrix P also suggests that the Al³⁺ ion corresponds closely to the diamagnetic precursor to the [AlO₄]⁰ center. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

蚌埠五河地区金矿床成矿物质来源及其年代学研究——以河口和荣渡金矿床为例

安徽省蚌埠五河地区河口和荣渡金矿床是华北地台东南缘蚌埠台隆和五河台坳邻接区域的两个金矿床。本文通过对比研究河口和荣渡金矿床的地质特征、流体包裹体地球化学、Re-Os同位素以及H-O-S同位素特征,来厘定其形成时代、探讨其成矿物质来源和成矿过程。河口及荣渡金矿床矿体主要呈细脉浸染状、脉状和网脉状赋存于古太古代西堌堆地层中,成矿过程可划分为4个阶段:(1)石英脉阶段(早阶段);(2)石英-黄铁矿阶段(早阶段);(3)石英-多金属硫化物阶段(中阶段);(4)碳酸盐阶段(晚阶段);其中,中阶段为金的主要矿化时期。河口和荣渡金矿床早阶段(石英脉和石英-黄铁矿阶段)的石英内发育富液两相包裹体(WL型)、富气两相包裹体(WG)以及少量的含子晶的气液固三相包裹体(S型),均一温度为322～412℃,盐度介于5. 56%～15. 67%NaCleqv之间,属于高温中低盐度流体体系;中阶段(石英-多金属硫化物脉阶段)石英内发育有富液两相包裹体(WL型)、富气两相包裹体(WG),均一温度为257～357℃,盐度介于3. 06%～7. 45%NaCleqv之间,属于中高温低盐度流体体系。从成矿早阶段到主成矿阶段流体的盐度和温度都发生了较大程度的降低,推测矿化过程可能是由流体温度和盐度的降低引起的。成矿阶段石英中流体水的δ~(18)O值介于5. 01‰～7. 90‰之间,δD_(V-SMOW)值介于-89‰～-65‰之间,表明两个矿床的成矿流体为主要为岩浆水。河口金矿床矿石中的黄铁矿δ~(34)S值介于3. 89‰～9. 65‰之间,荣渡金矿床方铅矿δ~(34)S值为0. 76‰,表明河口及荣渡金矿床δ~(34)S值具有岩浆硫的特征,后期由于岩浆去气作用或地层硫的参与使矿石中的δ~(34)S值升高。因此,这两个金矿床可能是与岩浆热液有关的热液石英脉型金矿床。对荣渡金矿床矿石的黄铁矿进行Re-Os同位素定年,确定金矿床形成于134±19Ma,结合前人对该地区所做的岩浆岩定年工作,推测荣渡金矿床的成矿可能与区内130Ma左右的岩浆活动密切相关。     

马庄山金矿床中石英特征与金矿化的关系

马庄山金矿床中自然金,除与多金属硫化物密切共生与伴生外,主要脉石矿物就是石英。历次的构造运动,多期的矿液活动及演化,无不在石英的宏观构造和微观构造上有直接或间接的反应。系统详细地研究这些构造,可以指示金矿的找矿方向及扩大金矿储量     

四川瀑布沟地区断裂破碎带的物质成分与断裂活动年代

本文主要研究断层泥中新生的粘土矿物特征,并研究断层泥中石英碎砾表面溶蚀的显微形貌结构特征。粘土矿物以X-射线衍射分析为主,石英碎砾则利用扫描电子显微镜对其显微形貌结构进行分析,并根据石英表面溶蚀的形貌特征来探讨断裂活动的相对年代     

五台地震科技中心水平摆倾斜仪对应强远震前兆异常特征分析

通过对数字石英水平摆倾斜仪最近几年积累数据的分析,发现在大多数中强远震发生前数小时,倾斜仪东西分量观测数据锯齿状异常、固体潮曲线出现畸变并伴有明显的抖动,这对异常识别和地震预测具有一定的参考价值。     

Investigating the optically stimulated luminescence dose saturation behavior for quartz grains from dune sands in China

The optically stimulated luminescence (OSL) signals from quartz have been widely used to estimate the equivalent dose (D_e) of environment radiation after the deposition of mineral grains. However, the usage of quartz is often limited due to the lower saturation behavior compared with feldspar. Saturation limits among quartz (defining the upper dating range) vary significantly. It is important to better understand the reason for various dose saturation behaviors of the quartz OSL signals. In this study, coarse quartz grains were extracted from the Taklimakan Desert and the Hunshandake sandy land in north China and the dose saturation behavior of quartz OSL signals were studied. Our results suggest that the quartz grains produce very different aliquot-specific dose response curves, showing the significant variability in dose saturation characteristics for OSL signals. Laboratory dosing, optical bleaching and heating experiments were designed to simulate their effects on the dose saturation behavior for the quartz OSL. The results demonstrate that cycles of dosing and optical bleaching have insignificant impact on the OSL dose growth curves, while the heating to high temperature (above 400 °C) can significantly change the dose saturation characteristics for the quartz OSL. Such results suggest that the different heating history of quartz might be an important factor for the variability in dose saturation characteristics for OSL signals. Additionally, the quartz grains from the Hunshandake sandy land exhibit lower dose saturation level for OSL signals, compared with that from the Taklimakan Desert. This can be explained that the quartz grains from Hunshandake sandy land are mainly of igneous origin, while the quartz grains from Taklimakan Desert are mainly of metamorphic origin.     

Interlaboratory comparison of cosmogenic 21Ne in quartz

We performed an interlaboratory comparison study with the aim to determine the accuracy of cosmogenic ²¹Ne measurements in quartz. CREU-1 is a natural quartz standard prepared from amalgamated vein clasts which were crushed, thoroughly mixed, and sieved into 125–250 μm and 250–500 μm size fractions. 50 aliquots of CREU-1 were analyzed by five laboratories employing six different noble gas mass spectrometers. The released gas contained a mixture of 16–30% atmospheric and 70–84% non-atmospheric (predominantly cosmogenic) ²¹Ne, defining a linear array on the ²²Ne/²⁰Ne-²¹Ne/²⁰Ne three isotope diagram with a slope of 1.108 ± 0.014. The internal reproducibility of the measurements is in good agreement with the formal analytical precision for all participating labs. The external reproducibility of the ²¹Ne concentrations between labs, however, is significantly overdispersed with respect to the reported analytical precision. We report an average reference concentration for CREU-1 of 348 ± 10 × 10⁶ at [²¹Ne]/g[SiO₂], and suggest that the 7.1% (2σ) overdispersion of our measurements may be representative of the current accuracy of cosmogenic ²¹Ne in quartz. CREU-1 was tied to CRONUS-A, which is a second reference material prepared from a sample of Antarctic sandstone. We propose a reference value of 320 ± 11 × 10⁶ at/g for CRONUS-A. The CREU-1 and CRONUS-A intercalibration materials may be used to improve the consistency of cosmogenic ²¹Ne to the level of the analytical precision.     

OSL dating of fine-grained quartz from Holocene Yangtze delta sediments

Holocene flood events in the Yangtze River are associated with variations in East Asian Summer Monsoon (EASM) precipitation, and so Yangtze delta sediments may preserve information about the frequency and magnitude of EASM precipitation. These flood/drought cycles of the EASM directly affect the living standards of East Asian population. However, despite its importance, little chronological control is available for the Yangtze Delta sediments; because biogenic carbonate only occurs sporadically, it has proved the difficulty to discuss sedimentation mechanisms and rates in any detail.In 2013 two sediment cores (YD13-G3 and H1) were taken from the Yangtze subaqueous delta to investigate precipitation history. In this study, we investigate the potential of quartz OSL dating of the fine silt fraction (fine-grained quartz; 4–11 μm) from these cores to estimate the depositional age of the sediments. We test whether: (1) Yangtze subaqueous delta sediments contain quartz with suitable characteristics for dating, and (2) quartz grains are well-bleached during/before the transportation process, by examining a modern analogue of suspended particulate matter, and by cross-checking with the doses derived from infrared stimulated luminescence (IRSL) signals (both IR₅₀ and pIRIR₁₆₀) from feldspar in polymineral fine grains. We find that both the quartz and feldspar luminescence characteristics are satisfactory (quartz dose recovery ratio 1.067 ± 0.004; n = 250, pIRIR₁₆₀ dose recovery ratio 1.01 ± 0.02; n = 151). Modern suspended particulate matter has measured quartz equivalent doses between 0.1 and 0.2 Gy, suggesting that this material was sufficiently bleached during/before transportation to allow dating of Holocene sediments (mean dose rates of ∼3 Gy ka⁻¹). OSL ages of 44 samples from the 2 cores show apparently rapid accumulation at ∼6 ka between 9.65 and 5.50 m in core H1 and ∼2 ka throughout core G3 and between 5.0 and 0.0 m in core H1. The pIRIR₁₆₀ signals suggest less light exposure of the core top sediments and of those from the transition layer between ∼6 ka to ∼2 ka, although there is no evidence for incomplete bleaching of quartz. The question remains as to whether significant deposition took place only at these two times, or whether the record has been disturbed by erosion/reworking.     

Radiofluorescence of quartz: A review

Radiofluorescence (RF) is the luminescence emitted during exposure to ionizing radiation. Charged particles or high-energy photons can be used as stimulation sources, and different designs for measurement equipment have been published. Only few studies have successfully used the quartz RF signal for dosimetry and dating. However, RF is a valuable tool in deciphering charge trafficking in quartz crystals, and also provides information for identifying types of defects causing specific luminescence emissions. Based on models for charge transfer in quartz, RF is seen as resulting from direct recombination of electrons with holes captured in recombination centers (or vice versa) during ionizing irradiation. Competition between reservoir and luminescent centers explains the initial decay of the modeled RF curve followed by a steady rise and also the observed ‘pre-dose’ effect. Emission spectra have been found to be similar to thermoluminescence (TL) spectra, with prevalent emissions in the UV and further emissions for some samples in the blue-green and red range. The high intensity levels and the possibility of choosing longer accumulation times compared to TL and OSL are advantages of RF for spectral measurements. Relative peak intensities in the emission spectra change with dose and absolute intensities with dose rate. Investigating the RF signal with changing measurement temperature allows calculating physical parameters of individual emissions that control thermal quenching. The degree of thermal quenching varies between the emissions, with most intense quenching in the UV. Sensitization of RF by several orders of magnitude has been observed after annealing at 500 °C.     

Paramagnetic lattice defects in quartz for applications to ESR dating

Quartz is one of the minerals useful for ESR dating with many applications. Although the study has quite a long history since 1980's, ESR dating of quartz is still in a developing stage. After the basis in physics for the ESR signals in quartz is described, the current status of ESR dating of quartz is summarized for applications to fault gouge, tephra and sediment. Both basic studies for mechanisms and practical applications with independent age controls are still necessary to establish the protocols.