地热系统钙华和硅华定年方法研究进展及其应用

Dating methods for travertine and siliceous sinter in geothermal systems:Progress and applications

地热系统泉华以钙华和硅华为主,因其携带丰富的多时空尺度信息,可用于恢复古水文古气候演化史、重建区域水热活动演化过程及约束构造模式。对样品精准定年是开展相关研究的基础,常用定年方法包括放射性碳定年法(无机碳和有机碳)、铀系不平衡法、宇宙成因核素法、光释光法和电子自旋共振法等。我国热泉泉华定年最早使用放射性碳定年法,随后多使用电子自旋共振法与铀系不平衡测年法。系统总结了上述方法的基本原理、定年范围、适用条件、对样品的要求等,并探讨存在问题及未来发展方向。对于钙华样品,如纯度高,无外源碳影响,可采用基于有机碳的放射性碳定年法和铀系不平衡法;如固结程度高,也可使用电子自旋共振测年法;古地磁法值得今后深入探索。对于硅华样品,基于有机碳的放射性碳定年法和电子自旋共振测年法更适用,而铀系不平衡法和古地磁法可以尝试。此外,^(10)Be和^(26)Al等宇宙成因核素也可用于古泉华的定年。样品纯度低或数据校正困难以及定年范围有限是目前钙华和硅华常用定年方法存在的两个主要问题;前者可考虑利用离子探针技术对微观尺度上的高纯度方解石中U和Th的同位素进行测定来获取样品年龄;为避免微量样品代表性问题,应尽量获得均匀分布的多个测点数据。后者则可以通过尝试利用长半衰期宇宙成因核素来获得百万年尺度上的泉华年龄。新技术研究以及定量化校正样品纯度不够的影响是未来发展方向。

Sinters of geothermal system are usually dominated by travertine and siliceous sinter.With rich information on multi-temporal and spatial scales,they can be used to restore paleohydrological and paleoclimatic histories,recon-struct the evolutionary process of regional hydrothermal activity,and constrain tectonic modes.Accurate dating of sin-ters is the basis of relevant researches.Common dating methods include radiocarbon dating(based on inorganic and or-ganic carbon),uranium series disequilibrium dating,cosmogenic nuclide dating,optically stimulated luminescence(OSL)dating,and electron spin resonance dating.Radiocarbon dating has primarily used for the dating of hot-spring sin-ters in China,followed by electron spin resonance dating and uranium series disequilibrium dating,which have been ap-plied more widely.This study systematically reviewed the basic principles,dating ranges,applicable conditions,and re-quirements for samples of these methods.Furthermore,the existing problems and future development directions of these methods are proposed.For travertine samples,those with high purity carbon can be dated using organic carbon-based ra-diocarbon and uranium series disequilibrium dating,while well-consolidated samples can also be dated using electron spin resonance method.For these samples,paleomagnetic dating worth further exploration.For siliceous sinter samples,organic carbon-based radiocarbon dating and electron spin resonance dating are more suitable,while uranium series dis-equilibrium and paleomagnetic dating can be tried.Additionally,cosmogenic nuclide dating,such as those using ^(10)Be and ^(26)Al,can be used for paleosinter dating.The two main challenges of dating are sample with low purity problem(or difficult correction)and limited dating range.For the first challenge,the ion probe technique can be used to determine the isotopes of U and Th in the microscopically high-purity calcite to obtain the sample ages.To improve the representat-iveness of problem samples,it is necessary to measure as many uniformly distributed points as possible.These data can be used to determine the sinter ages on a million-year scale by obtaining long-half-life cosmogenic nuclides.Further de-velopment of dating methods will focuses on the research of new techniques and the quantitative correction of low-purity samples.