碎屑锆石U⁃Pb年代学定量物源分析的基本原理与影响因素——以现代河流砂为例

Principles and Biases of Quantitative Provenance Analysis Using Detrital Zircon U-Pb Geochronology:Insight from modern river sands

碎屑锆石U-Pb年代学数据获取快,物源对比精确度高,还可以估算源区剥蚀量,在定量物源分析方面具有显著优势,广受沉积学界青睐。但由于采样、实验过程中的不确定性,常常导致一些物源判别结果存在多解性,甚至产生了很多争议。从碎屑锆石U-Pb年代学定量物源分析的原理入手,综述了由于沉积水动力、母岩锆石产率、沉积再旋回、人类活动、以及数据获取与处理5方面因素对年龄谱可能产生的影响。结果表明,河流砂相比地层中的沉积岩,物源区母岩性质明确,运移路径非常清晰,可以进行锆石产率的准确测定,并能够同时开展混合模型正演和反演,是理想的定量物源分析研究对象。对开展基于现代河流砂的定量物源分析机理研究进行了展望,指出应用新技术、新方法开展小流域碎屑锆石U-Pb年代学研究是揭示锆石侵蚀、搬运和沉积过程行为机理的重要手段、也是构建定量物源分析方法的重要基础,将为规范开展沉积地层的物源研究提供重要的理论依据。

Detrital zircon U-Pb geochronology is a powerful tool for deciphering Earth’s sedimentary archive,answer⁃ing a large amount of research questions including sediment provenance and palaeogeographic correlations.Although sound reasons exist to conduct qualitative analysis,the advent of large-n acquisition techniques and readily available statistical tools have provoked a shift towards quantitative analysis as the preferred approach during the last two de⁃cades.The assumption for a geologically meaningful interpretation of inter-sample comparison through detrital zircon age distributions is that the analyzed samples are a true reflection of the sediment sampled,and this can be used as a proxy for the relative proportion of crystalline rocks in the source region.However,this foundational assumption may be undermined by a number of biases,leading to controversial interpretations and a risk of abuse of the method.In this paper,we demonstrate the principles of the quantitative provenance analysis using detrital zircon U-Pb geochro⁃nology and review five major factors that can add bias to the age spectrum and influence the provenance interpretation.The results show the significance to using modern river sand for testing the fidelity of the detrital zircon geochronology because the zircon fertility,hydrological sorting,and sediment mixing can be better constrained than ancient fluvial sequence.Based on the new technologies,including large-n sampling,rim-core dating,and three-dimensional zircon morphology,the quantitative relationship between sedimentary hydrodynamics and age spectra are established,pro⁃viding further instruction for the provenance study of deep-time sedimentary strata.