花岗岩风化壳在我国华南地区广泛分布,其形成和演化对地貌、生态环境和矿产资源等具有重要影响。作为风化壳最基本的参数之一,生成速率是理解风化壳形成和演化的关键因子。铀系不平衡法是当前广泛应用于确定风化壳生成速率的重要地球化学手段,由于稀释剂获取困难,我国开展的研究较少。本研究以江西龙南花岗岩风化壳剖面为研究对象,开展铀系不平衡研究以确定其生成速率。研究结果显示:风化壳剖面中U和Th的含量范围分别为(3.25~3.39)×10^-6和(41.46~47.67)×10^-6;活度比(²³⁴U/²³⁸U)_a、(²³⁰Th/²³⁴U)_a和(²³⁰Th/²³²Th)_a范围分别为1.008~1.023、1.063~1.112和0.239~0.271。通过铀系不平衡法对铀系同位素进行拟合后得到风化壳20~120 cm区域的演化时间约为841 ka,据此确定风化壳的生成速率约为1.2 m/Ma。本研究中,控制风化壳生成速率的因素主要是表层覆盖,气候和构造的影响很小。此外,由于风化壳的生成速率远低于宇宙成因核素确定的剥蚀速率,因此风化壳的演化状态为厚度逐渐减小的非稳态。

The granitic regolith, prevalent across South China, plays a pivotal role in geomorphological evolution, ecological dynamics, and mineral resource management. Understanding the formation and evolution of regolith hinges upon fundamental parameters such as production rate. The U-series disequilibrium method serves as a crucial geochemical tool for determining regolith production rates, yet its application in China has been limited due to the unavailability of a spike. In this investigation, the U-series disequilibrium method was employed to ascertain the production rate of granitic regolith in Longnan, Jiangxi Province. Results indicate U and Th contents in the regolith profile ranging from (3.25-3.39)×10^-6 and (41.46-47.67)×10^-6, respectively. Activity ratios of (²³⁴U/²³⁸U)_a, (²³⁰Th/²³⁴U)_a, and (²³⁰Th/²³²Th)_a vary between 1.008-1.023, 1.063-1.112, and 0.239-0.271, respectively. Consequently, utilizing the U-series disequilibrium method to fit uranium isotopes, the evolution time of the regolith within the 20-120 cm stratum is estimated at ~841 ka, with a regolith production rate determined to be ~1.2 m/Ma. Surface cover emerges as the predominant control factor over regolith production rate, with minimal influence from climate and tectonic activity. Furthermore, the regolith’s evolution state is identified as non-steady state, evidenced by a significantly lower regolith production rate compared to denudation rates determined by cosmogenic nuclides, resulting in gradual thickness reduction.