Uranium series disequilibrium constraints on the formation and evolution of granite regolith in Longnan, Jiangxi Province

doi:10.13745/j.esf.sf.2023.10.35

Abstract

Abstract:

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.

Key words: U-series disequilibrium, production rate, granitic regolith, evolution state, non-steady state

CLC Number:

JIA Guodong, XU Sheng, LIU Congqiang. Uranium series disequilibrium constraints on the formation and evolution of granite regolith in Longnan, Jiangxi Province[J]. Earth Science Frontiers, 2024, 31(4): 366-379.

Figures/Tables 10

Fig.1 (a) The potograph of regolith profile in this study; (b)-(i) the variation trends in depth profile of density, TOC content, pH, chemical index of alteration (CIA), and Na2O, quartz, kaolinite, gibbsite contents, respectively

Table 1 U and Th concentrations and activity ratios of (234U/238U)a, (230Th/234U)a and (230Th/232Th)a

深度/cm	w(U)/ (mg·kg)	±2σ/%	w(Th)/ (mg·kg)	±2σ/ %	(²³⁴U/²³⁸U)_a	±2σ/ %	(²³⁰Th/²³⁴U)_a	±2σ/ %	(²³⁰Th/²³²Th)_a	±2σ/%
22.5	3.25	0.07	42.78	0.56	1.026	0.005	1.112	0.017	0.265	0.003
55	3.35	0.07	47.67	0.62	1.001	0.005	1.108	0.017	0.239	0.002
85	3.31	0.07	41.46	0.54	1.015	0.005	1.091	0.016	0.271	0.003
85	3.39	0.07	41.61	0.54	1.008	0.005	1.074	0.016	0.270	0.003
115	3.34	0.07	43.11	0.56	1.008	0.005	1.063	0.016	0.254	0.003

Fig.2 The variation trends in depth profile of concentration of U (a) and Th (b), ratio of U/Th (c), and mass transfer coefficients of U (d) and Th (e)

Fig.3 The variation trends in depth profile of (234U/238U)a (a), (230Th/234U)a (b) and (230Th/232Th)a (c) activity ratios

Fig.4 The fitting curves of measurements of regolith samples obtained by gain and loss model with the bottommost sample collected from the regolith profile (a) and the theoretical bedrock (b) as the initial state, respectively

Fig.5 The frequency histograms of time information obtained by gain and loss model with the bottommost sample collected from the regolith profile (a) and the theoretical bedrock (b) as the initial state to fit measurements of regolith samples, respectively

Table 2 Parameters and production rates determined by the gain and loss model for different reference samples

参考样品	生成速率 P/(m·Ma^-1)	时间t/a	k₂₃₈/a^-1	f₂₃₈/a^-1	k₂₃₄/a^-1	f₂₃₄/a^-1	k₂₃₄/ f₂₃₈	f₂₃₄/ f₂₃₈	k₂₃₈/f₂₃₈	k₂₃₄/f₂₃₄
最底部样品	1.19	840 683	1.29×10^-6	2.54×10^-7	1.38×10^-6	3.46×10^-7	1.07	1.36	5.26	4.14
最底部样品	(0.02)	(14 175)	(8.29×10^-8)	(4.41×10^-8)	(1.10×10^-7)	(6.42×10^-8)	(0.03)	(0.10)	(1.21)	(0.88)
理想基岩	0.96	1 039 690	9.57×10^-7	4.25×10^-8	9.90×10^-7	9.33×10^-8	1.03	2.22	26.25	13.00
理想基岩	(0.02)	(19 999)	(3.28×10^-8)	(1.45×10^-8)	(5.73×10^-8)	(3.51×10^-8)	(0.05)	(0.52)	(49.07)	(27.80)

Fig.6 The relationship between k238/f238 and k234/f234 obtained by gain and loss model with the bottommost sample collected from the regolith profile (a) and the theoretical bedrock (b) as the initial state to fit measurements of regolith samples, respectively

Fig.7 Complications of regolith production rate determined by U-series disequilibrium method. Adapted from [17,19,21,46⇓-48,53⇓⇓⇓⇓⇓-59].

Fig.8 (a) Schematic diagram of the relationship between the regolith production rate and depth, depicting the humped decay pattern and exponential decay pattern, respectively; (b) schematic diagram of regolith production rate with time during the regolith evolution, where the black boxes in (b) represent the extent of regolith in this study. Adapted from [66-67].

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