江西龙南花岗岩风化壳形成和演化的铀系不平衡约束

doi:10.13745/j.esf.sf.2023.10.35

摘要/Abstract

摘要：

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。

关键词: 铀系不平衡法, 生成速率, 花岗岩风化壳, 演化状态, 非稳态

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

中图分类号:

贾国栋, 徐胜, 刘丛强. 江西龙南花岗岩风化壳形成和演化的铀系不平衡约束[J]. 地学前缘, 2024, 31(4): 366-379.

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.

图/表 10

图1 本研究中风化壳剖面实际采样图(a)和密度(b)、总有机碳(TOC)含量(c)、pH(d)、CIA(e)以及Na2O(f)、石英(g)、高岭石 (h)、三水铝石(i)含量在风化壳剖面中随深度的变化趋势图

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

表1 U和Th的含量以及活度比(234U/238U)a、(230Th/234U)a和(230Th/232Th)a

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

图2 风化壳中U(a)和Th(b)的含量以及U/Th比值(c)、U(d)和Th(e)的传质系数随深度的变化趋势

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)

图3 风化壳中活度比(234U/238U)a (a)、(230Th/234U)a (b)和(230Th/232Th)a (c)随深度的变化趋势

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

图4 分别以风化壳剖面采集的最底部样品(a)和理论基岩(b)为初始状态,应用得失模型对风化壳样品的测量值进行拟合

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

图5 分别以风化壳剖面采集的最底部样品(a)和理论基岩(b)为初始状态,应用得失模型对风化壳样品的测量值进行拟合后得到时间信息的频率直方图

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

表2 不同参考样品使用得失模型得到的相关参数及对应的生成速率

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)

图6 分别以风化壳剖面采集的最底部样品(a)和理论基岩(b)为初始状态,应用得失模型对风化壳样品的测量值进行拟合后得到的参数k238/f238和k234/f234之间的关系

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

图7 应用铀系不平衡法确定风化壳生成速率数据集(据文献[17,19,21,46⇓-48,53⇓⇓⇓⇓⇓-59])

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

图8 风化壳演化示意图(据文献[66-67]) a—风化壳生成速率随深度变化的驼峰衰减模式和指数衰减模式示意图;b—风化壳生成速率随风化壳演化模式图,其中的黑色方框代表本研究中风化壳包含的范围。

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].

参考文献 61

[62]	GABET E J, MUDD S M. A theoretical model coupling chemical weathering rates with denudation rates[J]. Geology, 2009, 37(2): 151-154.
[63]	RIEBE C S, HAHM W J, BRANTLEY S L. Controls on deep critical zone architecture: a historical review and four testable hypotheses[J]. Earth Surface Processes and Landforms, 2017, 42(1): 128-156.
[64]	NORTON K P, MOLNAR P, SCHLUNEGGER F. The role of climate-driven chemical weathering on soil production[J]. Geomorphology, 2014, 204: 510-517.
[65]	GILBERT G K. Report on the geology of the Henry Mountains[M]. Washington: US Government Printing Office, 1877.
[66]	CARSON M A, KIRKBY M. Hillslope form and process[M]. Cambridge: Cambridge University Press, 1972.
[67]	AHNERT F. Some comments on the quantitative formulation of geomorphological processes in a theoretical model[J]. Earth Surface Processes, 1977, 2(2/3): 191-201.
[1]	MONTGOMERY D R. Soil erosion and agricultural sustainability[J]. Proceedings of the National Academy of Sciences of the United States of America, 2007, 104(33): 13268-13272. PMID
[2]	ARKLEY R J. Soil moisture use by mixed conifer forest in a summer-dry climate[J]. Soil Science Society of America Journal, 1981, 45(2): 423-427.
[3]	GRAHAM R C, TICE K R, GUERTAL W R. The pedologic nature of weathered rock[M]// SSSA Special Publications. Madison: Soil Science Society of America, 2015: 21-40.
[4]	DREVER J I, STILLINGS L L. The role of organic acids in mineral weathering[J]. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1997, 120(1/2/3): 167-181.
[5]	KUMP L R, BRANTLEY S L, ARTHUR M A. Chemical weathering, atmospheric CO₂, and climate[J]. Annual Review of Earth and Planetary Sciences, 2000, 28(1): 611-667.
[6]	BARRIOS E. Soil biota, ecosystem services and land productivity[J]. Ecological Economics, 2007, 64(2): 269-285.
[7]	BORST A M, SMITH M P, FINCH A A, et al. Adsorption of rare earth elements in regolith-hosted clay deposits[J]. Nature Communications, 2020, 11(1): 4386. DOI PMID
[8]	徐夕生, 王孝磊, 赵凯, 等. 新时期花岗岩研究的进展和趋势[J]. 矿物岩石地球化学通报, 2020, 39(5): 899-911, 1069.
[9]	洪大卫, 王涛, 童英. 中国花岗岩概述[J]. 地质论评, 2007, 53(增刊1): 9-16.
[10]	BEAULIEU E, GODDÉRIS Y, LABAT D, et al. Impact of atmospheric CO₂ levels on continental silicate weathering[J]. Geochemistry, Geophysics, Geosystems, 2010, 11(7): G3.
[11]	DONNADIEU Y, GODDÉRIS Y, RAMSTEIN G, et al. A ‘Snowball Earth’ climate triggered by continental break-up through changes in runoff[J]. Nature, 2004, 428(6980): 303-306.
[12]	DONNINI M, FRONDINI F, PROBST J L, et al. Chemical weathering and consumption of atmospheric carbon dioxide in the Alpine Region[J]. Global and Planetary Change, 2016, 136: 65-81.
[13]	ROYER D L, BERNER R A, MONTAÑEZ I P, et al. CO₂ as a primary driver of Phanerozoic climate[J]. GSA Today, 2004, 14(3): 4-10.
[14]	邓晓东, 李建威, 陈蕾, 等. 风化壳⁴⁰Ar/³⁹Ar年代学研究意义: 进展, 问题与展望[J]. 地质论评, 2008, 54(4): 494-504.
[15]	李德文, 崔之久, 刘耕年. 风化壳研究的现状与展望[J]. 地球学报, 2002, 23(3): 283-288.
[16]	孙承兴, 王世杰, 刘秀明, 等. 风化壳剖面的定年研究[J]. 矿物岩石地球化学通报, 2000, 19(1): 54-59.
[17]	CHABAUX F, BLAES E, STILLE P, et al. Regolith formation rate from U-series nuclides: implications from the study of a spheroidal weathering profile in the Rio Icacos watershed (Puerto Rico)[J]. Geochimica et Cosmochimica Acta, 2013, 100: 73-95.
[18]	CHABAUX F, RIOTTE J, DEQUINCEY O. U-Th-Ra fractionation during weathering and river transport[J]. Reviews in Mineralogy and Geochemistry, 2003, 52(1): 533-576.
[19]	ACKERER J, CHABAUX F, VAN DER WOERD J, et al. Regolith evolution on the millennial timescale from combined U-Th-Ra isotopes and in situ cosmogenic ¹⁰Be analysis in a weathering profile (Strengbach catchment, France)[J]. Earth and Planetary Science Letters, 2016, 453: 33-43.
[20]	DOSSETO A, HANNAN-JOYNER A, RAINES E, et al. Geochemical evolution of soils on Reunion Island[J]. Geochimica et Cosmochimica Acta, 2022, 318: 263-278.
[21]	JIA G D, CHABAUX F, VAN DER WOERD J, et al. Determination of regolith production rates from ²³⁸U-²³⁴U-²³⁰Th disequilibrium in deep weathering profiles (Longnan, SE China)[J]. Chemical Geology, 2021, 574: 120241.
[22]	TAYLOR S R, MCLENNAN S M. The continental crust: its composition and evolution[M]. Oxford, London, Edinburgh, Boston, Palo Alto, Melbourne: Blackwell Scientific, 1985.
[23]	CUI L F, LIU C Q, XU S, et al. Subtropical denudation rates of granitic regolith along a hill ridge in Longnan, SE China derived from cosmogenic nuclide depth-profiles[J]. Journal of Asian Earth Sciences, 2016, 117: 146-152.
[24]	LIU W J, LIU C Q, BRANTLEY S L, et al. Deep weathering along a granite ridgeline in a subtropical climate[J]. Chemical Geology, 2016, 427: 17-34.
[25]	ZHANG Z J, MAO H R, ZHAO Z Q, et al. Sulfur dynamics in forest soil profiles developed on granite under contrasting climate conditions[J]. Science of the Total Environment, 2021, 797: 149025.
[26]	ZHANG Z J, LIU C Q, ZHAO Z Q, et al. Behavior of redox-sensitive elements during weathering of granite in subtropical area using X-ray absorption fine structure spectroscopy[J]. Journal of Asian Earth Sciences, 2015, 105: 418-429.
[27]	范春方, 陈培荣. 赣南陂头花岗岩体Nd-Sr同位素特征及其意义[J]. 地质找矿论丛, 2000, 15(3): 282-287.
[28]	RIHS S, GONTIER A, LASCAR E, et al. Effect of organic matter removal on U-series signal in clay minerals[J]. Applied Clay Science, 2017, 147: 128-136.
[29]	BOSIA C, CHABAUX F, PELT E, et al. U-series disequilibria in minerals from Gandak River sediments (Himalaya)[J]. Chemical Geology, 2018, 477: 22-34.
[30]	WILLIAMS R W, COLLERSON K D, GILL J B, et al. High Th/U ratios in subcontinental lithospheric mantle: mass spectrometric measurement of Th isotopes in Gaussberg lamproites[J]. Earth and Planetary Science Letters, 1992, 111(2/3/4): 257-268.
[31]	WEYER S, ANBAR A D, GERDES A, et al. Natural fractionation of ²³⁸U/²³⁵U[J]. Geochimica et Cosmochimica Acta, 2008, 72(2): 345-359.
[32]	CHENG H, EDWARDS R L, HOFF J, et al. The half-lives of uranium-234 and thorium-230[J]. Chemical Geology, 2000, 169(1/2): 17-33.
[33]	JAFFEY A H, FLYNN K F, GLENDENIN L E, et al. Precision measurement of half-lives and specific activities of U235 and U238[J]. Physical Review C, 1971, 4(5): 1889.
[34]	RICHTER S, ALONSO A, DE BOLLE W, et al. Re-certification of a series of uranium isotope reference materials: IRMM-183, IRMM-184, IRMM-185, IRMM-186 and IRMM-187[J]. International Journal of Mass Spectrometry, 2005, 247(1/2/3): 37-39.
[35]	CARPENTIER M, GANNOUN A, PIN C, et al. New thorium isotope ratio measurements in silicate reference materials: A-THO, AGV-2, BCR-2, BE-N, BHVO-2 and BIR-1[J]. Geostandards and Geoanalytical Research, 2016, 40(2): 239-256.
[36]	SIMS K W W, GILL J B, DOSSETO A, et al. An inter-laboratory assessment of the thorium isotopic composition of synthetic and rock reference materials[J]. Geostandards and Geoanalytical Research, 2008, 32(1): 65-91.
[37]	MATTHEWS K A, MURRELL M T, GOLDSTEIN S J, et al. Uranium and thorium concentration and isotopic composition in five glass (BHVO-2G, BCR-2G, NKT-1G, T1-G, ATHO-G) and two powder (BHVO-2, BCR-2) reference materials[J]. Geostandards and Geoanalytical Research, 2011, 35(2): 227-234.
[38]	RIHS S, GONTIER A, VOINOT A, et al. Field biotite weathering rate determination using U-series disequilibria[J]. Geochimica et Cosmochimica Acta, 2020, 276: 404-420.
[39]	NESBITT H W, YOUNG G M. Early Proterozoic climates and plate motions inferred from major element chemistry of lutites[J]. Nature, 1982, 299: 715-717.
[40]	TIEH T T, LEDGER E B, ROWE M W. Release of uranium from granitic rocks during in situ weathering and initial erosion (central Texas)[J]. Chemical Geology, 1980, 29(1/2/3/4): 227-248.
[41]	DEPAOLO D J, MAHER K, CHRISTENSEN J N, et al. Sediment transport time measured with U-series isotopes: results from ODP North Atlantic drift site 984[J]. Earth and Planetary Science Letters, 2006, 248(1/2): 394-410.
[42]	KIGOSHI K. Alpha-recoil thorium-234: dissolution into water and the uranium-234/uranium-238 disequilibrium in nature[J]. Science, 1971, 173(3991): 47-48. PMID
[43]	FLEISCHER R L. Isotopic disequilibrium of uranium: alpha-recoil damage and preferential solution effects[J]. Science, 1980, 207(4434): 979-981. PMID
[44]	FLEISCHER R L. Alpha-recoil damage and solution effects in minerals: uranium isotopic disequilibrium and radon release[J]. Geochimica et Cosmochimica Acta, 1982, 46(11): 2191-2201.
[45]	VIGIER N, BURTON K W, GISLASON S R, et al. The relationship between riverine U-series disequilibria and erosion rates in a basaltic terrain[J]. Earth and Planetary Science Letters, 2006, 249(3/4): 258-273.
[46]	SCHOONEJANS J, VANACKER V, OPFERGELT S, et al. Coupling uranium series and ¹⁰Be cosmogenic radionuclides to evaluate steady-state soil thickness in the Betic Cordillera[J]. Chemical Geology, 2016, 446: 99-109.
[47]	DOSSETO A, MENOZZI D, KINSLEY L P J. Age and rate of weathering determined using uranium-series isotopes: testing various approaches[J]. Geochimica et Cosmochimica Acta, 2019, 246: 213-233.
[48]	MA L, CHABAUX F, PELT E, et al. Regolith production rates calculated with uranium-series isotopes at Susquehanna/Shale Hills Critical Zone Observatory[J]. Earth and Planetary Science Letters, 2010, 297(1/2): 211-225.
[49]	SUN J, XU W B, FENG B. A global search strategy of quantum-behaved particle swarm optimization[C]// Proceedings of IEEE conference on cybernetics and intelligent systems, Singapore. New York: IEEE, 2004: 111-116.
[50]	ŠAMONIL P, PHILLIPS J, DANĚK P, et al. Soil, regolith, and weathered rock: theoretical concepts and evolution in old-growth temperate forests, Central Europe[J]. Geoderma, 2020, 368: 114261.
[51]	HEIMSATH A M, DIBIASE R A, WHIPPLE K X. Soil production limits and the transition to bedrock-dominated landscapes[J]. Nature Geoscience, 2012, 5: 210-214.
[52]	HEIMSATH A M, DIETRICH W E, NISHIIZUMI K, et al. The soil production function and landscape equilibrium[J]. Nature, 1997, 388: 358-361.
[53]	MA L, CHABAUX F, WEST N, et al. Regolith production and transport in the Susquehanna Shale Hills Critical Zone Observatory, Part 1: insights from U-series isotopes[J]. Journal of Geophysical Research: Earth Surface, 2013, 118(2): 722-740.
[54]	MATHIEU D, BERNAT M, NAHON D. Short-lived U and Th isotope distribution in a tropical laterite derived from granite (Pitinga River Basin, Amazonia, Brazil): application to assessment of weathering rate[J]. Earth and Planetary Science Letters, 1995, 136(3/4): 703-714.
[55]	DEQUINCEY O, CHABAUX F, CLAUER N, et al. Dating of weathering profiles by radioactive disequilibria: contribution of the study of authigenic mineral fractions[J]. Comptes Rendus De L’Académie Des Sciences - Series IIA: Earth and Planetary Science, 1999, 328(10): 679-685.
[56]	DOSSETO A, TURNER S P, CHAPPELL J. The evolution of weathering profiles through time: new insights from uranium-series isotopes[J]. Earth and Planetary Science Letters, 2008, 274(3/4): 359-371.
[57]	DOSSETO A, BUSS H L, SURESH P. Rapid regolith formation over volcanic bedrock and implications for landscape evolution[J]. Earth and Planetary Science Letters, 2012, 337/338: 47-55.
[58]	DOSSETO A, BUSS H L, SURESH P O. Rapid regolith formation over volcanic bedrock and implications for landscape evolution[J]. Earth and Planetary Science Letters, 2012, 337: 47-55.
[59]	GONTIER A, RIHS S, CHABAUX F, et al. Lack of bedrock grain size influence on the soil production rate[J]. Geochimica et Cosmochimica Acta, 2015, 166: 146-164.
[60]	DIXON J L, VON BLANCKENBURG F. Soils as pacemakers and limiters of global silicate weathering[J]. Comptes Rendus Geoscience, 2012, 344(11/12): 597-609.
[61]	FERRIER K L, KIRCHNER J W. Effects of physical erosion on chemical denudation rates: a numerical modeling study of soil-mantled hillslopes[J]. Earth and Planetary Science Letters, 2008, 272(3/4): 591-599.