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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >First Ar-40/Ar-39 geochronology of lateritic manganiferous pisolites: Implications for the Palaeogene history of a West African landscape
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First Ar-40/Ar-39 geochronology of lateritic manganiferous pisolites: Implications for the Palaeogene history of a West African landscape

机译：红土锰铁云杉岩的第一个Ar-40 / Ar-39年代学：对西非景观古近纪历史的启示

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A pisolitic superficial formation deriving from the geochemical degradation of a previous massive manganiferous crust has been investigated in West Africa using Ar-40/Ar-39 laser probe analysis on different generations of cryptomelane from Fe-Mn pisolites and their embedding lateritic matrices. Prior to the Ar-40-Ar-39 geochronological analysis, a detailed petrographical study has allowed the identification of successive assemblages from Mn-pisolites to (Fe,Mn)-pisolites embedded in Mn-rich and Fe-rich matrices, respectively. The age spectra obtained for pisolites, including cores, cortices and matrices enables the definition of three age clusters around 56-59, 44-47, and 24 27 Ma. The first two age clusters with the petrographical and geochemical results obtained on the different assemblages indicate that the development of the pisolitic crust first induced Mn-leaching and Fe-Al accumulations due to warmer and wetter climatic conditions in the age interval 56-47 Ma that characterizes the Tertiary greenhouse effect period propitious to bauxite formation in West Africa. The absence of Ar-40/Ar-39 ages between 44 and 27 Ma only means that geochemical conditions for cryptomelane crystallisation were not fulfilled, but could still be favourable to Fe- and/or Al-oxy-hydrox ides formation, before drier climatic conditions became rather effective at the Oligocene period to sustain mechanical erosion rather than chemical weathering processes. The age cluster 24-27 Ma indicates however a reactivation of the manganiferous lateritic weathering late Oligocene. The Ar-40/Ar-39 dating results are discussed in terms of climatic condition changes during the Palaeogene that were favourable to the genesis of either Al- and Fe- or Mn-oxyhydroxides in the course of development and evolution of the pisolitic formation. The Ar-40/Ar-39 dating also provides a new highlight to large-scale geomorphological patterns of West Africa. (c) 2005 Elsevier B.V. All rights reserved.
机译：在西非，使用Ar-40 / Ar-39激光探针分析了Fe-Mn辉石岩中不同世代的隐甲烷及其嵌入的红土基质，研究了源自先前大量锰壳的地球化学降解的岩溶表层地层。在进行Ar-40-Ar-39地质年代学分析之前，详细的岩石学研究已经确定了从分别包含在富锰和富铁基体中的锰-辉石岩到（Fe，Mn）-辉石的连续组合。从岩芯岩（包括岩心，皮质和基质）获得的年龄谱能够确定三个年龄群，分别为56-59、44-47和24 27 Ma。在不同组合下获得的岩石学和地球化学结果的前两个年龄群表明，由于年龄在56-47 Ma之间的气候变暖和湿润，火山岩壳的发育首先引起Mn淋溶和Fe-Al积累。表征了西非铝土矿形成的第三纪温室效应时期。没有Ar和Ar-39年龄介于44和27 Ma之间，这仅意味着不能满足隐甲烷结晶的地球化学条件，但仍可能在干燥气候之前有利于Fe-和/或Al-羟基氧化物的形成。条件在渐新世时期变得相当有效，可以维持机械侵蚀而不是化学风化过程。然而，年龄群24-27 Ma表示锰红土风化风化晚渐新世的再活化。根据古近纪期间气候条件的变化讨论了Ar-40 / Ar-39的定年结果，这些气候变化有利于铝和铁或锰或羟基氧化氢氧化物在岩溶形成和发展过程中的发生。 Ar-40 / Ar-39年代测定法也为西非的大规模地貌格局提供了新的亮点。 （c）2005 Elsevier B.V.保留所有权利。

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