Goethite (U-Th)/He geochronology and precipitation mechanisms during weathering of basalts

Riffel, Silvana B., Vasconcelos, Paulo M., Carmo, Isabela O. and Farley, Kenneth A. (2016) Goethite (U-Th)/He geochronology and precipitation mechanisms during weathering of basalts. Chemical Geology, 446 18-32. doi:10.1016/j.chemgeo.2016.03.033

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Author Riffel, Silvana B.
Vasconcelos, Paulo M.
Carmo, Isabela O.
Farley, Kenneth A.
Title Goethite (U-Th)/He geochronology and precipitation mechanisms during weathering of basalts
Journal name Chemical Geology   Check publisher's open access policy
ISSN 0009-2541
1872-6836
Publication date 2016-04-11
Year available 2016
Sub-type Article (original research)
DOI 10.1016/j.chemgeo.2016.03.033
Open Access Status File (Author Post-print)
Volume 446
Start page 18
End page 32
Total pages 15
Place of publication Amsterdam, Netherlands
Publisher Elsevier BV
Language eng
Subject 1907 Geology
1906 Geochemistry and Petrology
Abstract (U–Th)/He geochronology of 33 goethite grains from in situ ferruginous duricrusts overlying the Paraná flood basalt in the Guarapuava region, Paraná, Brazil, reveals ages ranging from 3.6 ± 0.4 to 0.4 ± 0.1 Ma. Thirty-one grains from detrital fragments of ferruginous duricrust yield ages in the 6.2 ± 0.6 to 0.7 ± 0.1 Ma range. The results show that goethites from detrital blocks are generally older than those from the in situ ferruginous layers and that all the goethites from the Guarapuava sites are relatively young (≤ 6.2 ± 0.6 Ma). Goethites show variable U and Th contents depending on the mode of precipitation. Goethite that forms by progressive ferrugization and pseudomorphic replacement of basalts are rich in Th, U-poor, contain various mineral inclusions (e.g., kaolinite, rutile, ilmenite) and display a distinct “checkered” appearance. Colloform goethite precipitated within cavities and desiccation cracks contain higher U, show low Th/U values, and are free of mineral contaminants, suggesting direct precipitation from solution. The geochronological results reveal a dynamic evolution for the soils overlying the Paraná basalt, where recurrent duricrust formation and dismantling lead to the evolution of regional colluvial soil profiles while simultaneously preventing the evolution of chemically stratified lateritic weathering profiles. The modes of formation and U, Th, and He contents of goethite at Guarapuava provide useful constraints on the expected compositions and modes of formation of goethite formed on basaltic soils on Mars.
Formatted abstract
(U-Th)/He geochronology of 33 goethite grains from in situ ferruginous duricrusts overlying the Paraná flood basalt in the Guarapuava region, Paraná, Brazil, reveals ages ranging from 3.6. ±. 0.4 to 0.4. ±. 0.1. Ma. Thirty-one grains from detrital fragments of ferruginous duricrust yield ages in the 6.2. ±. 0.6 to 0.7. ±. 0.1. Ma range. The results show that goethites from detrital blocks are generally older than those from the in situ ferruginous layers and that all the goethites from the Guarapuava sites are relatively young (≤. 6.2. ±. 0.6. Ma). Goethites show variable U and Th contents depending on the mode of precipitation. Goethite that forms by progressive ferrugization and pseudomorphic replacement of basalts are rich in Th, U-poor, contain various mineral inclusions (e.g., kaolinite, rutile, ilmenite) and display a distinct "checkered" appearance. Colloform goethite precipitated within cavities and desiccation cracks contain higher U, show low Th/U values, and are free of mineral contaminants, suggesting direct precipitation from solution. The geochronological results reveal a dynamic evolution for the soils overlying the Paraná basalt, where recurrent duricrust formation and dismantling lead to the evolution of regional colluvial soil profiles while simultaneously preventing the evolution of chemically stratified lateritic weathering profiles. The modes of formation and U, Th, and He contents of goethite at Guarapuava provide useful constraints on the expected compositions and modes of formation of goethite formed on basaltic soils on Mars.
Keyword (U-Th)/He
Goethite
Landscape evolution
Paraná Continental Flood Basalts
Weathering geochronology
Q-Index Code C1
Q-Index Status Provisional Code
Grant ID 4862-06-6
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: School of Earth Sciences Publications
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