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(U–Th)/He geochronology of goethite and the origin and evolution of cangas

Monteiro, Hevelyn S. and Vasconcelos, Paulo M. and Farley, Kenneth A. and Spier, Carlos A. and Mello, Claudio L. (2014) (U–Th)/He geochronology of goethite and the origin and evolution of cangas. Geochimica et Cosmochimica Acta, 131 . pp. 267-289. ISSN 0016-7037.

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(U–Th)/He geochronology of 147 grains of goethite cements extracted from ferruginous duricrusts (cangas) developed on banded iron-formations from the Quadrilátero Ferrífero region, Minas Gerais, Brazil, records a history of protracted mineral dissolution–reprecipitation that started at ca. 48.1 ± 4.8 Ma and continues intermittently until the Present. A large majority of the samples (more than 30%) are younger than 2 Ma, revealing active mineral dissolution–reprecipitation in the recent past. Within cangas, goethite cements are younger near the surface and become progressively older towards the bottom of the weathering profile, indicating that iron is more effectively cycled in the parts of the weathering profile more strongly affected by biogenic activity. (U–Th)/He geochronology of 14 goethite grains from saprolites in the same profiles yield results ranging from 55.3 ± 5.5 to 25.7 ± 2.6 Ma. For a single weathering profile, goethite cements from cangas are invariably younger than goethite grains from the underlying saprolite, indicating that the duricrust and the saprolite behave as independent and separate systems responding to different environmental controls. Thorium shows conservative behaviour during goethite dissolution– reprecipitation, and it is enriched towards the surface of the weathering profile. Uranium, on the other hand, is preferentially leached from the surface into the saprolite or out of the weathering profile. Recurrent goethite dissolution–reprecipitation lends great textural complexities to cangas, but it is also responsible for its capacity to reheal when physically disrupted. This self-healing property accounts for canga’s role in armoring banded iron-formation landscapes.

Item Type:Article
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URLURL TypeDescription
Monteiro, Hevelyn S.0000-0002-3797-1869
Farley, Kenneth A.0000-0002-7846-7546
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EditorShuster, David L.Shuster-D-LUNSPECIFIED
Additional Information:© 2014 Elsevier Ltd. Received 5 August 2013; accepted in revised form 28 January 2014; available online 11 February 2014. We thank MBR-Vale for logistic support; UQ-CMM staff for help during microanalysis; and Lindsey Hedges for help with (U–Th)/He analysis. This project was partly funded by MBR-Vale and by UQ-AGES and the Brazilian Research Concil (CNPq), which sponsored Hevelyn Monteiro’s master studies. We are greatful to our external reviewers, Pieter Vermeesch, who suggested to use the HelioPlot software for ploting our (U–Th)/He ages in a log(Th/He) vs log(U.He) diagram, and Alexis K. Ault, who provided pertinent suggestions that helped to improve the organization and clarity of the original manuscript.
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Brazilian Research Concil (CNPq)UNSPECIFIED
Record Number:CaltechAUTHORS:20140429-083213616
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Official Citation:Monteiro, H. S., Vasconcelos, P. M., Farley, K. A., Spier, C. A., & Mello, C. L. (2014). (U–Th)/He geochronology of goethite and the origin and evolution of cangas. Geochimica et Cosmochimica Acta, 131(0), 267-289. doi:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:45263
Deposited By: Aucoeur Ngo
Deposited On:29 Apr 2014 16:47
Last Modified:09 Mar 2020 13:19

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