A Combined (U‐Th)/He and Cosmogenic 3He Record of Landscape Armoring by Biogeochemical Iron Cycling

A Combined (U‐Th)/He and Cosmogenic 3He Record of Landscape Armoring by Biogeochemical Iron... (U‐Th)/He geochronology and cosmogenic 3He in iron oxides reveal mineral precipitation ages as old as 55 Ma and exposure ages greater than 5 Ma for canga‐cemented plateaus in the Quadrilátero Ferrífero, Brazil, showing that lateritic profiles overlying banded iron‐formation (BIF) landscapes in tropical regions have a long history of surface exposure. The long‐term erosion history obtained from cosmogenic 3He on BIF plateaus confirms that relic surfaces persist in the landscape for millions of years. Combined 3He and (U‐Th)/He dating shows that cangas are preferentially goethite cemented by biogeochemical reactions in the subsurface. Importantly, pebbles of hematite‐magnetite in colluvia or shallow creeks draining the canga‐cemented plateaus record a much longer exposure history than in situ canga blocks, showing that even older duricrusts, now eroded, once blanketed these plateaus. Physically stable but biogeochemically dynamic, cangas armor the landscape by pervasive and recurrent iron cycling and cementation, slowing down the delivery of weathered BIF or friable hematite–magnetite ore to erosion. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Geophysical Research: Earth Surface Wiley

A Combined (U‐Th)/He and Cosmogenic 3He Record of Landscape Armoring by Biogeochemical Iron Cycling

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Publisher
Wiley Subscription Services, Inc., A Wiley Company
Copyright
©2018. American Geophysical Union. All Rights Reserved.
ISSN
2169-9003
eISSN
2169-9011
D.O.I.
10.1002/2017JF004282
Publisher site
See Article on Publisher Site

Abstract

(U‐Th)/He geochronology and cosmogenic 3He in iron oxides reveal mineral precipitation ages as old as 55 Ma and exposure ages greater than 5 Ma for canga‐cemented plateaus in the Quadrilátero Ferrífero, Brazil, showing that lateritic profiles overlying banded iron‐formation (BIF) landscapes in tropical regions have a long history of surface exposure. The long‐term erosion history obtained from cosmogenic 3He on BIF plateaus confirms that relic surfaces persist in the landscape for millions of years. Combined 3He and (U‐Th)/He dating shows that cangas are preferentially goethite cemented by biogeochemical reactions in the subsurface. Importantly, pebbles of hematite‐magnetite in colluvia or shallow creeks draining the canga‐cemented plateaus record a much longer exposure history than in situ canga blocks, showing that even older duricrusts, now eroded, once blanketed these plateaus. Physically stable but biogeochemically dynamic, cangas armor the landscape by pervasive and recurrent iron cycling and cementation, slowing down the delivery of weathered BIF or friable hematite–magnetite ore to erosion.

Journal

Journal of Geophysical Research: Earth SurfaceWiley

Published: Jan 1, 2018

Keywords: ; ; ;

References

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