干旱区生态环境知识资源中心

(U-Th)/He geochronology and cosmogenic He-3 in iron oxides reveal mineral precipitation ages as old as 55Ma and exposure ages greater than 5Ma for canga-cemented plateaus in the Quadrilatero Ferrifero, 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 He-3 on BIF plateaus confirms that relic surfaces persist in the landscape for millions of years. Combined He-3 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.