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

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.

Age and evolution of diachronous erosion surfaces in the Amazon: Combining (U-Th)/He and cosmogenic 3He records

“…Low rates of erosion are confirmed by cosmogenic 53 Mn analysis (Fujioka et al, 2010). (U-Th)/He geochronology of supergene goethites and cosmogenic 3 He measurements from plateaus at the Quadrilátero Ferrífero (QF), Minas Gerais, Brazil corroborate the low rates of erosion for the ancient (> 70 Ma) plateaus (Monteiro et al, 2014(Monteiro et al, , 2018.…”

“…The results for exhumed hematite blocks presently cemented in cangas and for loose hematite pebbles yield compatible average rates of erosion of 0.20 ± 0.02 m.Ma -1 for N1 and 0.24 ± 0.2 m.Ma -1 for S11D, confirming that the plateaus are largely impervious to physical erosion for tens of millions of years. The relatively higher erosion rates obtained for partially hydrated hematite (e.g., 0.44 ± 0.06 m.Ma -1 , Table 2; Monteiro et al, 2018) suggest that the plateaus evolve by slow hydration and dissolution-reprecipitation, with some mass loss resulting from downward leaching of soluble species. These chemical processes are very slow, and the plateau suffers very little chemical and virtually no physical erosion.…”

“…Hematite grains may contain areas partially recrystallized to goethite (Monteiro et al, 2018), which potentially results in partial 3 He loss. Therefore, we interpret all results as minimum values and refer to all results as apparent exposure ages (aEA).…”

“…Here, we expand on those studies and apply a similar approach to that used by Monteiro et al (2014Monteiro et al ( , 2018 for the QF. We investigate, in addition to ages, possible mechanisms controlling the formation and preservation of ancient weathering profiles at Carajás.…”

Innovations in (U–Th)/He, Fission Track, and Trapped Charge Thermochronometry with Applications to Earthquakes, Weathering, Surface‐Mantle Connections, and the Growth and Decay of Mountains

“…Depth scale is approximate, as weathering profile thickness is variable and can be >100 m. Insets (b, c) show metallic, Fe oxide enrichment and a scanning electron microscope image of a goethite concretion (Allard et al, ), respectively. (d) Frequency of supergene goethite He and hematite He dates from northern Brazil (Allard et al, ; Monteiro, Vasconcelos, & Farley, ; Monteiro, Vasconcelos, Farley, & Lopes, ; Shuster et al, , ). (e) Temperature variations based on δ 18 O isotopic composition over the Cenozoic (Zachos et al, ).…”