A combined (U-Th)/He and cosmogenic 3He record of landscape armoring by biogeochemical iron cycling

(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 (QF), 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.

(U-Th)/He地质年代学与铁氧化物中的宇宙成因³He研究显示，巴西费拉迪圣多（Quadrilátero Ferrífero，QF）卡恩加（canga）胶结高原的矿物沉淀年龄可追溯至55 Ma，暴露年龄超过5 Ma，表明热带地区条带状铁建造（BIF）地貌上覆的红土剖面具有漫长的地表暴露历史。从BIF高原宇宙成因³He获取的长期侵蚀历史证实，残留地表在地貌中持续存在数百万年。³He与(U-Th)/He定年相结合表明，卡恩加优先由地下生物地球化学反应胶结的针铁矿构成。重要的是，汇流卡恩加胶结高原的崩积物或浅溪中的赤铁矿-磁铁矿卵石，其记录的暴露历史远长于原位卡恩加块体，这揭示即使是现已侵蚀的更古老硬壳（duricrust）也曾覆盖这些高原。卡恩加物理稳定但生物地球化学活跃，通过广泛且反复的铁循环与胶结作用保护地貌，减缓风化BIF或易碎赤铁矿-磁铁矿矿石被侵蚀的过程。