Isotopic evidence for iron mobilization during Paleoproterozoic lateritization of the Hekpoort paleosol profile from Gaborone, Botswana

Kosei E. Yamaguchi, Clark M. Johnson, Brian L. Beard, Nicolas J. Beukes, Jens Gutzmer, Hiroshi Ohmoto

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46 Citations (Scopus)

Abstract

The isotopic composition and elemental abundance of Fe in a lateritic weathering profile (drillcore Strata-1) formed on the ∼ 2.2 Ga Hekpoort basalt of the Pretoria Group in Gaborone, Botswana, document open-system behavior for Fe during paleosol formation. The δ56Fe values of the profile increase from ∼ 0‰ at the bottom (parental basalt) to + 1‰ in the mid-depth mottled (Fe-depleted) zone, then decrease to + 0.3‰ at the top of the laterite (Fe-enriched) zone. The reworked ferricrete and red beds that overlie the laterite have high δ56Fe values of + 0.2 to + 0.4‰, which are comparable to those of the underlying laterite. Because of the absence of a complementary low-δ56Fe zone, these results are not compatible with a model of simple internal redistribution (closed-system) of Fe within the paleosol column. Instead, these results are well explained through fluid-rock interaction involving transport of aqueous Fe2+ through the system during paleosol evolution. Production of positive δ56Fe values for Fe3+ oxides were simulated through oxidation of low-δ56Fe Fe2+aq using experimentally determined Fe3+oxide-Fe2+aq fractionation factors. The increased quantities of Fe2+ involved in formation of the Hekpoort paleosol relative to previous estimates based on a closed-system model require a commensurate increase in the abundance of an oxidant, such as atmospheric O2, to explain the Fe3+/Fe2+ ratios in the paleosol. Therefore, previous estimates of atmospheric O2 levels of 2.5 × 10- 4 to 9 × 10- 3 atm based on this section [W. Yang, H.D. Holland, The Hekpoort paleosol profile in Strata 1 at Gaborone, Botswana: Soil formation during the great oxidation event, American Journal of Science 303(2003) 187-220] are too low by an order of magnitude or more. These results demonstrate that Fe isotopes can provide important constraints on open- versus closed-system behavior of Fe in ancient weathering profiles, a distinction that is critical when calculating Fe mobility as a means for inferring surface redox conditions in the past.

Original languageEnglish
Pages (from-to)577-587
Number of pages11
JournalEarth and Planetary Science Letters
Volume256
Issue number3-4
DOIs
Publication statusPublished - 30 Apr 2007

Keywords

  • Fe
  • isotope
  • paleosol
  • weathering

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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