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

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.