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The Hekpoort paleosol profile in Strata 1 at Gaborone, Botswana: Soil formation during the Great Oxidation Event

Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138

The Hekpoort paleosol is a regional paleoweathering horizon developed on the approximately 2.25 Ga Hekpoort basalt of the Transvaal Supergroup, South Africa. A core (Strata 1) through the Hekpoort paleosol near Gaborone, Botswana, is the most complete section through this paleosol. An uppermost ferricrete layer (40 cm) is followed by a mottled zone (2.0 m), an iron-depleted sericite-rich zone (2.3 m), and a FeO-enriched, chlorite-quartz zone (13 m), which grades downward into the Hekpoort basalt. The near-constancy of the mole ratios of Al, Zr, Nb, Hf and Th to Ti indicates that these elements were essentially immobile during the formation of the paleosol. Mg, Zn and Mn were largely removed from the upper portion of the soil and were partly reprecipitated in the chlorite zone. Fe²⁺, Co, Ni, V, Cr and U were depleted in the mottled and sericite zones, and were enriched in the ferricrete layer and in the chlorite zone. The vertical distribution of the major and trace redox-sensitive elements in Strata 1 indicates that the Hekpoort paleosol developed during the Great Oxidation Event (G.O.E.). The atmosphere must have contained some oxygen, but apparently not enough to oxidize all of the Fe²⁺ released during weathering to Fe³⁺. A mass balance calculation suggests a value of P_O2 between 2.5x10^-4 and 9x10^-3 atm during the formation of the paleosol. Interpretations of the paleosol as a ground water laterite developed under a highly oxygenated atmosphere are difficult to reconcile with the distribution of the major and trace elements in Strata 1.

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