Characterization of enriched lithospheric mantle components in ∼2.7 Ga Banded Iron Formations: An example from the Tati Greenstone Belt, Northeastern Botswana

Abstract

Major and trace element, samarium (Sm)–neodymium (Nd) and lead (Pb) isotopic analyses of individual mesobands of five Banded Iron Formations (BIFs) and associated volcanic and sedimentary rocks from the Neoarchean Tati Greenstone Belt (TGB, Northeastern Botswana) were conducted in order to characterize the source(s) and depositional environment(s). Rare earth element (REE)–yttrium (Y) patterns of individual BIF mesobands showfeatures characteristic of other Archean BIFs with LREE depletion relative to MREE and HREE, positive La/La∗PAAS, Eu/Eu∗PAAS, Y/Ho ratios and no Ce/Ce∗PAAS anomalies. The REY patterns are comparable to modern seawater and together with low concentrations of high-field strength elements these features are indicative of an essentially detritus-free precipitation. Elevated Eu anomalies in the TGB BIFs are a general feature observed in ∼2.7 Ga BIFs worldwide and possibly result from widespread magmatic activity and associated high-temperature fluid fluxes to the oceans at around this time. Uranogenic Pb isotope data for the BIFs define correlation lines with slopes corresponding to apparent ages of ∼2.7 Ga which brackets the depositional timeframe. Pb isotope data on sulfides and Pb-stepwise leaching (PbSL) data on garnets define a correlation line with an apparent age of 1976±88 Ma. This age is similar to tectono-metamorphic events within the adjacent Limpopo belt. Elevated 207Pb/204Pb relative to 206Pb/204Pb ratios of BIFs are indicative of a high- (238U/204Pb) prehistory of their source materials which can best be modeled by a 3.0–3.2 Ga extraction of these sources from an older Archean mantle reservoir. The TGB BIFs show evidence of two periodically interacting water masses during the deposition. The first is characterized by elevated Sm/Nd ratios and a negative inferred εNd(2.7 Ga) value of −2.5 and is associated with high Fe fluxes. The second source, associated with high Si fluxes, is characterized by lower Sm/Nd ratios and a less negative inferred εNd(2.7 Ga) value of −0.4. While the association of high Fe concentrations and elevated Sm–Nd in BIF mesobands is characteristic of hydrothermal seawater input, the Sm–Nd isotopic characterization of this source, unlike other Archean BIFs, points to a significantly LREE enriched mantle source. This finding is compatible with the potential existence of a sub-continental lithospheric mantle reservoir beneath the Zimbabwe and Kaapvaal craton. The old (up to ∼3.5 Ga) Nd (TDM) model ages, particularly of iron-rich mesobands of the TGB BIFs, support such a scenario. In contrast, Si-rich solutes were likely derived from weathering of mafic continental crust.