Continentally-derived solutes in shallow Archean seawater: Rare earth element and Nd isotope evidence in iron formation from the 2.9 Ga Pongola Supergroup, South Africa

Abstract

The chemical composition of surface water in the photic zone of the Precambrian ocean is almost exclusively known from studies of stromatolitic carbonates, while banded iron formations (IFs) have provided information on the composition of deeper waters. Here we discuss the trace element and Nd isotope geochemistry of very shallow-water IF from the Pongola Supergroup, South Africa, to gain a better understanding of solute sources to Mesoarchean shallow coastal seawater. The Pongola Supergroup formed on the stable margin of the Kaapvaal craton ∼2.9Ga ago and contains banded iron formations (IFs) that represent the oldest documented Superior-type iron formations. The IFs are near-shore, pure chemical sediments, and shale-normalized rare earth and yttrium distributions (REYSN) exhibit positive LaSN, GdSN, and YSN anomalies, which are typical features of marine waters throughout the Archean and Proterozoic. The marine origin of these samples is further supported by super-chondritic Y/Ho ratios (average Y/Ho=42). Relative to older Isua IFs (3.7Ga) from Greenland, and younger Kuruman IFs (2.5Ga) also from South Africa, the Pongola IFs are depleted in heavy rare earth elements (HREE), and appear to record variations in solute fluxes related to sea level rise and fall. Sm–Nd isotopes were used to identify potential sediment and solute sources within pongola shales and IFs. The ϵNd(t) for Pongola shales ranges from −2.7 to −4.2, and ϵNd(t) values for the coeval iron-formation samples (range −1.9 to −4.3) are generally indistinguishable from those of the shales, although two IF samples display ϵNd(t) as low as −8.1 and −10.9. The similarity in Nd isotope signatures between the shale and iron-formation suggests that mantle-derived REY were not a significant Nd source within the Pongola depositional environment, though the presence of positive Eu anomalies in the IF samples indicates that high-T hydrothermal input did contribute to their REY signature. Isotopic mass balance calculations indicate that most (⩾72%) of the Nd in these seawater precipitates was derived from continental sources. If previous models of Fe–Nd distributions in Archean IFs are applied, then the Pongola IFs suggest that continental fluxes of Fe to Archean seawater were significantly greater than are generally considered.