More than an age: U-Pb dating constrains alteration of Precambrian carbonates

Abstract

The Campbellrand Platform (South Africa) is one of the best-preserved carbonate archives for studying Neoarchean marine environments immediately preceding the Great Oxygenation Event. These carbonates preserve depositional structure and textures, like a variety of dolomitized stromatolites and microbialites encased in herringbone calcite – a type of early marine calcite cement. The preservation of depositional textures in Precambrian carbonates is often used to argue for the retention of primary paleoenvironmental signals, which are, in turn, used to infer the conditions in ancient surface environments including the composition of seawater and climate. Here we tested this idea using laser-ablation ICP-MS U-Pb to independently date specific carbonate mineral fabrics from the ∼2.58 Ga Reivilo Formation and ∼2.52 Ga Gamohaan Formation at the Campbellrand platform in the Northern Cape Province of South Africa. U-Pb dates of dolomite fabrics are mostly younger than their expected stratigraphic age and their initial ²⁰⁷Pb/²⁰⁶Pb compositions follow the modeled terrestrial Pb-isotope evolution trend. These results suggested that dolomite fabrics have experienced open-system alteration in which diagenetic fluids carrying a terrestrial signature reset U-Pb dates and overprinted initial Pb-isotope compositions during deep diagenesis. In contrast, herringbone calcite fabrics yielded near-stratigraphic U-Pb dates that clustered around an average of 2403 (±93) Ma and initial ²⁰⁷Pb/²⁰⁶Pb values that plotted below the modeled terrestrial Pb-isotope evolution trend. These results revealed that Neoarchean herringbone calcite fabrics were minimally altered in a closed system with respect to Pb, at deposition or some 10-100 Ma later, and have remained closed with respect to both U and Pb since. The closed-system behavior of a highly sensitive system like U-Pb in herringbone calcites serves as an independent validation for the reliability of herringbone calcites as useful recorders of marine environments. U-Pb dating of ancient carbonates thus offers a complementary tool for petrographic analyses that can identify and evaluate the timing and nature of alteration (open or closed system) of primary geochemical signatures in Precambrian carbonates.