A 3.1 billion year old marble and the87Sr/86Sr of late‐Archean seawater

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Abstract
A marble band in the ≈ 2.75 Gyr old Ramagiri schist belt in the Dharwar craton of south India gave a Pb–Pb age of 3.075 ± 0.095 Gyr. The geochemical data, including high Sr and low Ba and Mn indicate seawater origin for the parent rock, and that there was insignificant geochemical exchange between the marble and the surrounding rocks. The calculated initial Nd isotopic composition and μ1indicate an older continental crustal source for the Nd and Pb. The initial87Sr/86Sr of the marble is 0.70128, which is higher than the calculated mantle value at ≈ 3 Ga. Although pre‐3 Gyr old marine carbonate rocks are thought to be buffered by mantle Sr, the Ramagiri marble contains evolved, crustal Sr. Despite this, the marble has the lowest measured87Sr/86Sr among carbonates and represents one of the least radiogenic periods in seawater Sr isotope composition.