A 3.1 billion year old marble and the Sr-87/Sr-86 of late-Archean seawater

A marble band in the approximate to 2.75 Gyr old Ramagiri schist belt in th e Dharwar craton of south India gave a Pb-Pb age of 3.075 +/- 0.095 Gyr, Th e geochemical data, including high Sr and low Ba and Mn indicate seawater o rigin for the parent rock, and that there was insignificant geochemical exc hange between the marble and the surrounding rocks. The calculated initial Nd isotopic composition and mu(1) indicate an older continental crustal sou rce for the Nd and Pb. The initial Sr-87/Sr-86 of the marble is 0.70128, wh ich is higher than the calculated mantle value at approximate to 3 Ga. Alth ough pre-3 Gyr old marine carbonate rocks are thought to be buffered by man tle Sr, the Ramagiri marble contains evolved, crustal Sr, Despite this, the marble has the lowest measured Sr-87/Sr-86 among carbonates and represents one of the least radiogenic periods in seawater Sr isotope composition.