Nd isotope record of ocean closure archived in limestones of the Devonian–Carboniferous carbonate platform, Greater Karatau, southern Kazakhstan

The neodymium isotope composition of micritic limestones from the Devonian–Carboniferous carbonate platform of the Greater Karatau (southern Kazakhstan) was investigated to test the ability of calcite micrite to archive Nd isotope signatures of seawater. The carbonate fraction that displays seawater-like rare earth element (REE + Y) signatures is often more radiogenic than the dispersed terrigenous material in the samples. Hence, its Nd isotope composition is interpreted to correspond to the seawater from which the micrite was precipitated. The seawater on the Karatau platform exhibited an extremely wide range of εNd(t) values from –9.3 to +4.3 (the most radiogenic value measured for past seawater to date) and very uniform Sm/Nd ratios, from 0.19 to 0.22, lying within the range characteristic for modern oceanic water. The temporal trend in εNd(t) values is interpreted to document the final closure of the Uralian–Turkestan Ocean. It shows that the subduction along Kazakhstan's active margin had already started at the beginning of the Tournaisian (c. 355 Ma), at least 23 Myr earlier than previously thought. The application of Nd isotope time series on biostratigraphically dated carbonates opens a new direction for geotectonic studies. This approach has the potential to provide useful constraints for the precise dating of the duration of geotectonic and volcanic events.