Strontium isotope dating influenced by Rubidium contamination from terrestrial material: A case study from the Cenozoic dolomite in the Xisha Islands, South China Sea

Strontium isotopes (Sr-87/Sr-86) are commonly used to assess the age of Cenozoic dolostones found on many isolated oceanic islands. The possible influence of Rubidium (Rb) content on the Sr-87/Sr-86 ratios, however, has generally been ignored. This critical issue is assessed by examining the Sr-87/Sr-86 ratios and elements (Rb, Sr, Al, Ti and K) of seventy-one samples that came from a ca 339 m thick cored interval of Cenozoic dolomite on the Xisha Islands, South China Sea. For the dolomite from the upper Pliocene Yinggehai Formation (180.5 to 308.5 m; average 23% dolomite) there is a positive correlation between the Sr-87/Sr-86 ratios and Rb concentrations, whereas for the dolomite from the lower late Miocene Huangliu Formation (308.5 to 519 m; average >98% dolomite) such a correlation is not evident. Combined with high-temperature experiments, correlation between Al, Ti, K, Rb and Sr-87/Sr-86 indicates that Rb containing old Rb-87 from terrestrial materials modified the Sr-87/Sr-86 ratios of the Yinggehai dolomite, but not those from the Huangliu dolomite. This is mainly attributed to the different sources of terrestrial dust carried by different East Asian monsoon systems between the late Miocene and Pliocene, and probably also influenced by the dolomitization process to some extent. To test for Rb-87 contamination, extrapolation of results from the Yinggehai dolomite and Huangliu dolomite indicate that concentrations of Rb < 1 ppm, Al < 1000 ppm, Ti < 15 ppm and K < 250 ppm in the dolomite indicates that terrestrial inputs are minimal and that the measured Sr-87/Sr-86 ratios can be used for determining the dolomitization age. Potentially, these concentration thresholds may serve as a screening criterion for assessing the reliability of Sr isotope chronostratigraphy of other island dolostones throughout the world.