A New Geochemical Tool to Screen Carbonates for Strontium Isotope Stratigraphy

Abstract

Strontium isotope stratigraphy (SIS) is a valuable dating tool used to date and correlate marine sediments by comparing the ratio of Sr isotopes in marine carbonates to a composite reference curve. However, the original 87Sr/86Sr ratios in carbonate rocks are easily altered by contamination with Sr that bears a different isotopic signature sourced from direct terrigenous contamination or from diagenetic fluids. This alteration renders isotopic ratios useless for correlation purposes. Traditionally petrographic techniques and major element analysis have been used to screen and exclude diagenetically altered carbonates, but these techniques may not produce results that are easily interpreted. To improve the reliability of SIS, fast, reliable and unequivocal geochemical tools are required to screen potential dating targets. We propose that trace element geochemistry that includes the rare earth elements (REEs) may provide an inexpensive tool for evaluating SIS targets. We present a suite of combined Sr isotope and trace element analyses from early Miocene and late Eocene carbonate platform samples from Sarawak, Malaysia. Samples have known ages from prior biostratigraphic analysis and represent differing depositional environments with varying terrigenous input and include differing diagenetic end members. It was hypothesised that allochems that retain a marine REE distribution are more likely to retain original Sr isotopic ratios. Direct shale contamination of carbonate allochems was monitored using the immobile elements Th and Zr. Non-marine REE patterns and elevated Mn concentrations were interpreted to reflect alteration in deep burial fluids. Comparison of the 87Sr/86Sr ratios of samples from these well-dated sections to the reference Sr isotopic curve show that the samples that preserve a marine REE signature and lack direct shale contamination fall along the reference curve whereas those that lack a marine REE distribution are shifted away from the curve, thus suggesting the wrong age. Hence, this relatively simple geochemical analysis can exclude at least some unreliable Sr isotopic samples prior to expensive Sr isotope analysis and thus improve the efficiency and cost effectiveness of SIS for correlation.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015