Effective leaching of argillaceous and dolomitic carbonate rocks for strontium isotope stratigraphy

Abstract

In recent years, various sample preparation methods have been developed to extract a primary Sr isotope signal from carbonate rocks. However, there has been little consensus around the best method due to natural variability in sample purity and mineralogy. For this study, we conducted systematic leaching experiments, focussing mainly on generally less favoured argillaceous and dolomitic limestones, using samples from the Mesoproterozoic Gaoyuzhuang Formation. Learning from previous studies, a multistep leaching approach was used to explore strontium isotope leaching systematics and extract near-primary isotopic ratios. For argillaceous and dolomitic limestone, the first 10%-30% dilute acetic acid leach, following an ammonium acetate (NH4Ac) prewash, yielded the lowest, demonstrably least altered seawater 87Sr/86Sr ratios. Subsequent dissolution steps exhibited significantly elevated 87Sr/86Sr, Rb/Sr, Al/Ca, and Mg/Ca ratios, indicating greater contributions from aluminosilicates and dolomite in the leachates. Lanthanide rare earth element and yttrium (REY) concentrations were also examined for each leaching step. In most cases, seawater-like REY patterns with the highest Y/Ho ratios (mostly > 36) occur in the first leaching step after ammonium acetate prewash, which is consistent with the Sr isotope study. However, one organic-rich sample (TOCtotal ~1.3%) with a near-seawater Sr isotope value exhibits a non-seawater-like REY pattern, possibly indicating that early diagenetic exchange during organic remineralization may influence the REY pattern of carbonate rocks without necessarily changing Sr isotopes. We applied previously proposed leaching cut-offs (preleach 30% ~ 40%, 60% ~ 70%), alongside the cut-off proposed from this study, for samples with various Mg/Ca and carbonate purities. We found that for the same argillaceous and dolomitic carbonate rocks, applying higher leaching cut-offs might cause sizeable error compared with targeting the first 10% ~ 30% directly after NH4Ac prewash. However, no significant differences were evident for high purity, and low Mg/Ca limestones, highlighting the importance of matching different sample types to the most appropriate dissolution method. Here we demonstrate that argillaceous, even partially dolomitized limestones could help fill gaps in the Precambrian seawater Sr isotope curve, especially where higher purity limestone successions are not readily available.