Diagenetic effects on strontium isotope (⁸⁷Sr/⁸⁶Sr) and elemental (Sr, Mn, and Fe) signatures of Late Ordovician carbonates

Abstract

Understanding the effect and extent of diagenesis on the isotopic compositions of Sr in marine carbonates is a critical prerequisite for their use to unravel past environments. Here, we explore the dominant controls on carbonate ⁸⁷Sr/⁸⁶Sr of a Late Ordovician section from the Monitor Range, USA. Our results reveal a distinct increase in ⁸⁷Sr/⁸⁶Sr from 0.70794 to 0.70830 in the mid-upper <i>D. ornatus</i> zone, which is markedly higher than the published datasets of contemporaneous samples with a relatively lower and stable ⁸⁷Sr/⁸⁶Sr ratio of ~0.7079. These elevated ⁸⁷Sr/⁸⁶Sr ratios suggest a local and post-depositional overprint and cannot be interpreted to reflect the ⁸⁷Sr/⁸⁶Sr of the coeval seawater. Furthermore, ⁸⁷Sr/⁸⁶Sr exhibits statistically significant positive correlations with geochemical indicators for diagenesis ([Mn], [Fe], Mn/Sr, Fe/Sr), indicating that diagenetic alteration is the principal control on the observed radiogenic ⁸⁷Sr/⁸⁶Sr values. Using a numerical model of marine diagenetic fluid-rock interaction, we demonstrate that the observed Sr isotopic and elemental data can be best explained by the chemical variations in bulk carbonates associated with diagenetic alteration. Our results highlight that diagenesis may significantly alter the pristine ⁸⁷Sr/⁸⁶Sr ratios of carbonates than previously thought, although the samples satisfy the stricter geochemical criteria of Sr isotope preservation ([Sr] > 300 ppm, [Mn] < 300 ppm, [Fe] < 1000 ppm, Mn/Sr < 0.2, Fe/Sr < 1.6), pointing to the need for more caution when using bulk carbonate ⁸⁷Sr/⁸⁶Sr as a tracer of paleoenvironmental changes.