Diagenesis of fibrous magnesian calcite marine cement: Implications for the interpretation of δ18O and δ13C values of ancient equivalents

Abstract

Fibrous, high Mg-calcite (HMC) marine cement from the Pliocene Hope Gate Formation, Jamaica occurs as 1–2 mm thick isopachous coatings on skeletal grains in local occurrences of Halimeda grainstone and is commonly syntaxially overgrown by clear, prismatic low Mg-calcite (LMC) interpreted as a mixing zone cement. The presence of inclusions at discrete levels in the fibrous marine cement produces internal growth banding wherein clear, inclusion-free zones alternate with cloudy, inclusion-rich zones. Clear zones have uniform Mg contents (13.7 ± 0.2 mol% MgCO 3) and stable isotopic compositions ( δ 18O = 0.8 ± 0.1%; δ 13C = 3.2 ± 0.1%) consistent with a marine origin. In contrast, inclusion-rich zones contain fine-scale intergrowths of LMC, dolomite, and HMC, and have carbon and oxygen isotopic compositions that define a linear mixing trend between compositions of inclusion-free HMC and prismatic LMC cement overgrowths. Petrographic observations indicate that the mixing trend defined by isotopic data from inclusion-rich zones is related to the addition of large volumes of secondary LMC as cement in primary pore space, as intergrowths among fibers of primary HMC marine cement, and/or as replacements of primary HMC crystallites. Dolomite is present in inclusion-rich zones in volumes generally compatible with amounts predicted to result from the closed-system stabilization of HMC to LMC and dolomite. In addition, isotopic and geochemical data from some areas of inclusion-rich zones are consistent with mass balance constraints assuming a closed-system, which require that bulk isotopic and geochemical compositions are retained. These relations suggest that small remnants of HMC in inclusion-rich zones of Hope Gate marine cement have altered to LMC and dolomite while isolated from system pore fluids, such that primary marine isotopic signatures were imparted on diagenetic products. Similar petrographic, geochemical, and isotopic relations have been noted in ancient occurrences, suggesting that δ 18O and δ 13C values from some former marine cements should be useful for the determination of isotopic compositions of original marine cements and ultimately for the determination of ocean chemistry in the geologic past.