THE EFFECTS OF MARINE DIAGENESIS IN DEEP-SEA CHERTS AS DEPICTED BY COMBINED PETROGRAPHIC, HIGH-PRECISION AND HIGH SPATIAL RESOLUTION ISOTOPE MEASUREMENTS

Ancient cherts have been extensively used to reconstruct the evolution of seawater δ¹⁸O and temperature over geological time. However, given the strong impact of marine diagenesis, reconstructing seawater from the isotope composition of cherts is not straightforward, resulting in ambiguity of interpretations. In this contribution, we present detailed isotope and petrographic investigation of deep-sea seafloor-drilled 135-40 Ma cherts. These cherts have not been exposed to subaerial conditions and regional metamorphism allowing to isolate the isotope effects of marine diagenesis. We combined triple O isotope data with extensive high spatial resolution in situ δ¹⁸O—¹⁶OH/¹⁶O measurements using secondary ion probe (SIMS). In addition, we use electron microprobe to characterize the preservation of silica in analyzed domains. These measurements are cross-calibrated using traditional δ¹⁸O measurements from petrographically diverse domains, and δD and H₂O wt. % measurements of selected samples. The bulk δ¹⁸O values in our collection range from 30 to 36 ‰. The mean SIMS δ′¹⁸O values agree with the values measured by bulk, however, exhibit a variability mostly >1 ‰. Further, individual samples display dependence on the presence of highly hydrous opaline domains that are monitored by the SIMS ¹⁶OH/¹⁶O values. When filtered by ¹⁶OH/¹⁶O and mapped SiO₂ content, the SIMS δ′¹⁸O data still reveal significant heterogeneities up to 7 ‰ in some petrographic features (e.g., matrix vs filled radiolarian skeletons), while other samples exhibit variability attributed to microscopic variations in mineralogical content of analyzed area. These results agree well with previous studies on deep-sea cherts and show that the temperature-dependent diagenesis drives the bulk isotope value away from the original silica-seawater equilibrium. This is particularly notable in the δ′¹⁸O—Δ′¹⁷O space, with chert data consistently shifted towards higher temperatures, especially in young and hot oceanic crust. Consequently, the effects of marine diagenesis of ancient cherts have to be considered when interpreting data with respect to hot Archean oceans.