Cherts are used to reconstruct the evolution of seawater δ18O and temperature over geological time. However, given the influence of marine diagenesis, reconstructing seawater from the isotope composition of cherts is not straightforward, resulting in ambiguity of interpretation. Here, we present a detailed isotope and petrographic investigation of deep-sea drilled 135–40 Ma cherts with focus on the effects of marine diagenesis. We combined triple O-isotope data with in-situ δ′18O-16OH/16O measurements using secondary ion mass spectrometry (SIMS). We also provide electron microprobe maps, traditional δ′18O measurements from petrographically diverse domains, and δD and H2O wt.% values. The bulk δ′18O values range between 29‰ and 38‰ in our collection, while SIMS δ′18O data reveal significant intra-sample heterogeneities up to 6‰ related to distinct petrographic features (e.g., filled radiolarian tests) and to micrometer-scale variations in silica forms. Further, the δ′18O—Δ′17O values of these seafloor-drilled cherts plot near and under equilibrium curve. Both triple-O and SIMS δ′18O results reflect diagenesis in presence of marine pore waters at temperatures higher than ambient seawater, which is especially appreciable in cherts deposited on young oceanic crust. Despite the relatively constant δ18O seawater values over last 135 Ma, the marine silica spanning between 0 and 135 Ma occupies a wide compositional space in the δ′18O—Δ′17O rather than an equilibrium curve. The δ′18O values of cherts from modern-seafloor positively correlate with the oceanic crustal age at the time of deposition, hinting at the importance of the heat flux in the diagenetic recrystallization of marine silica.