Oxygen isotopic compositions were determined for coexisting mixed-layer serpentine-chlorite (Sp-Ch) and illite-smectite (I-S) from 5 Tuscaloosa Formation sandstone cores sampled between 1937 and 5470 m burial depth. High gradient magnetic separation (HGMS) was used to concentrate Sp-Ch and I-S from the <0.5 μm fraction of each core sample into fractions with a range in the Sp-Ch: I-S ratio, and end-member δ18O compositions were determined by extrapolation. The Sp-Ch δ18O values range from + 10.4 to 13.7% and increase with burial between 3509 and 5470 m. The only exception is Sp-Ch from 1937 m, which has an anomalously high δ18O value of +12.6‰ The I-S δ18O values range from +16.1 to 17.3% and do not change significantly between 3509 and 5470 m burial depth.

Pore water δ18O compositions calculated from Sp-Ch and I-S values and measured borehole temperatures range from −2.6 to +10.3‰ The isotopically light values indicate that Sp-Ch formed at shallow burial depths in the presence of brackish to marine water and/or meteoric water. The depth-related increase in δ18O of Sp-Ch is attributed to oxygen exchange between mineral and pore water during diagenetic mineral reactions. Increasing δ18O values, in conjunction with XRD and SEM data, indicate that transformation of serpentine layers to chlorite layers and Ibb polytype layers to Iaa polytype layers occurred on a layer-by-layer basis when individual layers dissolved and recrystallized within the confines of coherent crystals. Possible explanations for the variation in I-S δ18O values include depth-related differences in pore water δ18O values present at the time of I-S crystallization, contamination by detrital 2M, mica and 1M polytype rotations that facilitated oxygen exchange.