OXYGEN-ISOTOPE COMPOSITIONS OF MIXED-LAYER SERPENTINE-CHLORITE AND ILLITE-SMECTITE IN THE TUSCALOOSA FORMATION (US GULF-COAST) - IMPLICATIONS FOR PORE FLUIDS AND MINERALOGIC REACTIONS

Oxygen isotopic compositions were determined for coexisting mixed-laye r serpentine-chlorite (Sp-Ch) and illite-smectite (I-S) from 5 Tuscalo osa Formation sandstone cores sampled between 1937 and 5470 m burial d epth. High gradient magnetic separation (HGMS) was used to concentrate Sp-Ch and I-S from the <0.5 mu m fraction of each core sample into fr actions with a range in the Sp-Ch:I-S ratio, and end-member delta(18)O compositions were determined by extrapolation. The Sp-Ch delta(18)O v alues range from +10.4 to 13.7 parts per thousand and increase with bu rial between 3509 and 5470 m. The only exception is Sp-Ch from 1937 m, which has an anomalously high delta(18)O value of +12.6 parts per tho usand. The I-S delta(18)O values range from +16.1 to 17.3 parts per th ousand and do not change significantly between 3509 and 5470 m burial depth. Pore water delta(18)O compositions calculated from Sp-Ch and I- S values and measured borehole temperatures range from -2.6 to +10.3 p arts per thousand. The isotopically light values indicate that Sp-Ch f ormed at shallow burial depths in the presence of brackish to marine w ater and/or meteoric water. The depth-related increase in delta(18)O o f Sp-Ch is attributed to oxygen exchange between mineral and pore wate r during diagenetic mineral reactions. Increasing delta(18)O values, i n conjunction with XRD and SEM data, indicate that transformation of s erpentine layers to chlorite layers and Ibb polytype layers to Iaa pol ytype 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 delta(18)O values inc lude depth-related differences in pore water delta(18)O values present at the time of I-S crystallization, contamination by detrital 2M(1) m ica and 1M polytype rotations that facilitated oxygen exchange.