Gulf Coast): Implications for Pore Fluids a N D Mineralogic Reactions

Abst rac t -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 p~m fraction of each core sample into fractions with a range in the Sp-Ch : I-S ratio, and end-member ~80 compositions were determined by extrapolation. The Sp-Ch ~180 values range from + 10.4 to 13.7%o and increase with burial between 3509 and 5470 m. The only exception is Sp-Ch from 1937 m, which has an anomalously high 8180 value of +12.6%o. The I-S ~tsO values range from +16.1 to 17.3%o and do not change significantly between 3509 and 5470 m burial depth. Pore water ~80 compositions calculated from Sp-Ch and I-S values and measured borehole temperatures range from -2 .6 to +10.3%o. 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 8180 of Sp-Ch is attributed to oxygen exchange between mineral and pore water during diagenetic mineral reactions. Increasing 8~80 values, in conjunction with XRD and SEM data, indicate that transformation of serpentine layers to chlorite layers and lbb polytype layers to laa 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 ~LsO values include depth-related differences in pore water ~80 values present at the time of I-S crystallization, contamination by detrital 2M~ mica and 1M polytype rotations that facilitated oxygen exchange. Key Words--Diagenesis, Illite-Smectite, Oxygen Isotopes, Sandstone, Serpentine-Chlorite, Tuscaloosa Formation.