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Secondary ion mass spectrometry (SIMS) was used to obtain microanalyses of oxygen isotope ratios on diagenetic quartz cements from the Travis Peak Formation, Texas Gulf Coast. These measurements on 15to 20-nm-diameter spots were made on standard polished thin sections, and the analyses allow new constraints on paleofluid isotopic compositions. Microanalyses of detrital quartz (13.7 ± 4.6%»; n = 76) compare well with conventional analyses on bulk samples (15.3 ± 3.4%o, n = 33), indicating good calibration of the microanalytical technique. The range of detrital quartz δ18O was 4 to 20%o, with larger detrital grains having lower values (~ 12%o) typical of a plutonic source and smaller grains (16 to 23%o) typical of a metamorphic source. Errors in individual analyses are 1.3—1.8%o (lcr).

Bulk analyses of authigenic quartz in whole rock samples indicated an average oxygen isotopic ratio of 19 ± 3.3%o SMOW (n = 33). SIMS results ranged from 22%o to 35%o, with an average of 26 ± 3.5%o (n = 34). The isotopic results can be interpreted in terms of the geologic burial history, suggesting that sediments are buried and uplifted through waters that are isotopically stratified, becoming ,sO-enriched with depth. Under these constraints, authigenic quartz with 8,80 values >29%o must have precipitated from 2%o meteoric water at temperatures £50°C. Most δ18O values range from 25-29%o, implying that precipitation volumes peaked between 60-90°C, from waters with a positive δ18O value. Fewer quartz cements with 8,sO values <25%o were found, suggesting that quartz precipitation declined at temperatures >90°C. The burial history curve shows that the formation reached this temperature at —60 Ma. Quartz precipitation might have declined due to hydrocarbon infiltration, transition metals inhibiting quartz precipitation or occlusion of pores causing diminished fluid flow.

The mass transfer of silica required to precipitate the observed abundance of quartz cement at temperatures between 60-90°C can be explained if Travis Peak paleofluids were actively converting. In this flow regime, the heterogeneity in 8,sO values of quartz cements would result from temperature variations during precipitation from a paleofluid having a δ18O value of approximately + 2%o. This fluid might represent a mixture of meteoric water (— 2%c) and deep-seated basinal fluids (+ 5%o). Hot fluids introduced along fracture zones associated with uplift of the Sabine Arch (—100 Ma), might have dispersed into the permeable Travis Peak sandstone and precipitated quartz upon cooling.

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