MILLIMETER-SCALE VARIATIONS IN CARBON AND OXYGEN ISOTOPES: NEW INSIGHTS ON VEIN GROWTH MECHANISMS AND PLUMBING SYSTEMS OF FAULT-RELATED FOLDS

Using a recently developed micro-sampler technique and CF-IRMS analysis, detailed d13C and d18O maps of 250 microgram carbonate samples have been constructed. Isotope data are supported by field and petrographic studies that track temporal and isotopic variations in fluids from which the carbonates precipitated. The samples, from the Sierra Madre Oriental, Mexico, are from a series of detachment folds that include a 3000 m thick sequence of Upper Jurassic through Cretaceous carbonates and siliciclastic unmetamorphosed materials detached above a 1000 m evaporitic sequence. Microscale stable isotope analysis was performed on 22 thin sections of host rocks and veins. Results reveal consistent intrasite variations in stable isotope composition, reflecting the heterogeneous spatial and temporal evolution of fluids within dominantly calcareous units. The largest d18O variations occur in samples from the Lower Cretaceous Cupido Formation. Within a nonfibrous vein the variation is up to 9‰ (from 15.3‰ to 24.5‰ VSMOW), related to the presence of distinct fluid events and changes in precipitation conditions. For another Cupido sample, a d18O gradient in a host rock fragment was mapped from 15.5‰ (next to the vein) to 28.5‰ VSMOW (1 cm within the rock). Based on d13C variations in a fibrous vein from the Upper Jurassic La Casita Formation, two fluid events have been identified, the first related to vein formation and the second to a later event. The largest d13C variations, up to 9‰ (from –17.2‰ to – 8.1‰ PDB), occur in the Upper Jurassic Zuloaga Formation. A d13C variation of 7‰ was also mapped in a Cupido vein from near the evaporitic unit. These early results indicate that microscale analyses are critical in interpreting complex patterns of isotopic variations in fluid-rock systems.