OXYGEN ISOTOPE THERMOMETRY OF QUARTZ–CALCITE VEINS FROM THE NUNCIOS FOLD COMPLEX, MONTERREY SALIENT, MEXICO

We report oxygen isotope analyses of quartz-calcite veins carried out to evaluate the thermal history of stratigraphic units of the Nuncios Fold Complex, Sierra Madre Oriental, Mexico. Variations in d¹⁸O of calcite and quartz and d¹³C of calcite within veins in siliciclastic units, are smaller than variations in veins within dominantly calcareous units. Oxygen isotope variation of 24 quartz samples, determined using laser extraction, were compared with oxygen isotope variation in paired coexisting calcite. The d¹⁸O values for quartz have the following narrow ranges: Taraises Formation, 20.3 ± 0.48‰; La Casita Formation 27.1 ± 0.53‰, VSMOW. Three generations of quartz in a single La Casita vein have d¹⁸O values of 20.44‰, 20.78‰, 21.31‰, VSMOW. Possible disadvantages in the use of veins for geothermometry include danger that the veins are not in temperature equilibrium with the host rocks, the possibility that multiple generations of fluid are involved, and the related possibility that quartz and calcite are not in isotopic equilibrium. We observe that, although d¹⁸O values in the two formations are highly distinctive, the isotopic fractionation factor between quartz and calcite (d¹⁸O [qz-cc]) is very similar: d=1.88±0.1 for the Taraises and 2.0±0.2 for La Casita. The small difference, if significant, may represent real temperature variation within the system. Veins offer the advantage that large crystals minimize the possibility of diffusion and retrograde reaction. Furthermore, during formation, vein minerals can react readily with fluid, mimicking hydrothermal experiments. (They could perhaps also mimic kinetic effects in those experiments). The theoretical-experimental isotope fractionation equation of Clayton and Kieffer (1991) gives reasonable temperature estimates of 235 - 260 degrees C for the Taraises Formation and 190 - 260 degrees C for La Casita Formation (consistent with fluid inclusion data). Temperatures of 340-410 degrees C, calculated using the Sharp-Kirschner (1994) equation, seem too high for this geologic situation.