Pre-Pennsylvanian low-magnesium calcite cements from the Mississippian Lake Valley Formation of south central New Mexico comprise three major cathodoluminescent zones. Previous work by Meyers and Lohmann (1985) suggested that interzonal isotopic and minor-element variation resulted from spatial and temporal exchange between meteoric water and marine carbonate within a single meteoric system, that the carbonate for cementation was derived from dissolution of metastable carbonate grains, and that any contribution from marine water was insignificant. Isotopic analysis of the first two cathodoluminescent zones, using detailed micron-scale sampling methods, revealed a record of simultaneous variation in delta 18 O and delta 13 C values at the intrazonal scale that is incompatible with compositional change anticipated from the evolution of meteoric fluids in response to progressive water-rock interaction. Rather, isotopic data, in conjunction with stratigraphic and petrographic constraints, suggest that regionally extensive mixing between marine and meteoric water along the basal margin of the phreatic lens is the process responsible for the cementation of much of this platform succession. Although trends of progressive 18 O and 13 C depletion in cement composition are compatible with precipitation from a fluid that evolved from a marine- to a meteoric-dominated composition, low Mg and Sr concentrations are problematic in the context of experimentally derived partition coefficients for calcite. Because of compelling evidence indicating a mixing-zone origin for these cements, however, we attribute low Mg and Sr contents to temperature and/or PCO 2 effects, or to differences in conditions under which calcite is precipitated.