Pelitic schists and quartzites in the Picuris Range of northern New Mexico exhibit mineral δ18O and δD compositions that indicate rock-buffered isotopic exchange during metamorphism at uniform physical conditions of T ∼ 530°C and P ∼ 4 kbar. Phase assemblages and major-element compositions among silicates and oxides are uniform within stratigraphic units, but they change abruptly across lithologic contacts, yielding distinctive mineral Mg/Fe2+ ratios and inferred f(O2) values. Mineral compositions reflect the pre-metamorphic compositions of individual rock units. O- and H-isotopic compositions of quartz and muscovite are also discontinuous across lithologic boundaries, showing intra-layer homogeneity and bulk-rock isotopic compositions retained from sedimentary protoliths. Uniform Δ18 OQu-Ms values indicate isotopic equilibrium at peak metamorphic conditions. Sharp discontinuities in mineral and fluid isotopic compositions reflect limited isotopic exchange between units. The isotopic system in these units was probably one of rock-buffered exchange, in which the sedimentary compositions of individual rock units exerted the dominant control on mineral and fluid isotopic composition over short distances in a relatively closed metamorphic system. Fluid migration during progressive metamorphic devolatilization in this simple system was probably non-pervasive, and it was probably influenced by contrasting rock permeability. Consequently, our study suggests that pervasive exchange between interlayered units may be uncommon in regionally metamorphosed terrains that show weak initial geochemical gradients. In contrast, the chemical and isotopic homogenization that attends pervasive fluid flow and high fluid fluxes may be restricted to settings characterized by extreme geochemical gradients, such as interlayered silicates and carbonates, or terrains that host plutonic hydrothermal systems.
- New Mexico
- Stable isotopes