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.
Bibliographical noteFunding Information:
Many of the samples used in this study were collected by Cheryl Rattel-Carson. Jim Borthwick, Mike Colucci, Klaus Damm and Dwight Deuring provided assistance in the stable isotope and microprobe laboratories at SMU. We have benefited greatly from enlightening conversations with Bob Criss, Jeff Grambling, Bob Gregory and Russ Harmon, who have helped to clarify our interpretations, but they are not responsible for any shortcomings in our thinking. Barb Dutrow, Jean Morrison and Peter Nabelek provided very helpful critical comments on the manuscript. This research was supported by the US National Science Foundation (EAR-8606489, EAR-8904777 and EAR-9019277 to M.J.H.).
- New Mexico
- Stable isotopes