Water Content in Orogenic Peridotites from the North Qaidam UHP Belt: Constraints on HP-HT Rheological Weakening and Exhumation Kinetics

Nominally anhydrous minerals (NAMs) profoundly influence the physical properties and rheological behavior of the deep Earth under high-pressure and high-temperature (HP-HT) conditions. In this study, we investigated the water contents of olivine, clinopyroxene, orthopyroxene, and garnet in orogenic peridotites (garnet-bearing dunite, garnet lherzolite, and garnet pyroxenite) from the Lüliangshan area in the North Qaidam ultrahigh-pressure (UHP) orogenic belt using Fourier Transform Infrared Spectroscopy (FTIR). After excluding identifiable spectral contributions from molecular water and hydrous alteration phases, the measured structural H2O contents are 545 ppm for garnet (with a maximum of 1916 ppm), 145 ppm for clinopyroxene, 68 ppm for olivine, and 40 ppm for orthopyroxene. This concentration sequence and the relatively low clinopyroxene/orthopyroxene structural water partition coefficient (DHcpx/opx = 1.42) indicate significant hydrogen diffusion and degassing during rapid exhumation. Large garnet porphyroblasts exhibit contrasting core-to-rim water distributions, reflecting variable hydrogen loss and/or addition during retrograde evolution. While highly variable bulk-rock H2O/Ce ratios record multi-stage mantle-wedge metasomatism, these measured garnet water contents also provide constraints on deep rheological behavior. By combining experimentally determined garnet water-solubility and water-sensitive creep relations, we performed a normalized sensitivity analysis using the measured sample-average garnet H2O contents (313–566 ppm). Relative to a 100 ppm water-poor reference state, the central model yields approximately 7.5–21.3-fold reductions in garnet creep strength, although the magnitude is sensitive to experimental parameters and extrapolation to natural UHP conditions. The observed Cpx/Opx water partitioning and intra-grain heterogeneity are also consistent with incomplete hydrogen re-equilibration during rapid early exhumation. These results link mineral-scale water distribution with the rheological and kinetic evolution of continental UHP rocks.