The retrograde evolution of a dolomite-bearing hydrous peridotite from the Ulten Zone (Italian Alps)

A dolomite-bearing chlorite-amphibole peridotite from the Ulten Zone (Eastern Alps) preserves the mineralogical and textural record of three metamorphic assemblages: the near-peak condition M1 stage (= garnet-amphibole) and the exhumation-related M2 and M3 stages (= spinel-amphibole and chlorite-amphibole, respectively). This metamorphic evolution supports the hypothesis that chlorite-bearing peridotites from the Ulten Zone represent the retrograde equilibration product of former high-pressure garnet-bearing peridotite. Multi-equilibrium thermobarometry, which involves also hydrous phases, dolomite and a C-O-H fluid phase, gives P ~ 1.8-2.2 GPa, T ≥ 800°C (M1) and P ~ 1.5-1.7 GPa, T ~ 730°C (M3). M2 stage lacks minerals suitable for multi-equilibrium computation. During the retrograde evolution, the CO2 content X(CO2) of the fluid phase increased from 0.1 to 0.5.