Fluid availability during high-grade metamorphism is a critical factor in dictating petrological, geochemical and isotopic reequilibration between metamorphic minerals, with fluid-absent metamorphism commonly resulting in neither zircon growth/recrystallization for U-Pb dating nor Sm-Nd isotopic resetting for isochron dating. While peak ultra-high pressure (UHP) metamorphism is characterized by fluid immobility, high-pressure (HP) eclogite-facies recrystallization during exhumation is expected to take place in the presence of fluid. A multichronological study of UHP eclogite from the Sulu orogen of China indicates zircon growth at 216 ± 3 Ma as well as mineral Sm-Nd and Rb-Sr reequilibration at 216 ± 5 Ma, which are uniformly younger than UHP metamorphic ages of 231 ± 4 to 227 ± 2 Ma as dated by the SHRIMP U-Pb method for coesite-bearing domains of zircon. O isotope reequilibration was achieved between the Sm-Nd and Rb-Sr isochron minerals, but Hf isotopes were not homogenized between different grains of zircon. The HP eclogite-facies recrystallization is also evident from petrography. Thus this process occurred during exhumation with fluid availability from decompression dehydration of hydrous minerals and the exsolution of hydroxyl from nominally anhydrous minerals. This provides significant amounts of internally derived fluid for extensive retrogression within the UHP metamorphosed slabs. Based on available experimental diffusion data, the consistent reequilibration of U-Pb, Sm-Nd, Rb-Sr and O isotope systems in the eclogite minerals demonstrates that time-scale for the HP eclogite-facies recrystallization is c. 1.9-9.3 Myr or less. This provides a maximum estimate for duration of the fluid-facilitated process in the HP eclogite-facies regime during the exhumation of deeply subducted continental crust.

A combined study of mineral O isotopes and hydroxyl contents was carried out for the contacts between ultrahigh-pressure eclogite and gneiss from main hole of the Chinese Continental Scientific Drilling Project in the Sulu orogen. While there is a large d 18 O variation from )8.3 to 7.3& for all minerals, different styles of mineral-pair fractionation occur at the boundaries of different lithologies. Both equilibrium and disequilibrium O isotope fractionations are observed between quartz and the other minerals, with reversed fractionations between omphacite and garnet in some samples of eclogite. This suggests that both eclogite and gneiss acquired their negative d 18 O values by meteoric-hydrothermal alteration of their protoliths at high temperatures before subduction, and that fluid-assisted O isotope exchange did take place across the boundary of different lithologies at local scales during amphibolitefacies retrogression. Fourier Transform Infrared Spectroscopy analysis yielded H 2 O concentrations of 50 to 1144 p.p.m. (by weight) for garnet and 139 to 751 p.p.m. for omphacite. The state of equilibrium or disequilibrium O isotope fractionations between omphacite and garnet are correlated with variations in their water content at local scales, indicating that the internally derived fluid plays a critical role in retrograde metamorphism during exhumation. The retrograde metamorphism results in mineral reactions and O isotope disequilibria between some of the minerals, but the fluid for retrogression was derived from the decompression exsolution of structural hydroxyl and thus internally buffered in the O isotope composition. A quantitative estimate suggests that a hand specimen (3 · 6 · 9 cm) of eclogite composed of 70% garnet and 30% omphacite can release 0.316 g water by the decompression exsolution of structural hydroxyl, which can form 14.4 g amphibole during exhumation. This provides sufficient amounts of water for the amphibolite-facies retrogression.

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