This review combines fluid inclusion data from (HP-)UHP rocks with experimental research and thermodynamic models to investigate the chemical and physical properties of fluids released during deep subduction, their solvent and element transport capacity, and the subsequent implications for the element recycling in the mantle wedge. An impressive number of fluid inclusion studies indicate three main populations of fluid inclusions in HP and UHP metamorphic rocks: (1) aqueous and/or non-polar gaseous fluid inclusions (FI); (2) multiphase solid inclusions (MSI); and (3) melt inclusions (MI). Chemical data from preserved fluid inclusions in rocks match with and implement “model” fluids by experiments and thermodynamics, revealing a continuity behind the extreme variations of physico-chemical properties of subduction-zone fluids. From fore-arc to sub-arc depths, fluids released by progressive devolatilization reactions from slab lithologies change from relatively diluted chloride-bearing aqueous solutions (±N₂), mainly influenced by halide ligands, to (alkali) aluminosilicate-rich aqueous fluids, in which polymerization probably governs the solubility and transport of major (e.g., Si and Al) and trace elements (including C). Fluid inclusion studies point to a reconsideration of the petrological models explaining deep volatile liberation, and their flux into the mantle wedge.

Minerals, fluid inclusions and stable isotopes have been studied in ultrahigh-pressure (UHP) OH-rich topaz–kyanite quartzites from Hushan (west of Dongai), in southern Sulu (China). The quartzites underwent a metamorphic evolution characterized by a peak stage (3·5 GPa and 730–820°C) with the anhydrous assemblage coesite + kyanite I, followed by an early near-isothermal decompression stage (2·9 GPa and 705–780°C) with growth of kyanite II, muscovite, and OH-rich topaz, and by decompression-cooling stages, represented by paragonite (1·9 GPa and 700–780°C) and pyrophyllite (0·3 GPa and 400°C) on kyanite (I and II) and OH-rich topaz, respectively. These rocks may exhibit unusually low δ¹⁸O and δD values acquired before undergoing UHP metamorphism. Five distinct fluid generations are recognized. Type I: concentrated peak solutions rich in Si, Al, and alkalis, present within multiphase inclusions in kyanite I. Type II: CaCl₂-rich brines present during the growth of early retrograde OH-rich topaz. Type III, IV, and V: late aqueous fluids of variable salinity, and rare CO₂ present during amphibolite- and late greenschist-facies conditions. A number of conclusions may be drawn from these relationships that have an effect on fluid evolution in deeply subducted continental rocks. (1) At a pressure of about 3·5 GPa alkali–alumino-silicate aqueous solutions, with compositions intermediate between H₂O fluid and melt (H₂O > 25 and ≤ 50 wt %) evolved from quartzites, probably generated by dehydration reactions. (2) During early decompression stages, at the transition from UHP to high-pressure (2·9 GPa) conditions, brines of external origin with higher water contents (82 wt % H₂O) initiated the growth of OH-rich topaz and muscovite. (3) The subsequent decompression, at P <2 GPa, was defined by a limited circulation of NaCl aqueous fluids, and CO₂ infiltration. Overall, fluid inclusions and stable isotopes highlight a metamorphic fluid–rock interaction characterized by internally derived intermediate aqueous solutions at UHP, followed by infiltration of Cl-rich brines with higher water activities.