Occurrences of hydrous and carbonate phases in ultrahigh‐pressure rocks from east‐central China: Implications for the role of volatiles deep in cold subduction zones

Minor epidote‐zoisite, phengite, glaucophane, nyböite, talc, magnesite, and dolomite occur as matrix phases or as mineral inclusions in some ultrahigh‐pressure (UHP) rocks from the Dabie‐Sulu terrane. Some of these phases contain inclusions of coesite or coesite pseudomorphs and appear to have been in equilibrium with coesite at the time of formation. Their occurrences in the UHP rocks together with experimentally determined and calculated phase relations indicate that they are stable at mantle depths in relatively low‐temperature environments. Because of the apparently dry nature of subducted continental protoliths of the Yangtze craton, small amounts of volatile components at depths exceeding 50 km along a cold subduction zone may have been stored mainly by these hydrous and carbonate phases. These minerals, in addition to some dense hydrous magnesian silicates, act as important carriers for H 2 O and CO 2 recycled at mantle depths. Available petrological and geochemical data support limited or no fluid flow in this region. At very high pressures and low temperatures, the subducted sialic crust evidently served as a desiccating agent. Partial melting of the subducting slab, therefore, may not have occurred, and near absence of volatile expulsion from the subducting slab to the overlying mantle wedge + continental crust may have inhibited large‐scale partial melting, accounting for the lack of a typical contemporaneous calc‐alkaline magmatic arc.