On the relationship between mantle metasomatism and partial melting — Evidence from mantle-derived lherzolite xenoliths from Nüshan, China

Studies of the mantle-derived lherzolites from Nushan show that in addition to CO2, there were present H2O and small amounts of CO, CH4, SO2, Cl and F in the initial mantle fluids derived from the asthenospheric mantle plume. The initial fluids accumulated in some regions of the mantle, resulting in lowering of the mantle solidus (and liquidus) and partial melting of the upper mantle. Melts formed from low-degree of fluid-involved partial melting of the upper mantle would be highly enriched in incompatible elements. Fluids and melts are all the metasomatic agents for mantle metasomatism, and the interaction between themand the depleted mantle could result in the substantial local enrichment of LREE and incompatible elements in the latter. In case that the concentration of H2O in the fluids (and melts) is lower, only cryptic metasomatism would occur; in case that the concentration of H2O is higher, the degree of partial melting would be higher and hydrous metasomatic phases (e. g. amphiboles) would nucleate. Under such circumstances, there would occur modal metasomatism