Recycled oceanic crust-derived fluids in the lithospheric mantle of eastern China: constraints from oxygen isotope compositions of peridotite xenoliths

The oxygen isotope compositions of minerals of peridotite xenoliths from the Subei basin, Eastern China were investigated to detect a possible crustal signature in mantle rocks. The δ¹⁸O values of olivine (ol), orthopyroxene (opx) and clinopyroxene (cpx) vary from 5.10 to 5.98‰, 5.46 to 6.78‰, and 5.06 to 6.31‰, respectively. Olivine and opx have δ¹⁸O values similar to-, or above the "expected" mantle range, whereas the δ¹⁸O values of cpx range from higher to lower than average mantle. These characteristics are inherited from the peridotite mantle source, and likely result from mantle domains that experienced fluid-assisted metasomatism. Coexisting high δ¹⁸O values of ol and opx and low δ¹⁸O values of cpx are unlikely to result from selective interactions with the fluids with different δ¹⁸O values during a single metasomatic event. The measured O-isotope values can be reconciled with a simple two-stage metasomatic model characterized by an early high-δ¹⁸O fluid/peridotite interaction followed by a late low-δ¹⁸O fluid infiltration. Model calculations of mineral diffusivities predict that the negative δ¹⁸O_cpx-ol values are unlikely to be preserved at mantle conditions for time interval>80Ma. Provided the investigated mantle xenoliths were entrained and brought to surface by ~9Ma basaltic eruptions, this also limits the occurrence of the late metasomatic event, and the subducting Pacific plate is the best candidate to provide such fluids.