Texture-temperature-geochemistry relationships in the upper mantle as revealed from spinel peridotite xenoliths from Wangqing, NE China

Spinel peridotite xenoliths from Wangqing, NE China, exhibit correlated variations in texture, temperature, geochemistry and radiogenic isotopes. Protogranular and transitional peridotites are less refractory than equigranular samples, which are predominantly hazburgites and clinopyroxene-poor lterzolites. Deformed harzburgites and lherzolites are variably enriched in light rare earth elements (LREE), and Ce/Yb-MgO variations indicate that these olivine-rich rocks have been preferentially infiltrated by metasomatic agents. Clinopyroxenes from the fertile protogranular and transitional lherzolites have high Nd-143/Nd-144 (epsilon(Nd) = 10-18) and low Sr-87/ Sr-86 (0.7020-0.7036) consistent with long-term time-integrated depletions (>1.34 Gy). By contrast, clinopyroxenes in the majority of the equigranular samples have higher Sr-87/Sr-86 (>0.7035) and less radiogenic Nd-143/Nd-144 (epsilon(Nd) <4.5) values. Mineral chemistry shows intra-grain and inter-grain heterogeneity, and the degree of heterogeneity decreases from protogranular to equigranular peridotites. Thermometric calculation further reveals a cooling event from >1200 degrees C to 835-930 degrees C at which all samples last equilibrated. Integration of these data suggests that the Wangqing xenoliths may represent fragments of the lithosphere that has been isolated from the asthenosphere for a long time (>1.34 Gy). This aged lithosphere was locally altered by asthenosphere-derived fluids, which in turn enhanced grain boundary migration and recrystallization, giving rise to coupled textural and geochemical variations. During the thermal erasion of the lithosphere that took place since the late Mesozoic, the base of lithosphere represented by the precursor rocks of the Wangqing peridotites became rheologically similar to the thermal boundary layer such that it was able to intrude with the asthenosphere diapirically the rigid uppermost lithospheric mantle at a time just before the host basalt eruption.