Olivine and melt inclusion chemical constraints on the nature and origin of the common mantle component beneath eastern Asia

Geochemical compositions of Cenozoic intraplate volcanic rocks in eastern Asia reveal a highly heterogeneous mantle beneath them. Basalts from the northeast Asia are characterized by EM1 (Enriched Mantle-1) isotopic compositions while those from the southeast Asia are dominated by EM2 (Enriched Mantle-2) isotopes, and they converge on a common mantle component (C). Up to now, the overwhelming majority of studies have focused on the nature and origin of the enriched mantle domains, yet that of the common mantle component remain poorly constrained. In this study, we report compositions of olivines and melt inclusions for Cenozoic Halaha-Chaihe basalts from the Greater Khingan Range (GKR) in northeast China to constrain their source and origin. These basalts have compositions at the intersection of the EM1–C and EM2–C arrays and can represent the common mantle component beneath eastern Asia. Olivine phenocrysts in Halaha-Chaihe basalts are rich in Ni (1320–3033 ppm), poor in Mn (1038–2486 ppm) and Ca (364–2244 ppm), and show high Fe/Mn (64–90) and low Mn/Zn (< 13) ratios. These compositions are consistent with a pyroxenite source. The Halaha-Chaihe primary magma compositions, which were calculated from melt inclusion by adding/subtracting olivine until equilibrium with olivine of Fo 88.6 , show high MgO (12.92–14.33 wt%) and TiO 2 (1.89–2.46 wt%), and low SiO 2 (46.2–47.88 wt%), CaO (7.59–9.66 wt%) and Al 2 O 3 (10.65–12.52 wt%) contents. Comparison between the inferred primary magma compositions and experimental partial melts further indicates that the mantle source was dominated by silica-deficient pyroxenites. This is supported by the CMAS projection, showing that all samples fall on the silica-poor side of the En-CaTs join. Geohygrometer, thermometer and oxybarometer calculations using olivine and melt inclusion indicate that the Halaha-Chaihe magmas were relatively wet (~ 1.02–2.83 wt% H 2 O), hot (~ 1050–1250 °C) and oxidized (FMQ + 1.2), and by inference that their mantle source was also relatively wet, hot, and oxidized. Seismic tomographic images show that the subducting Pacific slab is stagnant in the mantle transition zone (MTZ) with its western edge extending to the eastern margin of the GKR, approximately beneath the Halaha-Chaihe volcanic field. Integrating the geochemical and geophysical observations, we propose that the silica-deficient pyroxenite source of the Halaha-Chaihe basalts was most likely derived from melt-rock reaction or mechanical mixing between the subducted Pacific oceanic crust and ambient mantle in the MTZ, and that such zones are responsible for the extensive low-Vp anomalies and common mantle component presently observed beneath eastern Asia.