Typical Oxygen Isotope Profile of Altered Oceanic Crust Recorded in Continental Intraplate Basalts

Recycled oceanic crust （ROC） has long been suggested to be a candidate introducing en- riched geochemical signatures into the mantle source of intraplate basalts. The different parts of oceanic crust are characterized by variable oxygen isotope compositions （δ^18O=3.7‰ to 13.6‰）. To trace the sig- natures of ROC in the mantle source of intraplate basalts, we measured the δ^18O values of clinopyroxene （cpx） phenocrysts in the Cenozoic basalts from the Shuangliao volcanic field, NE China using secondary ion mass spectrometer （SIMS）. The δ^18O values of the Shuangliao cpx phenocrysts in four basalts ranging from 4.10‰ to 6.73‰ （with average values 5.93‰±0.36‰, 5.95‰±0.30‰0, 5.58‰±=0.66‰e, and 4.55‰±= 0.38‰, respectively） apparently exceed those of normal mantle-derived cpx （5.6‰±0.2‰） and fall in the typical oxygen isotope range of altered oceanic crust. The δ^18O values display the negative correlations with the Eu, Sr anomalies of whole rocks and erupted ages, demonstrating that （1） the ROC is the main enriched component in the mantle source of the Shuangliao basalts and （2） the contributions of ROC var- ied with time. The basalt with the lowest δ^18O value is characterized by a significant K positive anomaly, highest H2O/Ce and Ba/Th ratios, suggesting that the mantle source of basalts with low δ^18O can also in- clude a water-rich sediment component that may be the trigger for partial melting. Considering the continuous subduction of the Pacific slab, the temporal heterogeneity of the source components is likely to be caused by the Pacific slab subduction.

Zircon U-Pb ages and O-Hf isotopes of Quaternary trachytes from the East Sea:Implications for the genesis of low-δ^(18)O magmas

Quaternary intraplate magmatism formed several volcanic islands and seamounts,including Dokdo(DD),Ulleungdo(UD),Simheungtack(ST),Anyongbok,and Isabu in the southwest of the East Sea back-arc basin.In this study,we present whole-rock geochemical,zircon U–Pb age,and in situ O–Hf isotope data for the submerged volcanic rocks from DD,UD,and ST to provide new insights into the eruption timing of these volcanoes and constrain the magma evolution processes.All samples used in this study were trachytes and exhibited ferroan,alkalic,and metaluminous to weakly peraluminous characteristics.They showed light rare earth element(REE)-enriched patterns with(La/Yb)N ratios of 25.3–31.7 and mostly negative Eu anomalies in a chondrite-normalized REE plot.In addition,they were enriched in large-ion lithophile elements and high field strength elements;they exhibited positive Pb anomalies and strongly negative Ba,Sr,P,and Ti anomalies.The zircons yielded a weighted-mean 206Pb/238U age of 2.61,0.348–0.704,and 2.76–2.94 Ma for the DD,UD,and ST trachytes,respectively.All zircons exhibited lowerδ^(18)O values than normal depleted mantle values,regardless of the crystallization age and spatial distribution of volcanoes.Theδ^(18)O values showed no correlation with U contents or Th/U ratios,indicating that the lowδ^(18)O signatures were of primary magmatic origin.The Hf isotopic compositions of the zircons were relatively heterogeneous but predominately characterized by positive eHf values.Binary O–Hf mixing modeling revealed that low-δ^(18)O rocks with positive eHf values from the UD and ST volcanoes were derived from a hybrid source of recycled juvenile crustal materials with low-δ^(18)O and positive eHf signatures and an enriched mantle source with normalδ^(18)O and negative eHf values.The juvenile oceanic crust in the source was likely metasomatized by seawater at high temperatures prior to melting.In contrast,the felsic magma that formed the DD volcanoes may have assimilated with regional basement rocks(Triassic–Jurassic granitoids),resulting in increasedδ^(18)O values and decreased eHf values relative to those of the UD and ST volcanoes.Our study highlights the significant contribution of recycled oceanic crust materials to the generation of the Quaternary magmas.