New Perspective On The Origin Of EM1 Component In The Mantle

Since the name 'EMI' was first proposed in the mid-1980s,it has been the long-term focus of mantle petrologists and geochemists.However,the controversy over the origin of the EMI component is ongoing,and there are still many different explanations about it.In order to further explore the nature and origin of EMI,this thesis focuses on the classical continental and oceanic intraplate EMI end-member basalts.Through combining the emerging Mg stable isotopes with traditional geochemical methods(including major elements,trace elements,and Sr-Nd-Pb-Hf isotopes),this thesis conducted systematic geochemical studies on the EM1 endmember basalts and provided some new critical constraints on the origin of EM1.The Cenozoic potassic basalts from Northeast China(including Xiaogulihe,Wudalianchi,Erkeshan,Keluo and Nuominhe)show Sr-Nd-Pb-Hf isotopic compositions close to the EM1 endmember,and the most extreme isotopic compositions of these basalts are:206Pb/204Pb?16.5,143Nd/144Nd?0.5123(or ?Nd?-6.4),176Hf/177Hf?0.2825(or ?Hf?-10.1).Such extreme isotopic compositions,together with the Pb isotopic evolution modeling,indicate that there are some recycled ancient(about 2.2 Ga)crustal materials in the mantle sources of the potassic basalts.These potassic basalts have 826Mg values(-0.57‰ to-0.34‰)lower than normal peridotitic mantle(826Mg=-0.23±0.04‰),lower continental crust(?26Mg=-0.26 ± 0.06‰)and the sub-continental lithospheric mantle peridotite.The correlations between Mg isotopes and Sr-Nd-Pb-Hf isotopes indicate that the light Mg isotopic compositions(826Mg ?-0.6‰)are the intrinsic characteristics of the EMI component in the sources of potassic basalts.Such low ?26Mg values,combined with the extremely low 206Pb/204Pb ratios of EMI component,explicitly suggest that the ultimate origin of the EM1 component is recycled ancient carbonate-bearing sediments.In addition,the correlations between some element ratios(K/U,Ba/Th and Hf/Hf*)and radiogenic isotopes indicate that the EMI component in the sources of the potassic basalt is also characterized by exceptionally high K/U and Ba/Th ratios,low U/Pb ratio,and the positive Zr and Hf anomalies.The unusually high K/U and Ba/Th ratios require some specific geological processes to strongly fractionate the K-U and Ba-Th element pairs with very similar geochemical behaviors.Subducted sediments and the element fractionation caused by dehydration in subduction zones cannot explain such uncommon fractionation of K-U and Ba-Th.The combination of published experimental data and quantitative modeling shows that the extraction of small amounts of carbonate melts from the carbonate-bearing sediments in the mantle-transition-zone depths will strongly fractionate K and Ba from U and Th.K-hollandite and majorite in the residual solid assemblages will hold the majority of the whole budget of K,Pb,Ba,Zr and Hf,while some Th,U and REEs prefer to enter the carbonate melt,thus generating the residual sediment component with high(K,Ba,Pb)/(U,Th)ratios and positive Zr and Hf anomalies.The long-term isolated evolution of this sediment component in the mantle-transition-zone formed the specific EM1 component.Therefore,the extraordinarily high(K,Ba,Pb)/(U,Th)ratios and positive Zr and Hf anomalies indicate that the EM1 component in the sources of potassic basalts is derived from mantle transition zone.Pitcairn Island,which is located in the middle of the South Pacific,is a typical EMI-type ocean island.The shield-building lavas(Tedside volcanics)in this island show the most extreme isotopic compositions of Sr,Nd,Hf and Pb that define the EM1 component.However,the Sr-Nd-Pb-Hf isotopic compositions of the late-stage volcanic rocks(Pulawana,Christans Cave Formation,and Adamstown)show the mixing of depleted component and EM1.This study shows that the Tedside basalts have 826Mg values(-0.40‰ to-0.31‰)lower than normal peridotitic mantle(?26Mg=-0.23±0.04‰)and the late-stage lavas(?26Mg=-0.30 to-0.16‰).Such low?26Mg values are the lowest ones so far recorded in fresh oceanic basalts.The Sr-Nd-Pb-Hf isotopic compositions of Pitcairn and Rarotonga EMI-type ocean island basalts are correlated with the ?26Mg values of these basalts,and Pitcairn basalts show more extreme EMI-type isotopic features and lower 826Mg values than Rarotonga basalts.This indicates that the low ?26Mg values of Tedside basalts are inherited from the EMI component in the mantle source of these basalts.Recycled ancient lower continental crust and delaminated subcontinental lithospheric mantle cannot explain such low ?26Mg value of EM1.The low ?26Mg value of EM1,combined with previously published evidence yielding exceptionally unradiogenic Pb as well as sulfur isotopes affected by mass-independent fractionation,suggests that the Pitcairn EM1 component is most likely derived from late-Archean(2.5?2.6 Ga)subducted carbonate-bearing sediments.However,the low Ca/Al ratios of Pitcairn lavas are inconsistent with experimental evidence showing high Ca/Al ratios in melts derived from carbonate-bearing mantle sources.The carbonate-silicate reactions in the late Archean subducted sediments were suggested to exhaust the carbonates,but the isotopically light magnesium of carbonate was incorporated in the silicates,which then entered the lower mantle and ultimately became the Pitcairn plume source.Thus,the ?26Mg remains in the plume source as a tell-tale 'ghost' of the originally subducted carbonate.