山东地区新生代不同类型幔源样品中稀有气体同位素组成与成因

摘要

稀有气体是地球流体源区信息的有效示踪剂.利用热熔融法对山东地区幔源巨晶、包体全岩及其单矿物以及寄主玄武岩等幔源样品中稀有气体的测试表明,不同类型样品具有明显不同的稀有气体同位素组成,特别是He同位素比值差异明显.巨晶和包体单矿物中的n(3He)/ n(4He)值分别为0.87～11.31Ra、0.911 ～10.11Ra,均值分别为6.1Ra和3.73Ra,主体介于地幔一大气一地壳源区之间；包体全岩中n(3He)/ n(4He)值0.19 ～ 2.30Ra(均值为1.05 Ra),且均低于相应单矿物中该比值；而玄武岩中n(3He)/ n(4He)值全为10-8数量级,与地壳均值无异.各类幔源样品中n(4Ar)/n(36 Ar)值无明显差异,介于296.4～864.3,高于大气但远低于典型地幔值.所获有限数据中,不同类型样品中n(2Ne)/n(2Ne)值均高于大气,具幔源特征,其中包体全岩及其单矿物n(2Ne)／n(22Ne)值分别为10.19 ～12.42和10.55～11.80,无明显差异；玄武岩中该值介于11.07～13.10,总体高于包体全岩及其单矿物.上述比值特征的差异反映了不同类型幔源样品各自不同的成因和演化特征:巨晶和包体单矿物中的轻稀有气体同位素组成主要反映了由于古板块俯冲所导致的岩浆源区地幔一大气／地壳的昆合特征,个别样品中可能存在宇宙成因3He;除单矿物中所体现的源区混合特征外,包体全岩还存在放射成因4He;而玄武岩在喷出地表后丢失大量地幔稀有气体信息,其同位素组成主要体现了放射成因4He的影响.%Noble gases are efficacious tracers for the origins of the Earth volatiles.Noble gas isotopes in megacrysts,whole rocks of mantle-derived xenoliths and corresponding mineral separates,and host-basals from Shandong area are tested by heating-melt,and the obtained data indicate that obvious differences in isotopic compositions of noble gases,especially in helium are observed in different kinds of samples.Ratios of n (3 He) / n (4He) in megacrysts and mineral separates,respectively varying from 0.87 to 11.31 Ra and 0.911 to 10.11 Ra,and with corresponding average values of 6.1 Ra and 3.73 Ra,imply a double-source compound of atmosphere and mantle.Ratios of n (3 He) / n (4 He) in the whole rocks,ranging from 0.19 to 2.30 Ra (the average is 1.05 Ra)and being lower than those of corresponding mineral separates,suggest that the radiogenic outcome from intergranular ingredients is additional devotion besides primitive mixing of atmosphere--mantle in source area.And with the values of 10-8,the n(3He)/r(4He) ratios in basalt samples emphasize the role of radiogenic 4He.No clear differences in n(40Ar)/n(36Ar),which usually fall in 296.4 ～ 864.3 and are a bit higher than that in the atmosphere but far lower than that in the mantle,are detected in different kinds of samples.All ratios of n(20Ne) /n(22Ne) in different kinds of samples are higher than atmosphere,which show characteristics of mantle-derived origins,and as a whole,n(20Ne)/n(2Ne) in basalts (11.07 ～ 13.10) are slightly higher than those in the whole rocks and corresponding mineral separates,separately ranging from 10.19 to 12.42 and 10.55 to 11.80 and showing no visible differences.Those characteristics and differences of noble gas isotopes suggest different origins and evolution processes for different kinds of samples.