洋中脊玄武岩的Ba同位素组成及其应用

Re-visiting barium isotope compositions of mid-ocean ridge basalts and the implications

Ba同位素是一种新兴的示踪地壳物质俯冲再循环的工具。确定地幔的Ba同位素组成是Ba同位素地质应用的前提。然而,目前幔源岩浆岩Ba同位素的数据仍然很少。本文报道了30个海洋玄武岩的高精度的Ba同位素数据,包括25个大洋中脊玄武岩(MORB)和5个弧后盆地玄武岩(BABB)。这些玄武岩的δ^(138/134)Ba变化范围为−0.06‰~+0.11‰,没有跨区域性的系统差异。结合已发表的数据,全球MORB的平均δ^(138/134)Ba为+0.05‰±0.09‰(2SD,n=51)。基于具有(La/Sm)_(N)<0.8,低^(87)Sr/^(86)Sr(<0.70263)和Ba/Th(<71.3)的D-MORB样品,我们估算出亏损MORB地幔(DMM)的平均δ^(138/134)Ba为+0.05‰±0.05‰(2SD,n=16),显著低于先前报道的DMM组成(≈0.14‰)。如果应用新估计的DMM平均δ^(138/134)Ba,E-MORB的Ba同位素特征来源于上地幔广泛存在再循环的沉积物这一推论是不合理的。EMORB的Ba同位素组成可能反映了地幔中俯冲的蚀变洋壳和/或沉积物的贡献,需要根据地幔Ba同位素组成和其他地球化学信息进行进一步的制约。

Barium(Ba)isotopes can be used as potential tracers for crustal material recycling in the mantle.Determination of the Ba isotope composition of the depleted mantle is essential for such applications.However,Ba isotope data for mantle-derived basalts are still rare.In this study,we reported high-precision Ba isotope data of 30 oceanic basalts including 25 mid-ocean ridge basalts(MORBs)from geochemically and geologically diverse mid-ocean ridge segments and five back-arc basin basalts.The δ^(138/134)Ba values of these samples varied from−0.06‰to+0.11‰,with no systematic cross-region variation.Together with published data,we constrained the average δ^(138/134)Ba of global MORBs to+0.05‰±0.09‰(2 standard deviation,n=51).Based on depleted MORBs that have(La/Sm)_(N)<0.8,low ^(87)Sr/^(86)Sr(<0.70263),and low Ba/Th<71.3,we estimated the average δ^(138/134)Ba of the depleted MORB mantle(DMM)as+0.05‰±0.05‰(2SD,n=16)that is significantly lower than the DMM(≈0.14‰)reported previously.If a new estimation of the DMM is applied,it is unreasonable to infer that the Ba isotope signatures of the“enriched-type”MORBs(E-MORBs)could be attributed to pervasive sediment recycling in the upper mantle.We,therefore,conclude that the Ba isotope compositions of the E-MORBs could be sourced from the incorporation of subducted altered oceanic crust and/or sediments depending on the Ba isotope composition and other geochemical information of the local mantle.