大西洋中脊26°S玄武岩岩石成因及构造意义:来自Hf同位素的约束  

Petrogenesis and tectonic significance of basalts from the Mid-Atlantic Ridge 26°S:Evidence from Hf isotopes

作　　者：孟凡雪 赵梦云 孟元库 MENG FanXue;ZHAO MengYun;MENG YuanKu(College of Earth Science and Engineering,Shandong University of Science and Technology,Qingdao 266590,China;Laboratory for Marine Geology,Qingdao National Laboratory for Marine Science and Technology,Qingdao 266061,China)

机构地区：[1]山东科技大学地球科学与工程学院,青岛266590 [2]青岛海洋科学与技术试点国家实验室,海洋地质过程与环境功能实验室,青岛266061

出　　处：《岩石学报》2023年第8期2365-2378,共14页Acta Petrologica Sinica

基　　金：青岛海洋科学与技术试点国家实验室山东省专项经费(2022QNLM050201-3);国家自然科学基金项目(41876037);自然资源部深地动力学重点实验室开放基金(J1901-16)联合资助。

摘　　要：大西洋中脊是慢速扩张洋脊的典型代表。本文以大西洋中脊26°S地区脊轴及海山玄武岩代表性样品为研究对象,开展系统的Sr-Nd-Pb-Hf同位素研究,并结合已发表的数据,探讨研究区玄武岩成因及地幔源区性质和演化,旨在为认识地幔不均一性和地幔柱-洋脊相互作用方式提供关键证据。样品主-微量元素与Sr-Nd-Pb-Hf同位素分析结果表明,所有样品均显示同位素富集的N-MORB特征。此外,大西洋26°S玄武岩主微量元素和同位素具有较大的变化范围,且同位素之间呈现出良好的相关关系,表明其是亏损软流圈地幔熔融的结果,但有富集组分参与。结合元素和同位素特征以及Sr-Nd-Hf同位素定量模拟结果,富集组分可能为Tristan da Cunha地幔柱残余组分,显示EMⅠ型富集地幔特征。同位素定量模拟结果表明:海山玄武岩地幔源区组成为约90%~95%的亏损组分和10%~5%的富集组分;而脊轴玄武岩地幔源区富集组分较少(<5%)。点位6、7海山玄武岩样品显示高放射成因Pb同位素组成,符合Dupal异常边界条件。定量计算表明,造成其异常的原因可能与EMⅠ型组分参与有关,这与同位素定量模拟结果相吻合。本文研究的同位素不同程度富集N-MORB可能的成因机制为:远端地幔柱-洋脊相互作用,即Tristan da Cunha地幔柱距离洋脊>1000km,地幔柱在运移至大西洋中脊的过程中,岩石圈厚度明显变薄,为减压熔融的发生提供了良好条件,使残余地幔柱物质不相容元素亏损,但同位素组成保留源区富集的特征。地幔柱残余物质到达大西洋中脊下方后,参与洋脊地区减压熔融,最终形成研究区不相容元素亏损且同位素富集的N-MORB。因此,本文研究的同位素富集的N-MORB可能记录了远端柱-脊相互作用和洋脊之下富集地幔柱物质再熔融的过程,为认识地幔不均一性提供了新的岩石学和地球化学证据。因此,地幔柱-洋脊相互作用不仅It is well-known that the Mid-Atlantic Ridge is a representative of slow-spreading oceanic ridge.In this study,a systematic study of Sr-Nd-Pb-Hf isotopes have been carried out on representative basaltic lavas from the oceanic ridge-axis and seamounts of the 26°S Mid-Atlantic Ridge.Integrated with the published data,the origin of basalts and the properties and evolution of mantle source has been discussed,aiming at providing key evidence for understanding mantle heterogeneity and interaction between mantle plume and oceanic ridge.The analysis results of major-trace elements and Sr-Nd-Pb-Hf isotopes suggest that all the samples show isotopically enriched N-MORB characteristics.In addition,the major-trace elements and isotopes of basaltic lavas from the 26°S Mid-Atlantic Ridge have a large variation,and the isotopes show a good correlation,indicating an origin from melting of depleted asthenosphere with involvement of enriched components.Combined with elemental and isotopic characteristics and Sr-Nd-Hf isotope quantitative modeling,the enriched component might be the residual component of Tristan da Cunha mantle plume,implying the characteristics of EMI-type enriched mantle.The modeling shows that the mantle source of seamount basalt consists of ca.90%~95%depleted components and 10%~5%enriched components.However,the enriched components of ridge axis basalt are less(<5%).Additionally,the seamount basalt samples from sites 6 and 7 show highly radiogenic Pb isotopic compositions,which conforms to the boundary condition of the Dupal Pb anomaly.Quantitative calculations show that the anomaly might be related to the participation of EMⅠ-type components,which agrees with the results of isotopic quantitative modeling.The possible genetic mechanism for normal mid-ocean ridge basalts(N-MORBs)with enriched isotopes studied here might be the interation between the distal plume and oceanic ridge.The Tristan da Cunda mantle plume is more than 1000km away from the oceanic ridge.During the migration of the mantle plume to the M

关 键 词：地幔不均一性 大洋玄武岩 地幔端元 Sr-Nd-Pb-Hf同位素 柱-脊相互作用 

分 类 号：P588.145[天文地球—岩石学] P597.2[天文地球—地质学]