酸洗预处理对大洋玄武岩Sr-Nd-Pb-Hf同位素组成的影响  

Effects of acid leaching on the Sr-Nd-Pb-Hf isotopic compositions of oceanic basalts

作　　者：张少威 赵思宇 杨阳[1,3] 赵太平[3,4] ZHANG Shaowei;ZHAO Siyu;YANG Alexandra Yang;ZHAO Taiping(State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,Guangdong,China;College of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing 100049,China;CAS Center for Excellence in Deep Earth Science,Guangzhou 510640,Guangdong,China;CAS Key Laboratory of Mineralogy and Metallogeny,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,Guangdong,China)

机构地区：[1]中国科学院广州地球化学研究所,同位素地球化学国家重点实验室,广东广州510640 [2]中国科学院大学地球与行星科学学院,北京100049 [3]中国科学院深地科学卓越创新中心,广东广州510640 [4]中国科学院广州地球化学研究所矿物学与成矿学重点实验室,广东广州510640

出　　处：《地球化学》2022年第4期404-416,共13页Geochimica

基　　金：中国科学院青年创新促进会项目(2020349)资助。

摘　　要：玄武岩的Sr-Nd-Pb-Hf同位素可以用来识别地幔源区组成。全球90%以上的幔源岩浆产出于海底,因此目前学术界对地幔地球化学组成的认识绝大多数建立在对大洋玄武岩研究的基础上。然而幔源岩浆喷发到海底之后,会不可避免地与海水发生不同程度的相互作用,导致其同位素组成发生变化。X射线衍射分析显示,对大洋玄武岩样品进行酸洗处理,可以有效去除样品中的次生蚀变矿物,大大降低海水蚀变作用对大洋玄武岩同位素组成的影响。本文系统汇总了前人已发表的大洋玄武岩样品酸洗前后的Sr-Nd-Pb-Hf同位素数据,结合本文对西南印度洋洋中脊玄武岩样品开展的Sr和Nd同位素分析,发现酸洗前后大洋玄武岩的Sr、Pb同位素组成的变化较大,其变化范围明显超过实验测试的误差,而Nd-Hf同位素组成变化相对较小。通过汇总国际玄武岩标样BCR-2、BHVO-2等样品酸洗前后的Sr、Pb同位素数据发现,酸洗处理可显著提高标样数据的重现性。因此,当使用大洋玄武岩的同位素组成讨论其母岩浆及地幔源区地球化学组成时,我们建议在测试分析同位素前对样品和国际标样均进行酸洗处理。除此之外,本文总结了不同酸洗方法对酸洗效率的影响,推荐使用6 mol/L的HCl至少进行3次酸洗处理,每次20 min,以去除海水蚀变对大洋玄武岩Sr-Nd-Pb-Hf同位素组成的影响。Basaltic Sr-Nd-Pb-Hf isotopes can be used to identify mantle source compositions. More than 90% of mantle-derived magma on Earth originates from the seafloor;thus our current understanding on mantle geochemistry is predominantly based on studies that focus on these submarine basalts. However, upon the eruption of mantle-derived magma at the seafloor, these basalts inevitably interact with seawater, leading to variations in their isotopic compositions. X-ray diffraction analyses of submarine basalt samples before and after acid leaching suggest that acid leaching can remove the secondary minerals and eliminate the potential influence of seawater alteration on the isotopic systems of the samples. This paper presents a systematic overview on the effects of acid leaching on the Sr-Nd-Pb-Hf isotopic compositions of submarine basalts with data from literature and new Sr and Nd isotopic data of mid-ocean ridge basalts from the Southwest Indian Ridge, indicating that Sr-Pb isotopic compositions show much larger variations after leaching compared to Nd-Hf isotopes. Acid leaching on international standard samples, such as BCR-2 and BHVO-2, significantly improved the reproducibility of their Sr-Pb isotopic data. Therefore, when the Sr-Nd-Pb-Hf isotopic compositions of oceanic igneous rocks are used to constrain their parent magma and mantle source compositions, acid leaching pretreatment of the samples together with international standards is recommended. Furthermore, this paper discusses the influence of different leaching procedures on the leaching efficiency and suggests leaching glass chips or sample powders of submarine basalts in 6 mol/L HCl at least three times(20 min each) to remove the influence of seawater alteration on their Sr-Nd-Pb-Hf isotopic compositions.

关 键 词：大洋玄武岩 酸洗 Sr-Nd-Pb-Hf同位素 海水蚀变 

分 类 号：P736[天文地球—海洋地质] P597[天文地球—海洋科学]