海南岛玄武岩橄榄石斑晶中熔体包裹体示踪氟、氯深部循环

Deep cycling of fluorine and chlorine traced by melt inclusions trapped in olivine phenocrysts from the Hainan Island basalts

氟(F)、氯(Cl)等挥发分在地球不同圈层之间的循环迁移,对地球的宜居性、金属矿产资源的形成等具有重大影响,因而一直是学术界关注的焦点。近年来,幔源岩浆中早期结晶矿物(如橄榄石)中捕获的熔体包裹体,被认为最大限度的保存了其被捕获时的挥发分组成,为揭示地幔氟、氯组成提供了重要的研究载体。本文对中国南部海南岛玄武岩中橄榄石斑晶所捕获的熔体包裹体开展了电子探针原位氟、氯挥发分分析,发现其氟、氯组成具有较大的变化范围(F为86×10^(-6)~897×10^(-6);Cl为149×10^(-6)~779×10^(-6)),并具有偏高的F/P(0.15~0.70)和Cl/K(0.031~0.051)值,以及比亏损软流圈地幔来源熔体更为富集的氟、氯特征。对海南岛玄武岩中橄榄石斑晶的进一步研究发现,其地幔源区岩性为辉石岩或榴辉岩,而非橄榄岩。综上所述,本文提出研究区地幔的氟、氯富集过程可能与蚀变洋壳或沉积物的再循环过程密切相关,熔体包裹体的组成可用以揭示氟、氯的深部循环过程。

The cycling of fluorine,chlorine and other volatiles among different layers of the Earth plays important roles in the Earth habitability and formation of metallic mineral resources,and is thus widely concerned by academic society.Recently,melt inclusions hosted within the early-stage crystallized minerals(e.g.,olivine)in the mantle-derived magma are frequently recommended to have maximally preserved important information on compositions of volatiles in the primary magma,and they can be used as important research carriers for revealing the compositions of fluorine and chlorine in the mantle.Here we have analyzed the F and Cl concentrations of melt inclusions trapped in olivines from the Hainan Island basalts,South China using the EPMA.The results show that melt inclusions have large variations of F and Cl contents(F=86×10^(-6)~897×10^(-6);Cl=149×10^(-6)~779×10^(-6)),and have high F/P(0.15~0.70)and Cl/K(0.031~0.051)ratios,indicating that F and Cl compositions of the mantle source for these basalts are more enriched than those of the depleted asthenospheric mantle.The further study of olivine phenocrysts hosted in the Hainan Island basalts indicates that the lithology of the mantle source is pyroxenite or eclogite rather than peridotite.In summary,we have suggested that the enrichment process of fluorine and chlorine could be closely related to the recycling of altered oceanic crust or sediment,and the compositions of melt inclusions can be used to trace the deep cycling process of fluorine and chlorine.