Earth Science Frontiers ›› 2020, Vol. 27 ›› Issue (3): 123-132.DOI: 10.13745/j.esf.sf.2020.4.13

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Advances in rubidium isotope analysis method and applications in geological studies

ZHANG Zhuoying1,2(), MA Jinlong1,*(), ZHANG Le1, ZENG Ti3, LIU Ying1, WEI Gangjian1   

  1. 1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
    2. University of Chinese Academy of Sciences, Beijing 100049, China
    3. Key Laboratory of Ocean and Marginal Sea Geology, Chinese Academy of Sciences, Guangzhou 510640, China
  • Received:2019-06-28 Revised:2020-02-27 Online:2020-05-20 Published:2020-05-20
  • Contact: MA Jinlong

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Abstract:

Rubidium (Rb) is an alkali metal element with moderate volatility, fluid activity and high magmatic incompatibility, which is essential for tracing material provenance and providing valuable information on various geological processes. Meanwhile, 87Rb is a radioactive isotope which can decay to 87Sr with a long half-life (T1/2=4.976×1010 a). Therefore, Rb-Sr dating system has been widely used to determine geological age of long-time scale. Traditionally, Rb isotopic composition (87Rb/85Rb) of geological samples are considered constant for specific geological period. However, with advances in both Rb purification technique and Rb isotopic measurement by mass spectrometry, high-precision Rb isotopic compositions can be precisely determined. And the limited data show that geological samples can have different 87Rb/85Rb ratios due to isotopic fractionation during diverse geological processes. Questions still remain that whether Rb isotopic fractionation can provide additional clues for Rb tracing or affect precision of classical Rb-Sr dating method. To answer these questions, it is important to investigate δ 87Rb values of major geological reservoirs, discover Rb isotopic fractionation during various geological processes and explore the relevant mechanism of the fractionation systematically. Research in these areas, however, is extremely scarce to date. In this paper, we reviewed Rb isotope studies in the past 20 years, including analytical methods and fractionation mechanism, and provide our perspectives on relevant applications in the near future. The scope of our work and main findings are as follows: (1) We summarized previous studies regarding Rb chemical purification and instrumental measurement and compared the advantages and disadvantages of these techniques to show that careful chemical pretreatment and robust instrumental determination are the prerequisites for obtaining high-precision Rb isotopic ratios; (2) We compiled data on Rb isotopic compositions of extraterrestrial samples and briefly described Rb isotope application in cosmochemistry, e.g., as a moderate volatile element, Rb isotopes are promising in constraining accretion and evolution of the inner solar system; (3) Rb isotopes show potentials in solving many geological issues, such as improving classical Rb-Sr dating system, understanding differentiation process between crust and mantle, restricting continental chemical weathering processes, and advancing our understanding of Rb deposit formation.

Key words: Rb stable isotope, Rb chemical separation, MC-ICP-MS, cosmochemical process, promising application

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