Advances in rubidium isotope analysis method and applications in geological studies

doi:10.13745/j.esf.sf.2020.4.13

Abstract

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 (T_1/2=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

CLC Number:

P597.2

ZHANG Zhuoying, MA Jinlong, ZHANG Le, ZENG Ti, LIU Ying, WEI Gangjian. Advances in rubidium isotope analysis method and applications in geological studies[J]. Earth Science Frontiers, 2020, 27(3): 123-132.

Figures/Tables 5

References 67

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流程	树脂类型及体积	淋洗酸类型	实验流程及用酸量	最终残留元素	回收率	参考文献
1	AG50W-X8(8 mL)	2.5 mol/L HCl	9 mL 淋洗 3 mL 接取Rb	Na、K、Mg、 Ca等基体元素	约98%	[13,21,26]
1	Sr-Spec树脂 (200 μL)	3 mol/L HNO₃	蒸干上步接取溶液, 提取至1 mL上样, 1 mL接取Rb	Na、K、Mg、 Ca等基体元素	约98%	[13,21,26]
2	DGA (1.8 mL)	1 mol/L HNO₃	1 mL上样 5 mL接取Rb	K/Rb<2	约100%	[27]
	AG50-X12 (20 mL)	3 mol/L HCl	蒸干上步接取溶液, 提取至1 mL上样 69 mL淋洗 50 mL接取Rb
	AG50-X12 (10 mL)	3 mol/L HCl	蒸干上步接取溶液, 提取至1 mL上样 34 mL淋洗 15 mL接取Rb
	AG50-X8 (1 mL)	0.5 mol/L HCl	蒸干上步接取溶液, 提取至0.25 mL上样 26.75 mL淋洗 40接取Rb
3	Sr-Spec树脂 (200 μL)	2 mol/L HNO₃+ 0.07 mol/L HF 以及不同浓度HNO₃	收集分离Ba、Sr、Pb 过程中含Rb及其他基体元素的混合液	未明	未明	[31]
	Cs-Rb树脂 (50 μL)	2 mol/L HNO₃+ 0.07 mol/L HF	取100 μL混合液上样 150 μL淋洗
		1 mol/L HNO₃	600 μL淋洗
		6 mol/L HNO₃	1.5 mL接取Rb
4	Sr-Spec树脂 (1.5 g)	3 mol/L HNO₃	100 μL上样 4.4 mL淋洗 7.5 mL接取Rb	Na/Rb<2、 Ca/Rb<4	约100%	[28]

流程	树脂类型及体积	淋洗酸类型	实验流程及用酸量	最终残留元素	回收率	参考文献
1	AG50W-X8(8 mL)	2.5 mol/L HCl	9 mL 淋洗 3 mL 接取Rb	Na、K、Mg、 Ca等基体元素	约98%	[13,21,26]
1	Sr-Spec树脂 (200 μL)	3 mol/L HNO₃	蒸干上步接取溶液, 提取至1 mL上样, 1 mL接取Rb	Na、K、Mg、 Ca等基体元素	约98%	[13,21,26]
2	DGA (1.8 mL)	1 mol/L HNO₃	1 mL上样 5 mL接取Rb	K/Rb<2	约100%	[27]
	AG50-X12 (20 mL)	3 mol/L HCl	蒸干上步接取溶液, 提取至1 mL上样 69 mL淋洗 50 mL接取Rb
	AG50-X12 (10 mL)	3 mol/L HCl	蒸干上步接取溶液, 提取至1 mL上样 34 mL淋洗 15 mL接取Rb
	AG50-X8 (1 mL)	0.5 mol/L HCl	蒸干上步接取溶液, 提取至0.25 mL上样 26.75 mL淋洗 40接取Rb
3	Sr-Spec树脂 (200 μL)	2 mol/L HNO₃+ 0.07 mol/L HF 以及不同浓度HNO₃	收集分离Ba、Sr、Pb 过程中含Rb及其他基体元素的混合液	未明	未明	[31]
	Cs-Rb树脂 (50 μL)	2 mol/L HNO₃+ 0.07 mol/L HF	取100 μL混合液上样 150 μL淋洗
		1 mol/L HNO₃	600 μL淋洗
		6 mol/L HNO₃	1.5 mL接取Rb
4	Sr-Spec树脂 (1.5 g)	3 mol/L HNO₃	100 μL上样 4.4 mL淋洗 7.5 mL接取Rb	Na/Rb<2、 Ca/Rb<4	约100%	[28]

仪器	仪器分馏校正方法	精度(2 SD)	参考文献
VG Sector 54 TIMS		1.3%	[13]
VG Axiom MC-ICP-MS	外标元素加入法	0.5%	[13]
Micromass IsoProbe MC-ICP-MS	外标元素加入法	0.5‰~0.2‰	[21,26]
Neptune Plus MC-ICP-MS	SSB	0.25‰~0.06‰	[27-28]

仪器	仪器分馏校正方法	精度(2 SD)	参考文献
VG Sector 54 TIMS		1.3%	[13]
VG Axiom MC-ICP-MS	外标元素加入法	0.5%	[13]
Micromass IsoProbe MC-ICP-MS	外标元素加入法	0.5‰~0.2‰	[21,26]
Neptune Plus MC-ICP-MS	SSB	0.25‰~0.06‰	[27-28]