A helium stratified and ingassed lower mantle: resolving the helium paradoxes

• 作者: Honggang Zhu Xuefang Li Yingkui Xu
• 来源期刊: 地球化学学报(英文）
• 关键词: Helium paradox Thermal diffusion Mantle convection Adiabatic temperature gradient Primordial mantle Ingassed lower mantle
• 年，卷（期）: 2022,（1）
• 页码范围: 4-10
• 页数: 6页
• 语种: 英文
• DOI: 10.1007/s11631-019-00378-2

摘要

It is generally believed a variation of 3 He/4 He isotopic ratios in the mantle is due to only the decay of U and Th, which produces 4 He as well as heat. Here we show that not only 3 He/4 He isotopic ratios but also helium contents can be fractionated by thermal diffusion in the lower mantle. The driving force for that fractionation is the adiabatic or convective temperature gradient, which always produces elemental and isotopic fractionation along temperature gradient by thermal diffusion with higher light/heavy isotopic ratio in the hot end. Our theoretical model and calculations indicate that the lower mantle is helium stratified, caused by thermal diffusion due to ~ 400℃ temperature contrast across the lower mantle. The highest 3 He/4 He isotopic ratios and lowest He contents are in the lowermost mantle, which is a consequence of thermaldiffusion fractionation rather than the lower mantle is a primordial and undegassed reservoir. Therefore, oceanicisland basalts derived from the deepest lower mantle with high 3 He/4 He isotopic ratios and less He contents—the long-standing helium paradox, is solved by our model. Because vigorous convection in the upper mantle had resulted in disordered or disorganized thermal-diffusion effects in He, Mid-ocean ridge basalts unaffected by mantle plume have a relatively homogenous and lower 3 He/4 He isotopic compositions. Our model also predicts that 3 He/4 He isotopic ratios in the deepest lower mantle of early Earth could be even higher than that of Jupiter, the initial He isotopic ratio in our solar system, because the temperature contrast across the lower mantle in the early Earth is the largest and less 4 He had been produced by the decay of U and Th. Moreover, the early helium-stratified lower mantle owned the lowest He contents due to over-degassing caused by the largest temperature contrast. Consequently, succeeding evolution of the lower mantle is a He ingassed process due to secular cooling of the deepest mantle. This