Chemical structure of the Earth’s mantle defined by fast diffusion elements like helium
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Mass-dependent isotope fractionation of helium is believed to be too small to significantly change 3He/4He isotopic ratios in geological processes, comparing with the change by radiogenic 4He by α-particle (4He) decay of U and Th. In addition, He isotope fractionation caused by diffusion is expected to be also small, since D3He/D4He = 1.03 (Shuster et al. 2004), where D3He and D4He are diffusion coefficients of 3He and 4He, respectively. Thermal diffusion, also called the Soret effect, has been experimentally and theoretically proved to produce significant amount of elemental and isotopic fractionations (Huang et al. 2010; Li and Liu 2015; Richter et al. 2014; Walker and Delong 1982). However, confirmed natural large-scale processes with such effect have not been found, because mass diffusion rate is several orders of magnitude smaller than that of heat diffusion, thus mass diffusion could not achieve much fractionation before temperature gradient or contrast had vanished.
High 3He/4He ratios in OIBs are often regarded as evidences for an undegassed mantle source sampled by plumes (Kellogg and Turcotte 1990; Kurz et al. 1982; Moreira 2013; Mukhopadhyay and Parai 2019). However, this conventional model can’t explain why helium concentrations are several orders of magnitude lower than in MORBs (Anderson 1998). This paradox has led to a long-standing controversy about the chemical structure and dynamics of the Earth’s mantle. Although many models have been proposed, such as the predegassing (Hopp and Trieloff 2008) and disequilibrium degassing models (Gonnermann and Mukhopadhyay 2007), the basal magma ocean model (Labrosse et al. 2007), primordial noble gases in the core (Bouhifd et al. 2013; Porcelli and Halliday 2001), residue of melting (Class and Goldstein 2005; Parman et al. 2005), etc., it is still an open issue (Moreira 2013). Zhu and colleagues’ models are brand new and physically rational, and do provide some insights to solve this issue.
Interestingly, Zhu and colleagues also claimed that the helium stratified mantle was formed in the early Earth, and would lead an over-degassed deepest mantle in He due to largest temperature contrast. Thus later evolution of the deepest mantle is helium ingassed process (Zhu et al. 2019). The ingassed helium was probably radiogenic 4He. This may help to explain the apparent imbalance of heat and helium released by the Earth, the heat-helium paradox (Onions and Oxburgh 1983). Zhu et al. (2019)’s model is still a hypothesis yet. Future experimental works are desperately needed to provide data about the helium’s behavior in the deep mantle.
Nevertheless, the new model by Zhu et al. (2019) will be of value in understanding the chemical structure the Earth’s mantle (Fig. 1). Moreover, extrapolation of the new models would suggest that fast diffusion components, such as hydrogen (Demouchy 2010; Hae et al. 2006), would be also enriched in their light isotopes in the deepest mantle. This is consistent with the case of lavas in Baffin Island, which have not only the highest 3He/4He isotopic ratios found so far (Starkey et al. 2009; Stuart et al. 2003), but also the lowest D/H ratio (Hallis et al. 2015).