Yang, and 5 co-authors, reported in early 2021 that incorporation of small molecules in silicate melts may provide an important mechanism for storing noble gases in the deep Earth, yet the means by which chemically inert noble gases enter and are retained in silica-based materials is not understood. High pressure, room temperature sound velocity measurements on silica and natural basalt glasses in different pressure-transmitting media reveal that neon enters the structure of silicate glasses and enhances their elastic strengths, whereas an ethanol-methanol mixture does not.
Combined with literature data, we found the incorporation of small molecules into silica and basalt glasses is controlled by the void size distribution of the glass and size of the molecules. Pressure primarily reduces the size of noble gases, thereby increasing their solubilities in silicate melts and glasses.
The authors concluded that the geometrical packing and coordination of atoms in silicate melts and glasses are similar at ambient and high pressure conditions based on experimental observations (Williams and Jeanloz, 1988; Morard et al., 2020). Hence, the void-space distribution in melt structure is likely to be comparable and the authors support significant solubility of helium and neon in high pressure silicate melts (Fig. 3). Furthermore, the partition coefficients between minerals and melts for noble gases are in the order of 10−3 (Karato, 2016).
It is expected silicate melts should be an important host for noble gases. He and Ne are the 2nd and 5th most abundant elements in the solar system (Palme et al., 2014). For 3He/4He and 20,21Ne/22Ne isotopic ratios, the discrepancy of upper mantle material value from the atmospheric value has been a hot topic in geochemistry (e.g., Bekaert et al., 2019; Mukhopadhyay and Parai, 2019).
Most answers to this question require a deep primordial reservoir which has unique geochemical features and survives mantle convection for the last 4.5 billion years. It has been noticed that some patches of partially molten rock might exist at the core mantle boundary (e.g., Wen et al., 2001). These melts may be able to host large amounts of noble gases like helium and neon with primordial and less radiogenic features. Other than the core-mantle boundary, partial melting may also occur at the top of the lower mantle due to dehydration melting (Fu et al., 2019). These layers might be perturbed by mantle convection more often and host noble gases with more radiogenic features. Therefore, the observed difference in noble gases ratios in OIBs and MORBs could be possibly due to sampling different melt reservoirs for noble gases. The storage of helium or neon discussed here reaches conditions beyond the range of this experiment, and due to the complex coordination environment change of silicon at higher pressures (Wang et al., 2014), and the high temperature conditions in deep Earth, directly applying the authors' results to these conditions may not be suitable. However, the mechanism revealed in this study and previous studies (Clark et al., 2016; Sato et al., 2011; Weigel et al., 2012) (i.e. availability of interstitial void to compressed noble gases) is still valid and future structure simulation and void space analysis of heated silica/basalt glass at high pressures is needed. Geodynamic simulations are also needed to better estimate the degree of mixing during these processes.
Noble gas-silicate interaction may also have important implications for the composition of the atmospheres of other planetary bodies like Jupiter. It is found that the abundances of helium and neon in Jupiter’s atmosphere are significantly lower than other noble gases, when compared to solar composition (Fortney, 2010).
The authors' results suggest that this discrepancy could be related to interior processes in the planet, which account for the ‘missing’ helium and neon. Forming a He-Ne-silicate composite at Jupiter’s rocky core could be a viable option to lower the fraction of He and Ne in its atmosphere. Whether such a mechanism could explain the deficit of neon and helium in Jupiter’s atmosphere requires further experimental and computational work.
The authors demonstrate that the controlling factors for noble gas solubility in a silicate melt are the noble gas size compared to void size, indicating that data on the structure of silicate melts with natural compositions at higher P-T is crucially needed in order to estimate the storage capacity of noble gases in deep planetary interiors.
Original working URL: https://www.geochemicalperspectivesletters.org/article2105/
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