Noble gas incorporation into silicate glasses: implications for planetary volatile storage

H. Yang¹,

¹Department of Geological Sciences, Stanford University, Stanford, CA 94305, USA

A.E. Gleason^1,2,

¹Department of Geological Sciences, Stanford University, Stanford, CA 94305, USA
²SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA

S.N. Tkachev³,

³Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, USA

B. Chen⁴,

⁴Center for High Pressure Science and Technology Advanced Research, Pudong, Shanghai 201203, China

R. Jeanloz⁵,

⁵Earth and Planetary Sciences Department, University of California, Berkeley, CA 94720, USA

W.L. Mao^1,2

¹Department of Geological Sciences, Stanford University, Stanford, CA 94305, USA
²SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA

Affiliations | Corresponding Author | Cite as | Funding information

Geochemical Perspectives Letters v17 | doi: 10.7185/geochemlet.2105
Received 21 September 2020 | Accepted 6 January 2021 | Published 10 February 2021

Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0

Keywords: Brillouin spectroscopy, elasticity, noble gas, silicate glass, silica glass



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Abstract

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.

Figures

Figure 1 Sound velocities of vitreous silica under high pressure in different pressure media. M-E represents 4∶1 Methanol-Ethanol mixture. Errors of the velocities are estimated from statistical uncertainties arising from the peak fitting. Typical errors are less than 1.5 % and smaller than the size of the symbols. For both the V P and V S of silica in different noble gas media, we found a consistent trend for the acoustic velocities — He > Ne > Ar ≈ M-E. The abnormal velocity minimum at around 2–5 GPa can be attributed to the rearrangement of SiO4 tetrahedra in the vitreous silica structure (Clark et al., 2016).	Figure 2 Sound velocities of basalt glasses at high pressure. M-E-W: 16∶3∶1 Methanol-Ethanol-Water mixture, M-E: 4∶1 Methanol-Ethanol mixture. Errors of the velocities are estimated from statistical uncertainties arising from the peak fitting. Error bars smaller than the symbols plotted are not shown. Although the three basalt glasses have slightly different compositions, their degrees of polymerisation (NBO/T = 0.6, 0.9 and 0.8 for BCR-2, BIR-1 and KB, respectively) are quite similar (BIR-1, blue coloured line, Liu and Lin, 2014; BCR-2, triangular points, Clark et al., 2016). The extents of the velocity drops are very different among the glasses. We observed that BIR-1 has a 2 % drop for both V P and V S, BCR-2 has a 2.8 % drop for V P and a 7.2 % drop for V S, while KB in M-E has a 14 % drop for both V P and V S. We attribute this variation to be mostly due to the different pressure media used. H2O and Ne have a relatively small molecule size that can possibly penetrate into the structure of silicate glass and make it stiffer (Figs. 3, S-4).	Figure 3 Void size distribution of SiO2 and molecular size of common pressure media at ambient condition and high pressure. Top panel: Molecular size data at ambient conditions were adapted from Reid et al. (1987). Bottom panel: High pressure molecule sizes were calculated using equations of state (He: Loubeyre et al., 1993; Ne: Dewaele et al., 2008; Ar: Ross et al., 1986, H2O: Yoshimura et al., 2006).	Figure 4 Solubility of neon and helium in vitreous silica at high pressure. The two lines for each medium represent upper limit and lower limits, respectively. The lower and upper limits were estimated by (Vrigid – V normal)/Vgas and (Vrigid – Vsixfold)/Vgas, respectively, where Vrigid and Vnormal represent the molar volume of SiO2 glass in noble gas media and non-gas media conditions, accordingly; Vgas – the molar volume of noble gas; Vsixfold – the molar volume of six-fold-coordinated SiO2 glass (Sato et al., 2011). We do not have an accurate determination of volume of silica in Ne, instead we assume the volume change under pressure is same as the He case, as similar volume curves were suggested by integration method (Fig. S-3).
Figure 1	Figure 2	Figure 3	Figure 4

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Introduction

Mukhopadhyay, S., Parai, R. (2019) Noble Gases: A Record of Earth’s Evolution and Mantle Dynamics. Annual Review of Earth and Planetary Sciences 47, 389–419.

). Owing to their changing reactivity and volatility with pressure, noble gases are also useful geochemical tracers for the interior processes of planets (e.g., Sanloup et al., 2005

Sanloup, C., Schmidt, B.C., Perez, E.M.C., Jambon, A., Gregoryanz, E., Mezouar, M. (2005) Retention of Xenon in Quartz and Earth’s Missing Xenon. Science. American Association for the Advancement of Science 310, 1174–1177.

). However, how these noble gases are distributed among potential geochemical reservoirs, and how they alter the physical properties of their host with increasing depth (and therefore pressure) is still unclear. The storage of noble gases in quartz, ferropericlase, and bridgmanite at high pressure has been experimentally verified (Sanloup et al., 2005

; Rosa et al., 2020

Rosa, A.D., Bouhifd, M.A., Morard, G., Briggs, R., Garbarino, G., Irifune, T., Mathon, O., Pascarelli, S. (2020) Krypton storage capacity of the Earth’s lower mantle. Earth and Planetary Science Letters 532, 116032.

). Nevertheless, the partition coefficients of noble gases between minerals and melts are on the order of 10⁻³ (Karato, 2016

Karato, S. (2016) Physical basis of trace element partitioning: A review. American Mineralogist 101, 2577–2593.

), implying significant storage of noble gases in silicate melts. This deep storage could also potentially alter atmospheric composition. As silicate glasses and melts share structural similarities (Williams and Jeanloz, 1988

Williams, Q., Jeanloz, R. (1988) Spectroscopic Evidence for Pressure-Induced Coordination Changes in Silicate Glasses and Melts. Science 239, 902–905.

; Morard et al., 2020

Morard, G., Hernandez, J.-A., Guarguaglini, M., Bolis, R., Benuzzi-Mounaix, A., Vinci, T., Fiquet, G., Baron, M.A., Shim, S.H., Ko, B., Gleason, A.E., Mao, W.L., Alonso-Mori, R., Lee, H.J., Nagler, B., Galtier, E., Sokaras, D., Glenzer, S.H., Andrault, D., Garbarino, G., Mezouar, M., Schuster, A.K., Ravasio, A. (2020) In situ X-ray diffraction of silicate liquids and glasses under dynamic and static compression to megabar pressures. Proceedings of the National Academy of Sciences 117, 11981–11986.

), with glass being the kinetically hindered state of the corresponding melt, understanding the incorporation of noble gases into silicate glasses can shed light on their storage in natural silicate melts.

Noble gases are widely used as pressure-transmitting media in high pressure diamond anvil cell experiments. These gases are chemically inactive and display relatively low mechanical strength, and thus minimise pressure gradients and deviatoric stresses in the sample chamber (Klotz et al., 2009

Klotz, S., Chervin, J.C., Munsch, P., Lemarchand, G. (2009) Hydrostatic limits of 11 pressure transmitting media. Journal of Physics D Applied Physics 42, 075413.

). Use of noble gases as pressure-transmitting media presumes minimal interaction with the pressurised sample, yet there have been several reports that helium penetrates into the structure of silica glass at room temperature, enhancing both its incompressibility and rigidity (Sato et al., 2011

Sato, T., Funamori, N., Yagi, T. (2011) Helium penetrates into silica glass and reduces its compressibility. Nature Communications 2, 345.

; Shen et al., 2011

Shen, G., Mei, Q., Prakapenka, V.B., Lazor, P., Sinogeikin, S., Meng, Y., Park, C. (2011) Effect of helium on structure and compression behavior of SiO2 glass. Proceedings of the National Academy of Sciences 108, 6004–6007.

; Weigel et al., 2012

Weigel, C., Polian, A., Kint, M., Rufflé, B., Foret, M., Vacher, R. (2012) Vitreous Silica Distends in Helium Gas: Acoustic Versus Static Compressibilities. Physical Review Letters 109, 245504.

). Another study on basalt and enstatite glasses also indicates neon can enter their structure at high pressure (Clark et al., 2016

Clark, A.N., Lesher, C.E., Jacobsen, S.D., Wang, Y. (2016) Anomalous density and elastic properties of basalt at high pressure: Reevaluating of the effect of melt fraction on seismic velocity in the Earth’s crust and upper mantle. Journal of Geophysical Research: Solid Earth 121, 4232–4248.

).

Void space analysis of silica could shed light on the incorporation of noble gases into its structure. Theoretical simulations of the structure of silica glass and void size analysis have provided statistics on the interstitial space (i.e. the largest spherical site not occupied by Si or O) that could potentially be available for incorporating noble gases (Shackelford and Masaryk, 1978

Shackelford, J.F., Masaryk, J.S. (1978) The interstitial structure of vitreous silica. Journal of Non-Crystalline Solids 30, 127–134.

; Malavasi et al., 2006

Malavasi, G., Menziani, M.C., Pedone, A., Segre, U. (2006) Void size distribution in MD-modelled silica glass structures. Journal of Non-Crystalline Solids 352, 285–296.

). However, systematic study of the high pressure solubility of these gases in silica glass has been lacking, despite a number of studies on helium (Sato et al., 2011

Sato, T., Funamori, N., Yagi, T. (2011) Helium penetrates into silica glass and reduces its compressibility. Nature Communications 2, 345.

; Shen et al., 2011

; Weigel et al., 2012

Weigel, C., Polian, A., Kint, M., Rufflé, B., Foret, M., Vacher, R. (2012) Vitreous Silica Distends in Helium Gas: Acoustic Versus Static Compressibilities. Physical Review Letters 109, 245504.

).

To help clarify the mechanism of noble gas incorporation into amorphous silica and natural silicate glasses, we measured high pressure Brillouin spectra of silica and basalt glasses using different pressure-transmitting media at room temperature. The measured elastic properties of the material provide insight into the structural evolution and molecule incorporation of each glass with compression. Elasticity is a useful monitor of the solution process because in situ measurement of gas solubility is challenging at high pressure. Together with existing literature data, we provide a comprehensive review of noble gases migrating into silica glass and natural basalt glasses under pressure. We find that solubility is controlled by the atomic sizes of the noble gases relative to the size of available interstitial spaces in the silicate glasses. Pressure alters both factors, thereby affecting the solubility of noble gases and other volatile species in glasses and melts.

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Results

Details on sample synthesis, compositions, and data collection can be found in the Supplementary Information. We found that the sound velocities of silica glass depend on the pressure-transmitting medium (Fig. 1). We observed a drop in both compressional and shear velocities when increasing pressure between 1 and 3 GPa, followed by a slightly increasing or nearly unchanged velocity at higher pressures (Fig. 1). Silicate glasses with natural compositions share a similar framework structure with silica glass, and their velocity drop upon initial compression was also documented in other polymerised silicate glasses, such as basalt, jadeite and albite glasses (Liu and Lin, 2014

Liu, J., Lin, J.-F. (2014) Abnormal acoustic wave velocities in basaltic and (Fe,Al)-bearing silicate glasses at high pressures. Geophysical Research Letters 41, 8832–8839.

; Sakamaki et al., 2014

Sakamaki, T., Kono, Y., Wang, Y., Park, C., Yu, T., Jing, Z., Shen, G. (2014) Contrasting sound velocity and intermediate-range structural order between polymerized and depolymerized silicate glasses under pressure. Earth and Planetary Science Letters 391, 288–295.

). In contrast, depolymerised glasses like diopside or enstatite glass do not show a decreasing trend, but rather an almost pressure-independent velocity (Sanchez-Valle and Bass, 2010

Sanchez-Valle, C., Bass, J.D. (2010) Elasticity and pressure-induced structural changes in vitreous MgSiO3-enstatite to lower mantle pressures. Earth and Planetary Science Letters 295, 523–530.

; Liu and Lin, 2014

Liu, J., Lin, J.-F. (2014) Abnormal acoustic wave velocities in basaltic and (Fe,Al)-bearing silicate glasses at high pressures. Geophysical Research Letters 41, 8832–8839.

; Sakamaki et al., 2014

). These observations can be explained by the flexibility of SiO₄ tetrahedra networks. In the low pressure range, below 3 GPa, the SiO₄ tetrahedra in the glass rotate into the void space to form a high-density structure (Clark et al., 2016

). The rotation does not involve substantial compression of the interatomic bonds, so the elastic moduli of the material remain largely unaltered. Therefore, the velocities, given by the square root of the ratio of the moduli and density, decrease during this stage. However, after the void space is filled, tetrahedral rotation is replaced by the interatomic bond shortening, and the sound velocities then increase under compression (Clark et al., 2016

). Depolymerised silicate glasses, which contain larger ‘modifier’ cations like Mg²⁺, Na²⁺ or Ca²⁺, have less void space and consequently less flexibility. The densification may also involve some chemical bond shortening and lead to the unchanged velocity profile with increasing pressure.

**Figure 1** Sound velocities of vitreous silica under high pressure in different pressure media. M-E represents 4∶1 Methanol-Ethanol mixture. Errors of the velocities are estimated from statistical uncertainties arising from the peak fitting. Typical errors are less than 1.5 % and smaller than the size of the symbols. For both the V_P and V_S of silica in different noble gas media, we found a consistent trend for the acoustic velocities — He > Ne > Ar ≈ M-E. The abnormal velocity minimum at around 2–5 GPa can be attributed to the rearrangement of SiO₄ tetrahedra in the vitreous silica structure (Clark *et al.*, 2016
Clark, A.N., Lesher, C.E., Jacobsen, S.D., Wang, Y. (2016) Anomalous density and elastic properties of basalt at high pressure: Reevaluating of the effect of melt fraction on seismic velocity in the Earth’s crust and upper mantle. *Journal of Geophysical Research: Solid Earth* 121, 4232–4248.
).

For basalt glasses, sound velocities at high pressure are also influenced by the pressure media (Fig. 2). The BIR-1 sample has higher velocities in a neon medium, as compared with basalt in M-E and M-E-W (methanol: ethanol: water = 16∶3∶1); it also has an earlier transition pressure at which the velocities start to increase. Comparing the M-E and M-E-W cases, we see that water seems to lower the decreasing slope below 5 GPa, but it does not change the transition point for the change in velocity trends (Fig. 2).

**Figure 2** Sound velocities of basalt glasses at high pressure. M-E-W: 16∶3∶1 Methanol-Ethanol-Water mixture, M-E: 4∶1 Methanol-Ethanol mixture. Errors of the velocities are estimated from statistical uncertainties arising from the peak fitting. Error bars smaller than the symbols plotted are not shown. Although the three basalt glasses have slightly different compositions, their degrees of polymerisation (NBO/T = 0.6, 0.9 and 0.8 for BCR-2, BIR-1 and KB, respectively) are quite similar (BIR-1, blue coloured line, Liu and Lin, 2014
Liu, J., Lin, J.-F. (2014) Abnormal acoustic wave velocities in basaltic and (Fe,Al)-bearing silicate glasses at high pressures. *Geophysical Research Letters* 41, 8832–8839.
; BCR-2, triangular points, Clark *et al.*, 2016
Clark, A.N., Lesher, C.E., Jacobsen, S.D., Wang, Y. (2016) Anomalous density and elastic properties of basalt at high pressure: Reevaluating of the effect of melt fraction on seismic velocity in the Earth’s crust and upper mantle. *Journal of Geophysical Research: Solid Earth* 121, 4232–4248.
). The extents of the velocity drops are very different among the glasses. We observed that BIR-1 has a 2 % drop for both V_P and V_S, BCR-2 has a 2.8 % drop for V_P and a 7.2 % drop for V_S, while KB in M-E has a 14 % drop for both V_P and V_S. We attribute this variation to be mostly due to the different pressure media used. H₂O and Ne have a relatively small molecule size that can possibly penetrate into the structure of silicate glass and make it stiffer (Figs. 3, S-4).

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Discussion

Sound velocity data for amorphous materials can be very useful to calculate their density at high pressure (Zha et al., 1994

Zha, C., Hemley, R.J., Mao, H., Duffy, T.S., Meade, C. (1994) Acoustic velocities and refractive index of SiO2 glass to 57.5 GPa by Brillouin scattering. Physical Review B 50, 13105–13112.

). However, this method would fail if pressure media penetrates the sample (Weigel et al., 2012

Weigel, C., Polian, A., Kint, M., Rufflé, B., Foret, M., Vacher, R. (2012) Vitreous Silica Distends in Helium Gas: Acoustic Versus Static Compressibilities. Physical Review Letters 109, 245504.

). We calculated the P-V curve from velocities observed in different media and used this to examine whether incorporation of pressure media occurred in our experiments (see Supplementary Information for details). Neon and helium seem also able to penetrate into the silica structure while water and methanol molecules seem also to be able to penetrate into basalt glasses (Fig. S-3).

We compared the molecular size of the pressure media with the size of the interstitial space in the silica structure (Figs. 3, S-4). At ambient conditions, the sizes of helium, neon and water molecules are smaller than some voids in the silica structure. At higher pressures, the void size distribution generally shifts to a smaller volume, but the peak position moves only slightly and is still larger than 1.75 Å. On the other hand, the sizes of highly compressible gases decrease dramatically with increasing pressure, especially for helium and neon (Fig. S-4). These results indicate that helium and neon elevate the elastic stiffness of silica by supporting the structure in the void space, while molecules larger than argon are too big to be incorporated into silica and do not show this effect. Pressure makes these atoms smaller, enhancing the solubility of neon and helium into silica (Fig. 3). Although argon also becomes smaller at high pressure, it is still larger than most of the voids in silica. Its solubility is limited and does not influence the elastic properties significantly (Fig. 1).

**Figure 3** Void size distribution of SiO₂ and molecular size of common pressure media at ambient condition and high pressure. **Top panel:** Molecular size data at ambient conditions were adapted from Reid *et al.* (1987)
Reid, R.C., Prausnitz, J.M., Poling, B.E. (1987) The properties of gases and liquids. McGraw Hill Book Co., New York, NY.
. **Bottom panel:** High pressure molecule sizes were calculated using equations of state (He: Loubeyre *et al.*, 1993
Loubeyre, P., LeToullec, R., Pinceaux, J.P., Mao, H.K., Hu, J., Hemley, R.J. (1993) Equation of state and phase diagram of solid 4He from single-crystal x-ray diffraction over a large P-T domain. *Physical Review Letters* 71, 2272–2275.
; Ne: Dewaele *et al.*, 2008
Dewaele, A., Datchi, F., Loubeyre, P., Mezouar, M. (2008) High pressure--high temperature equations of state of neon and diamond. *Physical Review B* 77, 094106.
; Ar: Ross *et al.*, 1986
Ross, M., Mao, H.K., Bell, P.M., Xu, J.A. (1986) The equation of state of dense argon: A comparison of shock and static studies. *The Journal of Chemical Physics* 85, 1028–1033.
, H₂O: Yoshimura *et al.*, 2006
Yoshimura, Y., Stewart, S.T., Somayazulu, M., Mao, H., Hemley, R.J. (2006) High-pressure x-ray diffraction and Raman spectroscopy of ice VIII. *The Journal of Chemical Physics* 124, 024502.
).

Our measurements do not provide solubility values, but by comparing the gas and non-gas experiments we can make an estimate of this parameter (Sato et al., 2011

Sato, T., Funamori, N., Yagi, T. (2011) Helium penetrates into silica glass and reduces its compressibility. Nature Communications 2, 345.

) (Fig. S-3). In Figure 4, the upper limit is constrained by the maximum available space in the silica structure. This space is calculated as the volume difference between normal silica and the ultra-dense six-coordinated silica extropolated to lower pressure. On the other hand, the lower limit is given by the difference between gas and non-gas curves, assuming the ‘expansion’ should wholly or partly come from the volume of gas in the structure. Since the partial volume of a component in a mixture is smaller than the volume on its own (Bajgain et al., 2015

Bajgain, S., Ghosh, D.B., Karki, B.B. (2015) Structure and density of basaltic melts at mantle conditions from first-principles simulations. Nature Communications 6, 1–7.

), the real solubility should be higher than the lower limit here.

**Figure 4** Solubility of neon and helium in vitreous silica at high pressure. The two lines for each medium represent upper limit and lower limits, respectively. The lower and upper limits were estimated by (V_rigid – V _normal)/V_gas and (V_rigid – V_sixfold)/V_gas, respectively, where V_rigid and V_normal represent the molar volume of SiO₂ glass in noble gas media and non-gas media conditions, accordingly; V_gas – the molar volume of noble gas; V_sixfold – the molar volume of six-fold-coordinated SiO₂ glass (Sato *et al.*, 2011
Sato, T., Funamori, N., Yagi, T. (2011) Helium penetrates into silica glass and reduces its compressibility. *Nature Communications* 2, 345.
). We do not have an accurate determination of volume of silica in Ne, instead we assume the volume change under pressure is same as the He case, as similar volume curves were suggested by integration method (Fig. S-3).

Basalt glass is compositionally more complex than silica glass with the addition of other cations. These cations can be classified into two categories: the network formers like Ti, Al and network modifiers like Mg, Ca, Na and K. The two sets of cations have distinct effects on gas solubility. Network modifiers tend to form bonds between the bridging SiO₄ tetrahedra and lower the volume of void space. Their negative correlation with noble gas solubility has been experimentally observed (Tournour and Shelby, 2008a

Tournour, C.C., Shelby, J.E. (2008a) Neon solubility in silicate glasses and melts. Physics and Chemistry of Glasses 49, 8.

Tournour, C.C., Shelby, J.E. (2008b) Helium solubility in alkali silicate glasses and melts. Physics and Chemistry of Glasses – European Journal of Glass Science and Technology Part B 49, 207–215.

). On the other hand, network formers, which reside in the Si site, seem to have less of an influence on gas solubility.

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Geochemical Implications

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

Williams, Q., Jeanloz, R. (1988) Spectroscopic Evidence for Pressure-Induced Coordination Changes in Silicate Glasses and Melts. Science 239, 902–905.

; Morard et al., 2020

). Hence, the void space distribution in melt structure is likely to be comparable and our results here 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

Karato, S. (2016) Physical basis of trace element partitioning: A review. American Mineralogist 101, 2577–2593.

). It is expected silicate melts should be an important host for noble gases. He and Ne are the 2^nd and 5^th most abundant elements in the solar system (Palme et al., 2014

Palme, H., Lodders, K., Jones, A. (2014) 2.2 – Solar System Abundances of the Elements. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. Second Edition, Elsevier, Oxford, 15–36, doi: 10.1016/B978-0-08-095975-7.00118-2.

). 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

Bekaert, D.V., Broadley, M.W., Caracausi, A., Marty, B. (2019) Novel insights into the degassing history of Earth’s mantle from high precision noble gas analysis of magmatic gas. Earth and Planetary Science Letters 525, 115766.

; Mukhopadhyay and Parai, 2019

Mukhopadhyay, S., Parai, R. (2019) Noble Gases: A Record of Earth’s Evolution and Mantle Dynamics. Annual Review of Earth and Planetary Sciences 47, 389–419.

). 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

Wen, L., Silver, P.G., James, D.E., Kuehnel, R. (2001) Seismic evidence for a thermo-chemical boundary at the base of the Earth’s mantle. Earth and Planetary Science Letters 189, 141–153.

). 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

Fu, S., Yang, J., Karato, S., Vasiliev, A., Presniakov, M. Yu., Gavrilliuk, A.G., Ivanova, A.G., Hauri, E.H., Okuchi, T., Purevjav, N., Lin, J. (2019) Water Concentration in Single-Crystal (Al,Fe)-Bearing Bridgmanite Grown From the Hydrous Melt: Implications for Dehydration Melting at the Topmost Lower Mantle. Geophysical Research Letters 46, 10346–10357.

). 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

Wang, Y., Sakamaki, T., Skinner, L.B., Jing, Z., Yu, T., Kono, Y., Park, C., Shen, G., Rivers, M.L., Sutton, S.R. (2014) Atomistic insight into viscosity and density of silicate melts under pressure. Nature Communications 5, 3241.

), and the high temperature conditions in deep Earth, directly applying our 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

Sato, T., Funamori, N., Yagi, T. (2011) Helium penetrates into silica glass and reduces its compressibility. Nature Communications 2, 345.

; Weigel et al., 2012

Weigel, C., Polian, A., Kint, M., Rufflé, B., Foret, M., Vacher, R. (2012) Vitreous Silica Distends in Helium Gas: Acoustic Versus Static Compressibilities. Physical Review Letters 109, 245504.

) (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

Fortney, J. (2010) Peering into Jupiter. Physics 3, 26.

). Our 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. Our study demonstrates 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.

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Acknowledgements

We appreciate constructive comments from two anonymous reviewers. AEG and WM acknowledge support from NSF Geophysics (EAR0738873). RJ acknowledges support from UC CMEC. Work was performed at GeoSoilEnviroCARS which is supported by NSF (EAR – 1634415) and DOE (DE-FG02-94ER14466). Use of the COMPRES-GSECARS gas loading system was supported by COMPRES under NSF (EAR -1606856) and GSECARS. The Advanced Photon Source is operated for the DOE Office of Science by Argonne National Laboratory (DE-AC02-06CH11357).

Editor: Anat Shahar

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References

Bajgain, S., Ghosh, D.B., Karki, B.B. (2015) Structure and density of basaltic melts at mantle conditions from first-principles simulations. Nature Communications 6, 1–7.