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[SIT16-13] Incorporation of noble gases into potassium-rich silicate under lower mantle conditions
Keywords:missing xenon, lower mantle minerals, laser-heated diamond anvil cells, synchrotron X-ray, noble gases, reservoir
Noble gases are important geochemical tracers for exploring Earth's global volatile cycles and the evolution of Earth's atmosphere. In the atmospheres of Earth and Mars, xenon is more depleted than other noble gas elements relative to chondritic meteorites[1]. The "missing Xe" is believed to be captured somewhere inside the Earth, and various materials under crustal/mantle/core conditions have been examined experimentally and computationally, resulting in no obvious evidence for the Xe reservoir so far. Although the two major lower mantle minerals (ferropericlase, bridgmanite) are unlikely to account for the missing noble gases, the solubilities of heavy noble gases (Ar, Kr, and Xe) are expected to increase with pressure/depth based on recent studies of lattice strain modeling[2-4].
This study focuses on KAlSi3O8, which forms one of the most abundant minerals in the continental crust, pottasium-rich feldspar. KAlSi3O8 is stable below ~25 GPa as the Hollandite-I phase (Holl-I; liebermannite; tetragonal) and up to 130 GPa and >3000 K as Hollandite-II (Holl-II; monoclinic)[5]. These crystal structures have large square tunnels formed by four double chains of edge-shared octahedra, accommodating large cations of K+ (with a similar size to Ar). Holl-II KAlSi3O8 is also thought to be a potential host mineral for transporting K and incompatible lithophile elements (LILEs) with large ionic radii into the lower mantle through subduction. Therefore, we investigated the incorporation of Ar and Xe into KAlSi3O8 using laser-heated diamond anvil cells (DAC) to explore whether noble gas storage in Holl-II under lower mantle conditions could account for Earth's missing Xe.
To avoid Na impurities and dissociation reactions in natural feldspars, Holl-I KAlSi3O8 was synthesized using a multi-anvil press from a powdered mixture of K2CO3, Al2O3, and SiO2 at 15 GPa and 1450ºC for two hours. The obtained sintered sample was ground into a fine powder, mixed with Pt powder to serve as a pressure standard and laser absorber, and then loaded into DACs. Ar or Xe was used as the pressure-transmitting medium, and one experiment was run with no pressure medium for comparison. High-pressure and temperature in-situ experiments were conducted at beamline 13-ID-D (GSECARS), Advanced Photon Source, Argonne National Laboratory, USA. X-ray diffraction (XRD) patterns and Raman spectra were obtained up to 80 GPa, 3000 K. Pressure was determined by ruby fluorescence, diamond Raman, and the equations of state of Pt and noble gases.
The XRD patterns show a phase transition from Holl-I to Holl-II at around 25 GPa, with a wide pressure range of their coexistence. There is no obvious difference in the compression curve of Holl-II regardless of pressure medium. Raman spectra of a single phase of Holl-II were first obtained at >75 GPa, in a good agreement with a previous study. Some new peaks were also found. Preliminary EPMA measurements of the recovered samples show a significant loss of K and formation of a new Si-rich phase in the heated areas, whereas unheated areas show the initial chemical composition. The solubilities of noble gases into Holl-II at 50 GPa was estimated using lattice strain modeling, resulting in a higher solubility for Xe than for Ar. Although no volume expansion from noble gas incorporation was observed experimentally, it is quite possible that K+ sites in Holl-II are substituted with Xe, whose radius became similar to that of K+ under high pressure. Further analyses are required to quantify the amounts of noble gases in the recovered samples.
References
[1] Marty, EPSL 313–314, 56–66 (2012)
[2] Shcheka and Keppler, Nature 490, 531–534 (2012)
[3] Rosa et al., EPSL 532, 116032 (2020)
[4] Zhu et al., Earth Sci. Rev. 224, 103872 (2022)
[5] Hirao et al., PEPI 166, 97–104 (2008)
This study focuses on KAlSi3O8, which forms one of the most abundant minerals in the continental crust, pottasium-rich feldspar. KAlSi3O8 is stable below ~25 GPa as the Hollandite-I phase (Holl-I; liebermannite; tetragonal) and up to 130 GPa and >3000 K as Hollandite-II (Holl-II; monoclinic)[5]. These crystal structures have large square tunnels formed by four double chains of edge-shared octahedra, accommodating large cations of K+ (with a similar size to Ar). Holl-II KAlSi3O8 is also thought to be a potential host mineral for transporting K and incompatible lithophile elements (LILEs) with large ionic radii into the lower mantle through subduction. Therefore, we investigated the incorporation of Ar and Xe into KAlSi3O8 using laser-heated diamond anvil cells (DAC) to explore whether noble gas storage in Holl-II under lower mantle conditions could account for Earth's missing Xe.
To avoid Na impurities and dissociation reactions in natural feldspars, Holl-I KAlSi3O8 was synthesized using a multi-anvil press from a powdered mixture of K2CO3, Al2O3, and SiO2 at 15 GPa and 1450ºC for two hours. The obtained sintered sample was ground into a fine powder, mixed with Pt powder to serve as a pressure standard and laser absorber, and then loaded into DACs. Ar or Xe was used as the pressure-transmitting medium, and one experiment was run with no pressure medium for comparison. High-pressure and temperature in-situ experiments were conducted at beamline 13-ID-D (GSECARS), Advanced Photon Source, Argonne National Laboratory, USA. X-ray diffraction (XRD) patterns and Raman spectra were obtained up to 80 GPa, 3000 K. Pressure was determined by ruby fluorescence, diamond Raman, and the equations of state of Pt and noble gases.
The XRD patterns show a phase transition from Holl-I to Holl-II at around 25 GPa, with a wide pressure range of their coexistence. There is no obvious difference in the compression curve of Holl-II regardless of pressure medium. Raman spectra of a single phase of Holl-II were first obtained at >75 GPa, in a good agreement with a previous study. Some new peaks were also found. Preliminary EPMA measurements of the recovered samples show a significant loss of K and formation of a new Si-rich phase in the heated areas, whereas unheated areas show the initial chemical composition. The solubilities of noble gases into Holl-II at 50 GPa was estimated using lattice strain modeling, resulting in a higher solubility for Xe than for Ar. Although no volume expansion from noble gas incorporation was observed experimentally, it is quite possible that K+ sites in Holl-II are substituted with Xe, whose radius became similar to that of K+ under high pressure. Further analyses are required to quantify the amounts of noble gases in the recovered samples.
References
[1] Marty, EPSL 313–314, 56–66 (2012)
[2] Shcheka and Keppler, Nature 490, 531–534 (2012)
[3] Rosa et al., EPSL 532, 116032 (2020)
[4] Zhu et al., Earth Sci. Rev. 224, 103872 (2022)
[5] Hirao et al., PEPI 166, 97–104 (2008)