Basaltic glasses in the lower mantle: trace elements as markers of changes in local structure

Silicate melts play a key role in Earth’s evolution, influencing differentiation and planetary cooling. Understanding their formation and properties in the lower mantle is essential but remains directly directly inaccessible.

This work investigates structural changes in the coordination environment of strontium and yttrium in silicate glasses at high pressure. Laser-heated DAC experiments with X-ray absorption spectroscopy were performed at ID24, ESRF. Spectra were obtained for two simplified basaltic glass compositions of in the systems Albite-Diopside and Anorthite-Diopside. In the AbDi-glass, part of Si was replaced by Ge [1]. Both contained Sr and Y at various levels.

EXAFS analysis showed a two-stage pressure response for both elements: initial bond lengthening and coordination increase up to ~20 GPa, then bond shortening and densification. In both glasses, the change in the trend of the Y–-O and Sr–-O distances at ~20 GPa, coincides with the stabilization of higher coordination states (CN = 7 for Y, CN = 8 for Sr). Bond valence analysis of the determined bond lengths confirms the coordination transitions upon compression. The results are consistent with molecular dynamics simulations for Ca and Mg in basaltic melts [2] and experimental high-P data for Sr in aluminosilicate glass [1]. These coordination changes may affect crystal–melt partitioning at depth [3], e.g., during magma ocean crystallization or near the core–mantle boundary.

References:
[1] Krstulović et al. (2021), Chem. Geol. 560, 119980, https://doi.org/10.1016/j.chemgeo.2020.119980
[2] Karki et al. (2018), in Magmas Under Pressure, Elsevier, https://doi.org/10.1016/B978-0-12-811301-1.00016-2
[3] Ozawa et al. (2024), Sci. Adv. 10, eadp0021, https://doi.org/10.1126/sciadv.adp002