The effect of Iron on the structure and density of silicate melts under extreme conditions

Seismic observations of ultra-low velocity zones (ULVZs) at the core-mantle boundary (~2900 km depth) suggest the presence of a dense silicate melt layer at the base of the mantle [1]. Such a layer is commonly interpreted as a remnant basal magma ocean, preserved after metal-silicate differentiation and partial crystallization of the early Earth’s mantle. The existence of a stable melt layer at these extreme conditions has important implications for the chemical stratification of the lowermost mantle, the evolution of mantle convection, and the long-term storage of incompatible elements and volatiles [2],[3]. Geodynamic models and geochemical proxies support the potential for melt retention at the core-mantle boundary, yet the stability of silicate melts remains debated due to their typically lower density relative to surrounding crystalline phases [4]. Resolving this requires quantitative constraints on melt density and structure under lower-mantle pressures.

Experimental data addressing the effect of iron on silicate melt properties at relevant pressures, however, remain sparse because of the challenges associated with probing weakly scattering amorphous materials at extreme conditions. To address this, we conducted in situ synchrotron X-ray diffraction experiments on Fe-bearing silicate glasses of composition (Mg_1-xFe_x)SiO₃ (x = 0, 0.1, 0.2, 0.4) up to 135 GPa at the ESRF ID27 beamline. High-energy X-rays (55 keV) combined with an optimized multichannel collimator system [5] allowed data acquisition over an extended Q range, enabling detailed pair distribution function analyses. Mass density indicates a pronounced effect of Fe content above ~20 GPa, while the atomic density remains nearly constant. This is consistent with Fe substituting for Mg in the silicate structure. These observations provide experimental constraints on iron-induced density variations in deep silicate melts, informing models of melt stability at the base of the mantle.

To investigate the effect of volatiles on silicate melt structure and density, a new beamtime is scheduled on ID27 beamline at ESRF in January 2026. Depending on the outcomes, results on CO₂- and H₂O-enriched silicate glasses in the (Ca,Al,Na,Mg)SiO system will be presented. These experiments aim to provide novel constraints on the structural and density changes induced by volatiles in silicate melts at lower-mantle pressures.

Combined, these studies advance our understanding of the physical and chemical behavior of silicate melts at core-mantle boundary (CMB) conditions, addressing fundamental questions about melt stability and help to model the coupled effects of CO₂ and Fe, Mg and Ca at CMB pressures on the silicate glass density. Such constraints are critical for linking geophysical observations, geochemical signatures, and geodynamic models of Earth’s deep interior, providing new insights into the formation and long-term evolution of the basal magma ocean and its role in the Earth’s volatile budget. 

 

References :

[1] Labrosse et al., 2007. Nature, 450(7171), 866 869

[2] Hirose et al., 2002. Physics Of The Earth And Planetary Interiors, 146(1-2), 249-260

[3] Garnero, E. J., et al. (2016). Nature Geoscience, 9(7), 481-489

[4] Dragulet and Stixrude. Geophysical Research Letters, 51(12)

[5] Mezouar et al., 2024. High Pressure Research, 44(3), 171–198