Solidus melting of pyrolite and bridgmanite: Implication for the thermochemical state of the Earth's interior - Archive ouverte HAL Access content directly
Journal Articles Earth and Planetary Science Letters Year : 2022

Solidus melting of pyrolite and bridgmanite: Implication for the thermochemical state of the Earth's interior

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Abstract

Melting properties of the deep mantle remain controversial due to experimental difficulties; e.g., reports of solidus temperatures of mantle-relevant compositions span over ∼700 K at 2000 km depth. This situation limits our understanding of the thermochemical state of the Earth's interior. Using the laser heated diamond anvil cell (LH-DAC), we performed new experimental determination of the solidus profile of ultra-dry pyrolite and the solidus of two compositions of (Mg,Fe)(Si,Al)O3 bridgmanite (Bg). Melting was detected (i) from -the correlation between laser power and sample temperature, -changes of sample texture and -the level of visible light absorption, for all samples, (ii) using X-ray diffraction, for the MgSiO3 composition and (iii) after scanning electron microscope observations, for selected Fe-bearing samples. Special care was given to using ultra-dry experimental chambers and to determination of sample temperature. In particular, we discuss the wavelength-dependent thermal emission of silicate samples, which lowers the solidus by 100 to 300 K, compared to the grey-body assumption. The solidus of MgSiO3-Bg is in good agreement with previous reports using ab initio calculations and shock wave experiments. We observe a net decrease in the solid-liquid Clapeyron slope at 60(3) GPa and 4400(200) K, which can be related to rapid pressure-induced coordination change of Si in the melt. (Mg0.955,Fe0.045)(Si0.993,Al0.007)O3 Bg melts 600–800 K lower than MgSiO3-Bg. Its solidus evolves smoothly with pressure, suggesting progressive Si coordination change in the melt. In the pressure range investigated (24–135 GPa) Clapeyron slopes suggest rapid decrease of the volume of fusion, from 14 to 2% for MgSiO3 and from 9 to 3% for (Fe,Al)-bearing Bg, assuming congruent melting. By comparing the solidii of various silicates, it appears that the higher the number of cations, the less pronounced is the curvature of the solidus. This observation suggests that the relatively ordered structure of simple liquid compositions with a limited number of distinct network-modifying cations frustrates the coexistence of tetrahedrally and octahedrally coordinated Si polyhedral. The solidus of pyrolite presents a smooth evolution from 2200(100) K to 3950(200) K in the same pressure interval. This is very similar to our previous work on chondritic-type mantle. The new solidus is 200–300 K lower than that of KLB-1 peridotite, which can be related to more incompatible elements in pyrolite. It remains problematic that our solidus plots several 100 K higher than other recent measurements performed on pyrolite; we discuss the possibility of a higher water content in previous samples, compared to our experiments. Assuming a dry lowermost mantle, our results imply a core-mantle boundary temperature lower than 3950(200) K. Modeling the melting diagram at the core-mantle boundary suggests a pseudo-eutectic melt significantly depleted in SiO2, compared to the composition of the mean mantle.
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hal-03759424 , version 1 (21-11-2022)

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Rémy Pierru, Laure Pison, Antoine Mathieu, Emmanuel Gardés, Gaston Garbarino, et al.. Solidus melting of pyrolite and bridgmanite: Implication for the thermochemical state of the Earth's interior. Earth and Planetary Science Letters, 2022, 595, pp.117770. ⟨10.1016/j.epsl.2022.117770⟩. ⟨hal-03759424⟩
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