M A T T E R A T E X T R E M E S

S C I E N T I F I C H I G H L I G H T S

1 6 H I G H L I G H T S 2 0 2 2 I

PRINCIPAL PUBLICATION AND AUTHORS

Solidus melting of pyrolite and bridgmanite: Implication for the thermochemical state of the Earth s interior, R. Pierru (a), L. Pison (a), A. Mathieu (a), E. Gardès (a,b), G. Garbarino (c), M. Mezouar (c), L. Hennet (d), D. Andrault (a), Earth Planet. Sci. Lett. 595, 117770 (2022); https:/doi.org/10.1016/j.epsl.2022.117770 (a) Université Clermont Auvergne, LMV, CNRS-OPGC-IRD, Clermont-Ferrand (France) (b) Normandie Université, ENSICAEN, CEA, CNRS, CIMAP, Caen (France) (c) ESRF (d) Université d Orléans, ICMN, CNRS, Orléans (France)

REFERENCES

[1] R. Giampaoli et al., High Pressure Res. 38, 250-269 (2018). [2] D. Andrault et al., Phys. Chem. Mineral. 47, 10 (2020).

Bridgmanite, the most abundant mineral on Earth and which adopts a perovskite-type structure, was investigated first. The end-member component MgSiO3 has the highest melting point of the mantle minerals, except for pure SiO2, which is even more refractory (Figure 3). The melting curve of MgSiO3 increases steeply with pressure to reach about 4400 K at 60 GPa. Such a steep solid-liquid Clapeyron slope is typical of the presence of a low-density silicate melt above the melting curve. At even higher pressures, the melting curve flattens significantly and presents a smooth evolution to ~5200 K for 135 GPa. The change of Clapeyron slope indicates the densification of the atomic structure in the melt around 60 GPa, which is most certainly due to the achievement of octahedral SiO6 units [2].

Melting of the true mantle composition was then investigated with a focus on the pyrolite composition. Melting of pyrolite occurs at much lower temperatures than bridgmanite, which is compatible with the presence of several elements (O, Si, Mg, Fe, Al, Ca, Na and K) in

Fig. 4: Melting curve (solidus) of various silicates relevant to the Earth s mantle. Melting curves are reported of SiO2, MgSiO3 and (Mg,Fe)(Si,Al)O3 bridgmanites, three possible mantle compositions (peridotite, pyrolite and chondritic-type) and subducted mid-ocean ridge basalts (MORB). Note the increasing curvature of the melting curve with the decreasing number of cations in the silicate composition.

its forming composition. Still, detecting its melting is highly challenging, in large part because the melting occurs progressively with an increasing fraction of melt between solidus and liquidus temperatures. In addition, some elements tend to diffuse along the temperature gradient that is established in the laser-heated samples. Fortunately, the onset of melting can be detected very quickly at the centre of the laser hot spot, thanks to the use of the very intense and micro-focused X-ray beam. The solidus of pyrolite presents a smooth evolution from about 2000 K to 4000 K with increasing mantle depth. This evolution suggests progressive changes of the atomic structure in the melt with increasing pressure (Figure 4). By comparison with the melting curve of bridgmanite, this smooth evolution suggests a progressive change of the atomic structure in the melt with increasing pressure.

Measurements on different types of samples pave the way for thermodynamic modelling of the melting diagram of the deep mantle. At the conditions of the core-mantle boundary, such modelling suggests that mantle melting occurs following a classical eutectic behaviour, with a eutectic melt composition significantly depleted in SiO2, compared to the composition of the average mantle. In addition, the melting curves suggest a core-mantle boundary temperature lower than 3950(200) K, which is slightly colder than previous estimations.

Crystallisation pathways for carbon- rich planetary cores

High-pressure experiments on alloys in the ternary Fe-Si-C were performed in a laser-heated diamond anvil cell and coupled with synchrotron X-ray diffraction. The results provide new insights into

the possible crystallisation pathways for planets accreting in oxygen-poor (reduced) environments that potentially segregate C-rich cores.

Investigating the properties of Fe-bearing alloys at high pressures and temperatures provides insights into the composition and processes taking place in planetary cores.