Investigating Martian mantle melting: Insights into Shergottite origins, thermal evolution, and potential melting under the Tharsis region

The InSight Mars lander, which is NASA's 12th mission under the Discovery program, was dedicated to studying the deep interior of the planet. The lander successfully landed in November 2018 [1] and since the beginning of winter 2019, the SEIS broadband seismometer has continuously recorded the seismic activity of the planet until December 2022 [2-3]. The results obtained during the first two years of the nominal mission have enabled a single-station seismic analysis of the interior of Mars, covering the subsurface and upper crust [4], placing constraints on discontinuities within the crust and the depth of the crust-mantle boundary [5]. However, the ability to fully exploit the seismic records from the SEIS seismometer to produce accurate compositional, structural, and dynamical models of Mars depends critically on knowledge of the melting properties, density, viscosity, and sound velocity of the relevant minerals, liquids, and mineralogical Martian assemblages under appropriate thermodynamic conditions.

Here, we present an experimental investigation into the melting behavior of a simplified Martian mantle composition under high pressures and using in situ methods, extending up to 20 GPa. Our investigation reveals that the solidus of the Martian mantle is lower than those reported in the literature, primarily attributed to the presence of Fe³⁺ within the FMQ (Fayalite-Magnetite-Quartz) buffer. The presence of Fe³⁺ could facilitate the incorporation of water, thereby potentially lowering the melting point. This study highlights the intriguing stability of magnetite and garnet within a narrow pressure range of 3-4 GPa. This phenomenon holds significant implications for understanding the mineralogical composition of Mars' mantle and its geological processes. The melt produce at this pressure is identifies a probable or similar source for some basaltic shergottites characterized by a superchondritic CaO/Al₂O₃ ratio and low Al₂O₃ content, offering insights into the origins of Martian lavas. Our results also contribute to a new paradigm in understanding the thermal evolution of the Martian mantle. We propose a thermal model wherein crustal extraction occurs progressively over an extended duration spanning approximately 1 billion years. This model predicts a present-day mantle temperature of 1533 K and a corresponding lid thickness of 333 km. Finally, this study raises intriguing questions about the potential for ongoing partial melting beneath the Tharsis region on Mars. While the possibility of such activity persists, it is likely confined to very low melt fractions (<5%), underscoring the dynamic nature of Martian geological processes and the need for further exploration and analysis.

In the other hand, some efforts were dedicated to designing efficient multi-anvil assemblies capable of withstanding high pressures and temperatures while limiting the thermal gradient, to enable density and viscosity measurements of iron-rich ultramafic Martian melts but also ultrasonic wave propagation in partially molten sample representing the chemistry of the Martian mantle. Some preliminary results will be presented.

 

[1] Banerdt et al. Nature Geosci. 13, no. 3, 183–189. (2020).

[2] Giardini et al., Nature Geosci. 13, no. 3, 205–212. (2020).

[3] Horleston et al., The Seismic Record. 2(2), 88–99, (2022).

[4] Lognonné et al., Nature Geosci. 13, no. 3, 213–220. (2020).

[5] Knapmeyer-Endrun, B. et al. Science 373, (2021).