Insights into the Martian Interior: Geochemical Constraints on Mantle Dynamics and Magma Source Compositions

Recent remote sensing studies of Mars have revealed an exceptionally large (~4,000 km diameter) regional domal uplift in the Eastern hemisphere near Elysium Planitia, which is hypothesized to be supported by an actively upwelling giant mantle plume. Given its size, that plume head appears to be nearly three times larger than the Afar superplume on Earth, despite Mars' small size (i.e., Mars’ diameter is smaller than Earth’s core). The Elysium dome is intersected by a rift zone through which very young lavas (~2 Myrs to ~60 Kyrs old) erupted in large volumes and traveled long distances, indicating that the dome is an active geodynamic feature. Another recent discovery about Mars, based on data from NASA’s InSight lander, is an exceptionally thick (~400 km) Mantle Transition Zone (MTZ) located 1100 km below the surface, in direct contact with the core at a depth of ~1500 km. Therefore, Mars likely lacks a refractory and dense lower mantle, unlike Earth. This suggests that the 400 km thick Martian MTZ is the only zone from which mantle plumes can originate.

Along with majorite and pyroxenes, the MTZ contains wadsleyite and ringwoodite (i.e., high-pressure polymorphs of olivine), which have unique crystallographic and compositional features because their crystal structures can potentially trap water up to 2 to 3 wt.% and halogens, as well as some noble gases (e.g., neon). Although present in small amounts, these volatile elements may impart unusual flow properties to the MTZ by significantly reducing its viscosity and density, promoting upwelling. Once a part of the Martian MTZ begins to upwell, it is theoretically subjected to mineral phase transformations: ringwoodite and wadsleyite convert into wet olivine at depths shallower than about 1000 km, and wet olivine transforms into two hydrous minerals—amphibole and phlogopite—which are stable at pressures shallower than approximately 300 km in Mars.

Petrological evaluation of meteorite and Rover data compiled from the literature in this study indicates the presence of amphibole and phlogopite in the source of nearly half of Martian lavas, thereby confirming theoretical considerations presented above. Results from petrological melting models in this study indicate that primitive Martian lavas may have formed through the mixing of magmas with contrasting compositions from two sources: (i) a depleted mantle, possibly representing plume material from the MTZ, and (ii) a metasomatized lithosphere highly enriched in incompatible elements. Both sources contain hydrous minerals such as phlogopite and amphibole, as well as anhydrous minerals like olivine, pyroxenes, garnet, and spinel. These findings suggest the volatile-rich nature of this small planet's mantle. The higher halogen levels in Martian lavas relative to terrestrial lavas support this interpretation. In summary, the rheological, mineralogical, and compositional characteristics of the Martian mantle explain why plumes rising within Mars’ mantle are rich in volatiles and why they can grow much larger than those on Earth, disproportionate to Mars’ size. Based on these findings, this study proposes that Martian mega-mantle plumes may be low-viscosity, hydrous upwellings originating from its MTZ, driven by heat from the underlying core, which increases their fluidity.