Composition of the Martian Crust and Its Relationship to the Martian Mantle: A Post-InSight View

Abstract

Concerted exploration and a ten-fold increase in Martian meteorites reveal a mostly ancient (>4 Ga), lithologically diverse mafic crust (igneous SiO₂ ranging from ∼40%–70%), commonly exhibiting alkaline affinities and with felsic compositions restricted to ancient crust. Most crustal growth (∼80%) took place during the pre-Noachian, resulting from magma ocean processes. Later crustal additions continued at diminishing rates to the latest Amazonian (<200 Ma). Data from the first fully operational seismometer (InSight mission) indicate the crust is 49 $\pm$ 7 km thick on average (∼4 $\pm$ 1% of the primitive mantle), with an intracrustal discontinuity, of possible global extent, at about 20–30 km depth. InSight results also suggest a ∼150 km-thick molten silicate layer (MSL) at the base of the mantle, a possible remnant of the early magma ocean. The MSL represents a distinctive, incompatible (including heat producing) element-enriched geochemical reservoir, comparable in physical size to the crust. A model for Martian crustal composition indicates a mildly incompatible and heat-producing element-enriched basaltic composition (e.g., SiO₂ = 48%, K₂O = 0.5%, La_N/Yb_N ∼ 1.8) that becomes less incompatible element-enriched over geological time. The petrological nature of the crust likely varies in a complex way with depth that may explain the intracrustal seismic discontinuity. The crust contains ∼45 $\pm$ 10% of the most incompatible elements in the primitive mantle and this composition is consistent with a mass balance model among crust, MSL, “depleted” mantle and primitive mantle. A geodynamic model for Mars can be constructed that is consistent with geophysical and geochemical constraints currently available.