A plagioclase-rich protocrust on Mars formed by magma ocean crystallization.

Accretion and early evolution of rocky planets involved high temperatures that produced global magma oceans. Their cooling, outgassing, and crystallization controlled primary crust formation, atmospheric composition, and mantle dynamics. Using new estimates of martian mantle composition and depth, we experimentally simulated the solidification of the entire magma ocean. Under the relatively oxidizing conditions of Mars, crystallization produced a mantle assemblage of olivine/wadsleyite, orthopyroxene, clinopyroxene, garnet, and spinel and generated in the final stages a >30-kilometer-thick, buoyant protocrust dominated by plagioclase and quartz. These results differ notably from previous models and are consistent with remote sensing evidence for ancient plagioclase-rich, low-density crustal rocks. Our results redefine the density structure and potential overturn dynamics of the early martian mantle, establishing a new framework for interpreting geochemical signatures in martian meteorites and for guiding analyses of igneous rocks from future Mars Sample Return missions.