Volatile and Trace Element Storage in a Crystallizing Martian Magma Ocean

Abstract

Immediately following core formation on Mars, the planet underwent a magma ocean phase. Volatiles released from the magma ocean fostered a primitive atmosphere which modulated heat loss from the cooling planet through the greenhouse effect. The solidification and degassing of the magma ocean are therefore coupled. In this work, we investigate two important aspects of this evolution: (a) the dynamics of melt trapping at the freezing front of the residual mantle and (b) the oxidation state during crystallization. For crystallization rates applicable to the martian magma, compaction is inefficient, leading to large fractions of melt trapped together with the crystals accumulating in the residual mantle. The H₂O content of the martian residual mantle is strongly influenced by dynamic melt trapping. Following magma ocean crystallization, up to 55.4% of the initial H₂O in the magma ocean is sequestered in the residual mantle, with the rest outgassed to the surface. Dynamic melt trapping also limits variations in trace element concentrations and fractionations. Resulting variations in important isotopic parent/daughter ratios (Sm/Nd, Lu/Hf) cannot account for all of the isotopic diversity inferred for martian basalt source regions, hence requiring alternative mechanisms. The redox state of the magma ocean exerts a strong control on the total CO₂ content of the residual mantle and the time of crystallization. Under oxidizing conditions, the residual mantle stores 0.01% of the delivered CO₂ but under the most reducing conditions we examined, the residual mantle can sequester 80.4% in the form of trapped carbonated melt and graphite/diamond.