Possible formation of ancient crust on Mars through magma ocean processes

摘要

Models for Martian magma oceans of varying depths predict that decompression mantle melting, perhaps forming Mars' earliest crust, could occur during gravitationally driven solid-state overturn of cumulates following magma ocean solidification. When hot cumulates rise from depth during solid-state overturn, some regions melt adiabatically, producing basaltic to andesitic magmas. The resulting crust would be formed at between 30 and 50 Myr after planetary accretion, when magma ocean solidification and subsequent overturn are complete. Models of magma oceans deeper than ～1550 km consistently produce two separate magmatic source regions during overturn that create compositionally distinct magmas, consistent with both major and trace element data for SNC meteorites and the Martian crust. In a partial magma ocean between ～1550 and ～1250 km (～15 GPa) the only early magma produced is from a shallow pyroxene + olivine source; but if the magma ocean were less than ～1150 km (～14 GPa) deep, the underlying (undifferentiated or minimally differentiated) mantle rises sufficiently during overturn that it melts adiabatically and produces an early magma. Magma ocean models therefore produce specific predictions for the volumes and compositions of the most ancient crust produced by a range of initial magma ocean depths. The predicted crustal compositions and volumes for a whole mantle magma ocean are consistent with observations of Mars today.