Compositions of the Earliest Basalt Crusts on Terrestrial Planets Produced through Magma Ocean Processes

A magma ocean model may be appropriate for both Mars and the Moon, as well as other terrestrial bodies. Magma ocean solidification may produce a wide range of cumulate mantle compositions, from iron-poor earliest cumulates through iron-rich later cumulates, each containing a different inventory of trace elements. These solid cumulates are gravitationally unstable due to iron enrichment in evolving magma ocean liquids, and are likely to overturn to gravitational stability. During overturn hot buoyant cumulates rise from depth and may melt through adiabatic decompression, producing the planet's earliest basaltic crust. Different initial planetary compositions, magma ocean depths, and planetary masses produce different earliest igneous crusts.

In addition to source production through magma ocean processing, planets may have undifferentiated silicate material that existed beneath any magma ocean, but which may participate in overturn and thus melt, or may be mobilized for melting later. If the primitive material participates in overturn, it will rise to a level of neutral buoyancy midway between the earliest-solidifying buoyant magma ocean cumulates and the latest-solidifying dense, iron-rich cumulates.

Models of Martian magma ocean solidification that include pressure, temperature, and compositional effects on density produce predictions about when primitive material would be involved in earliest igneous crust production. Different initial magma ocean depths will enable production of an earliest crust from two cumulate source region, from one cumulate source region, or from a cumulate source region and the primitive undifferentiated material. After the process of overturn is complete, the planet also has a differentiated cumulate mantle. This differentiated mantle can contain a wide range of source regions for the production of later magmatism.