IMPACT OF THE MARTIAN LITHOSPHERE ON MANTLE MELTING AND MAGMA TRANSPORT

As Mars does not have active plate tectonics, any magmas reaching the surface must transit through the planet’s crust and lithosphere. These layers are expected to grow with time as the planet cools and that process should also affect magma chemistry. The most abundant martian meteorites, the shergottites, represent some of Mars’s youngest igneous rocks and thus provide information on recent mantle melting, but they are difficult to explain in the context of the expected thick, immobile crust and lithosphere.

Compared to older alkali lavas measured on Mars’s surface, the shergottites have higher silica contents and are tholeiitic, leading many near-primary shergottite compositions to be multiply saturated with olivine and orthopyroxene at pressures from 1.0 to 1.5 GPa. That range is comparable to the estimated pressure at the base of Mars’s lithosphere in upwelling zones. Experiments and thermodynamic modeling using the pMELTS algorithm suggest this pressure range is key for shergottite formation. Two end-member scenarios are that these pressures represent either melt trapping and equilibration pressures or the average of pressures across melting zones. If these pressures represent equilibrium, then they reflect melts pooling near the lithosphere base and residence at this depth enables that equilibration. However, equilibrium between melts and upwelling mantle is atypical for terrestrial magmatism and other geochemical indices suggest contribution from higher pressure melts. Alternatively, multiple saturation pressures in terrestrial basalts typically reflect an average of melting pressures over a column undergoing fractional melting. If this were the case for the shergottites, it would require melting continue above the expected lower bound of the martian lithosphere and therefore some process on Mars would be required to enable continued melting above this estimated lithosphere base.

Evidence for these processes may be found in shergottite pyroxene minor element abundances and recently discovered mineral inclusions that suggest crystallization near multiple saturation pressures. Each of these end-member scenarios has implications for our interpretation of martian igneous processes and I will discuss these for the generation of the shergottites and earlier martian magmas.