Evidence for pre-Noachian granitic rocks on Mars from quartz in meteorite NWA 7533

The surface of Mars has long been seen as a basaltic, monotonous world,
but observations in the past decade have revealed more petrological
diversity. Orbital and in situ rover investigations show that Mars developed
a silica-rich crust early in its history. This is supported by studies of
the Martian regolith breccia Northwest Africa (NWA) 7533 (and paired
meteorites). When and to what extent rocks on Mars differentiated, and
which geodynamical process could lead to this evolution, is still unclear.
Here we use petrology and in situ geochemical analyses to document the
presence of quartz in lithic clasts of NWA 7533. The clasts have a granitic
composition with a mineral assemblage dominated by quartz, potassium
feldspar and plagioclase. Such quartz-bearing clasts are the most evolved
silicic rocks yet recognized among differentiated Martian lithologies. These
clasts suggest the likely existence of pre-Noachian granitic rocks on Mars
that formed in the presence of water. In bulk composition they resemble the
oldest terrestrial rocks (Acasta gneisses, Canada) and also rocks from the
large Sudbury impact structure. Therefore, we suggest that the combined
action of hydrothermal activity and impact melting could have triggered the
formation of granitic rocks and evolved crust on early Mars and Earth.