Ancient plutonic processes on Mars inferred from the detection of possible anorthositic terrains

Abstract

The mineralogical diversity preserved on ancient terrains of Mars provides insights into the planet’s early geological state and subsequent evolution. The martian crust is predominantly composed of mafic rocks with low silica contents^1,2,3,4,5,6, with the exception of a few localized volcanic sequences that indicate some compositional evolution towards compositions richer in silicate minerals². Anorthosite, which is dominated by the silicate mineral plagioclase, is rare in the Solar System. It is thought to require an evolved magmatic source in which lighter elements have been concentrated. Anorthosite has been observed previously only on Earth within localized continental plutons of intrusive igneous origin⁷, and more widely on the Moon⁸ where the anorthositic highland crust is thought to derive from crystallization of a primordial magma ocean^9,10. Using near-infrared spectral data obtained by the Mars Reconnaissance Orbiter, we report the detection of iron-bearing plagioclase-rich rocks at eight sites in the southern highlands of Mars with a spectral signature consistent with ferroan anorthosites. The paucity of detections suggests a localized plutonic origin similar to terrestrial anorthosites, although a lunar-like global anorthosite crust on early Mars cannot be entirely excluded. Our detections of anorthositic compositions at several locations on the martian surface suggest that magmatic processes that produce highly evolved melts were active on ancient Mars.