What SNC meteorites tell us about Martian magmatism

The SNC meteorites record relatively recent (less than b.y.) basaltic volcanism on a hydrous planet with a core. This much was known about Mars even before the Viking mission. The SNCs tell that the basalts were unusually low in Al2O3 and, despite overall similarities in major elements, apparently sampled two isotopically distinct regions of Mars: one is relatively depleted in incompatible elements (Nahklite source) with a U-Pb model age of 4.3 b.y. and is probably a portion of the upper Martian mantle, whereas the other is relatively enriched in incompatible elements (Shergottite source) with a 4.5 b.y. U-Pb age and is probably Martian crust assimulated into the mantle-derived Shergottite magma. The SNCs tell that the overall abundances of moderately volatile elements, such as alkalies, are higher on Mars than on earth. Given extensive volcanism and a depleted mantle, the present day absence of a thick atmosphere is more likely due to catastrophic removal of a primordial atmosphere than lack of degassing.