MAGMA GENERATION IN MARTIAN MANTLE

The numerical modeling of the partial melting of Martian primitive man tle composition [1] yields liquid compositions significantly more alum inous as compared to shergottites. This cannot be explained by shergot tites being orthocumulates, because this hypothesis does not account f or the drastic reduction of the Al/Ti ratio from the chondritic value observed in SNC meteorites. The generation of shergottite-like magmas deep in the stability field of garnet in the subliquidus region of pri mitive mantle composition conflicts with the relatively high silica co ntents in shergottites requiring low pressures for their generation. T he preferred model includes loss of garnet from the magma generating p art of mantle early in the history of Mars (perhaps at the stage of ma gma ocean) and formation of shergottite-like parent magmas from this A l-deficient material at relatively low pressures.