The plutonic rocks of the Samail ophiolite in the southeastern Oman Mountains formed in a spreading-ridge magma chamber that was repeatedly replenished by primitive magma. An average composition of the parent magma for the Samail ocean crust is calculated by mass balance of the igneous stratigraphy. The average parent magma was in equilibrium with most early-formed cumulus minerals from the ophiolite and is very similar to primitive mid-ocean ridge basalt. The magma is basaltic—in contrast to the picritic melts proposed as parents for some ophiolites. Although the average parent magma composition is basaltic, field evidence suggests that individual batches of magma that fed the magma chamber varied in composition because of mantle processes. The primary mantle melts were probably picritic, but owing to reaction during ascent through the upper mantle and mixing or density entrapment within an open-system crustal-level magma chamber, these picritic melts were probably not erupted on the seafloor.