Two regions of the Mid-Atlantic Ridge corresponding to different stages of tectonic evolution have been studied. Isotopic compositions of basalts and ores from hydrothermal edifices of these regions were compared. Compositions of quenched glasses (MgO 7-9%) indicate a low degree of primary magma differentiation, which operated within a temperature interval of 1175-1250°C. According to these data, the shallowest depth of small intermediate magma chambers beneath the slow-spreading zones was estimated at 10-12 km.