Postcaldera mafic volcanism on Socorro Island, Mexico, an island located in the eastern Pacific Ocean on a segment of a mid-ocean ridge spreading center abandoned at ~3.5 Ma, dominantly comprises alkalic basalt, hawaiite, and mugearite with subordinate mildly alkalic-transitional basalt. Most major element data are consistent with differentiation of alkalic basalt to hawaiite and mugearite by up to 50% fractionation of plagioclase+clinopyroxene+olivine¿Fe-Ti oxides. High-field strength element abundances are consistent with this interpretation, and the narrow range in Zr/Nb suggests that parental magmas formed by similar degrees of partial melting of a relatively homogeneous mantle source. Sr and Nd isotopic ratios exhibit relatively narrow ranges (0.7031--0.7032, 0.5128--0.5130, respectively). Despite this, a subset of the alkalic basalts has negative Ce anomalies and abundances of P2O5, Ba, Y, and some rare earth elements in excess of those predicted by fractional crystallization models; these chemical features require open-system processes. The observed heterogeneities are perhaps best explained by assimilation of ocean crustal components by basalt in a shallow magma reservoir. Regional mantle heterogeneity is evident from elemental and isotopic data for mildly alkalic-transitional basalt from Socorro Island, mugearite from San Benedicto Island, and submarine basalts from near Socorro Island. Evidence presented here indicates that compositional variations in ocean island basalts can be introduced by ocean crustal contamination and underscores the importance of considering this source of chemical heterogeneity in basalts before characterizing associated mantle. ¿ American Geophysical Union 1995