We present major and trace element and Sr-Nd-Pb isotope results on Mesozoic (130--151 Ma) mid-ocean ridge basalt (MORB) recovered from five Deep Sea Drilling Project sites in the central and northwestern Pacific Ocean. Seawater alteration is responsible for much of the major element variability in these basalts, but magmatic variations are still discernible. Major element modeling of the least altered samples indicates that the basalts were generated by degrees and pressures of melting identical to those of modern Pacific MORB, and this, in addition to the similarity in spreading rates between the East Pacific Rise and Mesozoic Pacific ridges, suggests that the style of mantle upwelling and melting at spreading centers is spreading rate dependent. In general, the five Mesozoic MORB units, like Jurassic Pacific MORB from Ocean Drilling Program Site 801, are depleted in highly incompatible elements relative to average N-MORB and display a wide range in Nd and Pb isotopic ratios (&egr;Nd(T)=8.4--6-11.6; 206Pb/204Pbi=17.9--18.6). but have a low and uniform Sr isotopic composition (87Sr/86Sri=0.7023--0.7026). This isotopic variation can be explained by mixing a depleted mantle source with small amounts of recycled oceanic crust (HIMU). In contrast to the older MORB, mid-Cretaceous Pacific MORB (≈115--100 Ma) are moderately to strongly enriched in highly incompatible elements with an enriched mantle isotopic affinity. The shift in MORB composition coincides with the onset of effusive mid-Cretaceous intraplate volcanism in the Pacific and reflects widespread contamination of the Pacific upper mantle with materials derived from the plumes or plume heads responsible for mid-Cretaceous oceanic plateaus and seamount chains.¿ 1997 American Geophysical Union