GEOCHEMISTRY OF MESOZOIC PACIFIC MIDOCEAN RIDGE BASALT - CONSTRAINTS ON MELT GENERATION AND THE EVOLUTION OF THE PACIFIC UPPER-MANTLE

We present major and trace element and Sr-Nd-Pb isotope results on Mes ozoic (130-151 Ma) mid-ocean ridge basalt (MORE) recovered from five D eep Sea Drilling Project sites in the central and northwestern Pacific Ocean. Seawater alteration is responsible for much of the major eleme nt variability in these basalts, but magmatic variations are still dis cernible. Major element modeling of the least altered samples indicate s that the basalts were generated by degrees and pressures of melting identical to those of modern Pacific MORE, and this, in addition to th e similarity in spreading rates between the East Pacific Rise and Meso zoic Pacific ridges, suggests that the style of mantle upwelling and m elting at spreading centers is spreading rate dependent. In general, t he five Mesozoic MORE units, like Jurassic Pacific MORE from Ocean Dri lling Program Site 801, are depleted in highly incompatible elements r elative to average N-MORB and display a wide range in Nd and Pb isotop ic ratios (epsilon(Nd)(T) = 8.4-11.6; Pb-206/Pb-204(i) = 17.9-18.6) bu t have a low and uniform Sr isotopic composition (Sr-87/Sr-86(i) =0.70 23-0.7026). This isotopic variation can be explained by mixing a deple ted mantle source with small amounts of recycled oceanic crust (HIMU). In contrast to the older MORE, mid-Cretaceous Pacific MORE (approxima te to 115-100 Ma) are moderately to strongly enriched in highly incomp atible elements with an ''enriched mantle'' isotopic affinity. The shi ft in MORE composition coincides with the onset of effusive mid-Cretac eous intraplate volcanism in the Pacific and reflects widespread conta mination of the Pacific upper mantle with materials derived from the p lumes or plume heads responsible for mid-Cretaceous oceanic plateaus a nd seamount chains.