Oxygen isotope ratios of olivine in 23 tholeiites from the Hawaii Scientific Drilling Project (HSDP) core (15 from Mauna Kea, 8 from Mauna Loa) and three samples of outcropping subaerial or dredged submarine Mauna Kea lavas have been measured by laser fluorination. The Δ18O values are 4.6--5.4?, confirming previous observations that some Hawaiian lavas are derived from sources with Δ18O values lower than typical upper mantle (Δ18Oolivine≈5.2¿0.2?). The Mauna Kea-Mauna Loa transition marks a shift from Δ18O values lower than the mantle average in Mauna Kea olivines (~4.8) to more typical mantle values in Mauna Loa olivines. Lavas containing olivines with Δ18O values similar to the typical upper mantle are associated with more ''primitive'' or less depleted radiogenic isotope characteristics; i.e., with higher 3He/4He (>13 Ra), higher 87Sr/86Sr (>0.7036) and lower &egr;Nd (<6.5), and with 206Pb/204Pb ratios less than ~18.3. These relationships indicate that the Δ18O values of the relatively enriched source components of the Hawaiian plume sampled by Mauna Loa lavas are comparable to (or greater than) the mantle average. This conclusion is supported by Δ18O values of olivine from other high 3He/4He islands, which are also comparable to the upper mantle average. The low Δ18O values in Hawaiian lavas are derived from a source having more MORB-like, or depleted, He, Nd, and Sr isotope ratios, but more radiogenic Pb than is seen in the Mauna Loa lavas Assimilation of 18O-depleted lower oceanic crust from the underlying Pacific crust by hot, MgO-rich parental magmas or melting of older, recycled oceanic crust entrained in the Hawaiian plume are both possible sources of this 18O-depleted, MORB-like component in Hawaiian magmas. ¿ American Geophysical Union 1996