Oxygen isotope ratios in olivine from the Hawaii Scientific Drilling Project

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 ~18o values are 4.6-5.4 %o, confirming previous observations that some Hawaiian lavas are derived from sources with ~18o values lower than typical upper mantle (~18oolivine"' 5.2±0.2 %o). The Mauna Kea-Mauna Loa transition marks a shift from ~18o 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 ~18o 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 87srt86sr (>0.7036) and lower E~ (<6.5), and with 206pbf204pb ratios less than -18.3. These relationships indicate that the ~ 0 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 ~18o values of olivine from other high 3He/4He islands, which are also comparable to the upper mantle average. The low ~18o 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 180-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.