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 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 (δ¹⁸O olivine ≈ 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 ɛ_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.