Source characteristics derived from very incompatible trace elements in Mauna Loa and Mauna Kea basalts, Hawaii Scientific Drilling Project

We report trace element analyses by isotope dilution spark source mass spectrometry of selected Hawaii Scientific Drilling Project samples for Mauna Loa and Mauna Kea basalts and picrites. The trace element abundances have been corrected along olivine control lines to an arbitrarily assumed, common MgO content of 12.0%. The corrected patterns all show remarkable similarities. These include negative Th and U anomalies with primitive-mantle-normalized ratios of(Th/Ba)(n) = 0.60 +/- 0.12 and 0.50 +/- 0.06 and (Th/La)(n) = 0.50 +/- 0.04 and 0.52 +/- 0.04 for Mauna Loa and Mauna Kea, respectively. These results are similar to those previously found for both Mauna Loa and Kilauea lavas [Hemond et al., 1994b; Hofmann et al., 1993], and they show no significant differences between tholeiites and alkali basalts from Mauna Kea. Other trace element ratios are more typical of other ocean island basalts and include (Nb/Th)(n) = 2.13 +/- 0.08 and 2.13 +/- 0.14 and (Nb/U)(n) = 1.86 +/- 0.20 and 1.75 +/- 0.25 for the two volcanoes, respectively. Six Hawaiian volcanoes for which extensive sets of high-quality Ba-Th-U-La data exist (Mauna Kea, Mauna Loa, Kilauea, Loihi, Haleakala, and Koolau) are all distinguished by a characteristic depletion of Th and U relative to Ba and La. This distinctive, nonprimitive ''fingerprint'' is found both in tholeiites and alkali basalts, and it is clearly a property of the present-day Hawaiian source rather than the product of fractionation during (or after) partial melting. It also occurs on Iceland but is found neither in typical depleted-mantle (''N-type'' mid-ocean ridge basalt (MORE) source) compositions nor in typical representatives of EM1, EM2, and HIMU-type oceanic island basalts as defined by Zindler ann Hart [1986]. Thus, contrary to the still frequently iterated opinion that Hawaiian basalts are derived from mixtures of primitive and depleted mantle sources [McKenzie ann O'Nions, 1995, 1991], the Hawaiian plume must contain strongly fractionated mantle material, which is quite unlike primitive mantle or the source or residue of MORE (and therefore probably also unlike the oceanic lithospheric mantle). We speculate that the Hawaiian source contains significant amounts of recycled oceanic gabbros.