Isotope and trace element evidence for depleted lithosphere in the source of enriched Ko'olau basalts

We have measured the Hf and Nd isotopic compositions of 38 basalts from the Ko'olau drill hole, Hawai'i. The basalts show limited variations in both Hf-176/(177) Hf and Nd-143/Nd-144 (epsilon(Nd) varies from +4.2 to +7.3 and epsilon(Hf) from +8.0 to +12.3). Their correlated variation has an R-2 of 0.86. The data form an array with a slope of 1.2 on an epsilon(HF)-epsilon(Nd) isotope correlation diagram, while the slope of all Hawai'ian basalt data is 0.98. Both slopes are significantly shallower than that of the mantle array of 1.4 defined by ocean island basalts. Previous studies have shown that a shallow slope in Hf-Nd isotope space can be related to ancient pelagic sediments in the mantle source (Blichert-Toft et al. 1999; Salters and White 1998). However, the combined variations In Ko'olau basalts of Hf-Nd-Pb-Os isotopic compositions and trace element ratios, such as La/Nb, Th/La and Sr/ Nd, are not consistent with the simple addition of a sediment component to the mantle. We instead propose that the shallow slope on the Hf-Nd isotope correlation diagram for Ko'olau shield stage basalts can be better explained if the enriched endmember contains either an ancient oceanic lithosphere component or the high- Hf-176/(177) Hf component observed in the Salt Lake Crater (SLC) peridotite xenoliths (which also have a depleted lithosphere origin). Since Ko'olau basalts have high Os-187/Os-188 (0.135-0.160) and the SLC xenoliths have (OS)-O-187/(OS)-O-188 up to 0.13 (Lassiter et al. 2000) Os-isotopes are consistent with the latter being a component in the enriched Ko'olau source.