KAUA’I VS. NI’IHAU, NORTHWEST HAWAIIAN ISLANDS: A COMPARISON OF VOLCANIC AND GEOCHEMICAL EVOLUTION

The neighboring islands of Kaua’i and Ni’ihau display both similarities and striking differences as the two volcanoes progressed through shield, postshield, and rejuvenated stages of activity. Lavas of the two shields have incompatible element and isotopic compositions that are mostly Kea-like, except eastern Kaua’i that is Loa-like due to melting along the plume edge. Both islands include rare postshield lavas that overlap in age with shield basalts, but compositions are very different. Postshield rocks from Kaua’i are alkalic basalt to mugearite that require more of a depleted mantle component compared to shield basalts. In contrast, postshield lavas from Ni’ihau include (a) isotopically shield-like transitional basalts, and (b) unusual basanites that resemble Hawaiian rejuvenated-stage lavas in trace element composition but have shield-like isotopic compositions. Rejuvenated stage lavas cover much of both Ni’ihau and Kaua’i. The Koloa Volcanics on Kaua’i are MgO-rich alkalic basalts to melilitites, whereas the Kiekie Basalts on Ni’ihau are almost exclusively alkalic basalt (MgO 12.5-6%). Sr and Ba excesses and Pb isotope ratios in Koloa rocks vary roughly with Th concentration, consistent with contributions from two mantle components: the proposed Depleted Rejuvenated Component (DRC) with high Sr/Ce dominates at high % melting, and a mix of Loihi and Kea components at low % melting. In contrast, Kiekie alkalic basalts are all high-% melts that plot near the DRC end of a Ka’ula-Koloa mixing array in Pb isotope diagrams. The Sr-rich component facilitates melting to relatively high degrees. Kaua’i is characterized by a near-continuous transition from shield to postshield to rejuvenated composition volcanic products with significant changes in isotopic composition with time, whereas Ni’ihau has a large time gap between shield/postshield and rejuvenated stage activity and exhibits a restricted range in isotopic composition through the islands lifespan.