GEOCHEMICAL VARIATIONS IN LAVAS FROM KAHOOLAWE VOLCANO, HAWAII - EVIDENCE FOR OPEN SYSTEM EVOLUTION OF PLUME-DERIVED MAGMAS

The Kahoolawe shield volcano produced precaldera and caldera-filling t holeiites and mildly alkalic post-caldera lavas that petrographically and compositionally resemble such lavas from other Hawaiian shield vol canoes. However, Kahoolawe tholeiites display wide ranges in incompati ble trace element ratios (e.g., Nb/Th = 9-24, Th/Ta = 0.6-1.3), Sr-87/ Sr-86 (0.70379-0.70440), Nd-143/Nd-144 (0.51273-0.51298), and Pb-206/P b-204 (17.92-18.37). The isotopic variation exceeds that at any other Hawaiian shield volcano, and spans about half the range for all Hawaii an tholeiites. Quasi-cyclic temporal evolution of Kahoolawe tholeiites is consistent with combined fractional crystallization and periodic r echarge by primitive magmas. Ratios of highly incompatible trace eleme nts and Sr, Nd, and Pb isotopic ratios form coherent sub-trends that r eflect recurrent interactions between variably evolved magmas and two other mantle components whose compositions are constrained by intersec tions between these trends. The most MgO-rich Kahoolawe tholeiites are partial melts of a high Nb/Th (approximately 23.5) ascending plume, p ossibly comprising ancient subducted oceanic lithosphere. Slightly evo lved tholeiites experienced combined crystal fractionation and assimil ation (AFC) of material derived from a distinct-reservoir (Nb/Th appro ximately 9) or asthenospheric derivation. The most evolved tholeiites display compositional shifts toward a third component, having mid ocea n ridge basalt-like isotopic ratios but enriched OIB-like trace elemen t ratios, representing part of the lithospheric mantle (or melts there of). Periodic recurrence of all three magma variants suggests that eru ptions may have tapped coeval reservoirs distributed over a large dept h range. Kahoolawe provides new evidence concerning the nature of the Hawaiian plume, the distribution of compositional heterogeneities in t he suboceanic mantle, and the processes by which Hawaiian tholeiites f orm and evolve.