THE THOLEIITE TO ALKALIC BASALT TRANSITION AT HALEAKALA VOLCANO, MAUI, HAWAII

Previous studies of alkalic lavas erupted during the waning growth stages (< 0.9 Ma to present) of Haleakala volcano identified systematic temporal changes in isotopic and incompatible element abundance ratios. These geochemical trends reflect a mantle mixing process with a systematic change in the proportions of mixing components. We studied lavas from a 250-m-thick stratigraphic sequence in Honomanu Gulch that includes the oldest (approximately 1.1 Ma) subaerial basalts exposed at Haleakaka. The lower 200 m of section is intercalated tholeiitic and alkalic basalt with similar isotopic (Sr, Nd, Pb) and incompatible element abundance ratios (e.g., Nb/La, La/Ce, La/Sr, Hf/Sm, Ti/Eu). These lava compositions are consistent with derivation of alkalic and tholeiitic basalt by partial melting of a compositionally homogeneous, clinopyroxene-rich, garnet lherzolite source. The intercalated tholeiitic and alkalic Honomanu lavas may reflect a process which tapped melts generated in different portions of a rising plume, and we infer that the tholeiitic lavas reflect a melting range of approximately 10% to 15%, while the intercalated alkalic lavas reflect a range of approximately 6.5% to 8% melting. However, within the uppermost 50 m of section, Sr-87/Sr-86 decreases from 0.70371 to 0.70328 as eruption age decreased from approximately 0.97 Ma to 0.78 Ma. We infer that as lava compositions changed from intercalated tholeiitic and alkalic lavas to only alkalic lavas at approximately 0.93 Ma, the mixing proportions of source components changed with a MORB-related mantle component becoming increasingly important as eruption age decreased.