THE ISOTOPIC COMPOSITION OF POSTSHIELD LAVAS FROM MAUNA-KEA VOLCANO, HAWAII

The postshield eruptive stage of Mauna Kea volcano, Hawaii, can be divided into an early basaltic substage, the Hamakua Volcanics, containing picrites, ankaramites, alkalic and tholeiitic basalt, and a hawaiite substage, the Laupahoehoe Volcanics, containing only hawaiites and rare mugearites. Cumulate gabbroic xenoliths in Laupahoehoe Volcanics have isotopic ratios similar to the Hamakua Volcanics, and these gabbros provide constraints on the crustal evolution of Mauna Kea lavas. Because of the small variation in Sr-87/Sr-86 (0.70335-0.70362), Nd-143/Nd-144 (0.51297-0.51308) and Pb-206/Pb-204 (18.306-18.440), lavas from both substages must contain relatively fixed proportions of depleted, enriched and primitive mantle components. In addition, there is Sr, Nd and Pb isotopic overlap between tholeiitic and alkalic Hamakua basalts. However, the steep Pb-207/Pb-204 vs. Pb-206/Pb-204 arrays of postshield lavas from Mauna Kea, West Maui and Haleakala volcanoes and the existence of rare samples with high Pb-207/Pb-204, up to 15.548, requires an unusual component in some Hawaiian lavas. This component is unlikely to be derived from sediments or MORB lithosphere, and it may be a minor plume component. Lavas erupted during the postshield stage of Mauna Kea volcano do not define a systematic temporal trend of varying Sr-87/Sr-86 and Nd-143/Nd-144. This result contrasts with the temporal trend defined by lavas from Haleakala Volcano and provides evidence for important differences between the origin and evolution of different Hawaiian volcanoes. However, the Laupahoehoe Volcanics trend to lower Pb-206/Pb-204 ratios than the Hamakua Volcanics. As inferred for other Hawaiian volcanoes, this trend reflects a larger proportion of a depleted component in the youngest lavas. Finally, postshield lavas from Mauna Kea have relatively high Ce/Pb (40 +/- 4 in 28 Mauna Kea lavas) when compared with most oceanic basalts, and this ratio may be more variable in oceanic island lavas than previously recognized.