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 87Sr 86Sr (0.70335-0.70362), 143Nd 144Nd (0.51297-0.51308) and 206Pb 204Pb (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 207Pb 204Pb vs. 206Pb 204Pb arrays of postshield lavas from Mauna Kea, West Maui and Haleakala volcanoes and the existence of rare samples with high 207Pb 204Pb 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 87Sr 86Sr and 143Nd 144Nd. 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 206Pb 204Pb 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.
Bibliographical noteFunding Information:
We thank S. R. Hart and G. Triton for access to mass spectrometers and clean labs and for their interest in this project, K. Burrhus for help with maintenance of mass spectrometers at MIT and R. Fodor for kindly providing xenoliths IB and 15A. We thank two anonymous reviewers for their comments, Research was supported by NSF grants EAR 8419723 and EAR 875809 (F.F.) and EAR 86-07163 (G.T.).
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science