EVOLUTION OF MAUNA-KEA VOLCANO - INFERENCES FROM LAVA COMPOSITIONS RECOVERED IN THE HAWAII SCIENTIFIC DRILLING PROJECT

The lower 776 m of core recovered during the initial phase of the Hawa ii Scientific Drilling Project (HSDP) contains lavas erupted from Maun a Kea volcano. Tholeiitic and alkalic basalts, including an Fe-Ti rich flow, are intercalated in the upper 58 m of Mauna Kea lavas. Similar basaltic sections are subaerially exposed on the lower east flank of M auna Kea. The Fe-Ti rich lavas reflect large amounts of clinopyroxene, plagioclase, and olivine fractionation within the crust and upper man tle, but the range from tholeiitic to alkalic compositions reflects va riable extents of melting of a garnet-bearing source. Based on abundan ces of incompatible elements, the extent of melting for a basanitoid w as a factor of 2 less than that for nearly coeval tholeiitic lavas. Al l flow units in the lower 718 m of the HSDP core are tholeiitic lavas. Their variability in major element compositions reflect variable accu mulation of olivine. Incompatible element abundance ratios in these la vas reflect a complex temporal variation in extent of melting. Within the tholeiitic part of the core, lavas from 800 m to 950 m formed by t he largest extent of melting, whereas tholeiitic lavas from the bottom of the core and from just below the tholeiitic to alkalic transition formed by lower degrees of melting. Inferred melt compositions at 16% MgO show that the similar to 200 to 400 ka Mauna Kea lavas from the HS DP core and the <250 ka subaerial exposures define an inverse correlat ion between SiO2 and FeO contents. Based on experimental studies, this correlation is caused by differing pressures of melt segregation. Fur thermore, abundances of Nb and SiO2 are also inversely correlated in t hese calculated melts. In general, the younger lavas are relatively en riched in FeO and incompatible elements but are depleted in SiO2. Thes e trends are interpreted to reflect an overall trend of increasing pre ssure. of melt segregation and decreasing extent of melting with decre asing eruption age. There are, however, geochemical variations which i ndicate short-term reversals in this long-term trend. Previously, the geochemical trends accompanying the transition from tholeiitic to alka lic volcanism at Hawaiian volcanoes have been interpreted as reflectin g the effects of increasing distance from the plume axis. The long-ter m geochemical trends of tholeiitic lavas in the HSDP core also reflect migration of Mauna Kea away from the Hawaiian plume.