Depth of Post-Shield Alkalic Reservoir, East Maui, Hawaii

Post-shield alkalic lavas account for about 1% of the lavas on Hawaiian Islands. In eastern Maui, these lavas erupted on top of the voluminous shield-stage tholeiitic lavas as the island of Maui migrated northwestward with the Pacific plate beyond the Hawaiian plume axis. The alkalic lavas in East Maui belong to an older Kula and a younger Hana series. The latter erupted predominantly from vents near the Haleakala summit and along the southwest rift zone forming a series of cinder cones. Alkalic lava from Kolekole cinder cone on the southwest rift near the Haleakala crater have transitional compositions between Kula and Hana lavas. They predominantly consist of ankaramitic picro-basalts with reverse- or cyclic-zoned clinopyroxene and olivine phenocrysts that indicate magma mixing during formation. Calculated pressures of equilibrium between phenocryst-rims and their host lava corresponding with the minimum depths of the alkalic magma reservoirs are between 4.4 and 11.2 kb. The alkalic magma reservoirs at Kolekole may thus extend deep into the spinel lherzolite field of the upper mantle.