Depth of alkalic magma reservoirs below Kolekole cinder cone, Southwest rift zone, East Maui, Hawaii

Abstract

The Kolekole cinder cone is located on the southwest rift near the summit area of East Maui, adjacent to the Haleakala Crater. The erupted lavas consist of ankaramitic picro-basalts and basanites. The ankaramites contain 26 to 33 modal percent olivine and clinopyroxene phenocrysts. Athough the cores of the clinopyroxene phenocrysts are not in equilibrium with the bulk rock compositions, recovered matrix compositions through image analysis indicate that the rims of the clinopyroxene phenocrysts and plagioclase microphenocrysts are in equilibrium with the matrix in many samples. Equilibrium pressures calculated from clinopyroxene rim–liquid equilibrium, and from equilibrium compositions of multiple phase-saturated liquids, yield values between 4.4 and 11.2 kb. In addition, barometry based on clinopyroxene composition indicates that the cores of the clinopyroxene phenocrysts crystallized at approximately 2.5 kb higher pressures than the rims. Therefore, the alkalic magmas of Kolekole resided in reservoirs that extended well below the crust–mantle boundary. The low Mg-content of the calculated matrix compositions indicates that extensive high pressure fractional crystallization of parental magmas occurred in the lower crust and upper mantle magma chambers.