Petrogenesis of tholeiitic lavas from the submarine Hana Ridge, Haleakala Volcano, Hawaii

Hana Ridge, the longest submarine rift zone in the Hawaiian island chain, extending from Maui 140 km to the ESE, has a complex morphology compared with other Hawaiian rift zones. A total of 108 rock specimens have been collected from the submarine Hana Ridge by six submersible dives. All of the rocks (76 bulk rocks analyzed) are tholeiitic basalts or picrites. Their major element compositions, together with distinctively low Zr/Nb, Sr/Nb, and Ba/Nb, overlap those of Kilauea lavas. In contrast, the lavas forming the subaerial Honomanu shield are intermediate in composition between those of Kilauea and Mauna Loa. The compositional characteristics of the lavas imply that clinopyroxene and garnet were important residual phases during partial melting. The compositions of olivine and glass (formerly melt) inclusions imply that regardless of textural type (euhedral, subhedral-undeformed, deformed) olivine crystallized from host magmas. Using the most forsteritic olivine (Fo(90.6)) and partition coefficients K-DFe-Mg(ol-melt) and D-CaO(*ol-melt), the primary magma composition is constrained to have similar to16.7% MgO and similar to8.4 wt % CaO. Modeling calculations using MELTS show that olivine first crystallized at 1380-1390degreesC and 0.1-0.3 GPa, under slightly hydrous conditions (0.5-1 wt % water).