Petrogenesis of Tholeiitic Lavas from the Submarine Hana Ridge, Haleakala Volcano, Hawaii
ZHONG-YUAN REN 1 *, EIICHI TAKAHASHI 1 , YUJI ORIHASHI 2 AND KEVIN T. M. JOHNSON 3

1 EARTH AND PLANETARY SCIENCES, TOKYO INSTITUTE OF TECHNOLOGY, 2-12-1 OOKAYAMA, MEGUROKU, 152-8551, JAPAN
2 EARTHQUAKE RESEARCH INSTITUTE, THE UNIVERSITY OF TOKYO, 1-1 YAYOI, BUNKYO-KU, TOKYO 113-0032, JAPAN
3 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF HAWAII, HONOLULU, HI 96822, USA

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, subhedralÐundeformed, deformed) olivine crystallized from host magmas. Using the most forsteritic olivine (Fo90 6 ) and partition coefficients K ol melt DFe Mg and Dol melt CaO , the primary magma composition is constrained to have 16 7% MgO and 8 4wt% CaO. Modeling calculations using MELTS show that olivine first crystallized at 1380Ð1390C and 0 1Ð0 3 GPa, under slightly hydrous conditions (0 5Ð1 wt % water). KEY WORDS: Hawaii; Haleakala volcano; submarine Hana Ridge; petrogenesis; tholeiitic lavas.