Site 648

Abstract

Site 648 provided a unique view of the internal structure of an axial rift valley volcano. The uppermost 30 m consists of pillow la vas, which built the volcanic edifice. The lower lithologic units con sist of a vesicular basalt underlain by a massive holocrystalline ba salt. These lower units probably represent massive flows that ponded inside Serocki Volcano.Site 648 basalts are moderately evolved normal mid-ocean ridge basalt, saturated with olivine and plagioclase, which are geochemically similar to basalts from the 22°-25°N region. All basalts recov ered at Site 648 are fresh; incipient alteration occurs along fractures. Alteration minerals are smectite and amorphous iron-hydroxyoxide. BACKGROUND AND OBJECTIVES