HR: 0800h
AN: V31C-1455    [Abstracts]
TI: Pyroclastic Eruptions on the Northern East Pacific Rise
AU: * Clague, D A
EM: clague@mbari.org
AF: Monterey Bay Aquarium Research Monterey Bay Aquarium Research Institute, 7700 Sanholdt Road, Moss Landing, CA 95039 United States
AU: Davis, A S
EM: davisa@mbari.org
AF: Monterey Bay Aquarium Research Monterey Bay Aquarium Research Institute, 7700 Sanholdt Road, Moss Landing, CA 95039 United States
AU: Paduan, J B
EM: paje@mbari.org
AF: Monterey Bay Aquarium Research Monterey Bay Aquarium Research Institute, 7700 Sanholdt Road, Moss Landing, CA 95039 United States
AB: Glass sand-sized fragments were recovered from three sites along the northern East Pacific Rise using the ROV Tiburon. Abundant moderately-vesicular bubble-wall (like limu o Pele) fragments, and rare Pele's hair were recovered near the center of the Alarcon Rise at 2360-2400 m at 23\deg23'N to 23\deg26'N and just south of the Tamayo Fracture Zone at 3150-3190 m at 22\deg50'N. Less abundant, but morphologically similar, fragments were recovered at 2620 m at 20\deg50'N, near the 21\deg N hydrothermal vent sites. The samples were recovered in push cores and using a small 29-jar suction sampler designed to collect and filter glass particles. The particles in individual samples include numerous compositional types, suggesting widespread dispersal of the particles after their formation. Many of the bubble-wall fragments enclose abundant (10-15%) spherical to elongate, stretched vesicles. The particles provide stratigraphic information on the sequence of eruptions in an area and can therefore be used to determine eruption compositions in space and time. For example, at the site at 22\deg50'N, particles with a distinct composition were recovered from the flank of a small cone, establishing that the cone predates an eruption just to the south of the cone. Also at this site, glass particles collected on top of the cone have a range of compositions that, on average, are about 0.3% lower in MgO than pillow rind glasses from the same cone. This compositional difference indicates that the pillow basalt glasses average about 6\deg C cooler than the particulate glasses. Maicher and White (2002) proposed that bubble-wall fragments form from steam expansion where lava flows cover saturated sediment forming hydroclastic (secondary) eruptions. On the other hand, Clague et al. (2003) proposed that bubble-wall fragments from the Gorda Ridge were formed by discharge of coalesced magmatic gas bubbles through the erupting vent in pyroclastic eruptions. Four lines of evidence support the pyroclastic origin of the glass fragments on the East Pacific Rise: 1) bubble-wall fragments occur with Pele's hair fragments, 2) site at 22\deg50'N, like the sites on the Gorda Ridge, are at pressures greater than the critical pressure for sea water boiling so no vapor phase is produced upon heating, and 3) bubble-wall fragments occur on top of a monogenetic cone that should have been sediment-free when the glass particles formed, and 4) the abundant vesicles trapped in some bubble-wall fragments are evidence of high concentrations of magmatic gas in these lavas. Pyroclastic eruptions, most likely similar to strombolian eruptions, are ubiquitous along the slow-spreading Gorda Ridge and the fast-spreading northern East Pacific Rise demonstrating similar eruptive processes despite the different spreading rates and ridge morphologies.
DE: 8414 Eruption mechanisms
DE: 8450 Planetary volcanism (5480)
DE: 9355 Pacific Ocean
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting