HR: 0800h
AN: V21C-0732    [Abstracts]
TI: Vesicle evolution in primary volcaniclastic material from Loihi seamount, Hawaii, with implications for submarine basalt explosivity
AU: * Schipper, C I
EM: ianschipper@hotmail.com
AF: Geology Department, University of Otago, PO Box 56 Leith St., Dunedin, 9016, New Zealand
AU: Houghton, B F
EM: bhought@soest.hawaii.edu
AF: Department of Geology and Geophysics, University of Hawaii at Manoa, 1680 East-West Road, Honolulu, HI 98622, United States
AU: Gonnermann, H M
EM: helge@hawaii.edu
AF: Department of Geology and Geophysics, University of Hawaii at Manoa, 1680 East-West Road, Honolulu, HI 98622, United States
AU: White, J D
EM: james.white@stonebow.otago.ac.nz
AF: Geology Department, University of Otago, PO Box 56 Leith St., Dunedin, 9016, New Zealand
AB: The extent and timing of magmatic degassing in ascending basaltic melts has implications for the rheological and physical properties of the magma at the time of eruption, and thus are controls on eruptive style. They are relatively well understood for subaerial eruptions, but have not been rigorously assessed for submarine ones. We present a quantitative textural evaluation of vesicular fragmental basalt collected at ~1140 mbsl on Loihi seamount, Hawaii, which provides insight into the microtextural evolution occurring within individual clasts. Using these clasts, we determine rates of vesicle growth by quantifying the advancement of a seawater-induced cooling front. The work presented here builds toward a semi-quantitative model explaining the origin of primary fragmental submarine deposits that share common features with the products of explosive (Hawaiian and/or strombolian) subaerial eruptions. Representative lapilli-sized Loihi clasts have vesicularities ranging 47-69%, with a well-defined modal vesicularity of 55%. These values are significantly higher than previously reported for lava flows and pillow lavas on submarine Hawaiian basalt of similar composition, despite eruption depths exceeding 1 km. Clasts of modal vesicularity are found to have margin-parallel zones with vesicle and groundmass textures that mature from rim- to-core over several cm. Seawater-quenched rims are dominated by small close-packed sub-spherical vesicles (N_a=1.5×10³ cm^-2, median diameter ~400 μm), hosted in sideromelane glass, and grade through 1 or 2 intermediate zones to texturally mature cores dominated by fewer, large, amoeboid vesicles (N_a=1.7×10² cm^-2, median diameter ~1650 μm) in a tachylitic groundmass. We infer that the rims represent the texture of the magma at the time of eruption and fragmentation, and hence the starting point from which the more mature textures evolved during cooling. Calculation of conductive cooling rates for basalt clasts in contact with water (+/- steam) indicates that the cores of Loihi clasts took on the order 1 to 10 minutes (~10²-10³ s) to reach the glass transition temperature (T_g). Below T_g, glass yield strength in the solidified melt inhibited further expansion, migration, or coalescence of vesicles. Vesicle quantification through the margin-parallel zones allows us to assess the rate-limited relative roles of vesicle nucleation, growth, coalescence, and loss, as well as associated rates of groundmass crystallization.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8427 Subaqueous volcanism
DE: 8428 Explosive volcanism
DE: 8434 Magma migration and fragmentation
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting