HR: 1340h
AN: V13B-0551    [Abstracts]
TI: Petrogenesis of Andesites and Dacites From the Southern Juan de Fuca Ridge
AU: * Cotsonika, L A
EM: lcotsoni@ufl.edu
AF: University of Florida, Department of Geological Sciences, Gainesville, FL 32611 United States
AU: Perfit, M R
EM: perfit@geology.ufl.edu
AF: University of Florida, Department of Geological Sciences, Gainesville, FL 32611 United States
AU: Smith, M C
EM: msmith@geology.ufl.edu
AF: University of Florida, Department of Geological Sciences, Gainesville, FL 32611 United States
AU: Kamenov, G
EM: kamenov@ufl.edu
AF: University of Florida, Department of Geological Sciences, Gainesville, FL 32611 United States
AU: Stakes, D
EM: debra@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039 United States
AU: Ridley, W I
EM: iridley@usgs.gov
AF: US Geological Survey, Denver Federal Center, Denver, CO 80225 United States
AU: Wallace, P
EM: pwallace@darkwing.uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403-1272
AB: The Cleft segment and the ridge-transform intersection (RTI) of the Southern Juan de Fuca Ridge have been investigated during three cruises of MBARI's R/V Western Flyer beginning in 2000. A total of 53 rock cores and 276 precisely located rock or glass samples were collected during sixteen dives with the ROV Tiburon. These ROV dive samples and observations allow us to test models regarding the magmatic evolution of this segment and the relationships between specific tectonic and morphologic features and magmatic processes. An extremely wide range of N-type MORB lavas were recovered which are, on average, more evolved (lower MgO) off-axis, away from the present neovolcanic zone, and towards the RTI. ROV dive T735 investigated a region of unfaulted, curved volcanic ridges that overshoot the Blanco Transform and recovered 39 samples ranging from ferrobasalt to andesite and dacite (SiO2 = 50.1 to 66.9 wt.%; Mg# = 49.9 to 9.7). The highly evolved lavas were recovered from two large constructional domes comprised of unusually large pillow flows, and extremely blocky, vesicular flows - similar to some terrestrial silicic domes. Some of the andesite-dacite hand samples are extremely vesicular with elongate vesicles (1-10 cm) in a glassy matrix. The dacitic melts have an H2O content of ~1.7% with a vapor saturation pressure of ~240 bars. The large vesicles appear to be the result of the high H2O content, causing the exsolution of H2O and CO2 vapor. The andesitic and dacitic lavas also have elevated Cl- levels that range from 4000-6000 ppm. Mineral assemblages are dominated by microphenocrysts of ferroaugite and ferropigeonite, with lesser amounts of sodic plagioclase and FeTi oxides. Rare zircon, disequilibrium fayalite and myrmekitic textures of plagioclase and quartz are present. A few of the more magnesian phenocrysts (xenocrysts?) exhibit fine normal zoning whereas more Fe-rich crystals exhibit fine reverse zoning. Additionally, inclusions of quenched basaltic material appear within some of the evolved lavas. These samples represent an extensive and unique set of some of the most highly fractionated ocean floor rocks that have ever been recovered; particularly from such a well-documented setting. Fractional crystallization models which predict over 80% crystallization do not adequately explain the major element chemistry of the silicic lavas and most incompatible trace elements exhibit significant enrichments relative to predicted concentrations. The highly evolved nature of the dacites, crystal zoning patterns and the presence of basaltic inclusions suggest the lavas are the result of mixing between two crystal bearing end-members, i.e. a typical basalt and a rhyolite likely generated from partial melting of oceanic crust. Isotopic analyses (Sr, Nd, Pb) are in progress in order to better constrain the sources involved. Periodic propagation of mantle-derived melts or down-ridge diking events into the static RTI environment may lead to the generation of such silicic oceanic lavas.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1037 Magma genesis and partial melting (3619)
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 3619 Magma genesis and partial melting (1037)
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005