HR: 1340h
AN: V53A-0611    [Abstracts]
TI: Magma Reservoir Dynamics and Diverse Mantle Melting at the Southern East Pacific Rise: $17\deg$22'S-$17\deg$35'S
AU: * Bergmanis, E C
EM: bergmani@hawaii.edu
AF: University of Hawaii, 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Sinton, J M
EM: sinton@hawaii.edu
AF: University of Hawaii, 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Rubin, K H
EM: krubin@hawaii.edu
AF: University of Hawaii, 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Mahoney, J J
EM: jmahoney@hawaii.edu
AF: University of Hawaii, 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Bowles, J
EM: jbowles@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Gee, J S
EM: jsgee@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Smith, M C
EM: msmith@geology.ufl.edu
AF: University of Florida, 241 Williamson Hall, Gainesville, FL 32611 United States
AB: Geologic observations, isotopic, major and trace element and U-series disequilibria data indicate that seven compositionally distinct lava types are present within 2 km of the fast-spreading ($\sim$145 mm/yr) southern EPR between $17\deg$22'S and $17\deg$35'S. Geologic contacts observed during submersible dives indicate that these lava types are the products of at least four eruptions. These observations require a complex history of mantle melting, recharge, cooling, and eruption that varies considerably over along-axis distances of 24 kilometers and a timescale of several hundred years. The boundary of a young, $\sim$18km-long lava flow (Aldo-Kihi) is well constrained by submersible observations, three other geologic units are less well-defined. ($^{210}$Pb)/($^{226}$Ra) deficits of $\sim$5 % and magnetic paleointensity measurements indistinguishable from present-day values suggest the Aldo-Kihi lava and two other compositionally distinct units are $<$100 yrs old. Major-element variation in the Aldo-Kihi flow (MgO 7.7-8.4 wt %) is consistent with shallow-level fractional crystallization. However, isotopic, trace element and U-series disequilibria data require along-axis mixing of two chemically distinct parental magmas. Pb and Sr isotopes, incompatible element concentrations, MgO contents, Th/U ratios, and ($^{226}$Ra)/($^{230}$Th) disequilibria for Aldo-Kihi samples all peak near $\sim$$17\deg$30.6'S, the lowest values occur near $17\deg$26.4'S. This spatial compositional diversity within a single eruption is difficult to reconcile with propagation of a dike originating from a small area, and suggests near-vertical eruption from a magma chamber that is compositionally zoned along-axis. MgO values for the most recent four lava types are lowest between $17\deg$24.6'S and $17\deg$27.9'S where the axial magma reservoir is shallowest; the highest MgO values occur south of $17\deg$30'S. These observations indicate that the processes controlling magma temperature have persisted through several cycles of eruption and recharge. Situated at the apex of a dome-shaped isotopic peak extending from $15.8\deg$S to $20.7\deg$S, samples from this 24 km-long area show isotopic variability ($^{87}$Sr/$^{86}$Sr: 0.70256-0.70282, $\epsilon$$_{Nd}$: +8.1 to +9.3, $^{206}$Pb/$^{204}$Pb: 18.549-18.799) equal to 50 % of the entire range observed along the $\sim$1100 km-long EPR axis from $13\deg$S to $23\deg$S. These data extend the isotopic peak for axial lavas to values observed previously only in nearby off-axis seamounts and flow fields.
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
DE: 3035 Midocean ridge processes
DE: 1040 Isotopic composition/chemistry
DE: 1527 Paleomagnetism applied to geologic processes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting