HR: 17:00h
AN: V34B-05    [Abstracts]
TI: Geochemical and isotopic variations along the Gal\'{a}pagos Spreading Center, 90.8-$97.8\deg$W
AU: * Chazey, W J
EM: chazey@hawaii.edu
AF: University of Hawaii, Dept. of Geology and Geophysics 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Mahoney, J J
EM: jmahoney@hawaii.edu
AF: University of Hawaii, Dept. of Geology and Geophysics 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Sinton, J M
EM: sinton@hawaii.edu
AF: University of Hawaii, Dept. of Geology and Geophysics 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Christie, D M
EM: dchristie@coas.oregonstate.edu
AF: Oregon State University, College of Ocean Atmospheric Sciences, Corvallis, OR 97331 United States
AB: Basalts dredged during R/V Maurice Ewing Leg EW00-04 to the Gal\'{a}pagos Spreading Center were analyzed for major elements, trace elements, and Pb-Sr-Nd isotopic compositions via XRF, pneumatic nebulization ICP-MS, and ID-TIMS, respectively. Going westward along axis from $90.8\deg$W, basalts change from E- to T- to N-MORB (Cushman et al., G$^{3}$, 5, 2004); Sr and Pb isotope ratios become less radiogenic and Nd isotope ratios more radiogenic. The highest $^{87}$Sr/$^{86}$Sr (0.70302) and $^{206}$Pb/$^{204}$Pb (19.125) and lowest $\epsilon$$_{Nd}$ (+6.5) values are at $\sim$ $92\deg$W. An overall westward along-axis isotopic gradient is evident (cf. Verma et al., Nature, 306, 1983; Schilling et al., G$^{3}$, 4, 2003). However, a pronounced "step" is present in all isotope ratios at the large propagator at $95.5\deg$W; for example, basalts to the east have $\epsilon$$_{Nd}$ less than +8.0, whereas those to the west generally have $\epsilon$$_{Nd}$ greater than +9.0. Several incompatible element ratios display a broadly similar along-axis pattern; e.g., primitive-mantle-normalized (La/Sm)$_{pm}$, (Nb/La)$_{pm}$, and (Nb/Zr)$_{pm}$ all reach a maximum at $92\deg$W, and decrease around $95.5\deg$W. (Th/Hf)$_{pm}$ values decrease by a more than a factor of two across the propagator. One interpretation is that, over time, mantle derived from the Gal\'{a}pagos hotspot has moved westward in "pulses", gradually mixing with ambient N-MORB-source mantle, and that the propagator marks a boundary between mantle from two such pulses. In Nd-Pb and Sr-Pb isotopic space, our data form a linear array extending from a high-$\epsilon$$_{Nd}$, low-$^{87}$Sr/$^{86}$Sr Pacific N-MORB-type composition to a point between values for the Gal\'{a}pagos plume and Wolf-Darwin components proposed by Harpp & White (G$^{3}$, 2, 2001) to be two of the end-members in the mantle source of the Gal\'{a}pagos Islands. Our data may suggest that the hotspot-derived mantle that affects the spreading center represents a specific, favored mixture of these two components; alternatively, only the plume component is involved, but has a slightly different isotopic composition than previously postulated.
DE: 8434 Magma migration
DE: 3655 Major element composition
DE: 3035 Midocean ridge processes
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting