HR: 14:10h
AN: V33G-03    [Abstracts]
TI: Geochemistry of Alvarado and Sarmiento Ridges Suggests Widespread Galapagos Plume-Upper Mantle Interaction in the Miocene Eastern Pacific?
AU: * Castillo, P R
EM: pcastillo@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0212 United States
AU: Lonsdale, P F
EM: plonsdale@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0212 United States
AB: Alvarado and Sarmiento are 1-2 km high, ~400 km-long, NE striking volcanic ridges in the Peru Basin that lie 150 km and 250 km, respectively, southeast of the Grijalva Scarp. The latter marks the southern boundary between the younger Cocos-Nazca and older EPR-Farallon crusts. The two ridges were originally proposed as transform fault trails on the Farallon plate, but new evidence suggest that they were the result of fissure eruption during an initially (pre-24Ma) broad zone of Farallon plate rupture. The rupture later became focussed along the Grijalva fracture, splitting the Farallon plate at 23Ma to create the Cocos and Nazca plates and initiating the Cocos-Nazca spreading center. Lava samples from the ridges were dredged during the DANA 02 expedition aboard R/V Revelle in Fall 2003. The lavas are invariably basalts ranging from tholeiitic to alkalic in composition. The basalts have flat to highly incompatible element-enriched trace element patterns and although their isotopic ratios are not as radiogenic as those of some of the Galapagos hotspot lavas, they are neither as depleted as those of normal-MORB (87Sr/86Sr[25Ma] $>$0.7027; 143Nd/144Nd[25Ma] $<$0.5130; 206Pb/204Pb[25Ma] $>$18.66). Some of the ridge lavas are compositionally similar to enriched-MORB erupted along the GSC section close to the Galapagos hotspot, but the majority are more akin to the relatively depleted hotspot lavas erupted at the center of the Galapagos Archipelago. The new geochemical data suggest that the volcanic ridge lavas came from a fairly large, anomalous region of the upper mantle that may have been created by the interaction between a Galapagos plume and the depleted source of MORB.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 3640 Igneous petrology
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 1025 Composition of the mantle
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting