HR: 0800h
AN: V21B-0609    [Abstracts]
TI: Geochemistry of the Vance Seamount Chain: Off-axis Seamounts Along the Southern Juan de Fuca Ridge
AU: * Wendt, R E
EM: rwendt@ufl.edu
AF: Geological Sciences, University of Florida, P.O. Box 112120, Gainesville, FL 32611-2120, United States
AU: Cornejo, E A
EM: lcornejo@connect.carleton.ca
AF: Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada
AU: Perfit, M R
EM: perfit@geology.ufl.edu
AF: Geological Sciences, University of Florida, P.O. Box 112120, Gainesville, FL 32611-2120, United States
AU: Cousens, B L
EM: brian_cousens@carleton.ca
AF: Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada
AU: Clague, D A
EM: clague@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-9644, United States
AB: The Vance Seamounts are a NW-SE trending chain of eight seamounts (Vance A through G) at approximately 45° 15'N and 130° 20'W on the flanks of the southern Juan de Fuca Ridge (SJdF). The chain consists of truncated cone-shaped volcanoes (A, C, E/D, and F) and large areas of disorganized volcanic constructs comprised of numerous small cones and volcanic ridges (B and G). Geologic evidence suggests that the seamounts progressively increase in age from the southeast (G) to the northwest (A). Eruptive products include pillow lavas, sheet flows, massive flows, and hyaloclastites. During July-August 2006, samples of the seamounts were collected using the ROV Tiburon; volcanic glass and whole rock samples were analyzed for major element, trace element, and isotopic compositions. Analyses show there are significant geochemical differences in lava composition between the seamounts in the chain, as well as within individual seamounts. Overall, Vance lavas are more primitive and exhibit greater diversity than samples from the adjacent SJdF. Major element data indicate that all samples are subalkaline-MOR basalts, but a range of depleted to moderately enriched varieties exist (0.02-0.31 wt.% K₂O). Incompatible trace element data indicate samples range from very depleted to slightly enriched ([La/Sm]_N =0.354-1.07; [Ce/Yb]_N =0.498-1.59; Zr/Y=1.69-3.86). Variations in major and trace elements suggest differing sources and/or styles of partial melting, rather than shallow level processes like fractional crystallization as the reason for most of the geochemical diversity. Sr-Nd-Pb isotope data supports slight variations in source characteristics (⁸⁷Sr/⁸⁶Sr = 0.702477-0.702687; ²⁰⁶Pb/²⁰⁴Pb = 18.117-18.816; and ¹⁴³Nd/¹⁴⁴Nd = 0.513052-0.513181). Variations are just outside of analytical error for many of the seamounts, but fall along a general mixing trend between the Depleted Mantle (DM) and High μ (HIMU) mantle reservoirs. Mantle heterogeneity beneath the Vance Seamounts appears to be on a very short length scale, with each seamount showing significant incompatible trace element and isotopic variations that are somewhat correlated. The very depleted nature of many of the Vance samples compared to adjacent ridge samples is similar to observations made at other off-axis chains (e.g. Lamont Seamounts, Heck Seamounts), but the presence of E-MORB is unusual for both the SJdF ridge and other near-axis seamounts. These results support a "veined mantle'' model for the NE Pacific mantle. The off-axis seamount samples appear to record small scale heterogeneity of the underlying mantle because individual melt batches undergo only minor amounts of differentiation and mixing whereas the inherent mantle diversity is homogenized for on-axis ridge samples due to the more steady-state shallow magmatic processes that occur in sub-axial magma bodies.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3075 Submarine tectonics and volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting