HR: 17:15h
AN: V34B-06    [Abstracts]
TI: On Melting, Dehydration and the Geochemistry of Off-Axis Plume-Ridge Interaction
AU: Hall, P S
EM: phall@gso.uri.edu
AF: University of Rhode Island Graduate School of Oceanography, Box 200 South Ferry Rd, Narragansett, RI 02882 United States
AU: Kincaid, C
EM: kincaid@gso.uri.edu
AF: University of Rhode Island Graduate School of Oceanography, Box 200 South Ferry Rd, Narragansett, RI 02882 United States
AU: * Viso, R F
EM: rviso@gso.uri.edu
AF: University of Rhode Island Graduate School of Oceanography, Box 200 South Ferry Rd, Narragansett, RI 02882 United States
AB: Interaction between off-axis mantle plumes and the mid-ocean ridge system has been proposed to occur via pipeline-like flows along the base of the lithosphere based on of off-axis variations in geophysical properties such as seafloor morphology, gravity and bathymetry. Off-axis variations in geochemistry from places such as the Galapagos and the Easter - Salas y Gomez seamount system provide an important independent constraint on the nature of flow in off-axis plume-ridge systems. We present results from a series of two-dimensional numerical experiments in which synthetic melt compositions are calculated for a system in which a thermally buoyant, off-axis mantle plume interacts with a nearby ridge axis. These experiments incorporate melting and a number of related dynamical feedbacks, including energy loss to latent heating and viscosity increases due to dehydration during melting. Spatial gradients in synthetic melt properties are compared to observed spatial gradients in radiogenic isotopic ratios in rocks from the Easter - Salas y Gomez system in an effort to constrain the dynamics of mantle flow in off-axis plume-ridge systems. Results indicate that the observed gradients between the ridge axis and the plume require significant heating of the ambient mantle material adjacent to the plume. This heating allows ambient mantle to melt off-axis, creating complex structure in off-axis geochemistry, as seen in the observational data. When increases in viscosity due to dehydration during melting are considered a large viscous plug forms on the base of the lithosphere above the plume, deflecting plume material horizontally to the ridge axis at sub-solidus depths. This leads to extremely sharp spatial gradients in the geochemical properties of synthetic melts, at odds with the observational data. This suggests that the effects of dehydration on rheology are minimal for the case of off-axis plume-ridge interaction.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8162 Rheology--mantle
DE: 3210 Modeling
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting