HR: 1340h
AN: T33B-1366    [Abstracts]
TI: Partial melting and mantle dynamics at the Fifteen-Twenty Fracture Zone (Mid-Atlantic Ridge)
AU: Lagabrielle, Y
EM: Yves.Lagabrielle@um2.fr
AF: Geosciences Montpellier, Universite Montpellier 2, CC60 Place E. Bataillon, Montpellier, F-34095, France
AU: * Godard, M
EM: Marguerite.Godard@um2.fr
AF: Geosciences Montpellier, Universite Montpellier 2, CC60 Place E. Bataillon, Montpellier, F-34095, France
AB: During ODP Leg 209, a magma-starved portion of the Mid-Atlantic Ridge (MAR) was drilled in the vicinity of the Fifteen-Twenty Fracture Zone (FZ). Mantle peridotites sampled at Sites 1272 and 1274 (to the south and the north of the FZ, respectively) represent the most depleted peridotites ever sampled at a slow spreading ridge, Site1272 representing the most depleted end-member. They were formed by high degrees of melting (>17%). In addition to melting, Site1274 peridotites show evidence for late melt transport and melt freezing reactions. Sites 1270 and 1271, drilled in the vicinity of Site 1272, sampled highly reacted and impregnated peridotites with fertile compositions similar to peridotites from the mantle-crust transition in ophiolites. In the Fifteen-Twenty FZ area, the compositional variability of the mantle does not relate to different degrees of partial melting on each side of the FZ but to variable degrees of melt-rock interaction. The formation of highly refractory peridotites with, locally, high melt focussing areas implies a more actively convecting mantle than generally inferred below slow spreading centers. We propose different scenarios to explain our observations: (i) melting during a period of high magmatic activity followed by present-day exhumation during amagmatic rifting or (ii) formation of shallow micro-convection cells close to FZ-ridge intersections. In addition, the complex and dynamic ridge system that characterizes the Fifteen-Twenty FZ area has probably favoured the sampling of melts formed at different depths and exposure of parts of the upwelling mantle rarely exposed in oceanic mantle windows elsewhere.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 3621 Mantle processes (1038)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting