HR: 0800h
AN: T21C-0491    [Abstracts]
TI: Melting-impregnation or subsolidus reactions during upwelling of mantle lherzolites
AU: * Boudier, F I
EM: Francoise.Boudier@dstu.univ-montp2.fr
AF: University Montpellier II, Place Eugene Bataillon, Montpellier, 34095 France
AU: NICOLAS, A J
EM: Adolphe.Nicolas@dstu.univ-montp2.fr
AF: University Montpellier II, Place Eugene Bataillon, Montpellier, 34095 France
AB: A recurrent question is whether two pyroxenes-spinel or two pyroxenes-plagioclase clusters observed in mantle peridotites are derived from subsolidus reactions or melting reactions during decompression. The orthopyroxene-clinopyroxene-spinel clusters preserved in protogranular spinel lherzolite xenoliths derive by melt reactions from the garnet lherzolites. Subsolidus reactions would produce kelyphite rims. This incipient melting of garnet peridotite occurred when the local geothermal gradient crossed the solidus in the garnet lherzolite field during asthenospheric upwelling. In the case of orthopyroxene-clinopyroxene-plagioclase clusters, present in plagioclase lherzolite massifs, the textural record is locally masked by further plastic deformation. However thick pyroxenite banding of ariegite type in plagioclase lherzolite massifs have preserved the textural record of phase transformation from garnet pyroxenite to troctolite. These phase transformations are coeval with the equilibration of the enclosing peridotite in plagioclase lherzolite during asthenospheric upheaval. We conclude that in the considered plagioclase lherzolite massifs, the plagioclase derives from melt-impregnation reactions. The melting reactions may deliver enough melt to locally produce melt impregnations. However, impregnations may be derived from a deeper level, but, in either case, they point to asthenospheric conditions. The answer has major implications for rifting processes, because, if these reactions are produced in the solid state, the lherzolites can be derived from the continental lithosphere uplifted passively during the rifting. Conversely, if the reactions are the result of partial melting and melt-impregnation, the lherzolites must represent the rising asthenosphere associated with the rifting process.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3660 Metamorphic petrology
DE: 8020 Mechanics, theory, and modeling
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: Fall Meeting 2005