Volumetrically, the lithospheric mantle comprises the bulk of the continental lithosphere, yet the mechanisms by which the lithospheric mantle is deformed during rifting are unresolved. Stretching and thermo-mechanical erosion are often cited mechanisms for facilitating lithospheric deformation during continental rift development; however, the infiltration of melt into the lithosphere during rift development also results in chemical alteration of the lithospheric mantle. The purpose of this study is to test the potential mechanisms by which the continental lithospheric mantle is chemically altered during rift development. Here we present a study of mantle xenoliths derived from the lithospheric mantle in Ethiopia that has been deformed during rifting. We find that the lithospheric mantle beneath the this zone exhibits evidence of focused magma-lithosphere interaction, resulting in four distinct types of peridotite xenoliths: a) deformed xenoliths representing the pre-deformation lithospheric mantle; b) granular xenoliths representing overprinted lithospheric mantle; c) replacement dunite xenoliths, evidence of pervasive melt-lithosphere interaction; and d) cumulate xenoliths representing remnants of a metasomatic agent. The deformed xenoliths exhibit a high Mg# (>89) and exhibit little, if any, interaction with melt. The remaining xenolith groups exhibit lower Mg# (<89) suggestive of magma-lithosphere interaction. The high Ni content in olivine and depleted incompatible elements in orthopyroxene of the granular xenoliths are inconsistent with simple metasomatic enrichment and the existence of dunite with olivine (low Ca and Sc) and spinel (Cr# ~60) compositions is inconsistent with a cumulate origin, instead suggesting a replacive dunite. Our samples are derived from a zone of intensely sheared lithosphere and we suggest that melt channeling is preferred over chromatographic metasomatism due to melt focusing along steep topography on the lithosphere-asthenosphere boundary and shear-induced porosity.
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