Microstructures and lattice fabrics in the Hilti mantle section (Oman Ophiolite): Evidence for shear localization and melt weakening in the crust–mantle transition zone?

Abstract

Experimental studies have shown that olivine aggregates with ≥4% melt are significantly weaker than melt‐free aggregates. However, questions remain as to the importance of melt weakening in nature. In several studies, melt weakening has been invoked to explain patterns of mantle flow in the Oman Ophiolite. In this paper, we reinvestigate evidence for melt weakening in the Hilti mantle section using structural and microstructural methods. The average olivine grain size increases with depth below the crust–mantle boundary. This is related to a change from equigranular‐to‐porphyroclastic microstructures at shallow levels to coarse porphyroclastic microstructures at depth. A strong foliation and high degree of recrystallization in the upper part of the mantle section are interpreted as the strong imprint of localized deformation at stresses of ∼4–10 MPa. Lattice orientation data show that the high‐strain peridotites have recorded top‐to‐the‐west shear; reversed shear senses were found deeper in the section. Since there is petrographic evidence for melt in the high‐strain zone, strain localization was probably caused by a melt‐related weakening mechanism. High melt contents required for melt weakening suggest that melt accumulated just below the crust–mantle boundary. We conclude that melt weakening is probably related to enhancement of grain boundary rather than intragranular deformation processes. Effective viscosities <2 × 1016 Pa·s may locally exist in the uppermost mantle beneath ridges if melt weakening occurs. Although our results agree with those of previous studies in the Hilti Massif, we conclude that both the previously published active flow model and a ridge compression model can account for them.