Shear zones in the upper mantle—Relation between geochemical enrichment and deformation in mantle peridotites

Textural variations in mantle-derived spinel peridotites have previously been interpreted as evidence of the existence of asthenospheric mantle diapirs, indicating deformational heterogeneity on a large lateral scale (kilometres to tens of kilometres). However, many volcanic vents entrain both deformed and undeformed xenoliths, and field relations in peridotite massifs show the scale of alternation between deformed and undeformed peridotite to be small (centimetres to metres) because of the presence of numerous lithospheric shear zones. Some rare xenoliths contain both deformed and undeformed peridotite. There is also an apparent relation between deformation and the growth of metasomatic minerals; amphibole is often concentrated in strongly deformed zones in peridotite massifs and deformed xenoliths, although it is also found in crosscutting veins and aureoles around such veins. A relation can also be seen between deformation and indicators of geochemical enrichment. Clinopyroxenes from many deformed spinel peridotites show light rare earth element (REE) enrichment, whereas clinopyroxenes from undeformed spinel peridotites commonly have mid-ocean ridge basalt (MORB)-type light REE-depleted patterns. Sr and Nd radiogenic isotopic compositions of undeformed peridotites are generally MORB-like, and have low ϵSr and high ϵNd. In contrast, deformed peridotites, with or without amphibole, often have higher εSr and lower ϵNd values, indicating geochemical enrichment by large ion lithophile (LIL)- and light REE-enriched fluids or melts. These observations can be used to infer that the shallow mantle contains lithospheric ductile shear zones in which metasomatic fluids precipitated amphibole and clinopyroxene.