Upper mantle segregation processes: evidence from alpine-type peridotites

Abstract

Summary Studies of alpine-type peridotites indicate that the upper mantle is pervaded by various segregations whose origin cannot be exclusively attributed to the effects of crustal emplacement. The segregations typically occur as veins and layers (less than 1 m thick). They are common in the lherzolitic varieties of alpine-type peridotite but are of more restricted occurrence in true ophiolites. The segregations are represented by predominantly metamorphic mineral assemblages which span the garnet, spinel and plagioclase peridotite stability fields. Low-pressure segregations may remain igneous. Many segregations are varieties of aluminous clinopyroxenite with or without garnet, spinel or plagioclase. Segregations possess variable bulk chemistries and mineralogies so a number of different origins for them must be considered. The available evidence indicates that they are (i) metamorphosed trapped partial melts, melt residua or conduit platings, (ii) recycled and streaked-out ocean crust (via subduction zones), (iii) solid-state metamorphic differentiates or (iv) products of mantle metasomatism. The formation of segregations renders the mantle heterogeneous on a macroscopic scale, but this is combated in time by diffusive processes and segregation recycling at spreading centres, both of which contribute to its rehomogenization.