Metasomatic interactions in the lithospheric mantle: Petrologic evidence from the Lherz massif, French Pyrenees

To better understand the processes of metasomatism in the lithospheric mantle, we have undertaken a detailed petrologic study of wallrock adjacent to late stage mafic dikes at the Lherz massif. Traverse I contains a garnet-pyroxenite dike, along with homblendite dikelets which cut harzburgite. Traverses II and III contain an anhydrous garnet-pyroxenite dike and a hornblendite dike in Iherzolite, respectively, There is evidence for three successive metasomatic events in these samples: (1) crystallisation of Al-rich grain-boundary spinels +/- associated phases from a silicate melt, (2) formation of apatite related to carbonatite metasomatism, (3) Fe-Ti metasomatism related to the intrusion of garnet-pyroxenite dikes and hornblendite dikes. The first two events were diffuse in character, and although they penetrated a relatively large volume of peridotite, interaction with the peridotite was not pervasive, The result was the development of localised chemical gradients at the mm scale, The carbonatite metasomatism caused a net oxidation and chemical enrichment of the peridotite, Metasomatic effects related to the intrusion of the garnet-pyroxenite and hornblendite dikes are spatially restricted. Primary and secondary spinel and clinopyroxene compositions indicate that melt migrating from the hornblendite dikes was mostly responsible for pervasive metasomatism at the scale of < 5 cm. Further distant, up to similar to 15 cm, pervasive overprinting gives way to a zone where variable degrees of interaction have occurred. Little to no metasomatic interaction is observed adjacent to a thick anhydrous garnet-pyroxenite dike. Successive overprinting of one metasomatic event by another has led to the production of enriched peridotites with complex geochemical signatures. The variable extent of melt-peridotite interaction can lead to geochemical heterogeneities at a variety of length scales. Textural relations reveal little, if any, difference in the mechanism of infiltration of a silicate or carbonatite melt through harzburgite or Iherzolite.