Melt Inclusions Trapped in Mantle Minerals: A Clue to Identifying Metasomatic Agents in the Upper Mantle Beneath Continental and Oceanic Intraplate Regions

Abstract

Rock samples derived from the Earth's upper mantle commonly show indirect evidence for chemical modification induced by migrating fluids or melts . Such modi f ica t ion , or 'metasomatism' , can be recognized by the precipitation of exotic minerals such as phlogopite, amphibole, or apatite, and by the overprinting of the bulk compositions of the mantle rocks by a chemical signature involving the enrichment of incompatible elements. However, identification of the metasomatic agents occurring in the upper mantle is still a controversial topic. Here, we study the composition of the metasomatic agents more directly by examining trapped melt inclusions in mantle minerals from ultramafic peridotites from both continental and oceanic intraplate regions. The xenoliths samples come from Society islands (Tahaa , Tahit i) , Canary islands (Lanzarote, Hierro), Kerguelen island (Jeanne d'Arc Peninsula), Comores island (La Grille), New Mexico (Kilbourne Hole), Arizona (San Carlos), Germany (Dreiser Eifel), Italy (Mt Ible'0, France (Massif Central), Mongolia (Dariganga), Russia (Vitim Highland) and Vietnam. They are massive anhydrous spinel-lherzolites and harzburgites, with 60-81% olivine, 10-21% orthopyroxene, 0.5-11% clinopyroxene and 0.5-3% spinel. Textures vary from protogranular to porphyroclastic, often with a transition between the two. The xenoliths contain three types of genetically related inclusions hosted by olivine, clinopyroxene and orthopyroxene: silicate melt inclusions, carbonate-rich inclusions, and CO2 fluid inclusions. The inclusions are secondary in nature and form trails along annealed fracture planes in the sheared peridotites. CO2 inclusions and silicate melt inclusions joined by necks and multiphase inclusions consisting of both CO2-rich fluid and silicate-carbonate melts are visible, indicating a cogenetic relationship between fluid and melt inclusions.