A model of mantle metasomatism

Structural, lithologic, and geochemical relationships are integrated to provide a basis for modeling upper mantle metasomatism, the evidence of which is observed in mafic and ultramafic rocks of peridotite massifs and xenoliths from basalts and kimberlites. Mafic rock types associated with peridotite occur as dikes, and most metasomatic phenomena observed in peridotites appear to be directly related to the magmatic rocks. The chronology of mafic dike emplacement, established by crosscutting relationships, is the same in peridotite massifs and xenoliths in basalts, and progresses toward lower pressure assemblages. Metasomatic alteration of peridotite wallrock in the mantle occurs locally adjacent to the dikes, which are crystallized initial magmas or magmatic differentiates separated from their parent dikes. The alteration is caused both by diffusion and infiltration processes. Additional metasomatism (“cryptic”) can be caused by a CO 2-rich, light rare earth element (LREE)-enriched gas phase evolved from the mafic dikes and distributed through hydro-fractures in peridotite. Crosscutting relationships of mafic dikes indicate that local metasomatic events have occurred serially in the melting history of peridotite, so that superposition of different types of metasomatic effects is to be expected. The sequential melting and metasomatic events are interpreted as consequences of diapiric rise of the host peridotite. The type and number of melting-metasomatic events evident in xenoliths is thought to be limited by the level of diapiric rise at which eruption occurs.