Multi-stage history of compound mantle xenoliths from Western USA: Implications for metasomatic processes in the deep mantle.

The compound mantle xenoliths from Cima Volcanic Field and Chino Valley (Western U.S.A.) represent outstanding candidates to illustrate the processes that occur prior to their delivery to the surface by alkali-basaltic volcanism. The xenoliths share characteristics like pyroxene zonation, amphibole breakdown and formation of glass and armalcolite. Their petrogenetic evolution involved partial melting of the silicate minerals, infiltration of reactive melts and dissociation of minerals en route to the surface, suggesting that these rocks followed multi-stage histories that initiated deep in the mantle (>1.0 GPa) and proceeded during a very short period of time.