Textural development of amphibole during breakdown reactions in a synthetic peridotite

Amphibole in metasomatised mantle peridotites is commonly associated with inclusions and pools of silicate glass that contain microlites of spinel, olivine and clinopyroxene. There are two contrasting hypotheses for the origin of this association: 1) glass is the remnant of a metasomatic fluid or melt that is responsible for the crystallisation of amphibole. 2) Glass and the associated minerals are the result of amphibole breakdown during heating and decompression associated with the transport of the xenolith to the surface. Experiments over the range 1 atm to 1.5 GPa at temperatures from 950–1150 °C reproduce the textures, mineral assemblages and mineral and glass compositions observed in natural samples. Thus, it is most likely that the amphibole + glass + microlite assemblage in mantle xenoliths is an artefact of heating during transport rather than a clue to the agent of mantle metasomatism.