The Origin of Amphibole in Lherzolite Xenoliths from Nunivak Island, Alaska

Ten lherzolite xenoliths collected on Nunivak Island, Alaska, contain interstitial chromian pargasitic amphibole. Of 2000 lherzolite nodules examined from the maars of the island, however, 50 per cent contain relics of such an amphibole in the form of fine-grained zones of euhedral diopside, olivine and spinel in a porous Al-rich glass. These nodules are believed to have been amphibole-bearing fragments of the upper mantle, that partially melted during their ascent to the surface in the Nunivak basalts. The textural intimacy and chemical zoning exhibited by the interstitial amphibole and spinel in the lherzolite xenoliths indicate that although the amphibole predates the Nunivak basalts, it is a secondary aluminous phase. The amphibole was formed in the upper mantle during a pervasive metasomatic event caused by rising temperature and the infiltration of alkali-rich fluid, prior to the introduction of basaltic magmas.