Ultramafic and mafic xenoliths from Hierro,Canary Islands: evidence for melt infiltration in the upper mantle

Three groups of ultramafix xenoliths were collected from alkali basalt in the island of Hierro, Canary Islands: (1) Cr-diopside series (spinel harzbugite, lherzolite, dunite); (2) Al-augite series xenoliths (spinel wherlite, olivine clinopyroxenite, dunite, olivine websterite); (3) gabbroic xenoliths. The main textures are granoblastic, porphyroclastic and granular, but poikilitic textures, and symplectitic intergrowths of clinopyroxene (cpx) + spinel (sp)±orthopyroxene (opx)±olivine (ol) (in rare cases cpx+opx), occur locally. Textural relations and large inter- and intra-sample mineral chemical variations testify to a complex history of evolution of the mantle source region, involving repeated heating, partial melting, and enrichment associated with infiltration by basaltic melts. The oldest assemblage in the ultramafic xenoliths (porphyroclasts of ol+opx±sp±cpx) represents depleted abyssal mantle formed within the stability field of spinel lherzolite. The neoblast assemblage ol+cpx+ sp±opx±plagioclase (plag)±ilmenite (il)±phlogopite (phlog)] reflect enrichment in CaO+Al₂O₃+Na₂O+ FeO±TiO₂±K₂O±H₂O through crystal/liquid separation processes and metasomatism. The Al-augite-series xenoliths represent parts of the mantle where magma infiltration was much more extensive than in the source region of the Cr-diopside series rocks. Geothermometry indicates temperature fluctuations between about 900–1000 and 1200°C. Between each heating event the mantle appears to have readjusted to regional geothermal gradient passing 950°C at about 12 kbar. The gabbroic xenoliths represent low-pressure cumulates.