Fluid and silicate glass inclusions in ultramafic and mafic xenoliths from Hierro,Canary Islands: implications for mantle metasomatism

Fluid and solid inclusions have been studied in selected samples from a series of spinel-bearing Crdiopside-and Al-augite-series ultramafic (harzburgites, lherzolites, and olivine-clinopyroxene-rich rocks), and gabbroic xenoliths from Hierro, Canary Islands. In these samples several generations of fluid inclusions and ultramafic-and mafic-glass inclusions may be texturally related to different stages of crystal growth. The fluid inclusions consist of pure, or almost pure, CO₂. The solid inclusions in the ultramafic xenoliths comprise early inclusions of devitrified ultramafic glass, sulphide inclusions, as well as polyphase inclusions (spinel+clinopyroxene±glass±other silicates) believed to have formed from trapped basaltic melts. Vitreous basaltic glass±CO₂±sulphide±silicates are common as secondary inclusions in the ultramafic xenoliths, and as primary inclusions in the gabbroic xenoliths. Microthermometry gives minimum trapping temperatures of 1110° C for the early ultramafic-and mafic-glass inclusions, and a maximum of 1260–1280° C for late inclusions of host basaltic glass. In most samples the CO₂ inclusions show a wide range in homogenization temperatures (-40 to +31° C) as a result of decrepitation during ascent. The lowest homogenization temperatures of about-40° C, recorded in some of the smallest CO₂ inclusions, indicate a minimum depth of origin of 35 km (12 kbar) for both the Cr-diopside-and Al-augite-series xenoliths. The gabbroic xenoliths originate from a former magma chamber at a depth of 6–12 km.Contribution no. 100 of the Norwegian programme of the International Lithosphere Project