Evidence for fractional crystallization of periodically refilled magma chambers in Tenerife, Canary Islands

Major and trace element data on basanitic to phonolitic lavas of different ages and from different parts of Tenerife (Canary Islands), and their mafic silicates have been used to obtain more detailed information about processes taking place in crustal magma chambers associated with ocean island magmatism in Tenerife. Clinopyroxene phenocrysts in basanitic to phonolitic lavas consist of diopside-salite (referred to as Al-salite) with alternating normal and reverse zoning, and commonly contain a rounded or corroded core of homogeneous Na-rich diopside-salite (referred to as Na-salite). In general Al-salite contains lower amounts of rare earth elements (REE) and Y, and more Mg, Al, Ti, Cr, Sc and Ni than Na-salite. Variations in trace element concentrations and ratios are only weakly related to variations in mg-number. Petrographic and compositional relations among the lavas and mafic silicates are interpreted as the results of fractional crystallization in periodically refilled magma chambers (FCM processes). The FCM processes took place at temperatures of 1040-1260°C and pressures of about 0.2-0.5 GPa; that is, partly within the old oceanic crust and partly within the overlying sequence of Canary Islands lavas. FCM processes may lead to significant fractionation between incompatible trace elements whose ratios in mafic magmas are used to characterize their mantle source(s). Melts subjected to FCM processes will, furthermore, produce significantly larger masses of cumulates than melts of similar mg-number that have only been subjected to simple fractional crystallization.