Evidence for Fractional Crystallization of Periodically Refilled Magma Chambers in Tenerife, Canary Islands
Journal of Petrology
Major and trace element data on basanitic to phonolitic lavas of INTRODUCTION different ages and from different parts of Tenerife (Canary Islands), Melts in equilibrium with are and their mafic silicates have been used to obtain more detailed generally assumed to have suffered little or no modiinformation about processes taking place in crustal magma chambers fication by fractionation processes and to have chemical associated with ocean island magmatism in Tenerife. Clinopyroxene
... close to those of primary magmas. Ratios phenocrysts in basanitic to phonolitic lavas consist of diopside-salite of incompatible trace elements in such magmas are (referred to as Al-salite) with alternating normal and reverse zoning, assumed to be inherited from their mantle sources and and commonly contain a rounded or corroded core of homogeneous Naare frequently used to shed light on the trace element rich diopside-salite (referred to as Na-salite). In general Al-salite characteristics of these sources (e.g. Weaver, 1991). Until contains lower amounts of rare earth elements (REE) and Y, and more recently much of the attention given to ocean island Mg, Al, Ti, Cr, Sc and Ni than Na-salite. Variations in trace element basalts was focused on their mantle sources (e.g. Zindler concentrations and ratios are only weakly related to variations in mg-& Hart, 1986; Weaver, 1991; Chauvel et al., 1992; number. Petrographic and compositional relations among the lavas and Hofmann, 1997; Sims & DePaolo, 1997). mafic silicates are interpreted as the results of fractional crystallization However, a number of recent papers have shown that in periodically refilled magma chambers (FCM processes). The FCM ocean island magmas may undergo complex processes processes took place at temperatures of 1040-1260°C and pressures such as fractional crystallization at different depths, crystal 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 accumulation, magma mixing, contamination through processes may lead to significant fractionation between incompatible trace assimilation of hydrothermally altered basement, conelements whose ratios in mafic magmas are used to characterize their tamination by oceanic sediments, and contamination by mantle source(s). Melts subjected to FCM processes will, furthermore, melts generated in mantle lithosphere (e.g. Ablay et al., produce significantly larger masses of cumulates than melts of similar 1998; Class et al., 1998; Davis et al., 1998; Garcia et al., mg-number that have only been subjected to simple fractional crys-1998; Gee et al., 1998; Hoernle, 1998) . Before conclusions tallization. are drawn about the mantle sources of a given series of volcanic rocks, it is important to have detailed information about the shallow-level processes to which the magmas have been subjected, and the degree to which these processes have affected the trace element characteristics KEY WORDS: clinopyroxenes; trace elements; basalts; fractional crystallization; magma mixing; crustal structure of the magmas.