Melt evolution beneath thick lithosphere: a magmatic inclusion study of La Palma, Canary Islands

Abstract

Volcanism in the Canary Islands is notable for its highly alkalic character even in the shield building lavas of the currently most active island, La Palma. In order to understand better the processes responsible for this alkalic end of the compositional spectrum of ocean island basalts (OIB), we have studied primary magmatic inclusions in olivine and clinopyroxene from La Palma. The investigated primary melt and fluid inclusions are hosted by olivine and clinopyroxene from lavas of two ∼700-ka, continuous stratigraphic sections in the Fagundo and Izcagua barrancos of the main shield volcano Taburiente. Lavas from the two barrancos show contrasting whole-rock compositions: Izcaqua lavas are highly alkalic or “basanitic”, typical of La Palma lavas in general, whilst the Fagundo lavas are characterised by more silica-rich, “transitional”, compositions. Although whole-rock compositions of the lavas from the two barrancos show different degrees of silica undersaturation, they show a similar range of elemental abundances such as MgO. In contrast, the Fagundo melt inclusions show a wider range, to more mafic compositions, than those from the Izcagua samples. This is in keeping with more variable partial homogenisation temperatures and host mineral compositions in the Fagundo samples, which are also generally higher (1130–1260 °C) and more primitive ( F o 76–87) than in the Izcagua suite. Indeed, the Izcagua melt inclusions are hosted in relatively evolved liquidus mineral assemblages ( F o<82), and have (partial) homogenisation temperatures of 1080–1140 °C. The Izcagua samples lie at the differentiated end of a compositional array defined by the Fagundo melt inclusions suggesting that these basanites are derived by fractionation of more primary transitional melts. Fractionation of a clinopyroxene-rich assemblage can quantitatively account for production of an Izcagua-like melt from a mafic Fagundo parent. Modelling of phase equilibria suggests this process is consistent with differentiation at ∼12 kbar, which is also in keeping with clinopyroxene geobarometry in our samples. The basanitic lavas that largely comprise the shield building phase of La Palma have previously been interpreted by low degrees of melting as a result of the thick (Jurassic) lithospheric lid lying above the Canary mantle plume. Our study instead suggests deep fractionation as the major process leading to the highly alkalic nature of lavas in La Palma and possibly other ocean islands.