Isotopic variations within a single oceanic island: the Terceira case

Abstract

Since the early work of Gast et al.¹ it has been recognized that some isotopic variations exist on the scale of a single oceanic island^2,3, although the cause of such isotopic heterogeneity remains a matter of debate. For some authors, such as Grant⁴, the variations are related to local contamination in the island. Others, for example, Chase⁵, consider isotopic variation within an oceanic island to reflect mantle heterogeneity, so that it can be used to characterize the mantle and calculate the age of mantle differentiation (mantle isochron). This debate is extremely important in connection with the problem of the isotopic characteristics of the reservoir, the product of which built the oceanic island, and more generally for the structure of the mantle. Terceira is an island of the Azores Archipelago for which we have obtained several kinds of information, including geological⁶, geochronological⁷ and geochemical^8,9 data, thus enabling us to study its isotopic composition in a well known context. We report here a Sr and Pb isotopic analysis completed on the same samples as were analysed for the major and trace elements¹⁰, and K/Ar dating⁷. Our isotopic results are correlated with the emplacement age of the rocks. This observation is in agreement with a model of mixing between two different mantle components, as has previously been proposed^2,11,12 for other oceanic islands.