Sr-Nd-Pb isotope data for ultramafic xenoliths from Hierro, Canary Islands: Melt infiltration processes in the upper mantle

Abstract

We present here Sr, Nd, and Pb-isotopic data from harzburgite (group I) and dunite-pyroxenite (group II) suite mantle xenoliths from the island of Hierro, one of the youngest and westernmost of the Canary Islands. A progressive leaching technique has been developed and applied to the whole-rock powder samples in order to identify and remove as far as possible any recent additions (host basalt and/or sea-water). Isotopic analyses of the leached residues show significant systematic differences between these two suites. Dunite-pyroxenite suite xenoliths (olivine pyroxenites, dunites and wehrlites) exhibit a relatively small range of isotopic compositions (87Sr/86Sr from 0.70292 to 0.70315; 143Nd/144Nd from 0.51295 to 0.51302; 206Pb/204Pb from 19.18 to 19.40) compared to the harzburgite suite (87Sr/86Sr from 0.70295 to 0.70320; 143Nd/144Nd from 0.51285 to 0.51296; 206Pb/204Pb from 18.85 to 19.41). In all isotope correlation diagrams the leached dunite-pyroxenite suite xenoliths plot between the Hierro basalt field and a hypothetical depleted mantle suggesting that these xenoliths may have been strongly infiltrated by Hierro-type basalt. Progressive leaching of this suite of samples showed removal of a component with more enriched Sr (higher 87Sr/86Sr relative to depleted mantle) and Nd (lower 143Nd/144Nd) isotopic compositions that is probably host basalt glass. The leached harzburgite suite xenoliths extend to more enriched Sr and Nd isotopic compositions than Hierro-type basalt but always have more depleted Pb. This relationship can best be explained if this suite has been subject to infiltration by earlier magmas of the Canary Island suite (in particular, those from Gran Canaria show appropriate compositional ranges), although additional infiltration by Hierro basalt cannot be ruled out. The leaching experiments for this suite mostly show removal of a radiogenic Sr component only (? seawater) which supports the interpretation of early infiltration and subsequent recrystallisation and equilibration prior to the Hierro event. Isotopic data presented in this study show that complex interaction with percolating basaltic melts of varying composition was occurring in the upper mantle beneath Hierro prior to and during the volcanic event and was probably related to the generation of earlier Canary Island magmas.