SR-ND-PB ISOTOPE DATA FOR ULTRAMAFIC XENOLITHS FROM HIERRO, CANARY-ISLANDS - MELT INFILTRATION PROCESSES IN THE UPPER-MANTLE

We present here Sr, Nd, and Pb-isotopic data from harzburgite (group I ) and dunite-pyroxenite (group II) suite mantle xenoliths from the isl and of Hierro, one of the youngest and westernmost of the Canary Islan ds. 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 seawater). Isotop ic analyses of the leached residues show significant systematic differ ences between these two suites. Dunite-pyroxenite suite xenoliths (oli vine pyroxenites, dunites and wehrlites) exhibit a relatively small ra nge of isotopic compositions (Sr-87/Sr-86 from 0.70292 to 0.70315; Nd- 143/Nd-144 from 0.51295 to 0.51302; Pb-206/Pb-204 from 19.18 to 19.40) compared to the harzburgite suite (Sr-87/Sr-86 from 0.70295 to 0.7032 0; Nd-143/Nd-144 from 0.51285 to 0.51296; Pb-206/Pb-204 from 18.85 to 19.41). In all isotope correlation diagrams the leached dunite-pyroxen ite suite xenoliths plot between the Hierro basalt field and a hypothe tical depleted mantle suggesting that these xenoliths may have been st rongly infiltrated by Hierro-type basalt. Progressive leaching of this suite of samples showed removal of a component with more enriched Sr (higher Sr-87/Sr-86 relative to depleted mantle) and Nd (lower Nd-143/ Nd-144) 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 dep leted Pb. This relationship can best be explained if this suite has be en subject to infiltration by earlier magmas of the Canary Island suit e (in particular, those from Gran Canaria show appropriate composition al ranges), although additional infiltration by Hierro basalt cannot b e ruled out. The leaching experiments for this suite mostly show remov al of a radiogenic Sr component only (? seawater) which supports the i nterpretation of early infiltration and subsequent recrystallisation a nd equilibration prior to the Hierro event. Isotopic data presented in this study show that complex interaction with percolating basaltic me lts of varying composition was occurring in the upper mantle beneath H ierro prior to and during the volcanic event and was probably related to the generation of earlier Canary Island magmas.