Geochemistry of Mauritius and the origin of rejuvenescent volcanism on oceanic island volcanoes
Résumé
We report new trace element concentrations and Sr, Nd, Hf, and Pb isotopic ratios for basalts from the three lava series of Mauritius. Older Series lavas, which represent the shield-building phase of Mauritius, are isotopically similar to other lavas produced by the Réunion mantle plume. The Intermediate and Younger Series lavas, erupted after a hiatus of millions of years, have more depleted isotopic signatures than the Older Series. Incompatible element abundances and major element compositions suggest that the extent of melting was greatest for the Older Series, smallest for the Intermediate Series, and intermediate for the Younger Series. Volcanic evolution on Mauritius is thus broadly similar to the Hawaiian pattern, with lavas of the rejuvenescent phase being produced by small degrees of melting of a more depleted source than the shield phase. We propose that both shield and rejuvenescent phase magmas are derived from a lithologically heterogeneous or ``plum pudding'' mantle plume that consists of pods or veins of low-solidus-temperature material such as eclogite or pyroxenite embedded in peridotite matrix. The plums have a less depleted isotopic signature than the matrix. In the vertical stem of the plume, melting is zoned: at greatest depth, only the plums melt; at intermediate depth, both plums and matrix melt, and at shallowest depth, only the matrix melts. Shield stage magmas are hybrids of melts produced in all three zones. As the plume flattens out against the lithosphere and is dragged downstream of the stem, some melting continues due to buoyant lateral spreading of the plume. In this region, although both plums and matrix are melting, melts produced carry the isotopic signature of the matrix because incompatible elements in the plums were previously stripped during melting in the vertical stem of the plume. The relatively small degree melts produced in the melting tail, which give rise to rejuvenescent stage volcanism, thus carry the isotopic signature of the matrix. From a consideration of thermal and isotopic equilibrium, we judge the scale of heterogeneity to be about 102 m.
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