Geochemistry of Lavas from Mauritius, Indian Ocean: Mantle Sources and Petrogenesis

Abstract

Trace-element and Nd-Sr-Pb isotopic data for mafic lavas from the Mascarene island of Mauritius reveal that the shield-stage Older Series (with 143Nd/144Nd = 0.512827 to 0.512853, 87Sr/86Sr = 0.70412 to 0.70433, and 206Pb/204Pb = 18.771 to 19.024) is geochemically very similar to the shield-building lavas of the younger island of Réunion. The post-shield Intermediate and Younger Series lavas of Mauritius have systematically greater values of 143Nd/144Nd and lower 87Sr/86Sr ratios than the Older Series and Réunion lavas. Intermediate and Younger Series Nd and Sr isotopic ratios are closely similar (0.512874 to 0.512899; 0.70369 to 0.70401), whereas 206Pb/204Pb ranges from values as great as any for the Older Series and Réunion lavas to as low as 18.608. Most of the Intermediate Series lavas are strongly enriched in highly incompatible elements relative to moderately incompatible ones (e.g., chondrite-normalized La/Lu = 4.03-17.8 vs. 9.38-10.2 for the Older Series), but the Younger Series lavas display much less relative enrichment (e.g., chondrite-normalized La/Lu = 3.86-5.37) and have low concentrations of incompatible elements. Modeling suggests that variable partial melting and mixing between mid-ocean-ridge-basalt (MORB) source mantle and mantle with bulk-Earth-like Nd and Sr isotopic and incompatible-element ratios, and "C"-like Pb isotope ratios, can explain the general relationship between the shield and post-shield lavas. All Mauritius and Réunion lavas can be explained as 2-8% partial melts of a bulk-Earth-like source that had been 1.5-0.25% metasomatized by a 0.1% MORB-source melt. The range in Pb isotopic compositions requires mixing with highly variable Sr/Pb and Nd/Pb ratios in the end members; such variation, however, is not supported by measured Sr/Pb, Nd/Pb, etc. in the lavas. Some isotopic heterogeneity thus appears to be required in the end members and/or much of the range in Pb isotopes represents radiogenic ingrowth since the metasomatic event. Near-constant 207Pb/204Pb values over the range of 206Pb204Pb among the Older Series lavas suggest that the metasomatic event must have occurred within the last few hundred million years.