Geochemistry of lavas from Mauritius, Indian Ocean: Mande source's and petrogenesis

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 Nd-143/Nd-144 = 0.512827 to 0.512853, Sr-87/Sr-86 = 0.70412 to 0.70433, and Pb-206/Pb-204 = 18.771 to 19.024) is geochemically very similar to the shield-building lavas of the younger island of Reunion. The post-shield Intermediate and Younger Series lavas of Mauritius have systematically greater values of Nd-143/Nd-144 and lower Sr-87/Sr-86 ratios than the Older Series and Reunion lavas. Intermediate and Younger Series Nd and Sr isotopic ratios are closely similar (0.512874 to 0.512899; 0.70369 to 0.70401), whereas Pb-206/Pb-204 ranges from values as great as any for the Older Series and Reunion 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 Reunion 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 Pb-207/Pb-204 values over the range of Pb-206/Pb-204 among the Older Series lavas suggest that the metasomatic event must have occurred within the last few hundred million years.