Modeling the isotopic evolution in the Earth, and geochemistry of Mauritius Island, Indian Ocean

Mass balance of heat producing elements U, Th, and K, constrained by the observed heat flow, strongly favors layered mantle convection. In contrast to the earlier views that the mantle is layered at the 660-km seismic discontinuity, results obtained here favor an upper mantle mass fraction m ≈ 0.45, which corresponds to a boundary at a depth of about 1200 ± 200 km. Mass balance together with consideration of Pb isotope ratios suggests that the 232Th/238U (κ) ratio in the continental crust is ∼5, and that the “canonical” value of 3.87–4.42 is too low.; A flexible multi-reservoir forward transport model of the Earth incorporating key isotope decay systems reproduces the Pb isotope systematics of the depleted upper mantle. In particular, upper mantle having low 238U/ 204Pb (μ) and 232Th/238U (κ) ratios, but high time-integrated values as inferred from the Pb isotope ratios are reproduced, which are a consequence of preferential subduction of U and radiogenic Pb from the upper continental crust into the upper mantle. A ubiquitous feature in all the versions of the model is the early depletion of incompatible elements in the upper mantle, and their enrichment in the early continental crust. Both steady state and time-variant incompatible element concentrations and ratios in the continental crust and upper mantle are possible. The model also suggests that the optimal value of the bulk silicate Earth K/U ratio is close to 10,000.; Trace element abundances and Sr, Nd, and Pb isotopic compositions determined for Older, Intermediate, and Younger Series of basaltic lavas from Mauritius, Indian Ocean, confirm that Mauritius is the product of Deccan-Réunion mantle plume. Each of the three series lavas has a unique, but internally heterogeneous mantle source. Older Series lavas were produced by melting of material from the core of the plume. Intermediate and Younger Series lavas, which have more depleted isotopic signatures, were derived from melting of entrained asthenospheric mantle. Eruption of both Intermediate and Younger Series lavas was triggered by decompression of lithosphere and underlying mantle due to the removal of mass of the volcano following large-scale erosion of the island. High ratios of light to heavy rare earths suggest that Mauritius magmas were generated in the garnet stability field.