The Chemical Structure, Composition, Extent of Mixing, and the Origin of Heterogeneities in the Earth’s Mantle
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AbstractFormation of chemical heterogeneities and the extent of mixing in the mantle are relevant questions to understanding large scale processes that dominate the bulk silicate Earth, which can be addressed using different methods of analyses. A shear wave velocity model was employed to evaluate the composition of the lower mantle. The model takes into account the effect of minor elements i.e., the Ca-bearing lower mantle phase and the effect of Al on Fe-Mg partitioning between bridgmanite and ferropericlase. The fractionation of the lithophile 176Lu-176Hf and 147Sm-143Nd systems in a hypothetical magma ocean was also calculated to constrain the composition of the lower mantle. These two methods demonstrate that the lower mantle composition is likely pyrolitic, and that the mantle is not chemically stratified at the 660-km discontinuity. The reproducibility of 142Nd/144Nd measurements was improved to 2σ = ±1.6 ppm (from 2σ = ±5–6 ppm). The measurements demonstrate resolvable variability in a modern day mid-ocean ridge basalt (MORB) sample, implying preservation of mantle heterogeneity for >4 Gyr. The distribution and variability of 142Nd/144Nd throughout time, through stochastic modeling, suggest the presence of plate tectonics already operating in the Hadean. Mg isotope measurements and major element analysis of alkalic rocks from the Oslo Rift, show these HIMU (high time-integrated 238U/204Pb) basalts to be from the peridotitic mantle, as the first strong evidence against HIMU compositions as a tracer of recycled oceanic crust. In addition, the major element compositions and Mg isotopes suggest the involvement of CO2 sourced from the indigenous mantle, as opposed to carbonated lithologies or carbonatite metasomatism. Modeling of the time scale of homogenization of the late veneer and the evolution of 182W/184W isotope signatures of the terrestrial mantle show that the late veneer signature takes a long time to homogenize, during which it is possible to sample the mantle 182W/184W values post-core formation into the Archean. It is also possible to resolve the distribution of 182W/184W and 142Nd/144Nd isotope variations with the same mixing rate. The 182W/184W and 142Nd/144Nd distribution in the mantle are consistent with the same stirring rate that applies to the long-lived radiogenic isotope systems.
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