Origin of HIMU and EM-1 domains sampled by ocean island basalts, kimberlites and carbonatites: The role of CO 2-fluxed lower mantle melting in thermochemical upwellings

Abstract

Parent/daughter isotope ratios of the enriched mantle (EM) and high- μ (HIMU) mantle reservoirs sampled by ocean island basalt (OIB), kimberlite and carbonatite magmas are produced entirely by CO 2-fluxed melting in the lower mantle. The region of formation of the HIMU-EM complementary reservoirs is interpreted to be near the base of the lower mantle. By quantitative modelling of partial melting, using partition coefficients determined or inferred for lower mantle phases, we show that U/Pb, Rb/Sr, Sm/Nd, Lu/Hf, and Re/Os ratios that are characteristic of EM are associated with lower mantle carbon-rich partial melts, with the residues of melting evolving to HIMU compositions. Melting of lower mantle phases is most likely caused by carbonate-induced suppression of the liquidus in the vicinity of thermochemical upwellings. These mantle end-members likely originate from “pristine” mantle domains that are isolated from whole mantle convection within the lower mantle. Melts containing >1% CO 2 and residues are both variably buoyant, and allow separation in space of EM and HIMU. The HIMU component is a solid relatively refractory residue that can have a long residence time in the mantle and evolve to extreme isotopic compositions. In contrast the EM component is a liquid melt that can react with ambient mantle and thus not evolve to extreme isotopic compositions. Subsequent entrainment in thermochemical plumes transports EM and HIMU end-member isotopic compositions at different ascent rates to the site of typical OIB magmagenesis. This separate shallower melting domain is located near the top of the lower mantle and extends into the mantle transition zone. Our model thus resolves the long-standing conjecture regarding the origin of HIMU and EM reservoirs sampled by typical OIBs and continental flood basalts, and also by carbon-rich kimberlites and carbonatites. Contrary to the accepted theory, HIMU does not reflect involvement of hydrothermally altered oceanic crust, and EM does not require entrainment of continent-derived sediment introduced during subduction of oceanic lithosphere. By generating EM and HIMU through a single melting process involving pristine mantle, we provide an explanation for the presence of primitive rare gas end-member isotope ratios in magmas associated with mantle plumes. Our model also satisfies constraints on the character of mantle sources imposed by the Pb isotope paradoxes. Furthermore, our results show that the carbon in the deep mantle plays a very important role in Earth's bulk geochemical evolution.