Origins of ultralow velocity zones through slab-derived metallic melt

Understanding the ultralow velocity zones (ULVZs) places constraints on the chemical composition and thermal structure of deep Earth and provides critical information on the dynamics of large-scale mantle convection, but their origin has remained enigmatic for decades. Recent studies suggest that metallic iron and carbon are produced in subducted slabs when they sink beyond a depth of 250 km. Here we show that the eutectic melting curve of the iron−carbon system crosses the current geotherm near Earth’s core−mantle boundary, suggesting that dense metallic melt may form in the lowermost mantle. If concentrated into isolated patches, such melt could produce the seismically observed density and velocity features of ULVZs. Depending on the wetting behavior of the metallic melt, the resultant ULVZs may be short-lived domains that are replenished or regenerated through subduction, or long-lasting regions containing both metallic and silicate melts. Slab-derived metallic melt may produce another type of ULVZ that escapes core sequestration by reacting with the mantle to form iron-rich postbridgmanite or ferropericlase. The hypotheses connect peculiar features near Earth''s core−mantle boundary to subduction of the oceanic lithosphere through the deep carbon cycle.Ultralow velocity zones (ULVZs) occur as isolated patches near the core−mantle boundary (CMB) and are generally associated with the large low shear velocity provinces (LLSVPs) (1, 2). The nonubiquitous distribution of ULVZs gives evidence for thermal and/or chemical heterogeneities at the base of the mantle (3, 4). The density excess of ULVZs likely arises from iron enrichment (5–7), whereas the velocity anomalies may indicate partial melting (3, 4, 8, 9) or iron enrichment (5–7). Elucidating the origin of ULVZs is therefore important for understanding the thermal and chemical state of the CMB, which, in turn, holds a key to unraveling the evolution history and dynamics of deep Earth.Given uncertainties in the melting behavior of mantle rocks (10), elastic properties of relevant phases (11, 12), and iron partitioning between them (13, 14), the origin of ULVZs remains enigmatic. Partial melt of silicate composition has been widely considered as the origin of ULVZs because the presence of partial melt reduces shear wave velocity (Vs) effectively, and partial melt was found to be denser than coexisting solids at deep mantle conditions (e.g., refs. 4, 13, 15, and 16). Models involving silicate partial melt face several challenges. First, the solidus temperatures of silicate compositions happen to fall into the ±500 K uncertainty margin of the CMB temperature. Consequently, nonubiquitous partial melting of a silicate composition critically depends on thermal structure of the lowermost mantle, and the presence of chemically distinct components is often invoked to explain the occurrence of patchy melts near the CMB. Given the controversy over the mantle solidus (3, 8–10) and CMB temperatures (17), these models are still under debate and remain to be tested. Second, Nomura et al. (13) found that silicate liquid is, at most, 8% denser than the coexisting solids, and therefore only fully molten pockets can marginally match the density excess of ULVZs, which would then give rise to a vanishing Vs that is too low to match the seismic observations. Similarly, Thomas et al. (18) concluded that residual liquids produced in a whole-mantle magma ocean are not dense enough to remain at the CMB on geological timescales. Furthermore, Thomas and Asimow (19) showed that dense silicate melt must be removed from its equilibrium solid matrix to combine with a denser solid to match the density excess of ULVZs.Iron-rich solid phases such as wüstite, postbridgmanite, or iron silicide have been proposed as alternative origins of ULVZs (e.g., refs. 5–7, 10, and 20). Candidate sources for iron enrichment include the core or core sediments, residual liquids from a putative basal magma ocean, or subducted banded iron formation. It remains unknown or controversial if iron-rich solids with required composition and properties can be produced near the CMB. For instance, Knittle and Jeanloz (20) attributed ULVZs to FeSi and FeO as core−mantle reaction products although a subsequent study did not produce FeSi from reaction between bridgmanite and molten iron (21). Some of these models showed that simultaneous match of density (ρ), compressional wave velocity (Vp), and Vs can be achieved for certain compositions at 300 K (7), but recent theoretical studies concluded that, at high temperatures, iron-rich wüstite or bridgmanite could not reproduce both Vp and Vs (11, 12).Seismic tomography revealed that subducted slabs sometimes penetrated the transition zone to reach the CMB (22). Carbonates in the crustal portion of the slab may melt at the mantle wedge or in the transition zone and return to shallow depths (23, 24). On the other hand, the slabs that sank beyond the depth of 250 km are expected to contain metallic iron as a result of stabilization of ferric iron in pyroxene, garnet, or bridgmanite (25–27), plus elemental carbon or carbide through the reduction of carbonates by the metallic iron (28). To assess whether an iron−carbon mixture carried by slabs to the lowermost mantle would contribute to the origin of ULVZs, we investigated the melting behavior of the Fe−C system under the pressures of the lower mantle.