Primordial metallic melt in the deep mantle

Zhang, Zhou; Dorfman, Susannah M.; Labidi, Jabrane; Zhang, Shuai; Li, Mingming; Manga, Michael; Stixrude, Lars; McDonough, William F.; Williams, Quentin

doi:10.1002/2016gl068560

Cited by 41 publications

(20 citation statements)

References 50 publications

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“…In all cases, the amount of metal trapped is below 2%, significantly less than previously believed (1,19,30). Such small amounts of trapped metal may be required to explain the abundance of siderophile elements in Earth (45,46,49), although siderophile elements in Earth's mantle may also originate from late accretion (50). The strong hysteresis in the connectivity of the melt network, reported here, therefore provides a mechanism for percolative core formation under reducing con-ditions, assuming the planetesimal contains sufficient metal to exceed the percolation threshold.…”

Section: Hysteresis In Melt Network Connectivitymentioning

confidence: 76%

Percolative core formation in planetesimals enabled by hysteresis in metal connectivity

Ghanbarzadeh¹,

Hesse

Prodanović

2017

Proc. Natl. Acad. Sci. U.S.A.

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The segregation of dense core-forming melts by porous flow is a natural mechanism for core formation in early planetesimals. However, experimental observations show that texturally equilibrated metallic melt does not wet the silicate grain boundaries and tends to reside in isolated pockets that prevent percolation. Here we use pore-scale simulations to determine the minimum melt fraction required to induce porous flow, the percolation threshold. The composition of terrestrial planets suggests that typical planetesimals contain enough metal to overcome this threshold. Nevertheless, it is currently thought that melt segregation is prevented by a pinch-off at melt fractions slightly below the percolation threshold. In contrast to previous work, our simulations on irregular grain geometries reveal that a texturally equilibrated melt network remains connected down to melt fractions of only 1 to 2%. This hysteresis in melt connectivity allows percolative core formation in planetesimals that contain enough metal to exceed the percolation threshold. Evidence for the percolation of metallic melt is provided by X-ray microtomography of primitive achondrite Northwest Africa (NWA) 2993. Microstructural analysis shows that the metal-silicate interface has characteristics expected for a texturally equilibrated pore network with a dihedral angle of ∼85°. The melt network therefore remained close to textural equilibrium despite a complex history. This suggests that the hysteresis in melt connectivity is a viable process for percolative core formation in the parent bodies of primitive achondrites.

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Section: Hysteresis In Melt Network Connectivitymentioning

confidence: 76%

Percolative core formation in planetesimals enabled by hysteresis in metal connectivity

Ghanbarzadeh¹,

Hesse

Prodanović

2017

Proc. Natl. Acad. Sci. U.S.A.

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“…This is an interesting scenario for HPE‐enriched piles, as aluminous Ca‐perovskite is also shown to easily incorporate HPEs (Gautron et al, ; Perry et al, ). Other suggestions for the composition of LLSVPs include metallic iron (Zhang et al, ), hydrous phases (Jiang & Zhang, ), and enrichment in both Fe and Al (Fukui et al, ). For most of these scenarios, there is limited constraint on the resulting densities and seismic velocities in the P‐T space required for our study.…”

Section: Discussionmentioning

confidence: 99%

Effects of Heat‐Producing Elements on the Stability of Deep Mantle Thermochemical Piles

Citron

Lourenço

Wilson

et al. 2020

Geochem Geophys Geosyst

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Geochemical observations of ocean island and mid-ocean ridge basalts suggest that abundances of heat-producing elements (HPEs: U, Th, and K) vary within the mantle. Combined with bulk silicate Earth models and constraints on the Earth's heat budget, these observations suggest the presence of a more enriched (potentially deep and undepleted) reservoir in the mantle. Such a reservoir may be related to seismically observed deep mantle structures known as large low shear velocity provinces (LLSVPs). LLSVPs might represent thermochemical piles of an intrinsically denser composition, and many studies have shown such piles to remain stable over hundreds of Myr or longer. However, few studies have examined if thermochemical piles can remain stable if they are enriched in HPEs, a necessary condition for them to constitute an enriched HPE reservoir. We conduct a suite of mantle convection simulations to examine the effect of HPE enrichment up to 25× the ambient mantle on pile stability. Model results are evaluated against present-day pile morphology and tested for resulting seismic signatures using self-consistent potential pile compositions. We find that stable piles can form from an initial basal layer of dense material even if the layer is enriched in HPEs, depending on the density of the layer and degree of HPE enrichment, with denser basal layers requiring increased HPE enrichment to form pile-like morphology instead of a stable layer. Thermochemical piles or LLSVPs may therefore constitute an enriched reservoir in the deep mantle. Plain Language SummaryThe amount and distribution of radioactive heat-producing elements within the mantle exert an important control on the thermal evolution of the mantle and core. Determining the composition of the mantle and its rate of heat production is difficult because several lines of evidence suggest that the Earth's mantle is not homogeneous, containing reservoirs of unmixed material. Such reservoirs may contain material enriched in radioactive elements and could be primordial, remaining isolated from the surface since Earth's formation. One possible physical location for such a reservoir is within "piles" of compositionally distinct material in the deep mantle. Such piles have been suggested by seismic observations, but it is unclear whether piles can persist if they are enriched in radioactive elements that heat the piles, promoting their buoyant rise and entrainment into the convecting mantle. We use geodynamic models to explore the dynamics of a compositionally distinct basal layer enriched in heat-producing radioactive elements. We determine the conditions under which the layer can be organized into piles that remain stable over geological timescales. We find that piles can remain stable, and we are able to reconcile the dynamical requirements for stability with seismic observations using models of lower mantle physical properties.

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“…These regions, dubbed large low‐shear velocity provinces (LLSVPs), are detectable by their increased density and decreased shear wave velocities and are sharply defined—implying a geochemical rather than solely thermodynamic origin. Since their observation, several potential explanations have been proposed including thermal plumes [ Thompson and Tackley , ], iron‐rich melts [ Zhang et al ., ], and variable oxidation [e.g., Gu et al ., ], but additional mineral physics investigations are needed to evaluate the stability and material properties of candidate materials at lower mantle conditions to find a robust answer to this mystery.…”

Section: Geophysical Implicationsmentioning

confidence: 99%

Elasticity of ε‐FeOOH: Seismic implications for Earth's lower mantle

2017

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We have calculated the structure and elasticity of low‐spin ferromagnetic ε‐FeOOH to 140 GPa using density functional theory calculations with a Coulombic self‐interaction term (U). Using these data, the elastic moduli and sound velocities of ε‐FeOOH were calculated across the pressure stability of the hydrogen bond symmetrized structure (30 to 140 GPa). The obtained values were compared with previously published values for phase H (MgSiH2O4) and δ‐AlOOH, which likely form a solid solution with ε‐FeOOH. In contrast to these Mg and Al end‐members, ε‐FeOOH has smaller diagonal and larger off‐diagonal elastic constants, leading to an eventual negative pressure dependence of its shear wave velocity. Because of this behavior, iron‐enriched solid solutions from this system have smaller shear wave velocities than surrounding mantle and therefore are a plausible contributor to large low‐shear velocity provinces (LLSVPs) which exhibit similar seismic properties. Additionally, ε‐FeOOH has substantial shear wave polarization anisotropy. Consequently, if iron‐rich solid solutions from the FeOOH–AlOOH–MgSiH2O4 system at the core‐mantle boundary exhibit significant lattice‐preferred orientation due to the strong shear stresses which occur there, it may help explain the seismically observed SH > SV anisotropy in this region.

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Primordial metallic melt in the deep mantle

Cited by 41 publications

References 50 publications

Percolative core formation in planetesimals enabled by hysteresis in metal connectivity

Percolative core formation in planetesimals enabled by hysteresis in metal connectivity

Effects of Heat‐Producing Elements on the Stability of Deep Mantle Thermochemical Piles

Elasticity of ε‐FeOOH: Seismic implications for Earth's lower mantle

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