Solid–liquid iron partitioning in Earth’s deep mantle

Abstract

Melting processes in the deep mantle have important implications for the origin of the deep-derived plumes believed to feed hotspot volcanoes such as those in Hawaii. They also provide insight into how the mantle has evolved, geochemically and dynamically, since the formation of Earth. Melt production in the shallow mantle is quite well understood, but deeper melting near the core-mantle boundary remains controversial. Modelling the dynamic behaviour of deep, partially molten mantle requires knowledge of the density contrast between solid and melt fractions. Although both positive and negative melt buoyancies can produce major chemical segregation between different geochemical reservoirs, each type of buoyancy yields drastically different geodynamical models. Ascent or descent of liquids in a partially molten deep mantle should contribute to surface volcanism or production of a deep magma ocean, respectively. We investigated phase relations in a partially molten chondritic-type material under deep-mantle conditions. Here we show that the iron partition coefficient between aluminium-bearing (Mg,Fe)SiO(3) perovskite and liquid is between 0.45 and 0.6, so iron is not as incompatible with deep-mantle minerals as has been reported previously. Calculated solid and melt density contrasts suggest that melt generated at the core-mantle boundary should be buoyant, and hence should segregate upwards. In the framework of the magma oceans induced by large meteoritic impacts on early Earth, our results imply that the magma crystallization should push the liquids towards the surface and form a deep solid residue depleted in incompatible elements.

Continent-sized anomalous zones with low seismic velocity at the base of Earth's mantle

“…Although an abrupt change in partitioning of Fe into melt over mineral phases has been suggested at a depth of approximately 2,000 km (ref. 68), more recent studies have questioned this observation 69,70 . Also, it is not fully understood how the fractional melting and crystallization will affect contents of other major elements, such as Mg, Si, Al, Ca and Na, and therefore mineralogy of the structures of the former melts.…”

Reconciling magma‐ocean crystallization models with the present‐day structure of the Earth's mantle

“…In this scenario, a BMO is separated from the "surficial" MO due to an isochemical density crossover between solids and liquids and/or a crossover in the slopes of the mantle solidus and adiabat (Andrault et al, 2017;Thomas et al, 2012). As any potential crossovers are inferred to occur at very high pressures, if at all (Andrault et al, 2012;de Koker et al, 2013;Ghosh and Karki, 2016;Nomura et al, 2011;Stixrude et al, 2009), they may only be relevant for (near-)complete melting of the early Earth's mantle, e.g. in a giant-impact scenario.…”

“…MO crystallization timescales strongly depend on the nature and thickness of the proto-atmosphere, which ultimately controls the heat flux from Earth to space (Lebrun et al, 2013;Marcq, 2012). As long as some degree of chemical fractionation occurs and the MO freezes from the bottom upwards, the MO is predicted to become progressively Fe-rich as Mg-rich bridgmanite in the lower mantle (Andrault et al, 2012;Boukare et al, 2015;Fiquet et al, 2010;Nomura et al, 2011;Tateno et al, 2014), and other Mg-rich phases (mostly olivine and pyroxenes) in the upper mantle, are successively removed. The related Fe-enrichment in the coexisting cumulates leads to unstable compositional density stratification in the early mantle (Elkins-Tanton, 2008).…”

Fate of MgSiO 3 melts at core–mantle boundary conditions

“…Although the partitioning of iron (K D ) between solid and melts remains controversial at lower mantle pressures (9,25), it is agreed that iron is an incompatible element. Therefore, iron is enriched in the melt phase with a K D value as low as ∼0.4-0.3 at 25 GPa (26,27) and most likely even lower at higher pressures.…”

“…In this scenario, the mantle starts to crystallize at an intermediate depth, and a raft of crystals continues to grow toward both the bottom and top of the mantle. In the modern mantle, seismic observations highlight ultralow velocity zones (ULVZs) near the CMB (8) that may be caused by melting of deep mantle material (9), potentially feeding sources of hot spots. Alternatively, the ULVZs may be dense remnants from an initial BMO (10,11).…”