Deep divide in fate of iron
A large component of Earth's atmosphere comes from the interior, where the gas species are dictated by the redox state of the mantle. After formation of Earth's iron core, the mantle became several orders of magnitude more oxidized. Armstrong et al. conducted a set of experiments looking at the redox state of silicate melt representative of Earth's early magma oceans. They found that at some depth, iron oxide disproportionates into iron(III) oxide and metallic iron. The reduced iron sinks to the core, leaving an oxidized rocky mantle that emits carbon dioxide and water instead of more reduced species.
Science, this issue p. 903
Abstract
The composition of Earth’s atmosphere depends on the redox state of the mantle, which became more oxidizing at some stage after Earth’s core started to form. Through high-pressure experiments, we found that Fe2+ in a deep magma ocean would disproportionate to Fe3+ plus metallic iron at high pressures. The separation of this metallic iron to the core raised the oxidation state of the upper mantle, changing the chemistry of degassing volatiles that formed the atmosphere to more oxidized species. Additionally, the resulting gradient in redox state of the magma ocean allowed dissolved CO2 from the atmosphere to precipitate as diamond at depth. This explains Earth’s carbon-rich interior and suggests that redox evolution during accretion was an important variable in determining the composition of the terrestrial atmosphere.
Present-day noble gas abundances indicate that impacts caused extensive losses of Earth’s proto-atmosphere during accretion (1, 2). A substantial fraction of the atmosphere must therefore have formed by degassing of Earth’s interior (3, 4). The oxidation state of the upper mantle during the first 500 million years of Earth’s history had a major influence on the composition and evolution of the atmosphere, as it controlled the redox state of degassing volatile species (5–7). Before Earth’s metallic core was fully formed, the mantle was strongly reduced and would have degassed to produce an atmosphere dominated by the reduced gas species CO, CH4, and H2 (7, 8). If this state had persisted, these reduced species would have prevented the rise of atmospheric O2 (9). The upper mantle appears, however, to have been substantially more oxidized by the time the first minerals and rocks were formed. Redox conditions are quantified by the oxygen fugacity (
The main mechanism proposed to explain the increase in mantle redox state in the past has been oxidation by H2O accompanied by the loss of H2 to space (8, 15). Although this almost certainly occurred to some extent, the question remains as to whether there would be sufficient H2O left inside Earth after core formation to accomplish this. It is also unclear why Mars, a seemingly more volatile-rich planet than Earth, has an apparently more reduced primitive mantle (16–18). An alternative oxidation mechanism is based on FeO disproportionation caused by crystallization of bridgmanite, the dominant lower-mantle mineral. Experimental studies show that bridgmanite has a high Fe3+/ΣFe ratio when in equilibrium with iron metal (19–23). This implies that the equilibrium 3FeO = Fe0 + 2FeO1.5, involving ferric and ferrous iron components in mineral phases, shifted to the right as the lower mantle formed. This resulted in the disproportionation of FeO and the precipitation of iron metal (Fe0). Segregation of precipitated iron metal from the crystallizing lower mantle into the core could have raised the bulk oxygen content of the entire mantle after convective mixing (19). We show that the same FeO disproportionation mechanism must occur in silicate liquid at conditions approaching those of the lower mantle, and hypothesize that the increase in the oxidation state of Earth’s mantle was an inevitable consequence of the formation of one or more deep magma oceans.
We describe the
We can determine the sign of ΔV[1] by examining whether the Fe3+/ΣFe ratio of a silicate melt increases with pressure at a constant temperature and buffered oxygen fugacity. Previous studies performed up to 7 GPa (24, 25) indicated a positive ΔV[1], which is consistent with the 1-bar volumes and compressibilities (26), although it has been proposed that this may change at higher pressures (27). We extended these measurements through a series of multianvil experiments to 23 GPa. We chose a relatively polymerized andesitic silicate melt composition to facilitate glass formation when quenching at high pressures. We used two starting compositions so that we could approach the equilibrium Fe3+/ΣFe ratio both from an initially more oxidized and a more reduced composition. We equilibrated melts with a Ru-RuO2 buffer, placed in the sample capsule, that resulted in an
After equilibration at high pressure, we analyzed the Fe3+/ΣFe ratios of the quenched silicate melts using Mössbauer spectroscopy. Above 10 GPa, the silicate melt crystallized upon quenching instead of forming a glass. We assumed that the Fe3+/ΣFe ratios of the silicate melts were unmodified by crystallization. The Fe3+/ΣFe ratios we determined near the boundary between glass and crystallized melts were similar, and we did not have any multivalent elements in large enough concentrations to cause major changes in speciation through electron exchange during quenching (28).
We found an initial decrease in the Fe3+/ΣFe ratio with increasing pressure (Fig. 1), consistent with a positive ΔV[1], but the trend reversed above 10 GPa, indicating a negative ΔV[1]. We rationalized this behavior as being due to the compressibility of the Fe2O3 melt component becoming greater than that of FeO at high pressure. This could be caused by a pressure-induced change in coordination of Fe3+ in the melt (7, 29). We fit the data with a thermodynamic expression for Eq. 1 that describes the Fe3+/ΣFe ratio of the melt as a function of temperature, pressure,
We buffered the experimental oxygen fugacity either by the assemblage Ru + O2 = RuO2 (colored symbols indicate temperatures), which has an oxygen fugacity of ~ΔIW +8, or by equilibrium with Fe metal (gray squares), ~ΔIW –2. Downward- and upward-pointing triangles indicate initially fully oxidized and fully reduced starting materials, respectively. Results from previous studies are shown as open circles (24, 25). All starting compositions were andesitic except an experiment at 4 GPa that had a MORB melt composition (green diamond). The curves show the fit of our model to the experimental data. The gray curve is calculated for liquid iron metal saturation at 2373 K. The experimental temperature uncertainties are ~50 K.
The accretion of planetary embryos through giant impacts likely resulted in multiple phases of extensive or even complete melting of the proto-Earth (33–36). We used our model to calculate
We normalized the oxygen fugacity to the iron-wüstite buffer (ΔIW). The value of the FMQ (fayalite, magnetite, quartz) buffer is indicated by the red arrow. The present-day range in upper mantle
To calculate
If the precipitated metal segregates to the core, the net result is an increase in the Fe2O3 content of the silicate liquid. The separation of 0.1 weight percent metal to the core, followed by convective homogenization, would raise the Fe3+/ΣFe of the magma to 0.03 (Fig. 2), which is close to estimates of the present-day mantle (39). Greater Fe3+/ΣFe ratios may well have been reached through the separation of more iron metal to the core from progressively greater magma ocean depths, as the ratio of 0.03 estimated for the present-day upper mantle is probably lower than that of the bulk silicate Earth.
Our model shows that for a constant Fe3+/ΣFe ratio, maintained by convection, a gradient in melt
The removal of metal produced by FeO disproportionation may have raised the Fe3+/ΣFe ratio of the mantle even before core formation was complete. Equilibration with core-forming metal during accretion would have reduced mantle Fe3+/ΣFe ratios to very low values. If the later stages of Earth’s accretion, starting from a planetary embryo (i.e., a Mars-size body), occurred mainly through multiple giant collisions (33–36), FeO disproportionation within each of the resulting magma oceans would have raised the Fe3+/ΣFe ratio of the mantle once the impactor’s core had fully segregated. This implies that a H2O- and CO2-dominated atmosphere may have been maintained throughout the final stages of accretion. On the other hand, magma oceans on smaller bodies such as the Moon, Mars, and Vesta were of insufficient depth to cause disproportionation. This explains why their mantles are more reduced [closer to IW (16–18)], despite Mars forming from more volatile-rich, and therefore potentially more oxidized, material (42).
Our experiments were not able to address what happens to the redox conditions in magmas at much higher pressures, which could be relevant for impacts that melted the entire mantle. However, the compressibility of the Fe2O3 melt component rivals that of FeO as lower mantle pressures are approached, which may reverse the rising trend in melt Fe3+/ΣFe ratio with pressure. Our model shows some indication of this (Fig. 1) for the more oxidizing conditions. A larger unknown is the impact of electronic spin transitions involving both iron oxide components that could potentially influence the melt Fe3+/ΣFe ratio. These uncertainties are unlikely to negate the effect of FeO disproportionation, even if the latter were restricted to a depth interval near the top of the lower mantle, because the entire magma ocean would pass through this region as a result of convection. The metal produced would ultimately sink to the core, and the increase in Fe2O3 would be redistributed to the mantle as a whole through convective mixing.
A gradient in
The CO2 content (in mole fraction) of a CO2 vapor–saturated melt is shown by the blue curve (52); the black curves show the CO2 content of a diamond-saturated melt, calculated with two different methods (28, 52, 53). The magma CO2 concentration is a function of atmospheric CO2 partial pressure (7) but is potentially in the range 1 to 10 ppm, as indicated by the horizontal shaded region. The calculation is performed at 2273 K assuming an oxygen fugacity gradient constrained by a melt with a constant Fe3+/ΣFe ratio of 0.03. The CO2 content of the melt at diamond saturation drops with depth as
The increase in the oxidation state of the mantle before the end of accretion would also have influenced the conditions under which siderophile (iron metal–loving) elements partitioned into the core, particularly for impactors that were too small to influence mantle
Supplementary Materials
science.sciencemag.org/content/365/6456/903/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S7
Tables S1 to S6
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