(ORDO NEWS) — If subduction transports hydrogenous minerals deep into the Earth‘s mantle, they can “rust” in the iron outer core, forming huge reserves of oxygen that can later be returned to the atmosphere.
Iron on the surface of the Earth – whether simple nails or powerful beams – when exposed to moist air or oxygenated water, gradually enters into a chemical reaction known as oxidation.
The reddish-brown product of this reaction, rust, can be composed of various forms of hydrated (water-containing) iron oxides and oxide-hydroxides of iron.
In nature, red rocks found in the arid regions of the US Southwest and elsewhere owe their color to the mineral hematite, which contains iron oxide, while in wetter environments, iron ore minerals such as hematite erode to form the mineral goethite (FeOOH) containing iron oxide-hydroxide.
Deep below the Earth’s surface – more precisely, at a depth of 2,900 kilometers – is a mass of mostly molten iron that forms the outer core of the planet. Can it also rust?
In experiments, scientists have recently shown that when iron encounters moisture either in the form of water or in the form of hydroxyl-containing minerals at a pressure close to 1 million atmospheres, similar to that in the deep lower mantle, it forms iron peroxide, or a high-pressure form of iron oxide-hydroxide with the same structure as pyrite (i.e., pyrite-type FeOOH) [Hu et al., 2016, Mao et al., 2017].
In other words, the oxidation reactions in these experiments do form high pressure rust.
If rust is indeed present where the outer core meets the mantle (the core-mantle boundary, or CMB), scientists may need to rethink their understanding of the Earth’s interior and history.
This rust may shed light on the deep water cycle in the lower mantle and the mysterious origin of ultra-low velocity zones (ULVZs) – small, thin regions at the top of the Earth’s liquid core that significantly slow down seismic waves (Figure 1).
It will also help answer questions about the Great Oxidation Event (GOE), which marked the beginning of the formation of Earth’s oxygen-rich atmosphere approximately 2.5-2.3 billion years ago, and the Neoproterozoic Oxidation Event (NOE) 1 billion-540 million years ago, in as a result of which free oxygen in the atmosphere reached its present level.
Rice. 1. The coloration of red rocks on the surface of the Earth – as seen here near West Mitten Butte, East Mitten Butte and Merrick Butte in Arizona – is mainly due to oxidized iron minerals hematite and goethite (top).
Possible rust deposits in the core at the core-mantle boundary (CMB), at a depth of 2,900 kilometers below the Earth’s surface, may consist of iron oxide-hydroxide minerals with a pyrite-like structure.
This rusty material may explain the detection of ultra-low velocity zones (ULVZ) in seismic data. The detection threshold ULVZ indicates the resolution of modern seismic tomography.
But how do you know if rust has occurred in the MDB?
Seismic signatures at the core-mantle boundary
While we cannot mine minerals in MMB, we can explore them in other ways. If the core rusts over time, then a layer of rust can accumulate on the MDB, showing certain seismic signatures.
Laboratory studies show that oxide-hydroxide core rust (i.e. FeOOHx, where x is 0-1) can cause a significant reduction in the velocities of seismic shear waves (Vs) and compression waves (Vp) that pass through it, similar to how do rocks (or partial melts, if they are present) do in the CLI (Liu et al., 2017).
In fact, core rust can slow down seismic wave velocities by 44% for Vs and 23% for Vp compared to the mean seismic velocities versus depth presented in the preliminary Earth reference model.
Such significant reductions in velocities would make rust in the core visible on seismic tomography if it accumulates in piles more than 3-5 kilometers thick.
The difficulty lies in determining whether the seismic anomalies in ULVZs are caused by rust in the core or if they are of some other origin.
For example, partial melting, which is commonly believed to occur at the base of the lower mantle and is the cause of the WWWZ [Williams and Garnero, 1996], can lead to a reduction in seismic velocities similar to that caused by core rust.
Scientists should be able to use seismic tomograms to distinguish core rust from partial melting in the PMB. A seismic tomogram is usually created using a mathematical inversion process that allows the calculated and observed seismic waveforms to be matched.
The inversion process requires identifying possible mathematical solutions that fit the data and then choosing the “best” solution from them based on additional considerations.
Each possible mathematical solution corresponds to a separate set of model parameters related to the physical properties of the respective materials – for example, the relative differences in Vs, Vp and density between the material of interest and the average of the surrounding mantle around that material.
Rice. 2. Shown here are ranges (orange and red) of seismic velocity ratios (δlnVs:δlnVp) for various materials proposed as sources of ultra-low velocity zones: iron-rich oxide, (Fe0.84Mg0.16)O; pyrite of the FeOOH0 type. 7 (possible core rust composition); carbon-iron melts, (Fe-C) melts; silicate perovskite and mantle partial melts, (Mg,Fe)SiO3 + partial melts; and post-perovskite hard silicate, PPv (Fe0.4Mg0.6)SiO3
These differences can vary with the amount of material in the mantle, but each material typically exhibits a characteristic range of differential log Vs to Vp (δlnVs:δlnVp) [Chen, 2021] values that can be used to distinguish between materials on seismic tomograms (Fig. 2 ).
It is known from experiments in mineral physics that this ratio ranges from a lower limit of 1.2 to 1 to an upper limit of 4.5 to 1 for all possible materials explaining the origin of ULVZs.
Within this broader range, the ratio for core rust (pyrite of the FeOOHx type) ranges from 1.6 to 1 to 2 to 1 and is different from other materials.
Evidence of the origin of rust in the kernels
To date, seismologists in search of the CIL have sampled about 60% of the MDB and have identified about 50 seismic anomaly locations, up to 20% of the area of the MDB, that may represent the CRL.
Most of these regions are associated with large low shear provinces (LLSVPs) in the lower mantle and exhibit a δlnVs:δlnVp ratio of approximately 3 to 1, indicative of partial melting (Fig. 2).
However, some of them, located on the margins or outside the LLSVP under the Pacific Ocean, show the best ratio of 2:1 [Chen, 2021]. For example, the CLI on the northern boundary of the Pacific WSWB (about 9° N, 151° W) (Hutko et al., 2009) and the CLI cluster under northern Mexico (about 24° N, 104° W) .e) (Havens and Revenaugh, 2001) have δlnVs:δlnVp ratios that indicate the presence of pyrite-type FeOOHx.
A common feature of these ULVZs is that they are located in the MDB region, where the temperature is relatively low – several hundred kelvins lower than the average temperature within the LLSVP. Low temperatures suggest that these zones were formed by a mechanism other than melting.
Notably, the region beneath northern Mexico has been identified as the remnants of a deep subduction that occurred approximately 200 million years ago west of North and Central America, supporting the idea that water released from the subducting slab may have rusted in the outer core of the MDB.
Consequences of a rusty core
Core rust (FeOOH0.7) can form when a relatively cold subducting slab of hydrogenic minerals meets the outer core.
Under the action of mantle convection, rust deposits from this cold region can then migrate along the core-mantle boundary to a hotter region at the root of the mantle plume, where they can become unstable and decompose into hematite (Fe₂O₃), water (H₂O), and oxygen (O₂).
The dominant mineral in the Earth’s lower mantle, bridgmanite, is believed to have little water retention capacity. However, core rusting can lead to the formation of a large-capacity reservoir in the MDB – FeOOHx rust can contain about 7% water by weight (Tang et al., 2021).
Because core rust is heavier than the middle mantle, this reservoir of water will tend to remain in the MDB. Thus, water could theoretically be transported and stored directly outside the core, at least until mantle convection sweeps it away from colder regions near subducted plate remnants and renders it thermally unstable (Figure 3).
Whether and when this deep water will cycle back to the surface depends largely on the thermal stability of the rust in the core.
Some scientists, based on experimental work, argue that FeOOHx can only persist up to 2400 K at a pressure in the CMB, while others have observed the presence of FeOOHx at 3100-3300 K at a similar pressure.
But whatever the maximum temperature that FeOOHx can withstand, it is likely that when the core rust migrates to hotter regions of the MDB, following the flow of mantle convection, it decomposes into hematite, water and oxygen.
This process offers a possible alternative explanation for the oxygenation history of the Earth’s atmosphere.
Geological, isotopic and chemical evidence suggests that during the Archean eon the Earth’s atmosphere was mostly or completely anoxic.
After the Archean, the first entry of molecular oxygen into the atmosphere began about 2.4 billion years ago in the GOE. The second major rise in atmospheric oxygen, NOE, occurred about 750 million years ago, bringing its concentration closer to today’s levels.
The reasons for these events associated with an increase in oxygen content remain unclear. One possible explanation for GOE is the emergence of cyanobacteria, the early photosynthetic precursors of plants.
The NOE that occurred almost 2 billion years later is explained by a rapid increase in marine photosynthesis and an increase in the photoperiod (i.e., an increase in the length of daylight hours) [Klatt et al., 2021].
However, these explanations are far from perfect. For example, in addition to the large discrepancy in time between the appearance of cyanobacteria on Earth and the HOE, a number of studies point to the possibility that a significant increase in atmospheric oxygen at the beginning of the HOE was followed by a deep drop to lower levels that stretched over several hundred million years. So far, there is no convincing explanation for this rise and fall based on cyanobacterial photosynthesis.
Moreover, although it is widely believed that HOE increased atmospheric oxygen concentration only marginally compared to the increase during NO, laboratory experiments investigating the effect of photoperiod on net oxygen export from microbial mats hosting competing photosynthetic and chemosynthetic communities provide conflicting results. result of.
Experiments have shown that instead of more oxygen being released from such mats as a result of the increase in day length during the NOE period, increasing the day length from 21 to 24 hours during the NOE period could only lead to half the increase in oxygen observed with increasing day length to 21 hours. during the GOE period.
Thus, the changes attributed to cyanobacteria and photoperiod length do not provide a complete or consistent explanation for the increase in atmospheric oxygen during GOE or NOE, and alternative mechanisms for the origin of these events cannot be ruled out.
Subduction, migration, convection, eruption
Decades of research have not produced conclusive evidence of when plate tectonics began on Earth. However, some recent research suggests that subduction began to bring hydrogenous minerals into the deep mantle prior to 3.3 billion years ago.
And experimental studies have shown that hydrogenic minerals in subducting slabs are able to transfer water up to the MMB (Ohtani, 2019). If so, then rusting could have occurred as soon as the first ancient slab met the core.
The rust of the core could gradually accumulate in the MMB, giving rise to ULVZ. As it moved away from the colder subduction region towards the molten outer core, driven by mantle convection, this pile heated up and likely became unstable when it reached the hotter region where the mantle plume originated (Fig. 3).
Just as typical volcanic eruptions occur periodically, the decomposition of rust in the core under the influence of temperature could lead to periodic bursts of oxygen at the surface. Unlike the gradual increase in oxygen from cyanobacterial photosynthesis, such a surge could release oxygen faster than the surface environment could react and absorb it, causing a rapid initial rise and subsequent fall in atmospheric oxygen levels.
The accumulation of a large pile of rust in the core and its migration to the site of thermal decomposition could take much longer compared to the duration of magma eruptions on the surface.
Indeed, some of the heaps formed may not have reached a hot enough area to cause decomposition, and their negative buoyancy among the surrounding deep mantle would have kept them on the MNB. Geological evidence suggests that the Earth’s surface was completely covered by ocean until about 3.2 billion years ago.
The clean removal of water from the surface and its storage in the deep mantle in core rust may have contributed to the emergence of continents in the Archaean, although changes in surface topography caused by plate tectonics and the growth of floating continents also contributed to this emergence.
Potential paradigm shift
Although everyone sees that iron rusts on the surface of the Earth, unfortunately, no one can directly prove that the liquid iron core of the Earth, located at a depth of 2,900 kilometers below the surface, rusts in the same way. However, ongoing research will help scrape away layers of uncertainty and answer basic questions such as whether core rusting is the cause of GOE and NOE.
In particular, more laboratory experiments are needed to accurately determine the limits of thermal and compositional stability of core rust in equilibrium with molten iron under MMB conditions.
For example, it is necessary to investigate the equilibrium between rust and liquid iron at high pressure and high temperature. Other studies could examine the thermal stability of rust at high pressures. These experiments are complex but feasible with the current experimental capabilities of laser-heated diamond anvil cells.
In addition, more work is needed to determine when the subduction began and, in particular, when “wet subduction” began, in which hydrogenic minerals enter the deep interior.
Geochemical evidence suggests that wet subduction began no earlier than 2.25 billion years ago, not 3.3 billion. This late onset of wet subduction may cast doubt on the hypothesis that core rusting was the cause of the GOE.
Moreover, the question of whether mantle convection includes stratified circulation (i.e., individual convective cells in the lower and upper mantle), whole-mantle circulation, or some hybrid of these scenarios still needs to be clarified.
If the mantle is dominated by layered circulation, then subducting slabs will be prevented from penetrating into the lower mantle. Thus, in order for slabs – and the hydrogenous minerals they carry – to reach the IMB and potentially cause rusting, either general mantle or hybrid convection must exist (Chen, 2016).
If all the pieces of the puzzle fall into place, then the rusting of the core could indeed be Earth’s massive internal oxygen generator – and the next big oxygenation event could be on the way.
The possibility of such an event would raise all sorts of questions about what impact it could have on the environment, climate and livability in the future.
In the near term, confirmation that the Earth’s core is rusting will lead to a paradigm shift in our understanding of the planet’s deep interior and how it fundamentally affects conditions and life on the surface.
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