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It is well accepted that the outer core is made out of liquid iron and nickel, and as everything else it should tend to reach chemical equilibrium with its surrounding.

In particular, I would expect it to interact with the lower mantle through mixing and chemical reactions with the oxygen, silicates and other compounds. These interaction would result in the outer core slowly dissolving away into the mantle.

I would also expect this to happen in spite of the density gradient, that anyway should be less relevant down there, where the acceleration of gravity is much smaller than in the surface.

Why this dissolution doesn't happen? Why is the outer core stable?

As a bonus question inspired by this one: would any other blob of molten iron be stable anywhere in the mantle?

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  • $\begingroup$ You are effectively lifting iron and lowering less dense materials hundreds of km. I think the gravitational potential energy is greater than the energy of solution (or whatever the physical chemistry terminology). I unsuccessfully looked for a quantitative treatment while considering Why is the core of Earth in a reduced state? (Fe and Ni). $\endgroup$
    – Keith McClary
    Apr 15, 2019 at 18:12
  • $\begingroup$ @KeithMcClary Segregation of materials by density is very intuitive but chemical mixing although less intuitive is surprisingly "powerful". Otherwise, all the heavy water (aka deuterium oxide) in the oceans would be at the bottom of deep trenches, and it is not the case. Look at this question that talk about the same in the atmosphere earthscience.stackexchange.com/q/13144/11908 $\endgroup$
    – Camilo Rada
    Apr 15, 2019 at 18:50
  • $\begingroup$ In this discussion of Differentiation Mechanisms they seem to assume no solubility or chemical reactions. $\endgroup$
    – Keith McClary
    Apr 15, 2019 at 19:24
  • $\begingroup$ KeithMcClary very interesting text, thank's for bringing it up. And indeed, after browsing trough a few sections they seem to assume that "Liquid metal separates rapidly from liquid silicate", like oil and water, and they don't mix later. Very interesting. I wonder what's @Gimelist opinion about this. $\endgroup$
    – Camilo Rada
    Apr 15, 2019 at 21:38
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    $\begingroup$ In the oceans, in the absence of stirring, the equilibrium salt concentration goes up exponentially with depth, by a factor of e for each 10 km or so. I expect the equilibrium concentrations in the Earth would also have exponential behaviour with a comparable characteristic depth. Over thousands of km this seems to be the dominant effect ( The core would need to be much hotter to stir it up to the undifferentiated condition!). $\endgroup$
    – Keith McClary
    Aug 11, 2019 at 17:50

2 Answers 2

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From the comments:

And indeed, after browsing trough a few sections they seem to assume that "Liquid metal separates rapidly from liquid silicate", like oil and water, and they don't mix later. Very interesting. I wonder what's @Gimelist opinion about this.

You are very correct.

The answer is simple: the core and mantle are immiscible. The core doesn't dissolve into the mantle because it's not soluble in the mantle.

This is precisely why we have the core and mantle to begin with.

My answer here is very relevant: https://worldbuilding.stackexchange.com/a/123951/39999

Let's take a hypothetical very simple core and mantle composition:

The core is made of pure iron ($\ce{Fe}$), and the mantle is fayalite ($\ce{Fe2SiO4}$).

What happens when you chemically react the two by $\ce{Fe + Fe2SiO4}$? Absolutely nothing. There is no intermediate $\ce{Fe3SiO4}$. Does not exist. So they just stay as separate iron and fayalite. And because iron is much denser than fayalite, it stays down at the core, whereas the fayalite is the mantle above it.

Edit:

Here's a recent paper describing the overall redox conditions of Earth compared to its neighbours:

https://doi.org/10.1038/s41467-020-15757-0

May be of relevance.

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This is an interesting question that is maybe a little misguided. Instead of answering your question directly I'd like to draw your attention to some things that might get you to rethink your reasoning.

It turns out that what you're thinking of as the lower mantle (an assemblage of solid phases: mainly bridgmanite + ferropericlase + some other stuff) might not actually extend to the core-mantle boundary (CMB).

In 1942, a mathematician and geophysicist named Keith Bullen came up with a scheme where he labeled the layers of the Earth alphabetically (the crust is A and the inner core is G). In this scheme, the entire lower mantle was designated as a single layer (D). In 1950 Bullen decided to split the D layer -- the top ~1800 km became D' and the bottom ~ 200 km became D''. The D'' layer is associated with a complicated seismic discontinuity, and geophysicists are not entirely in agreement on how to interpret the discontinuity. However, there is general agreement that the seismic signals from the D'' cannot be explained by assuming that it is simply an extension of the lower mantle to the CMB.

My preferred explanation is that D'' represents a thermochemical boundary layer that includes partial melting. A more in-depth explanation can be found here.

Regarding your question about blobs of iron...It's unlikely that blobs of pure liquid iron would be neutrally buoyant anywhere in the mantle. If you're interested in a more technical explanation of how the density of iron changes with depth (i.e., pressure), you may want to look into finite strain equations of state. The Birch-Murnaghan equation of state is a good place to start.

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