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When pondering the hypothesis of the Iron catastrophe, that seem to be widely accepted nowadays. It surprises me that currently the crust still contain a significant amount of Iron (5.6% in weight).

I wonder if that much Iron is what didn't sink in the Iron catastrophe (just because the process wasn't perfect), or it can be explained by slow reloading with iron transported from the outer core and mantle to the crust trough convection and volcanic activity.

Or perhaps it cam from other sources like meteorites?

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  • $\begingroup$ Don't forget meteor impacts during the late heavy bombardment. (not an answer, just a possibility). I'd also add that the early oceans held some dissolved Iron that wouldn't sink. Much of the Iron we mine today is from the early oceans. $\endgroup$
    – userLTK
    Feb 8, 2018 at 18:57
  • $\begingroup$ @userLTK Good points, I'll add those considerations to the question. $\endgroup$
    – Camilo Rada
    Feb 8, 2018 at 18:58
  • $\begingroup$ @userLTK Thinking it again. Were there oceans during the iron catastrophe? I would think it was way too hot for liquid water on the surface. $\endgroup$
    – Camilo Rada
    Feb 8, 2018 at 19:11
  • $\begingroup$ Yes, that's a bit of a sticky subject and not one I really wanted to get into in detail. You asked Iron currently being 5.6% of the mass of the crust - some of that 5.6% came from Iron that was dissolved in the oceans prior to oxygen from photosynthesis pulling it out. I really just wanted to say - look at this possibility, I don't know enough to try to make an answer. $\endgroup$
    – userLTK
    Feb 8, 2018 at 19:17
  • $\begingroup$ considering 35%of the planet (on average) is iron and the crust is only about 5% iron you're not looking at a huge retention, keep in mind the iron is not always going to be heavier, it depends on if it is bonded to anything. $\endgroup$
    – John
    Feb 9, 2018 at 4:22

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Not all the iron sank to the core. Have a look at my answer to a previous question:

https://earthscience.stackexchange.com/a/7076/725

Your question is similar, but coming the other way. You're asking - why is there iron in the mantle/crust?

The answer is that there was enough oxygen left in the Earth after oxidising the other lithophile elements (Si, Mg, Al, Ca, etc) to oxidise some of the iron. And when iron is oxidised - it is not a metal anymore, but instead incorporated into rock forming minerals like olivine and pyroxenes.

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  • $\begingroup$ So all the siderophile elements attach themselves to iron before iron oxidizes? $\endgroup$
    – AtmosphericPrisonEscape
    Feb 9, 2018 at 13:31
  • $\begingroup$ @AtmosphericPrisonEscape yes. It's less about time, more about chemistry. Siderophile elements partition to metallic iron and do not partition into oxidised iron (silicates and oxides to be exact). $\endgroup$
    – Gimelist
    Feb 9, 2018 at 21:14
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    $\begingroup$ Ok, makes sense. So Iron is indeed a leftover of the iron catastrophe, and the reason why that fraction didn't sink is because it was oxidized. Thanks! $\endgroup$
    – Camilo Rada
    Feb 12, 2018 at 17:23
  • $\begingroup$ @CamiloRada exactly. $\endgroup$
    – Gimelist
    Feb 12, 2018 at 19:54

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