Ocean Chemistry Question

According to what I've read, the iron content in our aquariums should be
almost nil. When I lookied up what it is in the oceans, it was almost nil.
(0.000006 ppm).

Well here is my question for all you geology/chemist brains out there.

If the earth is made up of mostly iron, should there not be a high iron
content in the world's oceans? Why isn't there?

And that is my question of the day.

I am just curious, it is not important for my tank, as I don't even test for
iron.

GB