Wow, real data, thank you!!!

Very informative.

One question would be what the composition of Earth's early oceans was like. Am I right in having some vague sense that there used to be a lot more dissovled iron in our oceans, until that precipitated out into the banded iron formations about the time the atmosphere got oxygenated?

I'll answer my own question: yes, that seems to be about what the thinking is.

http://tchgrey.com/travel/info/geo/bifpaper.html

or the Google cache

http://www.google.com/custom?q=cache:4SsvmMmnlKUJ:tchgrey.com/travel/info/geo/bifpaper.html+%2Bferric+%2Bferrous+%2B%22banded+iron%22&hl=en&ie=UTF-8

And reading this short article on origins of banded iron formations, there sure are a lot of little bits that made me think of the Opportunity Formation -- chemically precipitated, soft, iron-rich deposits, probably put down in shallow sea or lagoon environments, often showing ripple and cross-bedding, sometimes with hematite-rich spherules embedded (thought to perhaps be microfossils or to have formed around microfossils -- these spheres are typically 5-50 micrometers, not a few mm, however). Of course, there are also bits that are NOT like our little outcrop, but it's still neat to be thinking of these sorts of analogies on another planet, and not just impact and volcanism!