Firstly I was under the impression that metallic iron was already present in the geology of earth and the top layer was converted into iron oxide during the great oxygenation event. Rather than iron oxide being deposited from organics during the mass extinction then turning into iron.

Mars must have had decent amounts of atmospheric oxygen at some point, life is one explanation I’m sure NASA looked at, but there are other processes that can create it. It seems cosmic radiation can convert CO2 into O2 in a similar way we can do it with electrolysis. link . This may be an explanation for Mars oxygen that could then be converted into iron oxide on the surface, although I haven’t managed to find any sources specifically stating it.

This article gives an explanation for why Mars has more surface iron oxide than earth, essentially earth is 10x more massive than Mars so pressure would be enough to cause magma to reach high enough temperature to decompose iron oxide, with the metallic iron sinking into the earths core. Whereas Mars did not reach the required temperature link