How to find air composition a VERY long time ago.

How to Analyse Air a VERY Long Time Ago.

THIS ARTICLE introduces a couple of papers which use an unusual technique for finding the composition of the atmosphere before it became oxygen rich.

The article rehearses various reasons to think that the atmosphere was CO₂

rich and oxygen to be virtually absent. But putting numbers to this has, so far, been impossible.

One way to estimate the atmospheric composition is the chemical reaction of minerals with the atmosphere. And the most spectacular of these reactions is when a meteorite passes through the atmosphere. 

And it is this which is the subject of the two papers discussed. And they come to broadly similar results. One estimates a CO₂ content of 64% and a temperature of 30ºC, the other 25 to 50% and a temperature somewhat cooler.

The micrometeorites were pure iron ones, collected from 2.7Ga old sediments from the Pilbara of Western Australia, veneered with magnetite (Fe₃O₄) and wűstite (FeO). This veneer records their passage through the ancient atmosphere. You could get a similar surface coating from a passage through today's oxygen rich atmosphere. But we know that oxygen was virtually absent; CO₂ is the only viable candidate.

HERE is another article reviewing the same material but with a nice movie of a micrometeorite falling to Earth. 

Unfortunately none of the sources tell us where and how the micrometeorites were found and identified.