Most scientists believe earth's early archean atmosphere was formed primarily by gases released from:

It was formed by gases released from volcanic eruptions. The oldest rock formations exposed on the surface of the Earth are Archean. Archean rocks are found in Greenland, Siberia, the Canadian Shield, Montana and Wyoming, the Baltic Shield, Scotland, India, Brazil, Western Australia, and southern Africa. In Archean volcanic activity was considerably higher than today. There were numerous lava eruptions.