Logical Flail on Earth
<Anyone who has taken basic Geology knows, the #1 gas emitted by volcanoes (on Earth of course) is water vapor.
Water which could've been subducted from an ocean filled with water supplied by comets and acquired during accretion.
I don't know that I've seen anything definitive on which is the dominant contributor but comets cannot be ruled out so easily.
Also, where would all the atmospheric nitrogen come from? Is there a significant ammount of N2 in volcano plumes?