I searched the US Berkeley Physics-Astronomy library for over a year and never found any research proving that the material in cometary tails is outgassed from the nucleus. Not one paper.
For that matter I never found any research proving that the material in the coma of comets outgassed from the nucleus. In the absence of evidence -- you have to conclude that these are assumptions.
You clearly subscribe to the ice hypothesis. But you are giving too much credit to those who presume to know.
Please post the evidence -- or papers -- that back up your assertion -- and I'll look at it.