Advice? One of my PhD supervisors is insisting on involvement with my article based off my Masters thesis. I'm a bioarchaeologist working on my PhD research in a field that incorporates biochemistry (stable isotope analysis) and bioarchaeology/anthropology. I have submitted my first peer-reviewed article based on my Masters thesis and am currently going through the revisions process. I casually mentioned this to my boss, who also happens to be one of my PhD supervisors, and he immediately asked to read the article and insisted that he wants to edit the stable isotope sections (he's a geochemist). I demurred, because the situation doesn't sit right with me, for several reasons:

First, he has absolutely nothing to do this article, as it is based on my Masters thesis research, which was done under a different supervisor and in a different country. Second, I'm not sure as to what his motives are. Does he simply want to help, or does he want co-authorship? I would rather not make him co-author, as, again, he has no business with my Masters research. Third, he's pushy. I have clearly stated that I am a bioarchaeologist/anthropologist who uses biochemical/geochemical methods (stable isotope analysis), not the other way around, yet he is still trying to push me towards geology. I also do not like this. But, because there is no other place in the country to do my research, I am here only for that reason. 

Does anyone have some advice? Am I overreacting, or am I right in feeling that he's out of bounds? 

Thanks!
The standard answer to all questions from graduate students applies here as well: Talk to your adviser. Have a frank conversation in which you bring up your concerns, have him explain to you why he wants to help, and then -- between reasonable adults -- come to a conclusion of the issue. 

You can speculate all day long about his reasons and maybe he about yours. But grown-ups can have a professional conversation that resolves these issues to everyone's satisfaction.