Tecnics 1200G platter cleaning

i am thinking its carbon graphite from the boston audio mat but that does not explain what looks like tarnishing around the edges of the platter

Mine still looks like brand new.

Dirty or oxidizing, there is a big difference. If it is oxidizing, chances are that it is not clear coated, therefore, you will need a metal polish.  Try polishing the platter's underside as a test before moving on to the top surface.

Try this: https://www.flitz.com/brass-copper-tarnish-remover/ 

"One man's dirt is another man's patina."I would just wipe off any dust and let the brass age gracefully.

I have a boston audio mat on it.  maybe its carbon graphite dust, not sure

Good question.  Without knowing more, I would just use a damp microfiber towel, rung out as far as it could go.  You don't want to use any chemicals that could stain the finish.  I don't know why the platter is getting dirty.  Dusty perhaps, but dirty?