I agree with the others about getting her depression treated somehow.

As for the arguing, when it gets heated, just tone down your own voice to a normal level, meaning a calmer level, and she most likely will also calm down in response. When one person acts to diffuse an argument, the other person cannot help but respond in kind.

Or, you can simply say, let's take some space right now until we have calmed down so that we can discuss this rationally and calmly. And stop the communication before it escalates into a heated argument or fight. All it takes is one person to diffuse and calm the situation.