There are lots of studies. Are any serious? Probably not at this time. That is, nothing is funded. The US is unlikely to do so, due to the fixation on the SLS which is basically a design that says "We don't need no stinkin depots". The Russians seem to have no great ambitions at this time. The Chinese are not yet at a sufficient state to consider this. Probably no one else in the world has a sufficiently serious program to be worth considering.
That is the direct answer. As an aside, there are many benefits to depots.
One that is important is that it opens a market for competition. That is, if someone needs a depot for a mission (be it a government or a company) they have just created a market. They need something. They are willing to pay for it. Now, companies can compete to provide payload to it.
Depots are great examples for this, since really the cargo being launched is dirt cheap. LOX, LH2, Kerosene, or water or whatever. So any launcher can deliver it.
The cheapest one will win. Reliability interestingly is not that important if the payload is cheap and the launcher cost is cheap, and there is no immediate time dependency.
Thus if the US for example, eschewed the SLS approach and went with a depot approach some interesting things could happen.
They could stimulate an industry for launch. Doesn't matter if it takes 100 Falcon 1e launches to fill the depot, if they are cheap enough, and launch frequently enough. Nor does it matter if you fill the depot with a Delta 4-Heavy launch. What matters is cost, and ability to deliver in time.