Uses of the phrase “estimated at”

In short, yes, that construction is entirely correct. And yes, both of the sentences that you wrote are correct as well.

This sort of construction can be seen all over the place. The first example that comes to mind is in a boxing ring:

Announcer: And in the blue corner, weighing in at over 200 pounds, we have Rocky the destroyer!

Here are a few more examples from Merriam-Webster:

Examples of estimate:
They estimated the distance at about three miles.
The cost of the project has been estimated at about 10 million dollars.

As long as we follow our rules with prepositions, we should be fine. After “at,” we have to have a noun, which we do in this case with “tons.”

Google Ngram confirms that both options are almost equally popular:

However, if you did want other options, this works as well:

with an estimated mass of 10 billion tons, most of which is water ice.