I am not aware of any actual measurements. To my knowledge, the area has not been been explored yet at all on the ground. Furthermore, the volcanic dome is probably of heterogeneous composition. However, you may be able to find information on the geology obtained by remote sensing, and could use data on similar terrestrial rocks (basaltic lava, volcanic ashes).

Thank you for the answer. Extrapolating properties of surface rocks will not work for me. The best will be to have gravity data.

volcanic rock density is often considered between 2800 and 3300kg/m3

Thank you for the answer. However, I'm looking for direct data on the Olympus Mons. It is only assumed that it is a regular volcano made from volcanic rock. But it's extraordinary size and shape allow to question this assumption.

So your question was about the mass and not about the density.

 -diameter of Olympus Mons : 550 km.

- height of Olympus Mons : 22 km 

=> volume = 1750000 km3

If we assume a density of about 3000 kg/m3 for the volcanic material, you obtain 5.23*10^18 kg for the mass of Olympus Mons.

I apologize if I was not clear. My question was not about its mass. I'm searching for any studies of its density. But not hypothetical, based on assumptions about the rock density. I would like to know if there were any gravity studies allowing to estimate its density based on measurements.

The Mars Global Surveyor probe was used to make some gravity measurements, you could try looking up any papers based on that.

http://science.nasa.gov/missions/mars-global-surveyor/