Zach
It's nearly impossible to give you a meaningful answer unless you give us a lot more information about exactly what you are seeking to do, and what you have tried so far.
Reading heavily between the lines of what you asked, my preliminary answer is no, it's not possible. That is assuming you're using the term "spectral signature" to mean a detailed spectrum such as in this graph: http://2.bp.blogspot.com/-zycoCXPneqI/UBf8Vz46NwI/AAAAAAAABU0/6z3mZZsPLRc/s400/ref+spectra+of+minera... - recording a signature with this much resolution would require a spectrometer with hundreds of channels, and Landsat 8 only has 11 relatively broad bands as explained here: Landsat 8 Overview « Landsat Science
On the other hand, those signatures are probably already known, so I'm not sure if this is what you were seeking.
If you really meant "can I extract spectral values in the Landsat 8 bands over a known site with exposed heavy metals?" then yes, of course, you can sample the pixel values at your site(s) using ArcGIS. I would argue that the result isn't truly a 'signature' but if your intent is to use Landsat 8 to attempt to find those same discrete spectral values at other sites, then effectively the answer is yes. You should be sure your Landsat scene is adjusted to yield surface reflectance values, and you'll want to learn about using the spectral classification tools in ArcGIS to find other target sites.
If you need more information, please try to be as detailed as possible (including which version of ArcGIS you are using) regarding your objectives.
Cody B.
