What tree is this?

Hey everyone! I have a tree that volunteered near my compost pile, which means it could be something blown in OR a growth from a pit or seed that was composting.
I have tried just about every tree identification guide I can find and I still can't figure out what this little guy is.
I would just let it grow and see if I get any flowers or anything, but it's happily growing about 4 inches from my foundation.
Any help in identifying the little trouble maker would be extremely welcome so I can figure out a sutible place to move it to.

Thank you in advance!!!

Perhaps a walnut species. The plant looks exactly like my own saplings. If it is a walnut it is a very interesting addition.

https://en.wikipedia.org/wiki/Juglans_regia

Good to keep but you would want to check its spot. It does not go well with many other plants (juglone - see other posts on this site) and with horses.

Looks like a green ash (Fraxinus pennsylvanica) to me.  I wouldn't have guessed they would be found in AZ, but perhaps it's trying to outrun the emerald ash borer.  

Thanks guys!
I don't think its a walnut as the leaves aren't quite as....long as the walnut trees I'm used to at least.

As far as the Green Ash theory.....I think you may be right on the money there. We have a local Ash called the "Arizona Ash" or the "Velvet Ash", but this definitely looks more like the green ash. As far as how it got in my yard, maybe it blew in from neighbors ornamental somewhere?

In any case, if it is an Ash, I'm not sure what to do with it. I only live on .2 acre of property, and I keep reading about how these suckers have shallow root systems.
Anyone have any idea what a safe distance to plant this from, foundations, sidewalks, driveways, plumbing, ect is?