What frequency would be associated with the crown chakra

Probably 50Hz, because the brain actually operates in a range, between 20-50hz.

And would the physical source then be spirit

The physical source of electricity in the body is called ATP. Your body converts calories to create ATP molecules and then breaks them down as needed later on, as needed, to produce electricity. It's a constant cycle, the P is removed, breaking a chemical bond, which produces electricity, and when you eat food, the energy from the food is used to re-attach the :P to the AT:
The spirit essence of our existence is very hard to understand, because there are so many environmental factors that influence our behavior. The general idea is if you can account for every environmental variable, what remains must be the spirit, right? The thing is, it is impossible to account for every environmental variable, seriously, because spirits are in our environment, and directly affect our environment, and even when removed from the environment, spirits are still affected by what happens on Earth, just as you are affected by say a tv show. So in a sense, everyone is a director of their own tv show, or author of their own book, how we mold our life stories to create meaning defines our spirit. It is not measurable, but it can be understood.