GEOCHEMISTRY: differences in fractionation between Precambrian Eon and Cenozoic Era

I am writing a term paper on the fractionation that occurred in the Precambrian Eon as compared to the fractionation that occurs today. I am focusing on K/Rb ratios or REE, I haven't decided yet. I have gathered research but I was wondering if someone could provide me a brief overview, allowing me to orient my mind to the information.

I know this is very specific and is allot to ask, but if someone could give me an overview I would greatly appreciate it.

Thank you in advance,

Michael