Explain reasons that some trace elements are not necessarily “trace”?

Question

Explain reasons that some trace elements are not necessarily “trace” in some igneous rocks in crust of the earth.

Answer 1

Well, in things like pegmitites, late stage crystalized rocks, you can have very high amounts of incompatable elements, like Li, Au, Ag, B, Zr, and many more. That's why you get minerals like beryl, lepidolite, tourmaline, etc.

Answer 2

Hmmm, I don't know if this is what you are looking for but there is a layer in the Earths crust that dates back to the dinosaurs that has unusual amounts of Iridium. It is not huge by any standard, but considering how little Iridium the is on Earth, the Iridium levels in this layer are comparatively extremely high.