There would be more then one type of contamination be possible during the lifecycle of a piece of mineral.
Also consider that something can accumulate or be washed out.
In a properly sampled igneous rock there should be a minimal chance of contamination during most of the lifetime of a mineral grain. I think the most vulnerable period would be in near surface conditions where weathering processes can occur. That is why proper sampling must be done.
Actually I think that one of the greatest potential sources for error would occur be in the lab where procedures must be subject to great care.