What do you mean specifically? Are you suggesting that the several isotopes of Sr behave differently chemically? If not, then how will you arrange to vary the ratio of their concentrations in the minute parts of a lava flow that form a single zircon?
What can happen, according to what I've read, is that a zircon can be pulled into a later flow and act as the nucleus for additional crystal formation. Then the isochron won't give any age, or rather it would give two if we had enough data points.
I mean specifically that you can't *calculate* this value and you can't *observe* this value. You *first* make an *assumption*(well several actually but I am simplifying), *then* calculate. The *assumption* comes *first*.
If you can't see that, then you can't see that.
Sounds like some people know no more science than the average lawyer or pony-tailed journalist.