No. Any lead-204 in a zircon was there when the zircon was formed. Period. Similarly, if you do a rubidium strontium date, any 84Sr, 86Sr and 88Sr was there at formation, since only 87Sr is a decay product. So using the non-radiogenic isotopes, you can calculate how much 87Sr was there at the beginning.
Excess helium in zircons has been measured. Google 'RATE project
I've seen it. In fact, I've analyzed the paper in detail. Now those are assumptions! They didn't find excess helium. They found more helium than they thought they should, based on some very dubious assumptions of helium closure temperatures, and some downright dishonest data analysis.
And your assertion that higher radioactive rates in the past would have overheated the earth is obviously based on an unmeasured assumption that the energy emitted is a function of the event rather than a function of time.
The Law of Conservation of Energy is not an assumption.
It is also disingenious to pretend that anomalous results do not occur in radiometric dating. They happen all the time and are thrown out if they do not meet 'a priori' assumptions.
All scientific work turns up anomalies. Good practice is to discard them only if there are independent reasons for doing so.
Now, what was it you were saying about 'no assumptions'?
I wasn't. I asked you to identify the assumptions. So far you're 0 for 3. Want to try again?
Right, and anomalous zircon data is thrown out as 'erroneous'.
"Even a suite of samples which do not have identical ages and initial 87Sr/86Sr ratios can be fitted to isochrons, such as areal isochrons. [p. 1] ...The theoretical basis of the classical Rb-Sr isochron is being challenged and some limitations of its basic assumptions are being revealed. [p. 2] As it is impossible to distinguish a valid isochron from an apparent isochron in the light of Rb-Sr isotopic data alone, caution must be taken in explaining the Rb-Sr isochron age of any geological system." [from Abstract, p. 1]"
[Y.F. Zheng, "Influences of the Nature of the Initial Rb-Sr System on Isochron Validity," Chemical Geology, Isotope Geoscience Section, Vol. 80, No. 1 (December 20, 1989), pp. 1-16 (emphasis added).]
You can't calculate how much 87Sr was present at the beginning without first *assuming* that it is relatively proportional w/ 84Sr, 86Sr & 88Sr. Because of this assumption, you are *assuming* the amount of 87Sr that was present when the isochron was formed. Maybe that doesn't qualify as an assumption in your mind, but it does in mine.
And yes, they did find excess helium. You do the same thing when you assume how much helium should be present. Why criticize your opponent for the same thing that you do?
And the Law of Conservation of Energy is not violated with a faster rate with lower energies per event. See Setterfield.
http://www.setterfield.org/zpe.htm#zpeandatom
When 3 of 8 isochron samples by Dalrymple return dates of 34 billion years, there are no good 'independent' reasons for discarding these anomalies.
(Dalrymple, G. B., 1984 How Old is the Earth?: A Reply to ‘Scientific’ Creationism In “Proceedings of the 63rd Annual Meeting of the Pacific Division, American Association for the Advancement of Science” vol. 1, pt. 3, Frank Awbrey and William Thwaites (Eds).)
You are the one who is backpedalling furiously. You went from 'no asumptions on radiometric dating' to a focus only on isochrons. That's a huge step backward as non-isochronic methods were once presented as reliable just as isochron methods are today.
The only difference is that science hasn't figured out all of the problems w/ isochron dating yet. But they are starting to come out and that's not good for you.