Initially, one has to have a uniform ratio of lead isotopes in the magma. Lead could easily reside in impurities and imperfections in the crystal structure. It would really be nice if geologists would just do a double blind study sometime to find out what the distributions of the ages are. Furthermore, here is a brief excerpt from a recent article which also indicates that isochrons often have severe problems. John Moffat of the physics department disagrees - light once travelled much faster than it does today, he believes. For example, it's not clear to me that we need to worry about isochrons or whether U and U dates etc. An online directory of dinosaur exhibits fro around the world.
Sialic granitic and mafic basaltic magma are separated from each other, with uranium and thorium chemically predestined to reside mainly in sialic magma and less in mafic rock.
The site yielded more than 1, bone fragments, an astonishing number in a field that often celebrates the identification of a single tooth. The latter may be explained away due to various mechanisms. Older rocks would have more time to absorb argon, and there was probably more argon coming through the earth at the time of the Flood and shortly thereafter than there is today.