This is a follow up question asked and answered (Radio-dating and the age of the earth)
The answer was given that the mineral Zircon is formed under high pressures and temperatures. How do we know, for example, most of the Zircon on Earth wasn't formed in some extraterrestrial event before the Earth formed?
It would seem strange to find that the oldest material found on Earth all formed in some extraterrestrial event that post-dates the ages of the oldest meteoritic material in the Solar System.
The Solar System is dated using the oldest solid material that can be found in ancient meteorites. This is dated using various techniques, not just the U-Pb decay chain (e.g. Rb-Sr; Sm-Nd). These ages all converge on an age for the Solar System of around 4.56-4.57 billion years since the first solids formed.
The oldest materials and minerals found on Earth are younger than this (the oldest zircons found yet are about 4.4 billion years old - Wilde et al. 2001), because the Earth solidified somewhat later. The hot formation of the Earth would have reset the clock on any materials incorporated into it at that time (see the same reference).
I can't see what event it could be that you propose enriched the Earth with zircon (note that zirconium is an element) but left no trace in the material that formed meteorites.
Edit: To address a possible point of confusion; the radioactive dating method tells you how long it is since the zircon mineral was formed, not how old the atoms of the chemical element zirconium are. All the atoms of zirconium were formed inside other stars by nuclear processes before the Solar System formed.
