World's Oldest Rock?

When they are dating zircons, they are using U-Pb dating, which Dr. Adequate covered briefly in Message 221 in the Geology thread.Yeah, technically the zircon is a mineral, not a rock. Last I checked the oldest rocks came from the Great Slave Lake area of northwestern Canada; Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada: (They used the Tera-Wasserberg from of the concordia diagram, which you may not find familiar).There have been reports of possibly older rocks, e.g. Neodymium-142 Evidence for Hadean Mafic Crust (free registration required for full text), but that's not generally accepted yet (Comment on Neodymium-142 Evidence for Hadean Mafic Crust).

I think he/she is saying that the lead could only come from radioactive decay, which is the mainstream view.There is, of course, a really weentsy amount of lead mechanically trapped as the crystal grew around them (or whatever). That has a different isotopic composition than radiogenic lead and some other detectable properties, so correcting for "common lead" is very common ;-), and changes the raw date by a fraction of a percent or so. Buth these days the bleeding edge is sub-1% accuracy and every little bit of improvement is just that little bit of improvement more.

If It's Tuesday, This Must Be Belgium. If it's zircons, it's U-Pb.

I think I understand what you mean. It it's important to you, contact the author.However, we know what the answer will be. For good reason. I'll expound if you are interested.