Joined: Feb. 2005
|Quote (afdave @ Aug. 16 2006,10:36)|
|LET'S TRY TO FIND ROCKS THAT CAN FIT OUR PRE-DETERMINED TIMESCALE|
"Sufficient parent and daughter isotopes to measure precisely?" What if it has NO daughter isotopes? Doesn't that mean it was formed VERY RECENTLY? i.e. not much time for daughter products to be formed? I mean, in theory, couldn't we go to a lab with a really hot furnace and make a zircon TODAY? in 2006? I don't know how hot the furnace has to be and how much pressure is required. Maybe beyond our capabilities. But if we could, the zircon should have ZERO daughter products, right?
|An ideal mineral is one that has sufficient parent and daughter isotopes to measure precisely, is chemically inert, contains little or no significant initial daughter isotopes, and retains daughter products at the highest possible temperatures. |
dating. (2006). In Encyclopædia Britannica. Retrieved August 16, 2006, from Encyclopædia Britannica Premium Service: http://www.britannica.com/eb/article-69760
|Is this article snippet telling me that there are plenty of samples with little to no daughter products ... and we are rejecting them as "undatable" because they don't have the right amounts of parent and daughter?|
No, it's not telling you anything at all like that. There's no mention of undateable samples, rejection of samples, or any "right" amount of parent or daughter. You're just projecting your preconceptions. In the rare cases when samples are rejected, they are rejected for clearly stated and objective reasons, not vague excuses.
In practice, there's no such thing as a sample with absolutely dead-nuts zero, unmeasurable, daughter product; there are eentsy weentsy (technical term) trace amounts of daughter product (lead) in all zircons at solidification, enough to throw off the dating results by a percent or two. That's not enough to give you any consolation; if we date a rock at 300 million years but it's really 295 million years old, it's still not young enough for you by a long shot. But it bothers geochronologists, so they develop correction factors for "common lead" (AKA "primordial lead"), to get the answers a little more precise. Most, but not all, of these techniques rely on measuring the amount of lead-204, which is not produced by radioactive decay, and therefore any lead-204 in the zircon was there at solidification. Knowing the amount of lead-204 and knowing the average ratio of lead-206 to lead-204 and of lead-207 to lead-204 allows us to calculate a very good approximation to the amounts of lead-207 and lead-206, which are produced by radioactrive decay, in the zircon at soldification. Since the correction is very small, the error involved in introducing it is also very small, well under 1% of the reported age.
Again obviously far over Davie's head, but the lurkers may be interested.
|Am I reading that right?|
Of course not, Dave, you should take that as given.