Joined: Feb. 2005
|Quote (afdave @ Sep. 26 2006,09:50)|
|ENCYCLOPEDIA BRITANNICA ON "DATING"|
Not a particularly reliable source. It's too dumbed-down to be really accurate.
|ROCKS ARE "DATED" PRIMARILY BY FOSSILS AND BY THE ASSUMPTION THAT EVOLUTION HAS OCCURRED|
I said this long ago and was laughed at of course, but it is still true and here it is confirmed again. After the "date" is "determined" from fossils, then it is "confirmed" or "calibrated" with RM dating if possible.
|[DATING ...] in geology, determining a chronology or calendar of events in the history of the Earth, using to a large degree the evidence of organic evolution in the sedimentary rocks accumulated through geologic time in marine and continental environments.|
That quote does not support your claim. Yes, stratigraphy and index fossils are involved in dating. No, the date is not determined from fossils, absolute dates are determined from radiometric dating and index fossils are used to correlate between locations.
|CHERRY PICKING CONFIRMED AGAIN: ONLY "CERTAIN" ROCKS|
If certain rocks are not qualified for RM dating, then how can we qualify ANY rock legitimately?
|Rubidium–strontium (Rb–Sr) dating was the first technique in which the whole rock isochron method was extensively employed. Certain rocks that cooled quickly at the surface were found to give precisely defined linear isochrons, but many others did not. Some studies have shown that rubidium is very mobile both in fluids that migrate through the rock as it cools and in fluids that are present as the rock undergoes chemical weathering. Similar studies have shown that the samarium–neodymium (Sm–Nd) parent–daughter pair is more resistant to secondary migration but that, in this instance, sufficient initial spread in the abundance of the parent isotope is difficult to achieve.[This is what JonF was griping at me about on the Snelling data...]|
By cross-correlation with other methods, some not radiometric.
Here's another quote I like ...
... implying, of course, that "Fossils are King" ... they are standard by which other methods are judged.
|Absolute dating > Major methods of isotopic dating|
Isotopic dating relative to fossil dating requires a great deal of effort and depends on the integrated specialized skills of geologists, chemists, and physicists. It is, nevertheless, a valuable resource that allows correlations to be made over virtually all of Earth history with a precision once only possible with fossiliferous units that are restricted to the most recent 12 percent or so of geologic time.
Not implying any such thing.
|IOW ... "We used to only be able to date rock layers with fossils and it only covered 12% of geologic time. Now we can obtain the same precision on the remaining 88% of geologic time."|
Not quite. Now we can obtain the same precision on near 100% of geologic time, and verify the fossil dating.
|Notice they did NOT say, "RM dating allows to VERIFY our fossil-based guesses about millions of years." No no no.|
Yes, yes, yes, Davie-doodles, your lack of reading comprehension is showing again. They explicitly said that RM dating allows us to verify our fossil-based data. "Isotopic dating ... is, nevertheless, a valuable resource that allows correlations to be made over virtually all of Earth history." "Virtually all of Earth history" ain't no stinkin' 88%, Davie-dip, it's 99.9% or so.
|CONCORDANCE OBTAINED BY "PROCESSING" INDIVIDUAL GRAINS?"|
Moving along through the "Dating" article from EB ... we see that U-Pb dating supposedly is superior to other dating methods ... then we read something rather surprising ...
Now am I reading this right?
|Absolute dating > Major methods of isotopic dating > Uranium–lead method > Double uranium-lead chronometers|
Figure 2: Concordia diagram.
From T.E. Krogh, Geochimica et Cosmochimica Acta, vol. 46; © 1982 Pergamon Press
The reason why uranium–lead dating is superior to other methods is simple: there are two uranium–lead chronometers. Because there exist two radioactive uranium atoms (those of mass 235 and 238), two uranium–lead ages can be calculated for every analysis. The age results or equivalent daughter–parent ratio can then be plotted one against the other on a concordia diagram, as shown in Figure 2. If the point falls on the upper curve shown, the locus of identical ages, the result is said to be concordant, and a closed-system unequivocal age has been established. Any leakage of daughter isotopes from the system will cause the two ages calculated to differ, and data will plot below the curve. Because each of the daughters has a different half-life, early leakage will affect one system more than the other. Thus there is a built-in mechanism that can prove or disprove whether a valid age has been measured. Historically it had been observed that the uranium–lead systems in the mineral zircon from unmetamorphosed rocks were almost invariably disturbed or discordant but yielded a linear array on the concordia diagram. Given a set of variably disturbed samples, an extrapolation to zero disturbance was possible (see Figure 2). More recently, it has been found that of all the grains present in a rock a very few still retain closed isotopic systems but only in their interior parts. Thus grains with a diameter comparable to that of a human hair, selected under a microscope to be crack-free and of the highest possible quality, have been found to be more concordant than cracked grains. In addition, it has been shown that most such grains can be made much more concordant by mechanically removing their outer parts using an air-abrasion technique (upper points in Figure 2).
Nope. I don't know why you bother to ask that, the answer is always the same.
| You're tellng me that we can pretty much pitch all the mineral isochrons done on individual grains because they are open systems? |
Nope. He's not talking about isochrons. He's talking about a particular set of elements, one of which (lead) is fairly volatile as elements that are solid at STP go. He's talking about metamorphic rocks, which are always tricky.
|We have to strip away the outer part of the grain in order for the "dating" to be concordant?|
Sometimes yes and mostly no. That article's pretty seriously out of date. Since then instrumentation advances and sample preparation advances have cut down the need to strip away the outer part of the grain. Sometimes we still have to strip away the outer part of the mineral grain for the dating to be absolutely concordant (but not often) BUT you don't understand what "concordant" means in this context. Here "concordant" means" "absolutely exact agreement", and "discordant" means that the two methods disagree. The air abrasion technique is used to get concordant samples when the discordance is very small and there's reason to suspect differential leaching of lead from different parts of the sample. Even with "discordant" results, it's clear approximately how old the rock is; the air-abrasion technique is used to take results that are already accurate to a few percent and get the accuracy down below 1%. With or without discarding the outer layer of metamorphosed zircons, the rocks are far older than you think.
One of the great things about U-Pb concordia-discordia dating is that it often returns a valid and accurate date when the sampels are discordant, sometimes even seriously discordant. The reasons why are well understood.
And, of course, the truth that Davie dare not address; different methods are susceptible to different possible problems, but the observed concordance between different methods that are not susceptible to the same problems is one of the many reasons we can be sure that the vast majority of our dates reflect reality.
What have we here? You think my selection criteria is biased or something? Well, guess what? You're right!! I am biased and guess what else? EVERYONE is biased. That's what I've been telling you for a long time.
|Bet I could guess the selection criteria, and they won't be neutral.|
Yup. but you're an ignorant liar ; I make mistakes once in a while, but at least they're honest mistakes. Are you going to post any pf the many questions you have been ducking and can't answer? Or are you going to cherry-pick to try to give the impression that your claims stand up to casual inspection? I know the answer ...