JonF
Posts: 634
Joined: Feb. 2005
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| Quote (forastero @ Dec. 10 2011,13:40) | | Quote (JonF @ Dec. 10 2011,11:10) | | Quote (forastero @ Dec. 10 2011,11:36) | | And if huge reserves of oil are similarly contaminated by radioactive isotopes of the uranium-thorium decay, then surely whole isochron samples are also uniformly contaminated and/or leached. Same with all the excess Argon. |
I will respond to this part. You are an incredible over-generalizer.
No, contamination of oil and coal by goundwater or radioactive isotopes has no connection to U/Th/Pb dating or K-Ar or Ar-Ar or isochron methods in general. In case you haven't noticed, groundwater does not flow into the interior of a rock, from which we extract samples. None of the radioisotopes used in geologic dating are produced by particles produced by the decay of any radioisotopes, as 14C is produced from 14N by the alpha particles from decay of U and Th. |
Hmm sounds more grandiose closed system assumptions on your part. Again it can mess with the isochron ratio, which plots the ratio of radiogenic isotopes to non-radiogenic isope against the ratio of the parent isotope.
Btw, water diffuses into magma all the time. The following is a common problem from of hydrothermal contamination.
"In some cases, gain or loss of Rb and Sr from the rocks is so regular that a linear array can be produced on the conventional isochron diagram and a biased isochron results from the altered rocks to give spurious age and initial Sr-87/Sr-86 estimates,"..... "As it is impossible to distinguish a valid isochron from an apparent isochron in the light of Rb-Sr isotopic data alone, caution must be taken in explaining the Rb-Sr isochron age of any geological system. "..."In conclusion, some of the basic assumptions of the conventional Rb-Sr isochron method have to be modified and an observed isochron does not certainly define a valid age information for a geological system, even if a goodness of fit of the experimental data points is obtained in plotting Sr-87/Sr-86 vs. 87Rb/Sr-86. This problem cannot be overlooked, especially in evaluating the numerical time scale. Similar questions can also arise in applying the Sm-Nd and U-Pb isochron methods."
Zheng, Y.F., "Influences of the nature of the initial Rb-Sr system on isochron validity," Chemical Geology (Isotope Geoscience Section), vol. 80, pp. 1-16, 1989.
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We've made a lot of progress since 1989, but I keep telling you that Ar-Ar and U-Pb are where it's at.
| Quote | | "Certain assumptions presupposes that the concentration of uranium in any specimen has remained constant over the specimen's life...groundwater percolation can leach away a proportion of the uranium present in the rock crystals. The mobility of the uranium is such that as one part of a rock formation is being improvised another part can become abnormally enriched. Such changes can also take place at relatively low temperatures." J.D. Macdougall, “SHIFTY URANIUM”, Scientific American, Vol.235(6):118 |
Exactly what assumptions is he referring to? How does this pose a problem for U-Pb concordia-discordia dating? Or are you just pulling this quote from some creo site without understanding it?
I know where I'd put my money ...
| Quote | | And how do you explain the well founded "excess argon problem" welling up from the earth's mantle? |
I already did, a few pages ago. Do at least try to keep up. We know that excess argon is rare because of that consilience that scares you so much, and because Ar-Ar is not affected by excess argon, and because Dalrymple studied historic lava flows and found that one out of 26 had enough excess argon to affect a K-Ar date of a few million years (and that sample contained xenoliths which any geologist would recognize as making dating problematic) and 18 of the 26 had no excess argon at all. Case closed.
Remember, if one geologic radiometric date is correct, your fantasy is refuted. You need to show that every single date ever produced is incorrect.
A little free info from Dalrymple:
"The results (table 1) show that 18 of the samples have 40Ar/36Ar values that are not significantly different from atmospheric argon. The total amounts of 40Ar found for the samples are, in most cases, much larger than the average extraction line blank, which indicates that most of the argon comes from the samples and not the equipment. For the 21 samples that have 40Ar/36Ar ratios less than 300, it is possible to calculate 95% confidence limits for the presence of excess 40Ar. If excess 4OAr is present in any of these 21 samples, it should be less than the amounts shown in fig. 1. These upper limits are comparable to those calculated for seven Holocene sanidine samples from the Mono Craters [6] and more than two orders of magnitude less than excess 40Ar from intrusive and metamorphic rocks [1, 2]. Of the eight samples with anomalous argon compositions, five have 40Ar/36Ar ratios greater than atmospheric argon and three less. The calculated amounts of excess argon and the resulting apparent potassiumargon ages are given in table 2. Duplicate analyses of both the Hualalai and Sunset Crater flows give repeatable amounts of excess 40Ar despite the fact that different amounts of atmospheric argon contamination in the experiments resulted in different values for 40Ar/36Ar (table 1). The occurrence of excess 40Ar in the Hualalai flow is not surprising, because this flow is noted for its abundance and variety of ultramafic xenoliths. Naughton et al. [10] and Funkhouser [11] found ages as high as 3.0 × 109 years for xenoliths from this flow and reported that fluid inclusions with a high 40Ar content are common in minerals in the xenoliths. The consistent excess 40Ar values for the Hualalai and Sunset Crater flows suggests that large single inclusions are not directly responsible for the excess argon in these flows, but instead that the 40Ar is distributed more uniformly throughout the samples. Whether the 40Ar resides in fluid inclusions or in mineral lattices is not known, although fluid inclusions are not apparent in the samples analyzed. ...
With the exception of the Hualalai flow, the amounts of excess 40Ar and 36Ar found in the flows with anomalous 40Ar/36Ar ratios were too small to cause serious errors in potassium-argon dating of rocks a few million years old or older. However, these anomalous 40Ar/36Ar ratios could be a problem in dating very young rocks. If the present data are representative, argon of slightly anomalous composition can be expected in approximately one out of three volcanic rocks. "
{emphasis added}
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