JonF
Posts: 634
Joined: Feb. 2005
|
| Quote (forastero @ Dec. 08 2011,05:26) | | Quote (JonF @ Dec. 05 2011,16:13) | | Quote (forastero @ Dec. 05 2011,16:28) | | Quote (JonF @ Dec. 04 2011,18:33) | | Quote (forastero @ Dec. 04 2011,13:42) |
Likewise, your isochrons are based on psuedocalibrations that dont exists and probably why JonF refuses to answer my long question about There are countless cases of discordance. |
I haven't seen any such question. You did ask how isochrons work and I pointed out that this is a terrible medium for teaching such things, and gave links (several times) to excellent explanations.
How ya doin' on telling me which of those quotes is your "cited ... proof that contamination is a major problem"? |
Arnt you the same fella that insisted that isochrons are calibrated with Milankovitch cycles? I dismissed it twice but you never respondeds |
No, I'm not that fellow. I don't know how isochrons correlate with Milankovitch cycles, but that correlation has nothing to do with contamination. And isochrons certainly aren't calibrated with Milankovitch cycles.
| Quote | As for contamination here is a good but I will look for some more
Mineral isochrons and isotopic fingerprinting: Pitfalls and promises Geology; January 2005; v. 33; no. 1; p. 29-32; 2005 Geological Society of America http://geology.geoscienceworld.org/cgi........i....29
Abstract: The determination of accurate and precise isochron ages for igneous rocks requires that the initial isotope ratios of the analyzed minerals are identical at the time of eruption or emplacement. Studies of young volcanic rocks at the mineral scale have shown this assumption to be invalid in many instances. Variations in initial isotope ratios can result in erroneous or imprecise ages. Nevertheless, it is possible for initial isotope ratio variation to be obscured in a statistically acceptable isochron. Independent age determinations and critical appraisal of petrography are needed to evaluate isotope data. . |
Still not contamination. Note that "Nevertheless, it is possible for initial isotope ratio variation to be obscured in a statistically acceptable isochron." Note also that almost all isochron dating is consilient with other methods such as U-Pb concordia-discordia, by far the most widely used method, and with Ar-Ar, probably the second most widely used method.
If you had any idea of how isochron dating works, you would know that errors due to initial isotope ratio mismatches are rare and why that is so.
You are wasting your time with isochrons. They have their uses, but if you want to discredit radiometric dating you need to be talking U-Pb and Ar-Ar.
I don't have a subscription to Geology, and they don't offer the option of purchasing a single article. Will you send me the PDF of the whole thing? I assume you're not just blindly copying what some creo website has to say ... hee hee hee. |
Of course they calibrate isochrones with Milankovich cycles and vice verse. From your very own Glen Davidson
http://www.schweizerbart.de/resourc....690.pdf |
Goodness me, there is an actual mention of calibration of Ar-Ar from Milankovitch cycles! I suspect that they really meant correlation. But how about the vice-versa?
{ETA} But see the next page ... they did really mean correlation.
| Quote | | U-Pb isochron dating methods depend upon major assumptions. 1. the lead isotopes were originally uranium but there is no way to know if some of the lead was already in the rock when it was formed--making it appear much older than it really is. Its a closed system but in reality floods are known to leach uranium out of rocks quite readily, which again makes the rock appear much older than it is. The same goes for other isotopes like potassium, which often makes modern lava flows are often dated as very ancient |
As I've pointed out many times, these are not problems with modern methods which detect such issues and often produce a valid age in spite of them
| Quote | Some problems with the 40Ar/39Ar technique.
Standard Intercalibration
In order for an age to be calculated by the 40Ar/39Ar technique, the J parameter must be known. For the J to be determined, a standard of known age must be irradiated with the samples of unknown age. Because this (primary) standard ultimately cannot be determined by 40Ar/39Ar, it must be first determined by another isotopic dating method. The method most commonly used to date the primary standard is the conventional K/Ar technique. The primary standard must be a mineral that is homogeneous, abundant and easily dated by the K/Ar and 40Ar/39Ar methods. Traditionally, this primary standard has been a hornblende from the McClure Mountains, Colorado (a.k.a. MMhb-1). Once an accurate and precise age is determined for the primary standard, other minerals can be dated relative to it by the 40Ar/39Ar method. These secondary minerals are often more convenient to date by the 40Ar/39Ar technique (e.g. sanidine). However, while it is often easy to determine the age of the primary standard by the K/Ar method, it is difficult for different dating laboratories to agree on the final age. Likewise, because of heterogeneity problems with the MMhb-1 sample, the K/Ar ages are not always reproducible. This imprecision (and inaccuracy) is transferred to the secondary minerals used daily by the 40Ar/39Ar technique. Fortunately, other techniques are available to re-evaluate and test the absolute ages of the standards used by the 40Ar/39Ar technique. Some of these include other isotopic dating techniques (e.g. U/Pb) and the astronomical polarity time scale (APTS).
|
So what? This is talking about improving the precision of the method. The errors are still not significant, not by many orders of magnitude, in the context of YEC.
K-Ar is not the sole method of dating the primary standard.
Newsflash: there are uncertainties in radiometric dates, as there are in any physical measurement.
| Quote | Decay Constants
Another issue affecting the ultimate precision and accuracy of the 40Ar/39Ar technique is the uncertainty in the decay constants for 40K. This uncertainty results from 1) the branched decay scheme of 40K and 2) the long half-life of 40K (1.25 billion years). As technology advances, it is likely that the decay constants used in the 40Ar/39Ar age equation will become continually more refined allowing much more accurate and precise ages to be determined. |
Yup. So what? They are just talking about reducing the already small uncertainty.
| Quote | J Factor
Because the J value is extrapolated from a standard to an unknown, the accuracy and precision on that J value is critical. J value uncertainty can be minimized by constraining the geometry of the standard relative to the unknown, both vertically and horizontally. The NMGRL does this by irradiating samples in machined aluminum disks where standards and unknowns alternate every other position. J error can also be reduced by analyzing more flux monitor aliquots per standard location. |
Yup. So what? They are just talking about reducing the already small uncertainty.
| Quote |
39Ar Recoil
The affects of irradiation on potassium-bearing rocks/minerals can sometimes result in anomalously old apparent ages. This is caused by the net loss of 39ArK from the sample by recoil (the kinetic energy imparted on a 39ArK atom by the emission of a proton during the (n,p) reaction). Recoil is likely in every potassium-bearing sample, but only becomes a significant problem with very fine grained minerals (e.g. clays) and glass. For multi-phase samples such as basaltic wholerocks, 39ArK redistribution may be more of a problem than net 39ArK loss. In this case, 39Ar may recoil out of a low-temperature, high-potassium mineral (e.g. K-feldspar) into a high-temperature, low potassium mineral (e.g. pyroxene). Such a phenomenon would great affect the shape of the age spectrum. |
Yup. So what? They are just talking about reducing the already small uncertainty.
| Quote | Problems and Limitations of the K/Ar dating technique
Because the K/Ar dating technique relies on the determining the absolute abundances of both 40Ar and potassium, there is not a reliable way to determine if the assumptions are valid. Argon loss and excess argon are two common problems that may cause erroneous ages to be determined. Argon loss occurs when radiogenic 40Ar (40Ar*) produced within a rock/mineral escapes sometime after its formation. Alteration and high temperature can damage a rock/mineral lattice sufficiently to allow 40Ar* to be released. This can cause the calculated K/Ar age to be younger than the "true" age of the dated material. Conversely, excess argon (40ArE) can cause the calculated K/Ar age to be older than the "true" age of the dated material. Excess argon is simply 40Ar that is attributed to radiogenic 40Ar and/or atmospheric 40Ar. Excess argon may be derived from the mantle, as bubbles trapped in a melt, in the case of a magma. Or it could be a xenocryst/xenolith trapped in a magma/lava during emplacement. |
All of those problems are possible, and are some of the reasons that K-Ar dating isn't used much anymore. Some of those problems are obviated by rational sample selections and processing. Of course, dates can be checked by comparing with other independent methods, and those checks indicate that he possible problems are rare.
In 40Ar/36Ar analyses of historic lava flows, Dalrymple tested whether 26 very young lava flows had excess argon. 18 of them did not. 8 of them had detectable excess argon, but only one had enough to affect an age of a few million years:
"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."
So excess argon is rare.
You need to demonstrate that the possible problems are near universal and, if you can do that, explain the consilience between different radiometric techniques and non-radiometric techniques. For example, Are Radioactive Dates Consistent With The Deeper-Is-Older Rule? (his source is available at http://pubs.er.usgs.gov/publica....86110).
Still waiting for some evidence that contamination is a problem.
|
