Yes this works fine up to and a little beyond such a date but comparisons between Radio-Carbon years and known historical years show a variance in conclusion such that at 21,000 RC years we differ by 3,000 years with other historical analyses. And by 50,000 RC years other methods demonstrate there to only have been 36,000 years (and the farther one goes back in time the broader the variance becomes).
Yes, they are still working on the calibration for the older ages, mostly through lake varves and coral bands. The last I read, they have a fairly robust calibration to about 37,000 years ago. At which point there appears to have been some variations in the atmosphere, for which they are attempting to determine a cause. Although the current calibration charts do go back past 50k years, the last 15k years or so are less certain, pending more research.
As for how far off the RC dates are from the calibrated dates, it doesn't really matter. That is what the calibration is for. To adjust for that variance. Essentially, all it tells us is how different the atmospheric content of C14:C12 was back then relative to what the content is now (more precisely, what it was in 1950).
I used to calibrate electronic equipment in the military. Say I got a multimeter in the shop which read 130 volts when I only applied 100 volts to it from the voltage standard. That's a significant difference, and it needs to be calibrated. So, while that 100 volts is being applied, I tweaked a potentiometer until the multimeter read 100 volts. After that, any time you measured 100 volts using the multimeter, you knew that it really was 100 volts. It doesn't matter that the meter read 130 volts prior to the calibration.
And that's what happens with carbon dating. MOST carbon dates given are the
corrected value. That is the default position. If the uncalibrated date is used, it will be noted, usually by the letters "BP" afterward, which stand for Before Present. When this happens, it is understood that the data is essentially a ballpark figure.
Now I realize most of this opinion is coming out of my head and old notes I have collected over the years and you are correct in that I know little about the technicalities of Isochron dating (my field of knowledge is biology) but I have been told by Professor Daniel Schrag of Harvard University in a discussion forum a few years back that there are other reliable, intelligent, published scientists who question the reliability in some cases and that in quality scientific presentation their views should be (and among real scientists is) respected even if disagreed with ( IMO contrary data or evidence of possible error in findings or in what results were expected, should not be discarded but reported along with the concluding scientists own position as well as their reasons for why they disagree with the possibly dissenting information…which is the case with most scientists and what they publish). The sad thing for me is that in forums like these dissenters are not respected or their views really looked at for what they suggest.
Well, obviously you understand that the conversation you had with Mr. Schrag is anecdotal. And I obviously can't view the context of said discussion.
That said, there are examples where the data has been questioned, no doubt. And that's great. It's how we learn more about the process. But the fact that there are issues with "some cases" does not indicate a fatal flaw for the process itself. Nature throws us curveballs sometimes, and we have to find out why. And as you said, reputable scientists should figure out why their data doesn't add up. This happens all the time. In fact, it is unlikely that the paper would get published WITHOUT hypotheses which may explain the data through further study.
The problem with dissenting opinions on message boards is that they are, by and large, made by individuals who are hopelessly ignorant of the process, and incapable of offering a reasonable hypothesis, or even in many cases to understand whether it is a problem or not. Furthermore, virtually all of these dissenting opinions are not new issues that they have come up with on their own. They have been passed around creationist circles for decades, and they have received them 2nd, 3rd, 4th, etc. hand. And the worst part is that the first hand origin of these dissenting opinions are based on misunderstanding (or outright ignoring) of the resolutions given in the scientific papers to begin with. In other words, they are issues that science has dealt with extensively, often 40+ years ago.
Many laymen posting on message boards simply cannot comprehend how complicated geophysics and geochemistry is. They think they can pick out a little flaw that refutes the whole process, while totally ignorant of the big picture and the fact that science has already discussed the issue and resolved it or determined it irrelevant, decades ago.
Also I once read (and noted) a discussion forum where one researcher named Bale (sorry I no longer have a specific reference, but read his assessment and then comment) who says that in isochron dating, ages are obtained from the slope of a line based on isotope ratios measured for different minerals of the same age. He said, “The theory is that, although the different minerals have different initial amounts of the radioactive parent, the same percentage of the parent will decay in each case, with the result that the measured isotope ratios will fall on a line but that there are different isochron systems. “
G. Brent Dalrymple, Ph. D., "Radiometric Dating, Geologic Time, and the age of the Earth: A Reply to 'Scientific' Creationism," February, 1982, published by the U.S Geological Survey, discusses the Rb-Sr isochron, which plots the ratio Sr(87)/Sr(86) as a function of the ratio Rb(87)/Sr(86) ratio:
"When a rock is first formed, say from a magma, the Sr(87)/Sr(86) ratios in all of the minerals will be the same, regardless of the rubidium or strontium contents of the minerals, so all of the samples will plot on a horizontal line." (page 32)
However, Brooks, James, and Hart (C. Brooks, D.E. James, and S.R. Hart, "Ancient Lithosphere: Its Role in Young Continental Volcanism," Science, Vol. 193, September 17, 1976, pages 1086-1094, have found that this statement is not always true. They studied 30 examples given in the peer reviewed literature, correcting where necessary…
So according to isochron theory, the resulting diagrams, called pseudo-isochrons, should always be horizontal lines. However, the authors report:
"Correlation theory and regression analysis indicate that most of these psuedoisochrons have slopes significantly different from zero at confidence levels up to 95 percent (in some cases up to 99.9 percent) and that they define excess "ages" ranging from 70 million years to more than 3000 million years." (see page 1087)
Did you read the article by Brooks, James, and Hart? Did you notice that they did exactly what you would expect a good scientist to do, and offer interpretations of the "deviant" data?
Your quote is dangerously close to a quotemine. This paper is not a refutation of Dalrymple's work. And it is not, in any way, laying doubt upon the method. Indeed, they say in the paper:
We believe that the pseudoisochrons contain significant age information and are in reality mantle isochrons. It is the purpose of this article to elucidate this precept, and show that Sr isotopic study of young mafic volcanics provides windows through which the chronologic evolution of subcontinental mantle can be viewed, and regional variations mapped.
There are a couple of key points to be aware of here: 1. They are able to determine when the initial Sr87/Sr86 ratios deviate from horizontal. 2. It is not suggested that these results are the norm 3. The pseudoisochrons are useful information, in and of themselves
So, basically I fail to see why it is a problem that pseudoisochrons
sometimes occur, when we have the ability to detect them when they exist. Furthermore, they are irrelevant on isochrons in which the pseudoisochron test is negative (i.e., when the initial ratios form a horizontal line), which happens the majority of the time.
So Bale asked we please note that “the upper limit of error magnitude would give a newly formed rock an "age" 75% of the believed age of the earth.” The paper states that the cause of the error (which is really just data which disagrees with their expected results and is not “contamination” at all which in SOME cases is an excuse used to throw out or disregard the contrary findings) “could be contamination” (could be does not equal IS), but also states a number of reasons for believing it is not, putting forth the idea that the results represent the age of the underlying mantle that was the source of the magma.
Great. As I stated in my first reply, they are talking about mixing. Please see Dalrymple's reply to the mixing complaint.
I think you have a bit of a misconception about why scientists have "expected results." It is because situations like these are the exception to the rule, not because of any preconceived bias.
They want to determine why they get an unexpected result. In this case, they considered contamination, but ruled it out for a variety of reasons, in favor of a better alternative. From that point forward, more tests can be done on their hypothesis. This is a 40 year old paper. Have you researched papers which cited this one to find out if anyone contributed more insight to their mantle source hypothesis? Radiometric dating has come a LONG way in the last 40 years.
The conclusion of the authors is:
"One serious consequence of the mantle isochron model is that crystallization ages determined on basic igneous rocks by the Rb-Sr whole rock technique can be greater than the true age by many hundreds of millions of years. This problem of inherited age is more serious for younger rocks , and there are well-documented instances of conflicts between stratigraphic age and Rb-Sr age in the literature." (see page 1093).
and the quote goes on to say:
Effective use of the mantle isochron concept requires knowledge of actual crystalIization ages (so that the measured isochron can be divided into its pre-and postcrystallization components) and determination of isochron parameters on rocks that have been subjected to minimal postmelting processes (fractional crystallization, wall rock contamination, and so forth). The concept of Rb-Sr mantle isochrons as we have presented it provides a new tool for understanding the petrogenesis of mantle-derived igneous
rocks and for mapping the evolution of the various segments of the earth's
mantle
So...the study does not:
This study shows the apparent results gave too ancient of a date for SOME SP<samples (thus demonstrating probable inadequacies within this very excellent method). However, knowing these discrepancies, the authors of the study still contended that the dates represented real dates for the earth's mantle (thus, the probable possibility was excluded from the conclusion).
But rather:
The concept of Rb-Sr mantle isochrons as we have presented it provides a new tool for understanding the petrogenesis of mantle-derived igneous rocks and for mapping the evolution of the various segments of the earth's mantle
Moving on...
See also “Calculation of 230 TH/U Isochrons, Ages, and Errors” shared by , K, R, Ludwig (Geological Survey, Mail Stop 963, Denver, CO 80225-0046, USA),and D. M. Titterington (University of Glasgow, Department of Statistics, Glasgow G12 8QW, United Kingdom) where it is pointed out that “when analytical errors are responsible for the scatter of points on a 230Th-234U-238U isochron diagram”, the isochron should be fitted by a technique that “weights the points according to their analytical errors and error correlations, and either takes into account the presence of some of the same data in two coupled XY isochrons or (equivalently not specifically) uses a single, three-dimensional XYZ isochron.” Which they know and honestly admit is “a method based on maximum-likelihood estimation” (thus not on established indisputable fact). Therefore other conclusions are possible, even probable, and may in fact be the case. (all parentheses mine)
Sorry, but I'm not going to spend more time on another paper which likely doesn't say what you claim it does.
And I also do not believe in a 6,000 year old earth, but I want you to know your comments are noted, I appreciate them, and I will explore these comments more fully.
Interesting. Do you have an opinion on the age of the earth?
Here I do have an issue which would be that because something does not fit the model or the expected results this does not necessarily imply contamination unless can show how and when and by what means something was contaminated…at least define what the contamination actually was and how it occurred (and theoretical speculation on this would be fine so long as it was admitted and understood to be theoretical speculation). And I did not say the system remains closed.
Scientists DO show how it was contaminated. If they don't, the papers wouldn't get published.
Just because message board posters do not explain the contamination, does not mean the scientists don't.
If there is data that is abnormal, they document their conclusions, and where necessary, indicate whether further testing needs to be performed. This is completely standard. The only exception is when the cause of the aberrant data has ALREADY been identified in previous studies.