Talk:Isochron dating

Basis for method

In this section it says "As time goes on, some amount of the parent decays into the radiogenic isotope of the daughter, increasing the ratio of the concentration of the radiogenic isotope to that of the daughter." This seems wrong to me, or am I mistaken? Dan Watts 04:58, 9 August 2005 (UTC)

Main equation

Excuse me if this is a silly question; but in the main equation of isochron dating,

${D+\Delta {P}_{t} \over D_{i}}={\Delta {P}_{t} \over P-\Delta {P}_{t}}\left({P-\Delta {P}_{t} \over D_{i}}\right)+{D \over D_{i}}$

it seems you can cancel out $D_{i}$ (by multiplying through) without any consequence besides simplification. It becomes

$D+\Delta {P}_{t}={\Delta {P}_{t} \over P-\Delta {P}_{t}}\left({P-\Delta {P}_{t}}\right)+D$

The equation is still true, its proof is one step shorter, we get rid of complications for $D_{i}=0$, and the article says that mass spectrometry can still measure the quantities appearing in the new equation (especially since it is able to measure $D_{i}$), which allows us to plot the graph used in the dating. It seems we gain nothing by adding $D_{i}$ to our considerations. If I'm not wrong, one of the following is the case: mass spectrometry can't really determine $D_{i}$, which seems unlikely; the article's math is inconsistent with geologists' usage, and should be fixed; or the article's math is consistent with geologists' usage, and I think there should be an explanation of why they haven't simplified the equation. (Surely there is a good reason.) Pietro KC 05:44, 11 February 2006 (UTC)

Hi Pietro.

• Di is never 0, so we don't have to worry about that case
• Di is always used in isochrons because it is easier to measure isotope ratios than absolute abundances. For example, in Rb-Sr dating ⁸⁷Rb decays to ⁸⁷Sr but one plots the ratio of ⁸⁷Rb/⁸⁶Sr against the ratio of ⁸⁷Sr/⁸⁶Sr . You are correct, however, when you say that the math is equivalent.
• On a related note, it is a bit of a strange way of presenting the mathematics behind isochron dating. It appears to be from a Creationist website, which probably explains the odd approach. If you have time, it would be great if you could redraft that part of the article so that it makes more sense. A pdf at this link explains the basics in terms a non-specialist can understand and has everything you need for an isochron article [[1]]. I don't have time to write it myself, but am happy to help proof and fact-check.

Regards, Rickert 23:40, 11 February 2006 (UTC)

I am not an expert on isochron dating, but I think Pietro had an excellent question, the answer of which illustrates a major weakness of this article. For the isochron equation to work $D$ is the initial concentration of the daughter isotope. Isochron is useful where this is not known. Therefore you can't just multiply out the $D_{i}$ to get rid of it, because what would be left on the right is an unknown quantity. As I understand it from reading just this article, isochron works by getting many samples from the same source, for example multiple different minerals in a single source rock. Then $D+\Delta {P}_{t} \over D_{i}$ is plotted versus $P-\Delta {P}_{t} \over D_{i}$ for all of those samples. Because each sample is chemically different, the amount of the various chemicals in each sample will be different, so these plotted points will not all be at the same locations. However, since they all came from the same source created at the same, the isotopic ratios in that source should be the same, particularly $D \over D_{i}$ would be the same for all the samples. Also, the same fraction of the parent isotope should have decayed for each sample, so $\Delta {P}_{t} \over P-\Delta {P}_{t}$ should be the same for each different type of sample. Therefore, since the isochron equation follows for each sample with these two values the same for each sample, all of the points should come up in a straight line. The weakness of the article is that it took me HOURS to figure this out. Why can't the article just say this: multiple samples with different compositions, so they plot with points at different locations, but the slope and intercept of the equation not being dependent on the different chemical composition of the distinct samples but on isotope ratios and decay ratios which would be the same for all so it gives a line... I do not understand this field well enough to make this change myself. And an actual isochron plot wouldn't hurt. — Preceding unsigned comment added by 69.231.139.172 (talk) 10:53, 31 May 2012 (UTC)

OK, finally got around to it. Many thanks for the link to the lecture notes. Please, send your thoughts and corrections. I tried to keep the explanation simple lest I frighten away the merely cursory curiosities, and because the article on mass spectrometry seems quite good. Pietro KC 00:06, 2 April 2006 (UTC) P.S. I don't think that talkorigins is a creationist web site. If I understand correctly, they used to be (or still are) a newsgrooup dedicated to that debate rather than the promotion of one side.

No description of the process is given

Nothing in the introduction of the article or the "basis for the method" section actually states what isochron dating is or how it determines what it does. All we're told is that it's a dating method, and the basis section lists facts without describing what the method does or how it follows from them. The only definition is the equation, which without context is useless for even a technical audience.Fyedernoggersnodden (talk) 04:34, 4 April 2012 (UTC)

Article unclear regarding isochron slope change over time

Since the ratio of the daughter and non-radiogenic isotopes is proportional to the ratio of the parent and non-radiogenic isotopes, the slope of the isochron gets steeper with time.
This does not look correct to me. the ratio of the daughter and non-radiogenic isotopes is proportional to the ratio of the parent and non-radiogenic isotopes has the slope of the isochron as the proportionality constant. This does not show that the slope of the isochron increases, it is just the formula for that slope in words. It would seem to me that from the isochron equation in the article: the slope should be (delta P) / (P - delta P) from the equation, and so the slope increases as delta P increases from that equation, and is zero when the rock is formed at delta P equal to zero. I don't see a way without using the isochron equation to determine this. Someone with better understanding of this than I might want to look at this sentence.69.231.139.172 (talk) 09:41, 31 May 2012 (UTC)

Using isochron to determine the age

The change in slope from initial conditions--assuming an initial isochron slope of zero (a horizontal isochron) at the point of intersection (intercept) of the isochron with the y-axis--to the current computed slope gives the age of the rock. The slope of the isochron, $\Delta {P}_{t} \over P-\Delta {P}_{t}$, represents the ratio of daughter to parent as used in standard radiometric dating.
The above says that the change in slope gives the age of the rock, which is confusing because this slope is not in time units etc. The following sentence regarding using this slope for formula in standard radiometic dating indicates that it does not directly give the age of the rock, but that it may be used to to determine the age. I suspect that the word gives above could be changed to is used to determine to clear up this confusion. Perhaps someone with a better understanding of this than I would like to make this change. 69.231.139.172 (talk) 09:56, 31 May 2012 (UTC)

Notice that this page needs to be rewritten

This whole page seems to have taken major step backwards when 'GeoGreg' made it much more complicated than it needed to be in 2005. Speaking as someone who uses isochrons nearly every day, this page does not relate to the real world and needs to be completely rewritten, perhaps by reverting to the version before GeoGreg (who seems to have removed himself). — Preceding unsigned comment added by Radiogenic (talk • contribs) 15:43, 23 September 2015 (UTC)