Talk:Isochron dating

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[edit] 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)

[edit] 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 Di (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 Di = 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 Di), which allows us to plot the graph used in the dating. It seems we gain nothing by adding Di to our considerations. If I'm not wrong, one of the following is the case: mass spectrometry can't really determine Di, 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)

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.

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