Talk:Age of Earth/Archive 3

Modern dating

I added a bit on the Canyon Diablo meteorite, as it seems that it was used as the only source for the age of the Earth within the article, and even I could see it wasn't particularly robust an argument saying a meteorite = the age of the Earth.

Hence, why I explain why we use proxy materials to give an accurate representation of the age of the Earth, which may not be readily apparrent to many. Basically, to explan it, most Earth samples are inaccurate (hence why it took so long for people to get it acceptably right). This is because most are weathered products of other rocks which have been churned through plate tectonics (assuming you even believe in plate tectonics) for the past however long. So, yes, terrestrial samples, if not very carefully selected will derive erroneous and spurious ages.

So we take aboard the assumption that the solar system formed from a primodrial disk of accreting stardust, and we run with it. So that's an assumption, ok? You who doubt can obviously latch onto it, but I've made that pretty clear in the text, so to do so will be gratuitous to say the least.

This solar nebula collapses due to gravity and forms the meteorites and the planetismals, which go to form th planets. The meteorites which escape this coalescence of the major planets thus trap their isotopic compositions when they form. A range of dates is given. This is because the accretion was not instantaneous. The 50 ma window is quite reasonable, really, to form the inner solar system.

We do not have samples (although they just arrived on that comet probe) which have not entered the Earth's atmosphere at 50,000 mph, but even if this has contaminated things, the dates from over 50,000 separate isotopic measurements from Earth, moon, mars and meteorites all show a 4.54 +/- 0.02 billion year date for the formation of the iron and silicate planets. The gas giants, who knows? But who cares? This is the age of the Earth we are talking about.

So, to sum up, when 4 or more independent lines of evidence point, within error, to 4.54 billion years, you do have to start assigning some age significane to he measurements. Regardless of whether you believe the maths are right for Pb-Pb or U-Pb, there is also robust Lu-Hf, and even Ar-Ar dates of 4.54 +/- 1% which have been returned fom meteorites which have been dated at 4.54 Ga by U-Pb and Pb-Pb.

in fact, there are Lu-Hf dates on terrestrial rocks (the Acasta Gneiss) which show, a) that the bulk earth formed at 4.54 Ga, and b) that the Earth's core segregated from the bulk silicate Earth by about 4.45 to 4.58 Ga. So, that is another terrestrial date which backs up the scientific age of the Earth.

Rolinator 10:48, 19 February 2006 (UTC)

Thanks to Rolinator for putting some of the classic Patterson work on the age of the earth to light in this article. I have a couple of quick comments. The section on the Canyon Diablo comes completely out of the blue with little context provided in the rest of the article. A quick and dirty paragraph or two on Patterson and his calculation of what is called the "geochron" would be great. In addition it should be made clear that the calculation in question is based on a combination of Pb from the Canyon Diablo and from terrestrial sediment, not just the meteorite. There are a few misleading statements about meteorites. Most meteorites (in particular iron meteorites like the Canyon Diablo) do not represent primordial compositions of the solar system: they are, in fact, highly fractionated. Some chondrites are believed to represent relatively primitive/primordial material, but even those aren't strictly primordial. Most chondrites have what are believed to be sub-solar alkali abundances. Finally, in the above comment, I think you are referring to the Hf-W system (not the Lu-Hf) when you describe constraints on core segregation.

For reference, there is a review paper by Youxue Zhang (Zhang, 2002. Earth Science Reviews 59(1-4), 235-263) [1] that explains a number of radiometric constraints on the age of the earth in very simple terms. This might be a good start for anyone who wants to review, in detail, some of the radiometric constraints. For those who don't have subscription access to the Elsevier journals online, I'd be happy to help them get a copy if they really want to work on this. Thanks again to Rolinator. Cheers,Rickert 18:17, 20 February 2006 (UTC)

Feel free to edit and improve on things. I'm no meteorite expert. Rolinator 23:25, 20 February 2006 (UTC)

Well, I'm not a meteorite expert either, but I do know that iron meteorites are not chondrites. ;) Seriously though, I don't have time right now to devote to the writing of actual articles (reviewing and fact-checking takes much less time than writing and can be done while I eat dinner). This project seems worthwhile so I hope to devote some time in a couple of months. As far as corrections go, the only improvement I could provide right now is to delete the entire section. As it stands, the relationship between this section and the age of the earth is confusing/misleading. An alternative, simply deleting the incorrect or misleading sentences would leave it gutted and garbled. Simpler arguments must be developed first, for example by drawing out the discussion on bracketing ages (i.e., the jack hills zircon data) followed by a simple explanation of Pattersons 1950's experiment. In the context of Patterson's experiment, the importance of iron meteorites (they record initial Pb isotopic ratios) can be discussed. The mathematics behind Pb-Pb dating can either be qualitatively touched on or developed in a thorough manner in an appendix (or even better, on the U-Pb page!). If someone wants to do this now, great. If it can wait, we should save the meteorite discussion for a time when someone can do it justice. Cheers, Rickert 00:00, 21 February 2006 (UTC)

(Request for mediation tag removed. It was rejected) Vsmith 04:52, 24 March 2006 (UTC)

So to state the obvious we really have no idea how old the earth is. Doesn't even sound like any method of calculations seem to disagree. So what does this mean? —This unsigned comment was added by 24.197.139.111 (talk • contribs) .

It means dating methods have a margin of error; which is known by everyone involved. The calculations do agree; to say they disagree is to infer they are wildly different. They are not. It would be more accurate to say they do not precisely/exactly agree/match. Again, this is to be expected and is hardly a new situation to experimental science. - RoyBoy ⁸⁰⁰ 21:47, 16 March 2006 (UTC)