Zircon growing at low temperature, near surface environments

Can any geologists give me an explanation of this new research which seems to have University of Glasgow geologists all excited? In particular, I'm keen to know the implications for this in radiometric dating of rocks and which "impossibel to date" rocks can now be dated. I got the info from "The Scotsman" today and haven't read any of the original work because, not being a geologist I wouldn't really know where to start.For "impossibel" read "impossible" [This message has been edited by Trixie, 03-16-2004]

I might or might not be able to explain, but I have no idea what you're talking about ... can you at least post a link to a popularized version of the story?

Sorry JonF. I'm not surprised I've confused you. I've confused myself too. It wasn't even in "The Scotsman" it was in "The Herald". I usually read the former, but today could only get the latter and obviously didn't notice a blind bit of difference. Oh dear!! Anyway, here's the link.http://www.theherald.co.uk/news/12030.htmlSorry about the confusion - I've had a hard day. 500 PCR reactions and not a single positive to show for my efforts

I am not a geologist, but this site may shed light on how zircons are being used to date sedimentary rock. Around here, we always say that we date the igneous rock. However, sedimentary rock can be dated as well using zircons. The zircons can give the age of the rock that was eroded and it seems that the helium dating can give an approximation of when the rock was eroded.As to zircons growing at low temps/near surface, I have no clue. It may be linked to sedimentary rock dating, or may not. Hopefully Joe Meert or Berkland are lurking in the shadows ready to dump volumes of pertinent info as we speak.

I'm a little confused by them saying that zircons grow at low temperatures in *slate.* I thought slate was metamorphosed shale, so that it would have seen some pretty warm temps to even exist. C'mon, geologists, enlighten us!

OK ... assuming that the story is fairly accurate (always a dangerous assumption) they are saying that tiny zircons can grow in slate, and then the slate may be dated by dating the zircons.Zircons are very useful for dating, for several reasons; probably the most important is the fact that they strongly reject lead, but do incorporate noticeable amounts of uranium, when they solidify. This makes them useful for concordia-discordia dating, which is a really powerful and accurate method (partly because the half-life of U is known with much higher accuracy than any other element). I don't know of any really good explanations of it on the Web. Very briefly:If you know that the initial Pb in a sample was zero, you can write an equation for the ratio of ²⁰⁶Pb to its parent isotope ²³⁸U in which the only unknown is time, and another equation for the ratio of ²⁰⁷Pb to its parent ²³⁵U in which the only unknown is time. You can then do a plot in which ²⁰⁶Pb/²³⁸U is the Y-axis and ²⁰⁷Pb/²³⁵U is the X-axis. If you consider time (age of the sample) varying from 0 to infinity, the two equations are parametric equations for a curve (called "concordia" but almost never "the concordia") on the plot: If we measure a sample and plot it and get a point on the curve, that is essentially doing two different dating methods and getting the same answer from both: a good and reliable thing.It turns out that, in many realistic scenarios, alteration of a sample moves the point representing that sample off the concordia curve, but different samples from the same source move different amounts and the points representing the different samples form a line (called discordia) which intercepts concordia twice; once at the age of the source and once at a point that means different things in different scenarios: where t₀ is the age of the sample. (This particular example represents loss of Pb, the most common alteration, because Pb is relatively volatile and the radiogenic Pb is in crystal areas damaged by decay of U).There's a little more detail at Radiometric Dating, and some more in "The Age of the Earth", G. Brent Dalrymple, Stanford University Press, 1991.So, the big deal of this discovery is finding zircons that formed at the same time as the shale so the shale can be dated by the most powerful mehtod available to us today.Secondary High-Resolution Ion Microprobes (SHRIMP) have been developed to make very sensitive and accurate analyses of incredibly small samples. See SHRIMP.I'll have a little more to say about dating sedimentary rocks in another reply.

We can get an upper limit on the age of sedimentary rock by looking at zircons; but there are techniques for dating sedimentary rock. There are minerals that form in between the grains of sedimentary rock when the rock lithifies, and these minerals can be dated. People have done K-Ar dating of such minerals, with mixed success, for many years. The latest hot thing is the ability of a Sensitive High Resolution Ion Microprobe to measure incredibly small samples, and the fact that "xenotime" (one of those minerals that forms in between grains at lithification) is suitable for concordia-discordia dating. See U-Pb SHRIMP Dating of Diagenetic Xenotime

Thank you very much, JonF. As always you explain it very well and your graphs are great. I'm now gonna have a think about it all and make sure I've understood it properly.I see now why they're so excited about it - to be able to use this method on shale for the first time will be great. Let's hope the report was correct! Once again, than you very much

You're welcome.I suyspect that what they're really dating is something growing on zircons or something like that, but probably the end result is the same.

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