Saturday, June 04, 2011

Born again zircons

Last summer, I spent a bit of time back at Geoscience Australia as a geochronologist for hire, helping out with some of their state and territory geochronology programs. The report from the study I SHRIMPed zircons for is now out, thanks to the awesomeness of the first author, Dr. Nat Koscitin.

In theory, zircons are forever. The oldest known Earth materials are zircons, and there are countless* papers describing how they accumulate overgrowths through numerous heating and melting events.

So I was intrigues when I saw the following embayments on one of the samples. It is obvious that the zircon structure has been altered somehow. Prior to analysis, I assumed that it would be metamict- zircon where the accumulated radiation damage has destroyed the mineral structure.

Embayed zircons (c) Geoscience Australia

Usually, metamictization allows the radiogenic lead to escape, leading to geologically meaningless uranium-lead ratios. So I was surprised when these inliers gave a beautiful age that was 25 million years (plus or minus five) younger than the igneous crystallization age.

Igneous ages (red) are distinctly older than embayment ages (blue), which are surprisingly (to me) tightly clustered (c) Geoscience Australia.

The inliers have very low, “metamorphic type” Th/U ratios, suggesting that the zircon was drastically recrystallized, if not completely destroyed and regrown. These zircons weren’t forever; they had their insides dissolved and reprecipitated when they were 25 million years old. I had a brief look through the literature, but didn’t find anything that looked particularly similar. So I flicked the interpretation back to the petrologists and/or fieldies in the text. They didn’t come to the rescue- at least not yet.

If anyone knows how these things form, what they mean, and where I can read about previous descriptions, I’d appreciate it. I hear that the Curtin University gang has seen something similar, but I don’t have a particular reference. Any tips out there?

* Well, hundreds at least


Emily J. Chin said...

I've been reading your blog for a while and really enjoy it (I'm a PhD student in igneous petrology in the states). I recently worked on some messed up and potentially born-again zircons as well. Mine were ancient detrital zircons from the N. American craton that ended up in metasediments in the lower crust. Thus, upper intercept ages of >1, 2 Ga, but young (~100 Ma) lower intercept ages. There aren't any embayments in CL, but what is commonly seen are bright CL cores. I read somewhere that this can happen to even ancient cores if they recrystallize at high T (in my case, granulite facies). It's interesting that just where your embayments start you have bright CL colors (but it looks like your SHRIMP spots are on the darker spots adjacent to the bright embayment rims). I've been wondering what a zircon that experienced Pb loss would look like under CL. Do you think yours could be recording some Pb loss diffusion profile in the CL (or does Pb not have much of an effect and its mostly U... i'm still relatively new to the world of zircons.. i work on mantle peridotites!). Also, are those red data points on your U-Pb plot the original igneous cores? If you plot the original igneous core and an embayment... maybe not on a Tera-Wasserburg diagram if the cores are really ancient, could the embayments be a record of Pb loss (meaning they would correspond to a lower intercept along a discordia in the conventional diagram). Anyway, neat zircons you have there!

Dan said...

Classic dissolution & regrowth textures, so typical of granitic gneisses. Would not be suprised if the relative Th & U contents have redically changed too. You can dissolve your zircon into a wobbly lump, and regrow it back to a happy-looking euhedral crystal, because the new zircon will rebuild on the original crystal's framework. For me, the smoking gun that distinguishes this from some kind of solid state recrystallisation process is the presence of little bright fringes on the cores - the ones Emily noticed too - I reckon they're tiny bits of recrystallisation to very low-U zircon on dissolved surfaces. If it was a diffusion front, you wouldn't see that - the rims have lower CL and thus higher U.

Of course, being born-again zircons, the two ages should be recalibrated to before and after the flood. With a 2 standard begats error.

C W Magee said...

But dissolved by what, Dan?

Emily, a picture is worth a thousand words. email me a CL image to the address decucible in the sidebar, and I'll have a look.

Note that several people, including Bob Pidgeon and Carol Frost, have looked into lead loss from zircons in low temperature arid environments, but I don't know how much has actually been published.

Dan said...

>But dissolved by what, Dan?

Now, that would be a question for Dr Nat ^^