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