Recently, I started picking detrital zircons from a Neoproterozoic sandstone for later U/Pb age dating. Some grains which, judging from shape, relief, colour etc., most probably are zircons, can be broken if vigorously touched with tweezers. Is it possible that radiation damage in old zircons can get so intense that the zircon grains lose their original mechanical resistivity?
Dear Dominik
The stability of very old zircons may be reduced as a result of isotropization caused by high content of uranium and thorium. Even brittle deformation in rocks and during transport can damage the zircon crystals.
Anyway, zircons and other heavy mineral grains must not be touched or taken with tweezers for isotope geochemical analyzes. Individual grains are best "picked" or transported with the head of a fox hair brush, or you can use a micro air pump to free the zircons from other grains. I have used this method for the enrichment of concentrates for my own zircon and apatite fission track dating. Try it yourself, it works fine with a little practice.
Best regards,
Guenter
Dear Guenter,
thanks for your answer. I will try the brush technique.
Best regards
Dominik
Yes, metamict zircons are rather easy to break. You probably don't want to analyse those zircons anyway, as they are likely to be nastily discordant (and often high in common Pb). Of course, if you want to be completely unbiased, you should analyse those zircons as well, but the interpretation of their U-Pb age would be difficult, You could try to chemical anneal them before analysis, which is a lot of work though.
Dear Marlina, thanks a lot for your useful answer. I will apply chemical annealing techniques to some of my zircons (those from volcanic rocks, for later ID-TIMS dating) but most zircons from sedimentary rocks I will date with LA-ICP-MS (for provenance studies) without earlier chemical annealing. However, it is good to know about the potential bias for provenance studies which might result from not using the metamict zircons.
Best regards
Dominik
Hi Dominik,
It's interesting to keep in mind that nature also introduces a bias: if the zircons are already metamict when they enter the sediment, then they have a reduced chance to survive the erosion-transport-deposition cycle, compared to a population of unaltered zircons. So zircons from pegmatites, which are often very high in U, are less likely to survive than zircons from some low-U diorite or tonalite. This of course also depends on the time elapsed between formation of the (high-U) zircon and erosion of the protolith, as the zircons will only become over time.
Nature always has some tricks up her sleeve to complicate matters!
Regards,
Marlina
Dear Jim,
many thanks for your answer. I will have a look at the Lee and Tromp paper.
Best regards
Dominik
Dear Dominik,
Indeed, radiation damage is supposed to cause some volumetric extension, which is one reason for cracks. These cracks lead to the decreased mechanical stability. The formation of such cracks is highly dependant from the U and Th contents of the zircon. Of course older grains were longer exposed to radiation and thus, show these type of cracks more often.
You can observe this effect even better in grains with old cores and young rims with different U and Th concentration. If the core suffers radiation demage it forces the rim to react, which means that it is cracked...
Maybe this is of interest for you: Gärtner et al. 2013, Morphology of zircon crystal grains in sediments – characteristics, classifications, definitions.
Best regards,
Andreas
Dear Andreas,
many thanks for your answer. I will have a look at your paper.
With best regards
Dominik