I will be collecting carbonatite samples for LA ICP MS. They will be ground in order to handpick zircon crystals for U Pb geochronology. I want to get 100 zircon grains. What sample weight should I take?
No specific sample weight. Depending on the rock type. Felsic rocks normally contain abundant zircon compared to mafic. As such, while a small sample size may suffice for granite, the same is not true for basalt.
However, even for your felsic samples, the expertise of rock pre-treatment vis crushing, picking "good grains" are important.
Dear Georges M. Kasay ,
Zircon is an accessory mineral in carbonatites rocks and its abundance vary in relation with the studied carbonatitic complex.
I suggest you to study your sample initially with a petrographic approach and a modal analysis under a petrographic microscope. Then, focus your study on zircon modal content. Zr is a depleted element on carbonatites and this traslate into just some accessory minerals on carbonatites.
I attached you a file with a different approach to U-Pb geochronology using zircon crystals, using detrital zircon (recognizing thei magmatic origin and provenance
Dear Gabriele Carnevale
Thanks for providing this useful and recent paper and for your answer. Yes, petrographic and modal composition will be very important in the first steps of the process.
Sorry, I did not read details of your rock type when I responded. The general answer still applies. I also agree with Azin Naderi
that 5 kg per sample will work. Weldone
Dear Azin Naderi
and Kayode Abdulgafar AmudaThanks very much for your answer. So much appreciated.
Dear Dr Leonid Shumlyanskyy
Thanks so much for sharing your answer and experience in geochronology. I will take into account your suggestion which is very useful. Yes, baddeleyite is commonly used as a geochronometer.
Another mineral that is useful for U-Pb dating in carbonatites and alkaline rocks is zirconolite (CaZrTi2O7), together with baddeleyite as indicated by Leonid Shumlyanskyy.
Dear Dr D.M. Evans
Thanks very much for mentioning another geothermometer in carbonatite.
As others have pointed out, your challenge is with carbonatite where the solubility of Zr is high in alkaline rocks combined with low Si to form zircon. In Si rich rocks it is possible to take the Zr whole rock composition and do a norm calculation to determine the weight of zircon in a rock and from there, with an assumption on grain size dimensions and density of zircons, calculate how many zircons one might anticipate from a rock sample.
Dear Dr Geoff Grantham
Thanks very much for this useful information. So much appreciated
100 zircon grains will be more than enough for igneous rocks, but a bit less would also be okay.
For detrital zircons U-Pb dating with LAICPMS, you need a 5-gallon bucket filled with sedimentary rock samples.
Dear Dr Hector R. Hinojosa
Thanks for mentioning this important information. How many grains would be a minimum?
From a statistical perspective this has been addressed by various authors - see references in "Detrital zircons as tracers of sedimentary provenance:
limiting conditions from statistics and numerical simulation." Chemical Geology 216 (2005) 249– 270. T Andersen.
Lots of different approaches, but at the end of the day, you can't go wrong with about 5 kg. Don't separate all of it, start with just 500 g. Don't do anything special, just crush and gold pan it, and if the sample is rich, the zircon will be there. A UV lamp can be used to see it in a dark room, or just go ahead and pick it under the stereomicroscope. If it is detrital, it won't get locked up in other grains. Baddeleyite is different, and the only good way is to extract using a Wilfley Table.
Dear Dr Ashley Gumsley and Dr Om Prakash Pandey
Thanks very much for your answers. This is so helpful and interesting. I will follow these steps that you have mentioned.
- Badges
- Science topic
- Similar topics
- Engineering
- Materials Engineering
- Powders