Dear Geology Community,
I am currently researching the dating of detrital zircon in metasediments. Due to the small size of my samples, traditional mineral separation methods may not yield a sufficient number of zircons. However, I have observed numerous zircons in thin sections, with at least 20 visible in each.
I am thinking of making multiple thin sections for each sample, to directly date the detrital zircon present in the matrix and as inclusions. My question is: Could this method potentially influence the representativeness of the dates obtained? Are there any published studies that have employed this method?
Please note that the option to return to the field for additional sample collection is unavailable.
Thank you in advance for your insights!
If they are metasediments, then they are metamorphic rocks.
Which is the grade of metamorphism?
Zircon separation will guarantee a good number of crystals to date.
But, working on thin sections will allow to evaluate age vs fabric.
Federico Lucci Thank you for your response. The metasediments have reached the granulite metamorphic facies.
It depends on the history of zircon which in all metamorphic rocks is subject to lithogenic recycling represented by zonation, metamictisation, X structure (see Puppin etc.). The edge over single grain dating in a heavy mineral separate is the knowledge about the host minerals and intergrowth which eases the interpretation of age data.
HGD
Certainly, in situ dating of zircons in thin sections has important limitations, especially for detrital rocks. But, the suitability of this methodology will depend on the aim of your study. The advantage, as was already mentioned, is the dating in the textural context. Since the rock has reached granulite facies, zircons likely have metamorphic rims. The size of zircon grains in thin sections is limited by the thickness of about 30 um, which can pose a risk of not exposing a complete cross-section of the grain. This can lead to the misidentification of core and rim structures. Additionally, the zircon population in the thin section may only represent a narrow range of grain sizes. The photo shows a heterogeneous grain size, meaning that coarser grains will not be part of the analyzed population. This is a critical aspect if the purpose includes provenance analysis. A suggestion would be a detailed textural analysis of the zircons on a thin section. Furthermore, a careful grain separation process with adequate crushing-grinding and gravimetric methods (water-based, e.g. Söderlund & Johansson, 2002, G3) will help you to get a good amount of zircons even from a hand sample as you have. Metasediments commonly contain plenty of zircon.
Dear Mehdi: first of all I don't understan why when sampling a granulite belt you could not get a larger amount of rocks. Why are these rocks so "limited"? Where is your work zone located? I live in a tropical country, Venezuela, and we have a huge (500 km x 120 km) granulite belt called Imataca Complex, of Mesoarchean age, with Neoarchaean metamorphism. When one goes there, there are huge outcrops where any amount of samples can be available. I'm talking about the Guiana Shield, which has a tropical climate, and it is mostly covered by dense jungle and rainforest. So...
Well, after this observation, I would think that the zircons in a thin section would not be representative of those presented in 5-10 kg of rock. Dating detrital zircon is a tricky business, those zircons come from different sources, have different ages, and moreover, can be zoned, with younger overgrowths. Usually this kind of dating is used to determine provenance of the sediment, but studying individual zoned crystals by TIMS or LA-ICP-MS, can reveal several stages of growth of the zircon which can be related to different orogenic events. The zircons with the youngest age found in the metasediment (or sediment) give an educated idea of the age of the original sedimentary rock, but it is always an approximation.
Regards, Sebastián. UCV Department of Geology, Caracas, Venezuela.
I think that a Granulite facies would require a different approach.
Talking of "detrital" zircons in such a UHT rocks is dangerous and misleading.
You should start to think about "inherited" vs. "granulite peak" vs. "post-granulite/exhumation".
Article Hercynian anatexis in the envelope of the Beni Bousera perid...
Thank you all for your contributions.
I recently came across a paper that emphasizes the use of zircon dating on thin sections: “Thin-section detrital zircon geochronology mitigates bias in provenance investigations” (https://doi.org/10.1144/jgs2021-07). Since my work involves migmatized rock, this method could help me avoid dating certain intrusions in my metasediments.
Responding to Cindy’s point, the thickness of my thin sections is 50 um, and I plan to prepare about 15 such sections. The grains in my metasediment are varied in size, which I agree could introduce bias during interpretation. The separation method you suggested seems intriguing, and I will look into careful techniques for mineral separation.
In response to Sebastian’s question, some of my samples were collected by a company, and we cannot revisit the site for additional samples.
Thank you for Federico the insightful paper you recommended. I concur that interpreting the detrital zircon age in UHT should be done with caution. However, we plan to date garnet, monazite, apatite, etc., which should assist us in determining the age of metamorphism and isolating the dates of the sources.
Thank you all for this enriching discussion.
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