As there are both, inherited detrital as well as insitu crystallised zircons in the volcano-sedimentary succession....So when there is no such big differences in the ages from the sediments and volcanics... How one should proceed??
Dear Subhojit
The obtained age on zircon crystallised from the magma will give you +/- the eruption age as well as the age of the sedimentation... it is obvious that these two events are more or less of the same age. Perhaps you were referring to inherited zircon eroded from older units, transported and deposited with the sediments. If they derived from a previous volcanic unit, it might need some effort to distinguish them from the ones crystallised from the volcanics. However, even the volcanis can have some older zircon crystallised in the magma chamber some 0.05 to 0.5 Myr (or perhaps older) before eruption. So it depends on how precise you need to know the age of volcanism and sedimentation... as more precise as more effort you have to put into it. You should carefully look at zircon from different samples, use Cl imaging followed by LA-ICP-MS U-Pb analyses to give you a general picture of age distribution your zircon grains. Depending on this picture, the amount and quality of zircon grains you have recovered and the excess to analytical facilities or to funding you might want to do CA-TIMS analyses to better constrain the age of the youngest zircon population in your samples. The latter method is very time consuming and cost intense, so you really have to have a good reason to find out the exact age .... additional Hf isotopes analyses on the zircon might help you as well to disintguish zircon from different origin and sources (eruptions?)
and in addition you can apply Ar-Ar dating e.g. K-feldspar ....
I hope this helped and not just confuses you
Axel
Dear A. Gerdes Sir,
Thanks for the valuable suggestion. Really it will help a lot...
Dear Subhojit
the age determination of volcanic rock depend on the time of volcanism. as you know the U-Pb methods is function of U half life which is calculated as common procedure of age determination. you can see the link below which show you interval of age determination og igneous rocks.
link: http://www.colorado.edu/geolsci/courses/GEOL5690/U-PbNotes.pdf
Regards
Massih
Dear Massih Afgahah Sir,
Yes.. That is right.. But when these volcanics are intercalated with the sediments its very difficult to distinghuish between the ages which are from volcanics and which are inherited /detrital in origin..
Hello Mr. Saha,
If your volcanic deposit has sanidine or plagioclase, Ar/Ar is a reliable method of dating volcanic deposits, more so than using U-Pb in zircon. Because zircons are highly resistant to weathering, and there is evidence that zircons can crystallize in the magma chamber in multiple phases, it is not always straightforward to infer that the age from a zircon crystal identifies an eruption age. It may have crystallized in the magma chamber long before erupting, and will therefore yield a spuriously old age. Sanidine crystallizes later than zircon and under conditions that better agree with an eruptive process; furthermore, due to the chemical properties of the daughter element of Argon, a noble gas that has a much lower closure temperature than an element like lead when in the solid phase, argon is reasonably assumed to be reset to zero concentration at the time of crystallization of sanidine. This is not always a reasonable assumption with lead in zircon, since lead is likely to remain in the solid phase up to very high temperatures. Thus, lead excess can be another reason for a spuriously old zircon age. For these reasons, I would recommend using Ar/Ar on a potassium-rich feldspar if you have this option.
Dear Miss Elizabeth,
Thanks a lot.. Yes its of rhyolitic composition... I will try to go for Ar/Ar method.. If anything more u can suggest then please..
Dear Subhojit,
I agree with miss Elizabeth and would also recommend you to use Ar/Ar method for this problem. But if it not possible and only U-Pb zircon facility is available then CL images of the zircons are useful, it reveals the texture and shape of the zircons which are helpful to identify their origin. In general the magmatic zircons are prismatic in shape and show well developed zoning, whereas in detrital sedimentary zircons the grains are mostly oval shaped with no zoning. Along with this U vs Th plot, Ti in zircon thermometry is also helpful in this regard. After separating the magmatic zircons with U-Pb also try Lu-Hf and try to estimate the epsilon Hf and TDMc. This will probably help you to constrain the magmatic crystallization age.
I hope this information will be helpful.
Chiranjeeb
Dear Chiranjeeb,
Thanks a lot for the suggestion. Will try to consider all the points and finally decide how i shall proceed further.. Thank you so much..
Dear Saha,
If you consider the provenance aspects, only a strictly volcaniclastic succession will have one only zircon (juvenile) family, but only if no other source (non juvenile) sourced the deposits. The most common epiclastic siliciclastic deposits will most frequntly include several zircon families, which sometimes have the same ages but different sources.
The way I believe is most suitable to unravel the juvenile vs epiclastic origin of your zircon grains is to have a coupled U-Pb and Lu-Hf zircon analysis with Cl and backscattered electron images, zircon typology, and petrographic and stratigraphic analysis. The zircon internal zonning only indicates that it is magmatic. You still have to look to some chemical constraints as the Th/U ratio.
Hope this will help, best wishes,
Felipe.
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