Zehra, I will recommend you to read the numerous papers on dating Precambrian zircons available on the internet, perhaps starting with the dating of oldest zircons found on earth from Jack Hills of Australia. When you read these papers you will realize that Precambrian zircons are likely to have preserved a long geological history in them, and therefore, an analytical technique capable of identifying different stages of growth on a zircon should be used while studying them. The papers available on the net will tell you that the instrument known as the SHRIMP has been the most successful in analyzing zircons because of its capability to analyze small spots on zircons. There is a full journal volume, Precambrian Research, vol 183 (2010) discussing the "Impact of SHRIMP on understanding the Precambrian". It will be useful to read that volume of papers. Thanks to SHRIMP, limitations on isotopic analysis of zircons have been limited. I think it will be useful for you to read the attached paper on "Atlas of Zircon Textures" by Corfu et al. to get an idea about structure of different zircon grains.

Atlas of Zircon Textures is attached.

Dear Perera, I know the book, Atlas of Zircon Texture and for my master degree I analysed detrital zircons with LA-ICPMS..Of course I downloaded many papers about the issue. I am just curious about the limitation of dating precambrian zircons. They may have so complex and polyphase geological histories. So how sholud we interpret them? what are experts recommendatitions?

Dear Zehra, An interpretation means giving a justifiable explanation for the isotopic observations you made. If your data set is showing complex relationships in a traditional concordia diagram one can suspect a polyphase history. Then you have to look for other lines of geological evidence supporting a polyphase history. If people have made previous age estimates, those will give you a starting point to think where your data stands w.r.t. to their data, whether your data suggest the same or a completely different history. The problem in geochronology is that there is no criteria to assess whether an age assigned to a given event is correct. I have posted a separate question on that topic. https://www.researchgate.net/post/How_reliable_is_geochronology_in_dating_the_past

One possible way to assign an age to an event is to target the product of that event and date it, but this is going to be successful only if later events have not disturbed the isotopic system. But this cannot be guaranteed. The best thing always is to start with the dating of the very last event, if a terrain is having a polyphase history. Have you not interpreted your LA-ICPMS data of your master degree?

As I mentioned before it is my master work means that I separated, analysed zircons and processed, interpreted my own data and they are relatively younger aged and basic geological events are young and almost known. The last metamorphic event was dated by using phengite-Ar/Ar ages. But I mean entirely different issue; if a zircon grain is old and may have many different geological events, like metamorphism, hydothermal activies e.x., how does this geological background affect the attitude of old zircon? Surely, SHRIMP is acceptable as a method for deformed grains. What is the pros and cons of precambrian zircons?

I think your question do not have to refer to precambrian zircons but zircons that may have been effected by many different events. Zicons may not record all the events they have gone through as zircons may or not grow (overgrowth) during or respond to (reset) later disturbing event, which complicates age interpretation of zircon with complex structures. Zircons (Precambrian or not) with complex dating results indicate records of multiple geological events. Correlating distinct age populations to specific event can be achieved resorting to other independent in situ dating work on minerals such as rutile, titanite, monazite and other geological evidence. The zircon has been proved an excellent datable target, but not the only one. Zircons may be forever, but may not be a panacea.

If the zircons dated independently confirm geological events inferred on the basis other dating techniques, then one can consider zircons as providing positive results. But if there are disagreements between zircon dates and other dates, then either zircon dates or other dates or both could be in error. As Ya-Nan Yang points out this could result from zircons not recording certain geological events, or the zircons may have responded to all events but were not completely reset, and therefore, are giving upper and lower Concordia intercepts which are not related to any of the events dated by other techniques. Since diffusion of Pb in zircons is believed to occur at temperatures excess of 900 Celsius, low U older zircons (with limited lattice damage) may not have recorded geological events that happened at low T. But one may be able find zircons newly grown during those events. In addition, one may occasionally come across reverse discordance, suggesting that there is excess radiogenic lead in the system. All these may be considered as negative aspects of zircon dating.

Hi everybody,

This may be a little late, but I thought I might be able to offer some comments. It would seem the issue is not specifically restricted to Precambrian zircons, but ones with complex, polyphase histories, typically preserved in Precambrian terrains but also in detrital zircon populations.

Like any technique, the first step is to know what you are analysing. Hence petrographic, followed by CL or BSE imaging of your population will provide you firstly with information regarding whether indeed your zircons are zoned (often, but by means always the case for Precambrian zircons). This will also then highlight metamict or fractured grains which must be avoided at all costs. Then, if possible, you can undertake EMP analysis on different zones to establish those zones with similar trace chemical characteristics to guide in targeting prior to U-Pb geochronology.

Then you should choose where in your grain to analyse, either by SHRIMP or LA-ICPMS, and then in interpretation, you should always aim to get a decent sized number of analyses from the same population- ie, if your zircons have rims, and cores, and all of the rims have similar lower Th/U ratios then you should try and get ~30 spots from various grains on these rims in order to constrain this population. So too you might want to constrain the cores and hence would then try and identify cores with similar Th/U as well as specific ages.

If this is very important, some people also do LA trace elements on top of the geochron spots in order to check other trace element indicators (P, Ce etc), and if you are particularly interested in whether true detrital populations are coming from the same source region, then LA-MC-ICPMS on top of the U-Pb spots for Hf isotopes will distinguish between source terrains of the same age but different provenance.

However all of these need to be done in the context of clear spatial control of the grain and avoiding metmict zones and fractures which could experience Pb gain or loss. Data sets are also typically filtered to exclude those analyses which are more than 10% discordant, however with a significant number of analyses these spots often themselves define discordia with intercepts that can provide significant information.

Finally, if you image your zircons and discover they are simple (and many Precambrian zircons are!) you are not restricted to in situ techniques, ID-TIMS will offer a more precise determination than in situ techniques.

I hope this (belated) answer is of some help...