There are two important reasons why this is true. One is that tephras are instantaneous deposits, geologically speaking. Second is that they contain microphenocrysts of zircon and sanidine that are radiometrically datable minerals using a single-crystal technique. The U-Pb ages of a particular zircon crystal can be compared against each other, so that there is a  built-in method of determining the accuracy. Sanidine is a good "bottle" for Ar using the Ar40-Ar39 technique and the  single-crystal sanidine age can be compared with the zircon single-crystal ages.

Hello,

yes and no, it depends.

Fisrt you will date the tephra itself and not the surrounding layers.

Secondly, you should be sure that the tephra is "pure" and not a remobilized one by erosion and redeposition.

This is a thoughtful answer. Thank you, Jerome.

Dear Paul,

All you get from even a 'pure' tephra is just the age of tephra itself and also a maximum and minimum age for the upper and the lower beds, respectively. You might have a cross-cutting dike or an intrusion to just bracket your age. 

Kind regards   

Dear Rasoul,

If you get an absolute age on an ancient tephra (i.e. tonstein) you are also getting the age of the immediate lower and upper beds.  This is because of the uncertainty in the age of the tonstein. Thus, if the age of the tonstein is 316.1 +- 0.8, this age covers the age of the immediate lower and upper beds.  I doubt that getting the age of a dike or intrusion will resolve the uncertainty because of the difficulty of getting highly accurate ages on such bodies.

Thanks for your comments. Best wishes, Paul Lyons.

The problem of contamination can usually be avoided by analyzing single zircons, the youngest ones date the tuff layer. (Of course, the older zircons also might be components of the tuff, but they are xenocrysts.) We did that for tuffs at the K-T (K-Pg) boundary in Siberia and N China and could show that previous  radiometric age determinations were biased by xenocrystal zircons and hence were older than the paleontological age- looking just at the youngest zircons gave a very good agreement between radiometric U-Pb age and paleontological age.

Dear Ulrich,

Many thanks for you comments on single-crystal zircon ages and the problem of avoiding xenocrystal zircons that can represent a considerable problem.  I remember we had to get the ages of about 12 zircons before we got the true age of one tonstein, which was the youngest age of the 12 zircons.  So you are right on with your comments, which are really appreciated. Would appreciate a pdf of your paper on the age of the K-Pg boundary based on single-crystal zircon age dating.

Rhyolitic tephra age dates can provide very useful information. Zircon and sanidine age dates should be complimented by major and trace element tephra analyses that can be used to correlate regional ash falls. Geochem analyses will also tell you if the tephra has been altered. Very important to mske sure that the tephra is unaltered and not reworked.

Thank you, Mark.

You are quite right about using geochemical analyses to correlate tephras and tonsteins.  We used both bulk geochemical analyses and microprobe analyses and had moderate success with the correlations. See my publications.  We also did counting of the volcanic minerals to try to gauge the amount of mixing of detrital minerals vs. the amount of volcanic minerals.  This gives one a gauge on the amount  of reworking.

Thanks for your insightful comments.  Best wishes,  Paul Lyons.

Dear Paul,

Thank you for reply and I should say it is an interesting topic. But I think, the term ‘immediate’ that you used for the upper and lower beds is rather a qualitative and speculative term and perhaps based on your field observations. It might have been taken a long period of time between the lower/upper beds deposition and the tephra. We do not know how active has been the basin, but you know it from its general stratigraphy and other information from the geological setting. You have also access to the petrography of the tephra sample that we do not. So you are the one who can judge how the ages should be used as geochronology is just a tool and most of the time can mislead geologists. 

I am not agree on the accurate age for a dyke or an intrusion with you. These are magmatic in nature and almost all the time they give an accurate age even better that a fallout tephra in a sedimentary basin that its genetic source in most of the time is not obvious.

Best wishes, Rasoul

Dear Rasoul, 

I  suppose when you are mentioning the accuracy of getting the age of a dike or intrusion you are talking about the SHRIMP U/Pb zircon ages of granites.  I may have been reading the wrong papers, but the ones that I have read the ages do not very high precision and, therefore, the accuracy can not be high.  I suppose the problem of xenocrystic zircons is again the troublemaker. 

Thanks so much for your lively discussion.  Best wishes, Paul Lyons.

I Think it could be a good proxy of the age of the sedimentary sequence but is also necesary know the depositional environment and the absence of mayor unconformities and hiatuses. Also, it depends of the method, U/Pb in zircon may not be entirely reliable because of the posibility the the zircon crystals to be xenocrystals, also the age must be considered, because if it is very young it will not work with U/Pb.

Hi Maria, you are quite right. Best wishes, Paul Lyons,.

-> Maria, if you do single zircon dating, you usually can identify xenocrysts: the youngest zircons usually form a coherent group (maybe only 30 % of the total) whereas the xenocrysts generally show a lot of scatter over wide ranges. Finally, if it is a tuff that was not reworked, the tuff has the age of the youngest zircon or is younger (if all zircons are xenocrysts, which is probably extremely rare for rhyolite).

Dear Ulrich,

You have raised valid points. Finding the youngest zircons in single-crystal zircon U/Pb dating  may be very labor intensive because the zircons are very refractory unless you use the SHRIMP method that is not very precise in some cases. Also the percentage of xenocrystic zircons may be very high (e.g. over 90 %). Appreciate very much your discussion.

 Best wishes, Paul Lyons.

Dear Martin,,

You and Maria have made valid comments. For example, when we dated the Fire Clay tonstein we had stratigraphic and geochemical controls on its correlations in the central Appalachian Basin.  Thus, we were positive that when we dated it, we dated all the correlatives in KY, WV, VA, and TN (see Lyons et al., An Appalachian isochron: A kaolinized Carboniferous air-fall volcanic-ash deposit (tonstein), Geol. Soc. America Bull., 1992, 104, 1515-1527).

Thanks very much for the discussion.

Best wishes, Paul Lyons.

Dear Mark, Thanks for the discussion, Mark. Best wishes, Paul.

yes, laser ablation is perfectly ok, we used only laser abaltion ICP-MS in our study of the K-T (K-Pg) boundary.

SHRIMP and laser ablation are similar analytical tools, both using mass spectrometry.

I have never seen a comparison of data  from the two methods,  I would expect that the precision is about same.  Maybe you have, Ulrich.

Thanks for the interesting discussion.  Best wishes, Paul Lyons.

Dear Paul,

I think it is better to distinguish the terms DATE and AGE as geochronologists do. A date is just a NUMBER you get in the lab regardless of what isotopic system has been used, and for transforming this number (date) into an age you require to attach a geological significance to this number. I consider your question is not about acquiring a date for a tephra layer (which can be done using different geochronological methods bearing different precisions/accuracies). The discussion however is about using a date as a meaningful geological age for its upper/lower beds. I think your question was that can we use the tephra’s date to determine the age of the strata? That is not an easy task given the complicated nature of geologic environment as some of contributors mentioned above. So, it is not about the date’s numerical precision/accuracy. (I should note that, I am well aware of how analytical precisions/accuracies can affect our interpreting geological age).

Regarding the dating of a tephra/granite/dyke, and considering that all are magmatic, I am still sure that geochronologists will be more doubtful about the tephra’s age, unless it is deposited in an entirely volcanic environment (a purely fallout ash)!

Regarding the xenocrysts we should keep this in our mind they also keep invaluable information that can shed light on our understanding about the geological processes.

Let’s think about an example;

Suppose your tephra is not deposited from a homogenous volcanic ash and its materials came from different sources (e.g. reworked), but it has only one population of nice and fresh feldspars (e.g. sanidine), how about if they are the minerals coming from a proximal source? Suppose other minerals came from distal sources are weathered or ugly which usually are not picked for dating. Then, if you date the feldspars you get only a number (date) for them. How about if I date zircons in your tephra and they show an age spectra from diverse sources? So, please do not call the xenocrysts troublemaker. I love xenocrysts as well as fresh and nice zircons, and I prefer to do different dating techniques to get a better age.

Nice answer, Rasoul,

When we worked on the Fire Clay tonstein, we did counts of the minerals (n = 1,000) in the residuum from HF digestion, particularly the volcanic quartz, which gives one a good idea how pure the tephra is.  Thus, if there are 95% volcanic quartz and 05% detrital quartz in the residuum we knew that it was reasonably pure.  For example, see: An Appalachian isochron: A kaolinized Carboniferous air-fall volcanic-ash (deposit). Lyons et al., Geol Soc. America Bull., 1992, 104: 1515-1527.

We used two dating methods for the Fire Clay tonstein.   For the 40Ar/39Ar single-crystal sanidine date we got 311.5 +-1.3 m.y (1 sigma, n = 11).  For the 206Pb/238U  we got a date of 314.6 +-0.9  (2 sigma), for the 207Pb/235U  we got a date  314.2 +- 1.3, and for the 207Pb/206Pb we got a date of 312.9 +-9.5. We considered the 206Pb/238U date of 314.6 +- 09, which is the most precise of the dates, to be the closest estimate of the true age of the Fire Cay tonstein. The difference between the mean Ar40/Ar39 sanidine date and the U/Pb zircon dates may be due to inaccuracies in the 40Ar and 238U decay constants or other factors, such as the loss of Ar from the sanidine. See: Radiometric  ages of the Fire Clay tonstein (Pennsylvanian (Upper Carboniferous) Westphalian , Duckmantian)  A comparison of U/Pb zircon single-crystal plateau ages,  and 40Ar/39Ar sanidine single-crystal plateau ages. Int. J. Coal Geology, 2006, 67: 259-266. 

Rasoul, you have raised a valid point  on the difference between date and age. Thank you.  Best wishes, Paul.

Thank you, Rasoul. Best wishes. Paul.

Dear Paul,

Thank you for opening such an interesting discussion and for your articles. I have learnt a lot. 

With best wishes,

Rasoul 

Dear Rasoul,

Thanks so much for participation in the discussion.  I too have learned from you and others. Best wishes, Paul.

Thank you Ulrich for your participation in the discussion.  Your work is very interesting,  Please keep me informed on your research. Best wishes, Paul.