Dear Dr. Towe, ... as described above, salt is a new medium/field for me. From what I have seen so far, one could conclude that generally there is no radiometric method which is cable of directly dating salt.

There are some examples, which were successful with certain minerals, but it is unclear if this can be generally applied and what they actually date.... most of them give ages much younger than the depostion age. 

 Coming to your question, radiometric ages generally not date directly the sedimentation, usually they date crystallisation of certain minerals or mineral assemblages (at "elevated" termperatures some system record the time a certain temperatures were achieved and not the crystallisation) – so most likely they date the diagenesis, later re-crystallization of the minerals, or a mixture of both processes. In case of a primary diagenesis, the radiometric should be very close to the sedimentation age.

i have no experience.

k. bente

Dr. Towe ... no NaCl is too pure and seem to be undateable by radiometric methods... Often the rockforming, major constituents are not used in radiometric geochronological method or at least are not the most promising choice...

the K-Ar / Ar-Ar might be an exception....

The target are more rare minerals containing K, such as Langbeinite and Polyhalite,...., which seem to form during salt metamorphism and re-crystallisation, respectively. They were dated by the K-Ar and Ar-Ar method...

I don’t see diagenesis as a problem rather a chance for the geochronologist to make something dateable. From my understanding all sedimentary rocks underwent diagenesis during burial, compaction causes re-crystallization in carbonates as well as in salt. This is the target for radiometric method, at least the ones using the isochrones method.

Dear Dr. Gerdes, 

Can fluid inclusion studies help to give approximation of salt age, when relating it to the burial history of the studied salts?

Dear Gerdes,

If your samples contain K-bearing chloride minerals (i.e., KCl, sylvite), you may apply K-Ar or Ar/Ar dating methods. Alternatively you may get stratigraphic data thtoughout underlying and overlying units.

How clean is the salt? Are there any interbedded, non-evaporite lithologies that contain any nannoplankton or forams that could be used to obtain ages? Which basin are you working?

Mumtaz Shah: these should be very rough estimates, or better relative ages, I assume. Nothing which gives hard evidence.

Tadiwos Chernet: what dateable minerals can be included in halite? Fossil inclusions will likely constrain sedimentation ages. However, in most cases the sedimentation age is known as their are other method ton constrain them ...

 Omer Bozkaya: my point was to find out if other people use radiometric method to date "salt". How they do it (method and minerals) ? With what sucess and what (process) do they date? Yes, there are indirect methods to contrain the sedimentation age... but do they agree with the radiometric age of the salt crystals? I assume that salt minerls (at least partially) re-crystallize during compaction (burial), and later due to deformation during ascent.  

I agree with more or less all what Friedhelm wrote, except for the last sentence. 

Yes, one has to be careful with the interpretation of the radiometric ages of salt, and yes definitely there are approaches which will yield only useless and questionable ages.. but this does not mean that there are no other methods that can give meaningful and let’s say correct ages, contributing at least a bit to the understanding of salt geology; dating secondary processes related to tectonic and ascent of salt.

It is challenging to work with salt, but sometimes you have to climb up the tree to find out that it is possible.... theoretical consideration are not always helpful.

I still hope that there are some experts around with expierience (and some with the interests) on dating salt ....   

Radiometric ages of minerals from different halite formations have produced various results, as noted above. Some show results generally consistent with the depositional age derived from other methods. Some radiometric ages of crystals from the Salado Formation in New Mexico are reasonably consistent with the inferred age of deposition. Other results may be indicating post-depositional events, but I haven’t run across a framework for differing minerals and techniques in various types of salt deposits that might provide a better guide as to how to generally interpret or propose different techniques for a specific purpose.

Textures and features have been described that are accepted as indicating “primary” halite in salt deposits that have undergone varying degrees of alteration since deposition. Lowenstein and Hardie (1985) provided a good framework, partially building on previous work, and subsequent studies have used these and added features/textures. We (Holt and Powers, 1990, 2011) recently described very coarsely crystalline halite (commonly called “recrystallized”) in the Permian Salado Formation, where syndepositional halite cements filled dissolution pits/pipes that developed on the subaerially exposed halite pan. This doesn’t make all coarsely crystalline halite “primary” or syndepositional, but it removes the automatic assumption that such crystals are automatically assigned a post-depositional origin. So, there are some indicators, but in heavily deformed rocks, these can be rare to (probably) non-existent.

I disagree with the notion that halite deposits are inherently open (permeable) deposits.  Potassic salts have long been considered the consequence of potassic metasomatism. Yet much evidence in at least some deposits contradicts this hypothesis. Lowenstein and Spencer (1990), for example, found KCl crystals within primary halite. Casas and Lowenstein (1989) found halite beds being cemented after deposition, limiting the effects of near-surface diagenesis. Fluid inclusion compositions have been interpreted for a long time as evidence of seawater chemistry at the time of deposition, and this would not be practical if the deposits were open to infiltration of unsaturated solutions. Beauheim and Roberts (2002) provided evidence of the extremely low permeability of Salado halite, which, if Darcy’s law has any applicability at all, indicates the system is not open to infiltration.

Your original question ended with the concern that “results/ages could be (always) interpreted without any doubts.” This is a universal issue, but salt results seem a little more “doubtful” at this time. My opinion is that suites of results will provide more confidence, but I don’t think some of these mineral systems are well understood for such things as the range of conditions under which Ar, for example, is retained/released. Or they are just pretty susceptible.

Henjes:

I agree that when you enter real  "metamorphic" phases (diapirs) that "open" and "closed" need a whole lot more attention. Within lesser deformed rocks, it may be possible to find "primary" halite preserved, and then the question will be whether those crystals or beds or lithosomes have preserved some primary chemistry, radiometric age information, etc.

I am currently exploring through lots of geophysical log and other data the shallow burial deformation of late Permian evaporites in the Delaware Basin of southwester US. The lower beds (Castile) show more deformation (pillow), and the textures/fabrics are generally suspect though they may not always differ much from apparently undeformed areas. Upward deformation decreases, and it appears there is syndeformational deposition in overlying evaporite-bearing Rustler. A polyhalite bed in the Rustler is partially involved in the apparent syndeformational deformation. I'm not sure we'll have proper samples to test to see if the radiometric age of the polyhalite will match the relatively well constrained Permian age, and I don't actually need the data. Would be nice.

Anyway, we've had quite a few attempts at radiometrically dating the late Permian evaporites of the Delaware Basin, and I've always thought some of the "spurious" data were telling me something important (other than that a particular mineral was poorly suited to such work), but there often are not enough data to decide if there is a pattern of import.

I do need to look at your Sm-Nd work, as it has escaped me. This hasn't been my primary interest for some time for me, but I should have seen your work.

Ah, me. Should have looked up your (Henjes-Kunst et al,, 2008) work first and your page to see your first name is Friedhelm. Sorry about that.

Friedhelm: thanks, the Leitner et al paper is very interesting and useful. It seems that Ar-Ar works very well for polyhalite... not just for young one...

 I understood -from my limited experience- that carbonates are rather rare in salt deposits and if they would be around I would try to date them by U-Pb .... instead of Sm-Nd ....

Considering the complexity having undisturbed primary (?) and recrystallised, deformed domains next to each other, or domains partly modified by the interaction with brines, I wouldn’t choose any bulk method with relative low spatial resolution ... Although I have to admit the Sm-Nd data in your 2014 paper looks very good! I wonder if the Rb-Sr method can be of some better use in salt, as there are various K (Rb) phases around as well a lot of Sr. 

Dennis: thanks for you input; very interesting as well. I agree with you interpretation that one should see primary (early diagenetic?) and younger domains recrystallised due to fluid flow, deformation, or any other process, next to each other. So during ascent /metamorphism only parts (small domains) get recrystallized ...

To understand better what happened as well as the behaviour (e.g. Ar retention) and usefulness of certain radiogenic isotopes systems, it would be good to apply different isotope systems (radiogenic for dating and source tracing, as well as stable) at high spatial resolution. It helps as well when one can be reproduced the age on different samples from the same location as well as from different locations.

Perhaps the polyhalite bed, when really deposited syndeformational in the evaporite bearing Rustler during Permian time could be an interesting target. I am actually exploring methods with which it might be able to date “this”.... and I need suitable testing material .... (I am not using Ar-Ar, but it would be interesting to compare with)

The Late Permian Salado Formation represents an interesting target because of the multiple depositional cycles that (ideally) show the spectrum from initial flooding with thin basal carbonate-rich laminae, overlain by sulfate that in many units is partially polyhalitized, succeeded by thin 10-20 cm interbeds of orange halite with blebs of polyhalite, and finishing with finer halite (some probably efflorescent) with multiple exposure layers, infiltrated clay, dissolution pipes, and a major argillaceous exposure surface. In the ore zone in the middle of the formation, there are 10 major ore zones with sylvite, some langbeinite, etc. Other cycles between ore zones may have some disseminated K-bearing minerals as well.

The clays show radiometric ages consistent with transport into the basin, although Bodine (1978) was certain on chemical/mineralogic basis that there are authigenic clays. Authigenic feldspars show up here and there that might have some utility for age data.

Some of the K-bearing minerals are probably very close to primary in all respects. From the very stratified nature of the polyhalitization (persistent zones within a sulfate bed, for example) and other features, I expect that this occurred in potassic brine as a "brine table". My colleague, Bob Holt, and I have been observing these things for a lot of years (check out some of the photos in appendices of Holt and Powers, 1990; air intake shaft mapping report in RG pubs).

The overall Salado Formation offers a lot of parallel targets for radiometric dating. Unless someone finds the silver bullet, I expect it'll make most sense if a range of techniques is applied systematically through the formation.

This still doesn't answer your original question. But at least you have the juices flowing for a couple people.

Hi. You may want to look at: 

http://www.sciencedirect.com/science/article/pii/S0016703713004857

Thanks for the link, at least for Neogene salt this is an interesting techinique.

Apart from it I can tell already that using U-Pb SSI (small scale isochrone) method things look very promising for Miocene to Mesocoic "salt"... . this is work in progress and I will write more when the first presentation at a conference and the first article is ready...