Geochronology - Science topic

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

To Matt Edgeworth, Dear Matt, many thanks for your comment. Please understand that as to the "hardline geologists" I won't discuss here ;-) But there is much more to answer: as you know we (the AWG) always considered, discussed and published also on the earlier human impacts and developments. We discussed, e.g., Ruddiman's and others'work, wrote papers like "A formal Anthropocene is compatible with but distinct from its diachronous anthropogenic counterparts", and focussed also on what we then tentatively named "The Anthropogenic Modification Episode" (AME) (incl. the Anthropocene sensu stricto on top). By the way, I also wouldn't mind calling the episode before the Anthropocene Epoch 'Pre-Anthropocene' or 'Proto-Anthropocene'. We wrote papers on the early development of human action, like our production and consumption paper (Zalasiewicz, Williams et al 2016, with those great figs 1-6, and with you as a co-author) etc., and of course our Technosphere paper (again with you) and then the 'Energy use through time-paper' (Syvitski et al. 2020). So that also always was part of our studies and discussions. And no, the AME or the Anthropocene is not an event (for me this is the most puzzling wrong use of the "event"-term and one really cannot name the Great Oxygenation 'Event', which took about 400 million years, an event but rather the Great Oxygenation Episode (the abbreviaton remains the same GOE). And once more, we wrote papers about the proper use of event, epoch, episode in stratigraphy (e.g. Waters et al. 2022).

Yes it was a long human development on the way to the Anthropocene (with a lot of milestones there), in which Homo sapiens developed from a culturally enhanced biological factor (early weapons, early domesticated dogs for hunting, active use of fire etc) to a geographic factor (settlement and agriculture in the Neolithicum etc., in a diachronous, regionally scattered way. It was then however only during the Great Acceleration of the midth 20th century and its effects on the Earth System, that humanity switched into a global geological and Earth System factor, with a near synchronous, globally correlatable start.

And by the way, attempts to precisely date a primary marker in a GSSP with an absolute date is nothing new in geology. It simply is a "proxy age" for the start of an Epoch. This is not different in the Pleistocene (there mostly magnetic polarity reversals were used for near exact absolute dating of associated geological markers), and the Holocene (with exemplary primary marker layers in the NorthGrip Ice-Core and the speleotheme core of Meghalaya, India). These also are "calendar-time" proxy approaches for associated climatic changes and other geosignals. Nothing different as to our GSSP-proposal for the Golden Spike of an Anthropocene Epoch. We have a great event array of geosignals (event here in the normal understanding of short-termed "incidents" or sharp well visible starts of new criteria). This is why we chose a layer of radioactive fallout from H-bomb-tests for precise dating. To me, this results in so much of added-value also for cooperation of geoscientists with archaeologists, historians, etc: there are now much more correlation possibilities also for archaeology (a lot of additional regional and possibly global event layers also can be detected within the Anthropocene succession, such as first occurrence of plastic types etc); In addition we now can also jointly evaluate a global archive of human activity (that is not set up by humans, hence not biased in any way) along with other archives established by humans (historical archives, e.g. fish landings, plastic production,satellite data etc) together with historians.

With all that in mind, it is even more such a pity that you have left the AWG group some time ago. I always liked our cooperation so much.

For geochronology, I agree with H.G. Dill to avoid solution processes whenever possible. You already have a fine to coarse powder that you can use with heavy liquid separation, clean up with acetone and hand pick the crystals after that. The main problem with acids is that you are dealing with undiuted version that have many drawbacks such as vapors, spill-over droplets, very strong reactions with powdered rocks and the non-least one, a higher health risk to the operator. These acids will eat through clothes and skin rapidly and one can end up in hospital for treatment if a pure HF droplet or vapor gets under the skin; it attacks the nerves and is extremely painful.

As for the effects of this rock powder dissolution method on the geochronology results, the problems can be many folds. Cracks in baddaleyite crystals can fill up with dirty acid filled with U and Pb from the dissolved minerals and a very thin (1-2 microns) layer of the surface can have partial leaching and/or contamination. This leaching can be offset by abrading the grains as with zircons in case of chemical geochronological analysis. The problem with using commercial grade is that you have no control on the amount of 204Pb you 'add' to the system through cracks and surface porosity. One mistake is to start with the assumption that baddeleyites are 'impermeable', which is not really the case. They have the same problems as zircons have.

The important thing is with which machine type you are measuring the iotopes, a 'classical' mass spectrometer from chemical dissolution and separation analysis or an ICPMS directly on polished grains?

Overall, the problem of baddaleyite separation and analysis are the same as with zircon, in both case the U and Pb are not that strongly attached to the crystal lattice, only the mineral lattice itself is strong enough to trap quite efficiently U and Pb. The best advice is to take the same precautions that one would use for zircon extraction and it should go well.

Not a geologist (at least in a long time), but from applying Bayesian statistics on shorter time scale and in other applications, I would say YES. Additionally, you may want to look into other statistical sampling and resampling techniques, such as Bootstraping.

Thank you for the answer Brian P. J. Stevens Sir.

IT depends on the zircon textures. If the zircon is still in a good shape, maybe yes. If the Zircon shows evidence of recrystallization and disequilibrium, this method can be applied but results must be evaluated carefully.

Thanks for these interesting papers

Carbonate mineralisation can be used to track thermal histories, utilising U-Pb geochronology (giving you time), in combination with clumped isotopes (giving you temperature).

These papers combine these two methods (and there are many others utilising the methods individually):

Mangenot, X., Gasparrini, M., Gerdes, A., Bonifacie, M. and Rouchon, V., 2018. An emerging thermochronometer for carbonate-bearing rocks:∆ 47/(U-Pb). Geology, 46(12), pp.1067-1070.

Pagel, M., Bonifacie, M., Schneider, D.A., Gautheron, C., Brigaud, B., Calmels, D., Cros, A., Saint-Bezar, B., Landrein, P., Sutcliffe, C. and Davis, D., 2018. Improving paleohydrological and diagenetic reconstructions in calcite veins and breccia of a sedimentary basin by combining Δ47 temperature, δ18Owater and U-Pb age. Chemical Geology, 481, pp.1-17.

Brigaud, B., Bonifacie, M., Pagel, M., Blaise, T., Calmels, D., Haurine, F. and Landrein, P., 2020. Past hot fluid flows in limestones detected by Δ47–(U-Pb) and not recorded by other geothermometers. Geology, 48(9), pp.851-856.

MacDonald, J.M., Faithfull, J.W., Roberts, N.M.W., Davies, A.J., Holdsworth, C.M., Newton, M., Williamson, S., Boyce, A. and John, C.M., 2019. Clumped-isotope palaeothermometry and LA-ICP-MS U–Pb dating of lava-pile hydrothermal calcite veins. Contributions to Mineralogy and Petrology, 174(7), pp.1-15.

Looser, N., Madritsch, H., Guillong, M., Laurent, O., Wohlwend, S. and Bernasconi, S.M., 2021. Absolute Age and Temperature Constraints on Deformation Along the Basal Décollement of the Jura Fold‐and‐Thrust Belt From Carbonate U‐Pb Dating and Clumped Isotopes. Tectonics, 40(3), p.e2020TC006439.

Pan, L., Shen, A., Zhao, J.X., Hu, A., Hao, Y., Liang, F., Feng, Y., Wang, X. and Jiang, L., 2020. LA-ICP-MS U-Pb geochronology and clumped isotope constraints on the formation and evolution of an ancient dolomite reservoir: The Middle Permian of northwest Sichuan Basin (SW China). Sedimentary Geology, 407, p.105728.

Hoareau, G., Crognier, N., Lacroix, B., Aubourg, C., Roberts, N.M., Niemi, N., Branellec, M., Beaudoin, N. and Ruiz, I.S., 2021. Combination of Δ47 and U-Pb dating in tectonic calcite veins unravel the last pulses related to the Pyrenean Shortening (Spain). Earth and Planetary Science Letters, 553, p.116636.

You have to be very careful with the material you use because of potential diagenetic effects. It is best to use skeletal material originally precipitated as low-magnesium calcite, such as oysters for example. Even with low-Mg calcite material you need to be careful to make sure it is pristine. There are a number of petrographic and geochemical steps that can be done to assure you are using unaltered samples. There are numerous papers out there. Here's a paper you can look at to get an idea of the steps done to get good Sr dates. Ortega-Ariza, D., Franseen,E.K., Santos-Mercado, Ramirez-Martinez, W.R., and Core-Suarez, E.E., 2015, Strontium-isotope stratigraphy for Oligocene-Miocene carbonate systems in Puerto Rico and The Dominican Republic: Implications for Caribbean processes affecting depositional history: The Journal of Geology, v. 123, p. 539-560.

Dear Dr. Gupta

IOGC (iron-oxide-copper-gold) deposits is a new-fashion classification scheme in economic geology with the number of old and well-known metal deposits categorized as such.

The majority of these deposits and their locus typicus of origin is located in the paleo- and meso Proterozoic settings of the old cratons in Australia and South America. The kick-starter was Olympic Dam a big animal found by the application of mega-shear zones by Western Mining during the early 1980s by Driscoll. While this blind ore body was a new discovery on the Stuart Shelf a lot of well-known deposit such as Kiruna were re-classified and also attributed to this type of deposit. A series of others hosted by Mesozoic and Cenozoic strata in Chile, Peru and different modern fold belts followed suit.

They contain substantial amounts of Fe, they are bound to felsic and intermediate intrusive rocks of the granite clan and lack a special zonation. Their origin is held to be linked to hydrothermal and metasomatic processes affecting large crustal sections so that the above features need not be restricted to one ore body but found within a metal district, see e.g. Kiruna where you have Fe ore bodies like Kiruna Vara, P-enriched ones, e.g. Henry ore body and Cu deposits like Viscaria.

The ore structure is said to be caldera- or maar like with strong faulting all around that controls the accumulation of magnetite and hematite. Brecciation is a common textural feature of IOGC deposits. There is still a lot of debate around this type of deposit as to its link to volcanic and granitic rocks and the related processes which are normally not closely linked in time and space with each other .

The ore mineralization contains chalcopyrite, pyrite, Fe oxides like magnetite and hematite, REE minerals such as orthite, U minerals and gold.

It is the common way of handling new discoveries in economic geology. Olympic Dam also often named as Roxby Downs named after its mining camp is a giant deposit and exceptional in own rights which stands for great success in exploration and could only be brought on-stream by the joint venture with another big animal from the oil and gas business. The logical process was that of finding a plethora of new IOGC deposits worldwide covering now the timespan from 2.0 Ga to 20 Ma simply by re-examining old animals and re-naming it. Thereby you can increase the values of old deposits simply by a hype. As a result of that the features were more and more watered down. And this is why at the beginning I used the term fashion also to shed some light on how the marketing of “economic geology” often works and how it is adopted by those riding the paper tiger in this business.

This is the opinion of a geologist working since more than 40 years in economic geology with facts and data and who is rather skeptical of all these new finds here and there. I was in the early 1980 in Australia when the “genuine” giant type was discovered by one of the smallest mining companies taking a simply handy-craft geological approach was successful and subsequently was draped around by a wealth of ideology.

H.G.D.

Monazite and zircon are often closely associated with each others in the source rock and in placer deposits, alike. In the study which is available on request for download from the RG server you will find typical X morphological habits and x-y plot for discrimination of their origin and their behaviour during transport:

Dear Harold

Thanks for your answer and the paper.

I know that ia the problem with Alunite, but there are not other minerals to be dating.

Looking at the type of research you do (dust, Loess) you are likely to deal with the smallest of zircons?!?!? The Wilfley table may lead to dust-sized zircon overwash if your rate of water is too high. The Gemini table can be set up so that zircon (the >100 um fraction) stays on the table and you can use a pipette to "suck up" the residual zircon. It may still be likely that some finer zircon flows off the table. The Jasper Canyon Research zircon table is designed so that zircon stays on the table and is collected by pipette. I just purchased one, but have yet to try it because of COVID19 restrictions. It is not a cheaper option but might be more effective. I would maybe ask the vendors to complete a trial sample on each system and see what results they give you.

Hi,

Extrapolation beyond a dated point is always dangerous since you don't have more information. If the sedimentation rate within the dated section remains quite stable and the sedimentology doesn't change too much you could very carefully assume a constant sedimentation rate and extrapolate linearly. However, you'll never be certain and it'll likely be picked up by reviewers as well if the age-depth model in the undated section is important for your overall discussion.

Extrapolation is possible in age depth model scripts such as Bacon ( https://chrono.qub.ac.uk/blaauw/bacon.html ) and Clam ( http://www.chrono.qub.ac.uk/blaauw/clam.html ).

Cheers,

Loic Piret

Dear Sean Freeborne: all the dating methods mentioned could work well in an amphibolite (I assume this Chortis amphibolite is in the amphibolite facies, and so it contains hornblende, biotite and garnet, and plagioclase, of course). But a usually overlooked mineral present in mafic and metamafic rocks is also baddeleyite, ZrO₂, a monoclinic zirconium dioxide, which can also be dated with the U-Pb method, just a zircon is. In southern Venezuela, and extending to Guyana, Surinam and Brazil, there exists a huge Large Igneous Province in the Guiana Shield: the Avanavero-Roraima LIP, which was intruded in Mesoproterozoic times as thick and extensive diabase sills (and possibly extruded as basalt too, but this volcanic facies has been totally eroded, obviously). Its dating with baddeleyite has given the most correct age of 1.78 Ga for the largest sill located at the base of Mount Roraima. Regards, Sebastian.

Dear A.B. Salman!

Some information concerning to the Nile Craton you may find in the following papers:

1) Mohamed G. Abdelsalama,*, Jean-Paul Lieegeoisb, Robert J. Stern 2002. Review The Saharan Metacraton Journal of African Earth Sciences 34 (2002) 119–136

2. Sultan, M., Tucker, R.D., El Alfy, Z., Attia, R., Ragab, A.G., 1994.U–Pb (zircon) ages for the gneissic terrane west of the Nile,southern Egypt. Geologische Rundschau 83, 514–522

3) R. J. Stern - A. Kr6ner • R. Bender - T. Reischmann A. S. Dawoud 1994. Precambrian basement around Wadi Halfa, Sudan: a new perspective on the evolution of the East Saharan Craton. Geol Rundsch (1994) 83: 564- 577

First Appearance Data of Protohertzina might be constrained to 545–540 Ma according to SSFs (including Protoherzina and other protocodonts) reported from Ust’-Yudoma Formation, Russia. see the ref below:

Zhu M , Zhuravlev A Y , Wood R A , et al. A deep root for the Cambrian explosion: Implications of new bio- and chemostratigraphy from the Siberian Platform. Geology, 2017, 45(5):459-462.

I suggest you contact Dr Abdul Rashid Lone who has done a PhD from University of Delhi. Currently he is working as an Assistant Professor at Kashmir University.

Thanks for your experience we are happy to work together

Several papers have touched on this recently, including:

In my opinion you need to look at your own data to inform this decision.

Nick

Hello Jorge,

Thank you for your contribution. I am aware of how ages are determined using the U-Pb isotopic system in zircon. I think, perhaps, you have misinterpreted my question or my question was misleading as it referred to a question about calculation of concordance/discordance. I refer to the concordia age as defined by Ludwig (1998), not specifically concordant ages and how they are calculated. I take the following quote from the Isoplot 3.75 manual (Ludwig, 2012) which provides a concise description of the corcordia age:

"In addition to the usual ²⁰⁶Pb/²³⁸U, ²⁰⁷Pb/²³⁵U, ²⁰⁷Pb/²⁰⁶Pb ages, you can use any pair of ²⁰⁶Pb/²³⁸U - ²⁰⁷Pb/²³⁵U or ²³⁸U/²⁰⁶Pb - ²⁰⁷Pb/²⁰⁶Pb ratios to calculate a single “Concordia Age” (Ludwig, 1998). A Concordia Age is the most-probable age for a data-point (or weighted-mean data-point) on a concordia-diagram, where the true location of the data point is assumed to fall precisely on the concordia curve."

Most detrital zircon studies will use the 'usual' ages, particularly utilising ²⁰⁶Pb/²³⁸U for ages younger than a certain cutoff (e.g. 1.0 Ga) and ²⁰⁷Pb/²⁰⁶Pb for ages older than the cutoff because the latter age tends to be more precise for older ages. However, using the concordia age removes the necessity for a cutoff.

I refer you to the papers referenced in the question and to:

Spencer et al 2016. Strategies towards statistically robust interpretations of in situ U-Pb zircon geochronology. Geoscience Frontiers 7, 581-589.

The above paper provides a discussion on the cutoff age used to switch from 206/238 ages to 207/206 ages.

My question is aimed at starting a discussion on the pros and cons of the adoption of the concordia age method for detrital zircon studies rather than using the more traditional 206/238 and 207/206 method.

Hi Cesar Alfredo Tarazona Velasquez:

There is no specific answer to your question, although such petrogenesis for the gabbroic to tonalitic suite is possible. First, conducting detailed bedrock mapping is a critic way to establish temporal and spatial relationships of the felsic pluton(s) in the field with the assistance of geochronology age determination. Second, determining alkalinity and aluminous saturation state of each phases. Third, building up possible petrogenitic linkage between these phases, e.g., partial melting, or fractionation, or absolutely unrelated. With these work done, then you may have better idea to interpret your geolochemical data and origin of the felsic plutonic rocks.

Best regards,

Xue-Ming Yang

For plagioclase there are a few determination charts, based on the extinction angle in the zone normal to (010) - the albite twin plane, as well as the cleavage plane= which function pretty well. Try fpr example this address: eps.mcgill.ca/~courses/c312/Labs/Optlab9-13.doc @ the McGill University

For clinopyroxene it's a bit more complicated because the optical properties depend on several compositional changes, the general idea is that the optical properties and the mineral assemblage offer some clues for the composition of certain phases. Sorry for the quality of typing in this box, I notice it has become very difficult to get what you type..

Dear Researchers

Thanks for the paper suggestions by you. Went through all of them and also a few more which took me a time of a couple of months to write back to you.

The idea developed from the papers are useful and quite interesting.

Baddeleyite is also pleochroic and might have a greenish hue apart from the reddish-brownish colours mentioned by Leonid. Please find attached some microscopic images in transmitted (plane polarized and crossed polars) and reflected light. Field width is 180 microns if not stated otherwise. Of course a SEM/EDX investigation would provide more information. The rock containing the Zr phases is a potassic syenite.

Christopher:

There is no straightforward answer to your question as the subject needs to be explained in detail. However, this link would provide you with basic and useful insights:

Best

Syed

@ Jorge de Moura - I'd rather try with Jean-Marc instead of Jean Jacques

Dear Dr. Nayak, I've attached an article about Sm-Nd dating technique. Also, if you have gabbro rock samples, you could dating it using U-Pb on zircon, probably, will need 2 to 5 kg, and you can use SHRIMP or LA-ICP-MS methods.

 Your project appears to be focussed on thermochronology and cooling of rocks, and how exhumation may be related to structural evolution.  Thus you may want to focus on fission-track dating, or helium dating of zircon or apatite.  Cooling ages will help constrain the timing of structures and deformation, but may have little bearing on the timing of intrusion. 

Dear Shakoor,

I still think your question is vague. You are asking for locating a needle in a hay stack. The Arabian-Nubian plate has a complex geologic history.  It is defined on its NE by the Zagros Thrust whereas in the north is juxtaposed against the Anatolian plate. The Arabian-Nubian shield is composed mainly of Neoproterozoic Basement, but there are older rocks going back to 3 billion years. These are overlain by a  Paleozoic to Late Tertiary and Quaternary cover, including one of the largest basaltic provinces.

Do you wish to know about the magmatism in the suture zones or the blocks that collided? From your initial question, I thought you were interested in knowing the age of the Arabian shield!

But having said that, I am no expert to give any suggestion. Why do you not get in touch with the earth scientists in King Abdul Aziz University in Jeddah?

Best, Qasim

Excuse me. I can not help it because I just search for stable isotopes of C and O

Good evening Michael,

I don't know if you still need information, but Miller, R. McG. (2008) is sold at the Geological Survey --> http://www.mme.gov.na/gsn/library/ We bought it several times during several field trips, and there still seem to be enough copies.

Maybe they can send you the three volumes.

Best regards,

Andreas

Munazzam Hello

Your question is not simple. At the same time, I propose that our paper (you can find it in RG DataBase) will be useful for you:

Eppelbaum, L.V. and Katz, Yu.I., 2016. Tectono-Geophysical Zonation of the Near and Middle East and Eastern Africa. International Journal of Geology, 10, 1-10.

Best

Lev

You can use Belousova's diagrams for magmatic zircons to determine the nature of your detrital zircons.

Try GFZ potsdam Germany. sure you ll find some one there.

The best is to write to Prof. Peter Zeitler at Lehigh University. In addition to himself, he had several students engaged on Nanga Parbat.

The other would be Prof. Mike Searle at Oxford University-specialist on Karakoram.

I do not know, but Dr Hafiz Rehman >hafiz@sci.kagoshima-u.ac.jp< group in Japan may also be approached.

Cheers

Please see the attached paper and the references therein. 

Hi Stefan,

In past unpublished studies, I have attempted to determine relative ages of corals using AAR, but have found the results to be far more variable than either mollusks or whole-rock sediment samples.  That said, if the samples are well preserved chemically and mineralogically, and from clear stratigraphic context, it might be worth a pilot study.  AAR will provide only relative ages, but should be able to distinguish Pleistocene from Holocene age samples.  A substantial database of radiometrically calibrated AAR samples would be needed to estimate numerical ages. 

Samples must be air dried and not oven heated to yield reliable results.  The temperature history of the ocean floor location might be available through other proxy methods.  

Best regards,

Paul Hearty

Leonid's comment is very useful. Basalt magmas are commonly believed to originate by partial melting of the upper mantle peridotites. The lower continental crust is generally no more mafic than basalt. It would require unrealistic amount of heat supply to cause high degree of melting to produce basaltic magma. 

Regards,

Qasim

Thank you very much.

I am potentially interested in collaborating. Info about our lab is here: https://sites.google.com/site/icpgeolucsb/

You can email me for more information: cottle@geol.ucsb.edu

You can put them in clean aluminum foil. Then it is possible to transfer the pack in boxes or bags, and send them to the lab.

I didn't try it out, but from your description it sounds like what most mass spec software programs also do:

calculate average and standard deviation of your population. Identify which points fall outside of the range from (average - 2*standard deviation) to (average + 2*standard deviation). Reject those points. Recalculate average and standard deviation without those rejected points. This is what you report.

Note: you are not allowed to repeat the exercise a second time in order to improve your uncertainty further. Then it becomes fudging.

Thank You very much Marwan Wartes

Dear Mr Gui,

Palmer and Slack raised expectation to clarify some  questions using the isotope composition of boron in tourmalines. They mentioned as potential controls over the boron isotopic composition of tourmaline the following factors: (1) composition of the boron source, (2) regional metamorphism, (3) water/rock ratios, (4) seawater entrainment, (5) temperature of formation, (6) secular variation in seawater. Trumbull and Chaussidon demostrated that in pegmatite-aplite

dikes the boron ratio is higher than in hydrothermal tourmalines and interpreted their findings as a result of fluid fractionation Apart from the strong dependance of the boron isotope ratios from fluid fractionation there are also some values that can be used as some kind of provenance marker. Values published by Marshall and Ludwig correspond to what is known from S- type granites that have originated by anatectic process from metasedimentary rocks. Martin and De Vito mentioned that the isotopes of boron in tourmaline and danburite reveal clues about the unusual geochemical environment, taking as an example a hybrid pegmatite system from Madagascar. At the end of listing these isotope results and their interpretation, which is by no means complete, the conclusion my be drawn that some indications pointing to a crustal source of boron exist but the wealth of factors on the control of boron isotope ratios renders this highly- sophisticated method of isotope studies less efficient than a classical chemical review conducted as a function of the geological setting.

Best regards

H.G.Dill

I run a U-Th/He and U-Pb geochronology lab. Not sure if this is exactly what you want. Let me know.

Bernard

Try John Barron at the USGS in Menlo Park.  He is a diatom expert and is active in recent climate change research

You may see the attached file. Also the following paper might be useful to your work.

A review of burial dating methods using 26Al and 10Be

Darryl E. Granger

Truly

ARK

It may be a problem to get a precise age, since many gabbros and basalts do not contain minerals with high Sm/Nd, which is what you need to get a well-defined isochron. If there is garnet around, you're in luck. Otherwise, it may be a futile exercise, depending on the expected age of the rocks. If you really have no idea of their age, and are happy to know whether they are ca. 500 or 1000 Ma old, take cpx (and/or hbl) and WR; plagioclase contains very little of anything. If you have an alkalu-gabbro, you could use apatite. If you want to know whether the rocks are 400 or 450 Ma, you may have a problem to get an age that is precise enough to be meaningful.

Thanks Baerbel and Shannon for your advice...

There is considerable good advice above.  But any hand picking will introduce additional bias in any analysis.  If you want the analysis to be as un-biased as possible, make sure your mineral separates are clean, and this may mean refining the separation process.  Then mount everything, image the mount, and then do a random walk from zircon grain to zircon grain.  

Bear in mind that part of your approach should be directed to what you are tying to accomplish.   You may, for example, select euhedral grains because you are interested in the provenance of first-cycle grains only.  Or perhaps only the rounded grains that may be recycled are of interest.    As you note, the euhedral grains are relatively easy to identify, and but zircons get increasingly more difficult to identify if they are broken, frosted, and they have radiation damage.   As such, it is very easy to bias your data by just focussing on the euhedral grains.

I just added a quick illustration above - this should be applicable to all types of isochrons, regardless of the isotopic system.

Perhaps you're too focussed on the epsilon notation? There are plenty of articles where Sr and Nd isotopic ratios have been used to say something about the origin of granites. However, it may not necessarily be true that granites 'image their sources' directly, and that each granite has one single unique source. They can be formed by partial melting of sedimentary rock, or of pre-existing igneous rocks, but a granitic magma could also be produced by assimilation + crystal fractionation of a more mafic magma. Sr and Nd isotopic ratios can certainly help to constrain the source(s) of granites - but major and trace elements are useful too.

HI,

A bit late, but info is always handy.

Are you familiar with Particle Induced X-ray Emission (PIXE) and Nuclear Activation Analysis (NAA)? These techniques are to a large extent standard-less and should be of great help to you. Let me have your location and I will direct you to people who WILL/MUST help you. In this regards also search publications of  Lakay, Gihwala and Peisach. Regards.

I agree with Daniela, that it is difficult to get some ages from serpentinites. However I know that the Stockli's team at Austin (JSG) is trying to develop a method to date serpentinites from magnetite...it seems that it works...but is mainly restricted to thermochronology..

E.H. Goldstein et al.: Magnetite (U-Th)/He Geochronology: Advancements in Sample Preparation and Application to Date Serpentinites, Geological Society of America-South Central, Austin, TX (2013)

Papering is also a good method to separate micas.  Get a normal clean piece of paper (not folded) and place another piece of paper on the table (to catch the sample).  Hold the papering paper at a gentle slope and sprinkle your sample at the top.  Gently shake tap the paper and the round grains will roll down, while the sheet-like flakes will stick to the paper.  Hands down the cheapest and quickest way to separate micas :)

Any method of Ar-Ar dating using sericite (laser probe, total fusion or step-heating) can be susceptible to 39Ar and 37Ar recoil effects, resulting in mixed too-old and too-young ages.  It's an artifact of the fast-neutron irradiation process to produce 39Ar from 39K, and the fast neutrons can kick some newly formed isotopes out of their positions (think of it like a cue ball hitting other balls in pool/billiards/snooker).  If your grains are generally <50 microns in diameter, and/or interlayered with K-poor minerals (e.g., chlorite in biotite, or K-poor clays in sericite) then you will get disturbed Ar-Ar ages.  This is a great paper to explain the problem:

Dear Christopher,

I am slightly puzzled by the fact that this particular stalagmite was shown to be in secular equilibrium (>500 ka) by U-230Th method. A simple calculation shows that if you start accumulating radiogenic lead in a speleothem initially having common Pb (206/204~18.5, 207/204~15.6, and 208_204~38.5) and 238U/206Pb=50, then in 3 Ma in a closed system its 207Pb/206Pb will be =0.68, which is a bit lower than your lowest measured 207Pb/206Pb value. Because 238U/206Pb value decreases with time, measured values of ~1000 in your sample may indicate that initial 238U/206Pb was even higher. For the initial 238U/206Pb=1000 207Pb/206Pb will be =0.15 in 3 Ma, which you do not see. Therefore, there are at least 2 natural reasons to explain your data:

(1) Your speleothem is younger than ~ 50 ka

(2) You deal with open U-Pb system. The first scenario would require COMPLETE loss of radiogenic Pb, which is highly improbable. The second scenario would require very recent considerable U gain, which would result in young 230Th/U ages.

Therefore, my suggestion would be (1) to double check U/Th dating results and confirm that the same materials were analyzed for U-Pb and (2) carefully check the accuracy of your U/Pb ration LAICPMS measurements during the analytical sessions when these data were collected.

I hope this would help in resolving your puzzle.

Best,

Leonid

Dear Zehra,

What it often observed is the presence of apatite or xenotime while rock is devoid of monazite. In this case, ihis can be explained by the incorporation of both the Phosphorus and the REE in one mineral instaead of another. To me, the correlation of the presence of tourmaline and the absence of monazite is much more speculalive even if tourmaline may incorporate REE.

Sincerely,

Thermochronology of zircons in a high grade metamorphic rock will NOT provide you any information on the depositional age of the protolith, as some have suggested! But will provide you with the history of post-metamorphic cooling, which can postdate metamorphism by hundreds of millions of years (!)

The correct procedure would be to carefully analyse the CL textures of the zircons and date cores and rims - if you find a coherent, younger age population from sector zoned zircon rims, and these ages correspond to low Th/U ratios (e.g. <0.02) then these ages may represent the timing of metamorphism.

The youngest zircon cores which are NOT metamorphic (e.g. oscillatory zoned with high Th/U ratios) may constrain the MAXIMUM age of the sedimentary protolith.

I recommend reading the following paper on zircon textures. Other papers in this volume will also help you a lot.

Springer just published an Encyclopedia of Scientific Dating Methods, covering all major techniques for the Earth Sciences. There is a search option and you possibly find all sorts of applications/limitations for Lu-Hf

As far as I know, Nudat is strongly connected to the Nuclear Data Sheets, i.e. ENSDF(Evaluated Nuclear Structure Data File). These are _evaluated_ data, i.e. recommended values. Naturally, not every publicated half life can and will be (immediatly) incoporated into ENSDF.

You can find all references in the ENSDF database, also on the nndc webpage.

Atlas of Zircon Textures by F. Corfu may be also helpful if you are going to work with zircons

Hi Nargess,

yes, this is the problem with ANY model age, in that it depends on both your sample and the model source you are comparing it to. Obviously you have mafic or ultramafic samples with very high Sm/Nd (and hence 147Sm/144Nd). In that case the model ages you calculate will be very sensitive to firstly the choice of reservoirs, and the references suggested by others in this discussion are all the best and most up to date. As you point out, it is how to actually interpret these values you are getting, and whether you can actually use the data (ages) calculated.

In essence it depends on what you want. In general we use these ages not, of course, as ages of the rock, but the time it was last in isotopic equilibrium with that reservoir: ie, the time at which we think it was extracted from that portion of the Earth. In that sense, I don't think many people would argue that there is a large, truly chondritic reservoir still inside the Earth that is regularly sourcing large amounts of magmatism on the modern Earth, so T CHUR is not likely to be particularly meaningful for geological applications rocks of Proterozoic or younger ages.

TDM is derived from the evolution of the Depleted Mantle which is the cumulative residue of the upper convecting mantle after extraction of the continents and MORB. Indeed, one isotopic approximation of the DM is to plot the Nd isotopic value of MORB through time, therefore tracking its evolution. What happens when one does this is that we see that there is no "single" DM reservoir, and we need to use a two stage evolution as discussed above. However, I digress- I simply want to illustrate what your data may be telling you, and I attach a graphic that I prepared for other purposes that can outline what your data are saying.

Since the DM reservoir is itself evolving through time as a function of its own Sm/Nd (and hence 147Sm/144Nd) ratio, if your samples have a similar Sm/Nd ratio to the reservoir, then they will define a line that, when you back calculate, will never cross the DM evolution (pale blue line in the figure). In fact it doesn't matter what the present day epsNd value of your sample is, if it has a very high Sm/Nd ratio it will form a sub parallel array with the DM evolution and hence give meaningless ages. Therefore you will not be able to either calculate a TDM or compare it with any of your other samples.

Another case is where your sample has a positive modern day epsNd, but a low Sm/Nd (and hence 147Sm/144Nd) and hence will evolve to give you a TDM, but no TCHUR (since it will never cross the CHUR evolution (dark blue line on figure).

Finally, I have included the "normal" case, say that of Continental crust (CC) which is negative at the present day and will project back to gave both a TCHUR and an TDM- with TCHUR always being younger than TDM. In this case you can start to compare potential sources, extraction ages and mixing, but these all assume a near or below chondritic Sm/Nd (and 147Sm/144Nd) ratio.

I have a pile more examples if you need/are interested, but I think your issue is not the choice of reservoirs, but the Sm/Nd ratio of you samples, rendering model age calculations meaningless.

The final thing you could try is to see whether they plot on an isochron diagram, hence testing whether your ultramafic samples were all derived from a source of the same age, but at different times. I have seen the is for some komatiites and high Mg dolerites which are clearly not of the same magma batch, but are still isochronous.

cheers, and sorry for the long exposition!

Bruce

Hi Thomas,

I also suggest my lab, the CEREGE in Aix-en-Provence, France. For exposure dating, you can contact Pr Didier Bourlès (bourles@cerege.fr) the head of our team or Dr Régis Braucher (braucher@cerege.fr).

We organize also the AMS 13th conference at the end of this summer 24-29 August 2014. http://ams13.cerege.fr.

Hi there,

This is going to depend exactly on the type of age calculated. Typical ages quoted for zircon would be

1) a mean age

2) an intercept age

3) a concordia age

In the case of a mean age it should be a weighted mean of your age of interest (usually 207/206 for old zircon quoted in your question). Weighted mean ages are pretty easy maths.

Intercept ages are used when you have 2 significant ages (sometimes more but 2 is a simple case) with discordant zircon spread between them. In this case it is a similar calculation but based on your tie line, not a single 'axis'

Concordia ages are calculated based on 'matching age sets' e.g U/Pb-Pb/Pb or U/Pb-U/Pb. These ages are fairly heavy maths to do by hand!

If you are doing lots of this work I would recommend reading the manual for Squid2 (which is a free download) produced by Ken Ludwig at the Berkeley Geochron Centre (download link below). The Squid program processes SHRIMP U-Pb data files and this manual has plenty of info on how it calculates ages.

There is also a GCA paper by Ken Ludwig that goes through lots of the maths in detail for concordia ages, and associated errors that need to be included, titled "On the Treatment of Concordant Uranium-Lead Ages". link below

That should get you started!

Cheers

Rich

pMC is percent modern carbon, with modern or present defined as 1950. Of course as calibration is needed anyway you could just apply the formula and give a negative radio carbon age of minus 2150 years bp, but that'd be rather confusing. Since the atmospheric atom bomb tests of the fifties and sixties atmospheric radio carbon values have gone haywire, although they're nearing normal again now. So for anything younger than 1850 AD or so laboratories often do not give an age but the raw radiocarbon measurement as percent of the standard. It is up to you to try and make sense of it in the context of sample.

Hi,

You can use Pb Mb to avoid Y interference, but you will loose precision as Pb Mb has a lower count rate than Pb Ma, approximately only 60% of Pb Ma... There are less interferences issue with Pb Mb, although you should check for 2nd order Ce La (minor interference but Ce is a major element in monazite!). Pb Mb is recommended for xenotime analysis (due to the HUGE amount of Y in xenotime), however I would stick with Pb Ma for monazite analysis.

In any case, a critical point about accurate measurement of Pb (which is considered as a trace element) is the background correction. The curvature of the background under both Pb Ma and Mb is significant and can lead to several 10th to 100th of ppm of error!

That being said, and if you decide to go for Pb Ma analysis, you have to be aware of a peak shift issue with Y Lg2,3 between monazite and YPO4 (a typical standard used for Y standardization and peak interference measurement); the shift for YLγ2,3 is measured to about –4.5 × 10–3 Å in monazite relative to YPO4 (~50 * 10-5 sin-theta). There are additional interferences to take into account, such as La La (2nd order) and Th Mz. For more information on precise and accurate monazite dating (including the problem of Y Lg2,3 shift) I can only recommend you to read...

Jercinovic, M.J., Williams, M.L., 2005. Analytical perils (and progress) in electron microprobe trace element analysis applied to geochronology: Background acquisition, interferences, and beam irradiation effects. American Mineralogist 90, 526–546.

and

Jercinovic, M.J., Williams, M.L., Allaz, J., Donovan, J.J., 2012. Trace analysis in EPMA. IOP Conference Series: Materials Science and Engineering 32, 1–22.

You can also subscribe to the Probe for EPMA forum and ask your EMP questions to a bunch of expert in the topic :) Check here:

Good luck,

Julien Allaz

Allegre attached !

The simple response (without seeing images or knowing more about the rocks) would be that true sector zoning in monazite reflects primary growth. The sector zoning is likely to represent relatively fast growth (as mentioned above). Variable attachment of different elements onto different growth faces will result in your 'sectors', and slow diffusion of elements within monazite retains the zoning. If you are really seeing zoning of that type it would strongly indicate a lack of secondary recrystallisation processes, which play havoc with your monazites in terms of zonation.

The most widespread substitution is the huttonite (ThSiO4) substitution: REE_P_ThSi, by which part of the phosphorus in the anionic groups is substituted by silicon. Also the (-2)CO3 and (-2)SO4 anionic groups might be involved in similar heterovalent substitution, but to a lesser extent. The huttonite substitution is actually "good" for geochronology, beacause it is the main mechanism of introducing Th in the monazite lattice (besides the cheralite/brabantite substitution -2REE_CaTh).

the Deccan basalt pile at Mahabaleshwar, Sahyadri (Western Ghats) region.

Along the Sahyadri Range (syn. Western Ghats) beside the west coast and

thins progressively eastward and southeastward, such that along the eastern fringes of the province. Whereas the lava pile in the Western Ghats region and in the interior areas of the province is made up almost completely of fairly evolved, sub-alkaline tholeiitic basalts.

It seems like mission impossible with luminescence, because the cobble has been exposed to light. Had it been taken from an excavation and not exposed, you could have tried the exposure dating developed by Reza Sohbati. But even if you now drill a core through the cobble and get a good profile, its interpretation will always be questionable.