l have dunite samples and want to do geochronological study for them. Can anybody tell me why it is not possible to do dating for dunite, and so is there any way to do dating for these ultrabasic rocks?
Sm-Nd is a possibility (Chassigny gave a good isochrone) but at least minor phases are needed. Re-Os is another option. This is a lot of work though.
Wow I never thought of this "drawback" of geochronology. But it makes sense I guess. Most of the dating methods use LIL elements and their daughters. dunite by definition is devoid of these. I wonder with the new generation of mass specs if this still would be a problem. Dating dunite nodules would make it more complicated. You can date their host only. Even if you can date the nodule, it may be hard to assess the role of high temperature residence in the mantle in making it a "closed system" required by the dating. But it is an interesting problem.
zircon dating, Hf–Sr–Nd–Os isotopes and PGE geochemistry should be possible. May be this artcle is hepful:
⩾3850 Ma BIF and mafic inclusions in the early Archaean Itsaq Gneiss Complex around Akilia, southern West Greenland? The difficulties of precise dating of zircon-free protoliths in migmatites
Victor R McGregord et al., Precambrian Research, Vol.117, Issues 3–4, 2002, 185–224
Hi Francis, I was wondering how one would account for a "closed system" behaviour for a ultramafic xenolith. It perhaps spends quite some time before it gets caught in an ascending magma.
Hi everybody, my samples include dunite and harzburgite. the dunites are made of about 95% olivine and in some samples these olivines are altered to serpentine and idingsite with iron oxides offcourse. there is no zircon . On the other hand the harzburgites composition is olivine, orthopyroxene and clinopyroxene with alteration product mainly serpentine. l will make PGE analyses for both types of rocks but using LILE as l think will not be usefull , do you agree with me? and if we say ok ,it is possible can anybody tell me where l can do this?
Thanks alot
This is difficult problem. If there is garnet, you can use Lu-Hf. But then the rock must have crystallized at a temperature which is below the closure temperature of the minerals (800C for ga and cox). See Bedini et al (epsl, 2003 on my web site). For Os, the closure temperature is even lower (300C), the system remains open, so a model age must be used. In any case, cleaning minerals is a long job.
Hi everybody,
That issue in a controversial topic, but, there are several methods for dating of such kinds of rocks.
1- the Sm-Nd is the best applicable suite for the timing. For this you need to plot the rsults on 147Sm/144Nd versus 143Nd/144Nd isochron to get the accurate dates, 2- If there is garnet in your rock units, so you can use the Lu-Hf method on separated garnet., and 3- more recently the U-Pb geochronology on the beidlleiyte mineral was successfully applied in mafic and ultramafic rocks.
Hope that these brief material could be useful for you.
Regards,
Hi Ayten,
Regarding to your question, because of very low concentration of elements such as K, Ar, Rb, Re, ... is not possible to date ultrabasic and ultramafic igneous (UM-UB) rocks by different methods, unless I have primary amphibole or biotite in our ultramafic rocks. In dating of UM-UB igneous rocks it is better gabbroic rocks are considered.
Regards,
Rahimsouri
If you find zircon or baddeleyite in a UM sample, you can be almost sure the rock is contaminated.
Thanks to all of you , for all these informations but it seems not possible to do dating coz l have no primary amphibole ,no garnet .
Can you describe the geology of the location where you obtained the Dunite.? Is it possible to get relative
dating by dating strata graphic layers above and below the Dunite by doing a literature search of geological
articles about the geology of the area that you obtained the Dunite? Geology has some world wide layers
such as the thin layer that formed 65 million years ago following a meteor impact. There were also world
wide sudden drops in sea level elevations, with five million to 20 million year continental settlement recovery
time periods that re-established sea levels to their former elevations. These time periods create distinctive
strata in that deposition is occurring in areas that were under water as the ocean levels encroached across
the land.
Mike Clark, Golden Colorado, USA
Since all of you deal with radioactive decay of parent to daughter products, you might be interested to obtain
a copy of CHART of the NUCLIDES. it is quite expensive in the 17th edition, but more reasonably priced in the
14th and 15th edition. You can obtain it from Amazon.com, or other sources. Just GOOGLE
Chart of the Nuclides.
Mike Clark
Hi Michael. The area is so complicated within Zagros Orogenic Belt in northern lraq and tectonically it is within sbduction zone(arc and back-arc) and these dunites and harzburgites are overthrusting the mafics mainly gabbro and pyroxenite dikes . l am now trying to make dating of the gabbro but till now l did not get the results coz l send the samples of the gabbro to Europe for analyses and dating . But now you said if l can make dating of the above and below strata,well , l have some pyroxenite dikes cutting these ultramafics ,so if l canfind some PRIMARY AMPHIBOLES is it possible to do dating for them and hence l can estimate the relative age?
Thanks alot
I also support the use of Nd-Sm dating on garnet (if present) as the best option.
The dikes could be slightly younger, or they could be vastly younger than the strata they cut through..
I would suspect that IF the composition is similar to the Dunite, then it is only slightly younger, but if it is significantly different in composition, then it could be vastly Younger than what it cuts through.
Think of the Earth as a combination pressure cooker, with the properties of a fractional distillation column.
As time passes, the temperature, pressure, and chemical composition of the mix changes
especially as the more easily melted, and lower density materials find their way to the top and exit
through the fissures long enough for the pressure to equalize. The remaining materials mix with other
types of rock, and the next time it exits from a fissure, the materials released are of a different composition
than the prior release.
Alternatively attempt to date the layers immediately above and immediately below the Dunite where it flowed horizontally across a surface (if possible). You can then bracket the age is being younger than what it is lying upon, but older than what it is lying under.
Mike Clark, Golden Colorado, USA
Thaks alot but as l said these dunites overthrusting the gabbros and they are alternating with harzburgite, and hence it seems not possible. l will try to make a geodynamical model based on the geochemical study and l think this is all what l can do. Thanks alot again
The reason is simple. Dunites contain serpentinized OL (~Fo90-92) plus Cr-Al spinel, and maybe a trace of OPX or CPX. None of those minerals contain enough of the nuclides that are useful for radiometric dating. If they did, we would be able to date oceanic crust and upper mantle from ophiolites and drilling.
That's the problem , so what is the solution? how can we estimate the age of such ultramafic rocks? there must be a way.
Re-Os on sulfide will give a model age. Precision is limited and sulfide closure temperature awfully low (300 C). But that may be the best we have.
may be this can be a way to determine age of these ultramafics , l will check to find some sulfide minerals in my samples if there are some. Thanks, we will keep in touch
Would it be useful to your study to at least know when your rocks reached mid- to upper-crustal levels? If your cross-cutting dikes have amphibole (or better yet, muscovite), you could do 40Ar/39Ar and at least get an idea of the last time your rocks were 400 C. It doesn't mean anything for the age of your dunites or harzbergites unfortunately, but a little thermochron could yield cooling rates that may help you constrain the most recent activity of process you're interested in.
You mention harzburgite with opx and cpx. If you have some fresh unaltered pyroxenes you could use Sm-Nd using mineral separates and whole rock data. I have worked with similar problem, and if you don't have fresh silcates, then it is very difficult to get age data. We tried whole-rock Sm-Nd for altered peridotite, but it didn't work.
Another comment, if I may. Your colleagues in tectonics, paleomag and structure may help you reconstruct the geologic hisotory of your study area. The results won't be as accurate or satisfying as a numerical answer. C'est la vie.
Also, make use of the secondary phases if you can. What is the serpemtine mineralogy? High-temperature antigorite or low-temperature lizardite? Would the serpentine stable isotope compositions help you determine whether serpentinization was pre- or post-obduction? (I'm assuming this is an ophiolite.)
In genetral, I think it might be useful to reframe your question about dates. Instead of fitting the rocks into absolute geologic time, ask about the relative timing of events in the rocks' history.
These harzburgites overthrust dunite , it is ok l have some fresh px, but do you think these pxs. can be used to determine the age of dunites? ln my opinian , no we can't use them for dating of the underlying dunites, what do you think?
the serpentine is antigorite, and these succession of the outcropes in the study area was determined before to be an ophiolite succession but l have some question marks about it when we went to the field for sampling coz we did not find volcanic rocks as previosly was determined by so many MSc. and PhD. as well as researchers works . l am now trying to look for volcanics and pillow lavas in my next field trip within this month , and hope to find some answers
Could you clarify what process(es) you are trying to constrain or what hypothesis you are testing? Geochronologists can produce 'dates', but what is important is how this information relates to processes.
Is this an ultramafic complex, such as an ophiolite where the dunite is directly associated with the px-bearing peridotite? Of course the dunite might represent residua from a melting event and the more fertile px-bearing be overthrusted derivatives. Even if this is part of a cumulate progression, the peridotites should have roughly the same magmatic time scale. The time gap between dunite and peridotites should generally not be great (if co-magmatic).
Dear all.. yes this is ultramafic complex within ophoilite and the hatzbutgite overthrusted the dunite and both are cut by pyroxenite dikes, but there must be a time gap between them even if they are co-magmatic, and even if we suppose that the time gap is negligable do you think we can date the peridotite?
As already mentioned, especially cpx may be a candidate for Nd-Sm work, even without sulfides. Harzburgites with only opx should be much less helpful.
If your dunite contains some garnet, then preferably use Nd-Sm method on garnet and the WR which could record the age of crystallization of the garnet either by tectonic movement or magmatism. If no, use this same method on cpx, opx separates and WR . This would work for you.
Martin's answer is fully appropriate. Nevertheless, let me ask: what is the event you want to date? The last melting event? Emplacement? Remember that the temperature at which elements stop jumping around is about 800°C for Nd or Hf in garnet, cpx, etc, and a meager 350 K for Os in sulfide. In other words, as long as a rock remains at temperatures >800K in the mantle, the clock is permanently reset and never starts. If the melting event coincides with emplacement at shallow level, which is probably the case of ophiolitic material,then we are in business and you can use chronology. As for dating dunite, garnet is likely to have been introduced into the rock well after the rock formed.
Thanks Francis, in fact l want to date the emplacement of the upper mantle sequences , in dunite there is garnet , chromite and orthpyroxene associated with the olivine ,offcourse amphiboles which are secondary after mafic minerals
chromite and orthopyroxene contain nothing. Lu-Hf on garnet is the way to go if cooling is fast.
Generally zircons are used to date igneous rocks, but basic rocks are very low in zircon. This is one of the reasons they are difficult to date. Try looking for information on dating using badellyite (e.g. Krogh et al., 1987)
Its available in this textbook: http://www.amazon.com/Swarms-Geological-Survey-Association-Special/dp/0919216331/ref=sr_1_1?ie=UTF8&qid=1371066919&sr=8-1&keywords=Mafic+Dyke+Swarms
Baddeleyite is common in residual liquids of cumulates and in interstitial melts (Zr is incompatible), typically dolerites. I am sure it is less common in residues.
The problem with dating ultrabasic rocks is that most elements used for geochronology are heavy, and heavy elements are amongst the last to be incorporated into crystal structures from a melt. They're also amongst the first to leave crystal structures during heating, so dates can easily be re-set, which is something you would need to be aware of in this kind of study. I have no better suggestions for methods than those above, but be careful to correctly interpret the significance of any dates you can produce. Any geochronological date is an interpretation of available evidence, it's not evidence itself.
You can make use of the primary inclusions such as garnets,chromite or phlogopite in the ultrabasic rocks for effective age determination-Os-Re or Rb-Sr or Sm-Nd systematics of any one of the selected inclusions could be tried to constrain the date of solidification of the ultrabasic rock.
You can try with Re - Os in sulfides enclosed in olivine, used to stimate age of melt depletion, but if the olivine is altered the system can be reset, This is an example: http://ac.els-cdn.com/S0024493707001211/1-s2.0-S0024493707001211-main.pdf?_tid=019609fe-e41f-11e2-9363-00000aacb35f&acdnat=1372883439_a6b9e15951ef9105a9762005f813bcf9
Zircon in mafic melts tends to dissolve not to crystallize. Thus, basalts contain rare in-situ magmatic zircons but capture xenocrysts as magmas rising upward. See Hanchar, J.M., Watson, E.B., 2003. Zircon Saturation Thermometry. Reviews in Mineralogy and Geochemistry 53, 90–112. Ping Jian, SHRIMP, Beijing
most mantle sequence have plagio granites which have Zircon and are suitable for U-Pb dating . you should try to find plagio granite which most of tethyan mantel sequences have this type of rocks.
good luck
Try Lu-Hf isotope system on mineral phase(s) , as Martin mentioned. we have gain
ages on the depleted peridotites from the subcratonic mantle: http://www.sciencedirect.com/science/article/pii/S001670371300152X;
http://www.sciencedirect.com/science/article/pii/S0024493712002873
Hope it will help.
Thanks alot. In fact there is leucogranite body about two meter thick hosted within dunite body. Now I am going to study this leucogranite and try to date it and will see
I realy don't know till now I am trying to figure out this .In fact this body is acidic rock and l am sure coz l examine thin section and recognied the mineral composition. It is composed of feldspar both K-feldspar and plagioclase with quartz and amphiboles and l told you this body is hosted by dunite. I will start to study this relation but l need to make chemical analyses and try to date it but all these need time for me coz here we have no available techniques so l shoul send the rock samples somewhere outside my country. Wish me good luck
Dear Martin, the study area is very complex , there are ophiolite succession and pods of chromite, pods of pentlandite in addition to this acidic pods and now one PhD student is working on the ultramafic rocks and he is doing the chemical analyses for major,trace and REE as well as PGE and we will try to find the connection between all these observed features therefore I wanted to know which radiometric method
is suitable for the determination of the age of the ultramafics
Dear Ayten,
Literature is full of nice studies demonstrating that plagiogranite form by remelting hydrous mafic rocks. But leucogranites and dunite are reactive assemblages (quartz+olivine) so they are unlikely to share a common ancestry. And I wish you good luck regardless of what you decide to do!
Francis
Thanks alot l will do the required analyses for both and see what to interprete all these but as l said l need time to send samples for analyses
Dear Ayten,
Please see our recent paper that demonstrates alkali-rich granites and granodiorite can be produced from basaltic upper oceanic crust and gabbroic lower oceanic crust.
Liu, W., et al.,Underplating generated A- and I-type granitoids of the East Junggar from the lower and the upper oceanic crust
with mixing of..., Lithos (2013), http://dx.doi.org/10.1016/j.lithos.2013.08.009
Dear Ayten,
it is not impossible but difficult, as um rocks do not very frequently include sutable minerals. But you can try to find a few baddeleyite or perovskite and apply LA-ICP MS or SHRIMP. There are a number of papers you can use for the methods and applications
Platinum group minerals from chromitites associated with the ultramafic sequence can be dated pretty successfully, allowing dating of the ultramafic sequence itself. If you can find such minerals in a placer of course... See:
Coggon, J. A., Nowell, G. M., Pearson, D. G., & Parman, S. W. (2011). Application of the 190Pt-186Os isotope system to dating platinum mineralization and ophiolite formation: an example from the Meratus Mountains, Borneo. Economic Geology, 106(1), 93-117.
http://gsecongeo.highwire.org/content/106/1/93.short
Dear Ayten,
If material suitable for dating are not present in your rocks in sufficient quantity, try an indirect approach. Try to bracket the age of your ultramafics by dating the rocks that you are able to date (i.e. the rocks that are older and younger than your target rocks).
Primary zircon in ultra depleted (strongly undersaturated in Zr) dunites is restricted in terms of host rock geochemistry. So I would not trust zircon dating results in dunite until people will show "in situ" zircon. I mean we should see in microscope zircon relationship with major rock-forming minerals like olivine, pyroxenes and chromite OR serpentine, chlorite.
I think the question should be what exactly do you want to date when dating ultramafic rocks? Lithosphere (re-)formation, refertilization, melt depletion, emplacement in crustal levels, deformation. Re-Os-dating is sometimes applied although several obstacles need to be considered (e.g. Rudnick & Walker, 2009).
Ultrabasic rocks are not a good source of zircons that could be used for U-Pb single-crystal zircon age dating. Such zircons are found in granites and rhyolitic volcanic rocks. As the rocks get more basic starting with granitic rocks, the zircons become more depleted in the rocks until there are none. Thus, there is no good method for dating ultrabasic rocks.
If you have a good hunt around in your samples you may find helpful accessory phases. As has been pointed out, zircon isn't going to be a great contender in ultrabasic samples, however there may be datable minerals that do not require the same Si activity, such as titanite or baddeleyite that can be used for U-Pb. However as always you would have to make sure you know their petrological context, ie what you are actually dating, for example if they are inherited or been affected by later events in the case of low blocking temperature minerals. Other than that the Ar route may be the best one.
The question itself is very generalized since ultramafic rocks can widely fluctuate in the mineral composition. Some minerals that may occur in the ultramafic rocks like amphibole, zircon or titanite can enable Ar-Ar, U-Pb or Re-Os methods of dating. However, there is a strong limitation in the validity of this dating since those minerals are restricted in a few ultramafic rocks and present in others mostly due to secondary processes such as metasomatism or hydrothermal process of the original rocks.
Best Sm-Nd method on minerals and rhenium-osmium method for platinoids in chromites. You can use Ar/Ar by phlogopite, if it is in your dunite. Zircon is extremely rare in the dunite and often he xenogenic
I have chromite in these dunite some are very diagnostic and some are rimmed by garnet , but the amphiboles are secondary and I think after all the useful discussions by all who gave me ideas how to treat these dunites , the best is to try Rn-Os.
Thanks to all
Dear Ayten,
Chromite usually have low Re contents and is not posible have an age, in most cases; but you can use a Trd and get a melt depletion age and use your Re - Os system like a tracer, when your rock is not disturbed by other process.
You can try to use Re - Os, if you find PGE inclusions in the chromite, but you have the risk of the system be disturbed by secondary processes, in your case the growth of anphiboles.
You can read more in Carlson et al., 2005 and Rudnik and Walker, 2011.
Dunite: Olivine-rich, So , you don't have suitable elements for dating in this rock, e.g., K, Sr, Rb, U, Zr .. etc. If you are lucky, then you can use Sm/Nd isochrone for several dunite samples, at least you can get a model age
As Jens said: What do you want to know when you date the mantle? The time of formation? how would it be useful in the petrologic context? The oldest preserved evidence of crust is ~4.4 billion years old - so your mantle started depleting by then at the same time. How does it help you to know that?
If you get data from the crustal rocks surrounding your ultramafic unit, they will provide you much more useful information.
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