Yes, the ɛNd vs. ɛSr plots can be used for distinguishing different granites. The following two articles including this plot.

B.W. Chappell, 1984. Source rocks of I- and S-type granites in the Lachlan Fold Belt, southeastern Australia. Phil. Trans. R. Soc. Lond. A 310, 693-707.

M.T. McCulloch and B.W. Chappell, 1982. Nd isotopic characteristics of S- and I-type granites. Earth and Planetary Science Letters, 58: 51-64

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.

Dear Marlina, I think you point is quite important. However, in my case I'm trying to distinguish between two granitic suites. Major elements are very similar, trace aren't. The isotope ratios should make my argument stronger (hopefully). Thank you for your answer.

Granites are the product of partial melting of crustal materials or the product of mixing of mafic with felsic magmas. The magma can assimilate crustal materials. So, apart from granites themselves, you also need to know the crustal compositons downthrough the crustal column, which is especially true in that you want to know the source rock. You also need to know sedimentary, metamorphic and pre-existing igneous rocks in addition to seismic wave profile through the crustal column. Nowadays, people are eager to publish papers. So, the Nd, Sr, Hf isotopes become the easiest shortcuts. However, petrography and geochemistry are still the basic things one must study. In your case, I think the two suites should have different major and trace element characteristics and, perhaps, petrographic features, if these two suites of rocks were indeed derived from different source rocks. But you need to be more patient and do somewhat painstaking works.

Dear Giulio,

pay attention in using these isotopic ratios for ancient rocks characterized by high Rb/Sr (hence high 87Rb/86Sr) ratios. If you do not have good and close constraints on their exact age of emplacement you can obtain questionable initial 87Sr/86Sr isotopic ratios (i.e., epsilon- Sr, only a matter of different notation).

For the rest I agree with Marlina. Granites may have very different origin and their geochemical message is strongly modified by secondary processes (AFC-type processes, mixing of different melt compositions and so on).

I continue to believe that the strongest constrain for granitic magma (but in general for any kind of magma) must come from major elements, then from trace elements and at the end from isotope geochemistry.

If a model works for isotopes but is in disagreement with major element abundance, simply throw away the model. Isotopes can be commonly easily changed (of course it depends on the elemental abundance of the daughter isotope), while major elements are strongly related to the real petrogenetic processes (source composition, degree of melting, fractionating phases). Unfortunately many researchers focus their attention on the isotopes only or over-emphasize the message from the isotopes.

michele

Dear Dr. Solferino,

Maybe my comments sound like the "vox diaboli" as I again refer to the "classical chain of operation" and have all the geological (e.g. textural analysis, structural geology) , mineralogical (accessory minerals), and chemical (e.g. trace element fractionation) methods tried before embarking on this journey, which I did by myself for mineral assemblages containing fluorite. You mentioned two separate orogenic events; this would force me to think over the use of structural geology from the small- to large scale. Why shouldn´t you make some progress by means of this method in your work, either? Can you give further details to the audience so as to enable them to provide you with a more profound piece of advice? Is it a Paleozoic orogen in Europe? I think sometimes it is more advantageous to go from the general aspects into the regional ones earlier and see if a method warrants to be used or not.

Best regards

Harald G. Dill

Dear all,

I completely support Dr. Dill's opinion. Of course, direct geologic evidences look preferable.

However, if a detailed analytical study seems necessary, I would recommend to add the microprobe analysis of Fe-Mg silicates to the methods listed above. Biotite composition is quite sensitive to variations in oxygen fugacity and potassium activity in magmas, and different granitic complexes (even located in the same area) frequently contain different biotites.

Here are a few references regarding the isotopic composition of granites and crust-derived volcanic rocks:

Dini, A., Gianelli, G., Puxeddu, M., Ruggieri, G., 2005. Origin and evolution of Pliocene-Pleistocene granites from the Larderello geothermal field (Tuscan Magmatic Province, Italy). Lithos 81, 1–31.

Tikhomirov, P.L., Kalinina, E.A., Kobayashi, K., Nakamura., E., 2008. Late Mesozoic silicic magmatism of the North Chukotka area (NE Russia): age, magma sources, and geodynamic implications.  Lithos 105, 329-346.

Parada, M.A., Nyström, G.O., Levi, B., 1999. Multiple sources for the Coastal Batholith of central Chile (31–34°S): geochemical and Sr–Nd isotopic evidence and tectonic implications. Lithos 46. 505–521.

About trace elements in granitic systems. In my opinion, they provide less useful information compared with that for mafic magmas. Common granitic phases like biotite, allanite, zircon, monazite etc. absorb elements which behave as strongly incompatible in a mafic environment. This hampers the conventional geochemical modelling, and even the qualitative analysis of spider diagrams for silicic rocks.

Dear All, thank you for your thoughtful answers. I am keeping it a bit mysterious cause it's a very novel idea, and I just got involved in the project. I wanted to check with the experts in the community about the validity of isotopic ratios in the understanding of granites. It seems that geological (field), structural and major element data, coupled with properly done geochronology are the best tools. I will make use of your important comments, and discuss the issue further with the project leaders. I apologize for not providing more details, after making you curious, but that's the level of confidentiality at the moment.

It had better to use the plot of (87Sr/86Sr)i vs εNd(t) in interpreting initial Sr-Nd isotope ratios for granite petrogenesis. Nevertheless, it had better to use these two isotope variables together with major and trace element variables, otherwise it is susceptible to misunderstanding of the isotope data. For instance, it is certain from lithochemistry that granites are derived from partial melting of mafic to felsic crustal rocks rather than ultramafic mantle rocks. In general, low (87Sr/86Sr)i granites would originate from the continental lower crust of low Rb/Sr ratios, which are commonly produced by granulite-facies metamorphism/anatexis. It is during such processes that fluid-mobile incompatible LILE such as Rb, U and Pb were extracted by aqueous fluids and hydrous melts to result in the low Rb/Sr ratios. In contrast, high (87Sr/86Sr)i granites would originate from the continental upper crust of high Rb/Sr ratios, which are commonly produced by partial melting of high K/Na rocks and their weathering. Thus, the Rb/Sr ratios of parental crust are a key to the (87Sr/86Sr)i ratios of daughter granites. On the other hand, negative εNd(t) granites would originate from the ancient crust that was extracted from the normal asthensospheric mantle in the ancient time (Archean to Paleoproterozoic). In contrast, positive εNd(t) granites would originate from the juvenile crust that was extracted from the normal asthenospheric mantle in the recent time (Phanerozoic, at most Neoproterozoic). Taken the (87Sr/86Sr)i ratios and εNd(t) values together, high (87Sr/86Sr)i and negative εNd(t) granites would originate from the ancient crust with significant differentiation in trace element geochemistry by partial melting and chemical weathering. In contrast, low (87Sr/86Sr)i and positive εNd(t) granites would originate from the juvenile crust without the significant differentiation in trace element geochemistry by partial melting and chemical weathering. However, there is a key misunderstanding in the interpretation of granite Nd isotope data, particularly for positive εNd(t) values which are erratically linked to a contribution from the mantle. We have clarified this issue in a response to a comment on our paper (Zhang S.-B., Zheng Y.-F. and Zhao Z.-F., 2010. Petrogenesis of continental igneous rocks: Reply to the comment by Qiu et al. on “Origin of TTG-like rocks from anatexis of ancient lower crust: Geochemical evidence from Neoproterozoic granitoids in South China”. Lithos, 116, 191-194.)

I agree with Marlina, Harald Dill and other people who have stressed the many complexities involved in granite genesis. I also agree in that field evidence, followed by careful petrographic study, is to be considered first in granite petrogenesis. I usually tell to my students that geochermistry (even more so isotope geochemistry) provides excellent information to those having obtained first solid geological and petrographic evidence. Without these (pre)requisites, geochemistry tends to be misleading. Accordingly, my answer to your question (i.e., Is it possible to use ɛNd-ɛSr to characterize granites' source rock?) must be: yes, it is possible; but this is no simple or straightforward task. Simply, it cannot be so.

This is a very good discussion and I concur with all of the points made above... isotopes are simply another (often the most elaborate) tool that is at your disposal to make an interpretation for the petrogenesis of your igneous suites.

Since you are potentially trying to distinguish between whether the magmatism was the result of one orogen as opposed to another, then age information, or in its absence relative field context (although granitic bodies can themselves often behave as rigid bodies during orogenesis and hence remain undeformed even though they have been through the whole orogenic cycle) is the best constraint. Isotopically there are many variable as discussed above which can produce non-unique solutions based solely upon isotopic studies. Hence any isotopes can simply value add to an understanding based on all of the other geological tools at your disposal.

However, if one had no other constraints, as a last resort, one could compare the age corrected isotopic ratios for the two suites of granites at the two times in question; if the ratios overlapped at one of the ages then that could be circumstantial evidence that the granites were derived from the same source at that time. However, even if they were different, this does not mean that they could not be part of the same orogenic event, as they could have been melting different sources at the same time- an excellent example of this is the Delamerian Orogen (South Australia) in which I-, S- and A-type granites are located in close proximity, each with different Nd and Sr isotopic signatures, depending on which portion(s) of the crust (and indeed this case mantle) was melting. 

The reason you see this sort of approach used for basalts regularly is that there are (theoretically at least!) far fewer mantle reservoirs available to source basalts, and there are generally (relatively) well constrained, and in the case of the DMM, well mixed and hence homogeneous with respect to these isotopic systems.

Hi,

There are two aspects of your question, (1) - “that ɛNd vs. ɛSr could be a proper tool to show (or to ascertain) that their source rock was not the same” If this is your objective – logically speaking you cannot do it. It is not the question whether the rock is granite or basalt, but because of its age, and as the rock has been metamorphosed the radiogenic isotopes are homogenized and new radiogenic isotopes have been produced. So the rock no longer reflects the isotopic composition of its source rock. Therefore, there would not be any correlation between the Nd and Sr isotopic ratios. This correlation ɛNd vs. ɛSr is done on the recent oceanic basalts (zero age rocks) under the belief that their isotopic ratios are a reflection of their source reservoir - the mantle (the most prevalent view). The linear array formed by this correlation passes through the point of the present-day bulk earth isotopic composition. This correlation therefore has been used to deduce the present-day 87Sr/86Sr isotopic ratio of the bulk earth. So in their view this correlation shows that the same process has affected both the isotopic ratios. In other words in both the cases the parent-daughter isotopes were fractionated coherently by the same process in some geologically long past time (about 1.5 – 2.0 Gyr ago). Other than that it does not give much information about the source reservoir (mantle). The other view is that this correlation is because of the mixing of source reservoir, one end-member being highly enriched and the other highly depleted.

However, my view is completely different (Gargi, 2012). In my view, this correlation is because of the fractionation of radiogenic isotopes with respect to their non-radiogenic counterparts. I believe that all radiogenic isotopes from the lightest to the heaviest are fractionated. So it is not a case of mass fractionation. Just as 87Rb and 87Sr in spite of having the same mass are fractionated from each other because of their different physical characteristics, similarly the radiogenic isotopes are fractionated at the time of generation of magma, because they are different physically from their non-radiogenic counterparts. The radiogenic isotopes thus mimic the behavior of their parent isotopes to some extent. So this correlation, because the linear array passes through the point of the bulk earth composition, shows that the mantle is undepleted, that is it has the isotopic composition similar to that of the chondritic meteorites. So the apparent depletion we observe in oceanic basalts is the result of magmatic differentiation and the accompanying fractionation of radiogenic isotopes. So that is why we do not find anywhere granites which are depleted, except those which are formed by fractional crystallization, and the mafic and ultramafic rocks which are enriched.

(2) The second aspect of your question – “Beyond dating, I thought that ɛNd vs. ɛSr could be a proper tool to show (or to ascertain) that their source rock was not the same.” Also “People use the ɛ-ɛ plots to characterize basalt source/origin all the time, but not for granites.”

As I stated earlier, the ɛ-ɛ plots are not used to characterize the source regions. They have been used only for recent (zero age) oceanic basalts/continental basalts, but again not for characterizing their source reservoirs. However, ɛNd and ɛSr values have been used individually to characterize source reservoirs. It is just similar to using initial Sr isotopic ratio or initial Nd isotopic ratio of a rock to characterize the source regions of a rock, but just by a different name. This could be used for any rock of any age. But in my view, the ɛNd value of a rock does not reflect the true characteristics of its source reservoir because it is modified by the fractionation of the radiogenic isotopes. In a nut shell, we cannot really characterize the source regions in the true sense by using the isotopic ratios. However, the ɛNd vs. ɛSr plots could be used only for those rocks which have been infused by hydrothermal solutions, and they have not been metamorphosed. So in such cases there is a possibility that these two entities could show correlation. So if in such a case if there is correlation, and if the linear array does not pass through the point for the bulk earth, it could give us the information about the source reservoir.

Dear Satya,

Although there is now some evidence that even heavy isotopes such as those of Sr and Nd may be fractionated from each other during geological processes there are two points to consider:

1. fractionation of stable isotopes (whether they are produced by radiogenic decay or not) is most pronounced at low temperatures; forming granites is not a low-T process.

2. When Sr and Nd isotopes are measured for studies such as Giulio's, any evidence of fractionation that might have been there is erased by normalising to a canonical number for the 88Sr/86Sr or 146Nd/144Nd ratio.

So Sr and Nd isotope ratios can be used to say something about a granite's source, with the caveats that Bruce and I have given before.

Dear Marlina,

Thanks for your query. I am very sorry for the delay in replying to your query.

I am very sorry to say that whatever you said, in fact, just the opposite of that is true. Let us start with your first comment : “Although there is now some evidence that even heavy isotopes such as those of Sr and Nd may be fractionated from each other during geological processes there are two points to consider:”

In fact the heavier isotopes, that is non-radiogenic isotopes heavier than Sulfur, such as 88Sr, 86Sr and 84Sr, or non-radiogenic isotopes of Nd, and other such isotopes are not fractionated. The mutual ratios of such non-radiogenic isotopes of seversal heavier elements have been measured time and again, and they are found to be constant without exception. I don’t know from where you got this information. Are you referring to some specific study on this? I would be very much interested to see the complete reference of that paper. Further, the measurement of of isotopic ratios by mass spectrometer such 87Sr/86Sr depends upon the fact that heavier non-radiogenic isotopes do not fractionate. If this were to be otherwise such analysis would be meaningless.

It looks like that you have failed to realize the distinction between radiogenic isotopes such as 87Sr or 143Nd and their non-radiogenic counterparts such as 84Sr, 86Sr, 88Sr, and 146Nd, 144Nd etc. You have put all the isotopes of an element in one group as heavier stable isotopes. All the isotopes of Sr may be similar chemically, but in terms of their physical traits or behavior,

the radiogenic isotopes are not like their non-radiogenic counterparts. Most likely this is because they have been produced by the decay of their parent atoms, and, therefore, have acquired some of the physical traits of their parent atoms. As a result the radiogenic isotopes mimic the behavior of their parent atoms to some extent. Thus the radiogenic isotopes are fractionated with respect to their non-radiogenic isotopes during magmatic processes.

Also, it looks like what you have in mind is the mass fractionation of isotopes of heavier elements as observed in the case of stable isotopes of lighter elements such as H, O, N, and S etc. Thus what I have shown in my paper is not a case of mass fractionation of heavier elements. What I have suggested is that all radiogenic isotopes regardless of their mass from the lightest to the heaviest, that is from 3He to 208Pb are fractionated relative to their non-radiogenic counterparts during magmatic processes. This is one factor which has led to the problem of excess radiogenic lead. However, there are other factors also to this problem, which I cannot discuss here. This kind of fractionation of radiogenic isotopes cannot be measured directly in any way. It can only be visualized by indirect evidence which I have shown in my paper.

Now let us consider your first point: “fractionation of stable isotopes (whether they are produced by radiogenic decay or not) is most pronounced at low temperatures; forming granites is not a low-T process.”

It looks like that you have clubbed all the isotopes which do not decay regardless of their mass, as stable isotopes. Generally, the isotopes of lighter stable elements such as H, O, N, S, etc. are generally categorized as stable isotopes. All those isotopes which do not decay are not clubbed together as stable isotopes, particularly if they have radiogenic isotopes also amongst them. You further mentioned that their fractionation is more pronounced at low temperatures. I think just the opposite of that is true. The fractionation of these lighter stable isotopes is more pronounced at higher temperature in my opinion. Anyway, I would like to get the reference of that. Moreover, the fractionation of the lighter stable isotopes is the result of mass fractionation. What I have suggested is not a case of mass fractionation, but fractionation of radiogenic isotopes regardless of their mass, with respect to their non-radiogenic counterparts because of their different behavioral properties; and this kind of fractionation takes place during magmatic processes. So there is no question of temperature involved here. Whenever magma is extracted, at whatever temperature, or there is crystal fractionation, the radiogenic isotopes are fractionated with respect to the rest of their non-radiogenic counterparts.

Regarding your 2nd comment: “When Sr and Nd isotopes are measured for studies such as Giulio's, any evidence of fractionation that might have been there is erased by normalizing to a canonical number for the 88Sr/86Sr or 146Nd/144Nd ratio.”

It is not clear what you are trying to say here. The measurement of isotopic ratios such as 87Sr/86 by mass spectrometer is based on the fact that the non-radiogenic isotopes such as 88Sr, 86Sr, and 84Sr are not fractionated. If this were not to be the case such measurements would be meaningless. So the fixed ratio between these isotopes forms the very basis of the measurement of 87Sr/86 ratio. That is how the 87Sr/86Sr are measured. I do not understand, fractionation of which kind of isotopes you are referring to. I think you are still thinking of fractionation between non-radiogenic isotopes. As I mentioned earlier there is no fractionation between non-radiogenic isotopes. So there is no question of any destruction of evidence of fractionation. With regard to the fractionation of 87Sr isotopes, as mentioned earlier, it cannot be determined directly. There cannot be any direct evidence of fractionation of 87Sr isotopes. So if there is fractionation of radiogenic isotopes such as 87Sr relative to all other non-radiogenic isotopes, it can only be perceived by indirect evidence.

Your further comment that Sr and Nd isotope ratios can be used to say something about a granite's source. Here again, I disagree with you because all magmatic processes are accompanied by fractionation of radiogenic isotopes. The latter process is reverse of what happens during metamorphism, that is, all radiogenic isotopes are homogenized during metamorphism. Similarly during magma generation, whether granitic or basaltic, all radiogenic isotopes are fractionated. Therefore, the magmatic rocks we analyze do not give the true picture of the isotopic composition of their source rock. The radiogenic isotopes mimic the behavior of their parent isotopes. This is the reason that all granitic rocks appear to be enriched, and all basic rocks appear to be depleted. It would be very hard to find an example of an enriched basalt and a depleted granite. Thus the mantle may appear to be depleted on the basis of the Sr isotopic ratios of the oceanic basalts, but in reality it is not, because the Sr isotopic ratios of oceanic basalts do not give the true picture of the isotopic composition of their source reservoirs because of fractionation. Thus we can never determine the true isotopic makeup of the source reservoir by the isotopic analysis of the rocks derived from it. But from the other evidence, I have suggested that the mantle is undepleted, that is it has chondritic isotopic composition.

Regards,

Satya Gargi

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Dear Satya,

The idea that radiogenic isotopes inherit traits from their parent nuclide is interesting, although contrary to the present scientific paradigm. However, I fear that we are hijacking the original question by Giulio, and would therefore suggest that you set up a separate thread if you would like to continue this discussion.

Regards,

Marlina Elburg

Dear Marlina,

Thank you very much for your interesting comments. First with regard to your comment about hijacking the original question - I don’t think so. What I said was part of the answer to your comment that there is now some evidence that even heavier isotopes of Sr and Nd are fractionated. My answer was that only radiogenic isotopes of all the elements from the lightest to the heaviest are fractionated. It was also an answer to the original question of Solferino about using the Sr and Nd isotopic ratios to unravel the isotopic characteristics of the mantle source reservoir. My answer to that was that we cannot get any information about any source reservoir because the isotopic ratios of source rocks get modified by the magmatic differentiation and the extraction of magma from the source reservoir because of the accompanying fractionation of radiogenic isotopes. So in my opinion it is not a new thread. But if you think so, I would suggest that you first read my paper (Gargi, 2012), and then if you want to make any comments you can start a discussion on that. However, you did not answer my question – I asked you to provide me the references regarding the evidence for fractionation of isotopes of heavier elements that you mentioned.

With regard to your other comment about the radiogenic isotopes, I agree that my views contradict the prevalent views. Yes, in that sense it indeed is a new paradigm because my views are diametrically opposed not only to the long held views about the behavior of radiogenic isotopes but with regard to the isotopic characteristics of the mantle also. You cannot imagine the kind of odds I faced in getting my paper accepted. It took me six years and rejection from 11 journals to get the paper finally accepted. The comments of the editors/reviewers could not be called scientific at all, but were just simple rants. Even the lay street people could have done better. The journal which finally accepted the paper kept my paper for more than a year without rejecting it.

Regards

Satya

Dear Satya,

You can google the terms "fractionation Sr isotopes weathering" and find what comes up regarding the fractionation of Sr isotopes during low-temperature processes if you're interested in the fractionation of the isotopes of the heavier elements.

Regards,

Marlina

Of course you can use Sr and Ne isotopes to locate the rock sources. There are even the most efficient parameters for that purpose. Just check on the litterature you have. In case you can't find on granitic rocks, let me know I'll send you some. e.g Mbassa et al. 2016