Igneous Petrology - Science topic

The answer from Prof. Michele Lustrino is the best you can have.

And i totally agree with him, petrographic analyses must be done by people involved in the research (and also in the field activity).

Thank you for your answer Saleh.

Best regards

Ioannis

Thermocalc can be used to do amphiboles (see the work of Chris Yakymchuk)

DEAR SEBASTIAN

GOOD MORNING. THANK YOU FOR GIVING ME THE CHANCE TO DISCUSS A MUCH DEBATED MATTER. THERE ARE AT LEAST 48 EXTRUSIVE (VOLCANIC) CARBONATITES APART FROM OLDONYO LENGAI (WOOLLEY & CHURCH 2005). EXAMPLES ARE THE GROSS BRUKKAROS VOLCANO (KURSZLAUKIS & LORENZ 1997) IN NAMIBIA AND THE CATANDA VOLCANO IN ANGOLA (CAMPENY ET AL . 2015). NO LAVA FLOW IS NEADED FOR A CARBONATITE VOLCANO TO OCCUR (E.G. DIATREME VOLCANOES). THE PIERIAN MOUNTAINS META-CARBONATITE DIKE DOESN'T LOOK LIKE AN OPHICALCITE (SEE ATTACHED GREEK OPHICALCITE PHOTO FROM LARISA) AND IT ISN'T ONE SINCE THESE ROCKS HAVE A MAGMATIC (MANTLE NOT CRUSTAL) SIGNATURE IN THEIR CARBONATE (SCHENKER ET AL. 2018). EXTRUSIVE CARBONATITES GENERALLY APPEAR IN CONTINENTAL RIFTS LIKE THE RIFTING OF THE PELAGONIAN ZONE ON THE PIERIAN MOUNTAINS. PLEASE COMMENT.

BEST REGARDS

IOANNIS

Dear Michael

Good morning. Thank you for your detailed reply. Regarding carbonatites in Greece Schenker et al 2018 have found a meta-carbonatite dike on the Pierian mountains (see photo attached).

Best regards

Ioannis

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.

Hi Ayda,

A detailed literature check is fundamental. After having collected the samples, the first think you should do is to prepare a thin section. Too often geologists forget or consider as a secondary process the detailed petrographic description, but it is really the first activity to do, before analysing the rock.

To collect rocks a simple Eastwing hammer is not sufficient. You need a 3-5 kg sledgehammer. Eastwing comes after, to reduce the size and the weigth of the fresh rock chunk.

Good luck for your studies.

This is an excellent question and I don't think that there's a good answer yet. In this paper:

We have a relatively simple (or simplistic) definition where fenites form outside the carbonatite, whereas assimilation reactions (which we term antiskarns) are inside the carbonatite. This has limitations in that it's not always easy to identify that a rock is actually a carbonatite, especially if carbonates are not the dominant minerals in it, and it appears as various veins or other challenging-to-interpret textures.

It also has limitations in that some carbonatites gradually grade to silicate rocks around them (particularly the ijolite family and related rocks) and there isn't a clear contact from which one can infer "inside" or "outside".

What about cases where there is a clear contact, but it seems as if small brittle cracks protrude from the carbonatite into its surrounding rocks, then filled with carbonatite melt, which then assimilates material from the country rock? Is it an antiskarn or a fenite?

I reckon that the mineral assemblages and elemental compositions will differ, but I am not sure how yet. We are still in the early days of understanding these systems so my recommendation to you would be that if you see something interesting, write something about it, and put it out into the community.

Garnet The shales of Arghun Mountain have the mineral assemblages of quartz, feldspar, mica and garnet. The chemical composition of plagioclase, biotite, muscovite, chlorite and garnet shows that plagioclase is rich in albite, white mica is rich in the final members of muscovite, chlorite is more rich in the final members of amethyst and sedite, and phlogopite-anilite is basic. Garnets are rich in almandene and spesartine. In this study, the pressure and temperature of the upper limit and the lower limit of rock metamorphism were calculated using conventional geothermal-barometric methods. Using the Fe-Mg cation exchange thermometer between garnet and biotite, assuming a pressure of 4Kbar, the highest calculated temperature is 615 and the lowest calculated temperature is 429 ° C, and for 8 Kbar the highest calculated temperature is 644 and the lowest calculated temperature is 452 ° C. Using the multiple mineralogy equilibrium method, the calculated pressure and temperature for the upper and lower limits of metamorphism were about 801 ° C and 9kbar pressure, and 450 ° C and 7kbar pressure, respectively. This complex has been affected by two metamorphic and metamorphic phases of rocks. The second deformation in the region has been accompanied by the peak of metamorphism. A decrease in temperature of about 351 ° C in exchange for a decrease in pressure of about 2 kbar is observed in the metamorphic clay rocks of the study area.

I am old-fashioned (geology-mineralogy-chemistry-advanced-level geosciences, .e.g., isotopes).. Therefore, in this case where only a chemical ratio forms centerpiece of a discussion of such a far-reaching issue, I can only respond in a way like that:

"One swallow does not make summer"

HGD

I work as a mineralogist and I am not a specialist in NASICON phases, but the reason that these substances have not yet been found in nature will most likely be the strong hydrating ability and oxygen affinity of Ti and Zr. In even slightly hydrothermal environment there is anatase, resp. zircon (resp. hydrated zircon phases, gelzircon etc.) strongly stable. It is necessary to assess under what conditions, from which input components NASICON are synthesized and whether it is at least a little realistic for a similar synthesis to take place in nature. However, some Zr and Ti phases with PO4 or SiO2 have been found in nature. It is possible to use the search at: https://www.mindat.org/chemsearch.php

Dear Xin Liu, China University of Petroleum - Beijing.

I recommend using the magnetic anomalies to estimate the Curie boundary, which indicates, direct or by analogy, the distribution of the thermal anomalies, present-day heat-flow, providing a clear marker for the thermodynamic effect in the crust and mantle. Therefore, I recommend first the use of magnetic anomalies, for example, from the EMAG2 datasets, it is free, and it has the wavelength (deep mantle large-scale structure) that you need in your research.

The knowledge of the magnetic anomalies in LIP (large igneous provinces) regions is a geophysical way, due that the mafic and ultramafic intrusions linked to those LIPs and their contrast in magnetic properties, magnetic susceptibility, using modeling and inversion.

Also, high densities contrast mafic/ultramafic rock compatible with serpentinized, also could show you in gravity anomaly inspection of a LIP, particularly regarding Bouguer complete anomaly map / Residual isostatic anomaly map.

I attached Jennifer Blanchard´s Marter of Science thesis, "Geophysical identification and characterization of mafic-ultramafic intrusions in plume centre regions", 2015, Carleton University, Ottawa, Ontario, Canada. I recommend reading in focus the Modeling methodology, it has wonderful examples.

Best regards, Mario E. Sigismondi

Hi Donald,

I recommend what Prof. Sebastián Grande suggested. To start with and understand from basics you may follow the book by Rollinson,

In addition, you may find many commonly used diagrams for the granite discrimination in numerous papers however below I mention several specific papers that can be useful too to look for granite discrimination if you have geochemical data e.g.,

1. Eby, G.N., 1990. The A-type granitoids: a review of their occurrence and chemical characteristics and speculations on their petrogenesis. Lithos 26, 115–134.

2, Eby, G.N., 1992. Chemical subdivision of the A-type granitoids: petrogenetic and tectonic implications. Geology 20, 641–644.

3. Pearce, J.A., Harris, N.B., Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J. Petrol. 25, 956–983.

4. Whalen, J.B., Curry, K.L., Chappell, B.W., 1987. A-type granites: geochemical characteristics, discrimination and petrogenesis. Contrib. Miner. Petrol. 95, 407–419.

5. Frost, B.R., Barnes, C.G., Collins, W.J., Arculus, R.J., Ellis, D.J., and Frost, C.D., 2001, A geochemical classification for granitic rocks. J. Petrol 42, pp. 2033–2048.

6. Frost, B.R., and Frost, C.D., 2008, A geochemical classification for feldspathic igneous rocks. Journal of Petrology, v. 49, pp. 1955–1969.

Hello dear;

I didn't research on Paleomagnetic studies, but i know 2 methods in order to behavioral study between two things. K-means clustering and Artificial Neural Network (ANN). you can read this combination method in this paper :

good luck

Metamorphism is an isochemical adjustment (be it contactmetamorphic, regional/dynamometamoprhic or burial metamorphic) to changing physical conditions pressure and temperature in the lithosphere. By definition the expulsion of carbon dioxide and water along with an increasing metamorphic grade is not involved in these processes. The lower limit called the very low-grade stage overlaps with the upper part of the diagenesis (around 200°C) and depends on the angle you look at this boundary using siliceous, organic or sulfidic matter. The upper limit is the onset of anatexis between 600 and 800°C which depends on the water content of the system. The metasomatism sensu stricto is a closed system.

Metasomatism is allochemical and an open system where in special zones at a certain P-T level a new mineral partly or wholly different in its chemical composition from the host mineral formed. These mineralizations may be caused by subcritical or supercritical solutions sparked by igneous bodies at different depth (contact -metasomatic / skarn) or in the course of burial or dynamometamorphic processes (see the isochemical analogues above). The replacement of preexisting rock-forming material occurs through chemically active liquids and gases from external sources.

I have not seen yet it but it is too interesting case

Biotite as a petrogenetic discriminator: Chemical insights from igneous, meta-igneous and meta-sedimentary rocks in Iran

DOI: 10.1016/j.lithos.2021.106016

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:

I recently published on high Fe-Ti amphibolites at Broken Hill, Australia.

Hi Aleksandra, there is an interesting paper by Raimbault et al. (1993) on REE contents of apatites and scheelite and they are also discussing low Tb concentrations. The details are: Raimbault et al. (1993) in American Mineralogist 78: 1275-1285. Cheers Daniel

The answer to your question is: not always. Post-collisional or intraplate magmatism sourced from a hybrid mantle (crust+DM) presents geochemical (incompatible elements) and isotopic characteristics of the crustal component. Couzinié et al. (2016) (Earth and Planet. Sci. Lett.; Vol. 546) have demonstrated that a small (~10-25%) crustal component, transported to great depths in subduction zones as sediment or mechanically eroded sialic crust, is able to metasomatize/hybridize depleted mantle so that a rock sourced (e.g. high-K basalts) from this hybrid mantellic domain could provide incompatible element signatures (including Nb-Ta troughs), as well as radiogenic isotopic values (i.e. Hf, Nd), that dominantly reflect the crustal component. Therefore, let's picture a hypothetical scenario. Subduction during the Paleoproterozoic would generate arc-related magmatism and metasomatize/hybridize large swaths of the suprasubduction mantle. Right after continental collision, orogenic extensional collapse would partially melt this hybrid mantle and generate basalts (and fractionates) with a crustal-like geochemical signature, and therefore Nb-Ta anomaly. Extensional processes could (and probably would) continue during much longer periods due to periodic extensional/rifting. Therefore, basalts with Nb-Ta anomalies and other geochemical fingerprints of subduction zones could be generate in post-collisional or intracontinental settings. Nevertheless, they need to be sourced from mantle that was previously enriched and metasomatized by a (minor) crustal component delivered by an active subduction zone. Hope this helps. Cheers.

sorry，I don't understand your statement.

Hi....a couple of links for your use...

http://faculty.washington.edu/stn/ess_312/notes/Trace_Elements_selected_slides.pdf

Hope this helps.

Dear Mr. Bhowmick,

Slags are artificial products representing Fe ore which is made up of the Fe ore mineral and a wide range of gangue from carbonate to silicate minerals (rock) and the slag-producing additives e.g. limestone. The entire process in blast furnace takes place at low pressure and high temperatures. As a consequence of that the natural analogue is a combination of rocks forming at high T and low P which is the mafic volcanic clan (different types of basalt, see e.g. the major mineraloids in slags are Fe-enriched olivine s.s.s., pyroxenoids, wuestite and native elements plus Ca-Mg components). As far as the metamorphic part is concerned the natural equivalent well presenting these conditions is the so-called sanidinite hornfels facies (contact metamorphic reactions of carbonates at low pressure and very high temperature). These processes result in the formation of monticellite, akermanite, melilite, tilleyite, spurrite , rankinite , merwinite and larnite. The pressure conditions are below 1 kbar and the temperature greater than 700°C.

Kind regards

H.G.Dill

You can use the Al content and Fe/(Fe+Mg) ratios of the recalculated amphibole analyses to estimate the P, T and fO2 conditions. The relevant literature you should consult is as follows:

Anderson J.L. and Smith D.R. (1995). The effects of temperature and O2 on the Al-in-hornblende barometer. American Mineralogist, 80, 549-559.

Hammarstrom J.M and Zen E. (1986). Aluminum in hornblende: An empirical igneous geobarometer. American Mineralogy, 71: 1297-1313.

Hollister L.S., Grissom G.C., Peters E.K., Stowell H.H. and Sisson V.B. (1987). Confirmation of the empirical correlation of Al in hornblende with pressure of solidification of calc-alkaline plutons. American Mineralogy, 72: 231-239.

Johnson M.C. and Rutherford M.J. (1988). Experimental calibration of an Aluminium-in-hornblende geobarometer applicable to calc-alkaline rocks. EOS, 69, 1511.

Schmidt M.W. (1992). Amphibole composition in tonalite as a function of pressure: An experimental calibration of the Al-in-hornblende barometer". Contrib Mineral Petrol, 110: 304-310.

In this tissue other than olivine and pyroxene we see a kind of fossil structure with high cooling rate

The geological map shows the distribution of the formations and their contact ( different kinds of rocks and faults).

There are three major types of maps they use: topographic, cross-sectional, and structural.

A structural map shows the geologic features of an area. Its appearance is similar to that of a topographic map, but a topographic map displays elevations of the Earth's surface and a structure map displays the elevation of a particular rock layer, generally beneath the surface. a geologic map shows the distribution of geologic features, including different kinds of rocks and faults.Rock units or geologic strata are shown by color or symbols to indicate where they are exposed at the surface. Isopach maps detail the variations in thickness of stratigraphic units.

A cross-sectional map shows the cross-section from the side.

So you can't talk about stratigraphic maps, but rather structural maps at different stratigraphic levels,

Ickert, 2013 would be good start.

"Algorithms for estimating uncertainties in initial radiogenic isotope ratios and

model ages" Chemical Geology, v340.

Mineral chemistry alone is not going to be able to tell you whether your amphibole is of primary magmatic or metamorphic in origin. Many amphiboles are stable over a wide range of temperature and pressure conditions, and a single species may have either a magmatic or metamorphic origin. You need to make some thin sections and examine your samples under a microscope. Examine the textures and relationships between the amphiboles and the other minerals in the sample. Look at the regional structural information from where your specimens came from. All this ground work needs to be done before you dive into putting your amphiboles under an electron beam!

Hi Jorge

I agree it's certainly possible. However, since XRF only detects the Fe content not the O content, the meaured Fe content will not change as a result of its oxidation state: this was the point I was aiming to make.

Kindest Regards

Paul.

Seeing you classify the pigeontie as ‘microlites’, I assume the rhyolite groundmass crystallized relatively rapidly? If you think this is the case, I would investigate the possibility that the pigeoite crystallized under metastable conditions. A search for ‘pyroxene composiions under rapid cooling conditions’ should pull up some relevant papers.

Dear Roberto:

Are you sure that hexagonal crystal in the charoite is reall quartz? Nepheline also forms sometimes perfect hexagonal prisms, and the alkaline nature of this rock suggests the possibility that it could be silica undersaturated, thus containing feldspathoids! I got a wonderful sample of charoite as a gift while assisting to the XXVIII International Geological Congress, in Moscow, on august 1984, in that country this colorful stone is used to make ornaments, ash trays, and the like. Among other things, including tourism and cultural activities (the Hermitage and Natural Science museums in Saint Petersburg), we made excursions all over the U.R.S.S. One 8 day-trip to Irkutsk, on Lake Baikal, sailing the western shore and going to the lapislazuli mines of the Czars (the phogopite books there were meter-sized, but I settled with a 20 cm and a 10 cm wide pseudo-hexagonal books), to the Primorsky Beach, where crops out a wonderful rapakivi granite so similar to the huge Parguaza rapakivi Granite outcropping in southern Venezuela..., to the outcrops of "grenvillian" high grade marbles, wih fresh forsterite, red spinel and phlogopite or scapolite, related with cordierite-sillimanite metapelitic gneiss ending with a famous nepheline syenite intrusion and skarn, with a dyke of alkali pegmatite, there the zircon and titanite crystals where visible at the naked eye!... Another excursion was to the Ukraine, where we visited the Korosten Anorthositic Complex and the Voilyn pegmatite field, there I got from the dumps a wonderful fist- sized microcline crystal, an 8 cm wide slice of bicolored topaz, and a graphic intergrowth of smoky quartz with cream-colored microcline, very showy. With regards, Sebastián.

Lots of earthquake stations today, therefore, lots of data to study using the velocity of the waves. Scientists know a lot more information than previously and there is more to study. The data displayed in the lecture suggested by Dr. @ Borko Bulajic are amazing!

Dear Chutian Shy,

Have you seen any oriented fabric in meta-gabbro. I want to know. If it is containing an oriented fabric where garnets are seen. Further, what is seen within the garner, is there any planar fabric inside the garner?

Sincerely ,

T K Goswami

There have been many researches and papers on the Degan Igneous Province in India, but the latest international conference does not currently exist. Only last year, an international academic conference was held in Chengdu, China, about the Emeishan igneous province. I have published the latest articles on the causes of the Degen Igneous Province, welcome to read and communicate.

Best wishes,

Liu Chenming

The newest paper:

Anczkiewicz, A.A., Anczkiewicz, A. 2016. U–Pb zircon geochronology and anomalous Sr–Nd–Hf isotope systematics of late orogenic andesites: Pieniny Klippen Belt,Western Carpathians, South Poland. Chemical Geology, 427, 1–16.

Dear all,

I forgot to mention that I'm dealing with detritus, unfortunately it is not always easy to get nice well shaped crystals as the 50-200 µm fraction is mostly broken after the jaw crusher and the mill (especially titanite and monazite). However thanks D Raju for the suggestion!

Dear Mastoi, I'm still separating the samples and I'll process them at the LAICPMS for REE once ready, for the time being Dr Henrichs suggested me to follow the review from Engi (2017) "Petrochronology Based on REE-Minerals: Monazite, Allanite, Xenotime, Apatite"(doi.org/10.2138/rmg.2017.83.12) which seems to be extremely useful for this topic.

regards

Cesco

Dear Irakli,

What kind of cooperation you need? If it is theoretical, then feel free to contact me. For analytical support, you can contact University of Granada, Spain. Kindly check the following link: http://www.ugr.es/~ibersims/ibersims/Welcome.html

The best is to see initial details of the construction of any discrimination diagram. Application of the diagram for drawing meaningful conclusions requires that you plot data of rocks of similar composition. I mean, if a discriminant diagram is designed through use of basaltic rocks, it should be used for rocks of basaltic composition. It may not be appropriate to draw conclusions by plotting rhyolite or peridotite analyses on it. I think that Mullen was dealing with basaltic rocks and applying the diagram to ultrabasic rocks is a choice you can decide yourself.

Considering the limited variability of topaz in terms of major elements, studying trace elements, both of your heavy minerals and suspected source areas, might be the most fruitful approach. Perhaps cathodoluminescence imaging may also give some information (e.g. Agangi et al., 2016. Relation between cathodoluminescence and trace-element distribution of magmatic topaz from the Ary-Bulak massif, Russia. Mineralogical Magazine, 80(5): 881-899.

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..

You may find some  information of use in my recent article for The Mineralogical Record published very recently.

Lyckberg Peter, 2017 The Mineralogical Record, Vol. 48, No. 5

September - October 2017 Gem pegmatites of northeastern Afghanistan

pg 610-675.  This special Issue is named "Afghan Pegmatites" and can be ordered for 25 USD plus shipping from http://www.minrec.org/detail.asp

You are also welcome to keep in touch. Tashakor.

Warm regards Peter Lyckberg

Good discussion. Thanks all. Probably, we can not use granite powder as a geopolymer source material since it is mostly crystalline. However, it can  be a filler or partially reactive material in geopolymer mixes if there are geopolymer source materials such as fly ash, etc

Zircon, xenotime and garnet can be responsible of HREE depletion in magmas. Of the three possible cases above you would expect, respectively: Ce, Hf, and Zr depletion and Eu anomaly decrease with differentiation (zircon fractionation);  a P depletion with differentiation (xenotime fractionation); and a LREE increase if garnet is fractionated.

I would expect Amphibole fractionation in a chondrite normalized REE 'spoon-shaped' pattern.

Different HREE contents in co-genetic magmas could be explained by accesory minerals fractionation

You can explore de different partition coefficients in this link:

Hope it helps.

For partial melting, it depends on the source. An amphibolite source in the deep crust would have a garnet-bearing residuum that would be depleted in plagioclase feldspar, so you might expect, for example high La/Yb and Sr/Y in the melt (the so-called adakite signature). At lowqwer pressure (less than 8 kb or so) garnet won't be stable and you will lose the garnet signature. If you have a biotite gneiss for a source, garnet will also be stable (even mid-crustal pressure) and the melt will be strongly enriched in LILE elements. I would look for these types of signatures before you start any kind of melting model. I would also point out that most granitoids, even S-types,  are not going to be simple partial melts, but complex mixtures of material derived from both crust and mantle.

Dear Roman,

Thanks for your answer!

Yuxiang

     Not today,but the petrological literature (mostly from Europe) till the mid to late seventies have used Niggli values for both igneous and metamorphic rocks. It would not be surprising if they are still used today. Niggli primarily used the values to describe magmas and not rocks. The Niggli norm/Barth Niggli norm is better than the CIPW norm, as it applicable to both metamorphic and igneous rocks. Certain parameters, for example, 'mg' approximates the Mg#, 'k' - alkalinity ratio, and 'w' - to calculate the oxidation of iron, are still useful.

     Are they still good parameters of geochemical association of rocks? Yes, if you construct a Niggli variation diagram and apply the interpretation indices devised by Niggli (as detailed in Petrochemical Calculations by Conrad Burri). The Niggli variation plot has the same limits as a Harker variation diagram - mainly applicable to co-magmatic rocks and set of lavas from a single volcano.   

Dear colleagues:

We need more hard facts. Tidal regimes, volcano-sedimentary sequences, the source of Cr and the host of Cr, as well as  exhalations are very difficult to be put together to give me a coherent picture. Or shall we cast aside the slogan "The present is the key to the past"  for these very old rocks ?  A chemical approach using the V/Cr ratio in the argillaceous layers (so-called fuchsite) may give an answer to the variation in the redox conditions. V increases with lowering of the Eh (more reducing) and Cr responds in the opposite direction (more oxidizing). If it gives a more coherent picture together with barite you get a bit closer to the large-scale environment. I would be very much reluctant to jump too fast into an interpretation of the depositional environment without proper knowledge of the mineralogy and chemistry. Nevertheless, a nice story, to think about.

Best regards

H.G.Dill

Dear Ebrahim: your questions has been already answered by several colleagues, however I must point out two things. First, fossils can only give relative ages, I usually tell my students this funny analogy: Dinosaur bones don't have a tag attached saying "I was born 75 Ma ago!"... So to calibrate relative paleonthologigal ages, and also paleomagnetic time scales, one should have in the first case, a basin which, at the same time, is fossil-rich and has interestratified volcanic tuffs, lava flows or sills, and in the second case, a complete sequence of lavas showing several paleomagnetic inversions, such as MORB lavas or plateau basalts. These fossil+igneous basins could be rift basins, fore-arc or back-arc basins. The igneous tabular bodies present in them can be dated (usually in bentonites from weathered rhyolite tuffs, basalt flows, or diabase sills), and help to give upper and lower age limits to the fossil bearing strata linked to them. And second, that the so called absolute age of a rock has been substituted by a quite more realistic term as apparent age. This is because of what Asoori stated: rocks can be affected by many alteration processes, from metamorphism to hydrothermal alteration to weathering, that isotopic systems can be severely altered and give apparent ages which are not the real crystallisation or metamorphism ages. Systems with soluble cations, such Rb-Sr, and K, or with radiogenic gases, such as Ar, are easily modified by these secondary processes. Therefore the best methods used nowadays for primary crystallisation ages of igneous and metaigneous rocks are those effected in zircon crystals. Zircon, being a mineral resistant to secondary alterations, usually holds the true isotopic content reflecting the radioactive decay of U isotopes to Pb isotopes. It is a sort of miniature "time capsule", whose zoning can even record several successive orogenic or thermal events. Other methods of dating involving inmobile trace elements are: Sm-Nd, Lu-Hf, and Re-Os, applicable to specific rock types, and even to some Cu or Pt ores.

In Paleozoic granites of the Coastal Cordillera of Venezuela K-Ar method in biotite calculated 30 years ago gave quite "young" ages of 33 Ma; 20 years ago Rb-Sr isochrones gave quite "old" ages of about 404 Ma; but more recent U-Pb ages in zircon (LA-ICP-MS) gave even older ages, reaching to almost 500 Ma. It's a difference of almost 100 Ma, quite significant, indeed. The young Tertiary age has been interpreted as the orogenic uplift age, the oldest age, 495 Ma as the crystallisatin age of the plutons, so the 404 Ma Rb-Sr age was an apparent age, which really doesn't relate to either event! Therefore, hydrothermal and/or low metamorphic grade alterations have happened in these rocks disrupting somehow the Rb-Sr isotopic system. That's why absolute ages are no longer called such, they are apparent ages. With regards. Sebastian.

Mineral / Rock ID from pictures is notoriously difficult, especially if more "exotic" materials are involved. Our department, the Department of Earth & Atmospheric Sciences Mineral Laboratory at the Metropolitan State University of Denver offers FREE non-destructive & certain destructive mineral identification services as part of our geoscientist training program and university community outreach. It is really free and by processing a sample of your rock you would aid in the training of our geoscience students. Takes usually one semester, but you would receive a fine, detailed report of various test results (XRF, XRD, physical & optical properties, etc.) and interpretations. Next batch of sample analysis starts beginning of February 2017. If you are interested, here is a link with details on the process and how to submit your sample: http://college.earthscienceeducation.net/MIN/MINID.pdf

I suggest to do analysis of individual minerals that may host REE in rocks.

Perhaps that makes sense. If rutile is present, then the magma is saturated with TiO2 and the concentration of Ti in the magma is fixed (the excess gets precipitated as rutile and other phases such as ilmenite). If this fixed concentration is 45 times less than the Ti content in rutile (in terms of weight percent, approximately 60%, assuming 100% TiO2 and no other components such as iron or Nb/Ta oxides), then there must be 1.33% Ti (or 2.2% TiO2) in the melt.

Iskandar

Amphiboles are a tricky lot. As commented here by the worthy experts, distinction between igneous and metamorphic hornblendes can best be made through careful study of the textures. But there are some rocks in high-grade metamorphic terrains in which even texture is not much helpful in deciphering the origin of the hornblende. Determination of origin on the basis of optical properties can be erroneous.

The term hornblende is a sack-name and includes hornblende, tschermakite, edenite, pargasite, magnesiohastingsite and their Fe analogues. Better to use hornblendic amphibole.  During the first meeting in 1973 with my PhD supervisor, Prof RA Howie (DHZ fame), I pointed out to equilibrium relationship between hornblende and clinopyroxene in a gabbro-norite (mafic granulite). He remarked with reference to the hornblende, "you mean amphibole".

My compliments to all, Qasim

Hi Majid. You can also check the work of Dr. K. Putirka. His spreadsheet for P-T calculations using Cpx composition considers the Nimis' geobarometer.

You can find interesting info on his website:

Partial melting of mantle lithosphere can directly produce andesitic melts, which has been documented by geochemical studies of Mesozoic andesitic rocks in eastern China (Chen et al., 2014, 2016). In this case, peridotite cannot serve as the lithology of mantle sources for andesitic magmatism. Instead, the partially melted are mafic lithologies such as garnet pyroxenite and hornblendite rather than ultramafic ones such as pyroxenite and hornblendite.

Chen, L., Zhao, Z.-F. & Zheng, Y.-F. (2014). Origin of andesitic rocks: geochemical constraints from Mesozoic volcanics in the Luzong basin, South China. Lithos 190, 220–239.

Chen, L., Zhao, Z.-F. & Zheng, Y.-F. (2016). Geochemical constraints on the origin of Late Mesozoic andesitic volcanics from the Ningwu basin in the Middle-Lower Yangtze Valley, South China. Lithos 254–255, 94–117.

Because of the incongruent melting of source rocks, the majority of felsic to mafic melts produced by partial melting of crustal or mantle lithologies are generally not in equilibrium with their residues. Although the melts of andesitic composition can be produced by partial melting of subducting mafic oceanic crust (eclogite), the oceanic-type eclogite is generally characterized by depletion in both melt-mobile incompartible trace elements such as LILE and LREE and their pertinent radiogenic isotopes. In comparison, the adakites of andesitic composition commonly show arc-style trace element signatures, precluding their derivation from partial melting of the subducting oceanic slab itself.

Dear feng

to have a temperature estimation (but always make attention to the results that you get) you can use these two geothermometers:

- Putirka, K., 2008. Thermometers and Barometers for Volcanic Systems. In: Putirka, K., Tepley, F. (eds.). Minerals, Inclusions and Volcanic Processes: Reviews in Mineralogy and Geochemistry. Mineralogical Society of America, 69, 61-120.

- Thy, P., Lesher, C.E., Tegner, C., 2013. Further work on experimental plagioclase equilibria and the Skaergaard liquidus temperature. American Mineralogist, 98, 1360-1367.

The first one is a book where you can find,  among the other, a geothermometer based on the cpx-liquid equilibrium. Prof. Putirka in his personal website provides the excel file with the algorithms (http://www.fresnostate.edu/csm/ees/faculty-staff/putirka.html).

The second geothermometer works using the equilibrium plagioclase-liquid 

as you can imagine from the title of the paper. 

Cheers, Fabio

Hi Ahmed,

in addition to the very informatif comments above, I'd like to share with you some more informations. As HFSE Nb, Ta, Zr and Hf are very resistant to alteration and therofore they likely used to truck and understand to evolution of magmas as well as they are useful in tectonic discrimination diagrams. Nb-Ta and Zr-Hf are chemicaly very similar twins and hence, they are expected to not fractionate from each other during diffrentiation, however such fractionation is often observed in magmatic rocks leading for example to positive or negative correlations between Nb/Ta and Zr/Hf ratios with increasing differentiation. As mentioned by Prof. W.L. Pohl one of the reasons could be a result of the fractional crystallization of Ti bearing phases like rutile, titanite and ilmenite (John et al. 2011; Mallmann et al. 2014). Some authors (Hui et al., 2011; Niu, 2012) propose the process of “mass-dependent fractionation during magmatism” to explain Nb-Ta and Zr-Hf fractionation in magmatic systems. They suggest that the lighter elements 90-96Zr and 93Nb would behave more incompatible than the heavier elements 174-180Hf and 181Ta in magmatic processes. The presence of halogens like F and Cl could play an important role  to facilitate the transport of the HFSE in the magma and the hydrothermal fluids, forming e.g. HFSE-F complexes which could lead to high enrichment of some HFSE (Agangi et al., 2010; Demartis et al., 2014; Sheard et al., 2012; Timofeev et al., 2015). High concentration of complexing ligands allowed HFSE to be transported in solution and to behave more incompatible as expected.

As concerns plagioclase, you may interpret the metamorphic reactions as the inverse of the crystallization from a liquid.

A plagioclase crystallizing from a melt has higher Ca/Na ratio than the melt (and this is known for about one century). With decreasing temperature plagioclase tends to become more sodic and with a Ca/Na in equilibrium with that of the host magma.

The metamorphic grade is usually related to changing temperature, because many (not all) the metamorphic reactions are rather good geothermometers and relatively poor geobarometers (of course there are many exceptions of this simple definition).

Assuming a basaltic protholith (e.g., with a labradoritic to bytownitic plagioclase) the first step of metamorphism requires the loss of Ca from the plagioclase structure. E.g., at relatively low T conditions (zeolite to prehnite-pumpellyite facies) and variable but generally low P, the anorthite component of the plagioclase goes to form new metamorphic minerals such as lawsonite, prehnite, laumontite, heulandite, all Ca-Al-Si hydrous minerals). If plagioclase remains in the low-grade metamorphic rock, it is rich to very rich in the albite component, with a nearly pure NaAlSi3O8 composition.

Increase of the metamorphic grade (read increase of temperature) plagioclase accepts more and more Ca in its structure. Ca may derive from other minerals not stable at higher grade conditions (e.g., epidotes or amphiboles). Under these circumstances, when the plagioclase reaches an oligoclase composition, low-medium grade conditions are reached (oligoclase amphibolite facies).

With further increase of temperature, the composition of the plagioclase reaches andesine (medium grade) to arrive to labradorite in the high grade metamorphic rocks (e.g., granulite facies).

In conclusion, the amount of Ca (or, better, the Ca/(Ca+Na) ratio) can be considered a rough estimate of the maximum temperature reached by a basaltic system that experienced the full metamorphic cycle (from Ca-rich basalt to Ca-poor low-grade metamorphic rocks, eventually again to Ca-rich granulite).

Cheers,

michele

Christian,

Start with the basics. First of all, verify that the pentlandite analyses are good quality, and that apparent Ni-depletion is not due to electron beam overlap on a nearby or underlying pyrrhotite or similar grain (apologies if you already checked this).

Secondly, determine the high-temp mineral paragenesis that the pentlandite is/was in equilibrium with. The Ni and Fe content will vary systematically depending on whether it is in contact or in equilibrium with troilite, hexagonal pyrrhotite or monoclinic pyrrhotite, or pyrite as the main Fe-sulphide mineral (see ref. Harris and Nickel in previous answer). These are the main high-temperature minerals that pentlandite would be in equilibrium with as it cools and exsolves from the monosulphide solid solution. These equilibria are controlled rather by fO2 and FS2 variations than P and T.

Thirdly, look to see whether this high-temperature assemblage has been modified or obliterated by low-temperature alteration (as seems likely in your case) - evidence of alteration of pyrrhotite to pyrite or Fe-oxides, presence of millerite or heazlewoodite, bravoite, awaruite etc. If the Ni depletion of pentlandite relates to particular assemblages, then you can start to develop a hypothesis of your own.

Another reference dealing with progressive serpentinization and talc-carbonate alteration and its effect on sulfides is that by Donaldson (1981) Economic Geology Vol. 76, p 1698. Redistribution of ore elements during serpentinization and talc-carbonate alteration of some Archean dunites, Western Australia.

Good luck.

Dave

 I suggest the textbook "Manual of Mineralogy by Klein and Hurlbut, 21/22 edition" this is the best introductory book for undergraduate of Mineralogy/Geology 

Dear Mr. Quelhas,

The following optical data are very diagnostic:

Xenotime (one crystallographic axis/ straight extinction) vs. monazite (two crystallographic axis/ oblique extinction)

birefringence (X : 0.095-0.100) vs. (Mo: 0.049-0.050)

In a heavy mineral mount, X often looks like a carbonate mineral displaying some kind of a "pseudo-pleochroism".

X often morphologically shows up as "squares", whereas Mo is "rhomb-shaped".

Best regards

H.G.Dill

I think that one of the factors is restite association in the mantle source at its melting.

Hi Juan--

If you observed this outcrop at the base of your ignimbrite, this is the basal vitrophyre of the ignimbrite deposit. A vitrophyre forms when a hot volcanic deposit (e.g. ash-flow tuff, rhyolite flow) comes into contact with the relatively cold surface of Earth. This sharp temperature gradient results in rapid cooling or quenching of the lowest part of the volcanic deposit, forming a glassy basal vitrophyre. I have observed vitrophyres up to 1-2 m-thick for ash-flow tuffs that were 100's of m thick. I have observed more chaotic and thicker vitrophyres (5-10 m-thick) in rhyolite flows, which can sometime cannibalize their own basal material during viscous flow.

The spherical feature you show in your photograph is called 'spherulitic texture'. This is common to the lower portions of ash-flow tuffs (ignimbrites). I have observed spherulitic texture in the black basal vitrophyre, but have also observed it higher up in an ignimbrite deposit, above the basal vitrophyre.

I suggest looking at this book for detailed discussion of both 'vitrophyres' and 'spherulitic texture':

Cas, R. A. F. and J. V. Wright (1987). Volcanic successions, modern and ancient.

Here are 2 papers I have authored about an extensive Miocene ignimbrite in the Gulf of California, which commonly displays a beautiful vitrophyre:

Bennett, S. E. K., et al. (2013). "Transtensional rifting in the proto-Gulf of California, near Bahía Kino, Sonora, México." Geological Society of America Bulletin 125(11/12): 1752-1782.

Bennett, S. E. K. and M. E. Oskin (2014). "Oblique rifting ruptures continents: Example from the Gulf of California shear zone." Geology 42(3): 215-218.

I also attach 2 photographs. The first photo shows a typical vitrophyre as the base of the Tuff of San Felipe. The second photo shows a close up of small spherulites in the vitrophyre of this tuff. Other locations of this tuff displayed spherulites up to ~5 cm in diameter.

I hope this information is useful for your interpretation of your ignimbrite!

Cheers,

Scott Bennett

U.S. Geological Survey

Dear Gerhard,  Thanks a lot for all these info. Regarding the large Laacher See zircons, I fully agree with your pegmatite hypothesis and I have scanned a photo of a zircon that supports this - indeed you have already suggested similar observations you have made yourself. On the other hand for the large zircons in the older tephrite lavas I am less convinced that the same process could work (no obvious resorbption features). I'll attach the two scans to illustrate the contrast asap, in the coming days most probably.

Thanks again for the discussion and for the ref.

Beste Grüsse,

Gerald

Hi everybody,

to me (and not only because I started science there), the Laacher See Tephra remains unique. Unlike the Alban Maar, that has had several eruptions separated by thousands of years and covering a large compositional range, the Laacher See Tephra is continuously zoned with much tighter compositional trends. At the final stages, the Laacher See erupted mafic phonolite together with basanite, that had just very recently mixing into the base of the magma chamber, and abundant cumulate and cognate syenite rocks, the latter suite contains hybrid syenite-carbonatite compositions, all have residual phonolitc interstitial glass.  Together with Axel Schmitt and a student of mine, we have dated zircons in these rocks, verifying earlier indications by U-series dating, that the crystal-rich mush / carapace had existed at least 20 ka before the eruption. (

Wetzel F, Schmitt AK, Kronz A, Wörner G (2010) In situ U-Th disequilibrium dating of pyrochlore at sub-millennial precision. Am Min 95: 1353-1356

Schmitt AK, Wetzel F, Cooper KM, Zou HB, Wörner G (2010) Magmatic longevity of Laacher See Volcano (Eifel, Germany) indicated by intrusive carbonatites. J Pet 50: 1053-1085

I envisage  that the eruption was fed from the remaining liquid core of an alkaline intrusive complex that was cooling outside in and was surrounded by crystal mush, syenites, carbonatite etc. 

The final stage of the eruption was strongly phreatomagmatic and "excavated" right into the alkaline intrusive complex that was still just above its solidus. The abundance of cumulate and syenite lithics stems from the fact that the phreatomagmatic explosions probably occurred at the base of a diatreme right inside the  shallow mush complex (but still at > 5 km depth).

Such a scenario is required by the observations and I am not aware of a similar case described in the literature.

Regards

Gerhard Wörner

Dear Paul, I think as already suggested by Martin Menzies, that the best should be to try to correlate as much as possible proximal and distal tephra. This can allow you at least to make a more confident attribution and dating of the possible source avoiding just one single data set and /or number date. Of coarse there are many possible sources but recently many papers on tephrostratigraphy have been published, especially in quaternary journals (eg.doi:10.1016/j.quascirev.2011.07.012; http://dx.doi.org/10.1016/j.quascirev.2012.09.009; http://dx.doi.org/10.1016/j.quascirev.2015.03.006; http://dx.doi.org/10.1016/j.quascirev.2014.04.002 and others) that could help to direct your effort. Data and literature related to very old volcanism are much less because often the proximal part is poorly known or unknown.  Have a good job.

Dear Raghav

There are several feasible explanations to the examples you have shown. As you are aware, degassing in a magma can be triggered by pressure drop, reaching the saturation concentration for each volatile component at different pressures and that is the explanation you suggest (upper parts of the dykes, at lower pressure). Other possibility is volatile exolution as the magma solidifies; the concentration of volatiles in the melt increases as a consequence of volume reduction of the melt phase, reaching saturation and exolving. In confined dyke systems, this second option seems more probable and assuming a contact-inwards cooling, the vesicles are going to be concentrated in the center of the dyke, irrespective of the orientation of the dyke and even at higher pressures.  

Best regards 

Dear Haozheng,

I could not agree more with Esther. In fact there are a few papers out there which undertook this "exercise" and could help you about the applied methodology and expected concentration levels of "water" in different NAMs from granulites.

Németh, Bianka, et al. "Melting, fluid migration and fluid-rock interactions in the lower crust beneath the Bakony-Balaton Highland volcanic field: a silicate melt and fluid inclusion study." Mineralogy and Petrology 109.2 (2015): 217-234.

Zhang, Li, Junfeng Zhang, and Zhenmin Jin. "Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny." Gondwana Research (2015).

Yang, Xiao‐Zhi, et al. "Water contrast between Precambrian and Phanerozoic continental lower crust in eastern China." Journal of Geophysical Research: Solid Earth (1978–2012) 113.B8 (2008).

In spite of some unjustified criticism on the methodology I would still recommend you with confidence to use the unpolarized infrared methodology (see references below) (this would save you a lot of time and effort and would not worsen significantly the accuracy of your quantitative results), or in some cases it would make possible to have any quantitative data at all). There are only a few absorbance conditions which should be met for accurate results. Note, however, that this methodology is rather limited if you would like to constrain the crystallographic orientation of particular absorbers in minerals.

Kovács, István, et al. "Quantitative absorbance spectroscopy with unpolarized light: Part II. Experimental evaluation and development of a protocol for quantitative analysis of mineral IR spectra." American Mineralogist 93.5-6 (2008): 765-778.

Sambridge, Malcolm, et al. "Quantitative absorbance spectroscopy with unpolarized light: Part I. Physical and mathematical development." American Mineralogist 93.5-6 (2008): 751-764.

You can have access to the digital spectra of several NAMs from granulites in the PULI spectral database for free and you have the opportunity to share your own spectra with the wider community ( puli.mfgi. hu ).

I hope that this was a help and good luck with your research,

Cheers,

Istvan