Metamorphic 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 both for your generous input,

What I mean by "hydrothermal alteration" is that, either during or after metamorphosis of this schist, there was fluid inclusion that altered the rock. My reasoning, as well as my mentors, is that the preferred orientation appears to be in a "spiral". This thin section is from 325' into a drill core, of a schist containing muscovite, biotite, garnet, quartz, and sillimanite. Attached is a photo of the thin section as a whole.

Garnet in sedimentary rocks is always metamorphic or magmatic in origin, or has been derived from reworked sediments. Magmatic and metamorphic garnets are rarely monomineralic but s.s.s. such as "Grandite" (grossular+andratite = Ca/Al+Ca/Fe) or "Pyralspit" (pyrope+almandine+spessatine = Mg/Al+Fe/Al+Mn/Al). As you might conclude from the attached file you need to get an idea of the chemical composition and in some cases also of the valence state of Fe (see almandine and andratite)

I wish you much success with the table below which might help to come to grips with this issue.

HGD

That is a strange question. As Ioan Pintea suggested, 100% is a high quartz value for a quartzite, but 59% is a high value for your meta-basic rocks, but 59% might be a low value for a quartzite. The categories low, medium and high will be different for each different rock. You need to understand the purpose for making these categories. Is there any purpose? If not, why are you doing it?

Felipe,

Nothing is ever going to drip into the mantle unless it is denser than the mantle. So, we all know that eclogitised mafic crust can sink into the mantle, but what about "huge packs of chlorite-schists and serpentinites"?! What are the density contrasts here with ambient mantle, with TTGs, and with the unaltered mafic crust?

Food for thought...

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.

Can anyone provide me thr spreadsheet for chlorite geothermometry calculation?

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 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

Francolite synonymous with carbonate fluorapatite or

called collophane when microcrystalline

is the principal component of many sedimentary

phosphate rocks (“phosphorite”). Brushite (41.24 wt.% P2O5 ) and

monetite (52.16 wt.% P2O5) are only widespread in guano deposits,

where locally rather exotic phosphate minerals accommodating N, O

and H bridge the gap between organo-mineralic chemical compounds

and inorganic chemical compounds (coastal region in Chile). They can be the source of your apatite when recrystallized.

Marine upwelling along the shelf edge is responsible for the formation of carbonate-hosted phosphates and phosphorites. Some phosphorites accumulated within high energy — nearshore zones. Some formed in the outer shelf deposits. The common minerals besides francolite are smectite, illite, opal, sepiolite, clinoptilolite, quartz , siderite, smectite and palygorskite.

When they undergo physical-chemical alteration, e.g., an increase in T apatite may recrystallize and form XX of different X shape from elongated to stubby XX (pisms). You should investigate your apatite s.s.s. and determine how much carbon dioxide, F , Cl and OH is still in the XX. The OH- and CO3- anion complex should decrease along with increasing T. Also have a look at the minerals associated with the phosphate. The minerals mentioned above only occur in diagenetically overprinted phosphate-bearing limestones.

There are many phosphate deposits hosted by calcareous rocks.

See: DILL, H.G. and KANTOR, W. (1997) Depositional environment, chemical facies and a tentative classification of some selected phosphate accumulations.- Geologisches Jahrbuch, D 105: 3-43.

HGD

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.

Hi Nasir,

in my view, the best way is to follow the approach by Lünsdorf et al. (2017): https://doi.org/10.1111/ggr.12178

There is a protocol which you can follow step wise. Fitting is done by using IFORS, that enables to reduce the operator bias (Lünsdorf & Lünsdorf, 2016).

Best,

Jan

Dear Ahmed Morad: I understand perfectly what is your proposal. The best way to show how marine sediments get into a subduction zone is to draw a detail of the hinge zone where the oceanic plate bends when foundering inside the mantle. In this hinge zone stretching occurs, and some expansive graben structures are formed (normal faults parallel to the trench), which trap sediments and lead them to subduction. Then those sediments are metamorphosed under high P/T conditions and become phyllites and schists, usually white schists, and even metacherts and fine grained marbles. There is a quite accesible outcrop in a main freeway near the city of Puerto Cabello (north-central Venezuela) of eclogite knockers inside a relatively monotonous micaschist. A close look at thin sections of this schist shows the presence of kyanite, garnet, white mica, and Mg-glaucophane, these are usually called white schists, since the glaucophane is almost colorless and retrogradly altered to talc! Some have carbonate too, and in nearby localities of this same high P terrane there are also metacherts and marbles. So, there's no doubt that some marine sediments are indeed subducted and transformed to high P metamorphic rocks in the subduction complex. Regards, Sebastian.

To sum up:

The recommendations of Guenter Grundmann , Harald G. Dill and Patricia Roeser provide the best results for my question.

The British Geological Survey shares its deposites data on their homepage:

If you want to search for the quantitative mineralogical composition of rock use the phrase "X-ray Diffraction" in the text search and the rock you search for.

In the pangaea-data warehouse the mineral composition of rocks are sourced from publications. The search for locations is also possible. The raw data can be downloaded or can be viewed as html.

Dear Samuel,

There is a yahoo group which is dedicated for Perple_X. You can find the link of the group at the website of it. It is better to ask your question there and certainly you will get the solution of your problem.

-Steve- gambist@gmail.com ... https://tinyurl.com/to8tsbt

sorry，I don't understand your statement.

Dear Craig, there is this oldish paper from the authoritative Howie (late too), who attibutes the color to thin graphite deposited along mineral grain boundaries. These would be beyond optical resolution, and won't show up in SEM either due to their elemental lightness combined with thinness, allegedly deposited from high-grade metamorphic fluid, allegedly rich in CO2. You should be able to locate the paper online.

Howie, RA (1967): Charnockites and their color. Journal of the Geological Society of India (8): 1-7.

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

Dear Mr. Morad,

Unless you have carried out a geological survey and studied the mineralogical association of your rocks the chemical studies and the use of any index based upon chemical data is meaningless. It is a repetive critics of mine.

It is good for riding the “paper-tiger” but in practice these figures are of no use. Because what it is all about, is the futile attempt to circumnavigate experience and knowledge in geology and mineralogy. To gather experience needs time and to pile up knowledge even more in combination with a series of physical and mental endowments.

We cannot delegate geosciences to the technicians in the chemical lab.

Sorry for this harsh critics which is a general statement rather a reference to your question.

Sincerely yours

Harald G. Dill

Mr, could find a way to calculate these parameters?

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.

I recommend Cemal's answer

Best Regards

Yes, redox matters especially if you deal with Paleoproterozoic ore – as you mention high-grade metamorphic ore, this is one of the possibilities. At the time, the ocean was weakly oxidized and stratified. This means that Mn4+ compounds formed in the oxic (upper) layer may be dissolved upon settling in reducing conditions, in the deeper layer. Initial minerals precipitated in the oxic layer are mostly Mn4+-oxides, subsequently (an possibly rapidly) converted into Mn2+-carbonates (as rhodochrosite) or Mn2+/Mn3+-silicates (as braunite, garnet, and much more). If you use a thermodynamic code/model, be careful to introduce initial conditions appropriate to the contemporaneous chemistry of the ocean, or of the layer of the ocean you’re concerned with. Cheers, Thierry

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

Dear Mr. Morad,

such chemical indices are attractive to those who do not know the side effects and cast aside other processes leading to similar datasets. The ratios pretend to be all-embracing if you ignore the "small print". In my opinion they should be used with a stark warning and always supported by other methods from different geoscientic disciplines such as geology, mineralogy and shallow geophysics.

Best regards

H.G.Dill

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.

It's been a while, but I've finally achieved an answer to my question. In the Euclidean plane, given points as two-combinations of n phase names and lines are three-combinations of the phase names there must be 1 point adjacent to 2n-4, 2 points adjacent to 2n-5, 3 points adjacent to 2n-6, ..., n-1 points adjacent to n-2.

Thanks Wentao Cao for taking time and listing the references. I appreciate your effort. 

you can read these papers

Pagé, P. and Barnes, S.J., 2009. Using Trace Elements in Chromites to Constrain the Origin of Podiform Chromitites in the Thetford Mines Ophiolite, Quebec, Canada. Economic Geology, 104: 997-1018.

Zhou, M. F., Robinson, P. T., Su, B. X., Gao, J. F., Li, J. W., & Yang, J. S., 2014. Compositions of chromite, associated minerals, and parental magmas of podiform chromite deposits: the role of slab contamination of asthenospheric melts in suprasubduction zone environments. Gondwana Research, 26, 262–283.

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.

In igneous rocks, especially mafic igneous rocks, primary amphiboles tend to be pargasite or pargasitic hornblende. The problem arises in that at magmatic conditions, there are processes (hydration crystallization) that produce amphibole by reaction between pyroxene and melt. These amphiboles are usually pargasite or hornblende. However, reaction can continue subsolidus yielding low-Al hornblende, actinolite etc. that are generally construed as "deuteric" or metamorphic. Finally, if you have wholesale replacement of pyroxene by green or blue-green hornblende, it is probably metamorphic. The condition of the feldspar can also yield a clue in these situations. Is it altered? Is it more albitic than you would expect for the rock composition?

Dear Dr. Majumdar:

You are on the right track. There are two types of U deposits, one called intramagmatic bound to alaskites and another named U metasomatites.

The first group hosts davidite, uraninite, thorite, bedafite, eudyalite, and pyrochlore and is categorized as low grade-large tonnage (e.g. Roessing 300 ppm U3O8). They are mainly located  in southern Africa. The second one encompasses uranium mineralizations which occur in structurally-deformed rocks altered by metasomatic processes, and associated with the introduction of Na, K and Ca. Their mineralogy resembles the afore-mentioned ones (U-, Th-, P- and REE –bearing minerals). Their reference deposits are located in Brazil and Cameroon.

Partial melting of a granodioritic source rock produced a restite-bearing I-type monzogranite in the magmatic stage. The granite ascended synorogenically.

Residual late-magmatic melts are enriched in K or Na causing

different generations of albitization which led to the replacement of plagioclase by albite, and the formation of a Zr-Ce-La mineralization.

The hydrothermal phase gave rise to the uraninite mineralization

U-albitite deposits are normally in S- and A-type granites.

Best regards

H.G.Dill

After you prove sedimentary nature of the gneisses and schists (e.g., based on relict textures), and that most elements are immobile during metamorphism, you can use a variety diagrams and indices.

Classification diagrams for meta-sedimentary rocks:

(a) SiO2/Al2O3–Fe2O3*/K2O diagram (after Herron 1988);

(b) SiO2/Al2O3–Na2O/K2O diagram (after Pettijohn et al. 1987);

(c) CIA–ICV (Chemical Index of Alteration vs. Index of Compositional Variability). Trends of fresh granites and fresh basalts are from (Lee 2002), subdivision on mature and immature, intense weathering and weak weathering sediments are from (Nesbitt and Young 1984; Cox et al. 1995).

(d) A–CN–K plot for meta-sedimentary rocks (after Fedo et al. 1995). Molar proportions: A-Al2O3, K-K2O, CN-CaO*+Na2O, CaO* = CaO-CO2-0.5xCO2-10/ 3xP2O5.

Indices for meta-sedimentary rocks:

The chemical index of alteration (CIA) (Nesbitt and Young 1984)

The CIW index (Harnois, 1988)

The PIA index (Plagioclase Index of Alteration) (Fedo et al. 1995).

The WIS (Weathering Intensity Scale) (Meunier at al., 2013)

References:

Cox, R., Lowe, D.R., and Cullers, R.L., 1995, The influence of sediment recycling and basement composition on evolution of mudrock chemistry in the southwestern United States: Geochimica et Cosmochimica Acta, v. 59, p. 2919–2940.

Fedo, C.M., Nesbitt, H.W., and Young, G.M., 1995, Unraveling the effects of potassium metasomatism in sedimentary rocks and paleosols, with implications for paleoweathering conditions and provenance: Geology, v. 23, p. 921–924.

Harnois, L., 1988, The CIW index: A new chemical index of weathering: Sedimentary Geology, v. 55, p. 319–322

Herron, M.M., 1988, Geochemical classification of terrigenous sands and shales from core or log data: Journal of Sedimentary Research, v. 58, p. 820–829.

Lee, Y.I., 2002, Provenance derived from the geochemistry of late Paleozoic–early Mesozoic mudrocks of the Pyeongan Supergroup, Korea: Sedimentary Geology, v. 149, p. 219–235.

Meunier, A., Caner, L., Hubert, F., El Albani A., Preˆt, D. , 2013. The Weathering Intensity Scale (WIS): An alternative approach of the Chemical Index of Alteration (CIA). American Journal of Science, v. 313, p. 113–143.

Nesbitt, H.W., and Young, G.M., 1982, Early Proterozoic climates and plate motions inferred from major element chemistry of lutites: Nature, v. 299, p. 715–717. 

Nesbitt, H.W., and Young, G.M., 1984, Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations: Geochimica Et Cosmochimica Acta, v. 48, p. 1523–1534.

Pettijohn, F.J., Potter, P.E., and Siever, R., 1987, Sand and Sandstone: New York, Springer-Verlag, 553 p.

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

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

Dear Vahid,

So thanks. 

Best Regards,

Zahra

Dear Dr. Santos:

The term schist is rather well established within the sequence of metamorphic rocks, originating from a  psammo-pelitic parent material with quartz, some feldspar and phyllosilicates. I will depict below the grain size, metamorphic stage and denomination of the rock arranged in the order of increasing p-T condition during metamorphism

1. Slate (< 0.1mm fine-grained) very-low grade stage

2. Phyllite (0.1 mm to 1.0 mm medium-grained) low grade stage

3. Schist (> 1.0 mm) may be homogeneous or spotted with porphyroblasts medium (to high grade). In some cases also of low-grade stage (see talc schist)

4.Gneiss (> 1.0 mm) layered, banded, flasered locally with "augen" or single porphyroblasts medium to high grade

There are transitions into calc-schists or calcsilicate-schists  in case of marls forming the protolith.

Maybe this is of help for your question.

H.G.Dill

Dear Alejandro,

Please have a look at these works.

Kind Regards,

Masoud Ovissi

Liu, Lin-Gin. "High-pressure phase transformations in baddeleyite and zircon, with geophysical implications." Earth and Planetary Science Letters 44.3 (1979): 390-396.

Scoates, James S., and Kevin R. Chamberlain. "Baddeleyite (ZrO2) and zircon (ZrSiO4) from anorthositic rocks of the Laramie anorthosite complex, Wyoming: petrologic consequences and U-Pb ages." American Mineralogist 80.11-12 (1995): 1317-1327.

Wall, C. J., et al. "Baddeleyite-Zircon Relationships in cumulates of the Archean Stillwater Complex: evidence from U-Pb geochronology and Hf isotope systematics." AGU Fall Meeting Abstracts. Vol. 1. 2010.

Kovalenko, N. I., and B. N. Ryzhenko. "Comparative study of the solubility of zircon and baddeleyite." Geochemistry International 47.4 (2009): 405-413.

Schärer, Urs, Jasper Berndt, and Alex Deutsch. "The genesis of deep-mantle xenocrystic zircon and baddeleyite megacrysts (Mbuji-Mayi kimberlite): trace-element patterns." European Journal of Mineralogy 23.2 (2011): 241-255.

Voznyak, Dmytro K., et al. "Baddeleyite segregations in zircon of the Azov zirconium-rare-earth deposit (Ukrainian Shield)." Mineralogia 44.3-4 (2013): 125-131.

Lewerentz, Alexander. "On the occurrence of baddeleyite in zircon in silica-saturated rocks." Dissertations in Geology at Lund University (2010).

Hi Tharanga,

Before answering the question of residual stress, one will have to answer the questions whether stress accumulates? and if so how? and what is the evidence to suggest that stress once accumulated and then later released? Also, if stress accumulates is there the necessity to release all that stress after the metamorphic rocks reached the surface? One can argue that stress was accommodated by re-crystallization to a new mineral assemblage and ductile deformation (strain) and therefore did not accumulate. What happens if the system of stress changes? There will be new deformation and re-crystallization. Does one want to argue that stress accumulates during successive stages of deformation? or whether all that was accommodated by re-crystallization and deformation? These are questions you have to look into closely before answering your main question.

I'm an analytical chemist with experience in graphite exploration, and field sample analysis. The method we use is: precisely weigh a powdered sample (0.1 to 1 gram aliquot); leach with dilute HCl to remove carbonate carbon; roast in Leco furnace at 425 C until weight is stable (removes organic carbon); ash the residue and detect graphitic carbon-derived CO₂ by IR.

Dear Nirmalya,

deformation conditions seem to be thus well constrained. Consequently, I think grain size could be a possible explanation to understand the obtained ages. One interesting point that you could evaluate is whether the so called "closure temperatures" are valid, specially for mylonitic rocks (e.g., have a look on Igor Villa´s papers). I´ve been working on this topic for the last year and I think it´s not as easy as it´s thought due to the complexity and interaction of different processes during mylonitization (deformation, metamorphism, magmatism, fluids).

In any case, I am still not pretty sure whether both micas are contemporaneous or not. For example, the biotites >400 microns, do they represent the grain-size reducted biotites or the remnants of "underformed" biotites? Based on what you mention, I would still think that biotite nucleated prior to muscovite, although both could be afterwards homogeneised in terms of 18O. In that case, this event shouldn´t have affected K-Ar isotopy. 

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

Dear Peng,

In attached link you will find a paper, reporting an occurrence of high pressure granulite in relation to a fault zone. Maybe it is the one, you are searching for.

Best Wishes,

Masoud Ovissi

Thanks!

As Marlina says, although in many cases garnet may have also formed during the subsolidus evolution of a migmatite, it commonly forms as a peritectic phase (solid product) of the melting reaction in both metabasic and metapelitic rocks (which are you looking at?). It forms by the incongruent breakdown of hornblende or biotite (which contain the H2O needed by the melt but contain way too much Fe, Mg and other stuff to dissolve) with quartz and feldspars that form the main chemical constituents of the melt. In such cases the garnet is usually spatially associated with leucosome pockets or veins, and may be relatively deficient in inclsuions relative to eralier formed garnet. Garnet (peritectic and subsolidus) can be transported long distances with the melt, as the work of Gary and others have shown (see attached paper for a more recent example where we suspect this happened), particularly if the garnet is small. However, as garnet commonly forms large grains (several miliimetres to centimetres across) it generally remains trapped (along with other peritectic minerals like opx, cpx, cordierite), within the residual source rocks (granulites).

Dear Mr. Xaba:

4 terms need to be distinguished: Paleosome, neosome, melanosome, and leucosome. As the prefix indicates paleo is old, neo new, leuco light, and melano dark. Whereas paleo and neo is timely, leuco and melano is the color. In practice this means that the neosome is the product of anatexis that can be formed by reactions that produce for example graitic melt + garnet during fluid absent melting. In this case the quartzofeldspatic part is the leucosome, together with garnet forming the neosome. If you don't have an additional melanocratic phase like garnet or cordierite during anatexis, leucosome and neosome is the same. On the other hand the melanosome is the restite that did not melt and is formed by dark minerals, whereas the paleosome is the original rock of which the melt was extracted. The latter two terms are very difficult to distinguish is field and often chemical analyses is necessary.

The answer given by Volker Kaminske is quite good, the oceanic lithosphere takes some 12-15 Ma to become brittle. But some precision should be made in the question issued by Carlos Ganade de Araujo itself. There's NO "MORB" lithosphere, MORB is a term specific for mid ocean ridge basalts, which make up oceanic crust, not lithosphere. The lithosphere under the oceanic crust  call the LID or oceanic mantle lithosphere, which comprises usually more than 90% of the thickness of an oceanic lithospheric plate, is not made of basalt, but of ultramafic rocks,such as peridotite, dunite and  pyroxenite, often serpentinized and tectonized.  

A good review of d18O of minerals and rocks in mafic eclogites is in the work of Miller, Buick and Cartwright et al.

Miller, J. A., Cartwright, I., Buick, I. & Barnicoat, A. (2001). An O-isotope profile through the HP-LT Corsican ophiolite, France and its implications for fluid flow during subduction. Chemical Geology 178, 43-69.

Miller, J. A. & Cartwright, I. (2000). Distinguishing between seafloor alteration and fluid flow during subduction using stable isotope geochemistry: examples from Tethyan ophiolites in the Western Alps. Journal of Metamorphic Geology 18, 467-482.

For d18O in garnet from eclogites nice works are below, not all of them are oceanic crust but you will see a variety of d18O due to oceanic alteration, interaction with serpentinite and sedimentary fluids during subduction.

Errico, J. C., Barnes, J. D., Strickland, A. & Valley, J. W. (2013). Oxygen isotope zoning in garnets from Franciscan eclogite blocks: Evidence for rock-buffered fluid interaction in the mantle wedge. Contributions to Mineralogy and Petrology 166, 1161-1176.

Page, F. Z., Essene, E. J., Mukasa, S. B. & Valley, J. W. (2014). A garnet-zircon oxygen isotope record of subduction and exhumation fluids from the Franciscan complex, California. Journal of Petrology 55, 103-131.

Russell, A. K., Kitajima, K., Strickland, A., Medaris Jr, L. G., Schulze, D. J. & Valley, J. W. (2013). Eclogite-facies fluid infiltration: Constraints from d18O zoning in garnet. Contributions to Mineralogy and Petrology 165, 103-116.

Rubatto, D. & Angiboust, S. (2015). Oxygen isotope record of oceanic and high-pressure metasomatism: a P-T-time-fluid path for the Monviso eclogites (Italy) Contributions to Mineralogy and Petrology in press.

Dear Mr. Huang,

which element do you want to use for this dating to get realiable geological results ?

Zoisite is Ca₂Al₃Si₃O₁₂(OH) and epidote has the chemical composition Ca₂ (Fe,Al)₃(SiO₄)₃(OH). There is only one mineral in this group of minerals, allanite or orthite which is related to the above s.s.s. Ca(REE,Ca)A_l2(Fe++,Fe+++)(SiO₄)(Si₂O₇)O(OH). The latter has also elevated U and Th contents and as such it is amenable for radioactive dating. I would direct my thoughts to garnet which is besides omphacite the most important constituent in eclogites and can tell you something about the age of formation. Sm/ Nd and Lu/Hf may be an option.

Best regards

H.G.Dill

What do your biotites look like? Are they igneous or metamorphic? Any traces of residual amphibole? Is it possible the amphibole in the orthogneiss has been completely replaced by biotite during metamorphism? If, as you state, magmatic phenocrysts are very rare, metamorphism must have been extensive.

Dear Sandun,

For your question 1:

What do you mean by chemographic projections of mineral assemblage? Do you mean the AFM type diagrams? If you specify what type of projection do you want? It will help others to give you correct answer or some clue.

You may have a look on the page below, you may find it useful

For question 2:

Again, if I understood your question correctly, it is hard to generate P-T-t path simply based on petrographic observations. You may infer though from the petrograhic evidence you have in your samples. However, for correct P-T-t paths you have to have chemical composition of mineral assemblages you are interested in, and their age (t). There are several programs which can be used for P-T (and t, provided you know the age). Two already suggested by Alexeev, and you may check the link above for GIBBS.

Also, you may check PERPLEX, might be easy to use.

Can you provide a photomicrograph of the rock (having  unusually high Mg content) and the list of minerals present?  Just academic curiosity. Thanks. KNRAO

The best way to obtain a numerical age for a sedimentary rock – other than through comparison of fossil content or magnetopolarity reversals with the geological timescale – is through the direct dating of volcanic ash layers (U-Pb and Ar-Ar techniques on mineral separates). The main difficulties with this approach relate to the mixing of grains of different age and significance, and alteration of the dated minerals. An approach that works really well in marine deposits of Oligocene-Miocene age is to measure Sr isotope ratios in microfossils and macrofossils. The seawater ratio is well established for that interval, and more or less linear. Glauconite dating is mostly a mess. Poor resolution.

Wouldn't it be an idea to start reading about (garnets in) migmatites in general and seeing whether you can apply these general ideas to your field area?

Dear Mr. Aminov,

you expect too much from a single set of trace elements normalized to a standard and overstretch the possibilities of such analyses. You have to take a more diversified approach also including REE (see first response of Dr. Towe), isotopes and last but not least carefully examine the lithology of the volcanic rock to see which processes affected this magmatic rock. It is not only the source that counts. On the way up a magma can also be contaminated.

Best regards

H.G.Dill

Dear Mr. Udagedara,

Using SEM-EDS is not suitable for accurate chemical analyzes of garnet.

Exact quantitative chemical analysis of garnet are possible only with an electron beam microprobe.

In almandine garnet may be present, the trace elements:

Sodium, potassium, chromium, vanadium and in very minor quantities, scandium, yttrium, europium, ytterbium, hafnium, thorium, uranium.

In pyrope garnet may be present, the trace elements:

chromium, iron, sodium (in eclogite), potassium, phosphorus, vanadium, nickel, scandium, ytterbium, lutetium.

Sulphur does not fit in the garnet crystal lattice. Measured sulfur contents come from tiny solid inclusions of sulfides such as pyrite, pyrrhotite, chalcopyrite, which are often found in garnet crystals.

Mg and Ca Sodium are interchangeable. Na and K are trace elements.

Best regards,

G. Grundmann

Secondary hematite micro powder is quite common in some rocks when even mild oxidation is takin place. You do not have to have high hematite content because red staining gives very very strong effect. Obviously Al can folllow Fe and vice versa. This may not be taking place in your sample but it is worthy to check measurinf Fe content. Fe Ka spectral line is very efficient and you can get detection limit around 0.01wt%.

Cheers, HK

Actually LOI means "loss on ignition", therefore the sample has reached a temperature in which all volatiles are eliminated, this includes moisture water plus bound water in micas, amphiboles, epidote, etc. [in the form of (OH)-], and in some rocks even CO2 produced by decomposition of carbonates. If no carbonate is present, then LOI is H2O+ plus H2O-. LOI heating I understand that reaches some 700°C.  

Dear Mr. Chatterjee,

Quartzites often contain apatite, monazite and / or zircon crystals. If you find in your quartzites grains that are at one tenth of a millimeter in diameter, and contain at least 10 ppm uranium, then you should try the fission track dating methods.

Please compare two of my publications dealing with the apatite fission track dating method.

Best regards,

Guenter Grundmann

But from this reference for a start, you can trace backward and forward more references that are relevent to the boninites in Papua New Guinea:

Not need for further explanation, your question has been answered through the discussion thanks to the input of all the participants. By the way, that figure is from one of my papers (Fig 3f): Campos Alvarez and Roser 2007. Geochemistry of black shales from the Lower Cretaceous Paja Formation, Eastern Cordillera, Colombia: Source weathering, provenance, and tectonic setting, Journal of South American Earth Sciences 23, 271–289, and simply reflects the abundance of clay minerals is indicated by low K2O/Al2O3 ratios (<0.3), similar with the ranges of clays (Fig. 3f) but significantly different from the higher ratios (0.3–0.9) typical of feldspars.........just keep it simple

you can visit Dima DD website to find updated program, Ufortunately, this is in Russian, but you can ask someone to translate.

I agree with the above. Moreover, the big trade-off will be with temperature (and pressure), too. Another significant uncertainty is anelasticity, which is a hard problem. A few % change in the mineral content will have a theoretical effect on the velocity, but whether you would be able to calculate a reliable value is a different issue. This type of thing has been done before, ignoring anelasticity (see below), but doing so ignores a significant effect on the velocities:

A composite geologic and seismic profile beneath the southern Rio Grande rift, New Mexico, based on xenolith mineralogy, temperature, and pressure

J.M. Hamblocka,b,!, C.L. Andronicosa, K.C. Millerb, C.G. Barnesc, M-H. Ren b , M.G. Averill b , E.Y. Anthony b

Tectonophysics 442 (2007) 14–48

doi:10.1016/j.tecto.2007.04.006

They provide extra calculation materials in a supplement and refer to a spreadsheet by Hacker and Abers (2004).

Dear Mr. Baghban:

Bivalent Sr can substitute for bivalent Ca. It is not an abnormally high amount of Sr in your andradite-dominated garnet s.s.s. and I advise to be very cautious and do not over-interpret one trace element alone. You have to look at these values in context with other elements chemically affiliated to them and the overall composition of the garnet. Extremely high values may be expected, as alkaline magmatic activity is involved in the skarn formation. Are there LREE-bearing minerals such as monazite or bastnaesite among the minerals ? On the other hand, take also a closer look at the limestone that provides the parent material for the exoskarn. What is the background.

Sr/Sr isotopes have often been used for genetic interpretation of garnet emplacement. See one paper below.

Sousa et al. (2013) Strontium isotope zoning in garnet: implications for metamorphic matrix equilibration, geochronology and phase equilibrium modeling.

Best regards

Harald G. Dill

The fact that you talk about a metamorphic belt may already exclude some tectonic settings. Apart from geochemistry and (metamorphic) petrology, structural information is crucial. 

As long as metamorphism is isochemical, i.e. no evidence for metasomatism, then I do not see a problem with the isotopic signature being preserved.

Hi There,

As Marlina pointed out your question is a little vague. My guess is that you are asking about reintegration of melt with residue to calculate the original composition of the rock that melted. 

Whilst this may be possible in some cases i think the likely scenario is that in many cases partial melting is open system. This means leucosomes represent residual cumulates that are left behind after the loss of some (in may cases unknown) volume of granitic melt that has now migrated to higher levels in the crust. 

As a result a mass balance approach will still not result in calculating the original bulk composition of the unmelted protolith. There is a JMG special issue on crustal melting that covers a large amount of current migmatite research (link below). As well as recent books/reviews e.g. Mike Brown, Ed Sawyer, there is also a lot of active research with modelling of open systems e.g. Chris Yakymchuk, Fawna Korhonen etc

Jakub,

It is rather difficult, at least for me, to reply to you answer, that appears to be too generic to be answered with a simple and clear statement on the state of art on you problem.

I give to you some informations that are based on my experience of the natural laboratory of the ophiolitic peridotites from the Alps and Apennines (N-W Italy), and, accordingly concern the rifting stages of opening of the fossil Jurassic slow-ultraslow Ligurian Tethys basin.

I can say that :

1) lherzolites (and particularly fertile spinel – to plagioclase lherzolites), most probably derived from the sub-continental mantle lithosphere during the rifting stages of the basin, are exhumed and eventually exposed at the sea-floor close to the Ocean-Continent Transition (OCT) Zones of the rifted margins;

2) harzburgites (and, particularly. depleted reactive spinel harzburgites) frequently associated to large volumes of impregnated plagioclase peridotites, progressively replace sub-continental lherzolites ocean-wards. They are interpreted as products of melt/peridotite reaction caused by porous flow infiltration and percolation of early asthenospheric melts during the pre-oceanic magmatic rifting stages of the future basin.

3) refractory harzburgites, formed by oceanic partial melting and parental to the subsequent oceanic MORB magmatism of the basin, are doubtly described at the sea-floor during the embryonic stages of oceanic spreading.

In our cases, evident chemical contrast exist between fertile lherzolites, reactively depleted harzburgites and refertilized plagioclase peridotites, depending on the processes of formation, both melting and melt/rock interaction.

I don’t know if more informations on these situations and geodynamic settings are useful to your problem.

In the case, write again and I’ll give you appropriated references.

Giovanni

Even better: first dry the powdered sample at 110 degrees to get rid of any adsorbed water. And I would think that 550 degrees is a bit low (I typically go for 800-900 degrees), but if there is a possibility of alkali-loss, then you may want to keep the temperature lower than that