Hi Carl,
Thanks for the summary, it was nice to see it all pulled together like that. Good luck with the LA-ICP-MS data processing. Which program are you using for that? I never got to do laser or REE work with my Tasmanian garnets, but I have recently done some Laser maps of garnets here in Sweden that show some beautiful Y and REE rhythmic zoning. The maps can be seen here: https://sway.com/178YlQb08Y57hEJ0?ref=Link
In order to see the zoning it was necessary to tweak the colour bar scale by setting the upper limit has been set to some value lower than the highest CPS recorded for that element, and any data greater than that max is red. This would give a smaller range of numbers over which to spread the colour range, which in turn made it possible to see the narrow zoning bands. I would play with each one, first setting the upper limit to some nice round number close to the maximum recorded counts per second and look at the result. Then drop that value by one order of a magnitude at a time (or fractions thereof) until I found an upper limit where only a very small percentage of the map is red (values above the upper limit of the scale bar).
I look forward to hearing if your "unzoned" garnets nonetheless show some REE zoning.
Cheers,
--Riia
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Dr. Riia M. Chmielowski
Senior Research Engineer/Laboratory Manager
Laser-Ablation ICP-MS Laboratory, LTU
SE-971 87 Luleå, Sweden
www.ltu.se/LaserICPMS
Hours: Mon-Thurs 07:30-12:30 or by appointment
Email: [log in to unmask]
Phone: +46 (0)920 492033
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http://ltu-se.academia.edu/RiiaChmielowski
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________________________________________
From: Metamorphic Studies Group <[log in to unmask]> on behalf of Carl Guilmette <[log in to unmask]>
Sent: Thursday, October 5, 2017 00:38
To: [log in to unmask]
Subject: Re: [geo-metamorphism] Garnet texture
Hi all,
As a summary:
The texture I inquired about has been ascribed by most to the "atoll garnet" type.
Occurences of similar textures include Cambrian Tasmania (Riia), Paleoproterozoic Reynolds Range, Australia (Tom Raimondo), Ordovician Connemara - Ireland (Bruce), Sesia Alps (Lukas), Ruitor Alps (Dominique) and Altai (Vasily).
The processes proposed to explain such textures are quite contrasted (Lukas): growth and dissolution, single event growth, polymetamorphic overgrowth, multiple reactions in a single cycle growth .
All the processes fall into two groups:
a) Preferential dissolution of garnet cores over rims during (retrograde) fluid infiltration (Jean-Luc, Yong-Fei)
b) "Pulsed" growth of garnet (Tom, Bruce, Lukas, Nuno, Simon, Vasiliy) due either to polymetamorphism or to successive garnet-producing reactions separated by a hiatus.
In most cases, there seems to be some degree of major element zoning preserved, hinting to a garnet that grew over a significant range of P-T.
In the example I showed (from the Paleo-pretorozoic New-Quebec Orogen, by the way), garnet has no major element zoning, suggesting either HT diffusional homogeneization or quick growth over a very restrained range of PT. The fact that garnet is post-kinematic with the oriented fabric preserved inside the "atoll" sounds like a solid argument against preferential core dissolution. In my opinion, it is also a reasonable argument against sequential garnet-producing reactions, but I can't swear that there was not early pre- or syn-kinematic garnet that underwent resorption and then overgrowth by a idiomorphic rim in a static environment.
We are currently treating the data for LA-ICP trace element analysis. I can't tell yet whether there is HREE zoning, for instance.
I really liked Nuno's comments that appeared (to me) more aimed towards growth kinetics rather than P-T complications. Fits the homogeneous composition.
I'll let the group know whether the garnets are HREE zoned or not as soon as we have the results.
Thanks a lot everybody
Regards
Carl
____________________________________________
Carl Guilmette, B.Eng. PhD.
Professeur Adjoint
Chaire de Leadership en Enseignement Virginia-Gaumond
Département de Géologie et Génie Géologique
Université Laval
1-418-656-2131 poste 3137
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De : Metamorphic Studies Group <[log in to unmask]> de la part de Dominique Laduron <[log in to unmask]>
Envoyé : 3 octobre 2017 14:52
À : [log in to unmask]
Objet : [geo-metamorphism] TR: [geo-metamorphism] Garnet texture
Dear colleagues,
We have worked in my laboratory on zoning of garnets and published 4 papers.
- de Béthune P., Laduron D., Martin H. et Theunissen K, 1968. Grenats zonés de la zone du Mont-Rose (Valle Anzasca, Prov. de Novara, Italie) . Schweis. Min. Petr. Mitt., 43, 437-454
- de Béthune P., Laduron D., Bocquet (Desmons) J., 1975. Diffusion processes in resorbed garnets. Contrib. Mineral. Petro., 50, 197-204.
- Desmons J., Laduron D. et de Béthune P., 1977. Grenats zonés se la nappe du Gand-Saint-Bernard et de la nappe piémontaise (Alpes Occidentales) . Mémoires de l'Institut géologique de l'Université de Louvain, 29, 327-347.
- Laduron D. et Desmons J., 1981. Résorption et néocroissance dans les grenats d'un micaschiste du Ruitor (Alpes Franco-Italiennes). Mémoires de l'Institut géologique de l'Université de Louvain, 31, 335-347.
If some are interested, i can send these articles, especially the two last not published in a courant review.
Cheers
D.Laduron.
________________________________________________________________________________
Dominique Laduron
Professeur émérite - Géologie - UCL
Place L.Pasteur, 3 - Bte L4.03.06
B - 1348 - Louvain-la-Neuve
Belgique
Bâtiment Mercator - local a.002
Les matins du mardi et du jeudi
Tél : 00/32/(0)10/472856
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