Here are some nice halos. Mostly in cordierite.
I gather that it's not just the amount of radioactive element, but that the energy of alpha particle emission from Th is higher than that from U.
In the picture, the annuli are around monazite, the compact halos that touch the crystal are around zircon.
I've measured the halos, and if I remember correctly the monazite halos are ~40 micron radius, the zircon halos around 25.
The rock is 1000 Ma old. You could say its age has matured them nicely, like a good wine.
Dave
Dave Waters, Metamorphic Petrologist
(1) Department of Earth Sciences, South Parks Rd, Oxford OX1 3AN
(2) University Museum of Natural History, Parks Rd, Oxford OX1 3PW
Email: [log in to unmask] Tel: +44 1865 272000
http://www.earth.ox.ac.uk/~davewa/ Direct: +44 1865 282457
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-----Original Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Richard Taylor
Sent: 02 March 2018 20:21
To: [log in to unmask]
Subject: Re: [geo-metamorphism] identifying altered cordierite
I would actually like to hear more opinions on the zircon halos, I feel I too have been drilled this way. It is likely that in lower grade rocks allanite and co are bigger culprits. And it is certainly the case we tend to tell people/student “zircon” when we really mean “actinide bearing minerals like zircon” and it’s most likely that’s the one students have heard of. Certainly in the case of higher grade pelitic rocks, that are allanite free, I would say the halos are a common feature in cordierite around zircon. I have found you can usually spot a monazite straight away in cordierite as it is surrounded by totally degraded ‘pinitised’ cordierite, whereas the zircon tends to just have a halo. I have seen studies where they say zircon won’t produce halos, but seen other studies and my own examples where it is clearly the case. Cordierite after garnet is the classic where zircon is produced as part of that reaction, and litters the cordierite and shows halos.
I would say it’s also worth noting that this halo has nothing to do with actual serious radiation damage, and therefore the alpha dose does not necessarily need to be excessive. Cordierite in particular shows almost no damage even at TEM scale in the pleochroic halo. The optical effect is likely attributed to odd excitation effects in the hydrous portion of the structure (or Fe 3+ related), hence why it’s limited to hydrous phases.
Would be interested to know if others feel this is “uncommon” around zircon.
Cheers
Rich
On 2 Mar 2018, at 20:06, Jochen Mezger <[log in to unmask] <mailto:[log in to unmask]> > wrote:
Really Bob? Do you want to imply that all these years I or we've been living a lie? (pleochroic halos around zircons...)
Good point, though.
:) Jochen
On Thu, Mar 1, 2018 at 12:59 PM, Robert Tracy <[log in to unmask] <mailto:[log in to unmask]> > wrote:
It is interesting that we are all trained to think that pleochroic haloes surround zircon, but they actually virtually always are caused by small crystals of monazite or allanite (mostly monazite). It’s a rare zircon that has enough alpha-particle oomph (a technical term!) to cause the typical lattice damage of a halo.
Dr. Robert Tracy
Professor of Geosciences
Director, Museum of Geosciences
Virginia Tech
Blacksburg VA 24061-0420
540-231-5980 <tel:(540)%20231-5980>
540-231-3386 <tel:(540)%20231-3386> (F)
On Mar 1, 2018, at 4:18 PM, Sebastián Verdecchia <[log in to unmask] <mailto:[log in to unmask]> > wrote:
Hi all, In addition to Dave's comments, in the last images from pdf file that attached Carl, is possible to observed haloes surrounding zircons (?).
On the other hand, we recognized And/Cd post-St/Gr/Ky in micaceous schist from Sierra de Ancasti of Argentina and interpreted as a decompression stage, from 590º-5kb to ca. 560-3.5 kb (Verdecchia et al JMG 2013, 31, 131-146).
Regards,
Sebastián
2018-03-01 17:39 GMT-03:00 David Waters <[log in to unmask] <mailto:[log in to unmask]> >:
Just what I was thinking!
Needs a clockwise P-T path.
Later Crd/And in St schist can happen in the Lhotse schists on Mt Everest, which loop through 5.5 kbar, peak in low 600s, return via ~3 kbar
Waiting for Dave Pattison now ...
Dave
Dave Waters, Metamorphic Petrologist
(1) Department of Earth Sciences, South Parks Rd, Oxford OX1 3AN
(2) University Museum of Natural History, Parks Rd, Oxford OX1 3PW
Email: [log in to unmask] <mailto:[log in to unmask]> Tel: +44 1865 272000 <tel:+44%201865%20272000>
http://www.earth.ox.ac.uk/~davewa/ <http://www.earth.ox.ac.uk/~davewa/> Direct: +44 1865 282457 <tel:+44%201865%20282457>
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-----Original Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask] <mailto:[log in to unmask]> ] On Behalf Of Bernardo Cesare
Sent: 01 March 2018 20:36
To: [log in to unmask] <mailto:[log in to unmask]>
Subject: Re: [geo-metamorphism] identifying altered cordierite
Carl
quite interesting because St-Crd isn't a typical barrovian assemblage and is quite uncommon.
I guess Dave Pattison will want to comment on this.
Bernardo
> <!-- p { margin-top: 0px; margin-bottom: 0px; }--> Dear metamorphic
> community,
>
>
>
> We're working on a suite of archean paragneiss (Opinaca
> subprovince, Superior Craton) that display a barrovian-type sequence
> of isograds (bt-gt-st-sill). In the field, we have noticed a
> blueish-greenish mineral in the pressure shadows of early kinematic
> staurolite, and thought it's either chlorite or cordierite. However,
> in thin section, the mineral has clearly been altered and shows a
> heterogeneous chemical composition (see attached document for pictures and probe analyses).
>
>
>
> We are looking for ways to confirm or infirm that the altered mineral
> was once cordierite.
>
>
>
> Any comments/advice/references will be greatly appreciated.
>
>
>
> Best regards
>
>
>
> Carl and Myriam
> ____________________________________________
>
> Carl Guilmette, PhD. Eng.
> Professeur Adjoint
> Chaire de Leadership en Enseignement Virginia-Gaumond Département de
> Géologie et Génie Géologique Université Laval
> 1-418-656-2131 <tel:(418)%20656-2131> poste 3137
--
*******************************************************************
Bernardo Cesare
Dipartimento di Geoscienze, Universita' di Padova
Via G. Gradenigo, 6, I-35131 PADOVA <https://maps.google.com/?q=Via+G.+Gradenigo,+6,+I-35131+PADOVA+ITALY&entry=gmail&source=g> ITALY <https://maps.google.com/?q=Via+G.+Gradenigo,+6,+I-35131+PADOVA+ITALY&entry=gmail&source=g>
Tel: ++39-049-8279148 <tel:+39%20049%20827%209148> Fax: ++39-049-8279134 <tel:+39%20049%20827%209134>
email: [log in to unmask] <mailto:[log in to unmask]>
Web site: http://147.162.183.153/bernardo/berning.html <http://147.162.183.153/bernardo/berning.html>
ACME Group: http://147.162.183.153/ACME
MicROCKScopica: http://www.microckscopica.org <http://www.microckscopica.org/>
InsiemePerWamba: http://www.insiemeperwamba.org <http://www.insiemeperwamba.org/>
*******************************************************************
"The only real failure is the failure to try, and the measure of success is how we cope with disappointment, as we always must."
From the
movie: The Best Exotic Marigold Hotel
--
Dr. Sebastián O. Verdecchia
CICTERRA (CONICET - UNC)
Facultad de Ciencias Exactas, Físicas y Naturales
Universidad Nacional de Córdoba
Av. Velez Sarsfield 1611. CICTERRA (Piso 2, oficina 2)
CP: X5016GCA - Córdoba (Capital)
Tel.: +54-351-5353800 <tel:+54%20351%20535-3800> (int 30218)
Provincia de Córdoba - Argentina
CICTERRA: http://cicterra.conicet.unc.edu.ar/ <http://cicterra.conicet.unc.edu.ar/>
--
Jochen E. Mezger
Term Instructor of Geology / Field Camp Director
Department of Geosciences
University of Alaska Fairbanks
Fairbanks, AK 99775-5780
U.S.A.
Phone: +1 (907) 474-7809
http://www.uaf.edu/geology/faculty/
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