The obvious implication is that host phases seem to have a non-isotropic susceptibility to radiation damage
Hi Robert,
Fission track analysis of apatite and zircon is always conducted on c-axis parallel grain sections for this reason (well, sort of): the track annealing rate depends on orientation within the grain.
So it may not be the susceptibility to radiation damage that is non-isotropic, but rather the ability of the crystal lattice to re-order itself.
Coincidentally, some of the earliest fission track studies by Naeser and co-workers were on micas. Would be interesting to know if anisotropic annealing really does apply to these phases as well.
Chris
--
Dr. Chris Mark
Research Fellow
Dept. of Geology, Trinity College Dublin
Museum Building, College Green
Dublin 2, Ireland
Email: [log in to unmask]<mailto:[log in to unmask]><mailto:[log in to unmask]>
http://www.tcd.ie/Geology/
On 2 Mar 2018, at 20:31, Robert Tracy <[log in to unmask]<mailto:[log in to unmask]>> wrote:
Jochen,
No implications - just sayin’ it outright. I remember in many courses as an undergrad and grad student being told by profs who were pretty good petrographers that those little high-relief guys were always zircons. Then when I started getting interested in monazite and began hitting them with the probe beam while doing recon, and finding that they were nearly all monazite, I reassessed.
When you think about monazite chemistry versus zircon chemistry, in terms of concentrations of radioactive elements, it’s pretty obvious. I’ve also observed that haloes in biotite and chlorite especially, but also in cordierite, are different shapes in thin sections and typically correlate with the host crystal orientation being observed. Someone may have written something on this, but I’ve never seen anything. The obvious implication is that host phases seem to have a non-isotropic susceptibility to radiation damage. For example, you will usually see haloes in basal-section, c-axis orientations of biotite that are perfectly circular, but haloes are more elliptical in sections that are more nearly perpendicular to the optic axis (or more correctly, the acute bisectrix of a mineral with 2V of 5° or so). This might make a nice little research project for a student.
Bob
Dr. Robert Tracy
Professor of Geosciences
Director, Museum of Geosciences
Virginia Tech
Blacksburg VA 24061-0420
540-231-5980
540-231-3386 (F)
On Mar 2, 2018, at 3:06 PM, 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/ Direct: +44 1865 282457<tel:+44%201865%20282457>
--------------------------------------------------------------------
-----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
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/
--
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/
