Hi Carl – I agree with Lukas that the rather uniform composition is very unusual for a garnet grain recording a complex growth history. However if these are upper amphibolite facies rocks the garnets may simply have homogenised by diffusion at some stage in their development. I figured (very poorly it has to be said) an example of a garnet with a texturally early core and a later overgrowth but no compositional zoning in my 1977(!) American Mineralogist paper on diffusion in garnet. In that example, diffusion in garnet began when staurolite + quartz (+sillimanite) was still stable, and the garnet overgrowth was produced by the second staurolite breakdown reaction.
Good luck
Bruce
Bruce Yardley
Emeritus Professor
School of Earth and Environment
University of Leeds
Leeds LS2 9JT
UK
Tel: +44 (0)7745 132560
From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Lukas Baumgartner
Sent: 30 September 2017 10:22
To: [log in to unmask]
Subject: Re: [geo-metamorphism] Garnet texture
Hi Carl,
I would call them atoll garnets, or at least incipient atol garnetss, and there is a hughe literature out there on how to form them: prograde growth-> dissolution inside; growth; mutiple metamorphic events; multiple reactions during an event. In a paper on the Sesia we have argued that it is due to growth change (Robyr et al., J. met.pet 2013). All of these processes normaly require garnet compositional changes between the core and the rim. I find these garnets quite fascinating, if there are indeed no compositional differences between the inner and the outer part (or at least nearly non). I see you have spot analysis - so is this realy the case r is it just the color scale range you choose?
Cheers
Lukas
***********************************
Lukas Baumgartner
professor in metamorphic petrology
Chair CASA
Institut de Sciences de la Terre
UNIL-Grange
Geopolis
University of Lausanne
1015 Lausanne
Ofice 4885
T: +41 21 692 44 46
F: +41 21 692 43 05
Secretary +41 21 692 43 00
************************************
On 29 Sep 2017, at 17:41, Carl Guilmette <[log in to unmask]<mailto:[log in to unmask]>> wrote:
Hi all,
I've attached photos and X-Ray maps of garnet grains from a gt-qtz-pl-bt-ilm+/-chl gneiss. They show a puzzling (to me) texture with a ubiquitous ring of polymineralic inclusions in the mantle. The inclusions have the same mineralogy, same grain size, same composition as the matrix.
Has anyone encountered such textures before? We're not sure they qualify as atoll garnets.
Thanks
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
<garnets.pdf>
