I have often seen changes in the density of inclusions at the same point in the porphyroblast (mainly andalusite) where the orientation of the inclusions changes.  But this is not uncommon (see the work of Scott Johnson).  These are contact metamorphic porphyroblasts, and we have attributed the change in inclusion orientation to changes in the deformation of the matrix as a result of the inflation of a pluton from a magma sheet.  The best example that I have seen are the andalusite porphyroblasts surrounding Papoose Flat pluton in California.  So, there may be a strain induced/assisted change in the mobility of elements?

 

 

 

 

Sven Morgan

Professor

Department of Earth and Atmospheric Sciences

Central Michigan University

Mt. Pleasant, MI 48859

U.S.A.

 

989-774-1082

989-774-2142 fax

 

 

From: Metamorphic Studies Group <[log in to unmask]> on behalf of Carl Guilmette <[log in to unmask]>
Reply-To: Metamorphic Studies Group <[log in to unmask]>, Carl Guilmette <[log in to unmask]>
Date: Wednesday, May 24, 2017 at 5:48 PM
To: "[log in to unmask]" <[log in to unmask]>
Subject: Re: Porphyroblasts with Inclusions

 

Hi all,

 

   Are we only talking subsolidus garnet growth? Peritectic garnet "overgrowing" subsolidus garnet cores in a relatively melt-rich environment is a drastic change in crystallization conditions. I bet Bernardo's examples are all supra-solidus rocks.

 

Cheers


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

De : Metamorphic Studies Group <[log in to unmask]> de la part de Stephen Daly <[log in to unmask]>
Envoyé : 24 mai 2017 16:34
À : [log in to unmask]
Objet : Re: [geo-metamorphism] Porphyroblasts with Inclusions

 

That was surely petro-poetry from Dugald!

 

Stephen

 

On 24 May 2017 at 21:22, Dugald Carmichael <[log in to unmask]> wrote:

Good discussion.  In my experience, poikiloblastic cores and relatively clear rims are common not only in garnet but also in many other porphyroblastic minerals. Surely this calls for a very general explanation.  I used to teach that the clear rims are best explained as due to the inevitable slowdown of the rate of T-increase that almost any metamorphic rock must experience as it approaches maximum T along its P-T-t path, simply because any large body of rock is hard to heat up and hard to cool down. During the early stage of growth, when the rate of T-increase is relatively fast, finite overstepping of the equilibrium T can induce finite local gradients of chemical potential and finite gradients of induced stress, which cause the secondary mass-transfer by which the matrix minerals either “get out of the way” or get surrounded and enclosed within the growing poikiloblast.  As the rate of T-increase declines towards zero, the overstepping of equilibrium T must also decline towards zero, and hence the local gradients of chemical potential and induced stress must also decline towards zero, and it is no longer possible for narrow salients of the poikiloblast to grow faster than the re-entrants (which due to interfacial energy are generally more stable than salients). At this stage P tends toward a constant and uniform value and the poikiloblast can grow only by infinitessimal additions to virtually planar interfaces with its surrounding minerals, thus providing the rock with its best possible chance of a close approach to thermodynamic equilibrium, ideally with the very slow rate of addition of heat to the system being the rate-determining step in whatever prograde net-reaction may be taking place.  

 

From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Frank Spear
Sent: May 24, 2017 10:34 AM
To: [log in to unmask]
Subject: Re: Porphyroblasts with Inclusions

 

Bernardo and others — 

 

I agree that these textures are common in garnet of low and medium grades where there is only one dominant reaction producing garnet (so one need not call on different garnet-producing reactions).

 

In the case where there is evidence for only a single P-T “event”, I believe the explanation is a simple geometric one. Whether garnet grows continuously along a prograde P-t path at conditions close to equilibrium or grows at nearly constant P-T conditions following significant overstepping, the volumetric rate of garnet production is nearly constant with time. So the radius in the core must grow faster than the radius late in the growth history. If one then makes the leap that it is the rate of garnet growth (in a radial sense) versus the rate at which the matrix can “get out of the way” by dissolution and transport away from the garnet, then it makes sense that the cores should be more inclusion-rich.

 

 

Regards to everyone,

 

Frank

 

 

 

Frank Spear

Professor and Department Head

Department of Earth and Environmental Sciences

Rensselaer Polytechnic Institute

110 8th St., Troy NY 12180

518-276-6103

 

 

 

On May 24, 2017, at 10:12 AM, [log in to unmask] wrote:

 

Bernardo and Juergen,

These garnet textures are common everywhere, and in a variety of metamorphic conditions, whether they are greenschist, amphibolite or blueschist. I would agree with Juergen that rather than a second garnet growth episode, it must be some more fundamental process that happens during growth of any garnet porphyroblast. I was thinking that initially, garnet growth could have been skeletal around matrix  quartz etc. (possibly controlled by initial nucleation site requirements), and then once the skeletal strands join up, garnet can simply nucleate on itself, rather than on other phases. My thoughts, for whatever they are worth!

Saibal Gupta.

----- Original Message -----
From: "Juergen Reinhardt" <[log in to unmask]>
To: [log in to unmask]
Sent: Wednesday, May 24, 2017 7:19:28 PM
Subject: Re: [geo-metamorphism] Porphyroblasts with Inclusions

Bernardo,

I suppose with ‘single event’ you mean a single progressive-metamorphic
episode, as there are multiple potential reactions garnets can grow from
(or continue to grow from), particularly in metapelites, depending on exact
bulk rock composition. Some may start growing under greenschist facies
conditions, some perhaps only at lower amphibolite facies conditions. The
garnet microstructures you show, with fine-grained inclusions in the centre
and an inclusion-poor to –absent rims/mantles, are very widespread from
what I have seen in many different metamorphic terrains. Therefore, I would
think such a structure does not require a truly poly-episodic P-T history,
but simply reflects a typical prograde growth pattern along a P-T path from
greenschist to mid-amphibolite facies conditions. With variations on the
theme looking at the blueschist you included. The fine-grained nature of
the core inclusions is in line with a former lower-grade state of the rock
matrix (like chlorite-quartz-muscovite) at the time of initial garnet
growth, even taking into account that some quartz would have been been
consumed by that first garnet-forming reaction. The explanation for the
inclusion-poor part is then another matter to discuss, and solutions have
been offered like slower growth rates, consumption of reactants, increased
diffusion rates at higher T, etc. Seeing that the structure is so common,
the conditions for the second garnet growth stage should then be the norm
rather than something exceptional.

Regards,

Juergen Reinhardt



Dept of Earth Sciences

University of the Western Cape

Bellville, 7535

South Africa

On Wed, May 24, 2017 at 2:33 PM, bernardo cesare <[log in to unmask]>
wrote:

Dear all,
following an earlier message on a similar topic, I’d greatly appreciate to
receive your opinion and see if there’s some kind of consensus on the
microstructural occurrence of inclusions in porphyroblasts.
In your experience, *in the case one can confidently assume a
porphyroblast grew in a single event*:
-do porphyroblasts often show an inhomogeneous distribution of inclusions,
with core-rim textures?
-if so, are inclusions more common in the cores or in the rims?

My experience (or my bias) is that the porphyroblasts (garnet in
particular) commonly show inclusion-rich cores rather than mantles or rims.
I am attaching a few examples from metapelites and metabasites, supra- and
sub-solidus. But I cannot be sure garnet formed in a single event in all of
them.

Your feedback is very appreciated!

Thank you very much in advance and
Best Regards,

Bernardo

*******************************************************************
*Bernardo Cesare*
Dipartimento di Geoscienze, Universita' di Padova
Via G. Gradenigo, 6, I-35131 PADOVA  ITALY
Tel: ++39-049-8279148 <+39%20049%20827%209148> Fax: ++39-049-8279134
<+39%20049%20827%209134>
email*:* [log in to unmask]
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


-- 


Disclaimer - This e-mail is subject to UWC policies and e-mail disclaimer 
published on our website at: 
https://www.uwc.ac.za/Pages/emaildisclaimer.aspx

 



 

--

Professor J. Stephen Daly
Director, National Centre for Isotope Geochemistry
UCD School of Earth Sciences
Science Centre West
University College Dublin, Belfield
Dublin 4, Ireland.
Tel: +353 1 7162327 (note change of office phone number); +353 87 205 1198
http://www.ucd.ie/geology/