JiscMail Logo
Email discussion lists for the UK Education and Research communities

Help for GEO-METAMORPHISM Archives


GEO-METAMORPHISM Archives

GEO-METAMORPHISM Archives


GEO-METAMORPHISM@JISCMAIL.AC.UK


View:

Message:

[

First

|

Previous

|

Next

|

Last

]

By Topic:

[

First

|

Previous

|

Next

|

Last

]

By Author:

[

First

|

Previous

|

Next

|

Last

]

Font:

Monospaced Font

LISTSERV Archives

LISTSERV Archives

GEO-METAMORPHISM Home

GEO-METAMORPHISM Home

GEO-METAMORPHISM  2001

GEO-METAMORPHISM 2001

Options

Subscribe or Unsubscribe

Subscribe or Unsubscribe

Log In

Log In

Get Password

Get Password

Subject:

Re: Contact aureoles

From:

[log in to unmask]

Reply-To:

No title defined <[log in to unmask]>

Date:

Mon, 22 Jan 2001 17:15:57 +0000

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (90 lines)

Colleagues - I am commenting on this one in full greybeard mode - drooling
over my keyboard as I stare mistily through thickened classes to identify
the letters.

It is a quarter of a century since I used the changes in zoning patterns in
prograde metamorphic garnets between c. 600 and 650C to make an empirical
estimate of major cation diffusion rates in garnet (which was reasonably
robust!), so it is nice to see that the idea of using natural garnets still
has credibility. In hindsight, I think that the major weakness (a major
weakness...) of my earlier work was that it did not allow for the small
number of garnets which seem to survive prograde metamorphism to high
temperatures while retaining low-T bell-shaped profiles, even in areas like
Connemara where 99.99% of grains have reset. What I now think is the case
is that in a few cases it is possible for garnets to grow in a dry setting,
or to be left hot and dry after they have grown, in contrast to the high
water fugacity environment that normally attends prograde metamorphism of
garnetiferous rocks, and that this may explain the occassionally more
refractory behaviour of garnet in the 650 - 750C range. Now if this hunch
is right, it means that the contact overprint experiment, if it works, is
likely to measure the slow dry diffusion rates, because the regional
garnet-bearing rock will have lost all pore fluid as it cooled from the
metamorphic peak, and the contact event will not have replaced it. Even if
water was given off by further dehydration at the contact metamorphic peak,
it is far from clear how effectively it would penetrate garnet, and so
affect diffusion rates. Thus I fear that the results obtained would be very
relevant to diffusion rates in thermally-overprinted regional garnets, but
not to prograde garnet growth.

Unfortunately it is also the case, that although there are countless
aureoles affecting garnet-bearing regionally metamorphosed rocks, they tend
to be either rather deep aureoles, in which case the thermal constraints
can be modelled with less precision, or else to be so shallow that all
garnet is destroyed at temperatures below the diffusion threshold. It is
not normal for garnet to survive at temperatures for volume diffusion to be
effective, without participating in reactions with the surrounding matrix
that result in recrystallisation, new growth, or other processes that
invalidate the experiment! In fact I cannot think of an example where this
natural experiment will work reliably, and I look forward to seeing if
anyone else can.

Sorry to be so negative....
Bruce Yardley


>Dear all,
>
>I am looking for a rapidly cooled metamorphic terrane, containing garnet,
>that was intruded at a much later date (in the hundreds Ma) by a granite
>(or other heat source). It would be nice if garnet was found in most of
>the isograds associated with the granite aureole. In addition, alteration
>associated with cooling and or hydrous fluids should be minimal.
>
>The reason for this specific setting is that I want to use the granite
>thermal overprint as a natural experiment to determine the closure
>temperature of garnet by looking at diffusion profiles in garnet. This has
>a much better chance of success than carrying out laboratory diffusion
>experiments because REE diffuse slowly.
>
>Cheers
>
>Alfredo
>
>Alfredo Camacho
>School of Geology,
>The University of New South Wales,
>UNSW SYDNEY, NSW 2052,
>AUSTRALIA
>
>Tel no: (02) 9385 (6142 or 6134)
>Fax no: (02) 9385 5935
>Email: [log in to unmask]
>
>


--------------------------------------------
Professor Bruce Yardley
School of Earth Sciences
University of Leeds
Leeds LS2 9JT
UK

Tel. 0113 233 5200 Fax 0113 233 5259
---------------------------------------------

NEW! GEOFLUIDS - an interdisciplinary journal for research on the role of
fluids in all aspects of the evolution of the Earth's crust. Details on:

http://www.blackwell-science.com/gfl

Top of Message | Previous Page | Permalink

JiscMail Tools


RSS Feeds and Sharing


Advanced Options


Archives

October 2019
September 2019
August 2019
July 2019
June 2019
May 2019
April 2019
March 2019
February 2019
January 2019
December 2018
November 2018
October 2018
September 2018
August 2018
July 2018
June 2018
May 2018
April 2018
March 2018
February 2018
January 2018
December 2017
November 2017
October 2017
September 2017
August 2017
July 2017
June 2017
May 2017
April 2017
March 2017
February 2017
January 2017
December 2016
November 2016
October 2016
September 2016
August 2016
July 2016
June 2016
May 2016
April 2016
March 2016
February 2016
January 2016
December 2015
November 2015
October 2015
September 2015
August 2015
July 2015
June 2015
May 2015
April 2015
March 2015
February 2015
January 2015
December 2014
November 2014
October 2014
September 2014
August 2014
July 2014
June 2014
May 2014
April 2014
March 2014
February 2014
January 2014
December 2013
November 2013
October 2013
September 2013
August 2013
July 2013
June 2013
May 2013
April 2013
March 2013
February 2013
January 2013
December 2012
November 2012
October 2012
September 2012
August 2012
July 2012
June 2012
May 2012
April 2012
March 2012
February 2012
January 2012
December 2011
November 2011
October 2011
September 2011
August 2011
July 2011
June 2011
May 2011
April 2011
March 2011
February 2011
January 2011
December 2010
November 2010
October 2010
September 2010
August 2010
July 2010
June 2010
May 2010
April 2010
March 2010
February 2010
January 2010
December 2009
November 2009
October 2009
September 2009
August 2009
July 2009
June 2009
May 2009
April 2009
March 2009
February 2009
January 2009
December 2008
November 2008
October 2008
September 2008
August 2008
July 2008
June 2008
May 2008
April 2008
March 2008
February 2008
January 2008
December 2007
November 2007
October 2007
September 2007
August 2007
July 2007
June 2007
May 2007
April 2007
March 2007
February 2007
January 2007
2006
2005
2004
2003
2002
2001
2000
1999
1998


JiscMail is a Jisc service.

View our service policies at https://www.jiscmail.ac.uk/policyandsecurity/ and Jisc's privacy policy at https://www.jisc.ac.uk/website/privacy-notice

Secured by F-Secure Anti-Virus CataList Email List Search Powered by the LISTSERV Email List Manager