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GEO-METAMORPHISM  March 2008

GEO-METAMORPHISM March 2008

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Subject:

Re: Geothermobarometry

From:

John Winter <[log in to unmask]>

Reply-To:

Metamorphic Studies Group <[log in to unmask]>

Date:

Sat, 29 Mar 2008 14:22:25 -1000

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (166 lines)

I quite agree, and have tried to bring this approach up-to-date in the
second edition re-write of my text. I simply thought that the table ought to
be updated as well for those interested. The option, of course, was to leave
out both the table and the independent equilibria approaches in favor of the
internally consistent methodology. I decided to leave them in because they
are both illustrative and still useful.

Oh, and thanks to those who had further additions on the second go-around.
John

John D. Winter
Department of Geology
Whitman College
Now at: PO Box 551735
             Kapa'au, HI 96755
(808) 889-6901
[log in to unmask]
http://www.whitman.edu/geology/winter/

-----Original Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask]] On
Behalf Of Antonio Garcia Casco
Sent: Saturday, March 29, 2008 2:13 PM
To: [log in to unmask]
Subject: Re: Geothermobarometry

Dear colleagues,

I appreciate John's and others' invaluable efforts in compiling up-to-date
lists of this important petrological tool. However, as practitioner of
thermobarometry, I usually make myself the question: to what extent a P-T
point calculated by means of two given calibrations has petrological
(=physicochemical) and geological meaning?

The risk of "standard two-reaction thermobarometry" is that, if the
petrologist uses two calibrations that are "internally inconsistent", the
simultaneous P-T solution also is "thermodynamically inconsistent". One
could argue that this makes the P-T result petrologically invalid and that
its geological value suspect.

To be aware of the details of the calibrations and solutions models used,
and of the extent of consistency among them, is not a simple task. This
bring us to the point of assessing the worth of using routinely lists of
thermobarometers instead of internally consistent thermodynamic databases of
end-member standard-state and solid-solution excess properties. To me, the
last option is wiser. First, it avoids worrying about the physicochemical
consistency of the result. Second, it helps making P-T results of different
authors directly comparable, for example if they investigate a petrological
problem (e.g., the blueschist-eclogite transition) or a geological problem
(the orogenic history of a region). This, in turn, would help in the
improvement of the td-databases themselves and, consequently, in the
precision of the calculations. Third, the "multiequilibrium method" can be
readily applied, and this method has a number of additional practical
advantages. The selection of a non-equilibrium mineral assemblage translates
into spurious P-T results which cannot be readily identified if only two
reactions are used to obtain P-T. However, by means of the analysis of the
associated errors, the multiequilibrium method gives the opportunity to
asses the extent of equilibrium of the selected mineral assemblage and to
check the quality (in terms of equilibrium) of a specific mineral
composition selected for calculations. Errors are as important as P-T
figures for petrological and geological interpretation. P-T errors arise
from uncertainties in the end-member standard-state and solid-solution
excess functions and in the mircroprobe analyses. These uncertainties, that
propagate to > +/- 25-50 ºC and > +/- 0.5-1 kbar, are common to all types of
thermobarometric approaches, but still larger errors may arise from the
practical complexities of mineral chemistry and mineral assemblages of a
single sample (equilibrium, diffusion, dissolution, re-equilibration,
partial equilibration,...). Minimizing these errors is easier using the
multiequilibrium method.

All the best
Antonio

----------------------------
Antonio Garcia-Casco
 Dpt. Mineralogy and Petrology (http://www.ugr.es/~minpet/)
 Andalusian Institute of Earth Science (http://www.iact.csic.es/)
 University of Granada
 Avda. Fuentenueva sn
 18002 Granada
 Spain
 Tel: (+34) 958 246613
 E-mail: [log in to unmask]
 http://www.ugr.es/~agcasco/personal/ "Home page"
 http://www.ugr.es/~agcasco/igcp546/ "Subduction Zones of the Caribbean"
 

-----Mensaje original-----
De: Metamorphic Studies Group [mailto:[log in to unmask]] En
nombre de John Winter
Enviado el: sábado, 29 de marzo de 2008 19:51
Para: [log in to unmask]
Asunto: Re: Geothermobarometry

Thanks, Frank. You are correct...and not the first to update me. They are
now in the list. J

John D. Winter
Department of Geology
Whitman College
Now at: PO Box 551735
             Kapa'au, HI 96755
(808) 889-6901
[log in to unmask]
http://www.whitman.edu/geology/winter/


-----Original Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask]] On
Behalf Of Frank Beunk
Sent: Thursday, March 27, 2008 10:19 PM
To: [log in to unmask]
Subject: Re: Geothermobarometry

John, am I correct when I didn't see Ti-in-Quartz, and Zr-in-Rutile, while
Ti-in-rutile is mentioned?

Watson, E.B., D.A. Wark & J.B. Thomas, 2006. Crystallization thermometers
for zircon and rutile. CMP, 151, 413-33.
Wark, D.A. & E.B. Watson, 2006. TitaniQ: a titanium-in-quartz
geothermometer. CMP, 152, 743-54.
Zack, T., R. Moraes & A. Kronz, 2004. Temperature dependance of Zr in
rutile: empirical calibration of a rutile thermometer. CMP, 148, 471-88.

cheers,
Frank

-------------------------------
Frank F. Beunk
Faculty of Earth and Life Sciences, Vrije Universiteit Dept. of Petrology De
Boelelaan 1085, room F062
NL-1081 HV Amsterdam, Netherlands
T +31-20-5987371
F +31-20-6462457
E [log in to unmask]



John Winter wrote:
>
> Thanks to those who replied to my request for updates to a table of
> thermometers and barometers. It's all-but-impossible to keep such a
> table current or complete, but here is my latest compilation, as
> promised, for those interested. The references are integrated into my
> overall text references, and will be difficult for me to extract from
> that huge list. Hopefully you'll know how to track any down that you
> want (or just wait for the second edition to come out!). Cheers. John
>
> John D. Winter
> Department of Geology
> Whitman College
> Now at: PO Box 551735
>
> Kapa'au, HI 96755
>
> (808) 889-6901
> [log in to unmask]
> http://www.whitman.edu/geology/winter/
>


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