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GEO-METAMORPHISM  March 2008

GEO-METAMORPHISM March 2008

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Subject:

geothermobarometry

From:

Eric J Essene 1 <[log in to unmask]>

Reply-To:

Metamorphic Studies Group <[log in to unmask]>

Date:

Sun, 30 Mar 2008 10:50:23 -0400

Content-Type:

text/plain

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text/plain (221 lines)

Antonio,
     My sense is that you think the bulk of "internally consistent"
data bases are reasonably accurate. I am not so convinced that is
the case for silicate and oxide phases with solid solutions where
data are extrapolated to far lower T than where they were measured
(or observed). I agree with Sumit that many data have clear errors
due to unreversed experiments. Suspects include amphiboles and
micas, for instance, phengite solid solutions, the solvus between
paragonite and muscovite, and what is up with actinolite - winchite -
glaucophane? The problem with a multi-equilibrium calculation is
that the phase compositions have to have equilibrated and then remain
unchanged afterward. At low T there is only a partial approach to
that state, and the state continues to be reset during early cooling
at high T. A convergence to a small range of P-T is not proof of
good data or a good result, because the program always minimizes the
results despite errors in the data. Errors mean that the "correct"
answer is not at the tightest convergence. Not only is there the
problem of assuming the a(H2O), quartz is often not present in
metamafic rocks, and authors fail to even report such a mundane phase
or include it explicitly in the reactions that are presented.
     The biggest problem with pseudosections is the assumption of a
fixed bulk composition, which is clearly not correct in some rocks.
The effect of removing a significant fraction of certain elements by
isolation of cores in the case of zoned minerals, and evidence of
continued introduction and removal of elements in other cases,
especially for HP and UHP rocks, is generally ignored.
     The Ti and Zr "thermometers" must have a P dependence, despite
several works ignoring the actual reactions and the effect of their
delta V. Both calibration and application has proceeded in those
systems without the presence of rutile, which is a rare phase in
granitic rocks (some S-type granites have rutile), and many other low
P systems. Assuming without demonstration that the a(TiO2) must be
high in the presence of ilmenite, sphene or titanian biotite is
rather like the argument that a(H2O) must be high because some
hydroxyl phase is found. As I once said at a Grenville meeting, that
argument is deeply flawed -- every silicate has 50% O but that
doesn't mean that f(O2) is high. I recommend a judicious use of
simple equilibria corrected for minor solid solution compared to the
results of more complicated calculations.
eric



On Mar 29, 2008, at 8:12 PM, Antonio Garcia Casco wrote:

> 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:GEO-
> [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:GEO-
> [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/
>>
>
>
> __________ Informacisn de NOD32, revisisn 2984 (20080329) __________
>
> Este mensaje ha sido analizado con NOD32 antivirus system
> http://www.nod32.com
>
>

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