Sumit, Eric,
I admit that the problem is complex, and that I (and you) have touched quite
different but related issues.
A) ASSUMING A PERFECTLY EQUILIBRATED ASSEMBLAGE.
A.1) The main concern of my message was METHODOLOGICAL aspects of
thermobarometry. The use of just one (well calibrated) reaction is perfectly
OK, but this gives a P-T curve [P(T) or T(P) information], not a P-T POINT.
If one wants to look at the P-T intersection of two reactions, for example
in order to construct a P-T path, the methodological aspect becomes
important. From this point of view, a thermodynamically consistent P-T POINT
needs of internally consistent thermobarometers. This methodological aspect
has nothing to do with the use of the multiequilibrium method.
A.2) A different issue, that I did not intend to be discussed, is the
quality of the calibrations and data-bases. Just two comments, by Holland
and Powell (1990, p.110):
"...geobarometry is less precise than its practitioners would have us
believe, and the problem usually lies not with the calibrations or with the
thermodynamic data used for the equilibria but with poorly-known
activity-composition relations for the end-members in the mineral phase."
and by Koziol and Newton (1989, p. 423):
"An error in the activity coefficient of a garnet component, specially a
dilute component such as grossular or pyrope in garnets from pelitic rocks,
can make a large difference in the calculated activity of that component,
which translates into sizable errors in the calculated temperatures and
pressures of recrystallization of natural assemblages".
Yes, activity models are the Achilles' heel of thermobarometry whatever
method is applied. And this has much to do with Eric's comments on the
complexities of solid solutions, the effects of non-accounted components
(even if relatively-low concentrated), and the effects of extrapolation to
low T of thermodynamic properties extracted from high-T experimental data.
Most (if not all) well calibrated thermobarometers apply to chemical "model
systems" which deviate substantially from the chemistry of natural systems.
Applying them imply (large) chemical extrapolation. Internally consistent
databases are constructed using the experimental data of many reactions
pertaining to different "model systems" and covering a wider range of P-T
than a single well calibrated model-system reaction. It is my opinion that
the mixing parameters of a complex solid solution extracted in this way are
more accurate (wouldn't say precise) than those extracted from a single set
of experiments of a single reaction within a given model-system at a given
P-T window, at least for the practical aspects of thermobarometry (i.e.,
routine application to a number of samples of different grade and
composition from a given metamorphic complex).
A.3) I agree that trace-element thermometers must have a P-dependence. This
is readily deduced from the composition of UHP minerals and textures
developed upon decompression. More work is needed on this and other aspects,
but these equilibria are promising since the excess functions of the solid
solutions should be (almost) independent of chemical composition (Henry's
Law), making the description of the thermometer relatively simple.
A.4) Activity of H2O. The fluid phase can be readily omitted from
multiequilibrium calculations.
B) EQUILIBRIUM PROBLEMS (RE-EQUILIBRATION, PARTIAL/LOCAL EQUILIBRATION,
"BACK-REACTION",...)
The effects of kinetics in mineral (sub)assemblages and mineral composition
is the Achilles' heel of routine Petrology, not only of routine
thermobarometry. Kinetic problems may prevent the use of two (standard
thermobarometry) or more (multiequilibrium method) reactions to get a P-T
point (different "blocking" P-Ts). This problem is common to both
approaches, and it may lead to the conclusion that only reactions taken
independently (if local or partial equilibrium is assumed) can be used. But,
since this approach gives P(T) or T(P) information, construction of a
precise P-T path can be almost imposible (because no single P-T point can be
determined). However, even if it may be just an academic exercise rather
than a real solution to the problem, the multiequilibrium approach can give
in certain cases some insights into equilibrium problems among the selected
(sub)asemblages.
Antonio
-----Mensaje original-----
De: Metamorphic Studies Group [mailto:[log in to unmask]] En
nombre de Eric J Essene 1
Enviado el: domingo, 30 de marzo de 2008 16:50
Para: [log in to unmask]
Asunto: geothermobarometry
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/
>>
>
>
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