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GEO-METAMORPHISM  March 2008

GEO-METAMORPHISM March 2008

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Subject:

Re: geothermobarometry

From:

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

Reply-To:

Metamorphic Studies Group <[log in to unmask]>

Date:

Sun, 30 Mar 2008 15:33:49 -0400

Content-Type:

text/plain

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text/plain (383 lines)

Antonio,
     The problem with the current Zr and Ti "thermometers" is that we
do not have the t.d. of the pure phase represented by the dilute
substitution (ZrTiO4 zircon, ZrO2 rutile, TiO2 quartz, and now
CaZrSiO5 sphene. It is not straightforward to establish what the
Henry's Law constant is, much less its P-T dependence. As I implied
before the experiments and applications should be done with some
buffered reaction, not with a reduced and uncertain a(TiO2). Henry's
law behavior may also change if zircon has high U, if rutile has high
Cr, Fe, Al, Nb and if sphene has high Al, Fe, OH, F.
    There are very few reversed experiments for most silicates below
700°C. Holland & Powell also included some properties of end-members
extrapolated from KD relations observed in nature that may have large
errors from the fulcrum effect.
eric



On Mar 30, 2008, at 2:35 PM, Antonio Garcia Casco wrote:

> 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:GEO-
> [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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>>
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>>
>
>
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>
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