Hi All
Sumit is right to emphasise that the physics is just fine and the problems reflect practical issues. Under metamorphic conditions, diffusion through mineral lattices is almost always a much slower form of transport than other options, even dry grain boundary diffusion, so if any other mechanism kicks in, calculations based on lattice diffusion will be misleading. And unfortunately classic methods of observation do not always allow us to easily distinguish grains which have undergone processes such as dissolution-reprecipitation which can readily wipe the record clean. More widely, I think the existence of anomalous occurrences such as features preserved through high-T overprints should be a warning to us that we do not actually understand everything perfectly as yet. It may be that our assumptions on time scales are wrong, at least some of the time, and it is also likely that transport through the grain boundary network is much more variable than we currently assume, depending on whether rocks are wet (heating) or dry (cooling), the rate of fluid release, wetting characteristics etc etc, - all this may affect the boundary conditions for what is going on in grain interiors.
So jobs in metamorphism for a few years to come......?
Bruce
Bruce Yardley
Emeritus Professor
School of Earth & Environment
University of Leeds
Leeds LS2 9JT
UK
Tel +44 (0)7745 132560
-----Original Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Sumit Chakraborty
Sent: 23 April 2019 10:09
To: [log in to unmask]
Subject: Re: [geo-metamorphism] Closure Temperature
Dear all,
This is a discussion that keep recurring - unfortunately, according to me. To question whether thermal energy moves atoms / ions is to question basically all of physics. The definition of temperature is the "average kinetic energy of particles", and motion of particles without kinetic energy is....very strange.
One needs to have a differentiated analysis. Fluids, are a form of material, and not energy - so, by themselves they do not do anything to affect kinetic energy. What is implied (but not explicitly analyzed) is that they affect the chemical energy (chemical potential of different components etc....leading to dissolution, transport etc.), which can then contribute to kinetic energy. One needs to deal with deformation in the same sense (strain energy, localized i.e. brittle vs. not so localized i.e. various forms of ductile, plastic etc. etc.).
And finally, how can the concept of closure temperature not make sense?
Things do freeze sometimes, don't they? So the question is to determine at what conditions that happens.
If one goes back to the original Dodson papers, one would find that the equations were set up for a very generic kinetic process that depends on temperature - did not have to be diffusion at all. It is only over time that diffusion has been associated one-to-one with the Dodson equation (resulting of course from the fact that Dodson himself took diffusion as the kinetic process to analyze as an example). Consider the case of fluids above. Fluids may facilitiate reactions - but those reactions do follow a kinetic rate law, and that is likely to follow an Arrhenius equation, and with that, is amenable to treatment by the Dodson equation and the concept of closure temperature. Again, to question that fluid reaction kinetics would not be temperature dependent is to question a large chunk of chemistry.
That, also addresses the point that Martin raises - closure calculated based on diffusion rates may not (and should not) reflect the closure of systems (natural or experimental) that were affected by other kinetic processes; however that does not mean that the concept of closure temperature is not valid, it just means we need to calculate it with the rate constants appropriate for the process in question. That is true of any equation in science.
I suggest that we be more careful about the larger implications of discussions we carry out, with some precision in our expressions.
Thanks,
Sumit
Am 23/04/2019 um 10:24 schrieb JAVIER RODRIGUEZ ALLER:
> Dear all,
>
> Once Greg has raised a question about it, I would like to take it to a
> general audience regardless the isotopic system or the metamorphic grade:
>
> What is the closure temperature? Does it make any sense?
> Is T by itself (kinetic energy) able to move cations in and out of a
> crystalline structure?
>
> Though I gave up studying metamorphic petrology more than 10 years
> ago, at that time there were quite a few pieces of evidence that
> isotopic systems may be reset at low grades (well below commonly
> assumed Tc), and that they may remain closed at T above empirically
> defined Tc, even for long-lasting metamorphic events.
> As it has already been suggested to Greg, and this seems to have not
> changed since I was a petrologist, you'd absolutely need to take melt
> (or fluids at lower T) and/or deformation into your equations to put
> the Tc concept to work. Moreover, you may take T out of the equations,
> and fluids +/- deformation will still give the right answer,
> irrespective of T.
>
> Respectfully,
>
> Javier Rodríguez Aller, PhD.
> Servicio de Geocronología y Geoquímica Isotópica
> SGIker- Facultad de Ciencia y Tecnología Universidad del País Vasco
> Barrio Sarriena s/n
> E48940 Leioa
> SPAIN
>
> [log in to unmask]
> http://www.ehu.eus/en/web/sgiker/geokronologia-eta-geokimika-isotopiko
> a-aurkezpena
>
> http://www.linkedin.com/in/javi-rodriguez-032a5360
>
>
> Gregory Dumond <[log in to unmask]> escribió:
>
>> These are a few questions about Lu-Hf garnet dating:
>>
>> 1) How well do we know the closure temperature of the Lu-Hf system in
>> garnet?
>>
>> 2) How reliable is this technique for dating granulites, especially
>> granulites that have experience protracted high-T metamorphism and/or
>> residence in the lower crust?
>>
>> If you have any references that pertain to the above questions, I
>> would sincerely appreciate them.
>>
>> With best regards,
>> Greg
>>
>> Department of Geosciences
>> University of Arkansas, USA
>>
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>
>
> Javier Rodríguez Aller
> SGIker - Geocronología y Geoquímica Isotópica Facultad de Ciencia y
> Tecnología Universidad del País Vasco
> E48080 Bilbao
> SPAIN
>
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--
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Sumit Chakraborty
Professor, Institut für Geologie, Mineralogie und Geophysik and Director, RUBION
ACS Earth and Space Chemistry, Associate-Editor
Ruhr Universität Bochum,
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Germany
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