Dear Yong-Fei,
The position of the Ab=Jd+Qz equilibrium boundary in P-T space is a
function of the activities of Ab in Plagioclase and of Jd in Omphacite.
Have a look at the attachment which I have constructed using variable
values for the equilibrium constant of this reaction assuming, for
simplicity, activity coefficients of 1. Here: Κ=𝑋𝐴𝑏𝑃𝑙/𝑋𝐽𝑑𝐶𝑝𝑥.
I have included the line shown in most metamorphic petrology textbooks (it
nearly coincides with the boundary calculated using a value of K=3) and a
typical subduction zone geotherm (10°C/km) for your reference. I hope it
helps!
Cheers,
Dimitri
Στις Πεμ, Αύγουστος 29, 2019 19:20, ο/η Ralf Milke έγραψε::
> And after all, it's not either Plg or Cpx that expands the stabilty field
> of eclogite, but it's the equilibrium between those two. Thus, both
> answers are correct.
>
> Ralf
>
>
>
>> Aley (and all),
>>
>> Thanks! That's what I get for replying to emails before my morning
>> coffee
>> *and* not checking things before pressing send. Either way, it's solid
>> solutions that are key to Yong-Fei's question.
>>
>> Thanks,
>>
>> Richard Palin
>>
>> Assistant Professor of Metamorphic Geology,
>>
>> Department of Geology and Geological Engineering,
>>
>> Colorado School of Mines,
>>
>> Golden, CO 80401, USA
>>
>> http://inside.mines.edu/~rmpalin/
>>
>>
>>
>> Tel: +1 (303) 273-3819
>>
>> “Rocks are records of events that took place at the time they formed.
>> They
>> are books. They have a different vocabulary, a different alphabet, but
>> you learn how to read them.” – John McPhee
>>
>>
>>
>> On Thu, Aug 29, 2019 at 9:11 AM El-Shazly, Aley <[log in to unmask]>
>> wrote:
>>
>>> Richard,
>>>
>>> Increased An component in feldspar would expand the stability field of
>>> feldspars to higher P and lower T. It is the solid solution in Cpx (the
>>> “augite-acmite” components) that expand the stability field of Cpx to
>>> lower
>>> P and higher T, allowing for some eclogites to perhaps retain
>>> plagioclase??
>>>
>>> Aley
>>>
>>>
>>>
>>> *From:* Metamorphic Studies Group <[log in to unmask]> *On
>>> Behalf Of *Richard Palin
>>> *Sent:* Thursday, August 29, 2019 10:54 AM
>>> *To:* [log in to unmask]
>>> *Subject:* Re: [geo-metamorphism] Eclogite question
>>>
>>>
>>>
>>> Yong-Fei,
>>>
>>>
>>>
>>> It's wrong to state or assume that the reaction Ab -> Qtz + Jd is the
>>> low-grade limit of the eclogite facies, as plagioclase feldspar in
>>> mafic
>>> rocks is not pure albite. The anorthite component expands the stability
>>> of
>>> the feldspar to lower pressure and slightly higher temperature, which
>>> then
>>> allows you to have "eclogites" at the P-T conditions you describe.
>>>
>>>
>>>
>>> See the figure attached from Palin and Dyck (2018) showing the classic
>>> Miyashiro metaomrphic facies (plus geotherms for Archean subduction
>>> zones).
>>> The low-P limit for amphibole-eclogite facies sits below the Ab -> Qtz
>>> +
>>> Jd
>>> transition because of the presence of Ca in the plagioclase in the
>>> protolith (basalt).
>>>
>>>
>>>
>>> Thanks,
>>>
>>> Richard Palin
>>>
>>> Assistant Professor of Metamorphic Geology,
>>>
>>> Department of Geology and Geological Engineering,
>>>
>>> Colorado School of Mines,
>>>
>>> Golden, CO 80401, USA
>>>
>>> http://inside.mines.edu/~rmpalin/
>>> <https://linkprotect.cudasvc.com/url?a=http%3a%2f%2finside.mines.edu%2f~rmpalin%2f&c=E,1,DRvqbhxV_z9_9943F-TIeH_8f5dWyYIZBk3ooeAfUMpj2WrlVRiAlkHJTdisO5ph2VXA7co54CPNXc12Fl0Paa9f5ijheo2OPA2VCCDy_oOFI7WWCrGSIUfwUyg,&typo=1>
>>>
>>>
>>>
>>> Tel: +1 (303) 273-3819
>>>
>>> “Rocks are records of events that took place at the time they formed.
>>> They
>>> are books. They have a different vocabulary, a different alphabet, but
>>> you learn how to read them.” – John McPhee
>>>
>>>
>>>
>>> On Thu, Aug 29, 2019 at 8:39 AM Yong-Fei?Zheng <[log in to unmask]>
>>> wrote:
>>>
>>> Dear eclogite colleagues,
>>>
>>>
>>>
>>> I was informed that during eclogitization of mafic rocks, plagioclase
>>> is
>>> not stable any more with decomposition of albite to jadeite and quartz,
>>> and
>>> diopside is transformed to omphacite by incorporating jadeite. In this
>>> regard, the mineralogical reaction of Ab to Jd + Qtz marks the lower
>>> limit
>>> of eclogite facies metamorphism. Furthermore, it is not only the
>>> boundary
>>> between eclogite and garnet amphibolite at lower temperatures of <900°C
>>> for
>>> the retrogression of pyroxene to amphibole (hydration) but also the
>>> boundary between eclogite and garnet pyroxenite at higher temperatures
>>> of
>>> >900°C for the exsolution of Na cations from omphacite. However, I am
>>> puzzled now by metamorphic conditions for the formation of some
>>> eclogites
>>> in a P-T range below the metamorphic reaction of albite to jadeite and
>>> quartz. Are such eclogites really produced by eclogitization at lower
>>> pressures but slightly higher temperatures? Or are they just metastable
>>> at
>>> reduced pressures? Any comment is welcome.
>>>
>>>
>>>
>>> Best regards,
>>>
>>> Yong-Fei
>>> --
>>>
>>> ***********************************************************************
>>>
>>> Dr. Yong-Fei Zheng
>>>
>>> Professor of Geochemistry
>>>
>>> School of Earth and Space Sciences
>>>
>>> University of Science and Technology of China
>>>
>>> Hefei 230026, China
>>>
>>> Tel & Fax: +86 551 63603554
>>>
>>> Email: [log in to unmask]
>>>
>>> **********************************************************************
>>>
>>>
>>>
>>>
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>>
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--
Ass. Prof. Dr. Dimitrios K. Kostopoulos
National and Kapodistrian University of Athens
School of Science
Faculty of Geology and Geoenvironment
Department of Mineralogy and Petrology
Panepistimioupoli, Zographou
Athens 157 84
GREECE
Tel.: +30 - 210 - 72 74 127
Fax: +30 - 210 - 72 74 883
e-mail: [log in to unmask]
http://uoa.academia.edu/DimitriosKostopoulos
https://scholar.google.gr/citations?user=PWLTr3gAAAAJ&hl=el
http://www.linkedin.com/pub/dimitrios-kostopoulos/27/aa4/11
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"Ever tried. Ever failed. No Matter.
Try again. Fail again. Fail Better"
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