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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