This discussion is becoming more interesting!!
Just to add more gas, and from the point of view of someone who abandoned
metamorphic petrology some years ago, apparently "clear" contacts are not
always evidence for equilibrium. Specially if minerals like amphibole and
pyroxene are involved, because they have very similar crystal structures:
Partial hydration of pyroxene may lead to optically "clean" contacts between
forming amphibole and consuming pyroxene. And it's not rare to see that
px->amp substitution (hydration) proceeds through continuous silicate chains
until it finds a physical barrier (i.e., crystal boundaries). So you may end
with one (new) amphibole grain in clear contact with one (old) pyroxene.
In my opinion, this process is quite common during retrogression
(fluid-assisted) of LT eclogites.
Javier Rodriguez Aller
Servicio de Geocronología y Geoquímica Isotópica
SGIker- Facultad de Ciencia y Tecnología
Universidad del País Vasco
[log in to unmask]
De: Metamorphic Studies Group [mailto:[log in to unmask]] En
nombre de Pavel Pitra
Enviado el: viernes, 19 de junio de 2015 11:15
Para: [log in to unmask]
Asunto: Re: Granulites without garnet
It could be argued that all rocks tend towards equilibrium and may possibly
be considered always at equilibrium at some local, albeit small, scale, even
in presence of reaction "disequilibrium" textures.
In this case pseudosections can still be used at least as indicators of the
qualitative metamorphic evolution of a rock. Even more so since they can be
calculated for any chosen local bulk rock composition.
On the other hand, accessory minerals are fine, but why should they record
better the conditions of an imperfectly equilibrated rock? On top of it,
they may be difficult to relate to geologic structures and hence to the
history of the rock.
Finally, the samples presented by Tiago seemed rather well equilibrated.
>Yes, pseudosections can tell us what may theoretically happen at
>thermodynamic equilibrium in metamorphic rocks. Unfortunately, they do
>not tell us what happened in bulk mineral assemblages of either
>thermodynamic requilibration or thermodynamic disequilibrium during
>metamorphism. In contrast, refractory accessory minerals may record
>these signatures out of the thermodynamic equilibrium in bulk mineral
>Dr. Yong-Fei Zheng
>Professor of Geochemistry
>School of Earth and Space Sciences
>University of Science and Tecnology of China Hefei 230026, China Tel &
>Fax: 0086 551 3603554
>Email: [log in to unmask]
>From: <mailto:[log in to unmask]>Pavel Pitra
>Date: 2015-06-19 15:43
>To: <mailto:[log in to unmask]>GEO-METAMORPHISM
>Subject: Re: Granulites without garnet
>Hi Simon (and all)
> >Pseudosection by all means, but remember the >essentials of the
> basic petrology.
>This is obviously true (I'm not a fanatic of
>pseudosections) and it is clear to all of us that cpx-opx-pl = LP mafic
>granulite, probably something like 800°C, 6 kbar.
>You may spend a month calculating and come to pretty much the same
>result... and ask whether it was worth it... but the problem is that
>without that effort (be it thermobarometry, PT grids, pseudosections,
>or something else) you cannot really argue. And perhaps sometimes it
>can lead to some surprising discoveries...
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