Hello again,
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.
Cheers,
Pavel
---
>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 assemblages.
>
>
>------------------------------------------------
>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...
>
>Cheers,
>
>Pavel
--
Pavel PITRA
Géosciences Rennes email: [log in to unmask]
Université Rennes 1 tel: (++33) 2.23.23.65.06
Campus de Beaulieu - Bat. 15 fax: (++33) 2.23.23.60.77
F - 35 042 RENNES CEDEX
FRANCE http://www.geosciences.univ-rennes1.fr/
http://www.geosciences.univ-rennes1.fr/spip.php?article67
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