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Dear all,

Enclosed an example from El Hoyazo, Spain, similar to the one reported by Kurt, with the control of the neighbouring phase on reaction development, but also WITH melt (glass visible in the upper part of the BSEM image). Ilmenite forms with Opx and melt because biotite has 6 wt% TiO2.

Sorry, Martin…

Cheers,

bernardo

Dear all,

This is an interesting topic and thank you all for the inspiring discussion. I add a photomicrograph (250 kb) to this discussion which could be interesting to you. It shows an Opx crust growing from biotite and replacing the mica by the prograde reaction Bt + Qtz => Opx + Kfs + H2O. Interestingly, the Opx crust only develops where Bt is (was) in direct grain contact to Qtz, no Opx grows along Kfs + Bt and Crd + Bt contacts. Biotite is the only hydrate mineral in this granulite, other minerals include Crd, Grt, Sap, and Sil. The texture can be interpreted in different ways. My favorite is: The small amount of H2O released by the dehydration of the mica does not saturate the grain boundaries with a fluid phase. Chemical communication distance along the grain boundaries is small with the consequence that SiO2 from Qtz does not make it to Bt + Kfs boundaries. The local texture and also the texture of the rock does not suggest the presence of a melt. So, the picture has it all what Martin Hand wanted to see.

Kurt Bucher

For more details see: Droop & Bucher-Nurminen 1984: Jour. Petrol., 25, 766-803.

*From:*Metamorphic Studies Group [mailto:[log in to unmask]] *On Behalf Of *Martin Hand

*Sent:* Wednesday, December 07, 2016 1:03 PM
*To:* [log in to unmask]
*Subject:* [geo-metamorphism] Subsolidus prograde opx in metapelite

Dear all,

Has anyone come across or know of examples where opx has grown during prograde metamorphism of metapelite at subsolidus conditions?

Martin Hand

Department of Earth Sciences

University of Adelaide

Phone : +61 (0)8 831 35324

http://www.adelaide.edu.au/geothermal/

http://www.adelaide.edu.au/directory/martin.hand

--
Kurt Bucher (Prof. Dr.)
Institute of Mineralogy and Geochemistry
University of Freiburg
Albertstrasse 23b D-79104 Freiburg Germany

Phone 49-761-203-6417 (direct) 6396 (general office) 6407 (FAX)
http://www.minpetro.uni-freiburg.de
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<Fig.7.14 Bt breakdown.jpg>

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Bernardo Cesare

Dipartimento di Geoscienze, Universita' di Padova

Via G. Gradenigo, 6, I-35131 PADOVA ITALY

Tel: ++39-049-8279148 Fax: ++39-049-8279134

email: [log in to unmask]

Web site: http://147.162.183.153/bernardo/berning.html

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MicROCKScopica: http://www.microckscopica.org

InsiemePerWamba: http://www.insiemeperwamba.org

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