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Journal of Petrology Advance Access originally published online on May 8, 2009
Journal of Petrology 2009 50(5):909-931; doi:10.1093/petrology/egp026
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© The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

An Experimental Determination of the Effect of Bulk Composition on Phase Relationships in Metasediments at Near-solidus Conditions

Fabio Ferri1,*, Stefano Poli2 and Daniel Vielzeuf3

1Dipartimento Di Geoscienze, Università Di Padova, Via Giotto 1, I-35137 Padova, Italy
2Dipartimento Di Scienze Della Terra, Università Di Milano, Via Botticelli 23, 20133 Milano, Italy
3Cinam, Cnrs, Campus De Luminy, F-13288 Marseille, France

RECEIVED JULY 4, 2008; ACCEPTED APRIL 7, 2009


  Abstract

Phase relationships in metapelites were experimentally investigated in the model system CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O on four synthetic compositions differing in their K2O content and K2O/Al2O3 ratio. Experiments were carried out at pressures ranging from 0·8 to 1·4 GPa and temperatures from 620 to 740°C under different fluid-buffered conditions. The stability of the assemblage garnet–biotite–staurolite/Al2SiO5 at the investigated conditions largely controls the absence of muscovite in K-poor compositions. At 1·2 GPa and 700°C staurolite is present with garnet + biotite + muscovite, replaced by Al2SiO5 at lower pressures through the reaction staurolite + muscovite + quartz = garnet + biotite + Al2SiO5 + H2O. In K-poor bulk compositions, garnet + biotite + staurolite + gedrite coexist up to 730°C. A compositional reversal of the Mg–Fe partitioning between garnet and staurolite is observed with decreasing pressure and is responsible for the singular equilibrium staurolite + quartz = Al2SiO5 + garnet + H2O that governs the high-temperature breakdown of staurolite. Melting is found to depend both on bulk composition and on fluid speciation. The wet solidus is represented by the reaction muscovite + anorthite + garnet + quartz + H2O = melt + biotite + Al2SiO5. Even though staurolite is never recovered at supersolidus conditions, the large pressure–temperature stability field of staurolite + muscovite and of staurolite + gedrite suggests that staurolite can be directly involved in melt production through fluid-present or fluid-absent reactions.

KEY WORDS: CKFMASH; experiments; gedrite; melting; staurolite; metapelite; metasediment

*Corresponding author. Telephone: +39-049-8272045. Fax: +39-049-8272010. E-mail: fabio.ferri{at}unipd.it


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