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Journal of Petrology Advance Access originally published online on August 18, 2007
Journal of Petrology 2007 48(10):1955-1972; doi:10.1093/petrology/egm045
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© The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Equilibration Scales in Silicic to Intermediate Magmas—Implications for Experimental Studies

Michel Pichavant1,*, Fidel Costa2, Alain Burgisser1, Bruno Scaillet1, Caroline Martel1 and Stéphane Poussineau1

1Institut des Sciences de la Terre D’orléans (ISTO), UMR 6113 CNRS-UO, 1A Rue de la Férollerie, 45071, Orléans Cedex 02, France
2CSIC, Institut de Ciènces de la Terra ‘Jaume Almera’, Lluis Solé I Sabaris S/N, 08028 Barcelona, Spain

RECEIVED JANUARY 19, 2007; ACCEPTED JULY 10, 2007


  Abstract

Experimental phase equilibrium studies are increasingly being used for the determination of intensive variables (P, T, fH2O, fO2 ) in silicic to intermediate magmas. In contrast, silicic igneous bodies are now perceived as open, periodically recharged, systems involving only limited chemical equilibration. Thus, the use of laboratory-determined crystal–liquid equilibrium data needs clarification. Here we review the field, petrological and geochemical evidence concerning states and scales of chemical equilibrium in silicic magma bodies. It is concluded that total chemical equilibrium is generally not the rule. However, a subsystem in local equilibrium (the reactive magma) can be identified. Equilibration scales in silicic magmas are rate-limited either by diffusive flux in crystals (DICL regime) or by diffusive flux in the melt (MD regime). The recognition that equilibrium in magmas is limited to a reactive subsystem requires phase equilibrium studies to be chemically scaled. Experiments, either of total or partial equilibrium type, should aim at a close reproduction of equilibrium states specific to natural systems. The laboratory reconstruction of the natural equilibrium states guarantees a precise determination of the pre-eruptive parameters and a reliable application of the experimental data to active volcanic systems.

KEY WORDS: silicic magmas; chemical equilibrium; timescales; experimental studies

*Corresponding author. Telephone: 00 33 2 38 25 53 95. Fax: 00 33 2 38 63 64 88. E-mail: pichavan{at}cnrs-orleans.fr


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