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The Reactive Magma Concept, Local Equilibrium, and Experimental Studies of Silicic to Intermediate Magmas

Abstract : Experimental phase equilibria are being increasingly used for the determination of pre-eruptive conditions of subvolcanic igneous bodies. Yet, numerous petrologic studies of silicic to intermediate rocks have documented evidence for disequilibrium either between phenocrysts or between phenocrysts and melt. Thus, the applicability of equilibrium experiments requires clarification. Although bulk equilibrium is generally not attained in magmas, in all cases it is possible to identify a subsystem of the magma at local chemical equilibrium (hereafter designated as the reactive magma). This is because magmas, as other natural systems, tend to approach chemical equilibrium. Local equilibrium can be proved from tests of element partitioning between coexisting phases, thermobarometric calculations, or comparing compositions of phenocrysts and melts with phases from equilibrium experimental studies. Reactive magma volumes can vary by orders of magnitude between magmatic systems, depending on physical processes and their timescales, and on equilibration kinetics. Because equilibration kinetics are much faster in melts than in crystals, sub-aphyric chemically homogeneous (on a whole-rock basis) igneous bodies (eg, Novarupta 1912 high silica rhyolite) have large reactive magma volumes, and are probably close to bulk equilibrium. By opposite, smaller reactive magma volumes are to be expected for crystal-rich bodies. The recently erupted Mt. Unzen dacite illustrates one extreme case of phenocrysts being all out of equilibrium with the interstitial melt. Another indication for small reactive magma volumes is chemical zonation of matrix glasses (eg, Laacher See Tephra). Reactive magma volumes also change with perturbations (of either physical or chemical nature) imparted to the igneous body. For pre-eruptive conditions of equilibration to be determined experimentally, starting materials must closely approach the reactive magma composition appropriate for the pre-eruptive history of the igneous body. Certain discrepancies between experimental phase and phenocryst assemblages in recent studies likely result from a poor matching between starting compositions and the appropriate reactive magma. The reactive magma concept is useful for the planning and interpretation of experimental studies on silicic to intermediate magmas.
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Contributor : Nathalie Pothier <>
Submitted on : Monday, September 11, 2006 - 2:23:50 PM
Last modification on : Friday, April 5, 2019 - 8:08:29 PM

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Michel Pichavant, Fidel Costa Rodriguez, Bruno Scaillet, Caroline Martel, Stéphane Poussineau. The Reactive Magma Concept, Local Equilibrium, and Experimental Studies of Silicic to Intermediate Magmas. American Geophysical Union, 2005, San Francisco, United States. pp.V11A-02. ⟨hal-00092563⟩

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