The signature of devolatisation: Extraneous 40Ar systematics in high-pressure metamorphic rocks

Smye, Andrew J. and Warren, Clare J. and Bickle, Mike J. (2013) The signature of devolatisation: Extraneous 40Ar systematics in high-pressure metamorphic rocks. Geochimica et Cosmochimica Acta, 113. pp. 94-112. DOI 10.1016/j.gca.2013.03.018

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Abstract

Abstract The validity of using the 40Ar/39Ar system for thermochronology relies on the assumption that the source mineral is surrounded by a grain boundary reservoir defined by an effective 40Ar concentration of zero. However, the presence of extraneous 40Ar ( Ar e ) in metamorphic rocks shows that this assumption is invalid for a significant number of cases. Ar e is common in micas that have equilibrated under (ultra-)high pressure ( ( U ) HP ) conditions: metasediments from six Phanerozoic ( U ) HP terranes yield apparent 40Ar/39Ar phengite ages ≲ 50% in excess of the age of peak ( U ) HP conditions, whereas cogenetic mafic eclogites yield ages up to ∼ 700% older despite lower K2O concentrations. A model is developed that calculates Ar e age fractions as a function of variable mica–fluid K D , bulk K2O and porosity under closed system conditions. Measured Ar e concentrations in mafic eclogites are reproduced only when porosities are ≲ 10 - 4 volume fraction, showing that mafic protoliths operate as closed systems to advective solute transport during subduction. Porosities in eclogite-facies metapelites are ≲ 10 - 2 , reflecting loss of significant volumes of lattice-bound H2O relative to mafic rocks during subduction. Retention of locally-generated 40Ar in mafic eclogites shows that the oceanic crust is an efficient vehicle for volatile transport to the mantle.

Item Type:	Article
Uncontrolled Keywords:	2013AREP; IA65;
Subjects:	05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions:	05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title:	Geochimica et Cosmochimica Acta
Volume:	113
Page Range:	pp. 94-112
Identification Number:	10.1016/j.gca.2013.03.018
Depositing User:	Sarah Humbert
Date Deposited:	25 May 2013 14:14
Last Modified:	23 Jul 2013 10:06
URI:	http://eprints.esc.cam.ac.uk/id/eprint/2784

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