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

Smye, Andrew J.; 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.

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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 (Are) in metamorphic rocks shows that this assumption is invalid for a significant number of cases. Are 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 Are age fractions as a function of variable mica–fluid KD, bulk K2O and porosity under closed system conditions. Measured Are 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: Journal Item
Copyright Holders: 2013 Elsevier Ltd.
ISSN: 0016-7037
Project Funding Details:
Funded Project NameProject IDFunding Body
Not SetNE/E0114038/1NERC (Natural Environment Research Council)
From subduction to sand: Quantifying the balance between tectonic and surface processes during early continental collision and UHP rock exhumationNE/H016279/1NERC (Natural Environment Research Council)
Not SetNot SetJackson Postdoctoral Fellowship
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM) > Environment, Earth and Ecosystem Sciences
Faculty of Science, Technology, Engineering and Mathematics (STEM)
Item ID: 39306
Depositing User: Clare Warren
Date Deposited: 22 Jan 2014 11:42
Last Modified: 07 Dec 2018 10:21
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