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

Andrew J. Smye, Clare J. Warren, Mike J. Bickle

Research output: Contribution to journalArticle

22 Citations (Scopus)

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 (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 K D, 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.

Original languageEnglish (US)
Pages (from-to)94-112
Number of pages19
JournalGeochimica et Cosmochimica Acta
Volume113
DOIs
StatePublished - Jul 5 2013

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Metamorphic rocks
metamorphic rock
Porosity
porosity
Solute transport
subduction
thermochronology
phengite
Minerals
metapelite
Volume fraction
Grain boundaries
metasediment
grain boundary
eclogite
solute transport
mafic rock
Phanerozoic
protolith
Rocks

All Science Journal Classification (ASJC) codes

  • Geochemistry and Petrology

Cite this

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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 (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 K D, 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.",
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The signature of devolatisation : Extraneous 40Ar systematics in high-pressure metamorphic rocks. / Smye, Andrew J.; Warren, Clare J.; Bickle, Mike J.

In: Geochimica et Cosmochimica Acta, Vol. 113, 05.07.2013, p. 94-112.

Research output: Contribution to journalArticle

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