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.

DOI: https://doi.org/10.1016/j.gca.2013.03.018

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. 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 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.

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