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

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.