Age of oceanic plates at subduction and volatile recycling

Abstract

The age of the subducting plate as it enters the trench controls the maximum depth of volatile transport by the downgoing plate. As the slab descends and heats up, decarbonation and dehydration reactions cause alteration minerals and sediments to release volatiles. Our calculations show that subducting oceanic plates <11 m.y. old in oceanic arcs and <34 m.y. old in continental arcs heat up so rapidly that no H2O or CO2 can return to the asthenosphere. Instead, these volatiles rise into the over‐riding lithospheric plate. CO2 and H2O are released differently during subduction. A thickly‐sedimented plate subducting beneath an oceanic arc will return H2O to the asthenosphere only if the subducting plate is older than 11 m.y. and CO2 only if it is older than 25 m.y. If Archaean oceanic lithosphere had a maximum age of 30‐50 m.y. and an average age of 10‐18 m.y., then the amount of volatile recycling to the asthenosphere could have been much lower than at present, despite a greater total consumption rate.