The tempo of subduction-related magmatic activity over geological time
is episodic. Despite intense study and their importance in crustal
addition, the fundamental driver of these episodes remains unclear. We
demonstrate quantitatively a first order relationship between arc
magmatic activity and subduction flux. The volume of oceanic lithosphere
entering the mantle is the key parameter that regulates the proportion
of H2O entering the sub-arc. New estimates of subduction zone H2O
budgets over the last 150 million-years indicate a three- to five-fold
increase in the proportion of H2O entering the sub-arc during the most
recent global pulse of magmatism. Step changes in H2O flux enable
proportionally greater partial melting in the sub-arc. Similar magmatic
pulses in the ancient Earth could be related to variability in
subduction flux associated with supercontinent cycles.