Deep hydrous melting of the mantle wedge recorded by Mg isotopes.

Mg isotopic signatures in arc magmas reveal up to 50% of Mg originates from deep, water-saturated melting at the slab-mantle interface in subduction zones.

• Why it matters: Understanding the source of arc magmas is crucial for constraining Earth's deep volatile cycle and the role of the mantle wedge in geochemical processes, yet the influence of the slab-mantle interface remains uncertain.
• What they did: The study analyzed Mg isotopic data from multiple arcs, combined with a global magma compilation, and used Monte Carlo isotope mixing models to estimate the contribution of deep melting processes.
• The result: Findings suggest that the base of the mantle wedge, enriched by volatiles and influenced by slab geometry, significantly contributes to arc magmatism and the Earth's water cycle.