We report δ^98/95Mo${\delta }^{98/95}\mathrm{Mo}$ between −0.72 and +0.07‰$+0.07\mathrm{‰}$ in different subduction zone lavas.

Slab fluids lead to arc lavas with high δ^98/95Mo${\delta }^{98/95}\mathrm{Mo}$ compared to the mantle.

Melts from subducted sediments lead to arc lavas with lower δ^98/95Mo${\delta }^{98/95}\mathrm{Mo}$.

The nature of the slab component controls δ^98/95Mo${\delta }^{98/95}\mathrm{Mo}$ in arc lavas.