Across‐Arc Molybdenum Isotopes Variations in the Kamchatka Subduction Zone System

Molybdenum (Mo) isotopes are emerging as a valuable tracer of magmatic processes across diverse geological settings. Here we present Mo isotope data from the Kamchatka arc in the NW Pacific, spanning basaltic lavas along a SE – NW transect from the volcanic front to the back-arc. Most of the studied volcanic centres are situated above the subducting Hawaii–Emperor Seamount Chain.

Our results reveal systematic variations in δ⁹⁸/⁹⁵Mo correlated with trace element ratios (Ce/Pb, Ce/Mo, Nb/Zr, La/Sm) and ¹⁴³Nd/¹⁴⁴Nd across the arc. Lavas from the arc front exhibit higher δ⁹⁸/⁹⁵Mo and lower Ce/Pb, Ce/Mo, Nb/Zr, and La/Sm compared to back-arc lavas. As a subducted sediment contribution can be ruled out, we interpret these trends as reflecting a change in mantle source composition from arc front to back arc.

At the arc front, high δ⁹⁸/⁹⁵Mo and low Ce/Pb and Ce/Mo point to a dominant slab-derived fluid component. In contrast, back-arc lavas show signatures more typical of an enriched mantle, with mantle-like Ce/Pb and Ce/Mo, and elevated Nb/Zr and La/Sm. Notably, the combined δ⁹⁸/⁹⁵Mo, Nd, and Pb isotope signatures of back-arc lavas closely resemble those of modern Hawaiian ocean island basalts.

These observations suggest that the back-arc mantle may have been modified by an enriched asthenospheric component akin to that beneath Hawaii, potentially introduced by the subducted Hawaii–Emperor Seamount Chain.