Molybdenum isotopes in mafic igneous rocks record slabmantle interactions from subarc to postarc depths

Abstract

Abstract Arc volcanic rocks erupted above oceanic subduction zones generally have heavy molybdenum (Mo) isotopic compositions, which are attributed to contributions from oceanic slab–derived fluids with isotopically heavy Mo at subarc depths. Given that mafic igneous rocks with light Mo isotopic compositions have been rarely identified, it is unclear where light Mo isotope reservoirs reside, as these must exist due to oceanic subduction through geologic time. We present Mo isotope data from Mesozoic–Cenozoic mafic igneous rocks from the eastern North China craton, which were affected by continuous subduction of the oceanic slab. The Mesozoic mafic igneous rocks have island-arc basalt (IAB)–like features and high δ98Mo values of –0.15‰ to +0.09‰. In contrast, the Cenozoic mafic igneous rocks have ocean-island basalt (OIB)–like features and low δ98Mo values of –0.53‰ to –0.19‰. In addition, these two suites of rocks exhibit systematic differences in Sr-Nd isotopic compositions and fluid- and melt-mobile element contents, indicating that their mantle sources contained crustal components derived from different depths. The heavy Mo and weakly enriched Sr-Nd isotopic compositions and IAB-like trace-element signatures were inherited from slab fluids (including oceanic crust–derived aqueous solutions and seafloor sediment–derived hydrous melts) liberated at subarc depths. The light Mo and depleted Sr-Nd isotopic compositions and OIB-like trace-element signatures were inherited from dehydrated slab–derived melts (with rutile breakdown) at postarc depths. Therefore, Mo isotope systematics of mafic igneous rocks are a powerful means with which to trace the geochemical fluxes from subducting oceanic slabs to the mantle wedge at different depths.