Coupled molybdenum and niobium depletion in continental basalts

Molybdenum concentrations have been determined for a suite of Late Cenozoic basalt lavas from the Basin and Range province of the western United States. Lavas older than about 5 Ma, which are thought to have had a lithospheric mantle source, are strongly depleted in Nb and Mo relative to La and Nd, respectively. Lavas younger than 5 Ma, which are chemically and isotopically similar to ocean island basalt, show no such depletion in either Nb or Mo. A close geochemical coherence between Nb and Mo is consistent with their similar ionic radii (Nb5+, 0.64; Mo4+, 0.65). Both elements are probably retained in residual rutile in a dehydrating subducted slab, while the overlying lithospheric mantle is enriched in large-ion lithophile elements (including light REE), contained in silica-rich hydrous fluids expelled from the slab.