Nb/B ratios and δ11B values of Mariana lavas reveal the involvement of two isotopically distinct slab-derived fluids in the magma genesis. The evidence of one type of fluid is observed in lavas of the islands of Anatahan, Alamagan, Agrigan, and Uracas, in which simple mixing between fluid with very homogeneous δ11B values (+5.40‰ ± 0.05‰) and the mantle wedge controls the magma compositions. In lavas of the islands of Guguan, Asuncion, and Pagan, however, the fluids that mixed with the mantle wedge are significantly heterogeneous in δ11B (+5.4‰ to +7.6‰) even within a single island. The compositional difference between these two types of fluid can result from different ratios of sediment to altered oceanic crust (AOC) in the slabs, and this hypothesis is consistent with the Sr, Nd, and Pb isotope data. We propose that the variable isotope compositions of the latter fluid may be derived from local changes in sediment/AOC ratios of the slab that are caused by subduction of the Magellan seamounts beneath the Mariana arc.