Two isotopically distinct fluid components involved in the Mariana arc: Evidence from Nb/B ratios and B, Sr, Nd, and Pb isotope systematics

Nb/B ratios and delta(11)B values of Mariana lavas reveal the involvement o f two isotopically distinct slab-derived fluids in the magma genesis. The e vidence of one type of fluid is observed in lavas of the islands of Anataha n, Alamagan, Agrigan, and Uracas, in which simple mixing between fluid with very homogeneous delta(11)B values (+5.40 parts per thousand +/- 0.05 part s per thousand) and the mantle wedge controls the magma compositions. In la vas of the islands of Guguan, Asuncion, and Pagan, however, the fluids that mixed with the mantle wedge are significantly heterogeneous in delta(11)B (+5.4 parts per thousand to +7.6 parts per thousand) even within a single i sland. The compositional difference between these two types of fluid can re sult from different ratios of sediment to altered oceanic crust (AOC) in th e slabs, and this hypothesis is consistent,vith 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 are.