Heavy Mo isotope composition of northern Bataan adakites, Philippines: Evidence for fore-arc subduction erosion?

The identification of an eroded fore-arc crust component in arc magmas is challenging due to the combined effects of mantle metasomatism and crustal assimilation-fractional crystallization. In this study, molybdenum (Mo) isotope compositions are used in conjunction with Sr-Nd-Hf isotopic and elemental data to identify eroded fore-arc crust components in adakites from the Cuyapo and Balungao volcanoes of the northern Bataan segment of the Luzon arc (Philippines). The Mo isotopic ratios (delta Mo-98/95, relative to the NIST SRM 3134 standard) of these adakites increase with increasing epsilon(Nd) (+4.3 to +5.6) and Ba/Nb (206-286). The low delta Mo-98/95 (-0.36 parts per thousand to -0.26 parts per thousand) in the Cuyapo adakites coupled with low Sr-Nd-Hf isotopic ratios suggests contributions from the residual slab, which lost isotopically heavy Mo during dehydration. Interestingly, the high delta Mo-98/95 (-0.18 parts per thousand to 0.00 parts per thousand) Balungao adakites have Mo-Sr-Nd-Hf isotopic ratios similar to those of the Luzon basement. Fractionated Nb/Ta (16-18) and high Sr/Y indicate the coexistence of melt with residual rutile and garnet +/- amphibole assemblages, corresponding to a source region (>similar to 45 km) below the present Luzon crust (similar to 33 km). This thus suggests an origin of heavy Mo from partial melting of eroded crust in the mantle wedge rather than in the upper-plate crust. Our work not only demonstrates that Mo isotopes may be a potential tracer of eroded crust but also highlights that lavas with combined high delta Mo-98/95, epsilon(Nd), and Ba/Nb emplaced at subduction zones with juvenile arc crust may be a result of subduction erosion.