The Influence of Ridge Subduction on the Geochemistry of Vanuatu Arc Magmas

The effects of buoyant ridge subduction have been researched for decades. However, it remains unclear how this process influences magma chemistry. Here we use a compilation of geochemical data, well‐established geochemical proxies (i.e., Ba/Nb, Th/Nb) and mantle redox modeling (i.e., V/Sc, Cu) to propose that the subduction of a ridge underneath the central portion of the Vanuatu arc causes shallow‐angle subduction and the development of a slab tear. We suggest that the shallow slab pinches out the asthenospheric mantle and bulldozes ancient metasomatized lithospheric mantle from the forearc toward the main‐arc. Slab‐fluxed melting of this bulldozed material could account for the along‐arc 87Sr/86Sr‐143Nd/144Nd variations of the Vanuatu magmas. The influx of hot sub‐slab material into the Vanuatu arc mantle wedge through a slab tear produces magmatism within the forearc. Modeling of V/Sc and Cu systematics suggest that the mantle source of the forearc magmas has a higher fO2 and Cu content than the source of the main‐arc and rear‐arc samples. The main‐arc and rear‐arc mantles were metasomatized by both slab‐derived fluids and melts. Whereas release of high Cu‐SO2 slab‐derived fluids caused oxidation and Cu enrichment of the forearc mantle. These systematics indicate a decrease in the fO2 of slab fluxes with increasing depth‐to‐slab and distance‐from‐trench. Our findings highlight the role of ridge subduction in controlling the along‐arc and across‐arc variations in the chemistry of Vanuatu arc magmas. This updated geodynamic model, based on geochemistry, is consistent with recent geophysical constraints and 3D numerical modeling. Plain Language Summary Subduction of oceanic plates (slabs) at convergent plate margins (trenches) causes the slab to heat up and release fluids, which causes the overlying mantle wedge to melt and produce magmatism. Some parts of the slab (e.g., ridges) are less dense than others and therefore subduct at shallower angles. This can cause (a) the subducting slab to tear and (b) the material above the subducting slab to be pushed away from the trench toward the main‐arc (a process termed “bulldozing”). It remains unclear whether the effects of ridge subduction can be detected using the geochemistry of subduction‐related arc magmas. We use a combination of geochemical data and modeling to suggest that the latitudinal variations in the chemistry of Vanuatu arc samples results from “bulldozing” effects associated with shallow angle rid