Seismic implications of mantle wedge plumes

We use a coupled petrological–thermomechanical model to investigate the dynamical effects of metamorphic reactions and melting on the seismic structure of thermal–chemical plumes beneath volcanic arcs. Plume generation is driven by the subduction of buoyant crustal rocks and expulsion of aqueous slab fluids that causes hydration and partial melting of the mantle wedge. The model demonstrates two chemically distinct types of plumes. Unmixed plumes initiate from the melting front within the mantle that arises as a consequence of infiltration of slab-derived water-rich fluids, whereas mixed plumes initiate from the slab itself and entrain both slab- and mantle-derived magmas. Mixed plumes explain magmas such as adakites with crustal signatures, while primitive arc tholeiites are attributed to unmixed plumes. As a consequence of the interplay between water content and temperature both positive and negative seismic velocity anomalies are associated with the plumes. Positive anomalies are prevalent close to the slab due to the lowered temperatures associated with regions of cold plume initiation. Negative seismic anomalies develop at shallower depth due to the partially molten rocks that form plume heads. Flat lying partially molten regions that form beneath volcanic arcs as a consequence of cold wet plumes are manifest by >20% variations in the local Poisson ratio, as compared to variations of ∼2% expected as a consequence of temperature variation within the mantle wedge. In contrast to models that attribute a purely thermal origin for mantle wedge seismic anomalies, the petrological–thermomechanical model is able to explain the strong seismic velocity variations existing beneath volcanic arcs.