Under this scenario, the rheology of oceanic mantle reflects its melting history. Because water preferentially partitions into melt, the residuum of mantle melting is predicted to be stronger and higher in seismic velocity than unmelted mantle material. In particular, the water content of olivine markedly affects the viscosity and seismic velocity of the upper mantle, with anhydrous olivine predicted to be more viscous (by orders of magnitude), and higher in seismic velocity, than partially hydrated olivine. Īlternatively, models of oceanic lithosphere that include the effects of compositional layering and melt formation and transport can produce a seismically sharp interface in the upper mantle.
Because conductive cooling is gradual and continuous, the higher seismic velocities of the lithosphere should grade into the lower seismic velocities of the asthenosphere over a length scale that is greater than the seismic wavelength typically used to map upper mantle discontinuities. By these models, the transition from lithosphere to asthenosphere occurs as minerals lose their strength at elevated temperatures and the lithosphere gradually thickens, approximately as the square root of the age of the plate. Under the simplest models, the formation of the oceanic lithosphere is by cooling, and a seismically sharp interface is not expected. However, the physical processes that give rise to a seismically detectable interface are controversial, as is the attribution of the G discontinuity to the LAB. This interpretation implies that the transition from lithosphere to asthenosphere, or from conductive to advective regimes, is sharp on the scale of a seismic wavelength. Seismic studies have documented a seismic discontinuity in the upper mantle beneath ocean basins, historically known as the Gutenberg or G discontinuity, which has often been interpreted as the “lithosphere-asthenosphere boundary” or LAB. The lithosphere and asthenosphere are further distinguished thermally by conductive versus advective heat transfer, respectively, because the lithosphere does not convect internally, whereas the asthenosphere does. Plate tectonics is broadly defined as the steady movement of colder, more rigid lithospheric plates over hotter, more ductile asthenosphere. We conclude that the G discontinuity beneath the archipelago does not mark the boundary between rigid lithosphere and convecting asthenosphere. Results from seismic imaging, the compositions of Galápagos lavas, and rare-earth-element concentrations across the archipelago require that mantle upwelling and partial melting occur over a broad region within the dehydrated and depleted layer. At the depth of the solidus for anhydrous mantle material, removal of remaining water creates a sharp decrease in velocity with depth this discontinuity may also mark a site of melt accumulation. The G discontinuity lies within a high-seismic-velocity anomaly that we conclude forms by partial dehydration and a gradual but steady increase in seismic velocity with decreasing depth after upwelling mantle first encounters the solidus for volatile-bearing mantle material. We attribute areas of shallow discontinuity depth to the formation of the dehydrated layer near the Galápagos Spreading Center and areas of greater discontinuity depth to its modification over a mantle plume with an excess temperature of 115 ± 30☌. We equate the depth of the G discontinuity to the maximum depth extent of anhydrous melting, which forms an overlying layer of dehydrated and depleted mantle. The discontinuity appears deeper beneath the portion of the Nazca plate that we infer passed over the Galápagos mantle plume than elsewhere in the region. The mean depth of the discontinuity is 91 ± 8 km beneath the southeastern archipelago and 72 ± 5 km beneath surrounding regions. An upper mantle seismic discontinuity (the Gutenberg or G discontinuity), at which shear wave velocity decreases with depth, has been mapped from S-to -p conversions in radial receiver functions recorded across the Galápagos Archipelago.
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