Hawaii, Boundary Layers and Ambient Mantle-The LLAMA Model (Invited)

Recent high-resolution seismic observations and geodynamic calculations show that midplate swells and volcanoes are controlled by processes and materials entirely in the upper boundary layer (<220 km depth) of the mantle rather than by deep seated thermal instabilities. The upper boundary layer (BL) of the mantle is fertile enough, hot enough and variable enough to provide the observed range of temperatures and compositions of midplate magmas, plus it is conveniently located to easily supply these. Seismic data show that the outer ~220 km of the mantle is heterogeneous, anisotropic and has a substantially superadiabatic vertical temperature gradient. This is the shear, and thermal, BL of the upper mantle. It is usually referred to as ‘asthenosphere’ and erroneously thought of as simply part of the well-mixed ‘convecting mantle’. Since it supports both a shear and a thermal gradient, the lower portions are hot and move slowly with respect to the surface and can be levitated by normal plate tectonic processes, even if not buoyant. The nature of BL anisotropy is consistent with a shear-induced laminated structure with aligned melt-rich lenses. The depth of the minimum isotropic shear velocity, Vs, under young plates occurs near 60 km and this rapidly increases to 150 km under older oceanic plates, including Hawaii; 150 km may represent the depth of isostatic compensation for swells and the source of magmas. The seismic lid thickens as the square-root of age across the entire Pacific but the underlying mantle is not isothermal; average subridge mantle is colder, by various measures, than midplate mantle. Thus, there are vertical and lateral temperature gradients in the upper ~200 km of the mantle and lateral gradients below the seismic lid. Mantle potential temperature at depth under the central Pacific, and elsewhere, may be ~200°C higher, without deep mantle plume input, than near spreading ridges. This is consistent with bathymetry and seismic velocities and the temperature range of non-ridge magmas. Magmas extracted from deep in a thick conduction layer are expected to be hotter than shallower oceanic ridge magmas and more variable in temperature. Mid-plate magmas appear to represent normal ambient mantle at depths of ~150 km. rather than very localized very deep upwellings. Shear-driven upwellings from the base of the BL explain midplate magmatism and its association with fracture zones and anomalous anisotropy, and the persistence of some volcanic chains and the short duration of others. The hotter deeper part of the surface BL is moving at a fraction of the plate velocity and is sampled only where sheared or displaced upwards by tectonic structures and processes that upset the usual stable laminar flow. If midplate volcanoes are sourced in the lower half of the BL, between 100 and 220 km depth, or below, then they will appear to define a relatively fixed reference system and the associated temperatures will increase with depth of magma extraction. Lithospheric architecture and stress control the locations of volcanoes, not localized thermal anomalies or deep mantle plumes. The upper mantle is Lithologically Laminated, Anisotropic, with Aligned Melt Arrays (LLAMA).