The core–mantle boundary layer and deep Earth dynamics

Abstract

Recent seismological work has revealed new structures in the boundary layer between the Earth's core and mantle that are altering and expanding perspectives of the role this region plays in both core and mantle dynamics. Clear challenges for future research in seismological, experimental, theoretical and computational geophysics have emerged, holding the key to understanding both this dynamic system and geological phenomena observed at the Earth's surface.The Earth acquired its primary layered structure-consisting of a molten metallic alloy core overlain by a thick shell of silicates and oxides-very early in its history, and the region near the coremantle boundary (CMB) surface has undoubtedly played a significant role in both the core and mantle dynamic systems throughout their subsequent 4.5-Gyr evolution. The CMB has a density contrast exceeding that found at the surface of the Earth, has contrasts in viscosity and physical state comparable to those at the ocean floor, and has much hotter ambient temperatures than found near the surface 1,2 . These factors, together with the requirement that significant heat be flowing from the core into the mantle in order to sustain the geodynamo (the core magnetohydrodynamic flow regime that produces the Earth's magnetic field), provide the basis for the conventional view that a significant thermal boundary layer exists at the base of the mantle, with a temperature contrast of 1;000 Ϯ 500 K (refs 2-4). With ambient temperatures averaging around 3,000 K, this hot thermal boundary layer is a likely source of boundary-layer instabilities, and it has been speculated that upwelling thermal plumes ascend from the CMB to produce surface hotspot volcanism such as at Hawaii and Iceland, transporting about 10-15% of the surface heat flux 5-7 . The large CMB density contrast favours a mantle-side accumulation of chemical heterogeneities derived from initial and continuing chemical differentiation of the mantle, possibly including downwelling subducted slabs of former oceanic lithosphere; distinctive chemical signatures of these heterogeneities may eventually be entrained into thermal plumes, resulting in unique hotspot chemistry 8,9 . There may also be a core-side chemical and thermal boundary layer, but the low seismic velocities and molten state of the core make it much harder to analyse any structure in the outermost core 2 .In the past decade, seismic tomography has provided steadily improving images of deep mantle elastic velocity heterogeneity, revealing the presence of significant large-scale (Ͼ2,000 km) patterns of coherent high-and low-velocity regions at all depths 10-12 , with enhanced lateral variations in the lowermost 300 km. Attributing the seismological variations to lateral thermal and chemical heterogeneity in the boundary layer suggests several mechanisms of coupling between the core and mantle, influencing the core flow regime as well as irregularities in rotation of the planet 2,13-15 . An experimental demonstration that chemical reactions may be continuing betwee...

Small‐scale convection in the D″ layer

“…These are associated either with topography of the D 00 discontinuity or lateral variations in seismic velocities in the D 00 layer [Weber, 1993;Lay et al, 1997;Liu et al, 1998]. Anisotropy has also been established in many regions of the D 00 layer [Lay et al, 1998b;Kendall and Silver, 1996;Matzel et al, 1996;Garnero and Lay, 1997]. This is in a drastic contrast with the mantle above it, which is isotropic [Meade et al, 1995] and is characterized by heterogeneities of much larger scale and smaller amplitude, most likely associated with the subducting plates [Grand, 1994;Grand et al, 1997;van der Hilst et al, 1997].…”

“…It is conceivable that mixing within the D 00 layer occurs mainly on length scales comparable with the thickness of the convective layer (200 -300 km) and the D 00 layer remains highly heterogeneous on larger scales. The anisotropy of the D 00 layer [Lay et al, 1998b;Kendall and Silver, 1996;Matzel et al, 1996;Garnero and Lay, 1997] can be due to redistribution and alignment of partial melt by small-scale convection or development of lattice preferred orientation in magnesiowüstite [Wentzcovitch et al, 1998]. (17).…”

Development of finite strain in the convecting lower mantle and its implications for seismic anisotropy

“…[4] Possible causes of D 00 anisotropy are discussed by Karato [1998aKarato [ , 1998b, Lay et al [1998aLay et al [ , 1998b, and Kendall [2000]. On one hand, SPO can result from oriented melt inclusions or heterogeneous material with a large contrast in elastic constants.…”

“…[2] Seismic observations reveal patches of seismic anisotropy at the base of an otherwise isotropic mantle [Lay et al, 1998a[Lay et al, , 1998bKendall, 2000]. A better understanding of the cause of this anisotropy may lead to a clearer picture of mantle chemistry and dynamics.…”

A nonlinear vacillating dynamo induced by an electrically heterogeneous mantle

“…There is evidence indicating that the Earth's lower mantle adjacent to the core-mantle interface is thermally and chemically heterogeneous (for example, Weber, 1993; Lay et al, 1998). Thermal variations at the base of the mantle impose a nonuniform thermal boundary condition that affects convection in the outer fluid core (Bloxham and Gubbins, 1987; Zhang and Gubbins, 1993; Sumira and Olson, 1999).…”