Heterogeneity and Stratification of the Mantle: What Do We Mean by Layered?

The Earth's mantle is stratified in viscosity, mineralogy, chemistry and elastic properties. The mantle is also laterally heterogeneous: the upper mantle beneath continents and oceans have different compositions, rheologies and histories. Seismic tomography shows lateral variations throughout the mantle; some of these appear to be thermal, others compositional.

Heterogeneities are formed by partial melting to form the oceanic and continental crust and by recycling the crust. Melting at mid-ocean ridges preferentially depletes the uppermost mantle. Ridges sample the top of the upper mantle with a time scale of 300My. The time scale for sampling the lower mantle is ten times longer. This leads to vertical gradients in chemistry in the mantle; the upper mantle contains depleted and recycled material, while deeper mixing takes place more slowly. Plumes may transfer material from deep in the mantle to the upper mantle and crust.

Heterogeneities are prominent near the transition zone, and seem to indicate subducted material spreading out and also extending deeper. The increase in viscosity in the lower mantle may control deeper subduction.

Many of the observations that are interpreted in terms of a layered mantle are consistent with a relatively low viscosity upper mantle that is stirred by plate motions, and a more viscous and slowly convecting deeper mantle that still communicates with the upper mantle and the surface. Modeling of Nd, Sr and Pb isotopes indicate a pervasive background matrix of small scale heterogeneities (including recycled crustal material) in the upper mantle similar in composition to FOZO. The lower mantle is less depleted, and contains fewer and larger heterogeneities from crustal formation. Basalt samples appear to be mainly mixtures of crustal residues and the background matrix, with occasional contributions from recycled material.