Phase transformations and their bearing on the constitution and dynamics of the mantle

The bulk chemical composition of the Upper Mantle ("pyrolite") is derived from experimental and petrological studies of the complementary relationships between basaltic magmas and refractory peridotites. The phase transformations which are experienced by pyrolite between depths of 100-800 km are reviewed in some detail, particularly with regard to their capacity to explain the seismic P and S velocity profiles throughout this region. The transition of olivine and pyroxene to β-(Mg,Fe) ₂SiO ₄ plus garnet provides a satisfactory explanation of the velocity changes associated with the 400 km discontinuity within the limits of error of the seismic velocity determinations. Seismic velocities between 400 and 650 km are likewise consistent with this region crystallizing as an assemblage of β,γ(Mg,Fe) ₂SiO ₄ plus garnet. The depth of the 650 km seismic discontinuity corresponds closely to the pressure at which (Mg,Fe) ₂SiO ₄ spinel disproportionates to MgSiO ₃ perovskite + (Mg,Fe)O magnesiowüstite. This transformation is completed over a narrow depth interval (<4 km) and is capable of explaining the seismic characteristics of the 650 km discontinuity. The elastic properties and density of the Lower Mantle are readily explained within their observational uncertainties by a pyrolite composition crystallizing as an assemblage of perovskites plus magnesiowüstite. A substantial change in chemical composition (e.g., an increase in SiO ₂ and/or a decrease in FeO) at the 650 km discontinuity is not required by available geophysical and petrological data. The near-chondritic ratios of involatile lithophile elements in pyrolite provide important boundary conditions for geochemical Earth models and place severe limitations upon hypotheses which invoke large-scale melting early in the Earth's history. They also imply that the Mg/Si ratio of the Lower Mantle is similar to that of the Upper Mantle. The geochemical evolution and dynamical behaviour of the mantle are strongly influenced by the petrological differentiation of pyrolite at mid-ocean spreading centres to form new oceanic lithosphere. The MORB basaltic crust is underlain by a layer of harzburgite. During subduction, these lithologies each respond to sequential phase transformations in a different manner, so that at any given depth, they may differ in density from surrounding pyrolite. Most importantly, between 650 and 750 km, both former basaltic crust and harzburgite are less dense (~0.15 and 0.05 g/cm ³, respectively) than pyrolite. Relatively young and thin subducted plates may have attained thermal equilibrium with surrounding mantle by the time they reach the 650 km discontinuity. Because of their buoyancy below 650 km, these plates would not be able to penetrate the 650 km discontinuity and instead are deflected laterally along the discontinuity. This process would eventually produce a layer of former basaltic crust (garnetite), buoyantly trapped on top of the 650 km discontinuity, which would partially isolate the convective systems of the Upper and Lower Mantle. In contrast, older and thicker oceanic plates may be sufficiently cool and strong to permit their differentiated upper layers to penetrate the 650 km discontinuity. However, the tips of these plates experience substantial buoyancy stresses at this depth which cause buckling. In consequence, the descending slab piles up and forms a large melange, or "megalith," of mixed former harzburgite and former oceanic crust with cross-sectional dimensions amounting to several hundred kilometres. Its integrity is maintained for a limited period (~ 10 ⁸ a) by high viscosity arising from its lower temperature as compared to surrounding mantle. The presence of megaliths may explain a number of geophysical observations including the complex structures present near the intersections of slabs with the 650 km discontinuity, which have recently been imaged by seismic tomography, as well as associated depressions in the depth of this discontinuity. The megalith functions as a "cold finger" in the Lower Mantle and may initiate a sinking convection current. After the cessation of subduction, the megalith gradually warms up, accompanied by reduction in its viscosity, and ultimately becomes entrained in the convective system of the Lower Mantle. Mantle convection is thus envisaged as a hybrid system, with a large degree of independent convection within both the Upper and Lower Mantle, combined with a more limited exchange of material between these regions. This behaviour is enhanced by the high viscosity of the Transition Zone as compared to the Upper and Lower Mantle. The origins of intraplate (hot-spot) magmas are considered in terms of the above model. Partial melting of the former basaltic crust of the slab at depths of 200-600 km refertilizes overlying depleted peridotite. This refertilized material, entrained by the sub-ducting plate, accumulates in a thin zone immediately overlying the garnetite layer on top of the 650 km