Phase Transformations and Differentiation in Subducted Lithosphere: Implications for Mantle Dynamics, Basalt Petrogenesis, and Crustal Evolution

Abstract

Mantle pyrolite differentiates at mid-oceanic ridges to form a layered lithosphere consisting of a basaltic crust, immediately underlain by harzburgite and further underlain by pyrolite which has experienced depletion only of highly incompatible elements (e.g., Rb, light REE). The body forces driving subduction are concentrated mainly in the upper cool, brittle layer of lithosphere, comprised of basalt and harzburgite. The lower layer of relatively ductile pyrolite is stripped off during subduction and resorbed into the upper mantle. This material, which is depleted in highly incompatible elements, provides a future source region of mid-ocean ridge basalt magmas on a timescale of 10⁹ years. The Nd, Sr, and Pb isotopic characteristics of MORBs are explained on the basis of this model. The slab, which sinks to ~600 km, is comprised mainly of former basalt and harzburgite. These differentiated layers undergo a significantly different series of phase transformations to those experienced by mantle pyrolite. The characteristics of these phase transformations and their influence on the density contrast between the slab and surrounding pyrolite are reviewed in detail. They cause the cool subducted lithosphere to remain denser than surrounding mantle to 600-650 km. Below 650 km, former basaltic crust remains denser than surrounding mantle, whereas former harzburgite becomes relatively buoyant. The resulting non-uniformity in stress distribution causes the slab to buckle and to accrete to form a large, relatively cool, ovoid "megalith" of mixed former harzburgite and basaltic crust, sitting on the seismic discontinuity at 650 km. The megalith (dimension >300 km) is heated mainly by thermal conduction, and thermally equilibrates with surrounding mantle on a timescale of 1-2 b.y. Partial melting of entrained former basaltic crust ensues. The resultant liquids contaminate surrounding regions of former harzburgite, rendering them fertile in the sense of future capacity to produce basaltic magmas. As the megalith warms up, its viscosity falls, and large dense blocks of former oceanic crust (now depleted in incompatible elements by partial melting) sink into the lower mantle. Newly fertile, former harzburgite is now buoyant. Diapirs of this material separate and rise into the upper mantle, becoming incorporated in the lithosphere, and experience small degrees of partial melting to produce the alkaline basaltic suite. In oceanic regions, the rising diapirs are responsible for "hot-spot" alkaline volcanism whereas, in continental regions, the upwelling diapirs cause doming and rifting, also accompanied by alkaline activity. The residual components of the diapirs become permanently incorporated into the sub-continental lithosphere. This is a cumulative process and is ultimately responsible for the development of the chemical, physical, and isotopic characteristics of the sub-continental lithosphere. The U-Pb, Nd-Sm, and Rb-Sr characteristics of alkaline (and calcalkaline) associations can be explained in terms of the model. The geochemical evolution of the continental crust and its formation from the mantle by multistage irreversible differentiation processes are also examined within the framework of the model. The extraction of highly incompatible elements (e.g., Rb, K, Ba, U) is essentially decoupled from the extraction of the major elements of the crust (Si, Al, Ca, Fe, Mg, Na). The incompatible elements derive via MORB source regions from a reservoir comprising the upper 650 km of the mantle. The sialic elements, on the other hand, are derived from much more localized reservoirs within this region, namely the mantle wedges overlying subduction zones. The final products of long-term irreversible differentiation of mantle pyrolite are the sialic continental crust, the sub-continental lithosphere of depleted peridotite, and the former oceanic crust, which is consigned ultimately to the lower mantle.