dissolution
; garnet
; low temperature
; lower mantle
; mantle discontinuity
; oceanic lithosphere
; pyroxene
; seismic tomography
; stagnation flow
; subduction zone
; transition zone
英文摘要:
Oceanic lithosphere sinks into Earth's mantle at subduction zones. However, seismic tomography shows that the sinking slabs of lithosphere often stagnate in the lower part of the mantle transition zone, at depths less than 660 km, where rocks undergo pressure-induced phase transitions and become denser. Greater pressures are required to induce phase transitions in cold slabs compared with the hotter ambient mantle at the 660 km discontinuity, and so, at the boundary between the transition zone and the lower mantle, the slabs are buoyant. The slabs may also contain low-density minerals that could contribute to their buoyancy. Here we use laboratory experiments to analyse the rate of dissolution of the common slab mineral pyroxene into garnet, at pressures and temperatures representative of the lower part of the mantle transition zone. We find that the majorite component in garnet - a product of the transition from pyroxene into garnet - is one of the slowest-diffusing components in Earth's mantle. At the relatively low temperatures of the slab, this slow diffusion inhibits the dissolution of pyroxene into garnet, so that the slab remains buoyant relative to the ambient mantle and stagnates. However, at the base of the mantle transition zone, pyroxene undergoes another phase transformation to the mineral akimotoite, which causes a sudden increase in slab density. We conclude that the slab is likely to penetrate into the lower mantle eventually.
Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany; Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Carl-Zeiss-Promenade 10, D-07745 Jena, Germany; Materialwissenschaftliche Elektronenmikroskopie, Universität Ulm, Albert-Einstein-Allee 11, D-89081 Ulm, Germany
Recommended Citation:
Van Mierlo W.L.,Langenhorst F.,Frost D.J.,et al. Stagnation of subducting slabs in the transition zone due to slow diffusion in majoritic garnet[J]. Nature Geoscience,2013-01-01,6(5)