Physics
Scientific paper
Oct 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994pepi...86....5r&link_type=abstract
Physics of the Earth and Planetary Interiors (ISSN 0031-9201), vol. 86, no. 1-3, p. 5-24
Physics
35
Earth Mantle, Geochemistry, Petrology, Seismology, Subduction (Geology), Tectonics, Basalt, Lithology, Minerals, Peridotite, Perovskites, Spinel
Scientific paper
Recent seismic evidence suggests that subducted slabs experience resistance to further descent when they encounter the 660 km seismic discontinuity. Several possible causes of this resistance are evaluated. It is concluded that the chemical composition of the lower mantle is similar to that of the upper mantle, and that compositional change is therefore unlikely to be the cause of resistance to slab penetration. The proposal that a large increase of viscosity at the 660 km discontinuity impedes descending slabs is also rejected. However, three other factors are identified, each of which is capable of causing substantial resistance to descending slabs: (1) the negative slope of the transformation of silicate spinel to Mg-perovskite+magnesiowuestite; (2) differentiation of oceanic lithosphere into basaltic and depleted peridotitic layers, causing the slab to be buoyant compared with surrounding mantle pyrolite between depths of 660-800 km; (3) the accumulation of former oceanic crust to produce a gravitationally stable layer of garnetite (about 50 km thick) on top of the 660 km discontinuity. The combined effects of these sources of resistance provide a filter for subducted slabs. Those slabs with seismic zones extending below 600 km may possess sufficient negative buoyancy and strength to overcome the barriers and penetrate into the lower mantle. However, the resistance causes strong buckling and plastic thickening of these slabs, which accumulate to form huge blobs or 'megaliths' underneath the 660 km discontinuity. In contrast, slabs with seismic zones extending no deeper than 300 km possess much smaller degrees of negative buoyancy and strength and hence are unable to penetrate the 660 km discontinuity. Slabs of this type are recycled within the transition zone and upper mantle. Mixing and petrological homogenization processes are less efficient in the transition zone than in the upper mantle (above 400 km). The transition zone is composed mainly of ancient slabs of differentiated oceanic lithosphere, with discrete lithological domains of former oceanic crust, former harzburgite and former lherzolite. However, in the upper mantle, these lithologies have been homogenized via convective mixing and partial melting to form a uniform pyrolite composition. Seismic velocity gradients in the heterogeneous mixture of lithologies of the transition zone are higher than they would be in homogeneous pyrolite, because of the survival of clinopyroxene to a depth of about 500 km. This may contribute towards high seismic velocity gradients observed in the transition zone. The behavior during partial melting of heterogeneous lithologies from the transition zone is also quite different from the partial melting of homogeneous pyrolite. The transition zone provides the principal source region for alkaline intraplate basalts world-wide (including ocean island basalts) whereas the homogeneous pyrolite source region of the upper mantle yields mid-ocean ridge basalt magmas.
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