Mantle composition

The fact that much of the oceanic crust is made up of material of a basaltic composition derived from the upper mantle suggests that the upper mantle is composed of either peridotite or eclogite (Harrison & Bonatti, 1981). The main difference between these two rock types is that peridotite contains abundant olivine and less than 15% garnet, whereas eclogite contains little or no olivine and at least 30% garnet. Both possess a seismic velocity that corresponds to the observed upper mantle value of about 8 km s-1.

Several lines of evidence now suggest very strongly that the upper mantle is peridotitic. Beneath the ocean basins the Pn velocity is frequently anisotropic, with velocities over 15% higher perpendicular to ocean ridges. This can be explained by the preferred orientation of olivine crystals, whose long [100] axes are believed to lie in this direction. None of the common minerals of eclogite exhibit the necessary crystal elongation. A peridotitic composition is also indicated by estimates of Poisson's ratio from P and S velocities, and the presence of peridotites in the basal sections of ophiolite sequences and as nodules in alkali basalts. The density of eclogites is also too high to explain the Moho topography of isostatically compensated crustal structures.

The bulk composition of the mantle can be estimated in several ways: by using the compositions of various ultramafic rock types, from geochemical computations, from various meteorite mixtures, and by using data from experimental studies. It is necessary to distinguish between undepleted mantle and depleted mantle which has undergone partial melting so that many of the elements which do not easily substitute within mantle minerals have been removed and com bined into the crust. The latter, so called "incompatible" elements, include the heat producing elements K, Th, and U. It is clear from the composition of mid-ocean ridge basalts (MORB), however, that the mantle from which they are derived by partial fusion is relatively depleted in these elements. So much so that, if the whole mantle had this composition, it would only account for a small fraction of the heat flow at the Earth's surface emanating from the mantle (Hofmann, 1997). This, and other lines of geochemical evidence, have led geochemists to conclude that all or most of the lower mantle must be more enriched in incompatible elements than the upper mantle and that it is typically not involved in producing melts that reach the surface. However, seismological evidence relating to the fate of subducted oceanic lithosphere (Sections 9.4, 12.8.2) and the lateral heterogeneity of Layer D" suggests mantle wide convection and hence mixing (Section 12.9). Helffrich & Wood (2001) consider that the various lines of geochemical evidence can be reconciled with whole mantle convection if various small- and large-scale heterogeneities in the lower mantle revealed by seismo-logical studies are remnants of subducted oceanic and continental crust. They estimate that these remnants make up about 16% and 0.3% respectively of the mantle volume.

Although estimates ofbulk mantle composition vary in detail, it is generally agreed that at least 90% of the mantle by mass can be represented in terms of the oxides FeO, MgO, and SiO2, and a further 5-10% is made up of CaO, Al2O3, and Na2O.

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