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PDF (Chapter 24. the Nature and Cause of Mantle Heterogeneity) Chapter 24 The nature and cause of mantle heterogeneity The right to search for truth implies processes internal to the mantle. The fertility also a duty. One must not conceal any and fertility heterogeneity of the upper mantle are due to subduction of young plates, aseismic part of what one has discovered to ridges and seamount chains, and to delami­ be true. nation of the lower continental crust, as dis­ Albert Einstein cussed in Part II. TI1ese heterogeneities eventually warm up past the melting point of eclogite and become buoyant low-seismic-velocity diapirs that O verview undergo further adiabatic decompression melt­ ing as they encounter thin or spreading regions The lithosphere clearly controls the location of of the lithosphere. volcanism. The nature and volume of the volcan­ The heat required for the melting of cold ism and the presence of 'melting anomalies' or subducted and delaminated material is extracted 'hotspots,' however, reflect the intrinsic chemi­ from the essentially infinite heat reservoir of cal and petrologic heterogeneity of the upper the mantle, not the core. Melting in the upper mantle. Melting anomalies - shallow regions of mantle does not require an instability of a deep ridges, volcanic chains, flood basalts, radial dike thermal boundary layer or high absolute tem­ swarms- and continental breakup are frequently peratures. Melts from fertile regions of the attributed to the impingement of deep mantle mantle, recycled oceanic crust and subducted thermal plumes on the base of the lithosphere. seamounts, can pond beneath the lithosphere, The heat required for volcanism in the plume particularly beneath basins and suture zones, hypothesis is from the core; plumes from the with locally thin, weak or young lithosphere, or deep mantle create upper mantle heterogeneity. they can erupt. The stress state of the lithophere This violates the dictum of good Earth science: can control whether there is underplating and never go for a deep complex explanation if a shallow sill intrusion, or eruption and dike intrusion. simple one will do. Absolute mantle temperature has little to do with Mantle fertility and melting point variations, this. ponding, focusing and edge effects, i.e. plate tec­ The characteristic scale lengths - 150 km to tonic and near-surface phenomena, may control 600 km- of variations in bathymetry and magma the volumes and rates of magmatism. The magni­ chemistry, and the variable productivity of vol­ tude of magmatism may reflect the fertility and canic chains, probably reflect compositional het­ homologous temperature, not the absolute tem­ erogeneity of the asthenosphere, not the scales perature, of the asthenosphere. The chemical and of mantle convection or the spacing of hot isotopic heterogeneity of the mantle is, in part, plumes. High-frequency seismic waves, scatter­ due to recycling and, in part, due to igneous ing, coda studies and deep reflection profiles are MANTLE HOMOGENEITY: THE OLD PARADIGM 313 needed to detect the kind of chemical het­ on are products of these perceived constraints. erogeneity - blobs and small-scale layering - Absolute temperature, not lithologic diversity, predicted from the recycling and crustal delami­ is the controlling parameter in these models of nation hypotheses. geochemistry and geodynamics, and in the usual interpretations of seismic images and crustal thickness. Mantle homogeneity: the old Th e perception that the mantle is lithologi­ paradigm cally homogenous is based on two assumptions: (1) the bulk of the upper mantle is roughly Global tomography and the geoid characterize isothermal (it has constant potential tempera­ the large-scale features of the mantle. Higher ture) and (2) midocean-ridge basalts are so uni­ frequency and higher resolution techniques are form in composition ('the convecting mantle' required to understand the smaller-scale fea­ is geochemical jargon for what is viewed as tures, and to integrate geophysics with tecton­ 'the h omogenous well-stirred upper mantle') that ics and with mantle petrology and geochemistry. departures from the basic average composition The upper mantle is often regarded as being of basalts along spreading ridges and within extremely homogenous, based on low-resolution plates must come from somewhere else. The tomographic studies and the chemistry of mido­ only way thought of to do this is for nar­ cean ridge basalts. Both of these approaches aver­ row jets of hot, isotopically distinct, mantle to age out the underlying heterogeneity of the man­ arrive from great depths and impinge on the tle. The intrinsic chemical heterogeneity of the plates. shallow mantle, however, is now being recog­ The fact that bathymetry follows the square nized. This heterogeneity contributes to the iso­ root of age relation is an argument that the cool­ tope diversity of magmas and the scattering of ing plate is the main source of density varia­ seismic waves. Melting anomalies themselves - tion in the upper mantle. The scatter of ocean hotspots and swells - reflect lithologic hetero­ depth and heat flow - and many other param­ geneity and variations in fertility and melting eters - as a function of age, however, indicates point of the underlying mantle. The volume of that something else is going on. Plume influence basalt is related more to lithology of the shal­ is the usual, but non-unique, explanation for low mantle than to absolute temperature. Thus, this scatter, and for depth and chemical anoma­ both the locations of volcanism and the volume lies along the ridge. Lithologic (major elements) of volcanism can be attributed to shallow- litho­ and isotope homogeneity of the upper mantle spheric and asthenospheric- processes, processes are two of the linchpins of the plume hypoth­ that are basically athermal and that are intrinsic esis and of current geochemical reservoir mod­ to plate tectonics. els. Another is that seismic velocities, crustal Much of mantle geochemistry is based on the thicknesses, ocean depths and eruption rates are assumption of chemical and mineralogical homo­ proxies for mantle potential tempera­ geneity of the shallow mantle, with so-called tures. The asthenosphere, however, is variable normal or depleted m.idocean-ridge basalt in melting temperature and fertility (ability to (NMORB and DMORB) representative of the produce magma) and this is due to recycling homogeneity and depletion of the entire upper of oceanic crust and delaminated continental mantle source (the convecting upper mantle). crust and lithosphere. In addition, seismic veloc­ The entire upper mantle is perceived to be ities are a function of lithology, phase changes a homogenous depleted olivine-rich lithology and melting and are not a proxy for tempera­ approximating pyrolite (pyroxene-olivine-rich ture alone. Some lithologies melt at low tem­ rock) in composition; all basalts are formed by perature and have low seismic velocities with­ melting of similar peridotitic lithologies. Venera­ out being hotter than adjacent mantle. These ble concepts such as isolated reservoirs, plumes, can be responsible for melting and tomographic temperature-crustal-thickness relations and so anomalies. 314 THE NATURE AND CAUSE OF MANTLE HETEROGENEITY The isotopic homogeneity of NMORB has Earth, composed of materials with different strongly influenced thinking about the presumed intrinsic densities, will tend to stratify itself by homogeneity of the upper mantle and the inter­ density. Plate-tectonic processes introduce het­ pretation of 'anomalous' sections of midocean erogeneities into the mantle, some of which ridges. It is common practice to avoid 'anoma­ can be mapped by geophysical techniques. On lous' sections of the ridge when compiling MORB the other hand, diffusion, chaotic advection properties, and to attribute anomalies to 'plume­ and vigorous unidirectional stirring, are homog­ ridge interactions.' In general, anomalies along enizers. Convection is thought by many geo­ the ridge system - elevation, chemistry, physi­ chemists and modelers to homogenize the man­ cal properties - are part of a continuum and tle. Free convection driven by buoyancy is not the distinction between 'normal' and 'anoma­ the same as stirring by an outside agent. Melt­ lous' ridge segments is arbitrary and model ing of large volumes of the mantle, as at ridges, dependent. however, can homogenize the basalts that are erupted, even if they come from a heterogenous mantle. Mantle heterogeneity: toward There are numerous opportunities for gener­ a new paradigm ating (and removing) heterogeneities associated with plate tectonics (Figure 24.1). The tempera­ It is increasingly clear that the upper mantle is tures and melting temperatures of the mantle heterogenous in all parameters at all scales. The depend on plate-tectonic history and processes evidence includes seismic scattering, anisotropy, such as insulation and subduction cooling. Ther­ mineralogy, major- and trace-element chemistry, mal convection requires horizontal temperature isotopes, melting point and temperature. An gradients; cooling from above and subduction of isothermal homogenous upper mantle, however, plates can be the cause of these temperature gra­ h as been the underlying assumption in much of dients. The mantle would convect even if it were mantle geochemistry for the past 35 years. not heated from below. Radioactive heating from One must distinguish fertility from (trace ele­ within the mantle, secular cooling, density inho­ ment) enrichment, although these properties may mogeneities and the
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