Geophysical Constraints on Mantle Composition
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This article was originally published in Treatise on Geochemistry, Second Edition published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non- commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial Bina C.R., and Helffrich G. (2014) Geophysical Constraints on Mantle Composition. In: Holland H.D. and Turekian K.K. (eds.) Treatise on Geochemistry, Second Edition, vol. 3, pp. 41-65. Oxford: Elsevier. © 2014 Elsevier Ltd. All rights reserved. Author's personal copy 3.2 Geophysical Constraints on Mantle Composition CR Bina, Northwestern University, Evanston, IL, USA G Helffrich, University of Bristol, Bristol, UK ã 2014 Elsevier Ltd. All rights reserved. This article is a revision of the previous edition article by C R Bina, volume 2, pp. 39–59, © 2003, Elsevier Ltd. 3.2.1 Introduction 41 3.2.1.1 General Considerations 41 3.2.1.2 Bulk Sound Velocity 42 3.2.1.3 Acoustic Methods 42 3.2.1.4 Electrical Resistivity Estimation 43 3.2.1.5 Seismological Methods: Velocity Contrasts 43 3.2.1.6 Seismological Methods: Differential Travel Times 44 3.2.1.7 Seismological Methods: Spatial Localization of the Signal 44 3.2.1.8 Seismological Methods: Density Constraints 44 3.2.1.9 Seismological Methods: Scattering 45 3.2.1.10 Seismological Methods: Tomography 45 3.2.2 Upper Mantle Bulk Composition 45 3.2.2.1 Overview 45 3.2.2.2 Velocity Contrasts 46 3.2.2.3 Discontinuity Topography 46 3.2.2.4 Sharpness 47 3.2.2.5 Broadening and Bifurcation 48 3.2.3 Upper Mantle Heterogeneity 49 3.2.3.1 Subducted Basalts 49 3.2.3.2 Subduction-Introduced Volatiles 50 3.2.3.3 Plume Origins 51 3.2.4 Lower Mantle Bulk Composition 53 3.2.4.1 Bulk Fitting 53 3.2.4.2 Density Fitting 54 3.2.4.3 Depthwise Fitting 54 3.2.5 Lower Mantle Heterogeneity 56 3.2.5.1 Overview 56 3.2.5.2 Subducted Oceanic Crust 56 3.2.5.3 Heterogeneity Profiles with Depth 58 3.2.5.4 Lowermost Mantle 59 3.2.6 Future Prospects 60 Acknowledgments 61 References 61 3.2.1 Introduction questions that we shall seek to address in this chapter. For brevity, we will limit the discussion to seismological and mag- 3.2.1.1 General Considerations netotelluric studies of the mantle. Direct sampling of mantle rocks and minerals is limited to Most seismological constraints on mantle composition are tectonic slices emplaced at the surface (Chapter 3.4), smaller derived by comparison of values of seismic wave velocities xenoliths transported upward by magmatic processes inferred for particular regions within the Earth to the values (Chapter 3.5), and still smaller inclusions in such far-traveled measured in the laboratory for particular minerals or mineral natural sample chambers as diamonds (Chapter 3.5). Because assemblages, with such comparisons being made under com- P T of such limited direct access to mantle materials, knowledge of parable regimes of pressure ( ) and temperature ( ). The pri- mantle structure, composition, and processes must be aug- mary parameters of interest, then, are the compressional mented by geophysical remote sensing. What can these obser- (or P-) wave velocities (VP) and the shear (or S-) wave velocities vations tell us about the major element composition of the (VS). These wave velocities are simply related to the density (r) upper mantle? How can they constrain possible differences in and to the two isotropic elastic moduli, the adiabatic bulk chemical composition between the upper and lower mantle? modulus (KS) and the shear (or ‘rigidity’) modulus (G), via 2 2 What light can they shed upon the nature of velocity heteroge- VP ¼[KS þ(4/3)G]/r and VS ¼G/r, respectively. Similarly, neities in both the upper and lower mantle? It is these magnetotelluric studies yield estimates of electrical resistivity Treatise on Geochemistry 2nd Edition http://dx.doi.org/10.1016/B978-0-08-095975-7.00202-3 41 Treatise on Geochemistry, Second Edition, (2014), vol. 3, pp. 41-65 Author's personal copy 42 Geophysical Constraints on Mantle Composition (re) integrated over some range of depth. The resistivity may the measured amplitudes of various arriving P- and S-waves, but be compared to measured values for particular substances these are much more sensitive to complex properties such as and the mantle’s composition or state (partially molten, anelasticity than are simple travel times. Seismological observ- V V fluid-saturated, etc.) inferred in that depth range. ables, then, directly yield P and S, but static compression experiments directly yield Vj. It is common, therefore, to seek to combine seismological V and V models to generate a V 3.2.1.2 Bulk Sound Velocity P S j model for a region, for comparison to mineralogically con- V Straightforward measurements of elastic properties of mate- strained j values. Such combinatoric procedures can introduce V V rials can be made via high-pressure static compression experi- additional errors, however, in that the P and S profiles used ments, in which X-ray diffraction (XRD) is used to measure may often reflect seismic ray paths or frequency bands that differ the molar volume (V), or equivalently the density (r), of a from one to the other. Indeed, special source–receiver geo- material as a function of pressure (P). The pressure dependence metries (such as those whose epicentral distances are so small of volume is expressed by the ‘incompressibility’ or isothermal as to ensure nearly overlapping ray paths) may be necessary to K K ¼V @P @V ¼ @P @ V bulk modulus ( T), where T ( / )T r( / r)T. obtain robust estimates of j values (Bina and Silver, 1997). This isothermal bulk modulus (KT) measured by static Nonetheless, comparisons to Vj, rather than to VP and VS,are compression differs slightly from the aforementioned adia- commonly used to constrain compositions in the deep interior, batic bulk modulus (KS) defining seismic velocities in that KT because (as noted later), mineralogical values for G (and hence describes resistance to compression at constant temperature, for V ) grow more problematic with increasing depth. S such as is the case in a laboratory device in which a sample is slowly compressed in contact with a large thermal reservoir such as the atmosphere. K , on the other hand, describes S 3.2.1.3 Acoustic Methods resistance to compression under adiabatic conditions, which prevail when passage of a seismic wave causes compression Direct laboratory measurements of V and V for materials can P S (and relaxation) on a timescale that is short compared to be made through acoustic methods, such as Brillouin spectros- that of thermal conduction. Thus, the adiabatic bulk modulus copy and ultrasonic techniques. Brillouin spectroscopy (or generally exceeds the isothermal value (usually by a few ‘Brillouin scattering’) determines acoustic velocities in a single percent), because it is harder to compress a material whose crystal through measurements of the Doppler shifts experi- temperature rises upon compression than one which is enced by visible light scattering from the interior of a single allowed to conduct away any such excess heat, as described crystal in which thermally induced acoustic waves are propa- K ¼K þT by a simple multiplicative factor: S T(1 ag), where a is gating. Because the technique employs single crystals, full the volumetric coefficient of thermal expansion and g is the elasticity tensors (rather than just isotropic VP and VS) can be thermodynamic Gru¨neisen parameter. determined, which are useful in studies of velocity anisotropy. Experimentally, then, the bulk modulus is the simplest The technique requires only small samples, and because visible parameter to measure, but the seismological parameters of light is employed, the samples may be placed in an optically V V primary interest, P and S, both involve the shear modulus transparent diamond-anvil cell (DAC) in order to make mea- as well. It is convenient, therefore, to define a new parameter, surements at elevated pressures and temperatures. DAC studies the ‘bulk sound velocity’ (Vj), which eliminates all depen- yielding VP and VS may be combined with XRD analyses yield- dence upon the shear modulus (G) through a judicious linear ing r, to allow for direct determination of KS and G (and combination of the squares of the two seismic wave velocities: hence V ) for single crystals (Zha et al., 1998a). j V 2 ¼K ¼V2 À V2 j S/r P (4/3) S. This new parameter (sometimes The second group of acoustic methods, the ultrasonic tech- thought of as the P-wave velocity of an ‘equivalent’ fluid, niques, require larger samples but can be performed on either for which G¼0) can be determined from dynamic (shock single crystals or polycrystalline aggregates. They require exper- 2 wave) compression data: Vj ¼ KS=r ¼ ðÞ1 þ Tag ðÞ@P=@r T . imental measurement (via interferometry) of the travel times The bulk sound velocity possesses another desirable feature of two consecutive ultrasonic echoes from the ends of a shaped in that it can also be constrained indirectly through chemical sample, combined with measurement of the length of the V V equilibrium experiments.