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Topography of the Transition Zone Seismic Discontinuities

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Topography of the Transition Zone Seismic Discontinuities TOPOGRAPHY OF THE TRANSITION ZONE SEISMIC DISCONTINUITIES George Helffrich Earth Sciences University of Bristol Bristol, England Abstract. Phase transformations in mantle mineralo- tool for the mantle in mind, we review the basic seismo- gies probably cause the transition zone seismic disconti- logical and phase equilibrium concepts underlying the nuities, nominally at 410, 520, and 660 km depth. Ther- detection and interpretation of the seismic wave arrivals modynamic principles govern phase transformations, associated with the transition zone discontinuities. Re- making them sensitive to changes in the mantle’s ambi- viewing the evidence for and against the phase transition ent conditions through the thermodynamic response: model, we conclude that it is viable and describe how Changes in temperature or composition shift the trans- discontinuities have been and can be used to probe the formation to a different pressure, creating topography physical and chemical state of the transition zone. on a level discontinuity. With this use as an exploratory 1. INTRODUCTION discontinuity depths as a tool, a tool that turns out to be surprisingly versatile, under continual refinement, and 1.1. Radial Earth Structure increasingly common in a research seismologist’s tool Discontinuities separate regions of smoothly varying kit. seismic velocity inside the Earth (Figure 1). Working outward from the Earth’s center, the discontinuities are 1.2. Detecting Discontinuities at the inner core boundary, then at the core/mantle Seismic discontinuities are detected by the additional boundary (CMB), then three midmantle discontinuities arrivals they cause in the seismic wave field by reflection at 410, 520, and 660 km depth, and then the Moho at and refraction of the direct wave at the discontinuity. about 35 km depth. (Terms in italic are defined in the Reflection typically leads to an earlier arrival than would glossary, following the main text.) Two of these discon- be expected in a uniform medium. Its timing relative to tinuities, the Moho and the CMB, correspond to signif- the direct arrival yields the discontinuity’s depth. A icant compositional changes in the stuff of the Earth: the refraction usually leads to more than one arrival, where boundary between a peridotitic mantle and an iron-rich only one would be expected in a uniform medium. In this core in the case of the CMB and between a mafic mantle case, one deduces the discontinuity depth from the dis- and a silicic crust in the case of the Moho [Williamson tance from the seismic source where the multiple arriv- and Adams, 1923]. This paper focuses on the mantle als appear. The Earth’s major discontinuities were ini- discontinuities in the transition zone, the region between tially discovered this way, and others continue to be about 400 and 800 km depth. Whereas in seismology’s discovered like this [Oldham, 1906; Lehmann, 1936; nascency new discontinuity discoveries progressively re- Song and Helmberger, 1998]. fined our basic knowledge of Earth structure, in its There are quite diverse methods for measuring dis- maturity the discontinuities themselves become investi- continuity depths, probably best described through gative tools rather than investigative targets. The goal of sketches (Figure 2) (see Shearer [1991] for more exotic this review is to show how changes in discontinuity depth ones). One way to characterize the methods is by the have been and can be used to investigate the mantle’s position where the seismic wave interacts with the dis- properties. The argument develops along two lines. The continuity: near the source, near the receiver, or else- first establishes what the major discontinuities at 410, where. Near-source studies provide the best spatial res- 520, and 660 km depth are. Significantly, the topography olution because there is little opportunity for the seismic on the discontinuities gives strong evidence to discrimi- wave field to spread out before interacting with the nate among different ideas about how they arise. The discontinuity (its Fresnel zone width is small). Spatial second strand describes the sort of scientific craftsman- resolution for near-receiver studies is worse since the ship that can be achieved using the spatial variation of interaction is at least 410 km from the receiver. Interac- Copyright 2000 by the American Geophysical Union. Reviews of Geophysics, 38, 1 / February 2000 pages 141–158 8755-1209/00/1999RG000060$15.00 Paper number 1999RG000060 ● 141 ● 142 ● Helffrich: DISCONTINUITY TOPOGRAPHY 38, 1 / REVIEWS OF GEOPHYSICS Figure 1. P and S wave speed and density profile in the Earth [Kennett et al., 1995]. Jumps at 410 and 660 km depth demark the transition zone discontinuities. Right panel provides an expanded view of the Earth’s upper 800 km, which includes the transition zone. The 520 is not featured in this model. tions elsewhere in the mantle have correspondingly Earth [Bernal, 1936]. The transition zone could repre- poorer spatial resolution. sent a broad transition, mixing material with low-pres- Seismic wave frequency affects the size of the Fresnel sure properties and high-pressure ones [Meijering and zone and thus its spatial resolution. Figure 3a diagrams Rooymans, 1958]. the Fresnel zone of the near-receiver conversion P660s. In contrast, Williamson and Adams [1923] argued for ϳ Its Fresnel zone is 500 km wide at the discontinuity, a varying composition with depth. In their proposed blunting its ability to resolve narrow structures. In con- Earth structure the greatest departure from the contem- trast, Figure 3b shows the near-source conversion S660P porary view was a gradual change in composition below for an earthquake at 500 km depth. Its 100- to 200-km about 1600 km depth through a mixed silicate and iron Fresnel zone at the 660 arises due to the proximity of the region. Below the shallow crust’s silica-rich layer and source to the discontinuity under study. With a narrower above this mixed region, they found that self-compres- probe of the discontinuity, correspondingly smaller scale sion of material at constant composition explained best structure is visible (as suggested by larger reported to- the density profile they inferred from the seismic travel pography using short-period studies, to be discussed time tables available at the time. Birch [1952, 1961] below). reexamined this idea later, and while he did not rule out compositional changes as a possibility, he was not a 2. NATURE OF THE 410, 520, AND 660 proponent for two reasons. First, there was a contradic- tion between transition zone thermal gradients inferred The enduring idea that discontinuous changes in from the seismic velocity structure and from mantle heat wave speeds in the transition zone correspond to phase flow. Second, a compositional change violated the then changes in mantle minerals arose almost simultaneously known velocity-density systematics developed by Birch with the discovery of the discontinuities. On April 24, [1961]. Ringwood’s [1958a, b] experiments on the silicate 1936, H. Jeffreys read a paper on seismic velocities in the olivine polymorphs yielded an appropriate seismic wave mantle in which he noted that the kinks in travel time speed and density for transition zone material. The curves at 20Њ epicentral distance marked discontinuous simplest explanation lay in attributing the transition changes in seismic wave speeds. In the discussion that zone velocity gradients to phase changes rather than to followed, J. D. Bernal reminded the audience that V. M. compositional changes. Goldschmidt’s work on the germanate analog to the These two views framed the debate vigorously pur- silicate mineral olivine underwent pressure-mediated sued by the seismology and mineral physics communities phase transitions to denser structures. Bernal speculated until quite recently. The crucial test to distinguish be- that the discontinuous increases in seismic wave speeds tween the competing ideas arose among those studying could be due to changes in the structure of olivine the way the discontinuities influence mantle convection brought about by pressure increasing with depth in the [Christensen and Yuen, 1984; Kincaid and Olson, 1987]. 38, 1 / REVIEWS OF GEOPHYSICS Helffrich: DISCONTINUITY TOPOGRAPHY ● 143 Figure 2. Seismic wave interaction geometries with the discontinuities in the transition zone. P (solid lines) and S (dashed lines) ray paths leave source (stars) and reflect off or convert type at discontinuity (long-dashed line). The interactions illustrated in the top panel take place either near the receiver (Pds) or midway between the source and receiver (ScSdScS, PЈdPЈ, PdP). The bottom panel shows the interactions that occur near the source (SdP and sdP). Any mass flux through a discontinuity will entrain a halo mantle [Jeanloz and Thompson, 1983; Katsura and Ito, of surrounding material with it. If the discontinuity is 1989; Ito and Takahashi, 1989]. compositional, the body moving through it will not Another way to discriminate between the two discon- readily mix with the material on the other side of the tinuity explanations is to compare the observed seismic discontinuity due to its high viscosity. Therefore the characteristics with ones calculated from the material discontinuity will bulge downward or upward in the properties of the substances supposedly lying athwart direction of mass flux. Alternatively, if the discontinuity the discontinuity. The seismic observables are the reflec- results from a phase change, the thermodynamic param- tion amplitudes from the discontinuity as a function of eters governing the phase change dictate the discontinu- frequency. This approach requires both good material ity’s position. Provided that heat can diffuse in or out of property data (density, P wave speed, and S wave speed) the region at rates comparable to material motion, and and thermodynamic data (to calculate the discontinuity’s that metastability does not play a role, the discontinuity thickness or transformation interval in pressure), the will adopt its equilibrium position in the mantle, moving province of mineral physics, experimental petrology, and upward or downward either with or against the mass flux solid solution modeling.
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